Back to Search Start Over

Caught in the act? Distraction sinking in ammonoid cephalopods

Authors :
Neil H. Landman
Christian Klug
Royal H. Mapes
University of Zurich
Landman, Neil H
Source :
Swiss Journal of Palaeontology. 138:141-149
Publication Year :
2018
Publisher :
Springer Science and Business Media LLC, 2018.

Abstract

Two specimens of the Late Mississippian ammonoid cephalopod Metadimorphoceras sp. were recovered from the Bear Gulch Limestone in Montana. This unit was deposited in the lowest part of the Big Snowy Basin, where the bottom waters are thought to have been strongly oxygen deficient. The two nearly equally sized specimens are impressions with soft tissues preserved as brown carbonaceous smears. Diagenetic processes destroyed their aragonitic shells. The preserved soft tissues are interpreted as mandibles, remains of food in the crop, and, possibly, ovaries and eyes. The specimens are on their sides, aperture-to-aperture, and probably the male is on the left and the female is on the right. The specimens are thought to have been in the process of copulation when they died. Copulation by most (all?) externally shelled cephalopods (extinct ammonoids and fossil and extant nautiloids) was probably in a head-to-head, aperture-to-aperture position. This was probably governed in part by restricted accessibility to the female reproductive organs due to the presence of the shell and the ability of both animals to partly withdraw into their shells during copulation. The shell protected them from predators during copulation. In coleoids, which lack an external shell, copulation is a more rapid affair due to the greater vulnerability from predators including other coleoids. We suggest that the fossils from the Bear Gulch Limestone and similar finds of paired ammonoids preserved together with interlocking apertures, and including soft parts in the body chamber, represent examples of ammonoid behavior frozen in time. The two ammonoids were probably too pre-occupied with copulation to notice that they were sinking into the hypoxic bottom waters of the basin and facing suffocation (distraction sinking).

Details

ISSN :
16642384 and 16642376
Volume :
138
Database :
OpenAIRE
Journal :
Swiss Journal of Palaeontology
Accession number :
edsair.doi.dedup.....70f94ddee96caec7365467e155ba6629
Full Text :
https://doi.org/10.1007/s13358-018-0176-7