New information on the neurocranium of Archeria crassidisca and the relationships of the Embolomeri

Abstract

Abstract Embolomeres have been considered candidate stem-amniotes by many authors, but recent work has shown that anatomical and phylogenetic support for this hypothesis may be ambiguous. Newer phylogenetic approaches focus on the anatomy of the endochondral bones surrounding the brain (neurocranium) and associated dermal investing bones, so accurate description of the neurocranium of tetrapods, and particularly embolomeres, is critical to assess the phylogenetic context of amniote origins. I here amend prior descriptions of the neurocranium of the abundant Early Permian embolomere, Archeria crassidisca, with a description of a previously unreported partial braincase. This specimen reveals features broadly associated with stem-tetrapods of the fin-to-limb transition and recently described in aïstopod stem-tetrapods, including a cranial notochordal remnant, a possible buccohypophyseal canal, and anteroventrally projecting saccular fossae that extend deep into the basioccipital. I present an amended description and reconstruction of the neurocranium and a phylogenetic analysis integrating these codings, which recovers embolomeres more robustly within the tetrapod stem. I review characters previously used to assign embolomeres to the amniote stem and find this evidence is ambiguous or incorrect. I then outline strong character support for a stem-tetrapod position of embolomeres, and implications for the ancestral body plan of crown-group tetrapods.