Article: The amphibian Albanerpeton arthridion and the Aptian–Albian biogeography of albanerpetontids

Albanerpeton arthridion is rediagnosed and redescribed based on jaws, frontals, atlantes and humeri from the Lower Cretaceous (uppermost Aptian–middle Albian), Antlers Formation of Oklahoma and Texas, USA. Frontals described herein for the first time for A. arthridion confirm that the species belongs in the type genus Albanerpeton, making it the geologically oldest reported member of the genus. A. arthridion is interpreted as the most primitive species of Albanerpeton and to have been relatively small. Reduced body size is proposed to be derived and to have developed at least twice within the genus. A. arthridion and an indeterminate species reported herein, the latter known by fragmentary jaws from the paracontemporaneous Cloverly Formation, Wyoming, USA, are the geologically oldest records of unequivocal albanerpetontids in North America. These occurrences provide a minimum age of latest Aptian for the establishment of albanerpetontids on the continent and indicate that the Western Interior Seaway, which opened in the latest Albian, would not initially have prevented the movement of albanerpetontids across the continent.