Description of the lower jaws of Baculites from the Upper Cretaceous U.S. Western Interior

Abstract

We report the discovery of lower jaws of Baculites (Ammonoidea) from the Upper Cretaceous U.S. Western Interior. In the lower Campanian Smoky Hill Chalk Member of the Niobrara Chalk of Kansas, most of the jaws occur as isolated elements. Based on their age, they probably belong to Baculites sp. (smooth). They conform to the description of rugaptychus, and are ornamented with coarse rugae on their ventral side. One specimen is preserved inside a small fecal pellet that was probably produced by a fish. Another specimen occurs inside in a crushed body chamber near the aperture and is probably in situ. Three small structures are present immediately behind the jaw and may represent the remains of the gills. In the lower Maastrichtian Pierre Shale of Wyoming, two specimens of Baculites grandis contain lower jaws inside their body chambers, and are probably in situ. In both specimens, the jaws are oriented at an acute angle to the long axis of the shell, with their anterior ends pointing toward the dorsum. One of the jaws is folded into a U-shape, which probably approximates the shape of the jaw during life. Based on the measurements of the jaws and the shape of the shell, the jaws could not have touched the sides of the shell even if they were splayed out, implying that they could not have effectively served as opercula. Instead, in combination with the upper jaws and radula, they constituted the buccal apparatus that collected and conveyed food to the esophagus.

Correlation of shell and aptychus growth provides insights into the palaeobiology of a scaphitid ammonite

“…The main arguments against the hypothesis of a secondary opercular function of aptychi were based on the misfit between the ammonite apertures and associated aptychi. For instance, Morton (1973) found that the aptychi of Jurassic Sonninia did not provide a perfect fit to the aperture; the same was stated for Late Cretaceous Baculites by Larson & Landman (2017). As to scaphitid aptychi, Lehmann (1972) noted a significant dissimilarity between the shape and size of the aptychus and whorl section near the aperture in a specimen of Jeletzkytes nebrascensis (Owen, 1852) which served as the basis for the interpretation of aptychi as jaw elements by Meek & Hayden (1864) and Meek (1876).…”

“…8), offers an opportunity for testing the opercular hypothesis for Hoploscaphites constrictus crassus . The main arguments used in previous discussions on the subject based on testing potential matches or mismatches between the shape and size of the ammonite apertures with those of the associated aptychi (Trauth 1927–1936; Morton 1973; Lehmann & Kulicki 1990; Keupp & Mitta 2013; Larson & Landman 2017). Accordingly, lines 1–6, previously identified in the specimen ZPAL Am.…”

“…The terminology accepted for aptychi that are attributable to Hoploscaphites constrictus crassus is explained in Figure 2A–B. The terms anterior, posterior, ventral, dorsal, left and right refer to the aptychi as they were oriented during life of the animal (compare Larson & Landman 2017).…”

Recent advances in heteromorph ammonoid palaeobiology

“…The outline of the lower jaw spread open matches the shape of the whorl cross section ( Baculites , Scaphites ), which led to the suggestion that this structure had an opercular function. By contrast, Larson & Landman (2017) and Landman & Waage (1993 b ) argued that the jaws of Baculites , even when fully splayed out, did not touch the inner surface of the conch tube. Accordingly, they rejected an operculum function of the lower jaw in Baculites .…”