Evolutionary and developmental patterns of Cretaceous uncoiled ammonites
Final Report Abstract
The underexplored palaeobiology, intraspecific variability of heteromorph ammonoids (=ammonoids forming a conch with detached whorls (open coiling) or non-planispiral coiling) was detailed within the present DFG project. Therefore, a multidisciplinary approach was applied beginning with the development of a suitable set of morphological characters to describe the conch shape based on modern deep-sea cephalopod Spirula with an internal conch resembling heteromorphs. During this first phase of the project a software was developed in order to reduce the amount of time required to collect morphological data. Further, we used pathological forms, i.e. shells that show injuries, and regarded them as morphological end-members showing the limit of morphospace occupation. In a second project the set of Spirula conchs was extended to 21 specimens from all over the world, mainly including “normal” (not injured) specimens. The first relevant outcome was the recognition of two crypto-species complexes for Spirula. It turned out that parts of the Indo-Pacific population migrated towards the Atlantic via the Agulhas leakage which is active since the closure of the Isthmus. The Atlantic forms represent neotenous forms with a trend towards more closely coiled shells. During the second phase heteromorph ammonoid conchs have been analysed. Despite the original plan to collect data for all available type material covering a broad taxonomic range the project was slightly changed focussing on material from a single genus, namely Aegocrioceras with about 800 horizontally collected, already technically prepared specimens. The first morphological analyses of heteromorph ammonoids comprise 85 Aegocrioceras specimens from a single bed. Two species have been identified A. bicarinatum and A. semicinctum. The analyses led to a profound description of their intraspecific variability and revealed a large morphological overlap. Accordingly, both species were regarded as a sexual couple belonging to one species. Together was this publication the CONCH software was released as freeware. In the second part the sample number increased to 320 Aegocrioceras specimens from seven distinct beds. We conclude that Aegocrioceras was derived from Tethyan Crioceratites and gave later rise to Boreal Crioceratites. Further, we simplified the internal systematics of Aegocrioceras accepting only three out of ten species (A. raricostatum, A. spathi, and the A. bicarinatum – A-semicinctum complex). The following conclusions for heteromorph ammonoid palaeobiology, based on their fossil record, phylogenetic bracketing, and physical as well as virtual models, had be drawn: a) heteromorphs had ten arms, a well-developed brain, lens eyes, a buccal mass with a radula and a smaller upper as well as a larger lower jaw, and ammonia in their soft-tissue, b) Heteromorphs likely lacked arm suckers, hooks, tentacles, a hood, and an ink sac, c) all Cretaceous heteromorphs share an aptychus-type lower jaw with a lamellar calcitic covering, differences in the radular tooth morphology and size in heteromorphs suggest a microphagous diet, stomach content of heteromorphs comprise planktic crustaceans, gastropods, and crinoids suggesting a zooplanktic diet, d) forms with a u-shaped body chamber (ancylocone) are regarded as suspension feeders, whereas orthoconic forms might have consumed benthic prey in addition, e) Heteromorphs could achieve near-neutral buoyancy regardless of conch shape or ontogeny, f) Heteromorphs with a u-shaped body chamber are more stable hydrodynamically than modern Nautilus and were unable to substantially modify their orientation by active locomotion, i.e. they had no or limited access to benthic prey at adulthood, g) pathologies on Ptychoceras corroborates an external shell and rejects the endocochleate hypothesis, h) Devonian, Triassic, and Jurassic heteromorphs had a preference for deep-subtidal to offshore facies but are rare in shallow-subtidal, slope, and bathyal facies. Early Cretaceous heteromorphs preferred in deep-subtidal to bathyal facies. Late Cretaceous heteromorphs are common in shallowsubtidal to offshore facies, i) Oxygen isotope data suggest rapid growth and a demersal habitat for adult Discoscaphites and Baculites. A benthic embryonic stage, planktic hatchlings, and a habitat change after one whorl is proposed for Hoploscaphites. Carbon isotope data indicate that some Baculites lived throughout their lives at cold seeps.
Publications
- (2018) Blister pearl-formation in Spirula spirula. 10th International Symposium „Cephalopods Present and Past“, Fez, Morocco
René Hoffmann, Robert E. Lemanis, Helmut Keupp, Lena Wulff, Kenneth de Baets, Christian Klug
- (2018) CONCH – a new software for quantitative morphological analyses of ammonoid shells. 10th International Symposium „Cephalopods Present and Past“, Fez, Morocco
René Hoffmann, Peter Goeddertz, Manuel F. G. Weinkauf
- (2018) Morphological disparity and ontogeny of the endemic heteromorph ammonite genus Aegocrioceras. 16th Swiss Geoscience Meeting, Bern 30th November 2018
Manuel F. G. Weinkauf, René Hoffmann, Kurt Wiedenroth, Peter Goeddertz, Kenneth de Baets
- (2018) Traumatic events in the life of the deep-sea cephalopod mollusc, the coleoid Spirula spirula. Deep-Sea Research Part I, 142: 127-144
René Hoffmann, Robert E. Lemanis, Lena Wulff, Stefan Zachow, Alexander Lukeneder, Christian Klug, Helmut Keupp
(See online at https://doi.org/10.1016/j.dsr.2018.10.007) - 2018) Morphospace analyses of the endemic heteromorph ammonite genus Aegocrioceras (Lower Cretaceous, Hauterivian, NW-Germany. EGU, Vienna, Austria
René Hoffmann, Kenneth de Baets, Kurt Wiedenroth Manuel F. G. Weinkauf
- (2019) Morphological disparity and ontogeny of the endemic heteromorph ammonite genus Aegocrioceras (Early Cretaceous, Hauterivian, NW-Germany). Palaeogeography, Palaeoclimatology, Palaeoecology 520: 1-17
René Hoffmann, Manuel F. G. Weinkauf, Kurt Wiedenroth, Peter Goeddertz, Kenneth de Baets
(See online at https://doi.org/10.1016/j.palaeo.2019.01.020)
