The and anterior limb of unguicaudatus (Reptilia, ) from the upper () 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 . 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 and the .

ADDITIONAL KEY WORDS:-Functional morphology - - 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 fauna collected from the 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 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 (Pinna, 1979; Pinna & Nosotti, 1989) and the thalattosaur (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 (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 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 (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 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 containing the single genus Drepanosaurus, mainly for statistical purposes. No family diagnosis was 252 S. RENESTO

CO U

U

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B

Figure 4. Interpretations of the pattern of the shoulder girdle and anterior limb of Drepanosaurus: A, according to Pinna, 1984; B, proposed interpretation. Abbreviations: cl, clavicle; co, coracoid; h, humerus; icl, interclavicle, r = radius, sc = scapula, u = ulna, ul = ulnare, i = intermedium. given. Carroll (1988) retained the family Drepanosauridae and classified it as Diapsida incertae sedis, while Evans ( 1988) considered Drepanosaurus as an isolated genus of uncertain affinities. More recently Berman & Reisz (1992) included Dolabrosaurus (a small reptile collected in the Chinle Formation of New Mexico) in the Drepanosauridae, and provided a diagnosis for the family. The skeleton of Dolabrosaurus is very incomplete and some important differences can be recognized. Judging from the illustrations of the paper by Berman & Reisz (1992), however, the morphology of the caudal vertebrae is strikingly similar to that of Drepanosaurus, supporting the hypothesis that the two genera may be related. The family Drepanosauridae is tentatively retained here and the generic diagnosis is given for Drepanosaurus, amending that of Pinna (1980).

Genus Drepanosaurus Pinna, 1980 Diagnosis. Small reptile (the only known specimen being 40 cm in length, excluding the head and the neck); vertebral column consisting of 25 dorsal, 2 sacral and 37 amphicoelous caudal vertebrae; centra not notochordal and moderately elongate, with high narrow spines and short transverse processes; the neural spines of the first five dorsal vertebrae expanded distally, and forming a functional unit; pre- and postzygapophyses straight and horizontally directed; caudal vertebrae somewhat longer than the more posterior dorsal and sacral ones, bearing tall, narrow neural spines slightly expanded distally and deep, narrow, anteriorly bent haemapophyses; prezygapophyses of the caudal vertebrae long, sharp, with vertical, laterally enveloping articular facets, postzygapohphyses rounded and somewhat upwardly bent; haemapophyses of the anterior portion of the tail attached to the posterior rim of the ventral margin of the centrum, haemapophyses of the posterior region of the tail attached to the anterior rim of the ventral margin of the centrum; last caudal vertebra modified to a sharp, hooked spine; shoulder girdle consisting of a very tall and narrow scapula and a small rounded coracoid; pelvic girdle consisting of THE SHOULDER GIRDLE OF DREPANOSAURUS 253 a tall, wide ilium, gently bent cranially, and a large puboischiadic plate; the thyroid fenestra does not reach the symphysis; humerus about 707" the length of the femur, hollow; radius short and stout, with a hollow shaft; presence of a wide, flat bone contacting the proximal head of the radius, possibly an expanded ?ulna; two stout and elongated ?carpal bones; phalangeal formula for the manus, (?2), 2, 2, 2, 2; metacarpals and phalanges long, strong; the last phalanx bearing sharp hooked claws; claw of the second digit very long and wide, as high as the shaft of the radius; femur slender, hollow, the distal articular areas for the tibia and the fibula convex and divided by a distinct embayment; tibia and fibula Downloaded from https://academic.oup.com/zoolinnean/article/111/3/247/2691415 by guest on 27 September 2021 straight and enclosing a well developed spatium interosseum; tarsus consisting of an L-shaped astragalus fused to a wide calcaneum, this latter with a well developed lateral process; presence of a rounded foramen; a wide centrale and five distal tarsals present, the fourth being the largest and contacting both the centrale and the astragalo-calcaneum; metatarsals stout, the first being the widest, the fifth metatarsal not hooked; phalangeal formula for the pes 2, 3, 3, 3, 3; phalanges long, strong and bearing long, sharp and hooked claws.

Drepanosaurus unguicaudatus Pinna, 1980 (Figs 1, 2) Drepanosaurus unguicaudatus Pinna, 1980 pp. 181-192, fig. 1, (figs 2, 3 = Megalancosaurus preonensis Calzavara, Muscio & Wild, 1980) Drepanosaurus unguicaudatus Pinna, 1984, pp. 8-28, figs 1-12 Drepanosaurus unguicaudatus Pinna, 1986, pp. 1 127-1 132 Diagnosis. The same as for the genus, of which it represents the only known species. HOLOTYPE. Number 5728 of the catalogue of the Museo Civic0 di Storia Naturale 'E. Caffi' Bergamo, Italy.

MORPHOLOGICAL DESCRIPTION As specified in the introduction, those osteological characters involved in the new reconstruction of the shoulder girdle are redescribed here.

Shoulder girdle The shoulder girdle of Drepanosaurus consists of a tall, narrow scapula and of a rather short, rounded coracoid (Figs 2, 3, 4B, 5B). The scapula shows an expanded ventral portion bearing a very high strap-like scapular blade somewhat anteriorly directed, resembling slightly that of Chamaeleo. The outline of the coracoid cannot be reconstructed with confidence, since it is covered by the humerus, some ribs and some fragments of (?)cervical vertebrae.

Anterior limb The humeri are well developed. They are hollow and their walls have collapsed and are extensively fractured, obscuring many details (the supposed epiphyses for the articulation of the radius and ulna are probably fragments of the head of these bones), so that little can be said about their structure. The entepicondylar foramen was probably absent, while an expansion, just proximal 254 S. RENESTO Downloaded from https://academic.oup.com/zoolinnean/article/111/3/247/2691415 by guest on 27 September 2021

A B

Figure 5. Reconstructions of the shoulder girdle of Drcpanosaurus unguicaudntus:A, according to Pinna, 1984; B, proposed reconstruction. Abbreviations as For Fig. 4.

to the distal head of the bone, can be recognized in the left humerus; this may tentatively be considered to be a stout supinator process. The bones of the fore-arm are crushed and compressed, making their identification difficult; the following descriptions must be considered as tentative. The radius is short; both ends are expanded, especially the proximally one, and the shaft is hollow. The proximal articular surface is slightly concave; the distal head cannot be observed, being covered by metacarpals and other bones of the hand in both the right and left limb. The identification of the bones considered by Pinna (1980, 1984) as the coracoids raises problems. They are flat bones, with a beam-like outline, somewhat thickened at both extremities. Traces of the insertion of muscles can be seen along the convex margin. On both the right and left anterior limb, this bone contacts the proximal head of the radius forming a concave articular area that houses the distal head of the humerus. Distally it meets the bones considered by Pinna (1980, 1984) as the radius and ulna. It can be suggested that the supposed ‘coracoid’ is an enormous process of the radius. Its presence could be at least in part explained by the development of stout supinator extensor muscles for manoeuvring the huge claw of the second digit. This process mught have been crushed and separated from the radius during fossilization. If this hypothesis is the correct one, the bones described by Pinna (1980, 1984) as the ‘radius’ and ‘ulna’ are in fact the single ulna, which bears a stout lateral process (considered as a separate element by Pinna, 1980, 1984) that ends in a narrow but tall olecranon. The presence of the latter structure implies the development of a strong muscle triceps, required to extend the forearm. The humerus is much more slender, relative to the radius and ulna, than one might expect. In addition, this reconstruction does not explain why the ulnae are both separated from the humerus and lie in the same position relative to the large, flaring process of the radius. Perhaps this process for the radius displaced the ulnae from their articulation with the humerus, but if this were the THE SHOULDER GIRDLE OF DREPANOSAURUS 255 n Downloaded from https://academic.oup.com/zoolinnean/article/111/3/247/2691415 by guest on 27 September 2021

U

A

Figure 6. Reconstruction of the anterior limb of Drcpanosaurus unguicaudatus Pinna: A proposed reconstruction; B, according to Pinna, 1984. case pronation and supination of the hand would have been impossible, and a well developed olecranon would be unnecessary. An alternative hypothesis can be made: the flat bone might be an expanded ulna. Thus the bones previously identified as the ‘radius’ and ‘ulna’ might be elongated carpal bones ?ulnare and ?intermedium. The problems are even greater with this hypothesis: the structure of the radio-ulnar joint resembles a suture, so pronation and supination of the hand would have been impossible, unless the elongate carpal bones provided compensatory movements. In addition it is extremely difficult to hypothesize which modifications occurred in the musculature to accommodate this functional requirement. The first hypothesis seems the more reliable, but there is some evidence that the bones identified by Pinna (1980, 1984) as the radius and ulna are indeed separate elements and that these latter do not contact the humerus. For this reason the second hypothesis is preferred here (Figs 4A, 6A), despite the functional problems it raises. On the other hand, the reconstruction proposed by Pinna (1984, 1986), shown here as Figure 6B renders the autopodium too slender for manoeuvring the deep claw. Only further, better preserved findings can reveal whether this new hypothesis about the forearm is also incorrect and, allow more reliable reconstructions of the anterior limb of Drepanosaurus. A small, rounded bone (a distal carpal?) can be observed near the proximal md of the fifth metacarpal of the left hand. Both the metacarpals are narrow, slender bones with expanded distal ends; the phalanges are elongated and stout and bear processes for the insertion of powerful flexor muscles. According to Pinna (1984) the phalangeal formula for the manus should be 3, 2, 2, 2, 2. The first digit, however, is partially covered by the large claw of the second one and the distal head of the supposed first phalanx is very similar to that of other 256 S. RENESTO metacarpals, so that a more probable formula is: (?2), 2, 2, 2, 2. Each digit ends with a long, sharp, hooked claw; that of the second digit is greatly expanded dorsally (its height equals that of the shaft of the ulna) and laterally compressed.

PUNCXIONAL MORPHOLOGY Despite the highly specialized characteristics shown by the skeleton, it is not easy to infer the mode of life of Drepanosaurus. Pinna (1980), first considered this reptile to be a fast running predator, then later (Pinna, 1984, 1986) concluded Downloaded from https://academic.oup.com/zoolinnean/article/111/3/247/2691415 by guest on 27 September 2021 that the wide claws were digging tools and consequently interpreted all other morphological peculiarities as adaptations for a fossorial life. Even the hooked spine at the tip of the tail, according to this fossorial hypothesis, was useful to prevent the animal being pulled forwards during digging. Usually, however, digging prop with the tail, applying the weight of the entire body on their digging structures (Hildebrand, 1974, 1985). True fossorial animals, like the moles, show modifications of the whole body (Hildebrand, 1974, 1985; Lessertisseur & Saban, 1967), which are not present in Drepanosaurus. Other digging animals, like pangolins, anteaters and armadillos, do not lead a true fossorial life, but loosen soil or termite nests in search of food (the hook and pull diggers, sensu Hildebrand, 1974, 1985). These latter show modifications mainly in the girdles and in the anterior limbs, often showing a great development of one or more claws. There are, however, many diff'erences between Drepanosaurus and these diggers. According to Hildebrand ( 1974, 1985) and Lessertisseur & Saban (1967), expert diggers diggers show stout skeletons, with strong, heavy rough bones to support considerable forces; the carpus is short and the metacarpals and phalanges, excluding the ungual ones, are also short and broad (Fig, 7A-C), often with bony stops to prevent hyperextension; finally the claws are not only deep but also thick and heavy, and frequently the more lateral digits are reduced or even absent. The skeleton of Drepanosaurus is very lightly built, with hollow bones; the carpus is only partially known, but metacarpals, phalanges and claws are long relative to the radius, and highly mobile (Fig. 7D). No significant reduction took place in any digit; the claw of the second digit is deep, but rather thin and probably could not have been used for loosening soil or other hard materials without damage. The structure of the distal portions of the anterior limb is very odd and it is difficult to reconstruct its pattern and, consequently, to give a functional interpretation. The structure may possibly be related to the development of flexor and extensor muscles of the manus, particularly for manoeuvring the huge claw of the second expanded digit, and may provide clues about the mode of life of this reptile. Other skeletal features, however, help to suggest an alternative to the 'digger' hypothesis. The distal phalanges of the hands and feet of Drepanosaurus are longer than the proximal ones: the preungual phalanx is very long and its distal end has the shape of a large, rounded bicondylar ginglymus, around which the long hooked and sharp claw pivoted. This latter (excluding the ciaw on the second digit of the manus) shows a ventrally expanded posterior margin that allowed attachment of strong digital flexors (Fig. 8). These characters are considered as arboreal adaptations (Lessertisseur & Saban, 1967; Hildebrand, 1974; Cartmill, 1985) and are present in climbing reptiles, , and . They were considered by Wild (1984), Unwin (1988) and Wellnhofer (1991) as supporting THE SHOULDER GIRDLE OF DREPANOSAURUS 257

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B

Figure 7. A-C: The third metacarpal (stippled) and digit of the manus of three diggers; A, the pangolin Manzs; B, the anteatcr Myrmecophaga; C, the armadillo Priodontes (redrawn) from Hildebrand, 1985); D 8~ E, the second and third digit of the hand of Drepanosaurus; F, thc third digit of the hand of the pterosaur f'eleinosaurus (redrawn from Wild, 1978). Diggers show short and stout phalanges with bony stops and sesamoids, while Drepanosaurus shows more slender, elongated phalanges and sharp claws. Drawings not to scale. the hypothesis of climbing habits in pterosaurs, against the 'terrestrial running' model proposed by Padian ( 1983, 1984). The scapula of Drepanosaurus (Figs 3, 4B, 6B, 9B) resembles that of Zcarosaurus (Colbert, 1970), Chamaeleo (Nauck, 1938; Romer 1956; Peterson, 1971, 1984),

A D

B

F

Figure 8. The third toe of Drepanosaurus (A),of the climbing Colaples (B), of the sloth Choloepus (C), of !he pterosaur Pekinosaurus (D), of the terrestrial rauisuchid Ticinosuchus (E) and of the semiaquatic or fully aquatic prolacertiform Tunystropheus (F). B & C redrawn from Hildebrand, 1974; D, from Wild, 1978; E, from Krebs, 1965, F from Wild, 1973. Drawings not to scale. 258 S. RENESTO Downloaded from https://academic.oup.com/zoolinnean/article/111/3/247/2691415 by guest on 27 September 2021

G H I Figure 9. The left scapulae (external view) of A, Chamaelco (arboreal); B Drepunosaurus; C, Megalancosaurus (arboreal, Calzavara, Muscio & Wild, 1980; Renesto, 1994); D, Icarosaurus (arboreal glider, Colbert, 1970); E, ‘deep tailed swimmer’ (?aquatic, Olsen, 1980); F, (arboreal glider, Sharov, 1971; Haubold & Buffetaut, 1987); G, Sphenodon (terrehal); F TanysiropheuJ (predominantly aquatic, Wild, 1973); Endennasaurus (aquatic, Renesto, 1992). A and G redrawn from Nauck (1938). Drawings not to scale.

Longisquama (Sharov, 197 1; Haubold & Buffetaut, 1987). Megalancosaurus Renesto, 1994), which all exhibit adaptations towards arboreal life (Fig. 9). No opposable digits are present in Drepanosaurus, so this reptile did not grasp tree branches like chameleons, (if pronation and supination of the hand was allowed in some way) kept its feet held in a sprawling position, lateral to the body axis, with the claws clinging to the irregularities and crevices of the substrate. On large branches this climbing system is more secure than grasping digits and allows head-first descent from vertical trunks (Hildebrand, 1974; Cartmill, 1985). Such long claws (Fig. 10) might also be used for hanging from trees (the ‘grimpkurs a griffe’ of Lessertisseur & Saban, 1967) like sloths. Similarly, the sharp claws, the rod-like scapula, the opposable digits in the manus (Calzavara, Muscio & Wild, 1980; Renesto, 1994), the opposable ‘hallux’ in the foot and the prehensile tail of Megalancosaurus are all adaptations toward arboreal life (Renesto, 1994). Judging from the reconstruction published by Olsen (1980) the odd, small reptile from the Newark known informally as the ‘deep-tailed swimmer’ shows both pelvic and shoulder girdles very similar to those of Drepanosaurus and Megalancosaurus; in addition it shows very long haemapophyses on the tail and long slender limbs. It has been considered aquatic (Olsen, 1980), but the elongate slender Iimbs, and the morphology of the girdles, may contradict this interpretation and suggest arboreal adaptations (Hildebrand, 1974, 1985; Peterson, 1971; Romer, 1956). In Chamaeleo the elongated narrow scapula is slightly curled anteriorly, as in Drepanosaurus; this improves girdle rotation to THE SHOULDER GIRDLE OF DREPANOSAURUS 259 Downloaded from https://academic.oup.com/zoolinnean/article/111/3/247/2691415 by guest on 27 September 2021

., : *

Figure 10. Drepanosuurus unguicaudulus Pinna, the feet as preserved in the holotype; the left foot is exposed in plantar view, the right foot in dorsal view; in this latter the claws of the second and third digit are slightly superimposed upon each other and partially broken. Abbreviations: a, astragalus; c, centrale; ca, calcaneum; 1-5, distal tarsals; I-V, metatarsals. Scale bar equals I cm. increase step length (Peterson, 197 I, 1984), replacing the lateral undulation of the body, which would otherwise be a disadvantage when moving on narrow supports. This kind of scapula also allows more complex limb movements than in other (Peterson, 1984). Drepanosaurus probably did not walk along narrow perches, but the great mobility permitted by the strap-like scapula may have been associated with the large claws, As in Megalancosuurus (Renesto, 1994), the morphology of the shoulder girdle may be functionally related to the tall, expanded neural spines of the first dorsal vertebrae, which provided the insertion of those muscles related to limb elevation and girdle rotation, and of the transversospinalis system, which helps to raise the anterior part of the body off the substrate (Jenkins & Goslow, 1983; Rieppel, 1989; Tschanz, 1986). It may be hypothesized that, as in many climbing mammals, Drepanosaurus could have made a sort of tripod with the robust tail and the posterior limbs, keeping the anterior portion of the trunk raised and allowing the forelimb to be free to search food or to bridge gaps when moving from one support to another. Furthermore, the strange tail of Drepanosaurus (Fig. 1 I) might well be considered 2s prehensile. The morphology of the zygapophyses, with vertical, laterally mveloping articular facets, indicated that (excluding its base) lateral movements &ere limited, but good vertical movements, mainly ventral curling, would have 3een possible. In the distal portion of the tail, the shift of the insertion of the iaemapophyses from the posterior end of the ventral margin of the centrum to 260 S. RENESTO Downloaded from https://academic.oup.com/zoolinnean/article/111/3/247/2691415 by guest on 27 September 2021

Figure 11. Dreputlusuurus unguicuudu!us, reconstruction of the skeleton (only the left limbs and ribs are reconstructed, to avoid confusion). the anterior one, and the hooked spine at the tip, may be interpreted as devices for clinging to the substrate, that is, as supplementary hooks. These structures are consistent with climbing habits, in contrast to the hypothesis of aquatic life proposed by Berman and Reisz (1992) for Dolabrosaurus (these authors considered its Drepanosuurus-like tail as the main locomotory organ during swimming). The deep, curved and serrated haemal spines of the distal caudal vertebrae may have housed fibrous ligaments or similar structures to strengthen the hook, improving the function of the spine. The tails of many climbing mammals show curved ends with naked pads (often carrying dermatoglyphs, to improve adhesion) and often very short, strong flexor tendons prevent the hook from being opened (Hildebrand, 1974; Cartmill, 1985). The utility of the large claws for an arboreal animal remains a problem. Unfortunately, the absence of the skull and the difficulty of restoring the anatomy of the forearm do not permit reliable hypothesis on the feeding habits of this reptile. Its size allows it to be considered as either carnivorous or herbivorous (Fraser & Walkden, 1983). However it is difficult to imagine what the function of such large claws in an arboreal plant eater was. The pigmy anteater, Cyclopes, which is quite similar to Drepanosaurus in size, is highly specialized for arboreal life (modified hands, grasping feet, prehensile tail) and it also shows huge claws in the hands (stouter than those of Drepanosuurus). In Cyclopes these stout claws are used to rip the bark of trees in search of insects, which are its exclusive diet (Grassk, 1967; Moeller, 1989). Cyclopes may be used as a model for Drepanosaurus’ habits, particularly since available evidence (size, arboreal adaptations) is consistent with an insectivorous diet for Drepanosaurus. This reptile might have used its wide, sharp, shearing claws for the same purpose of Cyclopes. Plant fragments are known from the same locality that yielded the fossil. Furthermore, insect remains have been found a few metres above the fossiliferous horizon that yielded Drepanosaurus. These fossils comprise the zygopteran Italophlebia and the beetle Holcoptera (Whalley, 1986). Traces made by bark-boring beetles are known from other triassic localities (Boucot, 1990; Jarzembowski, 1990); it is conceivable that such insects were the prey of Drepanosaurus.

TAXONOMIC NOTE The taxonomic assignment of Drepanosaurus is quite difficult since most diagnostic features are unknown or are deeply modified. Pinna (1984)) on the THE SHOULDER GIRDLE OF DREPANOSAURUS 26 1 Downloaded from https://academic.oup.com/zoolinnean/article/111/3/247/2691415 by guest on 27 September 2021 B C

F

Figure 12. Comparison between the tarsi of Drepanosaurtu (A);of the primitive Galesphyrus (B); of the lepidosauromorph reptile Sauroslernon (C); of the prolacertiform Prolosaurur (D); of the eosuchian (E); and the sphenodontid Sphenodon (F). B, C & D from Carroll, 1988; E, from Wild, 1978; F, from Romer, 1956. Areas shaded black: the centrale; stippled, the fourth distal tarsal. Drawings not to scale. basis of the presence of fused astragalo-calcaneum and of its interpretation of the shoulder girdle, classified Drepanosaurus as an eolacertilian. More recently, Carroll (1988) and Evans ( 1988) considered it as Diapsida incertae sedis. Berman & Reisz (1992) considered the Drepanosauridae as belonging to the Lepidosauromorpha, mainly on the basis of the characters described by Pinna (1984) for Drepanosaurus and the presence of notochordal vertebrae in Dolabrosaurus. Following the phylogenetic models for the diapsid reptiles proposed by Benton (1985) and Evans (1988), and the revision by Laurin (1991), Drepanosaurus shares at least four neodiapsid characters: single coracoid, femur slender, distal articular surfaces on femur that level with each other, and femur more than 10% longer than the humerus. Other diagnostic characters present in Drepanosaurus (absence of a cleithrum, possible absence of the entepicondylar foramen, presence of a thyroid fenestra) can be observed in various neodiapsid lineages. In addition, Drepanosaurus shows two archosauromorph synapomorphies (vertebrae not notochordal, presence of lateral tuber on the calcaneum) . The prolacertiform character (elongated neck), quoted by Evans (1988) as present in Drepanosaurus, cannot be confirmed, since the neck is well preserved only in one of the supposed ‘juvenile individuals’ which belong to Megalancosaurus (Renesto, 1994). On the other hand the fused astragalo-calcaneum is typically considered as a synapomorphy of lepidosaurs. The pattern of the tarsus of Drepanosaurus is problematical, however (Figs 10, 12). The astragalo-calcaneum 262 S. RENESTO is rather different from that of lizards and sphenodontids, and a distinct, well developed 'heel' is present, as in many archosauromorph reptiles. In addition, some primitive features can be detected (presence of a wide lateral centrale, of a fifth distal tarsal and a straight fifth metatarsal) that can also be seen in the lepidosauromorph Saurosternon, as well as in some . As already suggested by Evans (1988), the fusion of the two bones of the ankle might not be homologous with that of the lepidosaurs, and may have evolved independently. The tarsus of Dolabrosaurus is poorly known but, undoubtedly, the astragalus and the calcaneum are separate elements (Berman & Reisz, 1992), and according to Downloaded from https://academic.oup.com/zoolinnean/article/111/3/247/2691415 by guest on 27 September 2021 the authors, this may represent the primitive condition for the Drepanosauridae. Scarcity of diagnostic elements prevents giving any reliable cladogram, but Drepanosaurus (and possibly also Dolabrosaurus) can be considered as belonging to the Neodiapsida with some confidence, its taxonomic position lying probably before the archosauromorph-lepidosauromorph dichotomy.

ACKNOWLEDGEMENTS I am deeply indebted to the referees, Dr S. Evans, London, and Dr N. Fraser, Cambridge, not only for their helpful comments but also for their kind suggestions on the style. Drs M. J. Benton, Bristol, and A. Tintori, Milano, read preliminary versions of the manuscript at different stages, their comments and encouraging advice greatly improved this paper. Dr D. B. Norman, Cambridge revised the final version. To all of them, my sincere thanks. I wish also to thank Drs R. Wild, Stuttgart, and D. Unwin, Bristol, for the helpful discussion about the climbing abilities of the pterosaurs. Thanks are also due to Dr A. Paganoni, Bergamo, for permission to examine the specimen, and to Mr M. Pandolfi, Bergamo, for final preparation of the specimen. Field work was financially supported by MPI grant to C. Rossi Ronchetti, Milano.

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