THE DIMETRODON DILEMMA: REASSESSING POSTURE IN SPHENACODONTS

Historically, the “pelycosaur” grade synapsid Dimetrodon has been reconstructed with sprawling, body-dragging posture. This reconstruction persists, despite the limited lateral flexibility of the Dimetrodon vertebral column and the narrow gauge of its trackways. The goal of our research is to re-evaluate the posture and locomotion of Dimetrodon, by comparing body and trace fossil measurements to modern analogues.

We collected postcranial skeletal dimensions from the Dimetrodon milleri holotype (MCZ 1365) and compiled Dimetropus pace angulations from the literature. We compared these data with analogous measurements from 23 extant mammalian and reptilian taxa, which were collected from NMNH specimens or the literature, and included Didelphis virginiana, Pecari tajucu, Iguana iguana, Alligator mississippiensis, and Tachyglossus aculeatus. Linear measurements from the limbs, pectoral girdles, and pelvic girdles, as well as pace angles, were used to predict both postural grade and femoral abduction angle for Dimetrodon.

Discriminant function analyses of postural grades recovered Dimetrodon with dual-gait taxa, capable of sprawling and semi-upright gaits. We found pace angle to be the only statistically significant predictor of femoral abduction angle in the 7 extant taxa of the 23 for which it was known. We then used pace angle in a linear regression to estimate the femoral abduction angle of Dimetrodon. This femoral abduction angle estimate must be treated with caution, due to limited sample size and the fact that lateral trunk bending increases pace angulation in sprawling taxa. Although these results suggest that Dimetrodon was not a sprawling “belly-dragger,” a larger sample size, greater variety of taxa, and phylogenetic context are required to fully characterize the posture of this synapsid.