THE FOSSIL RECORD AND PALEOECOLOGICAL SIGNIFICANCE OF NORTH AMERICAN AMPHISBAENIANS AND THEIR BURROWS: THE INTERACTION OF FOSSORIAL REPTILES WITH THE SUBSTRATE

The Amphisbaenia are an extant order of elongate, limbless, fossorial reptiles that inhabit equatorial regions. The fossil record of the North American Amphisbaenia extends from the Paleocene to the Pleistocene, with the greatest abundance in the Eocene to Oligocene continental deposits of northern Colorado, western Nebraska, southern South Dakota and Wyoming. The trace fossil record of these burrowing reptiles, however, is unknown.

The deposits of the Eocene Bridger and Wind River formations of Wyoming and the Oligocene White River Formation of northeastern Colorado and Wyoming contain nearly complete amphisbaenian fossils consisting of articulated cranial and postcranial skeletons. While many of these fossils occur in siltstones and conglomerates interpreted as channel deposits, the more complete fossils occur in laterally equivalent calcareous paleosols. The cranial morphology of these fossil amphisbaenians is characterized by a large, horizontally flattened, spade-shaped snout closely resembling that of the extant North American species, Rhineura floridana. These extant amphisbaenians use their spade-shaped head to excavate semi-permanent to permanent burrow complexes within dense soils. These tunnels are occupied and maintained by the amphisbaenian its entire life and used for dwelling, hunting, and breeding. Despite the complete nature and abundance of the amphisbaenian fossils collected from Colorado and Wyoming, their preserved burrows have not been previously documented. Casting and studying the burrow networks of extant taxa facilitates the recognition of fossil amphisbaenian burrows in Tertiary deposits. The knowledge of the morphology of these trace fossil may also lead to the extension and expansion of the stratigraphic and geographic range of amphisbaenians.

The burrow architecture of amphisbaenians can be used to infer the paleoenvironmental conditions of their habitats. The physiological and behavioral responses of extant amphisbaenians to variations in temperature, moisture, and other environmental factors have been studied in the laboratory. The resulting morphology of their burrow networks also varies in response to these changes. The morphology of the fossil burrows, when compared to those of extant taxa, then provides insight into paleoenvironmental conditions.