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

Two specimens of Haresiceras (Haresiceras) montanaense (Reeside, 1927) were recovered from the Satan Tongue of the Mancos Shale on the eastern side of the San Juan Basin in Rio Arriba County in northern New Mexico. An early form and a late form of the species were found in the lower Campanian Scaphites leei III and S. hippocrepis I zones, respectively.

The early form, a partial shell, has one lateral half preserving most of the outer whorl and the other half preserving a complete, septate inner whorl. The whorl section is well-compressed, and coiling is very involute with a very small umbilicus exposed on the inner whorl. The venter and flanks are flattened, and, on the inner whorl, they meet to form a sharp, angular ventrolateral shoulder. On the latter half of the inner whorl, sigmoidal ribs strengthen and arch forward on the outer flank, bend slightly backward before the ventrolateral shoulder, connect to tiny ventrolateral nodes, then cross the venter with forward arching. On the body chamber, ornament changes to strong, straight, sharp-edged primaries that connect to small, nodate ventrolateral tubercles and are separated by four straight secondaries. The late form, an inner whorl, differs mainly from the early form in its smaller ventrolateral nodes.

The NMMNH specimen that is the early form is best assigned to Haresiceras (H.) montanaense because the inner whorl is identical to that species, and the chronologic age is the same as the early form of that species. However, the suture is closer to Haresiceras (Mancosiceras) mancosense (Reeside, 1927) in the symmetrical trifid lateral lobe and the less extended second lateral saddle. Also, the venter of the body chamber seems more similar to H. (M.) mancosense than to H. (H.) montanaense. It may be an intermediate form low in the Scaphites leei III Zone, or a form resulting from intraspecific variation.

Haresiceras is a rare ammonite that is known only from the Western Interior of the United States. Initially, two specimens, one the holotype of Haresiceras (Mancosiceras) mancosense from the uppermost Santonian Desmoscaphites bassleri Zone in the Mancos Shale in San Juan County, were the only known occurrences of the genus from New Mexico (Cobban, 1964). Haresiceras (Mancosiceras) sp., likely H. (M.) mancosense, was later reported from the D. bassleri Zone in the Smoky Hill Member of the Niobrara Formation in Colfax County (Scott et al., 1986). This is the first report of Haresiceras (H.) montanaense from New Mexico.

References:

1. Cobban, W. A., 1964, The Late Cretaceous cephalopod Haresiceras Reeside and its possible origin: U.S. Geological Survey, Professional Paper 454-1, 21 p.
2. Reeside, J. B., Jr., 1927, The cephalopods of the Eagle Sandstone and related formations in the Western Interior of the United States: U. S. Geological Survey, Professional Paper 151, 87 p.
3. Scott, G. R., Cobban, W. A. and Merewether, E. A., 1986, Stratigraphy of the Upper Cretaceous Niobrara Formation in the Raton Basin, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Bulletin 115, 34 p.