Abstract
An accepted uniting character of modern cartilaginous fishes (sharks, rays, chimaera) is the presence of a mineralized, skeletal crust, tiled by numerous minute plates called tesserae. Tesserae have, however, never been demonstrated in modern chimaera and it is debated whether the skeleton mineralizes at all. We show for the first time that tessellated cartilage was not lost in chimaera, as has been previously postulated, and is in many ways similar to that of sharks and rays. Tesserae in Chimaera monstrosa are less regular in shape and size in comparison to the general scheme of polygonal tesserae in sharks and rays, yet share several features with them. For example, Chimaera tesserae, like those of elasmobranchs, possess both intertesseral joints (unmineralized regions, where fibrous tissue links adjacent tesserae) and recurring patterns of local mineral density variation (e.g. Liesegang lines, hypermineralized ‘spokes’), reflecting periodic accretion of mineral at tesseral edges as tesserae grow. Chimaera monstrosa's tesserae, however, appear to lack the internal cell networks that characterize tesserae in elasmobranchs, indicating fundamental differences among chondrichthyan groups in how calcification is controlled. By compiling and comparing recent ultrastructure data on tesserae, we also provide a synthesized, up-to-date and comparative glossary on tessellated cartilage, as well as a perspective on the current state of research into the topic, offering benchmark context for future research into modern and extinct vertebrate skeletal tissues.
Highlights
The cartilaginous fishes—sharks, rays (Elasmobranchii) and closely related chimaera (Holocephali)—exhibit a uniting anatomical character: unmineralized cartilage forms the majority of the skeleton of both juvenile and adult animals
Since the only detailed information on modern tesserae comes from elasmobranchs, for each feature, we summarize elasmobranch data first to provide a standard for interpreting our observations on the skeletal tissue of C. monstrosa
We show that is tessellated cartilage a shared character between modern elasmobranchs and chimaera, and that multiple specific morphological characters unify tessellated cartilage and tesserae in these groups
Summary
The cartilaginous fishes—sharks, rays (Elasmobranchii) and closely related chimaera (Holocephali)—exhibit a uniting anatomical character: unmineralized cartilage forms the majority of the skeleton of both juvenile and adult animals. Müller [35]; Leydig [36,37]; Tretjakoff [38]; Lorch [24]; Ørvig [15]; Applegate [4]; Peignoux-Deville et al [39]; Bordat [17]; Clement [18]; Summers [40]; Ortiz-Delgado et al [34]; Egerbacher et al [31]; Dean et al [21,41,42]; Johanson et al [43]; Maisey [6]; Omelon et al [26]; Seidel et al [23,44]; Knötel et al [45]; Seidel & Blumer et al [3]; Huang et al, [46]; Atake et al [27]; Chaumel et al [47]; Marconi et al [48]. Despite immense variation observed in tesseral morphology and tiling pattern (e.g. [6,23]), a number of characters appear to be common to the tesserae of different modern shark and ray species [23,44]
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