New Mexico Geological Society Annual Spring Meeting — Abstracts

Three specimens of Hoploscaphites aff. H. nodosus (Owen, 1852) were recovered from the Pierre Shale in the Raton Basin of northeastern New Mexico. The shells were found at the same stratigraphic level as Baculites jenseni, hence they are dated as upper upper Campanian. H. aff. H. nodosus has previously been reported from the Nostoceras (N.) hyatti Zone in the Coon Creek Tongue of the Ripley Formation in Tennessee (Landman et al., 2010, p. 133, fig. 81). N. (N.) hyatti Stephenson is only known in the Western Interior from the B. jenseni Zone in the Pierre Shale near Walsenburg, Huerfano County, Colorado (Kennedy, 1993, p. 105). H. aff. H. nodosus has also been reported from the B. reesidei-B. jenseni zones in the Bearpaw Shale in Alberta, Canada and Montana, Nacotoch Sand in Texas, and from the B. reesidei Zone in the Lake Creek Shale Member of the Pierre Shale in Kansas, Saratoga Chalk in Arkansas and the Larimer Sandstone Member of the Pierre Shale in Colorado (Landman et al., 2010, fig. 78, p. 125, 127, 133, 135).

The NMMNH specimens are closest to Hoploscaphites nodosus, but differ from that species in three respects: 1) they are too large for that species, with the largest specimen (macroconch) having a LMAX of about 117 mm, which would be larger if the phragmocone was complete, 2) the rib density of the specimens is about half that of H. nodosus with ribs on the venter of the mid-shaft measuring 3-3.5 per cm, and 3) the flanks are flattened on the body chamber. Forms assigned to H. aff. H. nodosus are larger, more coarsely ornamented and have flatter flanks than H. nodosus (Landman et al., 2010, p. 127, 135). The largest NMMNH specimen has the aperture and apertural lip completely preserved with an aptychus (lower jaw) preserved in close association in the shale covering the outside of the aperture.

The largest NMMNH specimen is significant because ammonite jaws usually occur as isolated elements, but jaws inside or closely associated with body chambers are relatively rare. The NMMNH specimens are also important because this is the first report of an aptychus associated with Hoploscaphites aff. H. nodosus and the first report of this taxon from New Mexico.

References:

1. Kennedy, W. J., 1993, Campanian and Maastrichtian ammonites from the Mons Basin and adjacent areas (Belgium): Bulletin de l’Institut royal des Sciences naturelles de Belgique, Sciences de la Terre, v. 63, p. 99-131.
2. Landman, N. H., Kennedy, W. J., Cobban, W. A. and Larson, N. L., 2010, Scaphites of the “nodosus group” from the Upper Cretaceous (Campanian) of the Western Interior of North America: Bulletin of the American Museum of Natural History, no. 342, 242 pp.
3. Owen, D. D. 1852, Report of a geological survey of Wisconsin, Iowa, and Minnesota; and incidentally of a portion of Nebraska Territory made under instructions from the United States Treasury Department: Philadelphia, Lippincott, Grambo, 2 vols., 638 pp.