Grasping the shape of belemnoid arm hooks—a quantitative approach

Abstract

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 hookmorphotypes. 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 interindividual changes in micro-hook morphology (Engeser 1987).

Predatory behaviour and taphonomy of a Jurassic belemnoid coleoid (Diplobelida, Cephalopoda)

“…At least two different types of micro-onychites are present: Larger hooks (up to 4 mm long), where the uncinus is strongly bent as well as 1 to 5 mm long onychites, where the uncinus is just slightly curved and the shaft is straight 21 . Their black colour and structure suggest that the hooks are carbonized similar to other coleoids from the Posidonienschiefer 35 .…”

“…ca. 300 in total) 34,35 . The size of the arm hooks is diverse with smaller hooks at the distal ends and proximal bases of the arms and larger hooks in the middle.…”

Fossilized leftover falls as sources of palaeoecological data: a ‘pabulite’ comprising a crustacean, a belemnite and a vertebrate from the Early Jurassic Posidonia Shale

“…Microhook size varies between c. 0.4 mm at the arm tips and 4 mm in the middle. The morphology of micro-hooks for various species has been described elsewhere (Fuchs, 2006;Hoffmann et al, 2017;Reitner & Urlichs, 1983;Riegraf & Hauff, 1983;Schlegelmilch, 1998).…”

New evidence from exceptionally “well-preserved” specimens sheds light on the structure of the ammonite brachial crown

“…A surprising feature, however, is the broad variation in size and shape of the elements in Rhaeboceras halli . In extinct coleoids (e.g., Donovaniconida, Belemnitida, and Phragmoteuthida), some morphological variability has been reported 50 , yet no more than four morphotypes within a single individual have been identified. Besides, the morphological differentiation of these morphotypes appears to mainly be due to their curvature.…”