CYNODONTS FROM THE UPPERMOST BURGERSDORP FORMATION, SOUTH AFRICA, AND THEIR BEARING ON THE BIOSTRATIGRAPHY AND CORRELATION OF THE TRIASSIC CYNOGNATHUS ASSEMBLAGE ZONE

ABDALA, F.; HANCOX, P. J.; NEVELING, J.

JOURNAL OF VERTEBRATE PALEONTOLOGY

SOC VERTEBRATE PALEONTOLOGY

Lugar: Lawrence; Año: 2005 vol. 25 p. 192 - 199

0272-4634

The Burgersdorp Formation in the Beaufort Group of the South African Karoo Basin records a diverse and rich vertebrate fauna that is referred biostratigraphically to the Cynognathus Assemblage Zone. Recent collecting efforts, and taxonomic and biostratigraphic studies on this formation, resulted in a proposed subdivision of the Cynognathus Assemblage Zone into three subzones informally known as A, B, and C. Cynodont fossils constitute an important component of the fauna of the uppermost subzone C of the Cynognathus Assemblage Zone. As in the middle subzone B, cynognathids, diademodontids, and trirachodontids are represented in subzone C, although the latter group--instead of Diademodon--is the dominant taxon. The presence of Cynognathus in the faunas of the three subzones confirms that this genus is the correct choice as the index taxon of the assemblage zone. The large size attained by trirachodontid specimens from subzone C represents an unusual characteristic. One complete skull measured 160 mm in length, although fragmentary remains of other specimens indicate even larger sizes. These skull lengths are well above the 100 mm that characterize trirachodontids recovered from the older subzones A and B. The large size, and particularly the ovoid overall pattern of the upper gomphodont postcanines allows the assignment of the trirachodontid specimens from subzone C to Cricodon metabolus, previously known from the Manda Formation in Tanzania. The Cynognathus Assemblage Zone of South Africa is shown to be the most temporally complete Early-Middle Triassic nonmarine sequence in southern Gondwana, and is the template against which other sequences of similar age may be compared.