New Paleogene Fossils from the Western USA Document a New Paleocene Species and Temporal Range Extension into the Eocene for the Sirenid Salamander Habrosaurus
DEMAR, David G, Jr, National Museum of Natural History, Smithsonian Institution, Washington, D.C., USA; GARDNER, James D, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada
Habrosaurus is the oldest unequivocal, named genus of the paedomorphic eel-like salamander family Sirenidae and has a documented temporal record spanning the latter half of the Late Cretaceous and into the early Paleogene. Two named and one indeterminate species are known only from the Western Interior of North America, with largely non-overlapping temporal ranges. These include the nominal species H. dilatus (late Maastrichtian–middle Paleocene), H. prodilatus (middle Campanian–late Maastrichtian), and an indeterminate species of Habrosaurus (Santonian) identified on the basis of non-species-diagnostic atlantal centra. Differences in overall dental morphology, crown form, and degree and position of wear facets of the marginal teeth separate the two named species: H. prodilatus has chisel-like teeth exhibiting minimal wear, whereas H. dilatus has stout teeth with bulbous crowns that often show extensive occlusal wear. Here we report a new species of Habrosaurus from the lower Paleocene Tullock Formation, northeastern Montana, USA, represented by an incomplete left dentary with a unique dentition. The three preserved teeth are pristine and resemble those of H. dilatus and H. prodilatus in being non-pedicellate and having a constricted neck between the crown and shaft. However, the crowns principally differ from those of the named species as follows: i) incipiently mesiodistally tricuspid and labiolingually compressed, with a shallow, diamond-shaped lingual “basin” and a broadly rounded labial face; ii) covered in a thin distinct layer of semi-translucent enamel, with a sharp crest that extends mesiodistally across the occlusal surface; iii) mesially and distally, the crest extends basally a short distance onto the lingual face of the crown; and iv) wear facets poorly developed and restricted to the occlusal crest. Differences in the marginal dentition of these three diagnosable species of Habrosaurus imply successive shifts in feeding strategies from the Campanian to Maastrichtian and across the Cretaceous-Paleogene boundary, with the latter possibly being an ecomorphological response to the end-Cretaceous mass extinction.
Additionally, discovery of an atlantal centrum diagnostic of Habrosaurus from the Bridger Formation, southwestern Wyoming, USA, extends the known temporal range of the genus from the middle Paleocene forward into the middle Eocene. Thus, Habrosaurus overlapped in time and space with Siren dunni, the geologically oldest species of a modern sirenid genus.