New Mexico Geological Society Annual Spring Meeting — Abstracts

The Mud Springs Mountains, a few miles northwest of Truth or Consequences, contain a Pennsylvanian section more than 1700' thick (Kottlowski, 1960). The paleontology of the Pennsylvanian units is incompletely known and the mollusc faunas are unstudied. In the summer of 1984 two UNM geology students discovered unusually large bellerophontid shells in a thin, darkgray shale in the NW 1/4 sec. 30, T. 13 S., R. 4 W. (0.7 miles east of Mud Mountain). Subsequent collecting by me has produced more than 60 specimens, together with a small sample of the associated fossils. The stratigraphic, position of this assemblage is about 375' above the base of the Pennsylvanian and its age is Desmoinesian, based on Kottlowski's determination of the Atokan-Desmoinesian boundary at 225' and the presence of the characteristic Desmoinesian brachiopods Mesolobus and Antiquatonia hermosana in a limestone immediately below the gastropod unit. The bellerophontids, an undescribed species of Bellerophon (Bellerophon), are far larger than most members of this subgenus the largest individual has a length of about 80 mm and a maximum width at the anterior margin that slightly exceeds the length. These specimens are also characterized by a broad, conspicuously elevated median ridge containing the selenizone. The only other North American Pennsylvanian species that attains this size, Bellerophon giganteus Worthen, is based on steinkerns and the name is considered a nomen dubium (Yochelson, 1960). Steinkerns of B. cf. giganteus have been reported from the Missourian and Virgilian of the Nacimiento uplift in north-central New Mexico (Wood and Northrop, 1946). The Mud Springs Mountains specimens appear to be the first giant representatives of Bellerophon from the Pennsylvanian that are known from well-preserved shells. The shale unit that yielded these specimens contains a sparse, generally poorly-preserved, mollusc-dominated fauna. Other gastropods identified are Strobeus sp., Straparollus (Amphiscapha) sp., S. (Euomphalus) sp., Naticopsis sp., Stegocoelia (Taosia) sp., Worthenia cf. tabulata, cf. Paragoniozona multilirata, and Microdoma? n. sp. The last three species and all of the genera are also present in the Desmoinesian part of the Flechado Formation near Taos (Kues, 1984)