Unusual soft-tissue preservation of a crocodile lizard (Squamata, Shinisauria) from the green river formation (Eocene) and shinisaur relationships.

Abstract

We describe an unusual squamate fossil from the Green River Formation (Uintan, Eocene) from the Piceance Creek Basin, Colorado, USA. The new specimen, USNM PAL 540708, is a small fossil squamate skin lacking skeletal elements. It is preserved as a part and counterpart in fine-grained limestone. Recovery of a fossil organism's skin (not a shed, but a true skin) is unusual and is most often accompanied by bone preservation. Phylogenetic analysis of a combined morphology (phenotype) and genetic data set reveals that USNM PAL 540708 is a shinisaur and reaffirms that shinisaurs are more closely related to varanids than to Xenosaurus. Shinisaur fossils are very rare, with only three species having been described (Dalinghosaurus longidigitus, Bahndwivici ammoskius, and Merkurosaurus ornatus). Despite differences in the relative size of scales, the new fossil demonstrates that shinisaurs have remained unchanged in the distribution of scales and patterns of scale size during the Cenozoic. This, paired with the osteological similarity between another Green River fossil (Bahndwivici ammoskius) demonstrates considerable overall conservatism within shinisaurs over the past 50 million years.