New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

Masiaposites is a rare genus of Late Cretaceous ammonite, composed of two species that until now have only been described from deposits of Turonian age in Madagascar and Europe (e.g., Collignon, 1965; Amédro et al., 2016). We report a recently collected specimen of Masiaposites from the middle shale interval of the Juana Lopez Member of the Mancos Shale in Sandoval County, New Mexico. It was recovered from the Coilopoceras inflatum subzone in the upper part of the Prionocyclus macombi zone. The specimen is an incomplete but well preserved phragmocone with smooth, concave flanks, and a narrow, deep umbilicus. Diagnostic features of Masiaposites include the smooth carinate venter, the lack of ornamentation, and the rounded flanks that are swollen and convex around the umbilicus but concave to flat toward the shoulders. Initial comparisons reveal that our specimen shares more characteristics with the Madagascan species M. carinatus (Collignon, 1965) than with the European species M. kennedyi (Amédro and Devalque, 2014), but further study is needed to determine the species assignment of our specimen. The recently discovered specimen of Masiaposites is the first record of the genus in North America. It expands the Late Cretaceous paleogeographic range of the genus from Madagascar and western Europe to the Western Interior seaway.

References:

1. Amédro, F. and Devalque, C., 2014, Les Ammonites; in Robaszynski, F., Amédro, F., Devalque, C. and Matrion B., Le Turonien des massifs d'Uchaux et de la Cèze (S.E. France). Migration globale d'ammonites et conséquences sur la zonation internationale, rudistes et corrélations entre les massifs : Académie Royale de Belgique, Mémoires de la Classe des Sciences, Bruxelles, v. 2, 197 p.
2. Amédro, F., Robaszynsk,i F., Matrion, B. and Devalque, C., 2016, A North American ammonite fauna from the late Middle Turonian of Vaucluse and Gard, southern France: the Romaniceras mexicanum, Prionocyclus hyatti and Coilopoceras cf. springeri association: Acta Geologica Polonica, v. 66, p. 729 -736.
3. Collignon, M., 1965, Atlas des fossiles caracteristiques de Madagascar (Ammonites), XII (Turonien): Service Geologique, Tananarive, 82 p.