Abstract
The Palaeozoic record of chondrichthyans (sharks, rays, chimaeras, extinct relatives) and thus our knowledge of their anatomy and functional morphology is poor because of their predominantly cartilaginous skeletons. Here, we report a previously undescribed symmoriiform shark, Ferromirum oukherbouchi, from the Late Devonian of the Anti-Atlas. Computed tomography scanning reveals the undeformed shape of the jaws and hyoid arch, which are of a kind often used to represent primitive conditions for jawed vertebrates. Of critical importance, these closely fitting cartilages preclude the repeatedly hypothesized presence of a complete gill between mandibular and hyoid arches. We show that the jaw articulation is specialized and drives mandibular rotation outward when the mouth opens, and inward upon closure. The resultant eversion and inversion of the lower dentition presents a greater number of teeth to prey through the bite-cycle. This suggests an increased functional and ecomorphological disparity among chondrichthyans preceding and surviving the end-Devonian extinctions.
Highlights
IntroductionThe Palaeozoic record of chondrichthyans (sharks, rays, chimaeras, extinct relatives) and our knowledge of their anatomy and functional morphology is poor because of their predominantly cartilaginous skeletons
The Palaeozoic record of chondrichthyans and our knowledge of their anatomy and functional morphology is poor because of their predominantly cartilaginous skeletons
The Symmoriiformes is a widely distributed group of early chondrichthyans ranging from the Late Devonian through to the early Permian, and perhaps extending as far as the Cretaceous[1,2,3,4]
Summary
The Palaeozoic record of chondrichthyans (sharks, rays, chimaeras, extinct relatives) and our knowledge of their anatomy and functional morphology is poor because of their predominantly cartilaginous skeletons. We describe a symmoriiform chondrichthyan from the Devonian of Morocco and investigate the morphology and biomechanics of the superbly preserved jaws and hyoid arch These jaws display the classic ‘cleaver’ shape[17] palate seen in a vast array of early crown-gnathostomes, and especially among chondrichthyans[2]. The preservation of the hyoid and mandibular arches is such that a physical reconstruction of the feeding apparatus is possible, allowing an investigation of jaw movements (kinematics) in three dimensions[18], and a more detailed comparison with the feeding mechanics of living chondrichthyans[19] By these means, it may be possible to gain a better appreciation of symmoriid chondrichthyans as early specialists or () generalist predators, and add to an emerging picture of functional disparity and ecomorphological partitioning among these early members of the modern vertebrate biota[18,20]
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