New constraints on the evolution of the snake clades Ungaliophiinae, Loxocemidae and Colubridae (Serpentes), with comments on the fossil history of erycine boids in North America

The Medicine Pole Hills of North Dakota, USA, afford an excellent view of an Eocene fauna in the Rocky Mountain interior prior to the climatic deterioration of the earliest Oligocene. I describe the snakes of this locality using 179 isolated vertebrae from all parts of the vertebral column as well as cranial elements. The assemblage comprises four species: (1) a primitive burrowing snake (“anilioid”); (2) a small boid related to Ungaliophiinae (dwarf boas); (3) a mid-sized booid related to Loxocemus (Mexican Burrowing Python); and (4) a colubrid. The dwarf boa, Calamagras weigeli, is conservatively regarded as the earliest secure representative of the total clade of Ungaliophiinae, but the history of this clade may stretch considerably further back. The loxocemid, Ogmophis compactus, is only the second reported fossil of that clade. The colubrid is one of the earliest known and could represent the first appearance of colubrine “racers” in North America; it may have had an elongate tail, but this is not yet statistically clear. Full-column analysis and cranial elements prove crucial for the accurate higher-level identification of snake clades from which these isolated elements derive. The past assignment of most North American Paleogene taxa to Erycinae (sand boas) is further undone in this work; there is no well-founded record of an erycine in North America prior to the Miocene.