Proceedings Chapter
Reference
Grasping the shape of belemnoid arm hooks – a quantitative approach
HOFFMANN, René, WEINKAUF, Manuel, FUCHS, Dirk
HOFFMANN, René, WEINKAUF, Manuel, FUCHS, Dirk. Grasping the shape of belemnoid arm hooks – a quantitative approach. In: Castelltort, S.; Girardclos, S.; Gnos, E.; Samankassou, E.;
Lupi, M.; Martini, R.; Schaltegger, U. & Ariztegui, D. 14th Swiss Geoscience Meeting : Time in Geosciences: Knowledge for a New Beginning. Geneva : 2016.
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14th Swiss Geoscience Meeting, Geneva 2016
Grasping the shape of belemnoid arm hooks – a quantitative approach
René Hoffmann*, Manuel F. G. Weinkauf** & Dirk Fuchs***
*Institut für Geologie, Mineralogie & Geophysik, Ruhr-Universität Bochum, Universitätsstrasse 150, DE-44780 Bochum (rene.hoffmann@rub.de)
**Section des Sciences de la Terre et de l’Environnement, Université de Genève, Rue des Maraîchers 13, CH-1205 Genève
***Department of Earth and Planetary Sciences, Hokkaido University, Kita-10 Nishi-8 Kita-ku, JP-060-0810 Sapporo
Chitinous arm hooks (onychites) of belemnoid coleoids are widely distributed in Mesozoic sediments. Due to their relative abundance and variable morphology compared to the single, bullet-shaped belemnite rostrum, arm hooks came into the focus of micropalaeontologist as a promising index fossil group for the Jurassic–Cretaceous rock record and have been the target of functional,
ecological, and phylogenetic interpretations in the past (e.g. Reitner & Engeser 1982; Fuchs et al. 2013; Hammer et al. 2013). Based on three well-preserved arm crowns of the Toarcian diplobelid Chondroteuthis wunnenbergi, we analyzed the shape of a total of 87 micro-hooks. The arm crown of
Chondroteuthis is unique in having uniserial (rather than biserial) hooks. The first application of elliptic Fourier shape analysis to the arm weapons of belemnoid coleoids allows for the distinction of four micro-hook morphotypes and the quantification of shape variation within these morphotypes. Based on the best preserved arm crown, we reconstructed the distribution of morphotypes within the arm crown as well as along a single arm. Our quantitative data
support former observations that smaller hooks were found close to the mouth and at the most distal arm parts, while the largest hooks were found in the central part of the arm crown. Furthermore, we found a distinct arm
differentiation, as not every arm was equipped with the same hook-
morphotypes. Here, we report the functional specialisation of the belemnoid arm crown for the first time and speculate about the potential function of the four morphotypes. Our analyses suggest a highly adapted functional morphology and intra-specimen individual distribution of belemnoid hooks, serving distinct purposes mainly during prey capture, prey digestion, as well as reproduction. In this regard, hooks at the distal end of the arms show stronger curvature to effectively catch and hold the prey, while hooks closer to the belemnoids mouth are more suitable for prey dissection and for transporting food to the mouth. We speculate that this highly specialised arrangement is an adaptation of
Chondroteuthis towards its uniserial hook-arrangement. This is supported by the fact that belemnoids with biserial hook armament do not show inter- individual changes in micro-hook morphology (Engeser 1987).
14th Swiss Geoscience Meeting, Geneva 2016
Figure 1. Principal component analysis of Chondroteuthis wunnenbergi micro- hook morphology. Symbols represent the individual hooks, grey silhouettes represent the theoretical mean form of the respective 0.1 × 0.1 square. The positions of the four distinguished morphotypes are indicated.
REFERENCES
Engeser, T. 1987: Belemnoid arm hooks (onychites) from the Swabian Jurassic – a review. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen 176, 5–14.
Fuchs, D., Donovan, D. T. & Keupp, H. 2013: Taxonomic revision of
“Onychoteuthis” conocauda Quenstedt, 1849 (Cephalopoda: Coleoidea). Neues Jahrbuch für Geologie und Paläontologie Abhandlungen 270, 245–255.
Hammer, Ø, Hryniewicz, K., Hurum, J. H., Hoyberget, M., Knudsen, E. M. &
Nakrem, H. A. 2013: Large onychites (cephalopod hooks) from the Upper Jurassic of the Boreal Realm. Acta Palaeontologica Polonica 58, 827–835.
Reitner, J. & Engeser T. 1982: Zwei neue Coleoidea-Arten aus dem
Posidonienschiefer (Untertoarcium) aus der Gegend von Holzmaden (Baden- Württemberg). Stuttgarter Beiträge zur Naturkunde Serie B (Geologie und Paläontologie) 84, 1–19.
