New Mexico Geological Society Annual Spring Meeting — Abstracts

A bioclastic packstone bed in Pennsylvanian strata of the Madera Formation in the Cerros del Amado (sec. 35, T28, R1 E, Socorro County) produces teeth, denticles and fin spines of chondrichthyans, fragmentary bones of tetrapods and a marine invertebrate assemblage dominated by brachiopods, rugose corals and trilobites. The marine invertebrate and chondrichthyan fossils are well preserved and therefore autochthonous, but the very fragmentary tetrapod fossils must be allochthonous (washed in). Fragments of resilient fin spines suggest relatively shallow water in which wave action broke up and abraded the fossils.

The vertebrates present are Agassizodus (teeth), cf. Ctenacanthus (spines), Petrodus (dermal denticles=placoid scales) and indeterminate
fragments of both chondrichthyan fin spines and tetrapod bones. The Agassizodus-cf. Ctenacanthus-Petrodus association may be coincidental, but the absence of other chondrichthyans makes it suggestive. Petrodus (Mississippian-Pennsylvanian) is an organ-genus for enlarged placoid scales comparable to those of extant skates and rays. Such denticles are usually found isolated, but at least two patches of them are known (from the Pennsylvanian of Kansas and Indiana), and one of these has some of the scales modified to Listracanthus-type "spines."

Ctenacanthus sensu lato (Mississippian-Triassic) is known from many isolated spines and several holomorphs with the dorsal spines in place. However, the few teeth described from articulated specimens are cladodont. Agassizodus (Pennsylvanian--lliinois, Indiana; Lower Permian--New Mexico, Oklahoma; Upper Permian--E Greenland) is known from isolated teeth and one partial fish (Pennsylvanian-Indiana) that does not show a spine, but this may be due to incomplete preservation. The teeth clearly belong to a hybodont-grade elasmobranch. Similar teeth were described from eastern Greenland as Orodus and Fadenia, which may be synonyms of Agassizodus. Because isolated ctenacanth and hybodont spines are difficult to distinguish, and the teeth and scales are hybodont in character, the association and size of elements at the site in the Cerros de Amado suggest the presence in New Mexican Pennsylvanian seaways of an unusually large, hybodont-Ievel elasmobranch about 1.5-2 meters long