TAPHONOMY OF FISH PRESERVED WITHIN AMMONITE CHAMBERS FROM THE UPPER CRETACEOUS AUSTIN GROUP, COAHUILA, MEXICO

Unique association of small fish within ammonite body chambers occurs within at least five ammonite specimens collected from lithographic limestone and platy marlstone deposits from the Upper Cretaceous Austin Group at La Rosaria Quarry, near Múzquiz, Coahuila, northeastern Mexico. The fishes are preserved within the body chamber of the ammonites, preserved as compressed 2-dimensional internal moulds. Different ammonite species preserve fishes that appear to be of one type - preliminarily identified as Kansius sp. La Rosaria fauna includes planktic foraminifers, ammonoids, bivalves, crustaceans, fishes, marine reptiles (marine varanoids, crocodilians), a pterosaur, and plants. The La Rosaria locality consists of several limestone quarries, which are heavily quarried for building material. The locality is considered to be part of the Upper Cretaceous Austin Group, which is a widespread limestone unit distributed in northeastern Mexico and southern Texas. Preliminary studies suggest that sediments were deposited in an open marine shelf environment near the southern opening of the Western Interior Seaway. The locality is relatively new and no research (both on the stratigraphy and faunal systematics) has been published on the quarries as of yet. The ammonites reported herein are housed at the Collection of PASAC (Paleontólogos Aficionados de Sabinas, Coahuila) at Sabinas, Coahuila. It is unknown if the fish inhabited the ammonite while it was alive in a symbiotic relationship or if the ammonite shell served as a refuge for the fish after the animal had died. However one ammonite specimen has preserved 40+ small fish within its body chamber suggesting the ammonite shell was used as a refuge. Also, several additional ammonite specimens have dense cirripedian (Stramentum sp.) epibionts on the body chamber indicating post–mortem colonization. Unfortunately, due to the 2-dimensional preservation of the ammonites, it is unknown if the epibionts also inhabited the body chamber interior. Rapid burial must have occurred to entrap the fishes within the body chamber of the ammonite. These ammonite specimens are a unique record of ammonite shell use that has not been previously reported. Continued research and collecting of the La Rosario quarries will hopefully produce more specimens for study of this unique phenomenon.