Commensal worm traces and possible juvenile thalassinidean burrows associated with Ophiomorpha nodosa, Pleistocene, southern Brazil

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Palaeogeography, Palaeoclimatology,

Commensal worm traces and possible juvenile thalassinidean

burrows associated with Ophiomorpha nodosa,

Pleistocene, southern Brazil

Jordi M. de Gibert a,*, Renata G. Netto b,

Francisco M.W. Tognoli c, Marcelo E. Grangeiro b,F

a Departament d’Estratigrafia, Paleontologia i Geociencies Marines, Universitat de Barcelona. Martı Franques s/n, 08028 Barcelona, Spainb Laboratorio da Historia da Vida e da Terra, PPGeo UNISINOS. Av. Unisinos, 950, 93022-000 Sao Leopoldo RS, Brazil

c Curso de Pos Graduacao em Geociencias, Universidade Estadual Paulista. Av. 24-A, 1515 Bela Vista, CEP 13506-900, Rio Claro SP, Brazil

Received 21 September 2004; received in revised form 12 July 2005; accepted 19 July 2005

Abstract

The Pleistocene Chuı Formation at Osorio (Rio Grande do Sul, Brazil) consists of coastal marine and eolian sands, the former

containing abundant and well-preserved Ophiomorpha nodosa burrow systems. Detailed ichnological study has revealed

interesting features associated with them. Small-sized Ophiomorpha, here assigned to a new ichnospecies, O. puerilis, are

interpreted as possible burrows of juvenile thalassinidean crustaceans probably belonging to the same species as the producers of

largerO. nodosa. Additionally, helicoidal burrows with thick, concentrically laminated linings are associated with the walls of O.

nodosa. They are assigned to the new ichnospecies Cylindrichnus helix, and they are interpreted as dwellings of commensal

annelid worms. The association of these three ichnospecies constitutes a fossil example of the role of thalassinideans as

ecosystem engineers able to modify their environment and to create new space and resources usable by other organisms.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Ophiomorpha; Ontogeny; Paleoecology; Ecosystem engineering; Pleistocene; Brazil

0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.palaeo.2005.07.008

* Corresponding author. Fax: +34 934021340.

E-mail addresses: [email protected] (J.M. de Gibert),

[email protected] (R.G. Netto), [email protected]

(F.M.W. Tognoli).F Deceased, June 2005.

1. Introduction

Ophiomorpha is one of the best-known trace fossils

for paleontologists and sedimentary geologists due to

its abundance in Mesozoic and Cenozoic shallow and

marginal marine deposits. The ichnogenus designates

multiple-branching gallery systems of variable com-

plexity characterized by having a thick pelletal lining.

Early works revealed the striking similarity between

Palaeoecology 230 (2006) 70–84

Fig. 1. (A) Situation of Rio Grande do Sul State. (B) Simplified

geological map of Rio Grande do Sul State. (C) Geological map of

the Coastal Plain of Rio Grande do Sul (PCRS) showing the outcrop

distribution of the Barrier-Lagoon System III and the study area

(star). Modified of Tomazelli and Villwock, 2000.

J.M. de Gibert et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 230 (2006) 70–84 71

Ophiomorpha and modern burrows of thalassinidean

crustaceans (Weimer and Hoyt, 1964; Suguio and Mar-

tin, 1976; Frey et al., 1978). Ophiomorpha is a sub-

strate-controlled ichnogenus that occurs almost

exclusively in fine- to medium-grained sandy deposits

(Ekdale, 1992). Although very abundant in shallow

marine environments, some ichnospecies of Ophio-

morpha are known from deeper water settings (e.g.,

Tchoumachenko and Uchman, 2001). The extensive

literature regarding Ophiomorpha includes papers

focused on paleoenvironmental interpretation and ich-

nofabrics (e.g., Pollard et al., 1993; Anderson and

Droser, 1998), paleobiological and neoichnological

aspects (e.g., Frey et al., 1978; Miller and Curran,

2001) and evolutionary paleoecology (e.g., Bottjer et

al., 1988).

Despite being a very well known ichnogenus,

Ophiomorpha often occurs only as cross-sections

that do not allow in depth analysis of important

attributes, such as architecture or relation with other

traces. Some outcrops, however, offer the possibility

of such detailed studies, which reveal interesting fea-

tures of the paleobiology of the tracemaker. The

Pleistocene Chuı Formation in Brazil includes some

outcrops with those characteristics. Ophiomorpha

occurs here in well-sorted loose sands that allow care-

ful cleaning of the burrow systems. The study of

Ophiomorpha nodosa from the Chuı Formation,

besides providing the opportunity to analyze certain

aspects of its 3-dimensional architecture, has revealed

interesting new details about the paleoecology of this

trace fossil. Small-sized Ophiomorpha (O. puerilis

nov. isp.) and helicoidal Cylindrichnus (C. helix

nov. isp.) have been found in direct association with

Ophiomorpha nodosa. They are here interpreted as

burrows of juvenile thalassinidean crustaceans and

commensal worms, respectively. The objective of

this paper is to analyze these features in the light of

biological data on modern thalassinideans and their

burrows.

2. Geological setting

The Chuı Formation crops out in the Coastal Plain

of Rio Grande do Sul (Planicie Costeira do Rio

Grande do Sul, PCRS) in southern Brazil. This plain

extends for about 33,000 km2 along the eastern part of

the state of Rio Grande do Sul, parallel to the present

shoreline (Fig. 1). The PCRS was formed during the

Quaternary by the progradation of sediments deriving

from the western highlands. The proximal part of the

plain consists of alluvial fans fed by the Precambrian

Sul-riograndense Shield and by Paleozoic and Meso-

zoic sedimentary rocks and the volcanic plateau of

Serra Geral. The distal region is composed of four

lagoon-barrier depositional systems (Villwock et al.,

1986; Villwock and Tomazelli, 1995; Tomazelli and

Villwock, 2000). They are known as Lagoon-Barrier

Systems I to IV, and they were formed during high-

stand sea levels related to glacio-eustatic cycles during

the Quaternary. The oldest systems are located to the

west, while the youngest are situated to the east as a

result of the progradation of the plain. Systems I to III

are Pleistocene, and System IV is Holocene (Toma-

zelli and Villwock, 2000).

The Chuı Formation extends all along the coast of

Rio Grande do Sul and it belongs to System III (Figs.

1 and 2). Detailed sedimentologic study of this system

was accomplished by Tomazelli et al. (1982) and

Tomazelli (1985). From base to top, the barrier con-

sists of shallow marine, beach and eolian deposits

composed of quartzose, fine-grained, well-sorted

Fig. 2. Stratigraphic chart of the PCRS. Modified of Tomazelli and

Villwock, 2000.

J.M. de Gibert et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 230 (2006) 70–8472

sand. The lagoonal facies consist of silty and muddy

fine sands with carbonate and ferruginous concretions.

Radiometric dating of fossils in the marine sands

(Martin et al., 1982) and thermoluminescence dating

of eolian sands (Poupeau et al., 1985) coincide in

assigning an age of about 120 ky to System III.

Hence, it corresponds to the last transgressive peak

of the Pleistocene, known as the Cananeia Transgres-

sion (Suguio and Martin, 1978).

3. The Osorio outcrops

The outcrops studied for this project are located in

two adjacent quarries, Jazida Gomes and Transareia,

situated in the surroundings of Osorio in the north-

eastern part of Rio Grande do Sul (Fig. 1). The

sedimentology of these outcrops was previously stu-

died by Tomazelli et al. (1982).

Several sections were made to establish the strati-

graphy of the Chuı Formation at Osorio. A represen-

tative section (Gomes Quarry) is shown in Fig. 3. Two

main units are differentiated: a lower sandy marine

unit and an upper sandy eolian unit. They correspond

to facies B and A, respectively, of Tomazelli et al.

(1982). These authors described an additional under-

lying unit consisting of silty–muddy sands that was

only temporarily exposed.

The marine unit consists of well-sorted fine-

grained mainly quartzose sands with an observed

maximum thickness of 5.5 m. They exhibit horizontal

lamination, and low-angle, herringbone, and planar

cross-stratification, besides symmetrical ripples at

the uppermost part. Trace fossils (Fig. 4) are ubiqui-

tous in both quarries, the most obvious being exten-

sive Ophiomorpha nodosa systems occurring mainly

in the lower half of the unit. Very abundant small

Macaronichnus isp. (no more than 2 mm in diameter)

and rare Diplocraterion parallelum occur in the same

part of the unit. The upper half hosts abundant Rosse-

lia socialis. Ichnologic and sedimentologic data indi-

cate that this unit was deposited in a very shallow

subtidal setting.

The upper eolian unit comprises about 5 m of fine-

grained sands showing planar cross-lamination inter-

calated with several few-centimeter-thick horizons

having higher mud content. A strongly pedogenized

horizon consisting of reddish fine- to medium-grained

sandstones is found at the top of the unit. Insect

burrows are very abundant, including Krausichnus,

?Vondrichnus and wasp cells (cf. Celliforma) (Fig. 5)

(Grangeiro et al., 2003). Isolated meniscate burrows

(cf. Taenidium) are probably related to ?Vondrichnus

burrow systems. These traces originate in the paleo-

soil horizon (and some probably in the muddier inter-

calations as well) and penetrate into the eolian sands.

Root traces, otherwise, are rare or poorly preserved,

suggesting vegetation dominated by grasses and

bushes.

4. Ophiomorpha systems and associated structures

4.1. Ophiomorpha nodosa burrow systems

Ophiomorpha burrow systems in the Chuı For-

mation constitute complex labyrinths with horizontal

to vertical elements and multiple branching. Quali-

tative observations show that horizontal and subhor-

izontal elements are important as constituent of

mazes connected to less-branched vertical shafts

(Fig. 6B). Vertical elements are rectilinear, while

the rest are rectilinear to gently curved. The branch-

ing style of the mazes is mostly Y-shaped but also

T-shaped (Fig. 6A, C).

The burrows are circular in cross-section. They

exhibit a strongly developed pelletal lining consti-

tuted by mud with isolated quartz sand grains. The

lining is externally knobby but internally smooth. It

is constituted by ovoidal to spherical pellets 0.5 to 1

cm in diameter. The pellets are not aligned or clus-

tered, but they are homogeneously distributed around

the burrow without displaying any particular arrange-

Fig. 3. Representative section (Gomes quarry) of the Chuı Formation in the study area showing its main sedimentological and ichnological

characteristics.

J.M. de Gibert et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 230 (2006) 70–84 73

ment (Fig. 6D). The internal diameter of the burrows

ranges between 2 and 5 cm, being the average 3.2

cm and the mode 3 cm. The thickness of the lining is

very variable between 0.25 and 1.5 cm with an

average of 0.9 cm and a mode of 1 cm. Arrangement

and shape of pellets in the lining are diagnostic

features to differentiate among Ophiomorpha ichnos-

pecies (Frey et al., 1978; Bromley and Ekdale,

1998). The characteristics described above allow

assignment of the burrow systems from Chuı to the

ichnospecies O. nodosa.

Other elements have been found that allow com-

pletion of the 3-dimensional configuration of the O.

nodosa systems. Chambers are sometimes found

connected to the end of some galleries. These cul-

de-sac chambers are ovoidal in shape (Fig. 6E), with

a length of about 15 cm and an external diameter of

about 8 cm. They exhibit the same type of pelleted

lining as other elements of the burrow system.

Additionally, some elements with very distinct

characteristics have been recognized as well. They

are vertical cylindrical structures with an external

smooth wall and an internal narrow tube (Fig. 7).

The lining is very thick (8–14 mm) compared with

the diameter of the central tube (about 5 mm) (Fig.

7C, D). One specimen has been observed that shows

evidence of slight lateral shifting of the shaft (Fig.

7B–D). The maximum vertical length observed was

17.5 cm. These elements are connected to Ophio-

morpha burrows and are part of the same burrow

Fig. 4. Shallow marine trace fossils from the Chuı Formation. (A) Ophiomorpha nodosa. (B) Small Macaronichnus isp. and O. nodosa (right).

(C) Rosselia socialis. (D) Diplocraterion parallelum (right) and O. nodosa (left).

J.M. de Gibert et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 230 (2006) 70–8474

system (Fig. 7A). They are comparable with the

constricted burrow entrances of thalassinidean bur-

rows figured by Frey et al. (1978) and Pollard et al.

(1993).

4.2. Small Ophiomorpha

Adjacent to the large burrows, very small Ophio-

morpha locally occur (Fig. 8A). They have external

diameters between 1.6 and 3.3 mm (average 2.4

mm). The configuration of these small Ophiomor-

Fig. 5. Insect burrows from the continental part of the Chuı Formation.

pha is rather simple, consisting of a rectilinear or

gently curved tube, most times parallel to the large

adjacent Ophiomorpha, and sometimes ending in a

slightly enlarged chamber (Fig. 8D). The lining of

these burrows consists of cylindrical round-ended

pellets, about 1 mm in length and 0.5 mm in

diameter. The pellets consist of mud with isolated

quartz grains. They constitute a single-layer lining

(Fig. 8E) and display variable orientations that are

parallel, oblique or perpendicular to the burrow axis

(Fig. 8B, C).

(A) Krausichnus isp. (B) ?Vondrichnus isp. (C) Possible bee nest.

Fig. 6. Ophiomorpha nodosa from the Chuı formation. (A) Maze comprising several horizontal and subhorizontal elements. (B) Vertical shaft

connected to a horizontal maze. (C) Horizontal maze. (D) External aspect of the knobby lining formed by ovoidal to spherical silty pellets. (E)

Cul-de-sac chamber. Scale in centimeters.

J.M. de Gibert et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 230 (2006) 70–84 75

In base of the distinct morphology of the pellets,

the simple configuration of the burrows and their

diminutive diameter, these trace fossils are here

considered to represent a new ichnospecies of

Ophiomorpha and consequently named as Ophio-

morpha puerilis nov. isp. Formal diagnosis and dis-

cussion of this new ichnotaxon are given in the

Appendix.

4.3. Associated thickly lined helicoidal burrows

Burrows of a very different type also are intimately

associated with the Ophiomorpha systems. They are

attached externally to the Ophiomorpha galleries and

partly penetrating into its lining (Fig. 9). These bur-

rows consist of an internal, central or eccentric, open

(unfilled) tube, 3–4 mm in diameter, surrounded by a

Fig. 7. (A) Thick-lined, externally smooth vertical shafts representative of the entrance neck of the Ophiomorpha nodosa burrow systems. (B, C

and D) Longitudinal (B), top (C) and cross-section view (D) of an entrance shaft exhibiting lateral displacement. Scale in centimeters.

J.M. de Gibert et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 230 (2006) 70–8476

thick silty lining (Fig. 9A). The lining displays an

irregular concentric lamination. The external diameter

of these traces varies between 8 and 12 mm. They

exhibit irregularly curved configurations commonly

forming helicoids. They follow the path of Ophiomor-

pha nodosa burrows, both shafts and galleries.

On the basis of the structure of their linings, these

burrows are assigned to the ichnogenus Cylindrich-

nus. The distinct morphology has led us to erect a new

ichnospecies, C. helix, whose diagnosis and ichno-

taxonomic discussion are contained in the Appendix.

5. Discussion

5.1. Ophiomorpha nodosa

Weimer and Hoyt (1964) demonstrated the striking

similarity between the burrows of the callianassid

thalassinidean Callichirus major (formerly known as

Callianassa major) and the ichnospecies Ophiomor-

pha nodosa. This fact led many authors to inade-

quately establish an identity between the two.

Quaternary Ophiomorpha from the south Atlantic

coast have been referred as dfossil tubes of Cal-

lianassaT (Suguio and Martin, 1976), dfossil tubes ofCallichirusT (Suguio et al., 1985), or dcallianassidfossil tubesT (Tomazelli et al., 1982; Mouzo et al.,

1989). However, several species of callianassid and

upogebiid thalassinideans are known to construct bur-

rows with pelleted linings (Frey et al., 1978; Bromley,

1996). Other decapods such as some brachiurans

(crabs) and astacidae (crayfish) built pelleted chim-

neys protruding above the surface, but their dwellings

within the sediment are unpelleted (Chamberlain,

1975). Hence, pellet-lined burrows today seem to be

exclusively the product of thalassinidean shrimps.

Most modern thalassinideans are soft-bottom

infaunal dwellers and numerous studies on their bur-

row morphology have been published (e.g., Rodri-

gues, 1966; Dworschak, 1987; Dworschak and

Pervesler, 1988; Dworschak and Ott, 1993; Rodrigues

Fig. 8. Ophiomorpha puerilis nov. isp. (A) Specimen of O. puerilis (Op) attached to a larger O. nodosa (On). (B) Specimen exhibiting

longitudinal arrangement of pellets. (C) Specimen exhibiting transversal arrangement of pellets. (D) Holotype (MP 4796) showing the rectilinear

morphology with a slightly enlarged terminal chamber. (E) Broken specimen displaying the lining and the internal passive sandy filling. Scale in

millimeters except for panel (A) which is in centimeters.

J.M. de Gibert et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 230 (2006) 70–84 77

and Shimizu, 1992, 1997; Dworschak and Rodrigues,

1997). Unfortunately, most of these studies are based

on resin casts and focus on burrow configuration,

Fig. 9. Cylindrichnus helix nov. isp. (A) Holotype (MP 4802) attached to a

(B and C) Two other specimens associated to horizontal or subhorizontal

paying little attention on the features of the burrow

lining, which is a very prominent feature in the fossil

record.

n Ophiomorpha nodosa shaft. Arrow indicates the void central tube.

O. nodosa galleries.

J.M. de Gibert et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 230 (2006) 70–8478

Griffis and Suchanek (1991) studied the burrows of

44 modern species of thalassinideans and observed

that they construct species-specific burrows. These

authors proposed a simple model to classify their

architecture and trophic mode. Although this model

has been criticized (Nickell and Atkinson, 1995) for

being too simplistic, it provides useful basis on which

to analyze the Ophiomorpha from Chuı Formation at

Osorio.

Griffis and Suchanek (1991) classified thalassini-

dean burrows in six types. Type 4 is the one that

compares best with Chuı Ophiomorpha. The burrow

systems grouped within this type are dprimarily reti-

culate branches extending horizontally from a long

vertical shaft.T They have a smooth, hard burrow wall

and distinctively narrow shafts at the burrow opening.

All these features are found in the Ophiomorpha from

Osorio. The five species that the authors listed as

producers of Type 4 burrows belong to the family

Callianassidae: Callichirus major, Callianassa guas-

sutinga, C. jamaicense, C. louisianensis and Glyp-

turus jousseaumei. The first three species are known

from the modern Brazilian coast (Rodrigues, 1966;

Rodrigues and Shimizu, 1997), although they are not

present in the shores of Rio Grande do Sul, where

thalassinideans are today represented by the species

Sergio mirim.

The trophic significance of Type 4 burrows is not

clear as Griffis and Suchanek (1991) recognized. The

deep branching pattern suggests some sort of sedi-

ment processing (deposit feeding), but the hard walls

point to rather permanent burrows favoring an inter-

pretation of sea-grass harvesting (although no sea-

grass was found on them) or suspension feeding

behavior of the dweller. Pending new data, the men-

tioned authors interpreted these burrow systems as

being constructed by suspension feeders that may

complete their diet with some sort of wall grazing.

Considering all above, we interpret that the trace-

maker of the Chuı Ophiomorpha was a thalassini-

dean shrimp, probably a callianassid, with a

complex feeding behavior based mainly in sus-

pended material.

5.2. Ophiomorpha puerilis

The small Ophiomorpha puerilis and the larger

O. nodosa are both pellet-lined open burrows, and

so they share the same essential constructional fea-

tures. However, they exhibit two main differences.

The first is the shape of the pellets, subcylindrical

in the first, and ovoidal to spherical in the second.

The second difference is the unbranched and simple

configuration of the small burrows. These differ-

ences may have resulted from different burrow

architects but they may also be due to ontogenetic

changes within a single species. Both ichnospecies

occur not only in the same facies but also in direct

association with one another. Although burrow con-

nections between them have not been observed, the

occurrence of the small burrows on the surround-

ings of the larger ones suggests that connections

existed.

The pellets found in O. puerilis are similar in

morphology to the fecal pellets of some polychaetes

(Schafer, 1972; Bayuk and Radwanski, 1979). Never-

theless, annelid excrements are rather find as aggre-

gates, trails or burrow fills and not known as

forming part of a constructed lining. Pelleted linings

seem to be an exclusive feature of thalassinidean

burrows (see discussion above). Thus, although a

polychaete origin for O. puerilis cannot be comple-

tely ruled out, we believe that it is most likely that

this ichnospecies was produced by thalassinideans.

The tracemakers could have been juveniles of the O.

nodosa producers or a completely different species

of small-sized crustaceans. We favor the first hypoth-

esis given the direct association between the two

types of burrows and the information available on

the burrowing behavior of modern and ancient tha-

lassinideans (discussed below). Hence, we consider

that juveniles and adults of a single species were the

tracemakers of smaller and larger Ophiomorpha,

respectively.

Nevertheless, the distribution of sizes of Ophio-

morpha (puerilis and nodosa) from the Chuı Forma-

tion is not continuous. There is an important gap

between small (1.6–3.3 mm in diameter) and large

burrows (2–5 cm in diameter). Intermediate forms (8–

10 mm in diameter) are not absent but very rare.

Tamaki and Ingole (1993) and Berkenbusch and Row-

den (1998) recorded the existence of bimodality on

populations of Callianassa japonica in Japan and

Callianassa filholi in New Zealand, respectively.

The two size modes corresponded to juvenile and

adult animals and resulted from the existence of a

J.M. de Gibert et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 230 (2006) 70–84 79

period of recruitment during the year. However, in the

fossil record, we should expect this situation to have

been modified by the effect of time averaging. Hence,

the bimodality of the fossil Ophiomorpha from Chuı

may have been the result of a high mortality of

juveniles or of the abandonment of juvenile dwellings

to occupy the parental burrows.

Modern burrows of juvenile thalassinideans asso-

ciated with their adults have been reported by Forbes

(1973; Callianassa kraussi), Frey and Howard

(1975; Upogebia affinis), Swinbanks (1981; Callia-

nassa californiensis) and Tamaki and Ingole (1993;

Callianassa japonica). In the case of U. affinis, two

possible scenarios were suggested for this situation

(Frey and Howard, 1975): (a) larvae development

within the parent burrow or (b) settlement of plank-

tonic larvae within burrows. The first would imply

some sort of parental care in the sense it is described

by Clutton-Brock (1991). Thiel (1999) demonstrated

how permanence in parent burrows increases the

survival chances of juveniles of two amphipod crus-

tacean species. In the second scenario, the presence

of juveniles within burrows would simply be a sec-

ondary consequence of settlement in an area with

open holes leading into the burrows.

Most thalassinideans have planktonic larvae, but

Forbes (1973) showed that the larvae of C. kraussi

were not capable of swimming, and so he interpreted

that they stayed in the parental burrows until they

were capable of burrowing at the juvenile stage.

Thus, the burrows of the juveniles protruded as

extensions from their parental burrow. In Forbes’

(1973) and Frey and Howard’s (1975) papers, juve-

nile burrows were described as originating in adult

burrow chambers that could be interpreted as brood

chambers. The small Ophiomorpha from Osorio

occur isolated and not associated with adult burrow

enlargements. That supports the interpretation of set-

tlement of planktonic larvae within adult burrows.

Several examples of juvenile–adult association are

known in the fossil record of thalassinidean or other

crustacean burrows. From the Cretaceous of Utah,

Howard (1966) described associations of very small

and dnormal sizeT Thalassinoides, and Frey and

Howard (1975) recorded Ophiomorpha burrows

locally dencrustedT by clusters of tiny thalassinoid

burrows. Curran (1976) and Curran and Frey (1977)

interpreted bulbous enlargements in Ophiomorpha

burrows with small radiating tubules as dpossiblecallianassid brood structuresT in the Pleistocene of

North Carolina. Verde and Martınez (2004) recorded

similar structures in the Miocene of Uruguay con-

nected to Thalassinoides and Ophiomorpha burrows.

These authors erected a new ichnogenus and ichnos-

pecies, Maiakarichnus currani to name these struc-

tures. Gibert (1996) and Gibert et al. (1999)

interpreted the association of small and large Sinu-

sichnus sinuosus as connected juvenile and adult

crustacean burrows. Curran (1985) described small

Ophiomorpha shafts (5–7.5 mm in diameter) from

the Cretaceous Englishtown Formation of Delaware

(U.S.A.). These burrows were found in the same

facies as normal size O. nodosa systems although,

in contrast to the case of O. puerilis, not directly

connected.

Studies indicate the burrowing capabilities of

modern juvenile thalassinideans and demonstrate

that their dwellings may merge with adult burrow

systems. The fossil examples demonstrate that this

situation can be tracked back to the Cretaceous.

Hence, the example of the Pleistocene Ophiomorpha

from the Chuı Formation is an additional contribu-

tion to understanding the burrowing capabilities of

juvenile thalassinidean. Nevertheless, other hypoth-

esis (polychaete or different crustacean species origin

for O. puerilis) cannot be totally rejected.

5.3. Cylindrichnus helix

These trace fossils always occur adjacent to large

Ophiomorpha burrows and are partly excavated into

their lining. They have never been found in isola-

tion. This fact suggests that the association is not

casual.

This case is different from that of Ophiomorpha

puerilis. The thickly lined helicoidal burrows (Cylin-

drichnus helix) exhibit very different constructional

features and configuration from that of Ophiomorpha,

which makes it very unlikely that both have been

produced by the same species. C. helix was probably

produced by a worm, as it shows a concentric lining

comparable to that of the burrow of the terebellid

polychaete Amphitrite ornata (Aller and Yingst,

1978) and a helicoidal shape such as the traces pro-

duced by the capitellid Notomastus latericeus (Rein-

eck et al., 1967; Bromley, 1996). The thick lining with

J.M. de Gibert et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 230 (2006) 70–8480

irregular concentric layers suggests continuous re-

excavation of the burrow, probably by grazing on

the burrow walls.

The restriction of occurrence of C. helix to the

surroundings of Ophiomorpha linings suggests that

their tracemakers had a preference for this location.

Studies of the lining and surroundings of the burrows

of three different species of Callianassa (C. kraussi in

Branch and Pringle, 1987; C. trilobata in Dobbs and

Guckert, 1988; C. tyrrhena in Dworschak, 2000)

reveal that burrow linings are zones with high bacter-

ial biomass content that decrease rapidly with

increased distance from the burrow. Dworschak

(2000) pointed out that bacterial content in the inner

part of the lining of the burrow was one order of

magnitude higher than that on the sediment surface.

This dbacterial enhancementT can be explained by two

causes (Branch and Pringle, 1987): (a) irrigation of the

burrows allowing oxygenation deep within the sedi-

ment and (b) modification of the organic content of

the sediment by shrimp activity that may incorporate

this when constructing the lining.

Considering these possibilities, it seems likely that

the producer of C. helix inhabited the environs of the

Ophiomorpha burrows in order to benefit from (a)

access to oxygenated waters, (b) a high bacterial

content to feed on, and (c) protection by being

deep in the sediment far from epibenthic and shallow

endobenthic predators. By this interpretation, C.

helix may well represent an example of a fossilized

commensal relationship between its tracemaker and

the thalassinidean producing the Ophiomorpha.

Curran (1985) recorded a similar association of

worm-produced burrows and crustacean burrow net-

works in the Cretaceous Englishtown Formation. He

described two different types of traces associated to

Ophiomorpha nodosa. One of them consisted of sin-

uous, sometimes irregularly branching burrows exca-

vated within the lining of Ophiomorpha. The other

comprised polygonal-patterned burrow networks

sometimes found wrapping around or anchoring in

Ophiomorpha shafts. Curran (1985) interpreted the

first as a probable fossil example of commensalism.

Both types, despite being morphologically distinctive

from C. helix, may record a similar ecologic relation-

ship between a commensal worm benefiting from

constructing its dwelling adjacent to open burrow

systems of crustaceans.

6. Conclusions: thalassinideans as ecosystem

engineers

The association of Ophiomorpha nodosa, O.

puerilis and Cylindrichnus helix in the Pleistocene

Chuı Formation constitutes a fossil example of how

thalassinidean crustaceans modify their environment

favoring the occupation of new ecological niches.

Modern thalassinideans and other crustaceans play

a very important ecological role, particularly in shal-

low marine environments, as a result of their intense

burrowing activity and high-density populations.

They can be considered as true dphysical ecosystemengineersT in the sense of Jones et al. (1994, 1997).

They define ecosystem engineering as the physical

modification, maintenance or creation of habitats by

directly or indirectly controlling the availability of

resources to other organisms.

Curran and Martin (2003) have pointed out the

importance of modern callianassids as ecosystem

engineers in marginal marine environments. They

recorded how the burrowing activity of Glypturus

acanthochirus is responsible for the creation of a

highly mounded topography in intertidal carbonates

in the Bahamas. The mounds are stabilized by micro-

bial mats resulting in a modified substrate suitable for

colonization by other burrowing crustaceans. The

trace fossils from the Chuı Formation are an example

of this effect in deeper tiers. In this case, the activity

of the producers of O. nodosa created new living

space by increasing the area of the water–substrate

interface. Additionally, O. nodosa networks allowed

oxygenated seawater to circulate deep within the sedi-

ment and organic matter to be incorporated in the

burrow wall. Both factors favored microbial activity

in the surroundings of the burrows. This situation

allowed the producers of C. helix to exploit these

resources (oxygen and food) in a deep-tier niche

achieving protection from epifaunal and shallow

infaunal predators. On the other hand, planktonic

larvae of crustaceans could have entered the burrow

networks and constructed their juvenile burrows (O.

puerilis). In the case we considered O. puerilis as

produced by a different species than larger O. nodosa

(a hypothesis that cannot be completely ruled out, as

indicated above), the former would have to be inter-

preted in similar terms than C. helix, that is as the

work of a commensal organism.

J.M. de Gibert et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 230 (2006) 70–84 81

Hence, the construction of the O. nodosa burrow

systems indirectly resulted in the creation of new

space and resources suitable for larval recruitment

and/or commensal occupation. These two effects

have been recognized in rock units back to the Cre-

taceous, revealing the ecological role of thalassini-

deans (and other crustaceans as well) as ecosystem

engineers.

Acknowledgements

The authors wish to thank Marcelo Zagonel de

Oliveira for his assistance in the field. Tony Ekdale

and Richard Bromley contributed with their sugges-

tions to improve the manuscript. This paper is a con-

tribution to the projects 31.00.006/01-0 and 31.00.002/

04-4 of UNISINOS, 524415/1996-0 and 474345/03-3

of CNPq (Brazilian Scientific and Technological

Development Council), BTE 200-0584 of the Spanish

Government and GRC 2001/SGR/00077 of the Gen-

eralitat de Catalunya.

One of the authors, Marcelo E. Grangeiro, passed

away when this paper was close to be definitively

accepted. The rest of the authors wish to dedicate this

work to his memory.

Appendix A. Systematic Ichnology

A.1. Ophiomorpha puerilis nov. isp. Fig. 8

Diagnosis. Small (external diameter less than 4

mm), rectilinear Ophiomorpha, in some cases ending

in a slightly enlarged chamber, with a lining formed of

a single layer of cylindrical, rod-shaped pellets with

rounded ends.

Derivation of name. From the Latin dpuerilisTmeaning dyouthful,T djuvenile.T

Holotype. MP 4796 housed in the Museu de

Paleontologia da UNISINOS, Sao Leopoldo, Rio

Grande do Sul, Brazil.

Paratypes. MP 4794, MP 4798, MP 4799, MP

4804 housed in the same collection as the holotype.

Remarks. The diagnostic feature of the ichno-

genus Ophiomorpha is the presence of a distinct

lining made of agglutinated pelletoidal sediment

(Frey et al., 1978). This feature is present in O.

puerilis despite of its unusual small size for the

ichnogenus. Fursich (1973) and Schlirf (2000) sug-

gested that Ophiomorpha had to be considered a

junior synonym of Spongeliomorpha. Nevertheless,

we retain Ophiomorpha as a valid name because of

the obvious etho-constructional significance of the

pelleted lining.

Ichnospecies of Ophiomorpha are defined on

basis of the characteristics of their linings, particu-

larly the shape and distribution of the pellets (e.g.,

Frey et al., 1978; Bromley and Ekdale, 1998; Uch-

man, 2001). Thus, pellets are regularly ovoidal in O.

nodosa, O. boornensis and O. annulata but they are

differently arranged; uniformly distributed in the

first, grouped in pairs in O. boornensis, and orga-

nized in transverse rows in O. annulata. O. irregu-

laire bears irregular conical pellets, while O. rudis

exhibits a rather irregular distribution of the pelleted

lining. The elongated round-ended morphology of

the pellets in O. puerilis is very distinctive and not

found in any other ichnospecies of Ophiomorpha.

Additionally, its small size (one order of magnitude

smaller than the rest of ichnospecies) and its simple

configuration are also diagnostic features of this

ichnospecies.

Several other ichnogenera have been described as

constituted by pellets similar in size and morphology

to those seen in O. puerilis. Eiserhardt et al. (2001)

revised some of them to conclude that they can be

reduced to two main types: pellet-filled burrows

(Alcyonidiopsis) and epigenic pellet trails (Tomacu-

lum). Nevertheless, Uchman (1999) considered Toma-

culum as a junior synonym of Alcyonidiopsis as well.

Katto (1974) Sakoites yukioia from the Miocene of

Japan, which also superficially resembles O. puerilis,

is also described as a pellet-filled burrow and, so it

should probably be re-assigned to Alcyonidiopsis.

Tibikoia is described as forming irregular aggregates

(Bayuk and Radwanski, 1979). In any case, none of

those ichnotaxa are described as pellet-lined burrows

such as O. puerilis.

A.2. Cylindrichnus helix nov. isp. Fig. 9

Diagnosis. Cylindrichnus exhibiting irregular sinu-

soidal to helicoidal configuration.

Derivation of name. From the helicoidal burrow

morphology.

J.M. de Gibert et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 230 (2006) 70–8482

Holotype. MP 4802 housed in the Museu de

Paleontologia da UNISINOS, Sao Leopoldo, Rio

Grande do Sul, Brazil.

Paratype. MP 4803 housed in the same collection

than the holotype.

Remarks. The ichnogenus Cylindrichnus was

erected by Howard (1966) to designate burrows with

a thick, concentrically laminated lining. Despite the

common subsequent use of the ichnotaxon, Goldring

(1996) and Goldring et al. (2002) pointed out its

dubious validity. These authors demonstrated that

some concentrically laminated burrows, assignable

to Cylindrichnus, were passively filled (draught fill-

ing) while others had actually constructed linings. The

validity of the ichnogenus is pending of revision of

type (or near-type) material from the Cretaceous of

Utah. Meanwhile, we have decided to conservatively

include the new ichnospecies erected here, helix,

within the ichnogenus because it bears a distinct con-

centrically laminated lining, which is commonly con-

sidered as its diagnostic feature.

The sinuous to helicoidal pattern of the material

studied herein is not known from previous records of

Cylindrichnus (see Goldring, 1996 for revision) and

hence, justifies the erection of a new ichnospecies.

Other helicoidal burrows are obligatorily vertical

(Gyrolithes) or horizontal (Helicodromites, Helico-

lithus) and lack the thick concentric lining. The axes

of the helices in C. helix are parallel to the associated

Ophiomorpha shafts (vertical) and tunnels (inclined to

horizontal), and so, orientation is not diagnostic.

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