New Mexico Geological Society Annual Spring Meeting — Abstracts

New Mexico preserves a large number of E.early Permian tracksites that are important for several reasons, including: (1) large sample sizes from discrete localities; (2) localities from a spectrum of environments from mountain front to shoreline; and (3) localities that occur in intercalated marine/nonmarine sequences that can be crosscorrelated directly to the global marine biochronology. Study of these collections can yield important paleoecological data including evidence of locomotory evolution in tetrapods.

The most common large morphotype (pes length 100-150 mm) of vertebrate track from the Lower Permian red beds of New Mexico is Dimetropus. This Pennsylvanian-Permian ichnogenus is interpreted to have been the track of a sphenacodontid pelycosaur and is common in both North America and Europe.

Conventional reconstructions of sphenacodonts indicate a widely sprawling posture for the limbs, often with a dragging tail. Recent remounting of a skeleton in Denver has led to some rethinking of the posture of these animals (Small. B. S., 1993, Journal of Vertebrate Paleontology, v. 13, p. 57A). Large sample sizes of Dimetropus and long segments of individuals trackways from New Mexico suggest that a radical reassessment of the posture of these ubiquitous Early Permian animals is necessary.

There are no trackways of Dimetropus that indicate any belly-or tail-drag. This ia consistent pattern in a large sample (e. g., over 100 tracks from New Mexico Museum of Natural History and Science locality 846) that includes a spectrum of substrate conditions from relatively dry to wet. Furthermore, the Dimetropus trackmaker had a relatively narrow gait indicating a semi-erect posture rather than the sprawling stance shown in all reconstructions. The pace angulation is dependent on stride, but is typically 80-120°, and the ratio of pes length: trackway width is about 0.35:1

The New Mexico samples clearly demonstrates that the stance and gait of sphenacodontid pelycosaurs has been seriously misinterpreted and that this group of synapsids had achieved a higher level of locomotory sophistication than was previously realized.