Earliest triassic dicynodonts (Amniota, Therapsida) from Argentina: new data on the Permo-Triassic extintion and the “Lilliput effect”.

DOMNANOVICH, NADIA; MARSICANO, CLAUDIA

Teruel

Conferencia; 10th International Symposium on Mesozoic Terrestrial Ecosystems and Biota; 2009

A well-known phenomenon after a mass extinction is the occurrence of a survival fauna with generally much smaller body sizes than in the pre-extinction fauna. This temporary size reduction of the animals after an extinction event, called the “Lilliput effect”, was first recognized in marine invertebrate faunas, and has also been described for tetrapod faunas, after the Permian-Triassic extinction. In 1981 Bonaparte described three specimens of a new dicynodont (PVL 3963-3965) from the Quebrada de los Fósiles Formation (Puesto Viejo Group, Mendoza, Argentina), represented mainly by the preorbital regions of very large skulls. The specimens were originally identified as kannemeyeriids gen. et sp. indet., although subsequently in 1993 DeFauw assigned them to the Indian taxon Rechnisaurus cristarhynchus Roy-Chowdhury. However, a recent revision of the materials originally described by Bonaparte considered them as Kannemeyeriidae incertae sedis. The kannemeyeriids are a group of Triassic dicynodonts known, until now, only since upper Lower Triassic levels. Therefore, the presence of kannemeyeriids in the Quebrada de los Fosiles Formation of Argentina expands their temporal range into the lowest Triassic. This evidence supports recent phylogenetic analyses suggesting that multiple dicynodont lineages survived the End-Permian extinction and that fossils of large kannemeyeriiformes should be present in the latest Permian. The presence of large massive kannemeyeriids in the lowest Triassic contradicts the “Lilliput effect” described for dicynodont faunas after the Permo-Triasic extinction in other gondwanan areas. One possible explanation was recently proposed by Twitchett to explain the Lilliput effect: a temporary reduction in the biomass of higher trophic levels makes larger taxa rarer and thus less likely to enter the fossil record, whereas taxa that undergo a reduction in body size maintain their population densities and are more likely to be recorded as fossils.gen. et sp. indet., although subsequently in 1993 DeFauw assigned them to the Indian taxon Rechnisaurus cristarhynchus Roy-Chowdhury. However, a recent revision of the materials originally described by Bonaparte considered them as Kannemeyeriidae incertae sedis. The kannemeyeriids are a group of Triassic dicynodonts known, until now, only since upper Lower Triassic levels. Therefore, the presence of kannemeyeriids in the Quebrada de los Fosiles Formation of Argentina expands their temporal range into the lowest Triassic. This evidence supports recent phylogenetic analyses suggesting that multiple dicynodont lineages survived the End-Permian extinction and that fossils of large kannemeyeriiformes should be present in the latest Permian. The presence of large massive kannemeyeriids in the lowest Triassic contradicts the “Lilliput effect” described for dicynodont faunas after the Permo-Triasic extinction in other gondwanan areas. One possible explanation was recently proposed by Twitchett to explain the Lilliput effect: a temporary reduction in the biomass of higher trophic levels makes larger taxa rarer and thus less likely to enter the fossil record, whereas taxa that undergo a reduction in body size maintain their population densities and are more likely to be recorded as fossils.