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Evidences for a Semi Aquatic Life Style in the Triassic Diapsid Reptile Tanystropheus

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Evidences for a Semi Aquatic Life Style in the Triassic Diapsid Reptile Tanystropheus Rivista Italiana di Paleontologia e Stratigrafia (Research in Paleontology and Stratigraphy) vol. 124(1): 23-34. March 2018 EVIDENCES FOR A SEMI AQUATIC LIFE STYLE IN THE TRIASSIC DIAPSID REPTILE TANYSTROPHEUS SILVIO RENESTO1 & FRANCO SALLER2 1Dipartimento di Scienze Teoriche ed Applicate, Università degli Studi dell’Insubria, via Dunant 3, I 21100 Varese, Italy. E-mail: silvio.renesto@ uninsubria.it 2Via Weingarter 5 39010 Gargazzone (Bolzano) Italy. To cite this article: Renesto S.. & Saller F.. (2018) - Evidences for a semi aquatic life style in the Triassic diapsid reptile Tanystropheus. Riv. It. Pale- ontol. Strat., 124(1): 23-34. Keywords: Tanystropheus; Reptilia (Diapsida); Triassic; functional morphology; osteology; myology; locomotion; palaeoenvironment. Abstract. The paleoecology of the bizarre, long-necked tanystropheid diapsid Tanystropheus from the Middle and Late Triassic of Western and Eastern Tethys has been debated since the first discoveries. In the present work, osteological features related with gait and locomotion are reanalysed, and a reconstruction of the pattern of the musculature of the tail, pelvic girdle and hindlimb is proposed. The anatomical correlates, along with the inferred functional interpretation of the musculature support the hypothesis that 1) Tanystropheus was able to lift the body off the substrate when on land, 2) it lacked adaptations for continuous swimming, either tail-based or limb-based, 3) it was able to swim for by rowing with symmetrical strokes of the hind limbs. Rowing is a discontinuous swim- ming pattern that occurs in animals with a semi-aquatic life style which return frequently to emerged land, thus the proposed model is consistent with the hypothesis that Tanystropheus was a shoreline dweller rather than a fully aquatic animal. NTRODUCTION I freshwater environments. The life style of Tanystro- pheus, the largest and most bizarre of all tanystro- The Tanystropheidae are a clade of bizarre pheids, remained uncertain since its discovery. archosauromorph diapsids from the Middle and Tanystropheus is mostly known based on the Late Triassic (see Pritchard et al. 2015 and refe- Middle Triassic species Tanystropheus longobardicus of rences therein) that are characterized mainly by a northern Italy, southern Switzerland and Tanystro- long and rather stiff neck, made by eight (Macroc- pheus conspicuus of Central Europe (Peyer 1931; nemus) to thirteen (Tanystropheus) elongate cervical Wild 1973; Nosotti 2007). Tanystropheus meridensis vertebrae, depending on the taxon. The elongate (Wild 1980a) is known from a single specimen con- cervical vertebrae had low neural spines and long, sisting of a skull and partial neck, but it may be a rod-like ribs that run parallel to the ventral margin small individual of T. longobardicus (Fraser & Riep- of the centra. Tanystropheids were firstly included pel 2006). Remains from the Middle East (Rieppel in the Prolacertiformes, then in the Protorosauria, 2001) have been attributed to a different species, but these clades are now considered as paraphyle- T. haasi. Also nearly complete skeletons from Chi- tic in recent phylogenetic hypotheses (Pritchard et na (Li 2007; Rieppel et al. 2010) are virtually un- al. 2015; Ezcurra 2016). The clade Tanystrophei- distinguishable from T. longobardicus, according to dae comprise the genera Macrocnemus, Tanystro- their describers. Isolated Tanystropheus remains are pheus, Amotosaurus, Langobardisaurus and Tanytrachelos also reported from the Middle Triassic of North (Pritchard et al. 2015). Macrocnemus and Langobardi- America (Sues & Olsen 2015); some isolated cer- saurus are supposed to have been terrestrial animals, vical vertebrae from the Norian (Late Triassic) of possibly facultative bipedal runners (Rieppel 1989; Northern Italy have also been assigned to Tanystro- Renesto et al. 2002). Other genera, like the very pheus (T. fossai Wild 1980 a, b; Tanystropheus sp. Dalla small (about 20 cm long) Tanytrachelos ahynis (Ol- Vecchia 2000). sen 1979) have been considered as aquatic, living in Even if the genus comprises species of dif- ferent size, all shared the same body architecture Received: August 29, 2017; accepted: October 19, 2017 (Fig. 1). 24 Renesto S. & Saller F. Fig. 1 - Reconstruction of the skeleton of a large specimen of Tanystropheus longobardicus in walking posture. Scale bar equals 1m. PREVIOUS STUDIES Nosotti (2007), Tanystropheus lived in the sea and swam by lateral undulations of the trunk and tail. In Tanystropheus, the elongation of the neck, A different interpetation was proposed by which consists of 13 cervical vertebrae (Rieppel et Renesto (2005), who suggested that Tanystropheus al 2015), is extreme, since it could equal the length may have been a semi-aquatic reptile, that lived in a of the rest of the body, tail included (Peyer 1931; shoreline habitat. The hollow cervical centra, which Wild 1973; Cox 1975; Tschanz 1986, 1988; Bürgin et housed the neural canal (Wild 1973), and the relative al. 1989; Li et al. 2004; Fraser & Rieppel 2006). This flexibility of the thin cervical ribs may have allowed peculiar structure, along with the impressive overall the neck to be kept off the substrate to some extent size (the largest individuals of T. longobardicus reached when the animal walked on land (Fig. 1A). In addi- up to five meters in length) rendered the animal ra- tion, the presence of large muscles at the base of the ther weird in appearance and a palaeoecological and tail may have shifted back the centre of mass, helping functional enigma. Excluding earlier intepretations as to balance the body even with the neck slightly raised. a flying reptile, based on isolated elements (Nopcsa Renesto (2005) also pointed out that Tanystropheus la- 1923), the first reconstructions (e.g. Peyer 1931) de- cked adaptations for continuous swimming, both in picted Tanystropheus as a terrestrial, lizard-like animal, the limbs and in the tail. with a sprawling gait and a somewhat laterally flexible Rieppel et al. (2010: 1086, fig. 3) discovered neck kept close to the ground. Instead, Wild (1973) also that the thirteenth cervical vertebra of Tanystro- proposed that Tanystropheus could have raised the pheus (previously considered as the first dorsal) is neck off the substrate to a great extent, in a giraffe- modified and may have allowed some raising of the like fashion and that the individuals shifted habitat neck, as already suggested by Renesto (2005), based during ontogeny: the juveniles were terrestrial insec- on the slanting posterior surface of the centrum of tivores, while the adults were aquatic predators. the twelfth cervical vertebra. Subsequent studies (e. g. Tschanz 1986, 1988) Recently, Beardmore and Furrer (2017) compa- concluded that Tanystropheus lacked an adequate dorsal red the skeletal taphonomy of Tanystropheus with that musculature to keep the neck raised off the substrate of terrestrial and aquatic genera from the same de- as proposed by Wild, and that the elongated cervical posits and concluded that the “carcasses of Tanystro- ribs significantly stiffened the neck, thus preventing la- pheus originated in terrestrial or at least marginal and teral flexion and vertical bending. In addition, the ribs near-shore, shallow marine settings” (Beardmore & granted adequate support (by transmitting the weight Furrer 2017, p. 1). to the shoulder and anterior limbs) only if the neck was held in horizontal position. According to Tschanz’s re- construction, the stiff, rod-like neck of Tanystropheus AIM OF THE PRESENT WORK was a hindrance in a terrestrial environment and con- sequently a fully aquatic life-style was proposed. Ac- The presence of hooklets from belemnoid cording to Tschanz (1986, 1988), Rieppel (1989), and cephalopods in the stomach region of one speci- Life style of the Triassic diapsid reptile Tanystropheus 25 men (Wild 1974), was cited as a further evidence for life in an open marine environment. This lat- ter assumption however does not take into account that there is a great variety of extant decapodiform cephalopods (e. g. suborder Myopsida) that live in shallow waters (Bartol et al 2001; Anderson 2000) or reach shallow waters to spawn (Arkhipkin et al. 2004; Hatfield et al. 1990). Thus, evidence of cephalopod remains in the stomach region of a sin- gle, or few, Tanystropheus specimen does not neces- sarily support an open-marine life style, but it may suggest that Tanystropheus may have lived at the sho- reline and fed on aquatic animals in shallow waters. The neck of Tanystropheus, however is different from that of herons or other wading birds, lacking of the powerful ventral muscles for head projection pre- sent in those birds and it could not have been used to dart at preys from a S-shaped starting position. Thus, its hunting strategy and feeding mechanics must have been different. In the present work, a re-analysis of the oste- ology of Tanystropheus has been carried out. The stu- dy focused mostly on the post-dorsal sections of the vertebral column, on the pelvis and hind limb; Fig. 2 - Tanystropheus longobardicus, specimen MCSN 4451 A) Sacral the pattern of hindlimb and caudal musculature has region and base of the tail; B) interpretative drawing. Ab- been reconstructed and a model for Tanystropheus breviations in (B) are: cv, caudal vertebrae he, haemal spine; locomotion is proposed, which is consistent both il, ilium, is, ischium, pu, pubis, sv sacral vertebrae. Scale bar equals 30 mm. with feeding on aquatic prey and with an semi aqua- tic life-style in a littoral
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