FI.INCTIONAL MORPHOLOGY and MODE of LIFE of the Ij.TE TRIASSIC PIACODONT PSEPHODERMA ALPINUM MEYER from the CALCARE DI ZORZINO (LOMBARDY, N ITALY)

Home , Alaunian, Calcare di Zorzino, Placochelys, Psephoderma, Pseudodalatias

Rivista Italiana di Paleontologia e Stratigrafia volume 101 numero I pagine J7-48 Maggio 7995

FI.INCTIONAL MORPHOLOGY AND MODE OF LIFE OF THE Ij.TE TRIASSIC PIACODONT PSEPHODERMA ALPINUM MEYER FROM THE CALCARE DI ZORZINO (LOMBARDY, N ITALY)

SILUO RENESTO 8r ANDREA TINTORI

Key-uords: Psepboderma alpinum (Reptilia, Placodontia), De- donts, allowing the restoration of the oxic parr of the basins en- scription, Functional morphology, Palaeoecology. vironment as a normal marine one. The surface waters were in- fluenced by tides that possibly generated rarher strong local currents Riassunto. I1 ritrovamento di un nuovo esemplare completo close to the mouth of tidal channels crossing the platform. This del Rettile Placodonte Psephod.erma alpin*m Meyer, 1858 nel Calcare provided for a rich nectonic and benthic life in the more superficial di Zorzino, consente di migliorare Ia conoscenza dell'osteologia della waters: Psepboderna fed on bissate molluscs, such u Modiolus and, specie, pur non modificandone gli aspetti generali. Pur sulla base dei Isognomon, dwelling on shallow oxic bottoms ar rhe basins edge. due soli esemplari completi conosciuti, è testimoniata 1a crescita al- Due to water stratification, a long-lasting anoxic environmenr lometrica degli arti. Si ritiene che il carapace, dalla forma idrodina- developed in the deepest part of the basins themselves, allowing fine preva- mica e dotato di tre carene longitudinali, svolgesse un ruolo preservation (Konservat-Lagerstàtten) of allochthonous organism s. lentemente legato alla locomozione piuttosto che difensivo. La lunga coda e la piastra caudale servivano per l'equilibrio. Lo spazio privo di corazza sopra la regione sacrale permetteva a Psepboderma di nluo- Introduction. tare almeno come le Tartarughe Trionichidi, benchè in assenza di adattamenti particolari del cinto peivico e del femore. Psephoderma si nutriva di molluschi epibentonici che vivevano in banchi sui fon- The main fossiliferous unit for Norian vertebra- dali poco profondi, che circondavano le parti anossiche dei bacini di tes in northern Italy is the Calcare dí Zorzrno (Zorzi- deposizione del Calcare di Zorzino. Una ricca fauna di Pesci e inver- no Limestone) which was deposited in basrns surroun- tebrati che si ritrova associata ai resti dei Placodonti consente di ricostruire un ambiente marino normale, perlomeno nella zona ossi- ded by the huge Dolomia Pnncipale carbonate plat- genata dei bacini. infatti 1e acque di superficie erano influenzate dalle form (Tintori et al., 1985). Those basins were tens to maree, che generavano forti correnti nei pressi dello sbocco dei ca- hundreds of meters deep and several square kilometers nali che attraversavano la piattaforma, e dal moto ondoso, benchè limitato dalle dimensioni e dalla posizione interna alla piattaforma in area (|adoul, 1986; Jadoul et al., 1992, 1994). Their carbonatica dei bacini stessi. Sui fondali ossigenati e in tutta la lama centre was anoxic but the superficial waters and the superficiale d'acqua, ia vita era rigogliosa, anche se probabilmente margins provided an oxic environment (Tintori & poco diversificata per quanto riguarda gli invertebrati a causa della barrier.a ecologica rappresentata dalla piattaforma circostante. Le Zambell| 1980; Jadoul, 7986; Tintori, 1992), whích condizioni ambientali sulla piattaforma carbonatica erano decisamen- was inhabited by nectonic vertebrates and benthic in- te più avverse, generando faune molto povere in specie. A causa vertebrates (corals, echinoderms, brachiopods, moi- della permanenza di srratífícazione delle acque, in profondità si in- staurava un ambiente anossico che permetteva la conservazione degli luscs, crustaceans) that thrived on the sandy-muddy organismi (Konservat-Lagerstàtten). bottom. Durophagous fishes (Tintori, in press a, b) and placodonts fed these invertebrates. Abstact. The recent find of a new complete specimen of Ae- the on Further phod.erma alpinum Meyer, 1858 in the Norian Calcare di Zorzino toward the platform there was a crown of patch reefs (Zorzino Limestone), in northern Italy, adds to our knowledge of and organic mounds, sometimes giving rise to smail the osteology of this species. The new specimen is the largest so far islands covered by arboreal vegetation (|adoul et al., collected, reaching 180 cm in length. Allometric growth of the limbs during ontogeny is demonstrated. A new interpretation of its 1992; Renesto, 1994a). Life on the platform was rather palaeoecology and mode of life is given on the bases of both scarce, owing to the very shallow waters, which pro- functional morphology and palaeoenvironmental observation. The bably were warmer and more saline than normal ma- streamlined keeled carapace is interpreted as having a hydrodynamic rather than a defensive function. The long, stiff tail and, probably, rine ones (Tintori, 1992). the dorsal caudal plate served for balance. The gap in the carapace, This Norian fauna is far from being completely above the sacral region, ena5led Psephodermd fo swim at least as studied; field work, started tn 1978 under the guidance well as trionychid turtles, though no particular swimming adaptations of the pelvic girdle and of the femur are observed. Inverte- of one of us (A.T.), is stiil in progress (Tintori, lllS). brates and a rich fish fauna were found together with these placo- An attempted restoration of this vertebrate assemblage

Dipartimento di Scienze della Terra, Università degli Studi, Via Mangiagalli 34, 20133 Milano, Italy. Jó S. Renesto G A. Tintori

lists more than 40 genera of fishes and 10 of reptiles, specimen supplies additional information about the the latter assuming a great significance in the palaeo- anatomy of Psephoderma. It is exposed on the dorsal environmental interpretation of the area surrounding side and its size (tSo cm from the trp of the snout to the basins. Though in the middle of a marine environ- the end of the tail) renders it the largest complete Pse- menr, rerrestrial reptiles (\fild, 7978, 1,997; Calzavara phoderma so far known. et al., 1980; Pinna, 1984, 1987; Renesto, 7993, 7994a, It allows the description of humerus, radius, and b, c) are more frequently found than aquatic ones, sug- ulna; a new phalangeal formula for the pes and the gesting the close presence of land. So far marine repti- allometric limb growth is also recorded. The gap of les are only represented by the very rare Endennasau- the dorsal armour in the sacral region, is confirmed: it rers (Renesto, 1992) and the more cornmon placodont may have played an important role in the swimming PsEhoderma, subject of this paper because of the re- ability of PsEboderma, as shown in detail in the sec- cent preparation of a superb, cornplete specimen. tion on functional morphology. Psephoderma alpinum Meyer, 1858 ls an ar- Thc roe nf thp Zorzino Limestone fossiliferous moured placodont known from the Late Triassic levels can now be confidently stated as around the deposits of Europe since the last century (Meyer, Alaunian-Sevatian boundary (Middle-Late Norian) 1858; Curioni, 1863). However, the genus was poorly fladoul et al., 1994). Thus, as already assumed for known until the discovery of more complete specì- several years (see for instance Tintori, 1980; Jadoul, mens in the Norian and Rhaetian beds of Northern 1986), the Norian comprises also the Argillite di Riva Italy. On the basis of these specimens, Pinna (I976a, di Solto (Riva di Soito Shale) and the lower part of 1.978, 1980) established that other Norian-Rhaetian the Calcare dì Zu (Zu Limestone; Jadoul et al., tll+). placodont species erected on fragments of the skeleton Only the middle, which also yields Psepboderma (Placochelyanus stoppanii, Placochelys malanchinii and (Boni, 1948; Pinna, 1976b), and upper part of the Cal- Placochelys stoppanii along with Placodus zitteli and care di Zu, is now considered Rhaetian, together with probably Macroplacus raeticus and Placochelys alpissor- most of the overlying Dolomia a Concbodon (Concho- didae) are junior synonyms of Psephoderma alpinum. don Dolomite). Thus, Lombardian specimens of Pse- More recently, Pinna & Nosotti (tlAl) extensively de- phoderma must be considered ranging from Middle- scribed the first two nearly complete specimens from Late Norian to early Rhaetian, not only Rhaetian as the Zorzino Limestone, found in a locality (Endenna) stated by Pinna (tllo, fig. 1) and Pinna & Mazin close to the one yielding the newly prepared one. This (ree3, fig. 1).

Systematic Palaeontology

The osteology of Psepboderma alpinum has been thoroughly descnbed by Pinna (1976b, 19s0) and Pinna & Nosotti (1989), while the pattern of the armour was discussed by \lestphal (1976) and Pinna & Nosotti (1989). Thus, in the present paper, only new elements as well as those that contrast with former de- scriptions are discussed, and a new diagnosis, emend- ing and translating that of Pinna (tlz0b) is proposed.

Class Reptilia Subclass i nce rtae sed is Order Placodontia Owen, 1859 Family Cyamodontidae Nopcsa, 1923 Genus Psepboderma Meyer, 1858

Type species: Psephoderma alpinum Meyer, 1858

Fio 1 Skematic restoration of a basin margin (redrawn after Jadoul, 1986). The asterisk points to the supposed mollusk bank position, where Psephodrma and durophagous fishes Diagnosis (emended). Large, armoured placodont fed. A) Shallow water carbonate platform; B) tidal channel; reptile reaching at least 180 cm in total length. Skull C) ephemeral islands with arboreal covering; D) turbidite triangular, dorsoventrally flattened; premaxillae eden- flows; E) lower slope; F) undisturbed laminated sediments:fossiliferous levels. Vertical scale is exaggerated and tulous, very narrow and elongate, forming a sharp ros- the depth of oxic-anoxic waters boundary is not marked. trum; their posterior portions insert between the ante- Late Triassic Dlacodont from the Calcare di Zorzino 39 rior parts of the frontals; well developed maxillae, of the humerus; tibia and fibula longer than radius forming part of the anterior margin of the nasal open- and ulna, forming a well developed spatium inreros- ings; frontals narrow, outlining mosr of the dorsal seum; tarsus consisting of roundcd astragalus, cal- margin of the orbits; parietal unpaired with a well caneum and fourth distal tarsal; first metatarsal short developed foramen near its anterior margin; squa- and broadened, the remaining four straight. Phalanges mosals flat, large and with a well developed posterior of the foot short, the more distal ones rounded and process tapering distally. Quadratojugal stout, forming wider than long; phalangeal formula for the pes z (t), most of the lateral margin of the temporal fenesrra. 3 (2), 3, 3, 2 (1). Palatine bones larger than pterygoids. Mandible stout, Carapace rounded, stout, wide and very flat: with a large dentary, anteriorly repeating the shape of sacral region not covered by armour, followed by ex- the premaxillary rostrum, becoming wide posteriorly panded caudal plate just at the base of the tail, both where two large and flat teeth are presenr on each carapace and caudal plate consisting of small hexago- ramus of the mandible. Two flat, crushing reerh are nal osteoderms probably covered in life by horny present on the maxilla and on the palatine; the poste- scutes. Three longitudinal ridges, made up by keeled rior palatine tooth is enormously developed. osteoderms, are present on the carapace and on the Vertebral column consisting of 5 cervical, 15 caudal p1ate. No plastron on the ventral surface of the dorsal, 3 sacral and up to 6O caudal vertebrae. Cervical body, but the beliy is protected by about 25 stout vertebrae short, with low neurai spines, short zy- gasrrarla. gapophyses and well developed, laterally projecting Psephoderma alpinum Meyer, 1858 parapophyses. Caudal centra bearing low neural spines fext'fig.1-7 and short chcvrons. Shoulder girdle iittle developed, apart from the scapula that shows a stout, vertical 1858 Psephoderma alpinum Meyer, pp. 646-650. scapular blade. Pelvic girdle with a rather high and 1978 Psephoderma alpinum - Pinna, p. 343, p|.71-74 (cum syn.). stout ilium and rounded pubis and ischium. Humerus Material. Specimen ST82003 (Fig. 2) from Zogno 2, Zorztno stout, with a thick proximal head, and an enlarged, Limestone, Middle-Late Norian (Late Triassic), stored in the Museo flattened distal one. Femur of about the same length della Vicaria di S. Lorenzo, Zogno (Bergamo, haly).

Fig.2 - Psephoderma alpinun Meyer. Specimen ST82003, exposed from dorsal side. The specimen is 180 cm long; scale bar equals 20 cm 40 S. Renesto & A. Tintori

Measurements. elongate. Centra can not be seen, owing to the ovelap- In following tables (Tab. r-3) the measuremenls ratios the and ping of vertebral series. taken on specimen ST82003 are compared with those of specimen V527 described by Pinna t Nosotti (1989), Measurements are in mm Humerus. The shape of the humerus (Fig. :A) if not otherwise specified. does not agree with the hypothetic restorarion given by Pinna & Nosotti (1989, p. 30, fig. 10). In its proxi- Specimen sT82003 mal portion the bone is stout but less expanded than at its distal end; deltopectoral is well overall length 180 cm 125 cm the crest rather skull length from the tip developed and there are traces of the insertion of the of the snout to the occipital muscle latissimus dorsi. The shaft of the bone is quite condyle l)f mm narrow, especially in its middle part; the posterior skull width at orbit level 95 border is quite embayed, open area between carapace of the shaft while the ante-

érlu^-l (4uudr^^,.1^t PrdLE^t^.^ 40 79 rior one is only gently concave. In its distal portion, length of the bone becomes enlarged and dorsoventrally flat- presacral vert. column 500 296 tened; a well developed ectepicondylar groove is pre- length of the carapace 420 lJU sent near anterior margin. The is length of the temporal the ectepicondyle fenestra 52 L 41R small and no distinct areas for the insertion of exten- width of the temp. fenestra 27 L 29R sor muscles can be detected. The articular areas for the length of the humerus 107 L 110R 41.6 radius and for the ulna are gently convex and divided humerus proximal width 27L 25R by an embayment of the distal margin of the bone. humerus distal width 42L 40R 15.5 humerus cross section rrL 10R radius length 68R ulna length 71 L 21 femur length lo7 L 107R 40.4 femur proximal width 39R 15 femur distal width 36R 19.9 femur cross section 77L 9R tibia Iength 90L 8ZR 35.7 fibula length 82L 80R 30 metatarsal III length 42L 40R 74.4 tibia distal width 17L 15R 6.7 tibia proximal width tl D lu.f, diameter of the calcaneum 20L 20R q) diameter of the astragalum 19R 5.4

Tab. 1

Specimen sT82003 B c humerus/femur Fig. 3 - Psepbodetma alpinum Meyer. The left humerus from ventral humerus/radius 1.37 side (A), the right radius (B) and the left ulna (C) from femur/tibia 1.18 L f tt R dorsal view, as preserved in specimen ST82003. Scale bar radius/tibia o.82 equals 2 cm.

Tab.2 Ratios occurring among some limb sections. Radius and ulna (Fig. f B, C). These two bones are quite similar to each other and are about half Specimen sT82003 v527 the length of the humerus; their shaft is straight and humerus/ps 27 14 their ends show no expansions. The proximal artic- 21 73 ular area is convex while the distal one seems to be ulna/ps 14 7 tibia/ps 18 11 rather straight. metatarsal IIL/ps 8.4 5 Femur. The femur (Fig. +) is a stout bone, as long as the humerus and well expanded both at its Tab. 3 - Percentual ratios occurring between the length of some limb sections and that one of the presacral vertebral proximal and at its distal end. The ends are expanded column (ps). in different planes, so that the proximal and the distal articular areas lie in nearly perpendicular planes. The Description. proximal articular a.rea is broad and occupies the en- Ceraical aertebrde. In the examined specimen the tire head of the bone. The shaft is quite narrow in its cervical vertebrae 3-5 are visible, exposed on their dor- middle, then the bone becomes dorsoventrally flat- sal side; neural spines are low, the prezygapophyses tened and wide. The articular areas for tibia and are rather short (but this may be due, at least in part, fibula are, as in the case of the humerus, gently con- to compression) and stout, while the parapophyses are vex and divided by a small embayment. Apart for the Late Triassic pkcodont from the Calcare di Zorzino 4l

expanded distal articular area, the shape of the femur Caudal armour. Posterior to the dorsal caudal does not reveal particular adaptations to swimming. plate some osteoderms are present on both the dorsal Tibia and fibula (Fig. +). These two bones are and ventral edge of the tarl. The dorsal osteoderms are both robust and elongate. The tibia is somewhat keeled and are present up to the 22nd caudal vertebra; longer and stouter than the fibula, its proximal head is the ventral osteoderms are flat and more numerous, expanded and gently bent medially, enclosing a well reaching the 29th to 30th vertebra. They are circular developed spatium interosseum; the articular area for and flat, and decrease in diameter gradually toward the femur is probably convex. The fibula shows an the tip of the tail. expanded proximal head, followed by a narrow shaft; Sizes and proportions. As already pointed out, the the drstal end of the bone is not expanded and its ar- new specimen is the longest one known so far (1so ticular area is slightly convex; the proximal articular cm), far exceeding the supposed maximum length of area of the fibula cannot be detected. the species (about 725 cm following Pinna & Nosotti,

Fig. 4 - Psepltoderma alpinum Meyer. The right posterior limb of specimen ST82003: fe) femur; d tibia; fi) fibula; as) astragalus; ca) calcaneum; c) centrale; I-V) digits. Scale bar equals 4 cm.

Pes (Fig. a). The tarsus sho$/s the same pattern as the one described by Pinna & Nosotti (trsr), but the degree of ossification of each element is much higher. Therefore these bones are proportionally wider in large specimens. The metatarsals are stout and expanded at both ends, with convex articular areas; the first metatarsal is very short and wide, and bears a deep embayment on its lateral margin. As already noted by Pinna Er Nosotti (1989), the same fea- ture can be observed in some aquatic turtles; these latter show also a modified last metatarsal, while in Pse- phoderma all the other metatarsals are similar to'each other. The phalangeal formula for the pes is different from that given by Pinna & Nosotti (19s9): 2,3,3,3, 2, instead of 7,2,3,3, 1. This latter is based on specimen V527 in which apparently some terminal phalanges are fused with the preceding ones, giving the appearance of claw-bearing structures (Pinna 8r Nosotti, 1989). In our specimen, the second phalange of all digits and the last one of digits III, IV and V are Fig. 5 - Composite sketch of the outline ol Psepboderma based on specimen ST82003 (left) and VSZZ (rîght), with the body instead discoidal in shape and wider than long. Thus size reduced to the same length to show the positive al- the last three digits did not bear claws. lometry in the growth of the limbs. Scale bars equal 20 cm. 42 S. Renesto & A. Tintori

1989). Still longer specimens, perhaps more than 300 minimize drag. Propulsion by asymmetrical limb cm, could have existed, judging from the sizes of some movements has been retained mainly by those modern isolated teeth observed by one of us (A.T.). Using the reptiles in which the rigidity of the vertebral column measurements given by Pinna & Nosotti (1989), a prevents lateral undulation of the body axis, like positive allometry of the limbs is evident (see Fig. 5 freshwater turtles. and Tab. 3). Even if only two specimens are con- Plesiosaurs and marine turtles, however, sidered, the variation is probably due to the different adopted a more efficient swimming mechanism based growth stages rather than to other factors like sexual on symmetricai movements. This switch to a sym- dimorphism, as for example in pachypleurosaurids metrical pattern of limb movements (subaqueous fly- (Sander, 1989). In fact, the proportional difference re- ing and/or continuous rowing; Braun & Reif, 1985) mains the same for both anterior and posterior limbs required a fundamentai change of behavioural and as well as for different limb sections. In addition, no morphological features that, as far as it is known, can- other morphological differences between the two not be detected in Psepboderma, The limb girdles, in specimens can be traced. fact, seem to lack those adaptations that are charac- teristic of subaqueous flyers (Godfrey, 1984), for instance the presence of a ventrally elongated acromial Functional morphology and mode of life process in the scapula to form, with other bones of the pectoral girdle, a truss as in marine turtles, or the Morphological features. exceptionally expanded ventral elements present in plesiosaurs. Also, the proportions of the different was peculiar reptile: Psephoderma alpinum a very limb sections, together with their general shape, are wide the sku1l was flat, with a narrow rostrum and a those of an oar and not of a flipper. The most ex- and stout posterior region. The enormous rounded panded area is the distal portion of the pes, and this teeth served to crush the hard shells of the molluscs. represents the best adaptation for paddling (\febb & The dorsal carapace had an independent caudal plate, Blake, 1985). The tarsus and the pes are heavily mod- which is unique among vertebrates (a similar structure ified for aquatic life. Reduced ossification of tarsal ele- may have been present also in another placodont ments is frequently found in aquatic reptiles (Rieppel, genus: Cyamodus Pinna, 1992), fused to the proximal 1989), and ín Psepboderma only three rounded tarsal part of the long, stiff tail. elements are ossified. The toes are mainly formed by The postcranial skeleton reveals adaptations to rounded phalanges wider than deep, and feathering an aquatic life both in the limbs and in the flat and flexion of the toes was limited, rendering the pes carapace, the tail being not specialized as a propulsive a wide, stiff paddle. Because feathering of the toes is organ owing to its shape and stiffness. Prnna & the way in which modern rowers minimize drag Nosotti (lrar) hypothesized that swimming in Pse- during the recovery stroke, Psephoderma had to act in pboderma was a paraxial discontinuous rowing (sensu different way, perhaps keeping the paddle horìzon- Braun & Reif, 1985) rn which the posterior limbs tally oriented (this was made possible by the large de- played a major role. According to Pinna & Nosotti gree of rotation of the proximal head of the femur). (1989), swimming was slow and the carapace had no One aspect of the pes is rather puzzling: a tendency to hydrodynamic function, serving only as a defensive hyperphalangy is present both in subaqueous flyers device. Vestphai (tlto) stggested instead that the flat and in some rowers (Romer, 1956; Hildebrand, 7978; and keeled carapace of Psephoderma played a major ac- Braun & Reif, 1985; Carroll, 1985) and can be recog- tive hydrodynamic role. nized even in some freshwater tLtrtles (\flalker, 1923). Available evidence suggests that swimming in Psephoderma shows a reduced phalangeal formula in- Psephoderma may have been discontinuous (the distal stead. The functional significance of this adaptation is portion of the anterior limb remains poorly known unclear. However, the morphology of the phalanges however, making a reliable decision difficult). Accord- and the poor ossification of the tarsus testify that the ing to Carroll (1985), secondarily aquatic reptiles in- pes gave very little support to the body during walk- most herited from their terrestrial ancestors an alternate ing on land. Thus, Psepboderma had to spend of just (discontinuous) rather than a symmetrical movement its time in water or resting on a beach. of the limbs. This kind of locomotion is not very efficient for propulsion in water because it causes undula- Swimming efficiency. tion of the body, which decreases efficiency of paddling. Most reptiles use lateral undulation of the tail PsEhoderma swimming may have been more (in some cases of the whole body) for aquatic propul- efficient than was hypothesized by Pinna & Nosotti sion, and limbs are consequently reduced in order to (trsr): both the humerus and the femur are stout Late Triassic îlacodont frorn the Calcare di Zorzino bones with enlarged and flattened distal heads and the caudal plate had to have some hydrodynamic bear traces for the insertion of powerful muscles. The function, possibly togerher with the long tail. It may ilium is rather tall and shows traces of the insertion of have limited lateral undulatìon of the sacral and well developed iliotibialis and iliofemoralis muscles. caudal regions in order to minimize drag. The tail had The presence of stout transverse processes on the no propulsive role: it is long and heavy, but not later- more proximal caudal vertebrae suggests that also the ally compressed, as is usual for unduiatory swlmmers. caudifemoralis muscle must have been strong. Thus, The morphology of the zygapophyses (Pinna & the posterior limb of Psepboderffta was capable of Nosotti, 1989) reveals that lateral undulation was powerful adduction and retraction. In addition, the limited, while greater vertical mobility was possible. sacral gap in the dorsal armour increased freedom of If Psephoderîna was a rather efficient swimmer movement, so that the posterior lrmb of PsEhoderma (at least in comparison with other discontinuous could have described an arc, pivoting around the head. rowers), the long, heavy tail was undoubtedly useful Freshwater turtles must limit their hind limb move- for balance. It probably rendered the posterror por- ments to the horizontal plane, because of the massive tion of the body fairly massive, possibly shifting the plastron and carapace (\flalker, 1973). centre of gravìty of Psepboderma somewhat posterior may be hypothesized It that such adaptations in to the centre of lift (where the propulsive forces PsEhoderma allowed it to swim ar least as well as the apply; Fig. 6). If this hypothesis is correc, Pse- best swimmers among freshwater turrles, the Triony- phoderma might have tended to lift during swimming, chidae, even in absence of their peculiar specializa- making this reptile capable to reach rather quickly the tions like the shape of the femur (\lalker, 1973). If surface for breathing. Actually bottom feeding tet- this was the case, Psepboderma could have reached rapods have a specific gravity that is somewhat higher enough speed to exploit the hydrodynamic properties than that of water (Hildebrand, 1978), e. g., fresh- of its carapace. According also to Westphal Qlle) and water turtles have a specific gravtty ranging from I.O7 Pinna & Nosotti (1989), the flat c^rapace is stream- to 1.39 (Valker, 1973). Psephoderma, with its very flat- lined and bears three longitudinai keels, whose height tened body and heavy skeleton, had probably a high may have been increased by horny scutes. Such keels specific gravity, requiring no particular efforts to dive. are present in some well swimming turtles líke Cara- Reaching the surface to breath was more difficult, so tochelys and Dermochelys and are considered useful for that the combined action of the streamlined carapace, controlling water flow around and past the body of the caudal plate, and long tail was an important aid in the animal to increase stabiliry (\festphal, 1976; these vertical movements, especially if water energy Hridebrand, 1978). In the presence of water currents, raised the relative speed. The vertical run to the water the streamlined, very fiattened carapace allowed Pse- surface was not great because the moliuscs on which phoderma to stay at the bottom with little effort also Psephoderma fed thrived at 10-20 m of depth.

Fig. 6 Hypothetical positions of the centre of lift (CL) and of the centre of gravity (CG) in Ae- phoderma. in presence of currents and, if properly oriented, the Function of the armour. carapace may have exploited other hydrodynarnic ef- Pinna & Nosotti (1989) suggested that Pse- fects, acting as an hydrofoil producrng lift, and thus pboderma had the possibility of burrowing in the soft saving energy. It is likely that the limited swimming sediments by the use of the stiff tail. They compared ability assumed by Pinna & Nosotti (1989) for Pse- the mode of life of Psephoderma wíth that of the My- phoderma, was restricted mostly to the early growth liobatidae, durophagous rays with flat bodies that live stages, when the limbs were relatively shorter than in in shaliow water and dig themselves in using undula- adults. the Poorer swimming adaptation suggests a tory movements of the large, modified pectoral fins. shallower and closer land to habitat for the iuveniles. This comparison with the Myliobatidae should be The expanded plate caudal could hardly have avoided for many reasons, first of all the presence of a played a mere defensive role because the posterior carapace in Psepboderma ín contrast ro the highly flex- limbs were too long to be housed under it, as sug- ible body and fins of rays. If this reptile ever dug it- gested by Pinna & Nosotti (1939). As for the carapace, self into the sediments, it could have managed much 44 S, Renesto G A, Tintori

4_.

Fig,. 7 - Life rcstoratio n o{ Psepboderma alpinum in ìts environment, feeding on a Modiolus bznk better with its anterior and posterior limbs, as turtles striosuchus was not living nearby. Thus, at least adult do, rather than with a long, tapering, stiff tail. In ad- Psephoderma had no predators from which it had to dition, burrowing for defense seems rather unlikely escape by burrowing in the sea bottom. For this rea- for a large, armoured, lung breathing animal, in order son, the hypothesis of \lestphal (tfZe) suggesting the to escape predators much smaller and gill breathing. shift from an originally defensive toward an hydrody- Most freshwater turtles dig themselves in to prey in namic function for the Psephoderma carapace seems ambush, rathcr than to avoid predators (Arambourg 8r reasonable, even if a limited protective role was re- Bertin, 1958; Guibé, 1970). Furthermore no potential tained. predators on adult PsEhoderma are known in the Westphal (nle) noted that any attempt at a Norian fauna from the Alps. These faunas lack the phyiogenetic interpretation of the placodont armour is huge Late Triassic ichthyosaurs, perhaps the only ani- not reliable, since the hexagonal pattern is known mals capable of grasping and injuring thcm. The Zor- from the Lower Muschelkaik to the Triassic-Jurassic zino Limestone basins were far from the open sea, and boundary, that is for the entire placodont record. The the only connection were long tidai channels. \fe as- various arrangements of the osteoderms in placodonts sume that the channels represented ecological barriers are probably the answer to functional requirements to the entry of huge nectonic organisms. The largest related to different environments and modes of life. fishes rn the fauna, such as Saurichtbys, Birgeria and According to $ilestphal Qve) an exclusively defensive Holophagus are mainly piscivorous (Btirgin, 1990; Tin- role can be supported for Henodus Fluene, 1936, and tori in Boucot, 1990, tab. 23) and in any case, their Psephosaurus Fraas, 1896, while in other genera the sharp conical teeth were not able to crush the placo- need of mobility or hydrodynamic adaptations also in- uorll,.1^-t car.4r.n4.p apaLc, uvennr In young I ndividuals. fluenced the structure of the carapace. In the case of Durophagous fishes are very common in the fauna Psephoderma, the original defensive function was prob- (Tintori, in press a, b), but were usually rather small, ably of much smaller importance than the hydrody- and all of them had smail gape. Pterosaurs are known namic function. (\ilild, 1928), but their eventual predation on placodonts (hypothesized by Pinna & Nosotti, 1989;Mazin Mode of life. & Pinna, 1993) had to be restricted to hatchiings, as in modern marine turtles that are preyed by sea birds. \íhen at the bottom, PsEhoderma wandered in Another predator to be considered is the large gavial- search of food by walking slowiy iike many fresh- like phytosaur Mystriosuchus (Pinna, 1987). This ani- water turtles (in bottom walking very littie support ls mai is known from the Zorzino Limestone, but only required; \falker, 1973). It likely fed on molluscs that as an isolated skul1, without lower jaw nor postcranial thrived on the muddy bottom (Fig. Z), mainly Isogno' elements. Mystriosuchu.s is large and probably did not mon and Modiolus, both epifaunal forms attached by prey exclusively on fish, but it is uniikely that its long byssus. These molluscs are commonly found in the and sle nder jaws couid have crushed the masslve Zorzíno Limestone vertebrate beds, even as al- carapace of Psephoderma. Furthermore, on taphonomic lochthonous fossils, whrle shelly endofauna is absent. and palaeoenvironmental ground, we think that My- This suggests that Psephoderma used its "beak" not for Late Triassic placodont from tbe Calcare di Zorzino 45 searching in the sand or mud like a platypus or some individuals may have occasionally reached the central shore birds (Pinna & Nosotti, 7989; Mazin & Pinna, area of the basins, perhaps sinking into the anoxic, 1993), but rather to twist the molluscs off before deepest part just before or after death. This can at crushing the shells. The edentulous rostrum ls con- least in part explain why alrnost all the Psephoderma sidered here to be analogous to the anterior teeth of collected in the Zorzino Limestone have been found Placodus, Paraplacodus, and Cyamodus (Kthn, 1969), complete and fully articulated, while terrestrial rep- also used to feed on epifaunal molluscs. Lumps of tiles are sometime incomplete as a consequence of a fragments shell are very common together with the more or less long post-morrem rransport into superfi- vertebrate remains, but they can be attributed also to cial oxic waters (Renesto, 1993; \fild, 19ZS). durophagous fishes, such as Paralepidotus ornatus (Agassiz 1833-1844), which had a similar diet (Tintori, Achnoubdgements. press '\le in a, b) and was much more common than Pse- now feel obliged to rhe people who helped in collecting all phoderma. Owing to the different size (Paralepidotus placodonts specimens during more than 15 years of field work in the not exceeding 50 cm in standard length), these Zogno area under A.T. responsability. We are particularly indebted to G.F. and O. Pesenti and L. Paramarti for specimen V52Z (now ar the durophagous animals could well live together in the Museo Civico di Sroria Narurale, Milano), to C. Barbero for specimen same environment, perhaps preying on specimens of n. 8358 (Museo Civico di Scienze Naturali "Caffi", Bergamo) and again the same mollusc species, but in different growth to G.F. and O. Pesenti for this new specimen. O. Pesenti was also stages. responsible for the nice preparation of the same. Prol J.R. Nursall, Edmonton, Prof F. Vestphal, Tùbingen and Dr. M. Sander, Bonn The functional analysis of the complete speci- made critical reading of an earlier version of rhe manuscript, ro them mens and of the whole faunai assemblage, suggests our sincere thanks. that the comparison between Psephoderma and Mylio- Field work was supported by M.P.I.-M.U.R.S.T. 4ool" granrs ro C. Rossi Ronchetti, N. Fantini Sestini and M. grants batidae should be rejected, and allows the hypothesis Gaetani and by from the Bergamo and Milano Museum of Natural Hisrory. The that the mode of life of this reptile was much more photograph of the complete specimen is by F. Carminati; the similar to that of trionychid turtles, even if in a drawings are by S. Renesro and M.R. Torazzi Renesto. marine environment. Like trionychids, and most other placodonts, Psephoderma usually lived rather close to land and probably came up on the muddy- REFERENCES sandy islands only to iay eggs or rest close by the rvater. 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