Diabloroter bolti, a short-bodied recumbirostran ‘microsaur’ from the Francis Creek Shale, Mazon Creek, Illinois

The Carboniferous (Pennsylvanian; 309–307 Mya) ‘Mazon Creek’ Lagerstätte produces some of the earliest tetrapod fossils of major Palaeozoic lineages. Previously, the Mazon Creek record of ‘microsaurs’ was known from a single specimen. However, the lack of key anatomy, such as the skull, precluded a confident taxonomic assignment, thus only a suggested affinity to the microbrachimorph ‘microsaur’ Hyloplesion was determined. Recently several new tetrapod specimens collected from Mazon Creek have come to light, of which some have recumbirostran ‘microsaur’ affinity. Here we describe a new genus and species of short-bodied recumbirostran, Diabloroter bolti, on the basis of a unique combination of autapomorphies. Both parsimony and Bayesian phylogenetic methods recover the new taxon in the Brachystelechidae clade, as sister to a clade including Carrolla and Batropetes. We determine Diabloroter to be the earliest known member of Brachytelechidae and thus establishing a Carboniferous origin of the family. We also provide an updated diagnosis for Brachystelechidae. Finally, we comment on the evolutionary trends in the clade, including dental adaptations for a proposed algivorous diet in derived clade members.