A NEW SPECIES OF THE HETEROMORPH AMMONITE EUBOSTRYCHOCERAS FROM THE UPPER CRETACEOUS FRONTIER FORMATION, NATRONA COUNTY, WYOMING

The specimen (WDC-Fnc-001) was collected from the Wall Creek Sand member within the biofacies of the planispiral ammonite Prionocyclus germeri and Prionocyclus quadratus zone identifying the Latest Turonian. Also noted within the same facies was the uncommon heteromorph ammonite Eubostrychoceras matsumotoi. This association to another ammonite of the same genus has allowed for close comparison. Growth stages of E. matsumotoi are present in the facies for comparison and WDC-Fnc -001 is not believed to be a juvenile of a known genus. Specimens are not taphonomically distorted and are preserved as steinkerns. However, little of the original shell remains to observe.

WDC-Fnc -001 displays a broken early whorl suggesting a hamitid aspect the extent of which cannot be determined as the straight early shell is missing at the oblique turn. The body chamber retains the helically coiled, spiral septate whorls and the large apical angle, sparse ribbing and lack of ornamentation and tubercles.

The holotype is a dextrally coiled, reduced spire with an apical angle of 90 degrees. In WDC-Fnc -001, whereas the body chamber begins to descend, it cannot be determined if the apertural end would face backward or ascending as in E. matsumotoi. The ribs are slight, single and bend forward at a shallow angle and number 40 at the largest septate whorl. E. matsumotoi has sparse ribbing with a strong curvature that are more defined than in WDC-Fnc-001. Each whorl cross section is oval and not as rounded as in E. matsumotoi, though this may be a result of the higher apical angle.

WDC-Fnc-001 represents a new Nostoceratid specimen of the Frontier Formation that is different to E. matsumotoi. It also doubles the Eubostrychoceras species and increases the diversity of species at the very top of the upper Turonian for the Western Interior Seaway.