New Mexico Geological Society Annual Spring Meeting — Abstracts

Several specimens of the extremely rare (in North America) ammonite Reesidites minimus were collected from the upper part of the Upper Cretaceous (Turonian) Juana Lopez Member of the Mancos Shale east of Mesa Prieta, in Sandoval County, New Mexico. These Reesidites specimens occur over a stratigraphic interval ~ 10 m thick that ends at the topmost calcarenite bed of the Juana Lopez Member. This bed contains most of the Reesidites fossils in an unusually thick lens of dense, bioclastic packstone.  Prior to the records documented here, only two specimens of R. minimus from North America have been reported in the literature, both from New Mexico. Most of the New Mexico Museum of Natural History specimens of R. minimus were found in a bed of calcarenite that also yields fossils of Scaphites whitfieldi, Baculites cf. B. yokoyamai, fragments of Prionocyclus novimexicanus, Inoceramus perplexus and other bivalves and gastropods. This places R. minimus from the Juana Lopez in the study area in the S. whitfieldi Zone, which is early late Turonian in age.

The involute shells of the NMMNH specimens of Reesidites are very compressed, with small umbilici, and bear low, broad, slightly sinuous primary and secondary ribs that each connect to a ventrolateral clavus and terminate in a siphonal clavus on a fastigiate venter, as has been previously described in Reesidites minimus. The great majority of specimens of R. minimus from the Juana Lopez Member are juveniles, with only one definite impression of an adult recovered. 

The only specimens of Reesidites minimus from North America previously illustrated and/or described are USNM (National Museum of Natural History, Smithsonian) 414510 and 414511. USNM 414510 is in the Scaphites whitfieldi Zone in the D-Cross Tongue of the Mancos Shale in Socorro County, and USNM 414511 is in the Scaphites ferronensis Zone in the Mancos Shale, 18 m above the top of the main upper ledge of the Juana Lopez Member in Valencia County (Cobban and Kennedy, 1988, p. 68; Kennedy et al., 2001, p. 27). The new records we document are thus the first report of R. minimus from the Juana Lopez Member and from Sandoval County. Furthermore, they effectively quadruple the known occurrences in North America of this important biostratigraphic indicator. R. minimus is best known from Japan, but has also been reported from Armenia, Austria, Tunisia and New Mexico. It is interesting to note that the holotype specimen of R. minimus from Hokkaido, Japan was also found in a Scaphites bed.

References:

1. Cobban, W.A. and Kennedy, W.J., 1988, Reesidites (Cretaceous Ammonoidea) from the Upper Turonian of New Mexico: Neues Jahrb. Geol. Paläontol. Monatsh, p. 65-70.
2. Kennedy, W. J., Cobban, W. A., and Landman, N. H., 2001, A revision of the Turonian members of the ammonite subfamily Collignoniceratinae from the United States Western Interior and Gulf Coast: Bull. Amer. Mus. Nat. Hist., v. 267, 148 p.