Thursday, October 27, 2016

[Paleontology • 2016] Prospea holoserisc • A Burrowing Frog from the late Paleocene of Mongolia Uncovers A Deep History of Spadefoot Toads (Pelobatoidea) in East Asia

Prospea holoserisca  
Chen, Bever, Yi & Norell, 2016

Fossils are indispensible in understanding the evolutionary origins of the modern fauna. Crown-group spadefoot toads (Anura: Pelobatoidea) are the best-known fossorial frog clade to inhabit arid environments, with species utilizing a characteristic bony spade on their foot for burrowing. Endemic to the Northern Hemisphere, they are distributed across the Holarctic except East Asia. Here we report a rare fossil of a crown-group spadefoot toad from the late Paleocene of Mongolia. The phylogenetic analysis using both morphological and molecular information recovered this Asian fossil inside the modern North American pelobatoid clade Scaphiopodidae. The presence of a spade and the phylogenetic position of the new fossil frog strongly support its burrowing behavior. The late Paleocene age and other information suggestive of a mild climate cast doubt on the conventional assertion that burrowing evolved as an adaptation to aridity in spadefoot toads. Temporally and geographically, the new fossil provides the earliest record of Scaphiopodidae worldwide, and the only member of the group in Asia. Quantitative biogeographic analysis suggests that Scaphiopodidae, despite originating in North America, dispersed into East Asia via Beringia in the Early Cenozoic. The absence of spadefoot toads in East Asia today is a result of extinction.

Systematic Paleontology

Amphibia Linnaeus, 1758.
Anura Rafinesque, 1815.

Pelobatoidea Bolkay, 1919.
Scaphiopodidae Cope, 1865.

Prospea holoserisca gen. et sp. nov.

Etymology: Prospea means “before the North American spadefoot toad Spea”, and holoserisca means “silk”, referring to its discovery on the ancient Silk Road.

Figure 1: IGM 2/001, holotype of Prospea holoserisca.
 (a) The original specimen in rock matrix and jackets before preparation, preserved in part and counterpart; (b) the specimen in ventral and dorsal view after the preparation.

Holotype: IGM 2/001 (Institute of Geology, Mongolia, Ulanbaatar, Mongolia), a nearly complete specimen preserved as part and counterpart in grey sandy clay (Fig. 1a). The rock matrix was later removed and the specimen was embedded in resin (Fig. 1b; see the Supplementary Experimental Procedures: fossil preparation). The two halves of the holotype were combined digitally to reconstruct the whole skeleton (Fig. 2; Supplementary Movie S1).

Jianye Chen, Gaberiel S. Bever, Hong-Yu Yi and Mark A. Norell. 2016. A Burrowing Frog from the late Paleocene of Mongolia Uncovers A Deep History of Spadefoot Toads (Pelobatoidea) in East Asia.
Scientific Reports. 
6, 19209. DOI: 10.1038/srep19209