First occurrence of the urocordylid Ctenerpeton remex from the Mazon Creek Lagerstätte, Illinois.

Despite being one of the most complete records of a paleozoic ecosystem, tetrapod fossils at the Mazon Creek Lagerstätte remain relatively rare compared to other contemporary late Carboniferous localities, making new additions to the fauna particularly important. Here we report a new concretion from Mazon Creek that contains a series of articulated caudal vertebrae of the early tetrapod Ctenerpeton remex. It is differentiated from Ptyonius and other nectrideans by the characteristics of its neural and haemal spines and arches, most notably the anterior and posterior projections on the haemal arches, the accessory zygapophyses on the neural arch, and the size and shape of the spines compared to known specimens of other urocordylids and nectrideans. The addition of Ctenerpeton remex to the Mazon Creek tetrapod assemblage increases the known diversity of nectrideans in this fauna and potentially aids in our understanding of the different environments inhabited by urocordylids in the late Carboniferous.