The Dimetrodon dilemma: Reassessing posture in sphenacodontians and related non-mammalian synapsids
Date Thesis Awarded
Honors Thesis -- Access Restricted On-Campus Only
Bachelors of Science (BS)
Historically, the early synapsid Dimetrodon has been reconstructed with sprawling posture. However, referred trackways are narrower than most sprawling taxa and its spine lacks lateral flexibility, implying that this animal carried its body higher above the ground. The goal of this research was to re-evaluate the posture of Dimetrodon and other synapsids, by comparing body and trace fossil measurements to present-day analogues.
I collected linear measurements from limb and girdle bones of museum collections for Dimetrodon, other extinct synapsids, and extinct relatives of early amniotes. I also collected pace angles from the literature for taxa with associated trackways, where available. I compared these data with analogous measurements for 45 modern mammalian, reptilian, and amphibian species, which were collected from museum specimens or the literature. I analyzed these data using three multivariate statistical analyses to estimate posture in Dimetrodon and other fossil taxa in this study.
The results of this study suggest that Dimetrodon and its immediate relatives had a hindlimb abducted to 30°, similar to a Savannah monitor lizard or Virginia opossum. This study and previous work also suggest they had a unique style of locomotion compared to all other tetrapods. Dimetrodon lacks true modern functional analogues, likely due to the huge temporal and phylogenetic distance separating early synapsids from modern tetrapods. Additionally, Aulacephalodon, a more derived synapsid, was estimated to have a similar to more sprawling posture than Dimetrodon. This indicates that locomotor transitions in synapsids followed a process of mosaic evolution, rather than a continuous gradient from sprawling to upright posture.
Abbott, Caroline Patterson, "The Dimetrodon dilemma: Reassessing posture in sphenacodontians and related non-mammalian synapsids" (2019). Undergraduate Honors Theses. William & Mary. Paper 1425.
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