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FRAGMENTA MINERALOGICA E T PALAEONTOLOGICA 16. BUDAPEST 1993. p.
51-59
Lessiniella benettii gen. et sp. n., a giant Middle Jurassic
rhynchonellid brachiopod from the Southern Alps (Italy)
by Attila VÖRÖS
Abstract: Lessiniella benettii gen. et sp. n. (Rhynchonellida,
Brachiopoda) is described on the basis of specimens found in the
Callovian of the Lessini Mts. (Southern Alps, Italy). This very
large, smooth, gibbose rhynchonellid shows strong external
resemblance to cer-tain Early Paleozoic Pentamerida. The internal
features (strong dorsal median septum, long, radulifer crura) seem
to connect Lessiniella to the family Septirhynchiidae. An appa-rent
episode of gigantism among the rhynchonellids in the late Middle
Jurassic is pointed out; each Tethyan faunal province had its own
giant rhynchonellid (NW-European - "R." de-corata, Ethiopian -
Septirhynchia, Mediterranean - Lessiniella).
INTRODUCTION
In 1988, during the field-trip of the "Meeting on Bajocian
Stratigraphy" the participants were taken to the nice village of
Camposilvano (Lessini Mts., South-ern Alps, Italy) to a local
museum, the "Museo dei Fossili della Lessinia". The cu-rator and
owner of the Museum, Cavalière At t i l io BENETTI had even a
private cave behind his small house built at the foot of a cliff of
Jurassic limestones. The knight, after offering some drinks, kindly
showed us his rich fossil material collec-ted by himself from the
cave (Covolo) and from the wider surroundings. The col-lection was
focused on Jurassic ammonites for the pleasure of the scientific
com-munity; the neatly arranged windows of the exhibition showed
only one piece of brachiopod, but a bizarre piece, resembling a
gryphon's or a giant parrot's beak rather than a true
brachiopod.
This huge, smooth, pentameroid-like rhynchonellid brachiopod was
collected by Mr. BENETTI from the Callovian beds exposed in the
cave (Covolo di Camposil-vano) together with further four, but
smaller specimens of the same species. It was quite obvious that
the form was completely different from any other Jurassic
bra-chiopods.
The cave exposes a very condensed, Upper Bajocian to Middle
Oxfordian red limestone sequence in less than 8 m thickness (see
Clari et al. 1984). The approxi-mately 1 m thick Callovian
oncoidal-nodular limestone is bordered from below and above by
hard-grounds, involving stratigraphie gaps. A condensed and mixed
Lower Callovian ammonoid fauna from these beds is listed in Clari
et al. (1984).
Besides the curious, new rhynchonellid, there were a few dozens
of other bra-chiopods, collected by Mr. BENETTI from the same
horizon from the same locality. The majority of them seemed to be
determinable with some certainty:
Septocrurella ? defluxoides (Uhlig, 1881) Calvirhynchia cf.
contraversa (Oppel, 1861) Striirhynchia sp., aff. subechinata
(Oppel, 1863)
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Apringia atla (Oppel, 1863) Capillirhynchia sp., aff.
brentoniaca (Oppel, 1863) Nucleata tenuiplicata (Uhlig, 1881)
Karadagithyris sp. Dorsoplicathyris ? sp.
In addition to the hooked beak, the new rhynchonellid possesses
a robust constitution and a giant size (at least a single, possibly
gerontic specimen) and this recalls the similar relationships found
at some other late Middle Jurassic rhyncho-nellids (e.g.
Septirhynchia and "Rhynchonella" decorata). Apart from the strong
morphological differences, they seem to be quite distant from
paleobiogeographi-cal and phylogenetical points of view as well.
Nevertheless, the more or less synchronous appearance of these
exaggerated forms points to an episode of "rhynchonellid gigantism"
near the end of the Middle Jurassic. Therefore, in spite of the
sparse material and the regrettable monotypy, it seems to be
reasonable to erect a new genus on these unusual South Alpine
forms.
In this paper, the higher rank systematic categories developed
by Ager et al. (1972) will be used.
SYSTEMATIC DESCRIPTION
Order R H Y N C H O N E L L I D A Kuhn, 1949 Superümüy
RHmCHONELLACEA Gray, 1848
Family Septirhynchiidae Muir-Wood & Cooper, 1951
The status of this family was endorsed in the "Treatise" (Ager
1965) and in Ager et al. (1972), later on, however, Mancenido &
Walley (1979) lowered its rank to the subfamily level. The main
reason for doing this was the monotypic character of the taxon. By
the addition of Lessiniella, this reason vanishes and the status of
the group can be restored with special reference to the strange,
pentameroid shape of the taxa included.
Genus Lessiniella gen. n.
Derivatiö nominis: after the name of Lessini Mts. (Southern
Alps, Italy). Type species: L . benettii sp. n. Diagnosis: Large
sized rhynchonellids, valves strongly convex, gibbose, urn-
bones massive, beaks strongly incurved. Dorsal fold and ventral
sulcus weak, anterior commissure uniplicate to parasulcate.
Planareas well-developed. Dental plates and pedicle collar are
present. Cardinal process is formed by a blade-like extension of
the dorsal umbo. Septalium slightly overturned. Dorsal median
septum well-developed, long. Crural bases emerge dorsally, crura
radulifer.
Discussion: Lessiniella was included in the family
Septirhynchiidae after some hesitation. At first sight it seems to
be very different from Septirhynchia because of the lack of
ribbing. However the giant size and the gibbose, pentameroid
appearance are more important common external features. The
internal morphology of the two genera matches rather well;
difference is only in the ventral umbo, namely the absence of
median septum in Lessiniella. The genus Heteromychus Cooper (Cooper
1989, p. 41) seems to be transitional from this point of view. It
is
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ribbed and so similar to Septirhynchia that they are regarded as
members of the same subfamily (M. Mancenido, pers. comm.) but does
not possess ventral median septum; a feature shared with
Lessiniella.
The phylogenetic relationships (ancestry) of Lessiniella are not
clear (just as those of Septirhynchia) but the external resemblance
to some Early Paleozoic pen-tameroid genera is astonishingly
strong. As a matter of special interest, it may be mentioned that
the Silurian genera Harpidium and Lissocoelina show so close
ex-ternal similarity to the new genus as i f their photographs
shown in the Treatise (Amsden & Biernat 1965, figs. 416/5a, 6)
were made of the type specimens of Les-siniella.
Distribution: So far, Lessiniella is known only from the
Callovian of the Southern Alps (Italy).
Holotype: Museo dei Fossili della Lessinia (Velo Veronese,
Verona, Italy), In-ventory number: CC 100 Cv-B (Plate I . , Fig.
la-e). - Locus typicus: Covolo di Cam-posilvano (Lessini Mts.,
Southern Alps, Italy). - Stratum typicum: Callovian, red, nodular
linestone.
Derivatio nominis: after the name of Cavalière At t i l io
Benetti, who collected the type specimens.
Diagnosis: Large sized Lessiniella with no ribbing, poorly
developed dorsal fold and ventral sulcus, anterior commissure
parasulcate.
Material: 5 specimens, partly incomplete.
Dimensions (mm): length width thickness
*-Approximate
Description - External characters: This is a large Lessiniella
and tends to be giant sized in late adult or gerontic stage. The
general constitution is robust, gib-bose, with massive, incurved
beak. The outline is drop-shaped; the lateral margins are slightly
divergent, almost straight, the anterior margin forms a
half-circle. Both valves are strongly convex but the maximum
convexity is nearer to the ante-rior margin in the brachial valve,
whereas it is at the posterior one-third in the pe-dicle valve. The
degree of convexity seems to be constant or slightly decreasing
du-ring the ontogeny. The umbo is very massive, the beaks are
strongly incurved (even the concealed beak of the brachial valve,
as revealed by the serial sections). The character of the pedicle
opening and the delthyrium can not be seen exter-nally but the
cross-sectioned paratype showed a circular, hypothyridid foramen of
about 2 mm in diameter, facing posteriorly. The blunt beak ridges
are long and run to the antero-lateral extremities. The poorly
delimited planareas are rather deep. The lateral commissures are
sinuous; they run on the deepest line of the planareas; at
mid-length they bend dorsally, then become straight. At the
antero-lateral extremities the lateral commissures deflect
ventrally and immediately after-wards they pass into the dorsal
uniplication of the anterior commissure. In spite
Lessiniella benettii sp. n. (Plate I . , Figs. 1-3)
holotype sectioned paratype juvenile paratype
48* 34* 24
38* 27 18
36 25 19
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Text-fig. 1 - Serial transverse sections of Lessiniella benettii
(paratype, inv. no. CC 99 Cv-B, Museo dei Fossili della Lessinia,
Velo Veronese, Verona, Italy). Original length about 34 mm.
Distance from ventral umbo given in mm. The median septum
disappeared at 12.7 mm, the crura disappeared at 16.7 mm
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of its basically uniplicate character, the anterior commissure
can rather be termed parasulcate. This anterior commissure
corresponds to a dorsal fold and a gentle ventral sulcus The shells
are almost completely smooth, except the growth lines and a few,
irregular, longitudinal wrinkles on the planareas. Two longitudinal
ridges on the ventral valve and corresponding furrows on the dorsal
valve run from the umbo to the lateral deflections of the anterior
commissure.
Internal characters (Text-fig. 1) - Pedicle valve: The
delthyrial cavity is trapezoidal in cross-section, showing a narrow
ventral groove; the umbonal cavities are relatively small and
drop-shaped in cross section. The thin dental plates are divergent
ventrally and are reinforced with callous secondary shell material
in the incurved part of the umbo; closer to the plane of dentition
they become subparallel and loose contact with the floor of the
valve. The well-developed pedicle collar is about 5 mm long and 1
mm across, and follows the curvature of the beak, suggesting the
presence of an active pedicle, at least in the early adult stage
(the sectioned specimen was 34 mm in length). The whole umbonal
part is characterized by strong, irregular secondary thickening
with callus. The deltidial plates are fused, forming a symphytium.
The hinge teeth are massive, expand dorsally, partly crenulated.
Denticula are present. - Brachial valve: The incurved beak
continues in a slightly crenulated blade-like structure which may
be termed as cardinal process. The fused inner hinge plates form a
septalium which can be seen in a gently overturned position in the
cross sections, due to the curvature of the beak. The septalium is
supported by a well-developed, long median septum. The outer hinge
plates are massive, steeply dipping outwards. The inner surface of
the hinge sockets is crenulated. The crural bases emerge dorsally
and give rise to crura of radulifer type persisting to the
half-length of the valve. Adductor muscle impressions have not been
observed.
Remarks: This species stands alone within the genus Lessiniella
and there are no other Jurassic brachiopod species to be compared
with it.
Distribution: So far Lessiniella benettii is known only from the
Callovian of the Lessini Mts. (Southern Alps, Italy).
NOTES ON T H E PALEOBIOGEOGRAPHY A N D PHYLOGENY OF T H E LATE M
I D D L E JURASSIC G I A N T RHYNCHONELLIDS
Jurassic rhynchonellids are generally not very large, therefore
the almost synchronous appearance of the giant Septirhynchia and
Lessiniella in the Callovian is somewhat surprising. A third group
of very large rhynchonellids, the group of "Rhynchonella" decorata
(Schlotheim) appeared a little earlier (Late Bathonian). This
species was placed into the officially invalid genus "Isjuminella"
by Drot & Fischer (1966) and later to Burmirhynchia by Dardeau
& Laurin (1982). A better attempt was made by Taddei Ruggiero
& Ungaro (1983) who erected a new genus (Sardorhynchia) for
similar, giant rhynchonellids from the Bathonian-Callovian of
Sardinia and attributed, though not explicitly, "Rhynchonella"
decorata to Sardorhynchia.
Here we may speak about a late Middle Jurassic episode of
gigantism among the rhynchonellids. May the synchroneity involve a
common cause ?
Gigantism can be due to different environmental factors (optimum
circumstances, especially food-supply, isolation, deep-sea
environment) and to special phylogenetic relationships, therefore
it seems to be almost hopeless to find the
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appropriate cause. It is important to note that in our case, the
three groups of giant forms occur in different faunal provinces:
"R." decorata and Sardorhynchia in the NW European, Septirhynchia
and Heteromychus in the Ethiopian (or Abyssinian), Lessiniella in
the Mediterranean province (Text-fig. 2).
The success of "R." decorata may be interpreted as an excellent
adaptation to extremely shallow water carbonate environment (see
Dardeau & Laurin 1982) and this holds true for Sardorhynchia
too (Taddei Ruggiero & Ungaro 1983, p. 242). Septirhynchia was
also found in micrites, almost invariably with umbones downward;
therefore a special, semi-infaunal, unattached mode of life was
suggested to explain the rapid invasion of the the African
epicontinental seas (Mancenido & Walley 1979). A similar,
semi-infaunal habit may be supposed in the case of Lessiniella.
Though in the early adult stage it definitely had an active
pedicle, the strong, gryphaeoid beak of the biggest specimen might
have served as an anchor in the lime mud and the pedicle might have
been atrophied. However, in contrast to the very shallow,
epicontinental settings favourized by the group of decorata and
Septirhynchia, Lessiniella was found in pelagic micrites (Rosso
Ammonitico) where the deep water environment is evidenced by the
very poor benthos (only brachiopods) and the dense and very diverse
ammonoid population.
Text-fig. 2 - Middle Jurassic paleogeographic map showing the
distribution of three groups of giant rhynchonellids. Base map
compiled from Vörös (1984,1992). Distribution of "R." decorata
after Drot & Fischer (1966), including Sardorhynchia after
Taddei Ruggiero &
Ungaro (1983), Septirhynchia after Manceflido & Walley
(1979). Stippled: land, horizontally ruled: shallow sea,
cross-ruled: deep sea, blank: ocean
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The phylogenetic relationships of the mentioned taxa are also
different. "R." decorata and Sardorhynchia are typical
representatives of the Tetrarhynchiinae subfamily (Ager et al.
1972, Taddei Ruggiero & Ungaro 1983). Septirhynchia has its own
family or at least subfamily and is a rootless enigma (Mancenido
& Walley 1979), showing closer relationship to the Dimerellidae
than to the "true" rhynchonellid families (Ager et al. 1972). The
phylogenetic position of Lessiniella is even less clear but the
basic external and internal similarities seem to link it to the
family Septirhynchiidae. Ager et al. (1972) speculated on the
ancestry of this family and suggested the Stenoscismatacea as a
possible Late Paleozoic parent group. The internal features of the
two taxa are, however, very different, therefore a true
phylogenetic relationship seems to be highly improbable. Similar is
the case with Lessiniella: the Paleozoic forms with almost
identical exterior, mentioned in this paper, are totally different
internally and belong to another order (Pentamerida). Therefore,
Septirhynchia and Lessiniella are probably not "Lazarus taxa" but
can be regarded as cases of heterochronous homoeomorphy.
In conclusion, the assemblage of the giant, late Middle Jurassic
rhynchonellids is not homogeneous. "R." decorata and Sardorhynchia
are distinct from the septirhynchiids both in distribution and in
phylogeny and their seemingly coeval appearance might be merely
incidental. The other two giants (Septirhynchia and Lessiniella)
are apparently closer in morphology and in phylogeny, possibly
belonging to the same stem and may represent two offshoots adapted
to very different environments. The strong Paleozoic reminiscences
they show, recall the array of archaic features found in the
Jurassic brachiopods of the Mediterranean province (Ager et al.
1972, Vörös 1977, 1984). This Paleozoic "flavour" seems to be
connected to the Tethys, the "lost Eden" of brachiopods. As it was
suggested elsewhere (Vörös 1993) the western end of the oceanic
Tethys was an asylum and an evolutionary centre for the brachiopods
in the first half of the Mesozoic. It was favourable for the
maintenance or recurrence of archaic morphological features. In the
late Middle Jurassic, by the opening of the "Hesperian Strait"
between the Tethys and the Central Atlantic, the system of the
oceanic circulation in the Tethys fundamentally changed. This was
probably the last moment for the true Tethyan brachiopods to invade
newly opened areas and niches; Septirhynchia and, with less
success, Lessiniella took the advantage of this opportunity.
ACKNOWLEDGEMENTS
Thanks are due to Mr. At t i l io BENETTI (Camposilvano, Italy)
for loaning the original specimens of Lessiniella for study. Dr.
Miguel MANCENIDO (La Plata, Argentina) kindly revised and commented
a first version of this paper; his valuable help is acknowledged
herein.
REFERENCES
AGER . D. V (1965): Mesozoic and Cenozoic Rhynchonellacea. In:
Moore, R. C. (Ed.): Treatise on Invertebrate Paleontology, Part H :
Brachiopoda, University of Kansas Press: H597-H625.
A G E R , D. V , CHILDS, A . & PEARSON, D. A . B. (1972):
The evolution of the Mesozoic Rhynchonellida. - Géobios, 5 (2-3):
157-233.
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AMSDEN , T. W. & BIERNAT , G. (1965): Pentamerida. In:
Moore, R . C. (Ed.): Treatise on Invertebrate Paleontology, Part H
: Brachiopoda, University of Kansas Press: H523-H552.
C L A R I , P. A., MARINI, P., PasTORiNi, M . & PAVIA, G.
(1984): II Rosso Ammonitico Inferiore (Baiociano-Calloviano) nei
Mont i Lessini settentrionali (Verona). -Riv. It. Paleont. Strat.,
90 (1): 15-86.
COOPER , G. A . (1989): Jurassic brachiopods of Saudi Arabia. -
Smiths. Contribs. Paleobiol., 65:1-213.
DARDEAU , G. & LAURIN , B. (1982): Les rhynchonelles du
Bathonien du domaine Provencal: liaison entre l'installation des
peuplements et les modalités de la transgression Bathoniennedans
les Alpes-Marittimes. - Géobios, 15 (4): 469-489.
DROT , J. & FISCHER , J.-C (1966): Nouvelles observations
sur "Rhynchonella" decorata (Schlotheim), Brachiopode bathonien. -
Annls Soc. géol. Nord, 86:53-63.
MANCENIDO , M . O . & W A L L E Y , C. D . (1979):
Functional morphology and ontogenetic variation in the Callovian
brachiopod Septirhynchia from Tunisia. -Palaeontology, 22 (2):
317-337.-
TADDEI R U G G I E R O , E. & UNGARO , T. (1983):
Sardorhynchia crassa gen. nov., sp. nov. (Brachiopoda), from
Jurassic of Sardinia. - Boll. Soc. Paleont. Ital., 22 (3):
225-246.
VÖRÖS , A . (1977): Provinciality of the Mediterranean Lower
Jurassic brachiopod fauna: causes and plate tectonic implications.
Palaeogeogr., Palaeoclimatol., Palaeoecol.,21(l): 1-16.
VÖRÖS , A. (1983): Some new genera of Brachiopoda from the
Mediterranean Jurassic. - Annls hist.-nat. Mus. natn. hung., 75:
5-25.
VÖRÖS , A. (1984): Lower and Middle Jurassic brachiopod
provinces in the Western Tethys. - Ann. Univ. Sei. Budapest., Sec.
Geol., 24(1982): 207-233.
VÖRÖS , A . (1993): Jurassic microplate movements and brachiopod
migrations in the western part of the Tethys. - Palaeogeogr.,
Palaeoclimatol., Palaeoecol., 100: 125-145.
Author's address: Dr. Att i la VÖRÖS
Geological and Paleontological Department Hungarian Natural
History Museum H-1088 Budapest Múzeum körút 14-16. Hungary
EXPLANATION OF PLATE I.
Lessiniella benettii gen. et sp. n. (Callovian, Covolo di
Camposilvano, Lessini Mts., Southern Alps, Italy). Collected by A
BENETTI , deposited in the "Museo dei Fossili della Lessinia" (Velo
Veronese, Verona, Italy). Photographs by A K E V E (Budapest).
la-e: Holotype, inv. no. CC 100 Cv-B, X 1 2a-b: Paratype
(sectioned), inv. no. CC 99 Cv-B, X 1 3a-e: Paratype (juvenile),
inv. no. CC 101 Cv-B, X 1
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PLATE I .