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. 2013 Feb 27;9(2):20130057.
doi: 10.1098/rsbl.2013.0057. Print 2013 Apr 23.

Jaws for a spiral-tooth whorl: CT images reveal novel adaptation and phylogeny in fossil Helicoprion

Leif Tapanila  1 , Jesse Pruitt, Alan Pradel, Cheryl D Wilga, Jason B Ramsay, Robert Schlader, Dominique A Didier
Affiliations

Jaws for a spiral-tooth whorl: CT images reveal novel adaptation and phylogeny in fossil Helicoprion

Leif Tapanila et al. Biol Lett. .

Abstract

New CT scans of the spiral-tooth fossil, Helicoprion, resolve a longstanding mystery concerning the form and phylogeny of this ancient cartilaginous fish. We present the first three-dimensional images that show the tooth whorl occupying the entire mandibular arch, and which is supported along the midline of the lower jaw. Several characters of the upper jaw show that it articulated with the neurocranium in two places and that the hyomandibula was not part of the jaw suspension. These features identify Helicoprion as a member of the stem holocephalan group Euchondrocephali. Our reconstruction illustrates novel adaptations, such as lateral cartilage to buttress the tooth whorl, which accommodated the unusual trait of continuous addition and retention of teeth in a predatory chondrichthyan. Helicoprion exemplifies the climax of stem holocephalan diversification and body size in Late Palaeozoic seas, a role dominated today by sharks and rays.

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Figures

Figure 1.
Figure 1.
Reconstructions of Helicoprion since 1899. Earliest models (a–d) posited the whorl as an external defensive structure, but (e–l) feeding reconstructions dominate more recent hypotheses. Credits: (a) Woodward [2]; (b) Simoens [11]; (c) Karpinsky [12]; (d) Obruchev [7]; (e) Van den Berg in Obruchev [7]; (f) John [8]; (g) Carr [9]; (h) Eaton [4]; (i) Parrish in Purdy [10]; (j) Troll in Matsen & Troll [13] based on Bendix-Almgreen [5]; (k) Lebedev [6]; (l) Troll & Ramsay, this study. Configuration of gill slits and fins based on related fish, e.g. Caseodus and Ornithoprion [14].
Figure 2.
Figure 2.
Helicoprion specimen IMNH 37899, preserving cartilages of the mandibular arch and tooth whorl. (a) Photograph and (b) surface scan of fossil, positioned anterior to the right, imbedded in limestone slab. (c) CT model of specimen in lateral, (d) medial, (e) posterior, (f) oblique lateral, (g) oblique medial and (h) ventral views. Modelled tooth whorl (grey, black outline) surrounded by palatoquadrate (green), Meckelian (blue) and labial (red) cartilages. (d) Asterisks mark widened part of labial cartilage corresponding to successive root volutions. (f) Palatoquadrate removed to show scanned portion of root (dark yellow), tooth crowns (pale yellow) and tessellated cartilages of the inner whorl (purple). Arrow indicates direction of root growth and advancement to form spiral. (h) Right side of image mirrored to show paired jaw elements surrounding the whorl. bf, basitrabecular fossa; bp, basal process; c, cup-shaped portion of labial cartilage; ep, ethmoid process; lj, labial joint with base of root; pf, lateral palatine fossa; pp, process limiting jaw closure; qf, lateral quadrate fossa; qmf, quadratomandibular fossa; qp, quadrate process. Scale bar applies to all but oblique views (f–g).
See this image and copyright information in PMC

References

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    1. Eaton TH., Jr 1962. Teeth of edestid sharks. Univ. Kansas Publ. Mus. Nat. Hist. 12, 347–362
    1. Bendix-Almgreen SE. 1966. New investigations on Helicoprion from the Phosphoria Formation of south-east Idaho, USA. Biol. Skrifter udgivet af det Kongelige Danske Videnskabernes Selskab 14, 1–54

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