Ecology and Variation of Cretaceous and Recent Cephalopods

Abstract

Palaeodiversity, palaeoecology and organismic evolution are essential disciplines in palaeontology. Although major trends and changes in biodiversity through the Phanerozoic are more or less well known, much more work is needed to understand ecological details as well as all the involved biases linked with biodiversity analyses. As far as evolution is concerned, variation produces the raw material for selection, and thus is of great importance. Within this work, questions of evolutionary processes, morphological intraspecific variation and palaeoecology of cephalopods have been addressed because cephalopods are excellent model organisms to study such fields due to their abundance and morphological diversity. The first part of this (Chapter I and II) thesis documents cephalopod associations and their ecology from the Alpstein massif in Switzerland. It is based on remains of over 1000 macrofossil taxa. Although this region has been sedimentologically and geologically studied in great detail, the fossil associations are much less well-known. To fill this gap, all available cephalopod fossils from the Alpstein were examined comprehensively. In total, at least 100 ammonite taxa, 6 nautilid taxa and 4 coleoid taxa occurred in the Alpstein region. Macrofossil associations of various ages from the Alpstein were also examined in order to assess palaeoecological and environmental changes through the Cretaceous. To quantify these changes, two diversity indices of taxonomic richness and ecological disparity were employed. Results show that taxonomic richness and ecological disparity are decoupled, i.e., taxonomic richness did not show a strong fluctuation with around 20 families through time, whereas ecological disparity fluctuated strongly. These results imply that, when environmental changes occur, ecological disparity is more susceptible than taxonomic richness at family level. In the second part (Chapter III and IV), phenotypic intraspecific variation of cephalopods and its ontogenetic trajectories were investigated. Knowledge of intraspecific variability is a prerequisite to study diversity, palaeobiogeography and chronobiostratigraphy because these can be only carried out correctly with accurate taxonomic assignments. Ammonoids are extinct externally shelled cephalopods (ectococh-leates), which had high evolutionary rates and occur worldwide in great numbers. If complete, ammonoid conchs record their entire ontogeny because of their accretionary growth and thus are, like many other mollusks, ideal to study ontogeny. However, ammonoid intraspecific variation is often underestimated or ignored when a new taxon is introduced, which resulted in oversplitting in many cases. To understand intraspecific variation in ammonoids, Recent Nautilus was used as a reference since fossil materials introduce a set of additional biases (e.g., taphonomy, time averaging). A quantitative approach using 3D morphometry was employed to analyze the phenotypic intraspecific variation of three conch parameters of three geographically separated Nautilus populations through ontogeny. Results illustrate that there are commonalities in the pattern of ontogenetic change of intraspecific variation between the three populations: In early ontogenetic stages, variation is higher and decreases until it shows an increase again before maturity. This pattern was compared with that of some ammonoids and belemnites, which revealed that this ontogenetic pattern of intraspecific variation is common among some groups of cephalopods.The second part of this thesis (Chapter III and IV) also included applications of (destructive and non-destructive) tomography technique to fossil cephalopods. As in the above mentioned study on intraspecific variation of Nautilus,3D morphometry was carried out to examine several ammonoid species in order to understand ontogenetic patterns of phragmocone chamber volume increase and its intraspecific variation. When the results of ammonoids and Nautilus are compared, the ammonoids show a much stronger ontogenetic variation of chamber volumes than nautilids. The driving factors behind this variation will be part of my postdoctoral study. In the last part of this thesis (Chapter V), the roles of ammonoids in the marine food web and their incumbent replacement after their extinction at the end of the Cretaceous are discussed. Globally, Cretaceous ammonites occur in great numbers. This stresses the importance for the question for the post-Cretaceous re-occupation of this ecospace. First, reproductive rates of large Cretaceous ammonoids were reconstructed, which yielded numbers of up to 100 million eggs (hatchling size 0.5 to 1 mm) for an adult female ammonite with two meters conch diameter. Combining that large ammonites with a diameter exceeding half a meter are abundant globally, this implies a key role of ammonite (and also belemnite) hatchlings in the plankton of Mesozoic seas. With the end-Cretaceous mass-extinction, this ecospace became available. Remarkably, holoplanktonic gastropods resembling ammonite and belemnite hatchlings in form, size and habitat originated early in the Palaeogene and subsequently provided the ecological prerequisites for the evolution of large filter feeders such as mantas, whale sharks and baleen whales.