New Mexico Geological Society Annual Spring Meeting — Abstracts

A lower jaw was recently discovered embedded in a concretion in association with the Late Cretaceous (Turonian) ammonite Spathites puercoensis (Herrick and Johnson, 1900) in the Carlile Member of the Mancos Shale in Sandoval County, New Mexico. The jaw was revealed when the concretion was broken open. Ammonite jaws are beaks or mandibles consisting of upper and lower elements. The jaw apparatus and the radula (rasping tongue) were the primary feeding organs of ammonites. The lower jaw of New Mexico Museum of Natural History specimen P-79962 is mostly complete and well-preserved, but is almost entirely flattened. The more complete plate (wing) is divided into three parts by calcite veins in the gray limestone concretion. It is situated next to another element that appears to be the other plate of the lower jaw. They are both offset in the concretion from a well-preserved, but weathered, partial, adult phragmocone of the robust form of S. puercoensis. A small portion of shell material of the ammonite was exposed during preparation. The body chamber does not appear to be preserved but could be hidden within the concretion. The elements are likely the paired calcitic plates of the lower jaw called the aptychus (sensu stricto). There are small areas of black material exposed directly below the light colored outer layer of both plates that could be the remains of the inner chitinous layer. The more complete plate is approximately 52 mm long by 30 mm wide (W/L = 0.58), but is flattened. Most of the anterior and lateral margins and parts of the posterior margin are preserved. The specimen is triangular in outline and retains some of the original curvature on the lateral margins. The anterior margin is narrow and pointed. Surface sculpture consists of fine, closely-spaced co-marginal rugae paralleling the lateral margin and curving to parallel the posterior margin. The less complete plate, in juxtaposition to the other, but facing the opposite direction, has the posterior margin broken off. Approximately 1-2 mm below the surface of this broken margin, a layer of black material is exposed that could be the remains of the inner chitinous lamella. This plate has folds or creases along and near one of the lateral margins that could be the result of postmortem, plastic deformation. The two plates are not symmetrical with each other, but this could be due to the deformation. It is likely that this jaw belongs to the associated ammonite in the concretion. It is only the second reported occurrence of a jaw in the family Acanthoceratidae and the first occurrence of a jaw from the subfamily Mammitinae Hyatt, 1900 and the genus Spathites Kummel and Decker, 1954. It is also the first reported occurrence of an ammonite jaw from New Mexico.