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.T H E Albanerpetontidae are salamander-like, probable lissamphibians characterized by apomorphies such as a mortise and tenon intermandibular joint, fused frontals and highly modified first three vertebrae (Fox and Naylor 1982;Milner 1988; McGowan and Evans 1995; McGowan 1998).Albanerpetontids are known by isolated bones and rare skeletons from the Middle Jurassic-Miocene of Laurasia (Estes and Hoffstetter 1976; Estes 1981;Fox and Naylor 1982;Nessov 1988 Nessov , 1997;;Evans and Milner 1994;Milner 1994; McGowan and Evans 1995; Gardner and Averianov 1998) and have recently been reported from the Lower Cretaceous of North Africa (Broschinski and Sigogneau-Russell 1996, p. 149).Two albanerpetontid genera are presently recognized (McGowan and Evans 1995; Gardner and Averianov 1998) based on differences in the construction and contacts of the fused frontals (McGowan and Evans 1995; Gardner in press): the type genus Albanerpeton Estes and Hoffstetter, 1976 (sensu McGowan and Evans 1995), from the Aptian-Paleocene of western North America and Miocene of France (Estes and Hoffstetter 1976; Estes 1981;Fox and Naylor 1982;Gardner 1996); and Celtedens McGowan and Evans, 1995, an exclusively European genus that has been reported from the Bathonian-Albian (McGowan and Evans 1995; McGowan 1996; McGowan and Ensom 1997).Albanerpeton arthridion Fox and Naylor, 1982 is of interest because it is the geologically oldest member of the type genus and the oldest diagnosable North American albanerpetontid species.Fox and Naylor (1982) described A. arthridion based on the incomplete holotype premaxilla and three referred dentaries from outcrops of the upper Antlers Formation (early-mid Albian in age) exposed at Greenwood Canyon, Texas, USA.These four jaws, along with isolated trunk vertebrae and an atlas from the Antlers Formation, had earlier been referred by Estes (1969Estes ( , 1981) ) to Prosiren elinorae Goin and Auffenberg, 1958, a prosirenid salamander named on a trunk vertebra from the same formation.No other fossils have since been described for A. arthridion.In the absence of more diagnostic specimens, concerns have been raised about the validity of the species (McGowan 1994) and its generic affinities (Rage and Hossini in press).Herein I report on collections of cranial and postcranial bones of A. arthridion from the Antlers Formation of Texas and Oklahoma that provide critical new information about the osteology and relationships of this hitherto poorly known species.Material from Oklahoma also extends the range of A. arthridion down into the uppermost Aptian.I further report on paracontemporaneous albanerpetontid jaws from the Cloverly Formation of Wyoming and comment on the identity of a supposed Albanerpeton-