Development of the osteocranium in the suckermouth armored ... · Development of the Osteocranium in the Suckermouth Armored Catﬁsh Ancistrus cf. triradiatus (Loricariidae, Siluriformes)

Development of the Osteocranium in the SuckermouthArmored Catfish Ancistrus cf. triradiatus(Loricariidae, Siluriformes)

Tom Geerinckx,* Marleen Brunain, and Dominique Adriaens

Evolutionary Morphology of Vertebrates, Ghent University-UGent, K.L. Ledeganckstraat 35, Gent 9000, Belgium

ABSTRACT The development of the osteocranium of thesuckermouth armored catfish Ancistrus cf. triradiatus isdescribed based on specimens ranging from prehatchingstages to juvenile stages where the osteocranium is moreor less fully formed. The first bony elements that arise arethe opercle, jaws, and lateralmost branchiostegal rays, aswell as the basioccipital and parasphenoid in the skullfloor. The supracleithrum and the membranous and peri-chondral pterotic components form one large, double-lay-ered skull bone during ontogeny, without clear evidence ofthe involvement of a supratemporal. The Baudelot’s liga-ment ossifies from two sides, i.e., from the basioccipitalmedially and the supracleithrum laterally. The lower jawconsists of a dentary, mentomeckelian, and angulo-articu-lar, which all soon fuse. The parurohyal, formed by thefusion of a ventral sesamoid bone and a dorsal cartilageelement associated with the first basibranchial, is piercedby a vein, unlike in some other siluriforms. The interhyalcartilage disappears during ontogeny; medially of it, asmall sesamoid bone appears in a ligament. The largest,canal-bearing cheek plate is not homologous to the intero-percle. The results of the present research, with emphasison bone formations and homologies, are compared withstudies on related catfishes. J. Morphol. 268:254–274,2007. � 2007 Wiley-Liss, Inc.

KEY WORDS: bone; cartilage; catfishes; ontogeny; ossifi-cation; skeleton

Development of structures and the early life his-tory of fishes are closely related. Early life-historystages must also function as organisms, so the studyof ontogeny is an obvious necessity, if one wants tounderstand the integration of, e.g., the feeding ap-paratus (Lauder et al., 1989). The vital importanceof events occurring during early development is eas-ily overlooked (Orton, 1955).

The current study on the loricariid Ancistrus cf.triradiatus describes and discusses the developmentof the osteocranium in detail, in continuation of ear-lier work on the chondrocranium of the species(Geerinckx et al., 2005). Loricariids are well knownfor their remarkable niche occupation, i.e., thescraping and sucking of algae and other food itemsoff submerged substrates. Of some loricariid speciesonly the adult skeletal morphology has been exam-ined by Alexander (1965), Schaefer (1987, 1988,1997), Schaefer and Lauder (1986), and others.

Compared with the development of the siluriformchondrocranium, the development of the osteo-cranium has received remarkably less interest.Relevant publications on the development of thebony skull in catfishes discuss Ariidae (Bamford,1948; Tilney and Hecht, 1993), Callichthyidae (Hoe-deman, 1960a,b), Clariidae (Surlemont and Van-dewalle, 1991; Adriaens et al., 1997; Vandewalleet al., 1997; Adriaens and Verraes, 1998), Clarotei-dae (Vandewalle et al., 1995), Ictaluridae (Kindred,1919), Siluridae (Kobayakawa, 1992), and the sus-pensorium of Diplomystidae, Trichomycteridae, andsome other families (Arratia, 1990, 1992). The on-togeny of the Weberian apparatus in Clarias garie-pinus and Corydoras paleatus has been examinedby Radermaker et al. (1989) and Coburn and Gru-bach (1998), respectively. Homology of the anteriorvertebrae has been treated by Hoffmann and Britz(2006). The Weberian apparatus and complex verte-bra are only briefly discussed in the present paper.Taking into consideration their complexity, a thor-ough study of these structures would merit a sepa-rate paper.

Loricariids are an exceptionally interesting fishtaxon, as they are able to respire while attaching toa substrate with their sucker mouth. The added pos-sibility of feeding in their typical manner (aforemen-tioned) is even more unusual. Knowledge of theontogenetic origin of the osteocranium in loricariidsis lacking. The development of many bones (e.g.,jaws) is, however, highly interesting, as they arecrucially modified key structures in the sucking andscraping device that has evolved in this large anddiverse catfish group. Some of these key structuresmight well have had an influence on the diversitywithin the large loricariid family.

Contract grant sponsor: Institute for the Promotion of Innovationthrough Science and Technology in Flanders (IWT-Vlaanderen); Con-tact grant sponsor: FWO; Contract grant number: G.0355.04.

*Correspondence to: Tom Geerinckx, Evolutionary Morphology ofVertebrates, Ghent University-UGent, K.L. Ledeganckstraat 35,Gent 9000, Belgium. E-mail: [email protected]

Published online 13 February 2007 inWiley InterScience (www.interscience.wiley.com)DOI: 10.1002/jmor.10515

JOURNAL OFMORPHOLOGY 268:254–274 (2007)

� 2007 WILEY-LISS, INC.

Next to the description of several ontogeneticstages, we discuss our observations in the light ofpossible homologies or nonhomologies with struc-tures present in related taxa, in order to try to shedsome light on the problematic identifications of skel-etal elements in loricariids. Finally, some more func-tional–morphological considerations are made.

MATERIALS ANDMETHODS

This study focuses on one loricariid representative A. cf. trira-diatus (Eigenmann, 1918). Identification up to species level isproblematic, as the genus is in need of a thorough revision, andcomplete determination keys are not available.Commercially obtained specimens were bred in aquarium tanks

of 30–130 cm. Embryos and juveniles (there is no distinct larvalstage) were sedated in MS-222 and fixed in a paraformaldehyde–glutaraldehyde solution at different time intervals (Table 1). Forprehatching stages, egg membranes were removed prior to fixa-tion. Eight specimens were selected for serial sectioning (Table 1).Toluidine-stained 2-lm sections (5 lm for 33.5-mm specimen) wereprepared (Technovit 7100 embedding, cut with a Reichert-JungPolycut microtome) and studied using a Reichert-Jung Polyvarlight microscope. Other specimens were cleared in toto and stainedaccording to the method of Taylor and Van Dyke (1985). Examina-tion of the specimens was done using an Olympus SZX9 stereo-scopic microscope, equipped with a camera lucida for drawing. Asan aid to the drawings, dissections (e.g., removal of pectoral girdleor part of the splanchnocranium) were performed in the largerspecimens. Drawings figure all cartilaginous and bony elements ofthe skull, which are visible on the cleared and stained specimens.However, the study of serial sections of specimens demonstrates

that sometimes early ossification is not visible on stained speci-mens of the same or earlier length or age, a known artifact in intoto staining techniques (Vandewalle et al., 1998). In such casesthis is clearly mentioned in the text. All cleared and stained speci-mens have been deposited in the Zoology Museum of the GhentUniversity (UGMD 175351-369; see Table 1).

Bone terminology is mostly based on Harrington (1955), Patter-son (1975, 1977), Schaefer (1987), and Arratia (2003). We refer tode Beer (1937) and Geerinckx et al. (2005) for terminology ofchondrocranium parts.

RESULTS

No endochondral bone is encountered in A. cf. trir-adiatus up to the 20.7 mm SL stage. The skull iscomposed of perichondral and membrane bones(including dermal bones), as well as a few sesamoidbones and bones of compound origin.

5.6 mm Standard Length (Fig. 1)

Only one pair of bony elements is present in thisspecimen (in serial sections of the younger 4.8- and5.2-mm specimens only a cartilaginous skeleton isvisible). Near the posterior end of the cartilaginoussuspensorium, at the distal tip of the opercular pro-cess, the opercles have appeared as small bonysplints. No other bony structures are seen. At thefuture location of each premaxilla, however, fourtooth primordia can be recognized.

TABLE 1. Specimens of Ancistrus cf. triradiatus used in the present study

No. UGMD no. SL (mm) SkL (mm) Age (dPF) Method Staining Use

1 — 4.8 — 2 SS T Observation2 — 5.2 — 3 SS T Observation3 175351 5.6 1.24 4 C AB þ AR Drawing4 175352 6.0 1.39 4 C AB þ AR Drawing5 — 6.1 — 4 SS T Observation6 175353 6.3 1.47 5 C AB þ AR Observation7 175354 6.8 1.60 5a C AB þ AR Observation8 — 7.0 — 6 SS T Observation9 175355 7.4 1.82 6 C AB þ AR Drawing

10 175356 7.7 2.00 6 C AB þ AR Observation11 175357 8.0 2.16 7 C AB þ AR Drawing12 — 8.0 — 7 SS T Observation13 175358 8.5 2.38 7 C AB þ AR Observation14 175359 8.7 2.45 7 C AB þ AR Observation15 175360 8.9 2.51 8 C AB þ AR Observation16 175361 9.1 2.64 8 C AR Drawing17 175362 9.8 2.98 12 C AR Drawing18 — 10.2 — 14 SS T Observation19 175363 10.8 3.4 18 C AR Drawing20 175364 11.7 3.8 30 C AB þ AR Drawing21 — 12.4 — 43 SS T Observation22 175365 14.4 5.0 45 C AB þ AR Drawing23 175366 16.4 5.7 67 C AB þ AR Observation24 175367 20.7 7.3 96 C AB þ AR Drawing25 175368 25.0 10.3 160 C AB þ AR Observation26 175369 31.0 12.5 160 C AB þ AR Observation27 — 33.5 — 160 SS T Observation

AB, alcian blue; AR, alizarin red S; C, clearing; dPF, days postfertilization; SL, standard length; SkL, chondrocranial skull length(from tip of ethmoid plate to end of basis of occipital pilae, thus excluding tectum posterius); SS, serial sections; T, toluidine blue;UGMD, Universiteit Gent Museum voor Dierkunde (Zoology Museum of Ghent University).aImmediately after hatching.

OSTEOCRANIUM IN ANCISTRUS CF. TRIRADIATUS 255

Journal of Morphology DOI 10.1002/jmor

6.0 mm Standard Length (Fig. 2)Neurocranium. The lateralmost protuberances

of the otic capsule bear a small odontode, andanother small odontode is visible on the right post-otic process. Odontodes are extra-oral teeth, part ofthe body armor of several fish taxa (Bhatti, 1938;Reif, 1982). Serial sections of a 6.1-mm specimenshow precursor tissue of the future supportingbones, but no ossification.

Splanchnocranium. Thin premaxillae haveappeared, supporting four to five teeth. The maxil-lae arise as thin bony splints supporting the maxil-lary barbel cartilages, and develop articulation fac-ets for the growing but still short autopalatine (orpalatine) cartilages (pars autopalatina of Arratia,1990). The cleared and stained 6.0-mm specimenhas neither dentaries nor dental teeth; however, se-rial sections of the 6.1-mm specimen show the pres-ence of four short conical teeth at the location whereeach dentary is to be expected. Both the premaxil-lary and the dental teeth are still embedded in softtissue in the 6.1- and 7.0-mm specimens. Two thinbranchiostegal rays attach to the paired ventrocau-dal process of the hyoid bar. The position of theirinsertion suggests that they are the lateralmostrays III–IV. The opercles have grown somewhat,and a pair of odontodes is present in the skin cover-ing them.

7.4 mm Standard Length (Fig. 3)Neurocranium. On the taeniae marginales the

dermal frontals have started developing. They areaccompanied by two odontodes in the skin. Odonto-des can now also be seen on both postotic processes

of the otic capsule. A first indication of the para-sphenoid is seen in the posterior half of the hypo-physeal fenestra on the serial sections in the 7.0

Fig. 1. Dorsal view of the skull of Ancistrus cf. triradiatus(5.6 mm SL). bd-5, basidorsal of fifth vertebra; c-eth, cartilagoethmoideum; c-Meck, cartilago Meckeli; c-mx, cartilago maxilla-ris; c-pc, cartilago parachordalis; ch, ceratohyale; fn-hyp, fenes-tra hypophysea; fn-met, fenestra metotica; hh, hypohyale; hs,hyosymplecticum; lm-bot, lamina basiotica; not, notochord; o-op,os operculare; ot-cap, otic capsule; p-q, pars quadrata of palato-quadratum; pl-oc, pila occipitalis; pmx-t, premaxillary teeth; tr-cr, trabecula cranii.

Fig. 2. Skull of Ancistrus cf. triradiatus (6.0 mm SL). (a) Dorsalview. (b) Ventral view. (c) Lateral view. bd-5, basidorsal of fifth verte-bra; c-eth, cartilago ethmoideum; c-Meck, cartilago Meckeli; c-mx,cartilago maxillaris; c-pc, cartilago parachordalis; cb-IV, fourth cera-tobranchiale; ch, ceratohyale; cm-bc-a, commissura basicapsularisanterior; cop-a, anterior copula; eb-III, third epibranchiale; fn-hyp,fenestra hypophysea; fn-met, fenestra metotica; fr-tr-hm, foramentruncus hyomandibularis nervus facialis; hb-II, second hypobran-chiale; hh, hypohyale; hs, hyosymplecticum; ih, interhyale; lm-bot,lamina basiotica; not, notochord; o-mx, os maxillare; o-op, os opercu-lare; o-pmx, os praemaxillare; od, odontode; ot-cap, otic capsule; p-q,pars quadrata of palatoquadratum; pal, palatinum; pl-oc, pila occipi-talis; pr-post, processus postoticus of otic capsule; pr-v, ventral proc-ess of ceratohyale; r-br-III/IV, third/fourth radius branchiostegalis;sol-n, solum nasi; t-m, taenia marginalis; tr-cr, trabecula cranii.

256 T. GEERINCKX ET AL.


mm stage, although no bone is visible in the stained7.4-mm specimen. Also in these sections, the noto-chordal sheath, where the notochord enters theskull, is slightly ossifying, giving the onset of thebasioccipital (also not shown in Fig. 3b). In the 7.4-mm specimen, two odontodes are present on theskin near the lateral protuberance of the cartilagi-nous otic capsule. Small perichondral ossificationsof the otic capsule are seen underneath these lastodontodes, representing the first anlage of theautopterotics. Serial sections (7.0-mm specimen)show that initially odontodes are not in direct con-tact with the dermal frontals or perichondral autop-terotics, which are situated deeper in the skin.

Splanchnocranium. The premaxillae havegrown broader, providing space for about six teeth.The maxillae have also expanded, now touching thecartilaginous autopalatine cartilage bars. The den-taries are visible as curved bony plates. Seven toeight slightly curved conical teeth attach on theposterior half of these, pointing ventrally. In serialsections of a 7.0-mm specimen, thin perichondralossifications can be recognized on the hyoid bar andsuspensorium, representing the onset of the ante-rior ceratohyals and hyomandibulars; these are notyet visible in the stained 7.4-mm specimen. The firstsigns of the parurohyal have appeared in the serialsections of the 7.0-mm specimen: a small plate-likesesamoid ossification is present in the tendon ofeach half of the sternohyoideus muscle (thus onlyurohyal parts are present at the moment).

The branchiostegal rays III and IV have elon-gated; branchiostegal rays II have also appeared.

8.0 mm Standard Length (Fig. 4)Neurocranium. The frontals are now more sub-

stantial, covering most of the taeniae marginalesdorsally. Serial sections of an 8.0-mm specimenshow that the membranodermal components, cover-ing the taeniae, are already present, while the neu-rodermal components still have to arise, as well asthe supraorbital canals which they will surround.Between the otic fenestrae another pair of odonto-des has arisen. A thin sheet of perichondral bonepartially covers each half of the otic capsule roof:the supraoccipital thus originates as a paired struc-ture. This remarkable feature is confirmed by theserial sections of the 8.0-mm specimen. At thismoment, as well as in later stages, no evidence isseen of separately developing parietal bones. At nomoment during ontogeny is there a full separationbetween the perichondral and membranous parts ofthe parieto-supraoccipital (we use this name for thebone, but refer to the discussion for a more thoroughtreatment of this bone). The ossification center ofthe basioccipital is present as a bony sheath aroundthe cranial part of the notochord, now continuing asa perichondral ossification of the neurocranium floornext to it. The transverse processes of the complex

Fig. 3. Skull of Ancistrus cf. triradiatus (7.4 mm SL). (a) Dorsalview. (b) Ventral view. (c) Lateral view. c-eth, cartilago ethmoideum;c-Meck, cartilago Meckeli; c-mx, cartilago maxillaris; c-pc, cartilagoparachordalis; cb-V, ceratobranchiale V; ch, ceratohyale; cm-bc-a,commissura basicapsularis anterior; cm-bc-p, commissura basicapsu-laris posterior; cm-bv, commissura basivestibularis; cm-lat, commis-sura lateralis; cm-sphsep, commissura sphenoseptalis; cop-a, anteriorcopula; cop-p, posterior copula; eb-IV, epibranchiale IV; fn-ot-a, fenes-tra otica anterior; fn-ot-p, fenestra otica posterior; fn-sph, fenestrasphenoidea; fr-IX, foramen nervus glossopharyngeus (fenestra basi-capsularis posterior); fr-on, foramen orbitonasalis fr-X, foramennervus vagus; hb-I, hypobranchiale I; hh, hypohyale; hs, hyosymplec-ticum; ih, interhyale; lm-on, lamina orbitonasalis; not, notochord;o-den, os dentale; o-fr, os frontale; o-mx, os maxillare; o-op, os opercu-lare; o-pmx, os praemaxillare; o-puh, os parurohyale; od, odontode;ot-cap, otic capsule; p-q, pars quadrata of palatoquadratum; pal,palatinum; pl-oc, pila occipitalis; pns-ep, pons epiphysialis; porb-b,preorbital base; pr-post, processus postoticus of otic capsule; pr-prc,processus praecerebralis; pr-pt, processus pterygoideus; pr-v, ventralprocess of ceratohyale; r-br-II, second radius branchiostegalis; t-m,taeniamarginalis; tt-p, tectumposterius.



vertebra have appeared; they form the ventral floorof the swimbladder capsules.

The parasphenoid is now visible as a narrow bandof dermal bone in the lateral and posterior pe-rimeter of the hypophyseal fenestra. Small lateralnotches are left in the bone; together with fissuresin the trabecular bars they form the foramina forthe paired internal carotid arteries (Fig. 4a,b). Inthe serial sections, but not in the stained 8.0-mmspecimen, initiation of perichondral ossification ofthe prootics is observed.

The lateral protuberances of the otic capsule arenow well covered by the paired autopterotics. Thesupracleithra of the pectoral girdle have appearedjust posterior to the otic capsule floor, and areclosely associated with the dorsal, articulatory pro-cesses of the cleithra, as could best be seen in serialsections of the 8.0 mm stage. The sections also showthat the supracleithra appear separate from theautopterotics. The first, separate ossification of thesupracleithra could be recognized in serial sectionsof the 7.0-mm specimen (Fig. 5a).

Splanchnocranium. The premaxillae havebecome higher, with the eight erected teeth insert-ing on the anterior margin. The maxillae have de-veloped complete articulatory facets for the autopa-latine cartilages.

The dentaries now attach firmly to Meckel’s carti-lages, which have also started to ossify medially,forming the mentomeckelian bones (Fig. 5b; not visi-ble in Fig. 4). The dentaries are best developed pos-teriorly, and each one supports about ten conicalteeth. An angulo-articular bone is not present atthis time. Separate from the anterior ceratohyal,the hypohyal and the posterior ceratohyal are visi-ble on the serial sections of the 8.0 mm stage (A. cf.triradiatus only develops a ventral hypohyal). Thepaired sesamoid component of the parurohyal (i.e.,the urohyal) is growing, and a dumbbell-shaped car-tilage nucleus has separated from the hypohyalregion of the hyoid bar. This nucleus contacts therostral end of the anterior copula, which can be con-sidered the first basibranchial.

The medialmost branchiostegal rays I haveappeared, articulating with the medial ends of thebroad caudoventral processes of the hyoid bar. Theopercles are triangular elements, each composed ofa horizontal rod that bears an odontode almost half-way, and a ventral plate extending in the directionof the outer branchiostegal ray.

9.1 mm Standard Length (Fig. 6)Neurocranium. At the tip of the snout, the hypo-

ethmoid is present as a perichondral ossification ofthe ventral face of the ethmoid cartilage. The fron-tals and the now unpaired parieto-supraoccipitalhave started to cover parts of the postpineal fonta-nelle. The anterior tips of the frontals have reachedthe prepineal fontanelle. The autopterotics have

Fig. 4. Skull of Ancistrus cf. triradiatus (8.0 mm SL). (a) Dor-sal view. (b) Ventral view. (c) Lateral view. bb-III, third basibran-chiale; c-eth, cartilago ethmoideum; c-Meck, cartilago Meckeli; c-mx, cartilago maxillaris; cm-sphsep, commissura sphenoseptalis;fn-hyp, fenestra hypophysea; fn-ot-a, fenestra otica anterior; fn-ot-p, fenestra otica posterior; fr-f-olf, foramen fila olfactoria; fr-on,foramen orbitonasalis; fr-tr-hm, foramen truncus hyomandibula-ris nervus facialis; lm-on, lamina orbitonasalis; n-puh, cartilagi-nous nucleus of parurohyale; o-apt, os autopteroticum; o-boc, osbasioccipitale; o-ch-a, os ceratohyale anterior; o-den, os dentale;o-fr, os frontale; o-mx, os maxillare; o-op, os operculare; o-para, osparasphenoideum; o-par-soc, os parieto-supraoccipitale; o-pmx, ospraemaxillare; o-puh, os parurohyale; o-sc, os supracleithrum; ot-cap, otic capsule; pal, palatinum; pl-oc, pila occipitalis; pns-ep,pons epiphysialis; porb-b, preorbital base; r-br-I, first radiusbranchiostegalis; tr-pr-c-v, transverse process of complex vertebra;tt-p, tectum posterius; vc-c, vertebral centra of complex vertebrae;vc-VI, sixth vertebral centrum.



Fig. 5. Histological sections showing details of osteocranium ontogeny. All scale bars represent 200 lm. (a) Section of 7.0 mm stage,at level of supracleithral bone. The autopterotic will form around the otic capsule. (b) Section of 8.0 mm stage, at level of lower jaw,with early jaw bone formation. (c) Rostral tip of ethmoid cartilage of 10.2 mm stage, showing the various components of the meseth-moid bone. (d) Interhyal region of 12.4 mm stage featuring the sesamoid bone and the (reducing) interhyal cartilage. (e) Section of 33.5mm stage at level of largest cheek plate, where the preopercular canal enters it from the preopercle. (f–h) Sections of 12.4 mm stage, atdifferent levels in the parurohyal bone. bb-I, basibranchiale I; c-eth, cartilago ethmoideum; c-Meck, cartilage Meckeli; c-pc, cartilagoparachordalis; ch, ceratohyale; ch-pl, cheek plate; hs, hyosymplecticum; ih, interhyale; m-hh-inf, musculus hyohyoideus inferior; m-stern, musculus sternohyoideus; mes-pmx-c, mesethmoid-premaxillary cartilage; n-puh, cartilaginous nucleus of parurohyale; n-X,nervus vagus; o-ch-p, os ceratohyale posterior; o-cl, os cleithrum; o-den, os dentale; o-deth, os dermo-ethmoideum; o-heth, os hypo-eth-moideum; o-hm, os hyomandibulare; o-mm, os mentomeckelium; o-pmx, os praemaxillare; o-pop, os praeoperculare; o-puh, os paruro-hyale; o-sc, os supracleithrum; o-seth, os supra-ethmoideum; or-cav, oral cavity; ot-cap, otic capsule; pect-f, pectoral fin; pl-oc, pila occip-italis; pc, preopercular canal; ses-b, sesamoid bone; t-g, tooth germ; v-j-inf, vena jugularis inferior.

Fig. 6. Skull of Ancistrus cf. triradiatus (9.1 mm SL). (a) Dorsal view. (b,c) Ventral view of neurocranium and splachnocranium(removed). (d) Lateral view. c-eth, cartilago ethmoideum; c-Meck, cartilago Meckeli; c-mx, cartilago maxillaris; cm-lat, commissura lateralis;cm-sphsep, commissura sphenoseptalis; fn-hyp, fenestra hypophysea; fn-ot-a, fenestra otica anterior; fn-ot-p, fenestra otica posterior; fr-c-int,foramen arteria carotis interna; fr-IX, foramen nervus glossopharyngeus (fenestra basicapsularis posterior); fr-r-op, foramen ramus opercula-ris nervus facialis; fr-tr-hm, foramen truncus hyomandibularis nervus facialis; fr-X, foramen nervus vagus; lm-on, lamina orbitonasalis; n-puh, cartilaginous nucleus of parurohyale; o-apal, os autopalatinum; o-apt, os autopteroticum; o-boc, os basioccipitale; o-ch-a, os ceratohyaleanterior; o-ch-p, os ceratohyale posterior; o-den, os dentale; o-dpt, os dermopteroticum; o-fr, os frontale; o-heth, os hypo-ethmoideum; o-hh, oshypohyale; o-hm, os hyomandibulare; o-mpt, os metapterygoideum; o-mx, os maxillare; o-op, os operculare; o-para, os parasphenoideum; o-par-soc, os parieto-supraoccipitale; o-pop, os praeoperculare; o-pmx, os praemaxillare; o-puh, os parurohyale; o-sc, os supracleithrum; ot-cap,otic capsule; pns-ep, pons epiphysialis; pr-pt, processus pterygoideus; r-br-I, radius branchiostegalis I; sol-n, solum nasi; tr-pr-c-v, transverseprocess of complex vertebra; u-ph-j, upper pharyngeal jaw; vc-c, vertebral centra of complex vertebrae.

grown significantly along the lateral and posteriorwalls of the otic capsule. The dermopterotics appearas posterior projections of the posterior verticalwalls of the autopterotics. They form the onset ofthe roof of the swim bladder capsules. The supraclei-thra are now fused to the autopterotics and dermop-terotics.

Splanchnocranium. Premaxillae and dentariesnow form basket-like structures. Their ventral edgesare complete at the tooth-bearing side, but not yetat the other, mouthward side (posterior in premaxil-lae, anterior in dentaries). The posterior ends of theautopalatine bones have appeared. The tip of thepterygoid process of each suspensorium has startedto ossify perichondrally, and a small membranousbony sheet forms around it. These perichondral andmembranous elements constitute the anlage of eachmetapterygoid. From the rostral end of this bone, aligament stretches toward the ventralmost aspectof the posterior autopalatine ossification. A partialperichondral ossification of each hyomandibular isfaintly visible, with membranous extensions cau-dally (leaving an opening for the path of the opercu-lar branch of the facial nerve) and lateroventrally,in the direction of the preopercle. The long and slen-der membranodermal preopercles have appearedalong the central part of the suspensoria (the neuro-dermal gutter-like part is not yet present in thisstage). Posterior to the path of the inferior jugularvein the paired urohyal ossifications of the sterno-hyoideus tendon have fused. The dumbbell-shapedcartilage nucleus is in contact with the tendon ante-rior to the path of the vein. In this stage the parur-ohyal bone thus still consists only of a sesamoid(‘‘urohyal’’) component. In the branchial basketpaired bony plates have developed, lying againsteach fourth infrapharyngobranchial. These upperpharyngeal toothplates (or ‘‘jaws’’) already bear twopointed teeth each at this stage (Fig. 6c). Serial sec-tions of a 10.2-mm specimen demonstrate, however,that these rudimentary teeth are still covered bythe epidermal pharyngeal tissue, so they cannot yetbe functional.

9.8 mm Standard Length (Fig. 7)Neurocranium. Some additional bony struc-

tures have appeared, while the skull bones alreadypresent have become enlarged so as to form a morerigid support of the braincase. The frontals haveelongated and now connect the otic capsules and thesphenoseptal commissures, and have overgrown alarge part of the epiphysial bridge as well. The pari-eto-supraoccipital consists of a large plate, membra-nous as well as perichondral, and makes up a bonytectum between both otic capsules. Four bones pro-vide support of the skull floor in the midline. As thenotochord is relatively smaller, when comparedwith the 9.1-mm specimen, the basioccipital be-comes the most important supporting element in

Fig. 7. Skull of Ancistrus cf. triradiatus (9.8 mm SL). (a) Dorsalview. (b) Ventral view. (c) Lateral view. ap-o-cb-I, anterior process ofos ceratobranchiale I; B-l-sc, ossified Baudelot’s ligament pars supra-cleithralis; c-mx, cartilago maxillaris; comp-pt, compound pteroticbone; fn-ot-a, fenestra otica anterior; fn-ot-p, fenestra otica posterior;fr-f-olf, foramen fila olfactoria; fr-on, foramen orbitonasalis; fr-r-op,foramen ramus opercularis nervus facialis; o-aa, os angulo-articu-lare; o-apal, os autopalatinum; o-boc, os basioccipitale; o-ch-a, osceratohyale anterior; o-ch-p, os ceratohyale posterior; o-den, os den-tale; o-deth, os dermo-ethmoideum; o-fr, os frontale; o-heth, os hypo-ethmoideum; o-hh, os hypohyale; o-hm, os hyomandibulare; o-ipb-IV,os infrapharyngobranchiale IV; o-leth, os latero-ethmoideum; o-mm,os mentomeckelium; o-mpt, os metapterygoideum; o-mx, os maxil-lare; o-nas, os nasale; o-op, os operculare; o-para, os parasphenoi-deum; o-par-soc, os parieto-supraoccipitale; o-pop, os praeoperculare;o-pmx, os praemaxillare; o-prot, os prooticum; o-puh, os parurohyale;o-q, os quadratum; o-seth, os supra-ethmoideum; o-vm, os vomerale;tr-pr-c-v, transverse process of complex vertebra; u-ph-j, upper pha-ryngeal jaw.



the posterior skull floor. It can be discerned from thenotochordal ossification of the complex vertebra.The parasphenoid now fills the whole hypophysealfenestra, and has become rhomboid-shaped, as themedial cartilage in front of the fenestra has reduced.Only the two foramina for the internal carotidarteries remain. More anteriorly, cartilage reduc-tion has freed space for the membranous vomeralbone, suturing with the parasphenoid. Around theethmoid cartilage the various elements of themesethmoid are seen. The supraethmoid and thehypoethmoid are dorsal and ventral perichondralbones, respectively, connected only at the tip. Thehypoethmoid has a medioventral process, which isthe onset of the typical loricariid mesethmoid disc(Fig. 7c). Most of this process lacks a cartilaginousprecursor (Fig. 5c). On the dorsal side of the ethmoidcartilage, a dermal sheet, the dermethmoid, devel-ops. It covers the posterior part of the supraethmoid.Serial sections of a 10.2-mm specimen show thatthese three bony elements later fuse to form a tube-like mesethmoid bone around the ethmoid cartilage.

The lateral ends of the orbitonasal laminae haveossified perichondrally. Small membranous extensionsbear one odontode each. Thus the lateral ethmoids arecomposed of perichondral and membranous elements.The nasal bones, containing the rostral part of thesupraorbital canals, have arisen on top of the nasalsacs. In serial sections of the 10.2-mm specimen onlythe canal-supporting neurodermal element is alreadypresent. The neurodermal parts of the frontals havealso developed, being well visible in the anterior halvesof these bones. It is not certain whether the neuroder-mal parts of the frontals have arisen separately, orfrom ossification centra of the membranodermal parts.Only serial sections of a specimen between 8.0- and10.2-mm SL could show this.

The perichondral prootics are seen in the skullfloor. In the serial sections of the 10.2-mm specimenmost of the perichondral orbitosphenoids, pteros-phenoids, and autosphenotics are present and rela-tively well-developed, but they are not seen on thestained 9.8-mm specimen.

The compound pterotic bone complex has grownextensively; it consists of three main spatial elements:a largely perichondral casing covering the lateral andposterior walls of the cartilaginous otic capsule, adorsal dermal extension forming the roof of the swim-bladder capsule, and a ventral membranous extensionforming part of this capsule’s floor. The original compo-nents of this compound bone cannot be clearly distin-guished anymore. From the ventral extension, at thelocation where the supracleithrum could be identifiedin the 9.1-mm specimen, Baudelot’s ligament runsmedially toward the posterior ventral surface of thebasioccipital. It starts to ossify laterally, from the com-pound pterotic.

Splanchnocranium. The ventrocaudal pro-cesses of the autopalatine bones, on which the ex-tensor tentaculi muscles and the autopalatine-meta-

pterygoid ligaments insert, are well developed. Boththe upper and lower jaws have fully developed theirtooth-bearing baskets. Thus the premaxillae arecomplete, while the coronoid processes of the denta-ries are not yet completely developed. The latterbones have formed ventrolateral flanges toward theangulo-articulars. The ossification of the latterbones has started at the articulation facets for thesuspensoria, and extend rostrally. All three ele-ments of both suspensoria are now present. Themembranous sheets of the metapterygoids havegrown extensively dorsally and ventrally, giving thebones a triangular outline. The quadrates and hyo-mandibulars are now both present, and the canal-bearing preopercles have broadened. The preop-ercles surround the preopercular canals.

The largely perichondral anterior ceratohyalshave developed membranous sheets growing dor-sally from the anterior edge of the bones, while theposterior ceratohyals still lack any membranousextension. The interhyals have partly reduced (notvisible on Fig. 7), separating the suspensoria andthe hyoid bar. Various ligaments have developedalong the suspensorial-hyoid connection. In a liga-ment at the inner side of the rudimentary interhyalcartilages, sesamoid bones appear (Fig. 5d).

The urohyal has developed further. The pairedsesamoid bones in the sternohyoideus tendon arenow fused anterior and posterior to the inferior jug-ular vein. Anteriorly both tips of the sesamoid bonealmost touch the developing hypohyals posteroven-trally; posteriorly the bone reaches up to a quarterof the sternohyoideus length. The cartilage nucleusstill shows no sign of ossification. As seen in the se-rial sections of the 10.2-mm specimen, the remain-der of the partly reduced first basibranchial is stillcontinuous with this cartilage nucleus.

The branchial basket is still completely cartilagi-nous, except for the upper pharyngeal jaws andnewly formed anterior processes originating fromthe first ceratobranchials.

10.8 mm Standard Length (Fig. 8)Neurocranium. Between the 9.8 and 10.8 mm

stages, extensive ossification of the skull has takenplace. All neurocranial bones are now present. Inthe skull roof the various bones have grown closerand often already touch each other. The frontalshave completely overgrown the epiphysial bridgeand now separate the anterior from the posteriorfontanelle (in the chondrocranium these two fenes-trae are referred to as the prepineal and postpinealfontanelles). From this stage on, the supraorbitalcanals and the anterior part of the otic canals arequite visible in the stained specimens, runningthrough the nasals, frontals, and dermosphenotics.

The orbitosphenoids are now well developed, andcover the preorbital bases and the anterior halves ofthe trabecular bars. In rostral view they are L-



Fig. 8. Skull of Ancistrus cf. triradiatus (10.8 mm SL). (a) Dorsal view. (b,c) Ventral view of neurocranium and splachnocranium(removed). (d) Lateral view. B-l-boc, ossified Baudelot’s ligament pars basioccipitalis; B-l-sc, ossified Baudelot’s ligament pars supra-cleithralis; c-Meck, cartilago Meckeli; c-mx, cartilago maxillaris; comp-pt, compound pterotic bone; fn-sph, fenestra sphenoidea; fr-f-olf, foramen fila olfactoria; fr-on, foramen orbitonasalis; fr-v-j-inf, foramen vena jugularis inferior; ft-a, fontanella anterior; ft-p, fonta-nella posterior; o-aa, os angulo-articulare; o-apal, os autopalatinum; o-boc, os basioccipitale; o-cb-IV/V, os ceratobranchiale IV/V; o-ch-a,os ceratohyale anterior; o-ch-p, os ceratohyale posterior; o-den-m, os dento-mentomeckelium; o-eb-III, os epibranchiale III; o-exoc, osexoccipitale; o-fr, os frontale; o-hh, os hypohyale; o-hm, os hyomandibulare; o-leth, os latero-ethmoideum; o-mes, os mesethmoideum; o-mpt, os metapterygoideum; o-mx, os maxillare; o-nas, os nasale; o-op, os operculare; o-osph, os orbitosphenoideum; o-para, os parasphe-noideum; o-par-soc, os parieto-supraoccipitale; o-pop, os praeoperculare; o-pmx, os praemaxillare; o-prot, os prooticum; o-psph, os pter-osphenoideum; o-puh, os parurohyale; o-q, os quadratum; o-sph, os sphenoticum; o-vm, os vomerale; r-br-IV, radius branchiostegalisIV; tr-pr-c-v, transverse process of complex vertebra; u-ph-j, upper pharyngeal jaw.



shaped, with broad horizontal parts, reaching to-ward the parasphenoid, and narrower vertical partsforming the anterior margins of the orbits, and cov-ering the anterior parts of the taeniae marginales.The foramina for the ophthalmic branches of the tri-geminal nerves are enclosed by them. The ptero-sphenoids have covered the main parts of the tae-niae marginales. The sphenotic fenestrae are sur-rounded by the orbitosphenoids, pterosphenoids,and prootics. The prootics have now covered thebasiotic laminae, the posterior parts of the trabecu-lar bars, and a part of the anterior basicapsularcommissures. The lateral commissures of the chon-drocranium are ossified as well, providing theprootics with an anterior foramen. The parieto-supraoccipital has grown to a massive, U-shapedcompound bone, including a small posterior process.This bone, as well as the sphenotics and pterotics,take part in the closing of the two otic fenestrae thatcharacterize the chondrocranial skull of A. cf. trira-diatus (Geerinckx et al., 2005). The lateral eth-moids, grown extensively since the 9.8 mm stage,touch the frontals, but not yet the mesethmoid.Only posteriorly are the hypoethmoid and supraeth-moid parts of the mesethmoid still unconnected.The ventral mesethmoid disc has grown to the levelof the premaxillae, where a mesethmoid-premaxil-lary cartilage is present, as already shown by serialsections of the 10.2-mm specimen.

In the skull floor the basioccipital has started toform deep sutures with the parasphenoid. The ex-occipitals have appeared, and have foramina for theglossopharyngeal and vagal nerves. All paired skullfloor bones are still separated by broad cartilagezones; only the unpaired medial bones interdigitatewith each other, forming the main axial supportivestructure of the skull. The epioccipitals, developingas perichondral ossifications of the posterior skullwall, can be recognized in the 10.2-mm serial sec-tions. Two ventral processes project laterally fromthe posterior half of the basioccipital. These are themedial ossifications of Baudelot’s ligaments (analo-gous to the lateral, supracleithral parts of the liga-ments, the medial ossifications spread from thebasioccipital bone). From the ventral layer of thecompound pterotics, lateral trabecles have begun togrow dorsally. Thus, the lateral walls of the airblad-der capsules start to form.

Some neuromasts of the infraorbital canals haveappeared and invaginated; the first bony encapsula-tion of these canals occurs by the fifth infraorbitals, asshown by serial sections of the 10.2-mm specimen.

Splanchnocranium. The upper and lower jawsnow bear 12–15 teeth each. The posterior attach-ment facets for the retractor premaxillae muscles onthe premaxillae are now well developed. The dermaldentaries and perichondral mentomeckelia havefused. This is already the case in the (serially sec-tioned) 10.2-mm specimen. The angulo-articularbones have covered the lateral halves of Meckel’s

cartilages, leaving only small posterior regionsunossified, which will remain as such to the adultstage. The angulo-articulars acquire rostrodorsalmembranous sheets that provide space for thedeveloping adductor mandibulae muscles (whichalso insert on the coronoid processes of the denta-ries). More than half of the autopalatine cartilagesare now enclosed by the autopalatine bones. Themetapterygoids have developed lateral ridges.

The cartilaginous hyoid bar is now almost com-pletely replaced by bone. Both the anterior and pos-terior ceratohyals have developed anterior membra-nous extensions. These extensions are orientatedslightly dorsally, so that the hyoid bar actually con-sists of a horizontal perichondral plane and analmost vertical membranous plane. In the anteriorceratohyals, large notches are left for the passage ofthe arteries supplying the lateral parts of the mus-culus hyohyoideus inferior, originating from thehypobranchial artery (Fig. 8c).

The central regions of all epibranchials and cera-tobranchials have begun to ossify perichondrally.First signs of these ossifications are visible in serialsections of the 10.2-mm specimen. The second basi-branchial and the first hypobranchials are alsostarting to ossify. Strips of membrane bone havedeveloped against the fifth ceratobranchials. Theselower pharyngeal tooth plates are not (yet) continu-ous to the ceratobranchials, and already bear twoteeth each. Three to four teeth can be counted onthe upper pharyngeal toothplates (or jaws).

11.7 mm Standard Length (Fig. 9)Neurocranium. The outline of the original chon-

drocranium is not easy to make out anymore, asmost of it has now been covered by perichondralbone (Fig. 9). Also, the dermal bony elements haveoverlain or hidden the cartilaginous skeleton. Theotic fenestrae of the otic capsule roof are fully closedby the parieto-supraoccipital, sphenotics and pter-otics, and the anterior and posterior fontanelleshave severely shrunk as the parieto-supraoccipitaland the frontals have expanded. The mesethmoid isnow completely tube-like, with the hypoethmoidand supraethmoid parts fully connected at both leftand right sides. The mesethmoid now reaches thefrontals, but does not yet cover them. The lateralethmoids now enclose the nasal sacs on three sides,and touch each other below the dermethmoid roof.They have contacted the vomer ventrally. Themedial floor of the ethmoid cartilage reduces wherethe vomer covers it ventrally. The three bones bor-dering the sphenoid fenestrae, i.e., the orbitosphe-noids, pterosphenoids, and prootics, restrict the fe-nestrae by the formation of membranous bony sheetsat the perimeter. The compound pterotics havedeveloped spectacularly, expanding the dorsal andventral layers, as well as connecting them laterallyby means of trabecles carrying numerous odontodes



(the only connection between the dorsal and ventrallayers thus far was rostrally, at the level of the oticpilae). These trabecles leave various small fora-mina. The connections with the transverse proc-esses of the complex vertebra are reinforced bymeans of fine sutures dorsally and ventrally. Thebasioccipital, exoccipitals, epioccipitals, and medialparts of the pterotics are tightly connected to thesetransverse processes. The basioccipital and the par-asphenoid are deeply sutured. In the skin below theeye, three small canal bones are now present:infraorbitals IV to VI.

Splanchnocranium. As the jaw bones had al-ready attained their more or less final shape in the10.8-mm specimen, the only significant differencenow is an increase in size, and a closer contactincluding suturing between dento-mentomeckeliaand angulo-articulars. The autopalatine bones arenow completely formed; the cartilaginous rostralarticulatory heads for the maxillae remain cartilagi-nous during further ontogeny. The original outlineof the cartilaginous suspensoria is not visible any-more, as the proportion of membrane bone has in-creased and the cartilage has become reduced,except for the symplectic cartilages and the articula-tory heads for the neurocranium and the opercle(the articulatory cartilage at the facet for the lowerjaw is minute). The hyomandibulars have startedsuturing weakly with the pterotics dorsocaudally.The lateral ridges of the metapterygoids haveexpanded into large sheets. As the cartilaginousinterhyal connections between the suspensoria andthe hyoid bar have been lost, the ligamentous con-nections grow stronger. The final shape of the sesa-moid bones medial to the former interhyal locationsis cylindrical.

Lateral to the quadrates, at the rostral tip of thepreopercles, the preopercular canals now turn ven-trally. Thin neurodermal ossification is presentaround them. These are the first signs of the largerof two cheek plates that will develop in this region(Fig. 5e). Membranodermal elements are not yetvisible in this specimen.

The branchiostegal rays have grown and are nowall flattened (the lateral ones more than the medialones). Both anterior tips of the sesamoid urohyalbone almost touch the developing hypohyals poster-oventrally; posteriorly the bone reaches up to a thirdof the length of the sternohyoideus muscle. Thehypohyals acquire a depression near the anterior(par)urohyal tips. The middle shaft region of theceratobranchials and epibranchials have furtherossified, leaving the heads (and growth regions) stillcartilaginous. The fifth ceratobranchials and lowerpharyngeal jaws have fused.

14.4 mm Standard Length (Fig. 10)Neurocranium. Both fontanelles are now com-

pletely closed. The posterior growth of the parieto-

Fig. 9. Skull of Ancistrus cf. triradiatus (11.7 mm SL). (a) Dorsalview. (b) Ventral view. (c) Lateral view. B-l-boc, ossified Baudelot’sligament pars basioccipitalis; B-l-sc, ossified Baudelot’s ligamentpars supracleithralis; c-Meck, cartilagoMeckeli; c-mx, cartilago max-illaris; c-symp, cartilago symplecticum; comp-pt, compound pteroticbone; fr-IX, foramen nervus glossopharyngeus (fenestra basicapsula-ris posterior); fr-X, foramen nervus vagus; ft-a, fontanella anterior;ft-p, fontanella posterior; o-aa, os angulo-articulare; o-apal, os auto-palatinum; o-boc, os basioccipitale; o-ch-a, os ceratohyale anterior; o-ch-p, os ceratohyale posterior; o-den-m, os dento-mentomeckelium;o-exoc, os exoccipitale; o-fr, os frontale; o-hb-I, os hypobranchiale I; o-hh, os hypohyale; o-hm, os hyomandibulare; o-io-V/VI, os infraorbi-tale V/VI; o-leth, os latero-ethmoideum; o-mes, os mesethmoideum;o-mpt, os metapterygoideum; o-mx, os maxillare; o-nas, os nasale; o-op, os operculare; o-osph, os orbitosphenoideum; o-para, os parasphe-noideum; o-par-soc, os parieto-supraoccipitale; o-pop, os praeopercu-lare; o-pmx, os praemaxillare; o-prot, os prooticum; o-psph, os ptero-sphenoideum; o-puh, os parurohyale; o-q, os quadratum; o-sph, ossphenoticum; o-vm, os vomerale; r-br-IV, radius branchiostegalis IV;tr-pr-c-v, transverse process of complex vertebra; u-ph-j, upper pha-ryngeal jaw.



Fig. 10. Skull of Ancistrus cf. triradiatus (14.4 mm SL). (a) Dorsal view. (b,c) Ventral view of neurocranium and splachnocranium(removed). (d) Lateral view. ap-o-cb-I, anterior process of os ceratobranchiale I; B-l-boc, ossified Baudelot’s ligament pars basioccipita-lis; B-l-sc, ossified Baudelot’s ligament pars supracleithralis; c-mx, cartilago maxillaris; c-symp, cartilago symplecticum; ch-pl, cheekplates; ch-sp, cheek spines; comp-pt, compound pterotic bone; fn-sph, fenestra sphenoidea; fr-v-j-inf, foramen vena jugularis inferior; o-aa, os angulo-articulare; o-apal, os autopalatinum; o-boc, os basioccipitale; o-cb-V, os ceratobranchiale V; o-ch-a, os ceratohyale ante-rior; o-ch-p, os ceratohyale posterior; o-den-m, os dento-mentomeckelium; o-exoc, os exoccipitale; o-fr, os frontale; o-hh, os hypohyale; o-hm, os hyomandibulare; o-io-V, os infraorbitale V; o-ipb-III, os infrapharyngobranchiale III; o-leth, os latero-ethmoideum; o-mes, osmesethmoideum; o-mpt, os metapterygoideum; o-mx, os maxillare; o-nas, os nasale; o-op, os operculare; o-osph, os orbitosphenoideum;o-para, os parasphenoideum; o-par-soc, os parieto-supraoccipitale; o-pop, os praeoperculare; o-pmx, os praemaxillare; o-prot, os prooti-cum; o-psph, os pterosphenoideum; o-puh, os parurohyale; o-q, os quadratum; o-sph, os sphenoticum; o-spop, os suprapraeoperculare;o-vm, os vomerale; prfr-pl, prefrontal plate; r-br-IV, radius branchiostegalis IV; tr-pr-c-v, transverse process of complex vertebra.



supraoccipital also has elongated the skull (in theearliest stages the posterior end of the skull wasdemarcated by the tectum posterius). The majorskull roof bones have also started suturing. The der-mal lateral processes of the sphenotics, enclosingthe posterior portions of the infraorbital canals,have grown. On top of the lateral connections of thelateral ethmoids and the frontals, thin odontode-bearing dermal layers, the prefrontal plates, haveappeared. Beneath the skull floor, Baudelot’s liga-ments are almost completely ossified: only a thinregion of each ligament between the medial ossifica-tion from the basioccipital and the lateral ossifica-tion from the ventral ridge of the supracleithrum(compound pterotic) remains ligamentous at thismoment. Most infraorbital bones (usually six oneach side in A. cf. triradiatus, but sometimes onlyfive) are present, or at least an odontode can be seenindicating the future location of the bone (the sup-porting bone underneath is often more difficult tosee than the odontode itself). Minute dermal plate-lets carrying odontodes now appear in the cheekregion and behind the skull as well.

Splanchnocranium. The suspensoria have at-tained their approximate final shape by now: themetapterygoids have made contact with the colla-teral quadrate and hyomandibular, suturing withboth. They are ligamentously connected to the la-teral ethmoids (dorsally) and the autopalatine bones(rostrally). The membranous dorsal parts of the hyo-mandibulars have become completed and formrounded, thin sheets, supporting the eyes. The hyo-mandibulars articulate with the neurocranium atthe level of the contact between the collateralprootic, pterotic, and sphenotic. Just anterior to thisarticulation, the membranous parts of the hyoman-dibulars form two processes fitting into the serratelateral edges of the prootics. These serrations havedeveloped together with these hyomandibular pro-cesses. The hyomandibular-pterotic sutures haveexpanded to most of the contact zone between bothbones, which is the final adult configuration, thoughthe sutures will become stronger during further de-velopment. The preopercles approach the quadratesand the hyomandibulars, though are not yet fusedto any of these bones. Between the preopercles andthe most rostral margin of the pterotics the preoper-cular canals are now enclosed by the paired supra-preopercles, dermal canal-bones bearing odontodesjust like the infraorbitals. Anterior processes areformed on both opercles and point ventromedially.Near these processes, sturdy bony elements aredeveloping, bearing large spiny odontodes, the so-called cheek spines. The dermal cheek plates lateralto the quadrates bearing the end of the preopercularcanals have expanded a little, as membranodermalbony sheets supporting a few odontodes are addedto the neurodermal gutter-like components.

The adult shape of the parurohyal is more or lessreached. The cartilage nucleus dorsal to the sesa-

moid urohyal bone has condensed, and on its ventralside, bone formation connects it to this sesamoidpart (Fig. 5f–h). The bone now has a compound na-ture. Both parurohyal tips fit in holes of the hypo-hyal bones. It appears that the centers of the hypo-hyal cartilages have reduced so that the depres-sions, mentioned in the previous stage, now piercethe bones and have become holes. The bony firstceratobranchials and their spongiose anterior pro-cesses have fused; in earlier stages the processeswere only loosely connected to the first ceratobran-chial bones via the cartilage at the medial ends ofthe bones. The uncinate processes of the epibran-chials that have arisen since the bones started to de-velop are now well visible; the largest are borne bythe third epibranchials. The infrapharyngobran-chials III and IV show the first signs of perichondralossification.

20.7-mm Standard Length Specimen (Fig. 11)and Further Development

Neurocranium. As all neurocranial elementshad already attained their approximate final shapeat the 14.4 mm stage; this and later stages aremainly characterized by growth and reinforcementof the skull by means of further suturing of dermalbones, closer synchondral contacts between the peri-chondral bones, and heavier ossification of the vari-ous elements. As the skull floor bones have alsothickened, the bony recesses for the paired maculaecan be easily seen: the utriculus is borne by a recessin the prootic, while the more posteriorly situatedsacculus is enclosed by the basioccipital, and thelagena by both the basioccipital and exoccipital.Baudelot’s ligaments are completely ossified, pro-viding a direct bony connection between the basioccipi-tals and compound pterotics. The ligaments formcontinuous transverse ridges, though sutures can beseen in the positions where the sesamoid ossifica-tions from the basioccipital and supracleithral sidestouch each other. No teeth develop on the ventralsurface of the vomer. The prefrontal plates havegrown, and will become almost rectangular whenspecimens reach maturity. Of the major skull bones,the compound pterotics expand most during furthergrowth, broadening the posterior part of the skull.As seen in the 20.7-mm specimen, the lateral eth-moids have not yet closed completely anteriorly,leaving the anterior part of the nasal sacs less sup-ported. In the 33.5-mm specimen the closure isestablished.

The infraorbital bones have grown and almosttouch each other. The suprapreopercles and the pre-opercles have almost touched as well. So now the in-fraorbital and preopercular canals are more or lesscompletely enclosed in bone. The number of plate-lets in the snout region has increased. They can bedivided into prenasal plates (on top of the meseth-moid) and lateral plates (between the preopercular



and infraorbital canals). Lateral to the hyomandibu-lar and the pterotic bones, platelets will also soonappear (the first are observed in the 25.0 mm stage;most are present at 31.0 mm SL). A complete cove-rage of similar, overlapping plates appears underthe skin of the rest of the body, leaving only the bellyunprotected. In the 31.0-mm specimen thin scleroticbones have appeared, which support the eyeballsanteriorly and posteriorly.

Splanchnocranium. The upper and lower jawsnow carry about 30 teeth each, a number that willhave doubled when the animals reach adulthood.The preopercles and the suspensoria have started tofuse. In later stages it is very difficult to differenti-ate between these bones. A short stretch of the pre-opercles lies directly under the skin, and now car-ries a few odontodes. During further development,the preopercles broaden somewhat, further over-growing the quadrates and hyomandibulars. Theanterior processes of each opercle have grown, andthe articulation of the bone with the hyomandibularis reinforced by the presence of a serration on theopercle posterior to the articulation. A second,paired cheek plate has developed dorsal to the firstone. It consists of an odontode-supporting plateonly, thus not bearing a canal. The cheek spines arenow well visible. The rostral processes of the par-urohyal pierce the hypohyals; the horizontal sesa-moid sheet continues to grow until the posteriormargin is more or less rounded in the 25.0 mmstage, and reaches to almost half of the sternohyoi-deus length. The dorsal region of the parurohyalcartilage nucleus degenerates, and the ventralregion further ossifies, although part of it stays car-tilaginous even in adults. The very small third basi-branchial is ossified in specimens of over 50 mm SL,while the posterior copula, consisting of the fourthand fifth fused basibranchials, remains cartilagi-nous. The second hypobranchials remain cartilagi-nous as well, while the third to fifth hypobranchialsstay fused to the cartilage heads of the correspond-ing ceratobranchials. The anterior processes of thefirst ceratobranchials have broadened and grown tothe same length as the ceratobranchials. The tips ofthe cerato- and epibranchials do not ossify. Also, theinfrapharyngobranchials maintain their cartilagi-nous articulatory caps.

DISCUSSION

Ossification in A. cf. triradiatus starts as early asthe fourth day after fertilization. At 5.6 mm SL thefirst dermal bone can be recognized, i.e., the opercle.This is only 1 day after the formation of the firstchondrocranial elements (Geerinckx et al., 2005), or1 day before hatching occurs. The early appearanceof the opercle, followed by the formation of the firstbranchiostegal rays and most dentulous bones, is ageneral trend in siluriforms and other teleosts(Weisel, 1967; McElman and Balon, 1980; Tilney

Fig. 11. Skull of Ancistrus cf. triradiatus (20.7 mm SL). (a)Dorsal view. (b) Ventral view. (c) Lateral view. B-l-boc, Baudelot’sligament pars basioccipitalis; B-l-sc, Baudelot’s ligament parssupracleithralis; c-mx, cartilago maxillaris; ch-pl, cheek plates;ch-sp, cheek spines; comp-pt, compound pterotic bone; d-pl, der-mal plate; fr-f-olf, foramen fila olfactoria; o-aa, os angulo-articu-lare; o-apal, os autopalatinum; o-bb-III, os basibranchiale III;o-boc, os basioccipitale; o-ch-a, os ceratohyale anterior; o-ch-p,os ceratohyale posterior; o-den-m, os dento-mentomeckelium;o-exoc, os exoccipitale; o-fr, os frontale; o-hh, os hypohyale; o-hm,os hyomandibulare; o-io-I, os infraorbitale I; o-leth, os latero-eth-moideum; o-mes, os mesethmoideum; o-mpt, os metapterygoi-deum; o-mx, os maxillare; o-nas, os nasale; o-op, os operculare;o-osph, os orbitosphenoideum; o-para, os parasphenoideum;o-par-soc, os parieto-supraoccipitale; o-pop, os praeoperculare;o-pmx, os praemaxillare; o-prot, os prooticum; o-psph, os ptero-sphenoideum; o-puh, os parurohyale; o-q, os quadratum; o-sph, ossphenoticum; o-spop, os suprapraeoperculare; o-vm, os vomerale;prfr-pl, prefrontal plate; tr-pr-c-v, transverse process of complexvertebra complex vertebra; vc-c, vertebral centra of complexvertebrae.



and Hecht, 1993; Vandewalle et al., 1995, 1997;Adriaens and Verraes, 1998). Generally, the onset ofsplanchnocranial ossification is earlier than the firstappearance of neurocranial elements (e.g., de Beer,1937; Bamford, 1948; Surlemont and Vandewalle,1991; Tilney and Hecht, 1993; Vandewalle et al.,1994, 1995; Adriaens and Verraes, 1998). Within theneurocranium, dorsal and ventral elements appearmore or less simultaneously in A. cf. triradiatus.The ontogeny of some skull bones of both neurocra-nium and splanchnocranium of A. cf. triradiatus isbriefly treated, while a few merit a more thoroughdiscussion.

Neurocranium

The most important bone in the ethmoid region ofA. cf. triradiatus, the mesethmoid, has a complexdevelopment. The dorsal and ventral perichondralcomponents can be identified as the supra- andhypoethmoid bones. They are connected over therostral tip of the ethmoid cartilage. A dermal ele-ment, called dermethmoid or rostral by Patterson(1975), soon overgrows the supraethmoid andstretches further posteriorly. Although these partscan be discerned based on their perichondral or der-mal nature, as well as their location, they areactually connected more or less from the momentthey arise. The ethmoid cartilage shows a small ven-tral protrusion, but most of the ventral disc of themesethmoid does not form perichondrally aroundthe ethmoid cartilage, but is an extension of thehypoethmoid. It is mostly membranous, althoughcartilage can be seen in serial sections (Fig. 5c). Itsdevelopment is related to the formation of ligamentsto the maxillae, premaxillae, and mesethmoid-pre-maxillary cartilage (Geerinckx et al., 2005).

The nasal sac is bordered on all sides only by thelateral ethmoid, as in the loricariids Hypostomusplecostomus (Schaefer, 1987) and Pterygoplichthys(Howes, 1983), but unlike in Hypoptopoma, wherethe nasal sac is free anteriorly (Howes, 1983).

The membranodermal component of the frontalappears first, with the neurodermal (canal-bearing)component ossifying from it (or perhaps separately)somewhat later in ontogeny. First both parts can bewell distinguished, but later they fuse more inti-mately. This is often seen in other teleostean canalbones as well (Daget, 1964). Exceptions in A. cf. trir-adiatus are the infraorbital bones, nasal and canal-bearing cheek plates (see later), where the first ossi-fication occurs around the canal. The anterior partof the adult frontal is relatively narrow, while theposterior part is broad enough to reach the orbit(as in Otocinclus [Schaefer, 1997] and Farlowella[Retzer and Page, 1996] but unlike the situation inHypostomus plecostomus [Schaefer, 1987]).

In many siluriforms and other teleosts, the skullfloor bones appear earlier than the roof bones; usu-ally the parasphenoid is the first bone to arise, more

or less together with the basioccipital ossificationaround the notochord (Kobayakawa, 1992; Vande-walle et al., 1995, 1997; Adriaens and Verraes,1998). Hoedeman (1960b) noted a slightly earlier de-velopment of some roof bones, i.e., the frontals andpterotics. When the parasphenoid ossifies, it is a U-shaped dermal sheet in the perimeter of the hypo-physeal fenestra, also seen in Chrysichthys auratus(Vandewalle et al., 1995) and Clarias gariepinus(Adriaens and Verraes, 1998). In various teleosts,the early appearance of the parasphenoid has beenlinked to the necessity of protecting the braincasefrom the physical particularities of food passing inthe buccal cavity, from the moment of the transitionfrom endogenous to exogenous feeding (Verraes,1974; Vandewalle et al., 1997, 1999; Adriaens andVerraes, 1998; Wagemans et al., 1998; Gluckmannet al., 1999). In A. cf. triradiatus though, the boneappears at 7.4 mm SL or 6 days after fertilization.Exogenous feeding starts around 9 mm or 8–9 daysafter fertilization. It could be hypothesized that pa-rasphenoid ossification is related to the respirationmovements and buccal pressure differences duringrespiration. Mechanical loading might be an impor-tant factor in inducing ossification (Mabee andTrendle, 1996; Adriaens and Verraes, 1998).

In siluriforms the posterior skull roof is composedof one large bone, known as the compound parieto-supraoccipital in siluriforms. Argumentation for thedevelopmental fusion of paired parietal and supra-occipital ossification centra is given by Bamford(1948), Arratia and Menu-Marque (1981, 1984), andFink and Fink (1996). In some cases (Callich-thyidae, Clariidae) no developmental evidence wasfound (Hoedeman, 1960b; Adriaens and Verraes,1998). In the ontogeny of A. cf. triradiatus separateparietals are never present. The parieto-supraocci-pital arises as a paired, mostly perichondral bone.The development of the bone is somewhat compli-cated by the early presence of odontodes on the post-otic processes, and posterior to the location wherethe possible parietals could be expected. There,membranous bone is soon added to the perichondrallayer. The fact that a cleared and stained 8.0-mmSL specimen as well as serial sections of another8.0-mm SL specimen show one paired ossificationanlage for the parieto-supraoccipital, is unusual.This might be correlated to the presence of odonto-des on the skull roof, possibly hastening ossificationin the region of the bone that will support it. Wecan, however, only speculate on the exact cause ofthis ossification pattern. We apply the name pari-eto-supraoccipital, not supraoccipital alone, butwhether the parietals are fused to the supraoccipi-tals, or missing, can, however, not be concludedunambiguously from our data. The ossification cen-tra might be close to each other, masking a possibledouble origin. The ending of the parietal branch ofthe supraorbital canal between the frontal and thesphenotic could be interpreted as an argument that



the parietal is absent, but the branch might justhave been reduced as well. It is absent in variouscatfishes (Arratia and Huaquın, 1995). Arratia andHuaquın (1995) regarded the absence of a parietalbranch of the supraorbital canal as a synapomorphiccondition for loricarioids. According to their defini-tion, this branch commonly runs from the frontalinto (or above) the parieto-supraoccipital bone incatfishes, or does not reach it (thus might have beenreduced). Schaefer (1987) reported this branch inthe sphenotic in Hypostomus plecostomus, and inthe frontal in Otocinclus (Schaefer, 1997), where herather confusingly called it ‘‘posterior’’ branch, pos-terior to a ‘‘parietal’’ (¼ epiphysial) branch. Whetherthis branch has disappeared in all loricarioid taxaexcept loricariids, or has secondarily reappeared inthis family, remains to be verified.

According to Arratia (2003), a separate epioccipi-tal is missing in Nematogenys and most trichomyc-terids, and also in Hypostomus and in scoloplacids.It has, however, been noted in other loricariids andin trichomycterids (Schaefer, 1987, 1997; Schaeferand Aquino, 2000). In A. cf. triradiatus it appears asa perichondral ossification of the middle part of theoccipital pila. We favor the name epioccipital, andnot epiotic (Schaefer, 1997; Arratia, 2003), followingthe argumentation of Patterson (1975).

In siluriforms the identification of the bone usu-ally termed posttemporo-supracleithrum, in the pos-terolateral corner of the skull, is not easy. Arratiaand Gayet (1995) stated that there is no develop-mental evidence that the bone termed posttemporo-supracleithrum in siluriforms is a compound ele-ment. It could be the posttemporal or supracleith-rum alone, or the result of the early fusion of bothelements during the earliest moments of ossifica-tion. Adriaens et al. (1997) suggested that in Clariasgariepinus the cleithral notch and the attachment ofthe Baudelot’s or transscapular ligament on it indi-cate it is at least composed of the supracleithrum,while the presence of an anteroventral process con-necting the posterior element to the pterotic and adorsal oblique process attaching it to the epioccipitalmight indicate that the posttemporal is also part ofit (Adriaens and Verraes, 1998).

In Callichthyidae, Scoloplacidae, Astroblepidae,and Loricariidae, the (posttemporo)-supracleithrumis fused to the pterotic (Regan, 1911; Arratia, 2003).The detection of the separate bones in this ‘‘com-pound pterotic’’ in loricariids has been furthercomplicated. Analogous to the argumentation ofAdriaens et al. (1997), Aquino and Schaefer (2002)considered the cleithral articular notch and Baude-lot’s ligament on the compound pterotic as indirectevidence of the incorporation of the supracleithruminto the ventral aspect of the pterotic. As Lundberg(1975) did, they concluded that there is no real evi-dence indicating the incorporation of the posttempo-ral as well, but that does not mean this componentis absent in (all) loricariids. Aquino and Schaefer

(2002) also referred to Coburn and Grubach (1998),who suggested that loss or fusion of these elementsmay correlate to the loss of the first two occipitalvertebral segments which they observed in the de-velopment of Corydoras paleatus (Callichthyidae)(see later for an account on the anterior vertebrae ofA. cf. triradiatus). In A. cf. triradiatus the supra-cleithrum could be observed as a separate ossifica-tion, before fusing to the ventral layer of the pterotic(Fig. 5a). The cleithral dorsal process is closely asso-ciated to the supracleithrum, and the articulatorynotch will form on this part of the ‘‘compound pter-otic’’ bone. Baudelot’s ligament attaches and ossifiesfrom this point as well (in case the posttemporalalso would be incorporated, it would lie between thepterotic and the supracleithrum). To us, the name‘‘compound pterotic’’ seems most appropriate for thecomplex. The posttemporal is most probably neverpresent in A. cf. triradiatus.

The double-layered nature of the compound pter-otic is considered a derived characteristic for loricar-iids by Aquino and Schaefer (2002), who added thatthe dorsal layer of the bone in loricariids is not ho-mologous to that in other catfishes, as it does notbear the postotic canal. Also, this dorsal layer wouldappear prior to the ventral layer, which includes theneurodermal component enclosing the canal. Thisossification sequence of the dermopterotic is notseen as such in A. cf. triradiatus. The ventral layerdevelops slightly earlier, and is grown to almost itsfull extent when the dorsal, strictly membranoder-mal layer starts to reach over the swimbladder cap-sule. Both are continuous with each other and theautopterotic at the occipital pila from the beginning,but only late in development they are connected bytrabecles laterally.

The absence of a pterotic branch of the postoticcanal was considered a synapomorphy of loricarioidsby Arratia and Huaquın (1995), who noted its ab-sence in trichomycterids and nematogenyids. In apaper discussing the pterotic branch homology, how-ever, Schaefer and Aquino (2000) could identify thisbranch in all loricarioids except scoloplacids andastroblepids. It is present in A. cf. triradiatus aswell.

The ossification pattern of Baudelot’s ligament inA. cf. triradiatus is interesting. As in other cat-fishes, it stretches from the ventral face of thesupracleithrum toward the basioccipital, therebyforming a transverse ridge on the posteroventralskull floor. Two ossification centers are present in A.cf. triradiatus, one from the attachment point onthe basioccipital, and one from the supracleithrum.In adults the boundary between both parts can stillbe seen, at the level of the basioccipital-pterotic con-tact. The boundary is also seen in Hypostomus ple-costomus and Otocinclus vittatus (Schaefer, 1987,1997). Not much is known about the ossificationsequence of the ligament in other siluriforms (Finkand Fink, 1996), but in the ictalurid Trogloglanis



pattersoni only one large ossification seems to arisefrom the supracleithrum (Lundberg, 1982).

The identification of the complex vertebra inloricariids is problematic, as a reduction in numberof the anterior vertebral centra appears to haveoccurred. In literature on loricariid morphology,there appears to be a general consensus on the iden-tification of the sixth vertebra, which carries a pairof large ribs connecting the vertebral column withthe lateral dermal plates posterior to the head(Alexander, 1964; Chardon, 1968; Schaefer, 1987,1997). The earlier assumption of Bridge and Haddon(1893) that it might be the fifth was based on anunclear account of Reissner (1859). The sixth cen-trum is immovably sutured to the fifth, and its neu-ral spine sutures to the posterior process of the pari-eto-supraoccipital. Chardon (1968) distinguishedthe fifth centrum from the first four, the centra ofwhich must have become reduced significantly. In adevelopmental study of the Weberian apparatus inthe callichthyid Corydoras paleatus, Coburn andGrubach (1998) concluded that the first two verte-brae are missing, and the third and fourth lack basi-dorsals and basiventrals. The situation in A. cf. trir-adiatusmight be similar. The fourth and fifth centraoriginate as one long vertebral centrum. Their para-pophyses form the bony encapsulation of the swim-bladder. The basidorsals of the fifth vertebra reachtoward the cartilaginous tectum posterius. Theresulting complex of vertebrae (up to the fifth) has alength of twice that of vertebra six or seven. Arecent paper by Hoffmann and Britz (2006) dis-cusses the homology of the anterior vertebral centraamong otophysans. Contrary to the previous view, ithypothesizes that it is the fourth (not the fifth) basi-dorsal which contacts the tectum synoticum, andthus the fifth (not the sixth) which bears the largeribs and touches the parieto-supraoccipital process.It is the fourth vertebra that forms the os suspenso-rium, a feature of the fourth vertebra in otophysans(Hoffmann and Britz, 2006).

Six, or rarely five infraorbital bones are present inA. cf. triradiatus. Usually six are found in hyposto-mine loricariids, though only five are found in Hypo-ptopomatinae (Schaefer, 1997). In one specimen ofA. cf. triradiatus, there is no canal in the second in-fraorbital. Such a disjunct canal is slightly reminis-cent to the situation in certain Trichomycteridae,where most of the infraorbital canal is lost, exceptfor the part in the first infraorbital (Arratia andHuaquın, 1995). The shape and late ossification ofthe first bone of the infraorbital series, as well asthe absence of an antorbital branch of the infraor-bital canal, suggest the bone corresponds to infraor-bital I and not to the antorbital. It does not matchthe criteria Arratia and Huaquın (1995) used for theidentification of this bone in Diplomystidae.

The prefrontal plate might be homologous tothe supraorbital-like tendon bone Arratia (1987)described in Diplomystidae. Howes (1983) noticed

the resemblance in position between the loricariidprefrontal plate with the supraorbital bone of somenonsiluriform taxa, but they are most probably nothomologous (Fink and Fink, 1981; Howes, 1983).

The ossification of the infraorbital, nasal and sup-rapreopercle and the canal-bearing cheek plate dif-fers from the situation in other canal bones. First, aneurodermal tube of bone arises around the sensorycanal; only later the membranodermal component isformed against and on top of it. Adriaens and Ver-raes (1998) describe the same phenomenon in theinfraorbital, nasal, and suprapreopercular bones inClarias gariepinus.

The number of prenasal and lateral plates orscutes varies in different specimens, particularly,the prenasal plates are variable in both number andshape. Schaefer (1997) mentioned a more rigid pat-tern of these plates in Otocinclus.

Sclerotic bones are present in larger specimens,although Fink and Fink (1996) regarded their ab-sence as synapomorphic for Siluriphysi. Similarbones were also found in young Callichthys sp. byArratia (1987).

Splanchnocranium

During ontogeny, the tooth-bearing dentary of thelower jaw in A. cf. triradiatus fuses to the mento-meckelian (Fig. 5b) and angulo-articular bones. Inthe adult stage only a rudiment of Meckel’s cartilagepersists. As in other loricariids, as well as astroble-pids, callichthyids, and most trichomycterids, acoronomeckelian bone is absent (Mo, 1991; dePinna, 1993).

The double posterior process of the autopalatinebone acts as a double insertion point for the extensortentaculi muscle subdivisions. In the basal siluri-form Diplomystes and yHypsidoris a similar, buteven larger, single posterior extension is present,posterior to the articulatory facet with the lateralethmoid (Arratia, 1987; Grande, 1987). During earlyontogeny, no sign is found of a palatine splint boneas is seen in Otocinclus (Schaefer, 1997); it can, how-ever, be seen in adult A. cf. triradiatus (Geerinckx,personal observation). Schaefer (1997) considered ita dermal or sesamoid ossification, variably presentin loricariids.

The view of Howes and Teugels (1989), suggestingthe presence of dermal ento- and ectopterygoidsnext to the perichondral metapterygoid in some cat-fishes, is opposed to the hypothesis supported byAlexander (1965), Gosline (1975), Arratia (1990,1992), Fink and Fink (1996), and Adriaens and Ver-raes (1998), who reported only the metapterygoid tobe present, as a perichondral ossification of thechondrocranial pterygoid process, and having mem-branous outgrowths. The latter interpretation is fol-lowed in this paper. The interpretation of Hoedeman(1960a), with the metapterygoid being part of thehyomandibular ossification, is incorrect. Ectoptery-



goids are only found in some individuals within theDiplomystidae (Arratia, 1992). Sesamoid ento- orectopterygoids are also lacking in A. cf. triradiatus(they are present in several catfish families; Arratia,1992; Kobayakawa, 1992; Diogo et al., 2001).

The hyomandibular articulation with the sphe-notic, prootic and pterotic bones has also beenobserved in some trichomycterids (Arratia, 2003),but is uncommon in siluriforms (where usually onlyone or two of these bones are involved). In adultindividuals of A. cf. triradiatus the hyomandibulartrunk enters the hyomandibular bone at the medialside and leaves it at its lateroventral margin, medialto the preopercle, whereas it leaves the bone atthe lateral side in Hypostomus and Otocinclus(Schaefer, 1987, 1997).

Schaefer (1988) elaborated on the identity of thelargest, canal-bearing cheek plate, present in manyloricariids. He concluded that it is not homologouswith the interopercle of most other catfishes, as noother teleosts possess a canal in the interopercle,and this canal communicates directly with the pre-opercular canal terminus (his exit 5) in primitivesiluriforms. In the loricariid genus Delturus a trueinteropercle might be present, although the homol-ogy issue remains problematic (Armbruster, 2004).No ontogenetic stage of A. cf. triradiatus showsany sign of the interoperculo-mandibular ligament,which is assumed to be lacking in most loricariids aswell as in astroblepids (Schaefer, 1988; Armbruster,2004). Development of the cheek plate in A. cf. trira-diatus starts with a neurodermal gutter-like bonesurrounding the canal at 11.7 mm SL, followed bythe addition of a small odontode-bearing membrano-dermal component at 14.4 mm SL. Schaefer (1988)observed an opposite sequence in Sturisoma sp.: theodontode-bearing part arises before a canal isobserved in the bone.

In adults of A. cf. triradiatus the suprapreopercleis fused to the sixth infraorbital bone (Geerinckx,personal observation). This could not (yet) beobserved in any of the examined embryonic and ju-venile specimens. The infraorbital and preopercularcanals, however, remain separated. In Otocinclusboth canals sometimes share a pore between thesphenotic, pterotic, and posterior (fifth) infraorbitalbone (Schaefer, 1997; personal observation).

The cartilaginous interhyal connects the chon-drocranial hyoid arch with the hyosymplectic car-tilage (Geerinckx et al., 2005). In A. cf. triradiatusit is lost during ontogeny. The final articulationbetween the hyoid bar and the suspensorium isassisted by a series of ligaments. The loss of theinterhyal is also seen in Clarias gariepinus (Nawar,1954; Adriaens and Verraes, 1994). A cylinder-shaped sesamoid bone arising in a ligament at themedial side of the original interhyal location (Fig.5d) might well be unique for loricariids. It is presentin Hypostomus plecostomus and Otocinclus vittatus(though interpreted as an interhyal by Schaefer,

1987, 1997). It is hypothesized to act as a support,strengthening the articulation, which may well beneeded to resist the strong forces exterted by thesuction used by loricariids to keep the body attachedto substrates, often in fast flowing water. The ab-sence of the interhyal is shared by loricariids andscoloplacids (Bailey and Baskin, 1976).

The branchiostegal rays articulate with the ven-trocaudal process of the hyoid bar, which is a largeand cartilaginous extension of the hyoid bar at thelevel of the joint between the anterior and posteriorceratohyal. Arratia (1987) saw a similar situation inLoricarichthys sp., and a different situation in Cal-lichthys callichthys, where three separate cartilageelements connect the four branchiostegal rays withthe hyoid arch, while the rays articulate with theceratohyals directly in other catfishes including dip-lomystids.

The minuscule cartilage nucleus present in frontof the infrapharyngobranchial III in adult speci-mens (Geerinckx, personal observation) has notbeen found in any of the studied developmentalstages, and must, therefore, develop later than the12.4 mm stage (its apparent absence in the older intoto-stained specimens [14.4–25.0 mm] might bedue to very weak alcian staining). Alexander (1965)stated that the loricariid Hypostomus plecostomushas no pharyngeal teeth, as ‘‘it does not requirethem.’’ This stands against the observation bySchaefer (1987), who counted numerous teeth onboth the upper and lower pharyngeal jaw in thesame species, exactly as in A. cf. triradiatus. Thelower pharyngeal jaws arise independently as sup-porting plates for the pharyngeal teeth, and coalescesecondarily with the fifth ceratobranchials thatappear at the same moment, as in other siluriforms(McMurrich, 1884; Vandewalle et al., 1999). The de-velopment of the upper pharyngeal jaws has startedmuch earlier than that of the lower, a sequence alsoobserved in other siluriforms (Adriaens and Ver-raes, 1998).

In A. cf. triradiatus the parurohyal is pierced byone blood vessel, the inferior jugular vein (Fig. 5h).This vein receives blood from vessels draining thehyohyoideus inferior, protractor hyoidei and inter-mandibularis muscles, before ascending throughthe center of the parurohyal. It then receives severalmore veins from the sternohyoideus, before runningto the sinus venosus, crossing the ventral aorta atthe right side. This vein has also been reported byNawar (1955) in Clarias gariepinus, though notpiercing the parurohyal. Instead, in C. gariepinus adirect branch of the ventral aorta descends throughit, before sending branches into the hyohyoideus in-ferior (Adriaens and Verraes, 1998; personal obser-vation). In A. cf. triradiatus, the arteries irrigatingthe hyohyoideus inferior branch off from the aortaventralis and run above the parurohyal, enteringthe muscle more laterally. These strikingly differentconfigurations also differ from the situation in Nem-



atogenys, Trichomycterus, and Noturus, where it isthe hypobranchial artery that pierces the par-urohyal (Arratia and Schultze, 1990).

In Clarias gariepinus the first basibranchial isabsent (Nawar, 1954). In their developmental studyAdriaens and Verraes (1998) concluded it is mostlikely incorporated in the parurohyal. The presentstudy corroborates this hypothesis: serial sections ofsubsequent stages show that the first basibranchialsplits off from the next basibranchials, and becomesreduced. It remains, however, continuous to thedumbbell-shaped cartilage nucleus of the parur-ohyal. It is difficult to say whether it disappearscompletely or forms the dorsalmost part of themedial dorsal ridge of the parurohyal anterior to theforamen for the inferior jugular vein.

ACKNOWLEDGMENTS

Comments of two anonymous reviewers have sig-nificantly improved the manuscript.

LITERATURE CITED

Adriaens D, Verraes W. 1994. On the functional significance ofthe loss of the interhyal during ontogeny in Clarias gariepinusBurchell, 1822 (Teleostei: Siluroidei). Belg J Zool 124:139–155.

Adriaens D, Verraes W. 1998. Ontogeny of the osteocranium inthe African catfish. Clarias gariepinus (1822) (Siluriformes:Clariidae): Ossification sequence as a response to functionaldemands. J Morphol 235:183–237.

Adriaens D, Verraes W, Taverne L. 1997. The cranial lateral-linesystem in Clarias gariepinus (Burchell, 1822) (Siluroidei: Clar-iidae): Morphology and development of canal related bones.Eur J Morphol 35:181–208.

Alexander RMcN. 1964. The structure of the Weberian apparatusin the Siluri. Proc Zool Soc Lond 142:419–440.

Alexander RMcN. 1965. Structure and function in the catfish.J Zool Lond 148:88–152.

Aquino AE, Schaefer SA. 2002. The temporal region of the cra-nium of loricarioid catfishes (Teleostei: Siluriformes): Morpho-logical diversity and phylogenetic significance. Zool Anz 241:223–244.

Armbruster JW. 2004. Phylogenetic relationships of the sucker-mouth armoured catfishes (Loricariidae) with emphasis on theHypostominae and the Ancistrinae. Zool J Linn Soc 141:1–80.

Arratia G. 1987. Description of the primitive family Diplomysti-dae (Siluriformes, Teleostei, Pisces): Morphology, taxonomyand phylogenetic implications. Bonn Zool Monogr 24:1–120.

Arratia G. 1990. Development and diversity of the suspensoriumof trichomycterids and comparison with loricarioids (Teleostei:Siluriformes). J Morphol 205:193–218.

Arratia G. 1992. Development and variation of the suspensoriumof the primitive catfishes (Teleostei: Ostariophysi) and theirphylogenetic relationships. Bonn Zool Monogr 32:1–148.

Arratia G. 2003. Catfish head skeleton: An overview. In: ArratiaG, Kapoor AS, Chardon M, Diogo R, editors. Catfishes, Vol. 1.Enfield, USA: Science Publishers. pp 3–46.

Arratia G, Gayet M. 1995. Sensory canals and related bones oftertiary siluriform crania from Bolivia and North America andcomparison with recent forms. J Vertebr Paleontol 15:482–505.

Arratia G, Huaquın L. 1995. Morphology of the lateral line sys-tem and of the skin of diplomystid and certain primitive loricar-ioid catfishes and systematic and ecological considerations.Bonn Zool Monogr 36:1–110.

Arratia G, Menu-Marque S. 1981. Revision of the freshwater cat-fishes of the genus Hatcheria (Siluriformes, Trichomycteridae)

with commentaries on ecology and biogeography. Zool Anz207:88–111.

Arratia G, Menu-Marque S. 1984. New catfishes of the genus Tri-chomycterus from the high Andes of South America (Pisces,Siluriformes) with remarks on distribution and ecology. ZoolJahrb, Abt Syst (Oekol), Geogr Biol 111:493–520.

Arratia G, Schultze H-P. 1990. The urohyal: Development andhomology within osteichthyans. J Morphol 203:247–282.

Bailey RM, Baskin JN. 1976. Scoloplax dicra, a new armored cat-fish from the Bolivian Amazon. Occ Pap Mus Zool Univ Mich674:1–14.

Bamford TW. 1948. Cranial development of Galeichthys felis.Proc Zool Soc Lond 118:364–391.

Bhatti HK. 1938. The integument and dermal skeleton of Siluroi-dea. Trans Zool Soc Lond 24:1–102.

Bridge TW, Haddon AC. 1893. Contributions to the anatomy offishes. II. The air-bladder and Weberian ossicles in the siluroidfishes. Philos Trans R Soc Lond 184:65–333.

Chardon M. 1968. Anatomie comparee de l’appareil de Weber etdes structures connexes chez les Siluriformes. Ann Mus R AfrCentr Ser IN-8 169:1–277.

Coburn MM, Grubach PG. 1998. Ontogeny of the Weberian appa-ratus in the armored catfish Corydoras paleatus (Siluriformes:Callichthyidae). Copeia 1998:301–311.

Daget J. 1964. Le crane des teleosteens. I. Origine et mise enplace des tissus squelettogenes chez l’embryon. Mesomesen-chyme et ectomesenchyme. Mem Mus Natl Hist Nat Ser A Zool31:164–340.

de Beer GR. 1937. Studies on the Vertebrate Head. Oxford: Clar-endon Press.

de Pinna MCC. 1993. Higher-level phylogeny of Siluriformes,with a new classification of the order (Teleostei, Ostario-physi), PhD Thesis, City University of New York, 482 pp.

Diogo R, Oliveira C, Chardon M. 2001. On the homologies of theskeletal components of catfish (Teleostei: Siluriformes) suspen-sorium. Belg J Zool 131:93–109.

Fink SV, Fink WL. 1981. Interrelationships of the ostariophysanfishes (Teleostei). Zool J Linn Soc Lond 72:297–353.

Fink SV, Fink WL. 1996. Interrelationships of ostariophysanfishes (Teleostei). In: Stiassny MLJ, Parenti LR, Johnson GD,editors. Interrelationships of Fishes. London: Academic Press.pp 209–249.

Geerinckx T, Brunain M, Adriaens D. 2005. Development of thechondrocranium in the suckermouth armored catfish Ancistrus cf.triradiatus (Loricariidae, Siluriformes). J Morphol 266:331–355.

Gluckmann I, Huriaux F, Focant F, Vandewalle P. 1999. Postem-bryonic development of the cephalic skeleton in Dicentrarchuslabrax (Pisces, Perciformes, Serranidae). Bull Mar Sci 65:11–36.

Gosline WA. 1975. The palatine-maxillary mechanism in cat-fishes, with comment on the evolution and zoogeography ofmodern Siluroids. Occas Pap Calif Acad Sci 120:1–31.

Grande L. 1987. Redescription of yHypsidoris farsonensis (Tele-ostei: Siluriformes), with a reassessment of its phylogeneticrelationships. J Vertebr Paleontol 7:24–54.

Harrington RW Jr. 1955. The osteocranium of the American cyp-rinid fish, Notropis bifrenatus, with an annotated synonymy ofteleost skull bones. Copeia 1955:267–291.

Hoedeman JJ. 1960a. Studies on callichthyid fishes: (4) Develop-ment of the skull of Callichthys and Hoplosternum (1) (Pisces-Siluriformes). Bull Aquat Biol 1:73–84.

Hoedeman JJ. 1960b. Studies on callichthyid fishes: (5) Develop-ment of the skull of Callichthys and Hoplosternum (2) (Pisces-Siluriformes). Bull Aquat Biol 2:21–36.

HoffmannM, Britz R. 2006. Ontogeny and homology of the neuralcomplex of otophysan Ostariophysi. Zool J Linn Soc 147:301–330.

Howes GJ. 1983. The cranial muscles of loricarioid catfishes, theirhomologies and value as taxonomic characters. Bull Br MusNat Hist (Zool) 45:309–345.

Howes GJ, Teugels GG. 1989. Observations and homology of thepterygoid bones in Corydoras paleatus and some other cat-fishes. J Zool Lond 219:441–456.



Kindred JE. 1919. The skull of Amiurus. Ill Biol Monogr 5:1–4.KobayakawaM. 1992. Comparative morphology and development

of bony elements in the head region in three species of Japanesecatfishes (Silurus: Siluridae; Siluriformes). Jpn J Ichthyol 39:25–36.

Lauder GV, Crompton AW, Gans C, Hanken J, Liem KF, MaierWP, Meyer A, Presley R, Rieppel OC, Roth G, Schluter D,Zweers GA. 1989. Group report: How are feeding systems inte-grated and how have evolutionary innovations been intro-duced? In: Wake DB, Roth G, editors. Complex OrganismalFunctions: Integration and Evolution in Vertebrates. NewYork: Wiley. pp 97–115.

Lundberg JG. 1975. Homologies of the upper shoulder girdleand temporal region bones in catfishes (Order Siluriformes),with comments on the skull of the Helogeneidae. Copeia1975:66–74.

Lundberg JG. 1982. The comparative anatomy of the toothlessblindcat. Trogloglanis pattersoni Eigenmann, with a phyloge-netic analysis of the ictalurid catfishes. Misc Publ Mus ZoolUniv Mich 163:1–85.

Mabee PM, Trendle TA. 1996. Development of the cranium andpaired fins in Betta splendens (Teleostei: Percomorpha): Intra-specific variation and interspecific comparisons. J Morphol227:249–287.

McElman JF, Balon EK. 1980. Early ontogeny of white sucker.Catostomus commersoni, with steps of saltatory development.Environ Biol Fish 5:191–224.

McMurrich JP. 1884. On the osteology of Amiurus catus (L.) gill.Zool Anz 168:296–299.

Mo T. 1991. Anatomy and Systematics of Bagridae (Teleostei),and Siluroid Phylogeny. Koenigstein: Koeltz Scientific Books.216 pp.

Nawar G. 1954. On the anatomy of Clarias lazera. I. Osteology. JMorphol 94:551–585.

Nawar G. 1955. On the anatomy of Clarias lazera. III. The vascu-lar system. J Morphol 97:179–214.

Orton GL. 1955. Some aspects of ecology and ontogeny in thefishes and amphibians. Am Nat 89:193–203.

Patterson C. 1975. The braincase of the pholidophorid and lepto-lepid fishes, with a review of the actinopterygian braincase.Philos Trans R Soc Lond Biol Sci 269:275–579.

Patterson C. 1977. Cartilage bones, dermal bones and membranebones, or the exoskeleton versus the endoskeleton. In: AndrewsSM, Miles RS, Walker AD, editors. Problems in Vertebrate Evo-lution. London: Academic Press. pp 77–121.

Radermaker F, Surlemont C, Sanna P, Chardon M, Vandewalle P.1989. Ontogeny of the Weberian apparatus of Clarias gariepi-nus (Pisces, Siluriformes). Can J Zool 67:2090–2097.

Regan CT. 1911. The classification of the teleostean fishes of theorder Ostariophysi. II. Siluroidea. Ann Mag Nat Hist 8:553–577.

Reif WE. 1982. Evolution of dermal skeleton and dentition in ver-tebrates. The odontode regulation theory. Evol Biol 15:287–368.

Reissner E. 1859. Uber die Schwimmblase und den Gehorapparateiniger Siluroiden. Arch Anat Phys Wiss Med Leipz421–438.

Retzer ME, Page LM. 1996. Systematics of the stick catfishes.Farlowella Eigenmann & Eigenmann (Pisces: Loricariidae).Proc Acad Nat Sci Phila 147:33–88.

Schaefer SA. 1987. Osteology of Hypostomus plecostomus (Lin-naeus) with a phylogenetic analysis of the loricariid subfamilies(Pisces: Siluroidei). Contrib Sci 394:1–31.

Schaefer SA. 1988. Homology and evolution of the opercular se-ries in the loricarioid catfishes (Pisces: Siluroidei). J Zool Lond214:81–93.

Schaefer SA. 1997. The neotropical cascudinhos: Systematics andbiogeography of the Otocinclus catfishes (Siluriformes: Loricar-iidae). Proc Acad Nat Sci Phila 148:1–120.

Schaefer SA, Aquino AE. 2000. Postotic laterosensory canal andpterotic branch homology in catfishes. J Morphol 246:212–227.

Schaefer SA, Lauder GV. 1986. Historical transformation of func-tional design: Evolutionary morphology of feeding mechanismsin loricarioid catfishes. Syst Zool 35:489–508.

Surlemont C, Vandewalle P. 1991. Developpement postembryon-naire du squelette et de la musculature de la tete de Clariasgariepinus (Pisces, Siluriformes) depuis l’eclosion jusqu’a 6.8mm. Can J Zool 69:1094–1103.

Taylor WR, Van Dyke GC. 1985. Revised procedures for stainingand clearing small fishes and other vertebrates for bone andcartilage study. Cybium 9:107–119.

Tilney RL, Hecht T. 1993. Early ontogeny of Galeichthys felicepsfrom the south east coast of South Africa. J Fish Biol 43:183–212.

Vandewalle P, Huysseune A, Aerts P, Verraes W. 1994. The pha-ryngeal apparatus in teleost feeding. In: Bels VL, Chardon M,Vandewalle P, editors. Advances in Comparative and Environ-mental Physiology. Heidelberg: Springer-Verlag. pp 59–92.

Vandewalle P, Laleye P, Focant B. 1995. Early development of ce-phalic bony elements in Chrysichthys auratus (Geoffroy Sint-Hilaire, 1808) (Pisces, Siluriformes, Claroteidae). Belg J Zool125:329–347.

Vandewalle P, Gluckmann I, Baras E, Huriaux F, Focant B. 1997.Postembryonic development of the cephalic region in Hetero-branchus longifilis. J Fish Biol 50:227–253.

Vandewalle P, Gluckman I, Wagemans F. 1998. A critical assess-ment of the Alcian blue/Alizarine double staining in fish larvaeand fry. Belg J Zool 128:93–95.

Vandewalle P, Chikou A, Laleye P. 1999. Early development ofthe chondrocranium in Chrysichthys auratus. J Fish Biol 55:795–808.

Verraes W. 1974. Discussion on some functional-morphologicalrelations between some parts of the chondrocranium and theosteocranium in the skull base and the skull roof, and of somehead parts during postembryonic development of Salmo gaird-neri Richardson, 1836 (Teleostei: Salmonidae). Form Funct7:281–292.

Wagemans F, Focant B, Vandewalle P. 1998. Early developmentof the cephalic skeleton in the turbot. J Fish Biol 52:166–204.

Weisel GF. 1967. Early ossification in the skeleton of the sucker(Catostomus macrocheilus) and the guppy (Poecilia reticulata).J Morphol 121:1–18.



Development of the osteocranium in the suckermouth armored ... · Development of the Osteocranium in the Suckermouth Armored Catﬁsh Ancistrus cf. triradiatus (Loricariidae, Siluriformes)

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