UNDERSTANDING ANURAN BURROWS: NEOICHNOLOGY OF THE EASTERN SPADEFOOT TOAD, SCAPHIOPUS HOLBROOKI

The eastern spadefoot toad, Scaphiopus holbrooki, is a burrowing, terrestrial toad (Anura:Scaphiopodidae) that inhabits the southeastern United States. Scaphiopus holbrooki construct burrows to avoid extremes in temperature and moisture conditions. While they are prolific burrowers, the morphology of the burrows of spadefoot toads has not been well documented. Anurans first appear in the fossil record in the Lower Jurassic. Body fossils of aquatic anurans are found in the Jurassic and Early Cretaceous, however, the early fossil record of terrestrial anurans is less well known. The record of anuran fossil burrows is even more limited. This limited record may be a due to an inability to distinguish fossil toad burrows from those of other animals. This study describes the morphology of S. holbrooki burrows using controlled laboratory experiments under varied environmental conditions. Four individuals were housed in sediment-filled terrariums for intervals of 10 to 14 days. The toads were removed from the terrariums at the end of each experiment and the burrows were cast with plaster. Burrow casts were described both qualitatively and quantitatively. Maximum depth, total length, slope, width, height, circumference, and width-to-height ratio were measured from the casts and the scale independent values complexity and tortuosity were calculated. The burrows consisted of a single vertical to subvertical, cylindrical shaft leading to an ovoid chamber similar in size to the toad. The toads moved a minimal amount of sediment to the surface, but did backfill portions of the burrow shaft. Individual toads produced multiple burrows in a single experiment. Qualitative burrow morphology, however, was consistent. Using nonparametric statistical methods the quantitative properties of the toad burrows were compared to each other and to burrows of other animals produced in previously conducted experiments including scorpions, spiders, salamanders, and skinks. The properties of the toad burrows were found to be similar, but different from those of other animals. The results of this study will aid in the recognition of fossil toad burrows by providing an analog for comparison. It also demonstrates that toad burrows are distinct from burrows of other animals and can be differentiated using morphological properties.