Sunday, December 16, 2012

No evidence for erycine boas in North America before the Miocene

Extant erycine boas: An African  Exyx colubrinus, and the North American  Lichanura, and Charina
The central Rocky Mountain Interior has long been a focus of study for late Eocene early Oligocene fossils in North America. The Medicine Pole Hills of North Dakota has preserved abundant late Eocene fossils which provide a glimpse of the central North American fauna prior to the earliest Oligocene cooling. In a forthcoming article, Smith (2012) reports on the fossil 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 clade Ungaliophiinae, but the history of this clade may stretch considerably further back. The loxocemid, Ogmophis compactus, is the second reported fossil from 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, implying that it was arboreal, but this is not yet clear. Full-column analysis and cranial elements prove crucial for the accurate higher-level identification of snake clades from which these isolated elements derive.

This paper is of most interest because it clarifies the previous idea that Erycinae boas were common in the Paleogene of North America. Today in North America erycines are represented by the rosy boas (Lichanura) and the rubber boas (Charina). In the past many authors assigned most North American Paleogene snake taxa to the Erycinae (sand boas) Smith reports that there is no well-founded record of an erycine boa in North America prior to the Miocene. The mistake apparently resulted from the similarity of mid-body vertebrae in the extant North American Erycinae and the Paleogene snakes. Thus the Eocene North American snake fauna had small booid snakes, but they were in fact members of the loxocemid and ungaliophid clades, not the Eastern Hemisphere sand boas, the Erycinae.

Citation
Smith, K. T. 2012. 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. Zoologischer Anzeiger - A Journal of Comparative Zoology, http://dx.doi.org/10.1016/j.jcz.2012.05.006

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