New Mexico Geological Society Annual Spring Meeting — Abstracts

A neural spine from the lowermost Abo Formation (Wolfcampian) is the first recorded occurrence of the eupelycosaur Dimetrodon from the Fra Cristobal Mountains of Sierra County, southern New Mexico. The Dimetrodon fossil (New Mexico Museum of Natural History [NMMNH] P-40458) was collected from NMMNH locality 5382 at Red Gap, in a conglomerate bed approximately 6 m above the Abo Formation base. The paleoenvironment of Abo strata in the Fra Cristobal Mountains has been interpreted as an inland floodplain. The slender, elongate neural spine is preserved together with its neural arch atop two matrix blocks, where it is visible in right lateral aspect. The distalmost portion of the spine was not recovered. However, its preserved height, measured from the top of the prezygapophyses is 180 mm. At its base, the spine has an anteroposterior length of 14 mm, which narrows abruptly to 9 mm at a shoulder-like constriction, above which it decreases gradually to an anteroposterior length of 4 mm along the remainder of its height. The broken distal edge of the spine is transversely expanded (width = 8 mm), and displays the figure-eight cross sectional outline that is diagnostic of most species of the genus Dimetrodon . With a single exception, the oldest records of Dimetrodon are known from the Lower Permian Abo Formation, and pertain to small specimens of the genus from inland habitats. Thus, the Fra Cristobal Dimetrodon fossil reported here adds to the growing number of Dimetrodon records from the Abo Formation of New Mexico, and lends support to the phylogenetic hypothesis that the genus Dimetrodon evolved in an upland environment as a predator of small size.