New Mexico Geological Society Annual Spring Meeting — Abstracts

In 1980, Fracasso (Journal of Paleontology 54: 1237-1244) assigned localities yielding Sphenacodon material from megasequence one at EI Cobre Canyon in the Cutler Formation of north-central New Mexico a Missourian age, thereby extending the stratigraphic range of Sphenacodon well below the Wolfcampian. However, later authors determined these localities to be within Wolfcampian age sediments of megasequence two. Diagnostic neural spines from the upper part of the Red Tanks Member of the Bursum Formation in the Lucero uplift of central New Mexico represent the oldest definitive record of the subfamily Sphenaeodontinae. Their age is late Virgilian based on streptognathid conodonts collected from the middle third of the member that display primitive characters of the species complex that includes the index of the Virgilian-Wolfcampian boundary, Streptognathodus isolatus. No stratigraphic breaks occur in the upper third of the Red Tanks, strongly suggesting that the entire member, at least up to the base of the overlying Abo Formation, is of Carboniferous age.

As many as seven localities in the Red Tanks Member yielded elongate, anteroposteriorly expanded, laterally compressed neural spines diagnostic of the genus Sphenacodon. Three of these sites, in the Carrizo Arroyo South section, are approximately 34 m below the base of the Abo in beds of limestone conglomerate, whereas four additional sites in the Major Ranch area are within the uppermost 13 m of the member and occur in beds of grayish-green shale and reduced mudstone. Four partially articulated vertebrae were found preserved in a matrix of limestone-pebble conglomerate 8 m below the base of the Abo in the Red Tanks section, in close association with a bed containing large numbers of the bivalve Permorphorus. The slender, elongate neural spines are characterized by a circular cross section and an absence of anterior and posterior grooves, features diagnostic of the primitive species Dimetrodon milleri, otherwise known only from the Lower Permian (middle Wolfcampian) Archer City Formation (ex Putnam Formation) in Archer County, Texas.

Pelycosaurian abundance and diversity are typically low in Desmoinesian and Missourian strata of the Pennsylvanian. Although faunal proportions of ophiacodontids and varanopsids increase significantly during the Virgilian, the Red Tanks material represents the only pre-Wolfcampian record of the progressive sphenacodontine genera Sphenacodon and Dimetrodon.