(late Early Triassic) vertebrate fauna from Antarctica
The Cynognathus zone (late Early Triassic) vertebrate fauna from Antarctica WILLIAM
R. HAMMER
Department of Geology Augustana College Rock Island, Illinois 61201
During the last 2 weeks of the 1985-1986 austral field season, a new fauna of Late Scythian (late Early Triassic) age temnospondyl amphibians and therapsid (mammal-like) reptiles was collected from the Gordon Valley in the Beardmore Glacier region of the central Transantarctic Mountains (Hammer et al. 1986; Hammer, Ryan, and DeFauw 1987; Hammer; Collinson, and Ryan in preparation). The fossils occur in an 8-meter thick quartzitic sandstone near the base of the upper Fremouw Formation (Collinson and Isbell 1986; Hammer et al. in prepara tion). Of the nearly 50 specimens recovered from this assemblage, approximately half are identifiable to genus. To date, six genera occur in this fauna, three therapsids and three temnospondyls. The temnospondyl material, however, is more abundant and better preserved than the reptilian material. In fact, two of the three reptiles are represented by single incomplete cranial fragments. This bias is apparently taphonomic, since the fossils occur in a channel sand. The more highly aquatic temnospondyl amphibians were living in the channels while the terrestrial therapsids were on the floodplains. Thus, the reptilian material required transportation into the drainage system as large clasts while the depositional environment equaled the life environment for the amphibians. Two of the therapsids are closely related to South African forms. Age equivalence with the South African Cynognathus zone (Kannemeyeria zone, according to Keyser and Smith 1978), has been established by the presence of the South African cynodont (carnivorous therapsid) Cynognathus sp. (figure) and an incertae sedis member of the dicynodont (herbivorous therapsid) family Kannemeyeriidae (Hammer et al. in preparation). The third theriodont belongs to a new gomphodont cynodont (herbivorous cynodont therapsid) genus. It shows the typical, transversely widened postcanine teeth of a gomphodont, and
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it has a large canine and diastema (gap in tooth row behind the canines). The temnospondyls are all new genera. Representatives of the families Capitosauridae and Benthosuchidae occur, plus a third form which shows some characteristics of both families. These two groups are part of the same evolutionary complex, hence both are classified in the superfamily Capitosauroidea. The three antarctic representatives of this group apparently evolved as endemic forms on that continent. It is of particular interest that a benthosuchid occurs, since this family is not found in Africa and is very rare in the Triassic of Gondwanaland in general. It is possible that the more derived capitosaurs and the few benthosuchids from the other southern continents might be the result of an evolutionary radiation from the antarctic assemblage. Overall the temnospondyls in the fauna are more endemic than the reptiles, since they all represent new genera while at least two of the therapsids show strong affinities to South African forms. This is probably due to differing lifestyles, the more terrestrial reptiles could migrate freely over low topographic barriers that might have existed between Antarctica and Southern Africa during the Triassic, while the aquatic temnospondyls would be more restricted by drainage divides of any type. This research is supported by National Science Foundation grants DPP 86-11334 and DPP 86-14140.
References Collinson, J.W., and J.L. Isbell. 1986. Permian-Triassic sedimentology of the Beardmore Glacier region. Antarctic Journal of the U.S., 21(5), 29-30. Hammer, W.R., S.L. DeFauw, W.J. Ryan, and J.W. Tamplin. 1986. New vertebrates from the Fremouw Formation (Triassic), Beardmore Glacier Region, Antarctica. Antarctic Journal of the U.S., 21(5), 24-26. Hammer, W.R., W.J. Ryan, and S.L. DeFauw. 1987. Comments on the vertebrate fauna from the Fremouw Formation (Triassic), Beardmore Glacier Region, Antarctica. Antarctic Journal of the U.S., 22(5), 32-33. Hammer, W.R., J.W. Collinson and W.J. Ryan. In preparation. A new vertebrate fauna from Antarctica and its depositional setting. Keyser, A.W., and R.M.H. Smith. 1978. Vertebrate biozonation of the Beaufort Group with special reference to the western Karoo Basin. Annals of the Geological Survey of South Africa, 12, 1-35.
ANTARCTIC JOURNAL
Lateral view of Cynognathus sp. mandible from Gordon Valley.
988 REVIEW
11
R. HAMMER
Department of Geology Augustana College Rock Island, Illinois 61201
During the last 2 weeks of the 1985-1986 austral field season, a new fauna of Late Scythian (late Early Triassic) age temnospondyl amphibians and therapsid (mammal-like) reptiles was collected from the Gordon Valley in the Beardmore Glacier region of the central Transantarctic Mountains (Hammer et al. 1986; Hammer, Ryan, and DeFauw 1987; Hammer; Collinson, and Ryan in preparation). The fossils occur in an 8-meter thick quartzitic sandstone near the base of the upper Fremouw Formation (Collinson and Isbell 1986; Hammer et al. in prepara tion). Of the nearly 50 specimens recovered from this assemblage, approximately half are identifiable to genus. To date, six genera occur in this fauna, three therapsids and three temnospondyls. The temnospondyl material, however, is more abundant and better preserved than the reptilian material. In fact, two of the three reptiles are represented by single incomplete cranial fragments. This bias is apparently taphonomic, since the fossils occur in a channel sand. The more highly aquatic temnospondyl amphibians were living in the channels while the terrestrial therapsids were on the floodplains. Thus, the reptilian material required transportation into the drainage system as large clasts while the depositional environment equaled the life environment for the amphibians. Two of the therapsids are closely related to South African forms. Age equivalence with the South African Cynognathus zone (Kannemeyeria zone, according to Keyser and Smith 1978), has been established by the presence of the South African cynodont (carnivorous therapsid) Cynognathus sp. (figure) and an incertae sedis member of the dicynodont (herbivorous therapsid) family Kannemeyeriidae (Hammer et al. in preparation). The third theriodont belongs to a new gomphodont cynodont (herbivorous cynodont therapsid) genus. It shows the typical, transversely widened postcanine teeth of a gomphodont, and
10
it has a large canine and diastema (gap in tooth row behind the canines). The temnospondyls are all new genera. Representatives of the families Capitosauridae and Benthosuchidae occur, plus a third form which shows some characteristics of both families. These two groups are part of the same evolutionary complex, hence both are classified in the superfamily Capitosauroidea. The three antarctic representatives of this group apparently evolved as endemic forms on that continent. It is of particular interest that a benthosuchid occurs, since this family is not found in Africa and is very rare in the Triassic of Gondwanaland in general. It is possible that the more derived capitosaurs and the few benthosuchids from the other southern continents might be the result of an evolutionary radiation from the antarctic assemblage. Overall the temnospondyls in the fauna are more endemic than the reptiles, since they all represent new genera while at least two of the therapsids show strong affinities to South African forms. This is probably due to differing lifestyles, the more terrestrial reptiles could migrate freely over low topographic barriers that might have existed between Antarctica and Southern Africa during the Triassic, while the aquatic temnospondyls would be more restricted by drainage divides of any type. This research is supported by National Science Foundation grants DPP 86-11334 and DPP 86-14140.
References Collinson, J.W., and J.L. Isbell. 1986. Permian-Triassic sedimentology of the Beardmore Glacier region. Antarctic Journal of the U.S., 21(5), 29-30. Hammer, W.R., S.L. DeFauw, W.J. Ryan, and J.W. Tamplin. 1986. New vertebrates from the Fremouw Formation (Triassic), Beardmore Glacier Region, Antarctica. Antarctic Journal of the U.S., 21(5), 24-26. Hammer, W.R., W.J. Ryan, and S.L. DeFauw. 1987. Comments on the vertebrate fauna from the Fremouw Formation (Triassic), Beardmore Glacier Region, Antarctica. Antarctic Journal of the U.S., 22(5), 32-33. Hammer, W.R., J.W. Collinson and W.J. Ryan. In preparation. A new vertebrate fauna from Antarctica and its depositional setting. Keyser, A.W., and R.M.H. Smith. 1978. Vertebrate biozonation of the Beaufort Group with special reference to the western Karoo Basin. Annals of the Geological Survey of South Africa, 12, 1-35.
ANTARCTIC JOURNAL
Lateral view of Cynognathus sp. mandible from Gordon Valley.
988 REVIEW
11
