THE EARLY DIVERSIFICATION OF AMNIOTES ON LAND: THE ROLE OF ICHNOLOGY IN THE GONDWANAN CONUNDRUM

MARSICANO, C.A.; KRAPOVICKAS, V.

Mendoza

Congreso; IV International Palaeontological Congress; 2014

It has long been known that during the Permian a major transition occurred in terrestrial tetrapod faunas: the radiation of amniotes on land. Early Permian faunas, dominated by temnospondyl amphibians together with early amniotes, changed during the Middle-Late Permian with the onset of much diverse and complex terrestrial amniote faunas. These changes in diversity and occupancy of continental environments are fully documented by the well-known paleotropical Laurasian faunas. In contrast, little is known about the early history of amniotes in the Southern Hemisphere. Basically all the information available comes from Permian beds at several high latitude localities in southern Africa and South America, where some clades of terrestrial synapsids along with parareptiles are well documented as early as the Middle Permian (Wordian-Capitanian). Before that time, the evolutionary history of amniotes in Gondwana remains elusive, as their skeletal record is very scarce. The only exception is a group of aquatic parareptiles, the mesosaurids, known from Artinskian (Early Permian) levels from southwestern Gondwana (southern Africa, Uruguay, Brazil). The mesosaurids has recently proved to be the earliest viviparous reptiles, an evolutionary novelty related to their fully aquatic mode of life. In this context, our current knowledge of the Gondwanan amniote fossil record suggests their sudden appearance by the Early Permian in high latitudes represented by an endemic group of specialized aquatic parareptiles. This is followed by a gap of information of ca. 10 Ma until the Middle Permian, when amniotes suddenly became diversified and abundant in the region, occupying different terrestrial ecological roles. Nevertheless, if the known amniote ichnological record is considered, a substantial different scenario is depicted. In the last years, tetrapod footprints have been described from putative Lower Permian (Sakmarian-Artinskian) strata of Argentina (Patquia and Yacimiento Los Reyunos formations). In all cases, the footprints were preserved in sandstone beds formed in dune field environments (dune and interdune deposits) revealing the presence of different groups of small-to-medium sized desert dwellers. Accordingly, the combination of the ichnological and skeletal record suggests that amniotes were already widespread in the high latitudes of Gondwana by the beginning of the Permian with a complex ecological structure, including the colonization of deserts as well as the aquatic habitats. This new scenario also implies that amniotes evolved in Gondwana significantly earlier than previously thought, probably in coincidence with the establishment of amniote-dominated land habitats in the Northern Hemisphere.