EARLY LATE CRETACEOUS (CENOMANIAN) MAMMALS AND OTHER VERTEBRATES FROM THE MATA AMARILLA FORMATION OF SOUTHERN PATAGONIA (ARGENTINA)

MARTIN, T.; GOIN, F. J.; CHORNOGUBSKY, L.; GELFO, J.N.; SCHULTZ, J.

Congreso; SVP Meeting; 2013

Extensive screen-washing of 4.5 metric tons of matrix at the 3LAG0-locality near Tres Lagos, southwestern Santa Cruz Province, Patagonia has produced the southernmost known Mesozoic mammalian remains. The Mata Amarilla Formation of the Austral basin is represented in its lower part by shallow marginal marine to lagoonal mud-, silt- and fine-grained sandstones with intercalated tuff layers. Long considered to be of a Coniacian age, it has recently been radiometrically dated as middle Cenomanian. The 3LAG0-locality belongs to the upper part of the lower Mata Amarilla Formation with grey mudstones of an estuarine environment. The fossils occur in a bonebed of few centimeters thickness with abundant aquatic vertebrate remains such as hybodontiform and neoselachian teeth, holostean and teleostean scales and bones, dipnoan (Atlantoceratodus iheringi) tooth plates, pleurodiran turtle plates and bones, crocodile teeth, as well as much rarer terrestrial vertebrate remains such as questionable pterosaur teeth, and theropod and ?ornithopod teeth. The mammalian remains comprise a right lower molar of a stem dryolestidan ("paurodont") with two subequal roots, a disto-lingual portion of a right upper stem dryolestidan molar preserving stylocone, metacone and metastyle as well as an incomplete left lower docodont molar with a strongly enlarged and strongly buccocervically shifted cusp d. An additional docodont tooth fragment represents cusps X and Y of an upper molar, and a double-rooted single-cusped premolar is also attributed to docodonts. Although fragmentary, the mammalian teeth provide important information on Mesozoic mammalian evolution in South America as they bridge the gap between the Jurassic and the Late Cretaceous records on that continent. The lower stem dryolestidan ("paurodont") molar suggests close affinities to North American and European representatives. The docodont teeth are the first unambiguous record of that group in South America and represent their geologically youngest occurrence. The lower molar differs from all other known docodonts by its very large and bucco-lingually shifted cusp d which emphasizes the endemic South American mammalian evolution in the Cretaceous.