Details - Lower jaw of Spathites (Ammonoidea, Acanthoceratoidea) from the Upper Cretaceous (Turonian) of New Mexico

Lower jaw of Spathites (Ammonoidea, Acanthoceratoidea) from the Upper Cretaceous (Turonian) of New Mexico

Title

Lower jaw of Spathites (Ammonoidea, Acanthoceratoidea) from the Upper Cretaceous (Turonian) of New Mexico

Title Variants

Alternative: Lower jaws of Spathites

By

Landman, Neil H. , author

Foley, Michael P., , author
Lucas, Spencer G. , author
Sealey, Paul L., , author

Type

Book

Material

Published material

Publication info

New York, NY, American Museum of Natural History, [2019]

Notes

Caption title.

"April 24, 2019."

Local PDF available in high- and low-resolution versions.

A lower jaw was recently discovered in a limestone concretion in association with the Late Cretaceous (Turonian) ammonite Spathites puercoensis (Herrick and Johnson, 1900) from the Carlile Member of the Mancos Shale in Sandoval County, New Mexico. It is nearly complete and comprises the aptychus with a hinge along the midline. The better-preserved plate, the left (according to its position in life), is roughly triangular in shape with a broadly rounded lateral margin, a narrowly rounded posterior margin, and a weakly concave anterior margin. It is 26.2 mm wide and 33.0 mm long. Together, the left and right plates form an escutcheonlike shape that projects slightly forward at the apex. The ratio of jaw width to length (26.2 mm x 2 / 33.0 mm) equals 1.59. The aptychus consists of yellow-orange calcite and is covered with comarginal ribs that parallel the lateral and posterior margins and become more prominent toward the posterior end. It is likely that this jaw belongs to the associated ammonite and would have comfortably fit inside the body chamber, based on a comparison of the length of the jaw and the whorl height, suggesting that it functioned as a jaw, rather than as an operculum. It is the first report of an ammonite jaw in the genus Spathites and the first reported occurrence of an ammonite jaw from New Mexico.