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Thrinaxodon from Graphite Peak, Central Transantarctic Mountains

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Thrinaxodon from Graphite Peak, Central Transantarctic Mountains Thrinaxodon from Graphite Peak, Kenyon in the Cumulus Hills near the Shackleton Glacier, while the other two were from Graphite Peak. central Transantarctic Mountains, The Thrinaxodon maxilla (AMNH number 26101) from Antarctica Graphite Peak under consideration here is a negative impres- sion in very well cemented, siliceous siltstone. A positive latex peel was made for study purposes. The specimen shows the four right incisors typical of galesaurids plus one from the left WILLIAM R. HAMMER side. The right canine and five postcanine teeth are also pre- served. The postcanines show the typical tricuspid pattern of Department of Geology Thrinaxodon liorhinus, with a large central cusp flanked by an- Augtistana College terior and posterior cingulum cusps. The tooth bearing portion Rock Island, Illinois 61201 of the maxilla measures 4 centimeters in length. Posterior to this section the specimen consists of a poorly exposed thin edge of bone that extends for another 3 centimeters. Only the During the 1985-1986 austral summer, vertebrate specimens ventral edge of the right mandible is exposed, because its dor- were collected from both the late Early to early Middle Triassic sal portion is overlapped by the maxilla. upper Fremouw Formation (Hammer 1990; Hammer, Collin- Overall the specimen shows no features that would make it son, and Ryan 1990) and the Early Scythian (earliest Triassic) a new species of Thrinaxodon. It is, however, very fragmentary, lower Fremouw Formation (Hammer et al. 1986; Hammer 1989) and hence, it is only questionably referred to Thrinaxodon lior- in the Beardmore Glacier region. The primary vertebrate lo- hinus. calities in the lower member of the formation in this area were This research was supported by National Science Founda- Coalsack Bluff and Graphite Peak (see Hammer 1989; Hammer tion grants DPP 85-11334, DPP 86-14140, and DPP 84-18428 to 1990). The Graphite Peak material consists mainly of disarti- W.R. Hammer. The reconstruction was done by William J. culated to partially articulated specimens of Lystrosaurus mur- Ryan, III. rayi but includes a single right maxilla and mandible of Thrinaxodon (see reconstruction in the figure). Thrinaxodon is a small, carnivorous cynodont (Therapsida) AMNH denotes American Museum of Natural History. that shows a mixture of advanced and primitive cynodont features. It belongs to the family Galesauridae, and like other galesaurids, it has a reduced number of incisors and postcan- ines compared to more primitive cynodonts. A single species of the genus, Thrinaxodon liorhinus Seeley (1894) has been de- References scribed from both the basal Triassic Lystrosaurus Zone of South Colbert, E.H. and J.W. Kitching. 1977. Triassic cynodont reptiles from Africa and the lower Fremouw Formation of Antarctica (Col- Antarctica. American Museum Novitates, 2,611, 1-30. bert and Kitching 1977). Of the fourteen antarctic specimens Hammer, W.R. 1989. Lystrosaurus Zone (Triassic) vertebrates from the referred to Thrinaxodon by Colbert and Kitching (1977), 12 were Beardmore Glacier region, Antarctica. Antarctic Journal of the U.S., from a single locality, Thrinaxodon Col, on the flank of Mount 24(5), 40-41. Reconstruction of Thrinaxodon liorhinus. 1990 REVIEW 37 Hammer, W.R. 1990. Triassic terrestrial vertebrate faunas of Antarc- New vertebrates from the Fremouw formation (Triassic) Beard- tica. In T.N. Taylor and E.L. Taylor (Eds.), Antarctic paleobiology. more Glacier region, Antarctica. Antarctic Journal of the U.S., 21(5), New York: Springer-Verlag. 24-26. Hammer, W.R., J.W. Collinson, and W.J. Ryan, III. 1990. A new Trias- Seeley, H.G. 1894. Researches on the structure, organization and clas- sic vertebrate fauna from Antarctica and its depositional setting. sification of the fossil Reptilia. Part IX, section 1: On the Therosuchia. Antarctic Science, 2(2), 163-167. Philosophical Transactions of the Royal Society of London, series B, 185, Hammer, W.R., W.J. Ryan, J.W. Tamplin, and S.L. DeFauw. 1986. 987-1,018. Field collection for apatite fission track 85°W analysis, Ellsworth Mountains EDMUND STUMP and PAUL C. FITZGERALD Department of Geology Arizona State University Tempe, Arizona 85287-1404 We collected rock samples for fission track analysis in the Ellsworth Mountains during the 1989-1990 austral summer to provide an understanding of the tectonic history of this crustal block. In broader terms, the study will attempt to relate the Figure 2 uplift history of the Ellsworth Mountains to the tectonic evo- lution of West Antarctica. Because the put-in date was delayed, we had to abbreviate the original field plan, which included collections in the Her- ,-Nunatak .690± itage Range (figure 1). We were put in on 21 November 1989 Bastien Hills 5 kilometers west of Mount Gardner in the central Sentinel 79°S Minisa (aCU " Range (figure 2). Bowers Corner On this expedition, all samples were collected from quartz- ites of the Crashsite Group, mainly from the Howard Nunataks Formation (Craddock et al. 1986). Sample size was typically about 5 kilograms. Over the span of 4 days, we collected a "vertical" profile of 26 samples from the Vinson Massif covering 2,500 meters of elp relief from the summit (4,897 meters) to the head of Nimitz c3 Glacier (1,400 meters). On the first day, we covered the lower ridge line from the terrace to the west of Vinson Massif down to Nimitz Glacier, with access by snowmobile from the west side of the glacier (figure 2). On the second day, we covered the western ridge line of the Vinson Massif from about the 2,900-meter level down to the terrace at the foot of the massif. Access was by snowmobile down a drift on the north of the 100 km glacier draining the face of Vinson Massif. On the third day, 1 we drove by snowmobile to the head of this glacier and climbed to the 3,800-meter level of the summit plateau, placing a tent and collecting down from 4,000 meters to the snowmobiles. On the fourth day (30 November), we drove to the head of Figure 1. Location map, Ellsworth Mountains. (km denotes kilo- the glacier, climbed to the tent, where we rested and cooked meter.) a meal, and then continued to the summit. From there, we sampled down to the previously collected level. The summit A 400-meter profile (5 samples) was collected at Bowers Corner day took 18 hours, basecamp to basecamp. extending the Vinson profile to the lowest elevation in the Subsequently, the party traversed down Nimitz Glacier to Sentinel Range (690 meters). From there, collections were made Bowers Corner (figure 1) collecting on both sides of the glacier. on the northern side of Minnesota Glacier from Nunatak 690 38 ANTARCTIC JOURNAL.
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