3-D orientation and distribution of ammonites in a concretion from the Upper Cretaceous Pierre Shale of Montana

Abstract

One of the most common modes of preservation of ammonites in the Upper Cretaceous US Western Interior is in concretions. We examine an accumulation of ammonites from a single concretion in the lower Maastrichtian Pierre Shale of eastern Montana. The concretion is an oblate spheroid 50 cm in length and 26 cm in diameter, with its long axis parallel to the substrate. It contains approximately 90 ammonite specimens representing three species of Hoploscaphites including adults and juveniles. The concretion also contains other fauna, primarily bivalves and gastropods. A total of 33 ammonites, mostly adults, are concentrated in a cluster that spans 71 % of the length of the concretion (called the ''sculpture''). 3-D measurements of the ammonites in the sculpture reveal that (1) the shells dip at all angles, with a significant trend toward more horizontal from west to east; (2) the shells dip with a highly significant bias toward the east, suggesting a current from that direction; and (3) a highly significant number of the shells that are non-vertical face with their left side up. Most of the shells show lethal damage as indicated by missing pieces of body chamber. After settling to the bottom, the shells may have piled up against each other, creating a sediment trap. Other organisms such as scavenging gastropods may have been attracted to the site to feed on the stranded ammonite carcasses. The chambers of the ammonite phragmocones and even some of the body chambers are empty, suggesting relatively rapid burial. Oxygen and carbon isotopic analyses of the ammonite shells reveal that they preserve their original isotopic signature. The values of d 13 C of the carbonate cement in the concretionary matrix are much lighter than those in the ammonite shells and imply that cementation of the concretion occurred in association with the decomposition of organic matter.

Encrustation of inarticulate brachiopods on scaphitid ammonites and inoceramid bivalves from the Upper Cretaceous U. S. Western Interior

“…The faunal assemblage in the scaphite concretionary layer at Glendive, Montana, has previously been described by Bishop (1967) and Landman et al (2015). As the name implies, this layer is dominated by scaphites, mainly Hoploscaphites crassus and H. plenus.…”

“…A single concretion from the scaphite concretionary layer was recently analyzed by Landman et al (2015). They documented 90 scaphites, representing three species of Hoploscaphites, most of which are adults.…”

“…Such a bloom may have attracted adult scaphites into the region to feed on the plankton or on organisms higher up in the food web. Because most of the scaphite shells exhibit lethal injuries, Landman et al (2015) argued that they accumulated on the bottom as a result of predation, perhaps from a mosasaur or shark. The ammonites may not have been killed all at once, but possibly over a protracted period of days or weeks.…”

“…The accumulation of dead ammonites on the sea bottom acted in turn as a sediment trap, entraining additional shell debris. This accumulation also promoted the development of a local community including numerous scavenging gastropods that were attracted to the area to feed on the high concentration of scaphite carcasses (Landman et al 2015).…”

“…cf. B. baculus Meek and Hayden, 1861 Nektonic Planktivore Uncommon Hoploscaphites crassus (Coryell and Salmon, 1934) Nektonic Planktivore Very abundant Hoploscaphites plenus (Meek, 1876) Nektonic Planktivore Very abundant Hoploscaphites saltgrassensis (Elias, 1933 (Sohl, 1964) Epifaunal Carnivore Common Drepanochilus evansi Cossman, 1904 Epifaunal Carnivore Very Abundant Landman et al 2015). Uncommon: 1-5 specimens; common 6-20 specimens; abundant: 21-30 specimens; very abundant: >30 specimens…”