Body Size Estimation in Toads (Anura: Bufonidae): Applicability to the Fossil Record

Abstract

Organisms’ body size is an important biological trait that is related to the environment and constrained by physiology. It is also one of few biological characteristics that can be inferred from fossil specimens. Variation in body size both within and across fossil communities can provide insight into their response to past climatic events, as well as morphological or ecological evolution in specific taxa. Among vertebrates, frogs and toads (Anura) are of particular interest given their sensitivity to environmental variation. Here, we propose a method for estimating body size in toads, one of the most ecologically and taxonomically diverse frog families (Bufonidae) that is cosmopolitan in distribution and contains aquatic, terrestrial, arboreal, and fossorial species. We used computed tomography scans (CT-scans) of 36 living species of toads to digitally segment five bones that are frequently found as fossil (ilium, sacrum, urostyle, humerus, and radioulna). We took nine different measurements on those bones to be used as proxies for body size and for each specimen collected a measurement of body size, snout–urostyle length (SUL). We used ordinary least square regression analysis (OLS) with 95% confidence and prediction intervals to determine if those measurements are useful to estimate body size from isolated bones and in the fossil record. Our regression analyses indicate that these measurements can serve as proxies to estimate body size in bufonids (with coefficients of determination between 0.80–0.95). The measurements with the highest coefficient of determinations are those of the ilium and humeri, both of which are abundant in the fossil record and taxonomically informative. Last, we tested our method on examples of living and fossils toads of North and South America. Our method is the first quantitative approach to estimate size in toads based on isolated bones and enables us to continue to explore the correlation between size and ecology in toads in the past.