Since the discovery of the first Triassic terrestrial vertebrate fossil from Antarctica in 1967, three significant Mesozoic faunas have been identified and described from the southern Transantarctic Mountains. Early Triassic vertebrates occur at numerous localities in exposures near the Beardmore and Shackleton Glaciers. An early Middle Triassic assemblage has been described from two localities in the Gordon Valley discovered in 1985-86 and more recently (2003) from Fremouw Peak, both of these sites are in the Beardmore Glacier region. Finally, an Early Jurassic dinosaur fauna was found in 1990-91 on Mt. Kirkpatrick near the Beardmore Glacier.
In 1999 a short reconnaissance expedition to Southern Victoria Land near McMurdo Station led to the discovery of a few bone fragments from the Upper Triassic Lashly Formation at Mt. Dearborn. Consequently, there are now terrestrial vertebrates of four different ages from the Transantarctic Mountains. However, exposures of Upper Triassic rocks in Southern Victoria Land are not extensive and hence do not have much potential for producing significant collections of fossil vertebrates.
Vertebrate faunas of the Fremouw and Hanson Formations
The lower Fremouw Formation has produced a diverse vertebrate fauna from numerous localities near the Shackleton and Beardmore Glaciers. The bones occur at numerous levels in both fine-grained siltstones/mudstones and coarser channel sandstones to conglomerates. This fauna has been the most extensively collected and studied of the three well known terrestrial vertebrate faunas from the southern Transantarctic Mountains. This fauna includes the therapsids Lystrosaurus murrayi, Lystrosaurus curvatus, Lystrosaurus mcCaigi, Myosaurus gracilis, Thrinaxodon liorhinus, Eriolacerta parva, Pedaeosaurus parvus and Rhigosaurus glacials (figure 1). It also includes the prolacertid Prolacerta broomi, the procolophonid Procolophon trigoniceps, the temnospondyls Austrobrachyops jenseni, Cryobatrachus kitchingi and an indeterminate rhytidosteid. Fragmentary postcranial material of a large primitive rauisuchid has also been described from this unit.
Figure 1: Lystrosaurus © William Stout.
Vertebrates from the Upper Fremouw have been found at only three localities in the Beardmore Glacier area. Two of the localities were discovered in 1985-86 and occur along the Gordon Valley. During the recent 2003 field season a third site was found on Fremouw Peak. Most of the bones from the upper Fremouw occur within a conglomerate at the top of an 8 meter thick quartzose sandstone that forms a prominent platform for several kilometers along Gordon Valley. This deposit has been interpreted as being the result of a high viscosity, unchannelized flow that spread from a large channel across the floodplain transporting bones, wood and large clasts. This fauna shares a few taxa with the Cynognathus Zone of South Africa, including Cynognathus and a large kannemeyeriid. It has also produced a more derived cynognathid and a gomphodont that shares some features with the Middle Triassic traversodontids. Because of the fragmentary nature of these specimens it is uncertain whether or not they represent new genera. Two large capitosaurs occur with the synapsids from the Gordon Valley localities. The mixed nature of this fauna makes the age unclear, it could be slightly younger than the Cynognathus Zone of South Africa. During the 2003 season another large fragmentary capitosaur skull was collected at Fremouw Peak.
The first vertebrates from the Early Jurassic Hanson Formation (former the upper Falla Formation) were originally collected during the 1990-91 austral summer from a siliceous siltstone on Mt. Kirkpatrick in the Beardmore Glacier region (Hammer et al. 1994). During the 2003 season a field team returned and collected more material from the original site (figure 2). In addition a second locality was discovered about 30 meters higher in the section on Mt. Kirkpatrick.
Figure 2: Philip Currie and William Hammer excavating the remains of Cryolophosaurus in the Hanson Formation on Mt. Kirkpatrick.
To date the most complete specimen from this assemblage is the partial skeleton of the crested theropod Cryolophosaurus ellioti (figure 3). More fragmentary material belonging to a plateosaurid prosauropod, scavenging theropods and a Bienotheroides clade tritylodont have also been studied and described. A dimorphodontid pterosaur humerus is currently under study. Some of the material from the 2003 season is still under preparation; however, elements recently identified from the new Kirkpatrick site include the ilium and two articulated sacral vertebrae and ribs possibly from a true sauropod.
Figure 3: The crested theropod Cryolophosaurus ellioti © William Stout
The overlapping ages established for plateosaurs and Bienotheroides clade tritylodonts from other continents establish an Early Jurassic (Sinemurian-Pliensbachian) age for the Hanson fauna . Skull features of Cryolophosaurus indicate that it is a tetanuran theropod related to the Late Jurassic to Cretaceous tetanurans known mainly from the northern continents. However, much of the postcranial material retains plesiomorphic characters of more primitive theropods such as dilophosaurs. Since Cryolophosaurus is tens of millions of years older than other known tetanurans it is not surprising that it retains some primitive features.
Excavating on Mt. Kirkpatrick