Description Sphenacodon

heavily restored skull of s. ferox in field museum

the skull of sphenacodon similar of dimetrodon. narrow side side , vertically deep, indented notch @ front of maxillary bone in upper jaw. upper , lower jaws equipped array of powerful teeth, divided sharp pointed incisors [precaniniforms], large stabbing canines [caniniforms], , smaller slicing teeth [postcaniniforms]. orbit set high , far single opening (temporal fenestra) behind , partly below eye, characteristic of synapsids.

body proportions similar dimetrodon, large head, short neck, robust trunk, relatively short front , hind limbs, , tapering tail makes half animal s entire length. however, tops of neural spines along bone strikingly different in each genus. in dimetrodon, neural spines develop long, narrow, cylindrical projections support tall vertical dorsal sail ends near base of tail. in sphenacodon, neural spines enlarged retain flat-tipped, blade-like shape along , tail, , form crest rather tall sail. (the sphenacodontid genus ctenospondylus has blade-like neural spines, dorsal crest taller in sphenacodon, although not tall sail in dimetrodon.)

there evidence strong epaxial muscles along base of raised neural spines in both sphenacodon , dimetrodon, helping stiffen , strengthen backbone walking , lunging @ prey restricting side-to-side flexing motion. recent study of structure of neural spines on sphenacodon confirms upper parts not encased in thick muscular hump , instead protruded above layer of muscle form low dorsal crest. finds of sphenacodontid specimens in postmortem distortion of body caused dorsal spines overlap suggests spines not connected hard or particularly tough tissue. possible function of low, skin-covered crest in sphenacodon debated. thermoregulatory role seems unlikely, although taller crest in sphenacodon ferocior allometrically larger in s. ferox. recent research has favored display role tall sails in dimetrodon , edaphosaurus.

s. ferox , larger s. ferocior

both sphenacodon , dimetrodon have been depicted short limbs splayed outward @ 90 degrees body in wide pushup position , tail (and belly) dragging on ground, similar modern lizards , crocodiles. sprawling stance typical sphenacodon , dimetrodon skeletons mounted in museums. however, trackways called dimetropus (“dimetrodon foot”) match foot configuration of large sphenacodontids show animals walking limbs brought under body narrow, semi-erect gait without tail or belly drag marks. such clear evidence more efficient upright posture suggests important details anatomy , locomotion of sphenacodon , dimetrodon may not understood. preserved narrow dimetropus tracks found in parts of prehistoric trackways national monument in new mexico match smaller size of sphenacodon, genus known skeletal fossils in state, come small dimetrodon.