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)

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

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

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

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

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

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.

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

PLATE I .