Long bone histology indicates sympatric species ofDimetrodon(Lower Permian, Sphenacodontidae)

Abstract

ABSTRACTThe Briar Creek Bonebed (Artinskian, Nocona Formation) in Archer County is one of the richest sources ofDimetrodonbones in the Lower Permian of Texas, USA. Based on size, a small (D.natalis), an intermediate (D. booneorum), and a large species (D. limbatus) have been described from this locality. It has been proposed that these traditionally recognised species represent an ontogenetic series of only one species. However, the ontogenetic series hypothesis is inconsistent with the late ontogenetic state of the small bones, as suggested by their osteology and degree of ossification. Histological analysis of newly excavated material from the Briar Creek Bonebed has resolved some of the discretion between these two competing hypothesis, confirming the coexistence of a small (D. natalis) with at least one largerDimetrodonspecies. An external fundamental system is present in the largest sampled long bones identified asD. natalis. The histology ofD. natalispostcrania is described as incipient fibro lamellar bone. This tissue is a combination of parallel-fibred and woven-fibred bone that is highly vascularised by incipient primary osteons. The species status ofD. booneorumandD. limbatusremain unresolved.