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The Shoulder Girdle and Anterior Limb of Drepanosaurus Unguicaudatus

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The Shoulder Girdle and Anterior Limb of Drepanosaurus Unguicaudatus <oological Journal of the Linnean Socieg (1994), Ill: 247-264. With 12 figures The shoulder girdle and anterior limb of Drepanosaurus unguicaudatus (Reptilia, Neodiapsida) from the upper Triassic (Norian) Downloaded from https://academic.oup.com/zoolinnean/article/111/3/247/2691415 by guest on 27 September 2021 of Northern Italy SILVIO RENESTO Dipartimento di Scienze della Terra, Universita degli Studi, Via Mangiagalli 34, I-20133 Milano, Italy Received January 1993, accepted for publication February 1994 A reinvestigation of the osteology of the holotype of Drepanosaurus unguicaudatus Pinna, 1980 suggests that in earlier descriptions some osteological features were misinterpreted, owing to the crushing of the bones and because taphonomic aspects were not considered. The pattern of the shoulder girdle and fore-limb was misunderstood: the supposed interclavicle is in fact the right scapula, and the bones previously identified as coracoid and scapula belong to the anterior limb. The new reconstruction of the shoulder girdle, along with the morphology of the phalanges and caudal vertebrae, leads to a new hypothesis about the mode oflife of this reptile. Drepanosaurus was probably an arboreal reptile which used its enormous claws to scrape the bark from trees, perhaps in search of insects, just as the modern pigmy anteater (Cyclopes) does. Available diagnostic characters place Drepanosaurus within the Neodiapsida Benton, but it is impossible to ascribe this genus to one or other of the two major neodiapsid lineages, the Archosauromorpha and the Lepidosauromorpha. ADDITIONAL KEY WORDS:-Functional morphology - taxonomy - taphonomy - palaeoecology . CONTENTS Introduction ................... 247 Taphonomy ..... .............. 249 Systematic palaeontology . .............. 251 Genus Drepanosaurus Pinna, 1980 .............. 252 Drepannsaurus unguicaudatus Pinna, 1980. ............ 253 Morphological description . .............. 253 Functional morphology . .............. 256 Taxonomic note .................. 260 Acknowledgements ................. 262 References ..... .............. 262 INTRODUCTION The Norian vertebrate fauna collected from the Calcare di Zorzino and .rgilliti di Riva di Solto in northern Italy is of great interest: more than 40 enera of fishes (Tintori, personal communication) and 10 genera of reptiles ave been collected so far. The main fossiliferous unit, the Calcare di Zorzino, as deposited in intraplatform basins surrounded by the huge Dolomia 247 124-4082/94/070247 + 18 $08.00/0 0 1994 The Linnean Society of London 248 S. RENESI'O Downloaded from https://academic.oup.com/zoolinnean/article/111/3/247/2691415 by guest on 27 September 2021 Figure I. Drepanosaurus unguzcaudatus l'inna, Bergamo Museum of Natural History 5287, drawing of the holotypr, as preserved. Endenna, Norian. Scale bar = 2 cm. Principale carbonate platform. These basins were deep, several kilometres wide and their centres were anoxic (Jadoul, Berra & Frisia, 1992; Tintori, 1992). Despite the depositional environment, terrestrial reptiles are more common than marine ones (Wild, 1978, 1991; Pinna, 1980, 1984, 1986; Renesto, 1994), which are represented only by the placodont Psephoderma (Pinna, 1979; Pinna & Nosotti, 1989) and the thalattosaur Endennasaurus (Renesto, 1984, 1992). Among non-marine reptiles, fragments of the armour of Aetosaurus (Wild, 1991) and a phytosaur skull ascribed to the genus Myslriosuchus (Pinna, 1987a) have been found, but the most interesting discoveries are the oldest pterosaurs THE SHOULDER GIRDLE OF DREPANOSAURUS 249 (Wild, 1978, 1984), along with some genera unknown from other localities, such as the small archosauromorph Megalancosaurus (Calzavara, Muscio & Wild, 1980; Renesto, 1994) and the enigmatic Drepanosaurus unguicaudatus Pinna 1980, the subject of this paper. This latter (Fig. 1) is approximately half a metre long, with a bulky trunk, short, stout anterior limbs, and longer, more slender posterior ones. The tail was moderately long, strong, ventrally deep and laterally compressed. The main peculiarities of its skeleton are the enormous claw of the second digit of the manus and the hooked spine (a modified vertebra) at the end of the tail. Unfortunately the head and the neck of this reptile are unknown. The Downloaded from https://academic.oup.com/zoolinnean/article/111/3/247/2691415 by guest on 27 September 2021 genus Drepanosaurus was erected by Pinna ( 1980) on the basis of three specimens (the holotype and two other, smaller, specimens), all collected in the Zorzino Limestone from the same locality near Zogno, in the Bergamo Prealps (Lombardy, northern Italy). More recently, Pinna (1987b) ascribed to the same genus another small specimen collected in the Dolomia di Forni, a formation that is approximately coeval with the Calcare di Zorzino (Tintori, Muscio & Nardon, 1985), in a locality near Udine (Friuli, northern Italy). Many differences are present between the small specimens and the larger one, but they were considered by Pinna (1980, 1987b) to be due to different growth stages or poor preservation. The finding of another better preserved ‘juvenile’ from the Zorzino Limestone clearly demonstrates that the small specimens are not immature individuals of Drepanosaurus, but actually belong to Megalancosaurus (Renesto, 1994), so the former genus is represented by the holotype alone. An extensive description of the anatomy of Drepanosaurus unguicaudatus was published by Pinna (1984), and a summary was later published by the same author (Pinna, 1986). In the earlier description, the pattern of shoulder girdle and the anterior limb were probably misidentified. In the present paper these regions are redescribed; the diagnosis for the genus is amended; and new hypotheses about the mode of life and the taxonomic assessment of Drepanosaurus unguicaudatus are suggested. TAPHONOMY Pinna’s ( 1984) reconstruction of the shoulder girdle is erroneous because the crushing of the bones renders them very difficult to identify, and because their postmortem orientation has been misinterpreted. Pinna (1980, 1984, 1986) suggested that the shoulder girdle and part of both the forelimbs were disarticulated and rotated relative to the trunk in the same way (Figs 2, 3); thus the two halves of the girdle were separated from each other, exposed on the medial surface and tilted upside down. The coracoids were displaced dorsal to the scapulae and the tip of the long posterior stem of the supposed interclavicle reached the expanded neural spine of the third dorsal vertebra (Fig. 4A). However according to Pinna (1980, 1984), despite the suggested disarticulation of the shoulder girdle, the humerus did not lose its anatomical connection with the ‘shoulder girdle’, but rotated relative to the radius, the ulna and the manus. The latter bones apparently did not suffer any dislocation relative to one another. Even the phalanges, which are generally among the first bones to be disarticulated, remained in anatomical connection, as did all other preserved portions of the skeleton. It is also rather strange that the two halves of the 250 S. RENESTO Downloaded from https://academic.oup.com/zoolinnean/article/111/3/247/2691415 by guest on 27 September 2021 Figure 2. Drepanosaurus unguicaudalur Pinna, the anterior region showing the shoulder girdle and the anterior limbs. Scale bar = I cm. shoulder girdle were twisted in exactly the same way (both carrying with them the humeri), with the coracoids close to the proximal ends of the two autopodia. It has to be noted that practically no other vertebrate (from thousands of specimens) from the same locality has been found disarticulated (Pinna, 1986, Pinna & Nosotti, 1989; Renesto, 1984, 1992, 1994; Tintori, 1981, 1992; Tintori, Muscio & Nardon, 1985; Tintori & Sassi, 1991; Wild, 1978; Zambelli, 1975); some are incomplete, but the preserved portion nearly always retains the bones in anatomical connection. The most convincing argument against the reconstruction by Pinna (1984), however, is the nature of the supposed scapula and interclavicle. The supposed scapula shows a clearly hollow shaft, and the bone identified as the interclavicle THE SHOULDER GIRDLE OF DREPANOSAURUS 25 1 Downloaded from https://academic.oup.com/zoolinnean/article/111/3/247/2691415 by guest on 27 September 2021 Figure 3. Dreponosaurus unguicaudulus Pinna, the shoulder girdle and the anterior limbs (stippled). Scale bar = 1 cm. by Pinna (1984) is not symmetrical, nor is it unpaired. The connection between the ‘posterior stem’ and the anterior enlarged portion is sharply bent anteriorly with no traces of fractures, and a second ‘stem’ is clearly visible on the left side of the trunk of the animal (Figs 2, 3, 4B). These bones are indeed elongate and narrow scapular blades, and the supposed interclavicle is actually the right scapula, while the bone identified by Pinna (1984, 1986) as the scapula is the shaft of the radius. The bone described by Pinna (1984, 1986) as a clavicle is a portion of the coracoid, this last being covered by the right humerus, by the dorsal margin of the great claw and by the anterior dorsal, and fragments of the (?) cervical, vertebrae. The identification of the flat bone described by Pinna as the coracoid remains problematical; it might be an enormous process of the radius, or alternatively, a greatly expanded and flattened ulna. SYSTEMATIC PALAEONTOLOGY Class Reptilia Superdivision Neodiapsida, Benton, 1985 Family Drepanosauridae, Olson & Sues, 1986 Olsen & Sues (1986) erected the family Drepanosauridae
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