A symmoriiform from the Late Devonian of Morocco demonstrates a derived jaw function in ancient chondrichthyans

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.

Scarce ctenacanthiform sharks from the Mississippian of Austria with an analysis of Carboniferous elasmobranch diversity in response to climatic and environmental changes

“…The dental terminology employed here is explained in Figure 2 and the systematic scheme follows that of Ginter et al (2010). The phylogenetic analyses of Frey et al (2019Frey et al ( , 2020 which produced a different systematic scheme are controversial and we therefore prefer using a more traditional approach until those analyses are supported or rejected by future studies.…”

Diverse stem-chondrichthyan oral structures and evidence for an independently acquired acanthodid dentition

“…2018 [51], (d) Frey et al . 2020 [52]. Boxes as in Fig.…”

“…Not all topologies support this hypothesis, however: Frey et al . [52], for example, recover climatiids as remote from the chondrichthyan crown node, with an extensively whorl-based dentition borne on the jaw cartilages rather than dermal bones apparently developing independently or being lost in “Acanthodii”. The interposition of many non-shedding stem-chondrichthyan taxa between shedding chondrichthyans and shedding osteichthyans confirms that a shedding dentition evolved twice, in two different ways, in crown-gnathostomes [6,7,11,14].…”

“…In holocephalans (chimaeras and relatives), the dentition is modified to two upper pairs and one lower pair of non-shedding hypermineralised toothplates [31]. The elasmobranch condition of file-like teeth is present in stem-group elasmobranchs [32] and stem-group holocephalans [33], which alongside the file-like whorls of stem- chondrichthyans [17], implies that this arrangement is plesiomorphic for chondrichthyans. However, the joined tooth of stem-group chondrichthyans [17], and evidence that at least some stem-group elasmobranchs had non-shedding dentitions [34], indicating that this mode of tooth replacement was a later acquisition.…”

“…Further confounding this diversity of oral structures are competing and unsettled hypotheses of relationships for early chondrichthyans. While ‘acanthodians’ are now established as stem-group chondrichthyans [24,32,33, 38–42], there is limited certainty over the monophyly of ‘acanthodian’ subgroups. There is some evidence that an assemblage of diplacanthiform, ischnacanthiform, and acanthodiform taxa may form a clade or grade subtending the remainder of the chondrichthyan total-group [24].…”