NOT EVERYTHING IS BIGGER IN TEXAS: MICROMORPH AMMONITES OF THE TRANS-PECOS AREA

The Late Cretaceous (Turonian) Chispa Summit Formation of the Trans-Pecos area of Texas is known for big ammonites, some over 60 cm in diameter. However, micromorph ammonites from the area remain largely undocumented. Tiny pyritized micromorph ammonites (less than 2 cm in diameter) are numerous in prodeltaic deposits of the Prionocyclus hyatti Zone near the Chispa Summit type locality. The excellent preservation of the dwarves, juveniles, and nuclei permits the study of previously undescribed juvenile features, including ornamentation and sutural ontogeny. These taxa include Prionocyclus hyatti and Coilopoceras springeri as well as rarer Romaniceras (Yubariceras) ornatissimum, Puzosia (Puzosia) serratocarinata, Worthoceras cf. W. minor, and Metaptychoceras crassum. A new genus and species of progenetic dwarf (Nannospathites korusi) has been found, along with the second known specimen of Prionocyclites mite, another progenetic dwarf described by Kennedy (1988). This large collection of small ammonites contributes significantly to the understanding of the ontogenetic development of previously studied adult specimens, and expands the species diagnoses and the geographic ranges of rarer species.