CONIACIAN AMMONITE AND INOCERAMID FAUNAS OF THE AUSTIN GROUP OF NORTH TEXAS, USA

The resistant, fossiliferous limestones, chalks, and marls of the Coniacian to lower Campanian Austin Group are extensively exposed along a north-to-south arcuate belt from northeastern Texas to the Mexican border. Despite numerous stratigraphic and vertebrate paleontological studies, the ammonites and other invertebrates of the Austin Group have received little attention since Keith Young published his biostratigraphic framework for these strata in the 1960s. The lack of ammonite studies is mainly due to the scarcity of aragonitic taxa in the chalk-marl facies, which are dominated by inoceramid bivalves, some of which grew to over a meter in length. However, there are intervals where ammonites are relatively abundant, particularly in the upper Santonian and lower Campanian strata. The most abundant ammonites in the Austin Group are members of the Collignoniceratidae (Peroniceratinae, Texanitinae, Menabitinae), Baculitidae, and Scaphitidae. Nautilids (Eutrephoceras), non-inoceramid bivalves, gastropods, echinoids, and crustaceans occur in these strata, many of which remain undescribed. Thus, there is a strong argument for renewed study of the invertebrate paleontology and biostratigraphy of the Austin Group. Here, we report for the first time on diverse Coniacian ammonite and inoceramid faunas from a 19 m interval in the Austin Group of north Texas. The ammonite taxa in this interval include: Scaphites spp., Paratexanites cf. serratomarginatus, Peroniceras westphalicum, Prionocycloceras gabrielense, Placenticeras cf. kauffrarium, Phylcticrioceras trinodosum, Tridenticeras tridens, Glyptoxoceras ellisoni, Baculites cf. codyensis, and Neocrioceras (or Allocrioceras) sp. The ammonite and inoceramid fauna indicate that this 19 m interval in the Austin Group correlates to the upper middle to lower upper Coniacian. Several of the ammonites and inoceramids occur in the Western Interior and Europe demonstrating the broad geographic range for Coniacian faunas. This diverse fauna is unusual due to the preservation of aragonitic shells, which is rare in the limestones and chalks of the Austin Group. This finding advances our understanding of the Texas Coniacian and its connection to both the Western Interior and Europe.