The identification of Kopidosaurus perplexus shows the problems in the classification of lizards

Kopidosaurus perplexus fossil with its 3D reconstructions
An article published in the journal “Scientific Reports” describes a new species of lizard now extinct that lived in present-day Wyoming, in the USA, about 52 million years ago, in the Eocene period. Simon Scarpetta, a paleontology student at the University of Texas (UT) at Austin, named it Kopidosaurus perplexus because the various analyzes carried out on the available fossil skull give different results regarding its classification. The relationships also vary considerably depending on the type of analysis conducted, underlining the difficulties and uncertainties that may exist in both paleontology and biology field.

The lizard skull examined by Simon Scarpetta was discovered in 1971 in the Willwood Formation, Wyoming. Lizards are generally small reptiles with bones that can easily fragment after their death. The consequence is that it’s normal for paleontologists to discover only isolated and fragmented bones of a specimen with all the consequent difficulties in their study and classification. In this case, the fossil ended up in the collection of the Peabody Museum of Natural History at Yale, where Simon Scarpetta saw it in 2017, remaining intrigued enough to take it to his university to study it.

To perform an anatomical examination of the characteristics of the fossil lizard, Simon Scarpetta subjected the skull to a high-resolution CT scan at UT’s Jackson School laboratory. It’s a type of examination that is now common in the field of paleontology because it allows to obtain detailed 3D reconstructions without damaging the fossils. The top image (Courtesy Simon Scarpetta. All rights reserved) shows the fossil seen from different angles with its 3D reconstructions.

Simon Scarpetta’s research was complicated by the fact that there are different family trees of lizards. Over the years, various researchers conducted studies that led to reconstructions of the evolution of lizards with the creation of different ramifications in the existing and extinct groups. Simon Scarpetta considered three different family trees, illustrated in the bottom image (Courtesy Simon Scarpetta. All rights reserved). The groups to which Kopidosaurus perplexus most likely belongs are the ones indicated as H1 and H2, but their classifications are very different within the Iguania group.

In essence, the species Simon Scarpetta named Kopidosaurus perplexus could be plausibly assigned to different groups, depending on which family tree is used. In the field of paleontology, it’s normal to work with fossils of incomplete specimens, but in this case, there’s the further problem that genetic research also provides different reconstructions. Advances in genetic analysis techniques, which means not only DNA sequencing but also the analysis of mutations within a certain group of species, will increase the accuracy of the results but it’s a long and complex job.

Family trees for the Iguania group

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