Stable isotopes and predation marks shed new light on ammonoid habitat depth preferences

Marcin Machalski,Weronika Wierny,Oksana Malchyk,Krzysztof Owocki,Zofia Dubicka

doi:10.1038/s41598-021-02236-9

Abstract

Ammonoids are extinct cephalopods with external shells which predominated in many late Paleozoic and Mesozoic marine ecosystems. Stable isotope data from ammonoid shells constitute primary tools for understanding their palaeohabitats. However, in most sedimentary successions globally the aragonitic shells of ammonoids are dissolved during fossilisation process and therefore not available for geochemical studies. We overcome this taphonomic bias by analysing the better preservable calcitic elements of the ammonoid jaws (aptychi). We study moulds and aptychi of two successive members, temporal subspecies in our interpretation, of a scaphitid evolutionary lineage from a Late Cretaceous chalk succession in Poland. In order to reconstruct their habitat depth preferences, we apply the powerful combination of stable isotope data from aptychi and co-occurring benthic and planktic foraminifera with an analysis of predation marks preserved on scaphitid specimens. On this basis we conclude that the populations of the older subspecies led a nektic, and those of the younger subspecies, a nektobenthic lifestyle. The shift in habitat depth preferences took place probably as a response of local populations to the shallowing of the sea. Previous studies largely assumed stable depth preferences for ammonoid species, genera and even higher clades. Our study casts doubts over such generalizations by pointing out that ammonoids could have been more flexible in their depth-related behaviour than anticipated.

Highlights

Ammonoids are extinct cephalopods with external shells which predominated in many late Paleozoic and Mesozoic marine ecosystems
We study two successive members of the Hoploscaphites constrictus evolutionary lineage[21], based on specimens and samples from three intervals in the shallowing-upwards epicontinental chalk succession of late Maastrichtian age exposed at Chełm, P oland[23,24]
As an independent test of palaeothermometric inferences, we study predation marks preserved on the scaphitid moulds from Chełm[25]

Summary

Introduction

Ammonoids are extinct cephalopods with external shells which predominated in many late Paleozoic and Mesozoic marine ecosystems. The use of stable isotope thermometry of aragonitic ammonoid shells has yielded promising results for depth inferences[9,10,11,12] This method excludes from study those specimens that originate from carbonates in which the aragonitic shells were dissolved during fossilisation proces, leaving only natural moulds (steinkerns). Following the pionieering study by Kruta et al.[20], we use stable isotope thermometry of aptychi for the reconstruction of habitat depth preferences of a scaphitid, that is member of the ammonoid family Scaphitidae, from the European Boreal Chalk Sea. We target Hoploscaphites constrictus (Fig. 1), a species common in the Maastrichtian of Europe[21,22]. Quantitative proportions between the various types of such marks in fossil assemblages provide insights into the depth preferences of the ammonoid p rey[26,27]

Methods

Results

Discussion

Conclusion