Iguanian lizards from the Split Rock Formation, Wyoming: exploring the modernization of the North American lizard fauna

摘要

Terrestrial ecosystems in North America changed substantially during the Neogene. Previous authors have documented changes in plants and mammals but squamates have received comparatively little attention. Several Miocene lizard fossils were described previously from the Split Rock Formation (Wyoming), which was deposited just before or during the mid-Miocene Climatic Optimum (c. 18-14 Ma). I re-evaluate the identifications of some of the previously published fossil lizards from the Split Rock Formation and describe several new fossil lizards. I focus on the iguanians and revise the biogeographical and temporal context for the evolution of several clades. I establish a minimum age for crown Phrynosomatidae and describe the oldest known occurrence of Crotaphytidae. The fossils provide evidence of a relatively modern lizard fauna in central North America by similar to 17.5 Ma and support a substantial turnover from the Eocene to the middle Miocene. I also describe a new taxon, Caeruleodentatus lovei, with a distinctive dentary morphology. The revised iguanian lizard fauna is more diverse than previously described. I discuss difficulties and considerations with using apomorphies to identify fossils, biogeographical biases that affect fossil identifications, and the previous and future use of fossils from the Split Rock Formation in divergence time analyses. I recommend that comparative samples used for identifying fossils should not be intentionally limited by biogeography or stratigraphy, and underscore the importance of topology selection when constructing an apomorphy-based diagnosis. While apomorphy-based diagnoses may decrease taxonomic resolution of fossil identifications, they do not necessarily decrease our capacity to interpret the phylogenetic, biogeographical or ecological significance of fossils.