Browsing by Author "Keyser, A. W."

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EVOLUTIONARY TRENDS IN TRIASSIC DICYNODONTIA
(1974) Keyser, A. W.
Triassic Dicynodontia differ from most of their Permian ancestors in a number of specialisations that reach extremes in the Upper Triassic. These are ( 1) increase in total body size, (2) increase in the relative length of the snout and secondary palate by backward growth of the premaxilla, (3) reduction in the length of the fenestra medio-palatinalis combined with posterior migration out of the choanal depression, (4) shortening and dorsal expansion of the intertemporal region, (5 ) fusion of elements in the front part of the brain-case, (6) posterior migration of the reflected lamina of the mandible, (7) disappearance of the quadrate foramen and the development of a process of the quadrate that extends along the quadrate ramus of the pterygoid. It is thought that the occurrence of the last feature in Dinodontosaurus platygnathw Cox and Jacheleria colorata Bonaparte warrants the transfer of the species platygnathus to the genus Jacheleria and the erection of a new subfamily, Jachelerinae nov. It is concluded that the specialisations of the Triassic forms can be attributed to adaptation to a Dicroidium-dominated flora.
A new bauriamorph from the Omingonde Formation (Middle Triassic) of South West Africa
(Bernard Price Institute for Palaeontological Research, 1978) Keyser, A. W.
A new genus and species of bauriamorph therapsid from the Anisian Omingonde Formation (Middle Triassic) of South West Africa is described as Herpetogale marsupialis gen. et sp. nov. This new form is chronologically the latest known member of the therocephalian lineage. It is in many ways more advanced than Bauria, but it also displays a number of more primitive features . It is advanced in its reduced postcanine series, slightly larger secondary palate, smaller suborbital fossae, and more pronounced coronoid processes of the dentaries, displaying laterally very distinct fossae maJsetericae. A very distinct crista IacialiJ is developed in front of the orbit. Primitive features are the presence of a pineal foramen and complete postorbital bars. Conspicuous and no doubt specialised are the pronounced "cheek cavities" well demarcated above by maxillary overhangs and below by shelf-like expansions on the dentaries. This arrangement suggests "cheek pouches" for food storage reminiscent of a habit in modem primates and certain rodents. The skull is complete, very little distorted or damaged, with lower jaw in occlusion. Besides an account of the cranial morphology, attention is also given to jaw musculature and the taxonomic position of the Bauriamorpha.
A NEW TRIASSIC VERTEBRATE FAUNA FROM SOUTH WEST AFRICA
(BERNARD PRICE INSTITUTE FOR PALAEONTOLOGICAL RESEARCH, 1973) Keyser, A. W.
A new fauna of mammal-like reptiles is described from a continental sequence of sediments in South West Africa hitherto known as the Etjo Beds. These include representatives of the Anomodontia (two new forms and one known from the Karroo basin), Bauriamorpha, (one specimen comparable with a known form), Cynodontia (one new form and two known from the Karroo basin) and one small eriopoid amphibian. On the basis of this faunal assemblage it can be shown that there is a disconformity between the upper Plateau Sandstone Formation containing traces of Dinosaurs of possible Carnian-Norian age and a lower Omingonde Mudstone Formation with this new fauna, of essentially Upper Beaufort (Scythian/Anisian) age. The two formations are distinguished on lithological grounds. The Triassic sequence in the neighbouring Doros area is thought to be equivalent to the Plateau Sandstone Formation.
A RE-EVALUATION OF THE GENUS TROPIDOSTOMA SEELEY
(BERNARD PRICE INSTITUTE FOR PALAEONTOLOGICAL RESEARCH, 1973) Keyser, A. W.
The type specimens of Cteniosaurus platyceps Broom, Dicynodon acutirostris Broom, and Dicynodon validus Broom were re-examined and were found to be very similar in a number of features rarely encountered in other Anomodontia. The skull of the type of Cteniosaurus platyceps is described in some detail. It is concluded that the above species must be considered to be junior synonyms of Tropidostoma microtrema (Seeley).
A RE-EVALUATION OF THE SYSTEMATICS AND MORPHOLOGY OF CERTAIN ANOMODONT THERAPSIDA
(1972) Keyser, A. W.
The cranial morphology of a number of specimens assigned to the genera Oudenodon, Rhachiocephalus, Aulacephalodon and Pelanomodon was investigated (Keyser, 1969). It was found that the internal morphology and the general structure of the skulls show great agreement. Many of the differences between the genera can be associated with the size of the skull. The main differences between the genera lie in the specialisation of the biting mechanism and in the relative size and shape of the nasal and prefrontal bosses. It is suggested that the genera Oudenodon and Rhachio cephalus bit off their food with the sides of the horn-covered jaws while the broadnosed genera Aulacephalodon and Pelanomodon bit with the transverse anterior tips of the jaws. This difference in the morphology of the jaws is probably indicative of a fundamental dichotomy between the two groups of genera. A similar dichotomy has been suggested for Triassic dicynodonts by Cox (1965).