New Mexico Geological Society Annual Spring Meeting — Abstracts

In southwestern New Mexico, the limestone-shale member of the Lower Cretaceous U-Bar Formation produces ammonites of the Douvilleiceras mammillatum Zone of early Albian age. Here. we document the first record of numerous echinoids from a 20-m-thick interval of the upper part of the limestone-shale member, in the Big Hatchet Mountains. We collected over 800 irregular echinoids that we identify as Hemiaster cranium (Cooke) from five discrete stratigraphic intervals. Most specimens are well preserved, being the complete or nearly complete calcareous test, with only a small percentage showing signs of post-mortem crushing. The fossil assemblage we collected is almost completely limited to echinoid tests--we found no echinoid spines, and only a few small bivalves.

These subglobular, irregular echinoids are bilaterally symmetrical and characterized by being nearly heart-shaped, slightly antero-posterior elongated, and having a rounded posterior end. The posterior end is moderately inflated, being thicker than the anterior end. The apical system is located slightly anterior to the center. Five moderately deep and relatively short ambulacra (petals) contain two paired rows of slit-like pores in all but the single anterior-most petal, which has circular pores and is slightly deeper. The two posterior petals are very short. being about half the length of the anterior pair, and curve slightly toward the posterior. The oval periproct is higher than wide and is located high on the posterior side, being visible from above.

Most of the above characters are consistent with Cooke's 1946 description of Hemiaster cranium, however, these Big Hatchet specimens consistently differ in three characters. When viewed in profile. the posterior end is nearly vertical, not the 65° angle that Cooke reported for Hemiaster cranium. When viewed apically, the posterior end is also smoothly rounded, not flattened as in Hemiaster cranium, and the anterior end has a slightly heart-shaped indentation. not smoothly rounded as in Cooke's specimens. We attribute these differences to variation within the species, but additional research may warrant a species-level distinction.

Hemiaster cranium is best known from Texas, where it occurs in the Weno Limestone of the Washita Group. Because the Weno Lmestone is of middle to late Albian age, and the limestone-shale member of the U-Bar Formation is early Albian, we report here a downward extension of the biostratigraphic range of Hemiaster cranium.