Tritylodon

Abstract: In the year 1858 it was my good fortune to receive from Sir George Grey, K.C.B., Governor of the Cape of Good Hope, a fossil skull from the Triassic sandstone of the Rhenosterberg, which combined dental characters, indicated by the sockets and fragments of teeth, most resembling those of a Carnivorous Mammal, with the unequivocal cranial structure of a Saurian Reptile. This interesting evidence of the Vertebrate life of that geological period and locality is described and figured in my ‘Catalogue of the Fossil Reptilia of South Africa,’ 4to, 1876, p. 23, pl. xviii. figs. 6–11, as a species of an extinct genus, Galesaurus , and as the type of a Division of the Class Reptilia, subsequent accessions to which, also described and figured in the same work, led me, by similar modifications of the dentition, to group them in a distinct suborder termed Theriodontia ( op. cit. p. 15). The collection of fossils from the same formation, in the locality of ‘Theba-chou,’ Basuto Land, subsequently deposited in the Geological Department of the British Museum of Natural History, by Dr. Exton, which, together with the evidence of a mammalian genus ( Tritylodon ), included remains of the reptilian genera Kistecephalus and Batrachosaurus , has also furnished the subject of the present paper. Characters of the skull and teeth, more or less mutilated in the original specimen of Galesaurus , have been brought to light by careful removal of the adherent matrix from the present fossil, under the superintendence of my friend Mr. W. Davies, Assistant in

Abstract: An investigation of bone microstructure of nonmammalian therapsids has revealed distinctive signals pertaining to their ontogenetic growth and biology. Until now, histological studies of the nonmammaliaform cynodonts have focused only on postcranial material. Through the examination of micro-computed tomography (micro-CT) scans and serial thin sections, the current study provides a novel perspective on the structure and growth of the mandible of Tritylodon, a derived herbivorous cynodont from southern Africa. By tracking histological features across the serial thin sections, trends in relocation and modelling are documented for the growing Tritylodon mandible. For example, during growth, localized changes in the cross-sectional shape of the mandible occurred. Localized deposits of new lamellar and fibrolamellar bone on the lateral edge indicate widening of the mandible during different episodes of growth. The presence of radial channels indicates the deepening of the mandible at its anterior and posterior ends. The relocation of the paired mental foramina suggests that the mandibular body lengthened mainly in the posterior direction. The medial movement of a posterior postcanine tooth during growth and eruption is recorded in the histology. This histological assessment also documents the presence of Sharpey’s fibres in the cellular cementum of the first incisor, providing novel and unequivocal evidence that it was attached to the Tritylodon jaw by a periodontal ligament. This is the first comprehensive study that uses histological analysis to document the growth dynamics of the mandible of a nonmammalian therapsid, thus providing a unique perspective of localized mandibular growth in a fossil animal.

Abstract: . Tritylodon longaevus Owen, 1884 is one of the most common members of the Lower Jurassic faunas of the Karoo Basin. The cranial and dental anatomy of this taxon is well known, but its postcranium has not been previously addressed in detail. Our analysis shows that T. longaevus shares many postcranial features with other tritylodontids that distinguish them from other non-mammaliaform cynodonts. The correlation between taxon size and postcranial anatomical traits is briefly explored among tritylodontids, showing that few morphological differences among species correlate with size. Analysis of the purported oldest remains of Tritylodon, from the Norian Los Colorados Formation of Argentina, suggests that they cannot be unambiguously assigned to this taxon, circumscribing the record of Tritylodon to African localities.