Abstract: Evolutionary and Biogeographic Implications of an Antarctic Jurassic Dinosaur Fauna

William R. Hammer

The Jurassic vertebrate fauna from Mt. Kirkpatrick in the Beardmore Glacier region of Antarctica includes the new crested theropod dinosaur Cryolophosaurus ellioti, at least two genera of scavenging theropods, a plateosaurid prosauropod, a Rhamphorhynchoid (possibly dimorphodontid) pterosaur, and a large tritylodont. An early Jurassic (Pleinsbachian-Toarcian) age has been established for this fauna, based on the overlapping ranges of plateosaurid prosauropods and Bienotheroides "clade" tritylodonts from other dated faunas.

Features of the skull of Cryolophosaurus indicate it is not closely related to Early Jurassic ceratosaurs such as Dilophosaurus. Instead, the deep, narrow skull with a large antorbital fenestra and a pneumatic lacrimal suggests a close relationship to the tetanurans of the later Jurassic. In particular, the short length and orientation of the quadrate suggest it is a member of the same clade as Allosaurus from the Late Jurassic. Other members of this clade include Acrocanthosaurus from the Early Cretaceous of North America and Monolophosaurus from the Late Jurassic of China. Because all of the previously described allosaurids are Late Jurassic or younger, Cryolophosaurus extends the known range of this group back approximately 50 m y. This indicates the large tetanurans separated into distinct family clades much earlier than their time of peak diversity in the Late Jurassic-Early Cretaceous.

Cryolophosaurus also extends the known geographic range of the allosaurids. Previous finds have all been from the northern continents; there were no allosaurids from any part of Gondwana. Thus, Cryolophosaurus also indicates that allosaurids were Pangean in their distribution rather than restricted to the north. Other elements of the fauna are also of biogeographic significance.

While prosauropods were previously known from the Late Triassic to the Early Jurassic of all continents except Antarctica, the large plateosaurids were known only from northern continents, specifically Europe and China. The Antarctic prosauropod extends the range of these large plateosaurids into Gondwana. The pterosaur from Antarctica is represented by only a single humerus, thus its precise taxonomic affinities are uncertain. However, the proximal end of the specimen shows features that very distinctly place it among the primitive rhamphorhychoids, and most probably in the family Dimophodontidae. Rhamphorhynchoid pterosaurs are known mainly from the northern continents, particularly Europe, with one reported occurrence from the Upper Jurassic of India. Rhamphorhynchoids that belong o the Dimorphodontidae are all from Europe; thus, if the pterosaur is a dimorphodontid, it extends the range of this family into Gondwana for the first time. In addition, this is the first known occurrence of any pterosaur from a high paleolatitude fauna.

In summary, this discussion shows that Early Jurassic vertebrates from Antarctica have strong taxonomic affinities with mainly northern groups. This suggests that the geographic/climatic conditions that typified the Triassic Period and allowed for a very Pangean distribution of terrestrial vertebrate faunas continued into the Early Jurassic. The reason the Antarctic fauna includes the first reported Gondwana occurrences of so many forms is probably because very few Early Jurassic faunas are known from any of the southern continents.

AAPG Search and Discovery Article #90948©1996-1997 AAPG Distinguished Lecturers