Morphological disparity and ontogeny of the endemic heteromorph ammonite genus Aegocrioceras

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Morphological disparity and ontogeny of the endemic heteromorph ammonite genus Aegocrioceras

WEINKAUF, Manuel, et al.

WEINKAUF, Manuel, et al . Morphological disparity and ontogeny of the endemic heteromorph ammonite genus Aegocrioceras. In: Klug, Christian; Scheyer, Torsten & Cavin, Lionel. Abstract Volume, 16th Swiss Geoscience Meeting: 5. Palaeontology . 2018. p. 184-185

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184

Symposium 5: Palaeontology

Platform Geosciences, Swiss Academy of Science, SCNAT Swiss Geoscience Meeting 2018

5.16 Morphological disparity and ontogeny of the endemic heteromorph ammonite genus Aegocrioceras

Manuel F. G. Weinkauf¹, René Hoffmann², Kurt Wiedenroth³, Peter Goeddertz⁴ & Kenneth De Baets⁵

1 Département des sciences de la Terre, University of Geneva, Rue des Maraîchers 13, CH-1205 Genève (Manuel.Weinkauf@unige.ch)

2 Institut für Geologie, Mineralogie und Geophysik, Ruhr-University Bochum (RUB), Universitätsstraße 150, D-44801 Bochum

3 Am Hohen Holze, D-30823 Garbsen

4 Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Rhenian Friedrich-Wilhelm University Bonn, Nussallee 8, D-53115 Bonn

5 GeoZentrum Nordbayern, Friedrich–Alexander University Erlangen–Nuremberg, Schlossgarten 5, D-91054 Erlangen

Intraspecific variation based on heritable traits is considered an important motor for evolution (Hunt 2007) but is seldomly studied in palaeontology, and despite the fact that heteromorph ammonites often show a large range of intra-specific variation, that variation has rarely been quantified. This is unfortunate, since especially loosely coiled forms suffer from severe taxonomic over-splitting due to their large degree of morphological variation (De Baets et al. 2013). To that end, we herein quantify the intraspecific variation and ontogeny of 85 newly collected specimens of the Hauterivian endemic ammonite genus Aegocrioceras. All specimens have been collected from concretions in the newly exposed bed 82 in the clay pit Resse in NW Germany (Lower Saxony Basin, Mutterlose & Wiedenroth 2009). Data for the conch morphology (diameter, whorl height, whorl interspace, rib spacing) have been collected in 45° steps using the new software CONCH^©. All ammonites have been a priori identified as Aegocrioceras bicarinatum or Aegocrioceras semicinctum following Rawson (1975). Morphological disparity and the development over the ontogeny have been quantified using univariate and multivariate analyses (Fig. 1), and an automated species delimitation algorithm (Ezard et al. 2010) was applied. An assessment of the intra- and inter-specific variation and ontogeny of this population demonstrates the presence of just one species, viz. A. bicarinatum. Comparison with the type material of A. bicarinatum and A. semicinctum reveals large morphological overlap of different shell parameters and does not support a separation of both forms as independent species. The analysed population of A. bicarinatum has a high variation in shell morphology including whorl interspace, umbilical width, and whorl expansion. Further, the changes in intraspecific variation through ontogeny have been quantified and we postulate that the two “species” are probably males and females of the same species, showing sexual dimorphism.

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185

Symposium 5: Palaeontology

Platform Geosciences, Swiss Academy of Science, SCNAT Swiss Geoscience Meeting 2018

Figure 1. Principal component analysis (PCA) of A. bicarinatum/A. semicinctum from the clay pit Resse. Convex hulls encompassing all specimens of the respective species. Representative examples of the morphologies across the PCA space are shown.

REFERENCES

De Baets, K., Klug, C. & Monnet, C. 2013: Intraspecific variability through ontogeny in early ammonoids. Paleobiology 39, 75–94.

Ezard, T. H. G., Pearson, P. N. & Purvis, A. 2010: Algorithmic approaches to aid species’ delimitation in multidimensional morphospace. BMC Evolutionary Biology 10, Article 175.

Hunt, G. 2007: The relative importance of directional change, random walks, and stasis in the evolution of fossil lineages.

Proceedings of the National Academy of Sciences of the United States of America 104, 18404–18408.

Mutterlose, J. & Wiedenroth, K. 2009: Neue Tagesaufschlüsse der Unter-Kreide (Hauterive–Unter-Apt) im Großraum Hannover–Braunschweig: Stratigraphie und Faunenführung. Berliner paläobiologische Abhandlungen 10, 257–288.

Rawson, P. 1975: Lower Cretaceous ammonites from North-East England: The Hauterivian heteromorph Aegocrioceras.

Bulletin of the British Museum of Natural History and Geology 26, 131–159.