Late Paleozoic tetrapods in eastern Pangaea: when and how did they get there?

The distribution and dispersal patterns of Late Paleozoic tetrapods remain poorly constrained. Diverse assemblages of Carboniferous and Permian tetrapods are well documented across western Pangaea, whereas in eastern Pangaea (east of the Ural seaway), tetrapods only become abundant from the mid-Permian onwards. It is unclear whether this discrepancy reflects imprecise chronostratigraphy or a genuine absence of Carboniferous–Early Permian tetrapods in eastern Pangaea. Recently proposed vicariance models of tetrapod evolution incorporate several questionable datasets, and existing palaeogeographic reconstructions of western–eastern Pangaean connections remain controversial, requiring improved constraints and documentation. We report a CA–IDTIMS radioisotopic age of 292.14 ± 0.14 Ma (Sakmarian) from one of the earliest tetrapods (discosauriscid) in the East Pangaea localities in Tajikistan, that closely resembles discosauriscid assemblages from Kazakhstan. This age supports the recently proposed model for the development of the Precaspian Isthmus during the Asselian–Sakmarian transition, interpreted as the first terrestrial corridor enabling tetrapod dispersal from western to eastern Pangaea. Integrated analyses of regional paleogeography, tectonics, palaeoclimate, palaeobiogeography, palaeofacies, and heavy-mineral provenance indicate that tetrapod migration pathways evolved through time: the initial eastward dispersal occurred via the Precaspian Isthmus during the Asselian–Sakmarian transition, whereas subsequent migrations (late Kungurian, late Roadian, Wuchiapingian, and Changhsingian) proceeded both directly eastward from the East European Platform and through recurrent connections across the Precaspian corridor.