Neoichnology of the eastern spadefoot toad, Scaphiopus holbrookii (Anura: Scaphiopodidae): criteria for recognizing anuran burrows in the fossil record

Abstract

Anurans first appear in the Early Jurassic, and although terrestrial anurans are considered to be ancestral, their early record is less understood than that of aquatic forms. Many extant terrestrial anurans produce burrows to escape unfavorable environmental conditions. Fossil anuran burrows could, therefore, serve as proxies for the presence of anurans if their morphology was known. The eastern spadefoot toad, Scaphiopus holbrookii (Anura: Scaphiopodidae), belongs to one of four groups of burrowing terrestrial anurans. This study describes the burrowing behaviors of S. holbrookii as well as the qualitative and quantitative morphology of their burrows produced in laboratory experiments with varying sediment conditions. The toads used a hindlimb-first burrowing technique moving a minimal amount of sediment to the surface. 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 to the occupying toad. Impressions on the burrow walls were produced by the hindlimbs and feet of the toads. Qualitative burrow morphology was consistent between individuals and experiments. Quantitative properties of the toad burrows were compared statistically to each other and to burrows of scorpions, salamanders, and skinks. Spadefoot toad burrows were similar to each other and different from those of the other animals. 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.

Dryness affects burrowing depth in a semi-fossorial amphibian

“…Although most amphibians can use burrows opportunistically, some species are obligate burrowers (semi-fossorial), spending most of their life underground, and only emerging to forage and reproduce (Wells, 2010). The majority of semi-fossorial anuran species dig backwards (Emerson, 1976), and resulting burrows do not have a permanent structure due to gravitational collapse (Johnson and Hembree, 2015).…”