New bystrowianid chroniosuchians (Amphibia, Anthracosauromorpha) from the Triassic of Russia and diversification of Bystrowianidae

The first bystrowianids recorded in the Gamskian Horizon of the East European Platform, Vyushkoviana operta gen. et sp. nov. and Dromotectum abditum sp. nov. (Amphibia, Chroniosuchia), are described based on specimens from the Malaya Northern Dvina and Don river basins. The patterns of the dermal scute evolution observed in bystrowianids warrant the subdivision of the group into the subfamilies Bystrowianinae Vjuschkov 1957, Dromotectinae subfam. nov. and Axitectinae subfam. nov. The second and third show progressive reduction of the external (paraxial) zone of the interosteoderm articulation. New data supporting close proximity or phyletic succession of the chroniosuchid and bystrowianid types of the osteoderm design are revealed. The concept of the Chroniosuchidae as a paraphyletic group is rejected. Evolutionary significance of the key features distinguishing the bystrowianid subfamilies is analyzed.