Callistomordax

Schoch, Rainer R., 2008, A new stereospondyl from the German Middle Triassic, and the origin of the Metoposauridae, Zoological Journal of the Linnean Society 152 (1), pp. 79-113 : 104-105

PALAEOBIOLOGY OF CALLISTOMORDAX

Articulated skeletons of C. kugleri occur most frequently in mudstones that formed under estuarine conditions. The shallow Lower Keuper Basin, spanning most of Central Europe, was repeatedly flooded by the Tethys. Subsequent regression left a diversified landscape with brackish swamps, larger lakes, and saltwater marshes ( Beutler, Hauschke & Nitsch, 1999). Under these conditions, some larger water bodies existed long enough to permit algae, conchostracans, fish, and aquatic tetrapods to invade and form small ecosystems. Such a body of water probably existed at Vellberg, where mudstones rich in organic matter bear a large fauna of fish (hybodontiform Acrodus , the actinopterygians Saurichthys , Gyrolepis , Dipteronotus , and Serrolepis , and juveniles of the dipnoan Ptychoceratodus ) as well as diverse tetrapods (R. Böttcher, pers. comm.). Marine forms ( Nothosaurus , Neusticosaurus , and Psephosaurus ) are absent, whereas both ‘lacustrine’ temnospondyls and chroniosuchians, as well as terrestrial archosaurs and small choristodere-like diapsids, are abundant ( Schoch, 2002a; R. R. Schoch, unpubl. data). Furthermore, Callistomordax is found in various bonebeds with mixed lacustrine and marine faunas, but it is only represented there by isolated bones. At the type locality, and only in the type horizon, Callistomordax is much more abundant with numerous articulated finds, including complete ones such as the holotype, as well as specimens reaching only half the ‘adult’ skull length of 130–160 mm. Tiny single remains such as interclavicles and skull fragments suggest the taxon was present with small larvae reaching hardly 100 mm in total length. In the type horizon, a low degree of salinity (oligohaline state) is indicated by the presence of the ostracod Darwinula sp. and the bivalve Unionites brevis ( Schoch, 2002a) . None of the articulated specimens of C. kugleri has any gut or intestinal content. The available data on the fauna suggest that at least one actinopterygian, Serrolepis sp. , was autochthonous and probably highly abundant in the lake in which the deposit formed. A second taxon, a ceratodontid lungfish, is represented by juvenile skeletons often found in a semiarticulated but highly distorted, condition.

Callistomordax was obviously an able swimmer, as its elongated body proportions suggest, and there are no anatomical features in the vertebral column that would contradict that. The presence of disproportionately large, keeled fangs and the powerful adductor musculature, indicated by the shape of the subtemporal fenestrae, as well as the preglenoid process in the mandible, suggest it may have tackled larger prey items, perhaps rather active animals that struggled fiercely after initial capture. The presence of branchial denticles in the region between the pectoral girdle and mandible indicates the possibility of open gill slits, which would have played a role in underwater feeding. In combination with the lateral line sulci present throughout ontogeny, and the structure of the postcranial skeleton, Callistomordax is here concluded to have been a predominantly aquatic animal.