The Dimetrodon Dilemma

Dimetrodon was the apex terrestrial predator of the early to middle Permian. This genus is a “pelycosaur”-grade synapsid, and a member of Sphenacodontidae the most derived family of basal synapsids. This “sail”-backed ancestor of mammals is conventionally depicted with sprawling posture, but this may be incorrect. Its trackways lack belly-dragging marks and its spine exhibits limited lateral flexibility, contradicting sprawling reconstructions. Based on my preliminary work, Dimetrodon posture resembles dual-gait taxa (crocodilians) suggesting it was not sprawling; however, further analysis is needed to fully describe the posture of this genus. I expect Dimetrodon to be less sprawling than suggested by previous reconstructions, and instead adopt an intermediate posture (“high-walk”) when moving rapidly. To carry out this research I will be employing comparative anatomy of 50 extant tetrapod species (25 mammals, 23 reptiles, 2 amphibians) with 4 extinct genera of synapsid (Dimetrodon, the basal “pelycosaur” Ophiacodon, and therapsids Aulacephalodon and Lycaenops). Using limb bone, girdle bone, and trackway dimensions of both extinct and extant taxa, as well as the known posture grades and femoral abduction angles of extant taxa, I will predict posture grade and femoral abduction angle in the four extinct taxa, granting insight into the locomotion styles of mammalian ancestors. It is likely that traditional reconstructions of posture in early synapsids, like Dimetrodon, are outdated and in need of revision. If mammalian ancestors are found to develop upright posture earlier than previously thought, it could increase understanding of when and in which taxa we see these major “mammalian” traits begin to arise.

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