Palaeobiology, morphology and diversity of macrofaunas: A case study on Early Cretaceous ammonites
Final Report Abstract
The background for controlling mechanisms regarding fluctuations in the abundance, diversity and morphological changes of fossil invertebrates is a complex but seminal field. Particularly the nutrient and sediment supply may be a major point influencing macrofossil fluctuations and essential for reconstructing their environmental background, but is not yet in the focus of current research. In this project associations of mikrofossils are used in addition to sedimentological and geochemical data to reconstruct the palaeoenvironment for the early Cretaceous and to apply these data to the interpretation of ammonite ecology as a case study for other macrofossil groups. This is considered an urgent matter of research, since ammonites are among the dominant and most important invertebrates of their times and their position in the food web may be a key for understanding marine ecosystems on the Mesozoic shelf. Only a few pioneer studies support this hypothesis yet, underlining the urgency of the matter. According to the reviewer's hints about the approved proposal, one Lower Aptian key section was chosen to prove the method. A building site allowed us to collect several hundred ammonites bedby-bed and to obtain a few hundred micropalaeontological, geochemical and sedimentological samples. Our ammonite data show a differentiated pattern of occurrence and also of morphological changes of species during the time line, probably indicating that species were individually impacted by environmental change. The highest morphological disparity in ammonites of the genus Deshayesites in the section is believed to be multi causal. This may have been the result of an invasion of two species new to the habitat and thus facing, immediately, considerably higher competition in an environment just released from the low oxygen conditions during an early segment of Oceanic Anoxic Event 1a (OAE 1a) and also a rising sea level and palaeoclimate change. This is of importance beyond Deshayesites. It also challenges the current concept of highly morphologically variable species, questions our ideas about ammonite diversity and sheds a new light on using well-established index species for supraregional correlation. The interval contains the most prominent early Cretaceous Oceanic Anoxic Event, and thus allows to describe the long and short-term changes prior to, during and subsequent to the event and partly also a comparison on a supraregional scale. Unexpectedly a temporary claypit in North Germany allowed us to address our main scientific questions. This lucky incident made logistics much easier and obtaining data from a section on the doorstep finally turned out a very successful approach, since we collected much more material of relevance than we would have been able to collect abroad and did a much more extensive field work. A second surprise is that we obtained reliable, apparently not-altered δ18oxygen isotope values that we could obtain from calcareous foraminifera. These climate data for the succession will be of broad interest for the community since they belong to the chronostratigraphically oldest data sets on palaeoclimate of this type obtained.
Publications
- 2010. Umweltveränderungen im Erdmittelalter. Ammoniten berichten über Meeresspiegel, Plattentektonik, und Klima, in: Wefer, G., Schmieder, F. (Eds.). Marum-Bibliothek, Bremen, pp. 262-271
Lehmann, J.
- 2012. Early Aptian bay deposits at the southern margin of the Lower Saxony Basin: Integrated stratigraphy, palaeoenvironment and OAE 1a. - Acta Geologica Polonica 62, 35-62
Lehmann, J., Friedrich, O., von Bargen, D., Hemker, T.
- 2013. A new ammonite fauna from NE-Germany – evidence for an Early Albian cooling and the initial transgression in the Danish-Polish Trough. - Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 268, 199-235
Lehmann, J., Owen, H.G., Beckert, W.
- 2013. Aptian and Albian (Early Cretaceous) ammonites from glacial erratic of the Hamburg area, North Germany. - Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 270, 69-82
Lehmann, J.
- 2014. Benthic ecosystem response to the deposition of lower Aptian black shales in an epicontinental sea. - Cretaceous Research 51:208-224
Von Bargen, D., Lehmann, J.
(See online at https://doi.org/10.1016/j.cretres.2014.06.006) - Ammonite biostratigraphy of the Cretaceous – an overview. In: Klug C., Korn D., De Baets K., Kruta I., Mapes R. (eds) Ammonoid Paleobiology: From macroevolution to paleogeography. Topics in Geobiology, vol 44. Springer, Dordrecht, 2015, pp 403-429
Lehmann, J
(See online at https://doi.org/10.1007/978-94-017-9633-0_15) - Paleobiogeography of Early Cretaceous Ammonoids In: Klug C., Korn D., De Baets K., Kruta I., Mapes R. (eds) Ammonoid Paleobiology: From macroevolution to paleogeography. Topics in Geobiology, vol 44. Springer, Dordrecht, 2015, pp 229-257
Lehmann, J., Ifrim, C., Bulot, L. G., Frau, C.
(See online at https://doi.org/10.1007/978-94-017-9633-0_9) - Morphological variability in response to palaeoenvironmental change – a case study on Cretaceous ammonites. - Lethaia, Vol 49 Issue 1, January 2016, Pages 73-86
Lehmann, J., von Bargen, D., Engelke, J., Claßen, J.
(See online at https://doi.org/10.1111/let.12133)
