Burrowing is one of the many locomotor modes of frogs (order Anura) and is found within many clades. Burrowing is generally categorized into two groups: forward-burrowing and backward-burrowing. While forward-burrowing is more rare than backward-burrowing, we show that it has evolved independently at least eight times across anurans and is correlated with distinct features of the external and internal anatomy. The shape of the humerus is especially important for forward-burrowing, as many species use their forelimbs for digging. Using X-ray computed tomography data, we characterize shape variation in the humerus, including three-dimensional (3D) morphometrics, assess the morphology of muscles related to this variation in the humerus, and discuss the mechanical and evolutionary consequences of our results. We show that the humeri of most forward-burrowing frogs are morphologically distinct from those of non-forward-burrowers, including features such as a curved and thick diaphysis, the presence of a pronounced ventral crest, and relatively large epicondyles and humeral head. Our findings also suggest that pectoral muscle anatomy differs substantially among burrowing modes in frogs. This work provides a framework for predicting locomotor modes in taxa for which the natural history is poorly known as well as extinct taxa.

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The nearly 200 species of direct-developing frogs in the genus Eleutherodactylus (the Caribbean landfrogs, which include the coquís) comprise an important lineage for understanding the evolution and historical biogeography of the Caribbean. Time-calibrated molecular phylogenies provide indirect evidence for the processes that shaped the modern anuran fauna, but there is little direct evidence from the fossil record of Caribbean frogs about their distributions in the past. We report a distal humerus of a frog from the Oligocene (approx. 29 Ma) of Puerto Rico that represents the earliest known fossil frog from any Caribbean island. Based on its prominent rounded distal humeral head, distally projecting entepicondyle, and reduced ectepicondyle, we refer it to the genus Eleutherodactylus . This fossil provides additional support for an early arrival of some groups of terrestrial vertebrates to the Greater Antilles and corroborates previous estimates based on molecular phylogenies suggesting that this diverse Caribbean lineage was present in the islands by the mid-Cenozoic.