Evolution and palaeoecology of crocodiles in the mesozoic of Khorat plateau, Thailand

Abstract

The Khorat Plateau has yielded so far nine species in six genera of Mesozoic crocodilians, which belong to three difference families. Theriosuchus from the Phu Kradung and Sao Khua Formations, and a Teleosaurid, cf. Peipehsuchus from the Phu Kradung Formation, were recognized as new records in the Khorat Plateau. Siamosuchus phuphokensis from the Sao Khua Formation and Khoratosuchus jintasakulii from the Khok Kruat Formation were erected as new genera and species, whereas Theriosuchus sp. A from the Sao Khua Formation, "Goniopholis" sp. A and "Goniopholis" sp. B both from the Khok Kruat Formation possess unique characters, which are sufficient to recognize these taxa as new species, but they are in need of more preparation before they can be properly described. "Sunosuchus" thailandicus Buffetaut and Ingavat, 1980 and "Goniopholis" phuwiangensis Buffetaut and Ingavat, 1983 were considered as uncertain genera, based on insufficient diagnostic characters. The discovery of a Theriosuchus skull from the Phu Kradung Formation indicates that the distribution of this dwarf Atoposauridae was widespread from Europe through China and Southeast Asia and also suggests that Europe and Southeast Asia has already been connected at that time, but the exact time of the connection between these continents is not exactly known. For "Sunosuchus" and cf. Peipehsuchus, the phylogenetic positions of these taxa are still unresolved, because the former lacks the complete upper jaw, while the latter was described on the basis of isolated teeth only. These two taxa thus cannot be used to decipher the relationship of the faunas between Southeast Asia and China. The disappearance of long-snouted piscivorous crocodilians in the Sao Khua Formation may be caused by a palaeoclimate change from humid environment in the Late Jurassic to semi-arid environment in the Early Cretaceous. Ecological niche of the long-snouted crocodilians seems to be replaced by a fish-eating dinosaur Siamosaurus suteethorni, which is unknown before the deposition of the Sao Khua Formation This hypothesis is supported by the similarity of the enamel microstructure of S.suteethorni and the Mesozoic crocodilians. The broad, short-snouted crocodilians were still diverse in this formation, probably because of their more diverse diet. The Thai "Goniopholis" from the Khok Kruat Formation shows the existence of this form throughout the Early Cretaceous Khorat Plateau, but its phylogenetic affinity is still ambiguous. K. jintasakulii, also described from this formation, is the most advanced elongate-snouted Thai neosuchian, based on the position of the internal choanae, which is almost enclosed by the pterygoid.