BURROWS OF TERRESTRIAL ANURANS: NEOICHNOLOGY OF THE EASTERN SPADEFOOT TOAD, SCAPHIOPUS HOLBROOKI

Anurans first appear in the Early Jurassic, and although terrestrial anurans are considered to be more ancestral, their early record is less understood than aquatic anurans. Many extant terrestrial anurans produce burrows to escape unfavorable environmental conditions. The identification of fossil anuran burrows may, therefore, serve as a proxy for terrestrial anurans in the absence of body fossils.

The eastern spadefoot toad, Scaphiopus holbrooki (Anura: Schaphiopodidae) belongs to one of four groups of terrestrial anurans evolved to be highly specialized burrowers. This study describes the morphology of S. holbrooki burrows produced in laboratory experiments with varied sediment conditions. Four individuals were housed in sediment-filled terraria for 7 to 32 days. The toads were then removed and the open burrows were cast with plaster. Burrow casts were described both qualitatively and quantitatively including their depth, length, slope, width, height, circumference, width-to-height ratio, complexity, and tortuosity.

The toads used a hind-limb burrowing technique moving a minimal amount of sediment to the surface, yet portions of the burrow shaft were backfilled. Individual toads produced one to five burrows in a single experiment. Three distinct architectures were produced: isolated ovoid chambers, vertical shafts with ovoid chambers, and subvertical shafts with ovoid chambers. The shapes and sizes of the burrow chambers were similar those of the toad’s bodies. Surficial markings resembled the hind limbs and feet of the toads. Qualitative burrow morphology was consistent between individuals and experiments. Using nonparametric statistical analyses the quantitative properties of the toad burrows were compared to each other and to burrow casts of other animals including scorpions, spiders, salamanders, and skinks. Spadefoot toad burrows were found to be statistically similar to each other but different from those of other animals and could be differentiated using morphological properties.

The results of this study will aid in the identification of anuran burrows in the fossil record by providing an analog for comparison. Application of this data can improve the understanding of the evolution of terrestrial anurans, their behavior, and paleoenvironmental significance.