New early permian tetrapod fauna from Namibia

Abstract

The earliest tetrapods in Western Gondwana are the mesosaurs, a group of specializedendemic aquatic parareptiles known only from Artinskian strata of Namibia, South Africa,Uruguay, and Brazil. Recent finds from early Permian beds in northern Brazil revealed a newfreshwater fauna from tropical Gondwana. However, it was not until the Guadalupian (middlePermian) that diverse temnospondyl amphibian and amniote faunas became widespreadacross south-central Gondwana. A new source of evidence lies in the Carboniferous-Permiancontinental strata from the Huab Basin (Tsarabis, Huab and Gai-As formations) ofnorthwestern Namibia. These strata have yielded scattered tetrapod remains (temnospondyls,Mesosaurus) reported several years ago. The Gai-As Formation was deposited in a freshwaterrift valley lake at approximately 60°S palaeolatitude. The lake opened westwards into theParana Basin and eastwards it was closed off by river deltas. It is in this transition betweenaquatic and terrestrial environments that most of the fossils are preserved. To date, we havecollected 75 fossils of mainly fishes (actinopterygian and chondrichthyan) and temnospondylamphibians from a single lower Gai-As locality just above the Mesosaurus-bearing levels.Absolute zircon dates from ash beds in the upper Gai-As (265.5±2.2 Ma), and the underlyingMesosaurus-bearing beds (270±1 Ma) constrain the new fauna to the Roadian. At least threedifferent temnospondyl taxa are represented by partial remains of a long-snouted form, and ashort-snouted (parabolic) one with a highly vaulted skull-roof. The third taxon, whichincludes near-complete large (3 m-long) skeletons, presents character states that place it inthe edopoid clade, a basal temnospondyl group only previously known from theCarboniferous-early Permian of Euroamerica and the uppermost Permian of Niger. Theseinclude exclusion of the vomers and palatines from the interpterygoid vacuities, whichthemselves are relatively small and taper strongly anteriorly, and the presence ofintertemporal ossification in the skull table. This new Namibian fauna that lived in andaround saline to brackish water lakes and fjords, helps to fill a crucial ca. 10 Ma gap in thefossil record of tetrapods in the Southern Hemisphere and presents a previously unknownradiation of Laurasian lineages into the region during the Rodian.