Mineralization of the Callorhinchus Vertebral Column (Holocephali; Chondrichthyes)

Abstract

Members of the Chondrichthyes (Elasmobranchii and Holocephali) are distinguished by their largely cartilaginous endoskeletons, which comprise an uncalcified core overlain by a mineralized layer; in the Elasmobranchii (sharks, skates, rays) most of this mineralization takes the form of calcified polygonal tiles known as tesserae. In recent years, these skeletal tissues have been described in ever increasing detail in sharks and rays, but those of Holocephali (chimaeroids) have been less well-studied, with conflicting accounts as to whether or not tesserae are present. During embryonic ontogeny in holocephalans, cervical vertebrae fuse to form a structure called the synarcual. The synarcual mineralizes early and progressively, anteroposteriorly and dorsoventrally, and therefore presents a good skeletal structure in which to observe mineralized tissues in this group. Here, we describe the development and mineralization of the synarcual in an adult and stage 36 elephant shark embryo (Callorhinchus milii). Small, discrete, but irregular blocks of cortical mineralization are present in stage 36, similar to what has been described recently in embryos of other chimaeroid taxa such as Hydrolagus, while in Callorhinchus adults, the blocks of mineralization are more irregular, but remain small. This differs from fossil members of the holocephalan crown group (Edaphodon), as well as from stem group holocephalans (e.g., Symmorida, Helodus, Iniopterygiformes), where tesserae are notably larger than in Callorhinchus and show similarities to elasmobranch tesserae, for example with respect to polygonal shape.

Diversity and Evolution of Mineralized Skeletal Tissues in Chondrichthyans

“…Previous studies showed poorly developed cartilaginous tissue in the posterior vertebrae of juvenile H. colliei, where histology displayed no sign of modified cartilaginous matrix that could be linked to cartilage mineralization (Debiais-Thibaud, 2019). However, more recent publications pointed out the thin layer of mineralized matrix in the synarcual of the elephant shark Callorhinchus milii (Pears et al, 2020) and recognized thin tesserae in the adult C. milii and rabbit fishes Chimaera monstrosa (Pears et al, 2020;Seidel et al, 2020). In our H. colliei sample, we detect a similar thin layer of mineralized tissue in the synarcual of a hatchling and a juvenile specimen (Figures 4A,G).…”

“…The cells located on the perichondrial side of this line are enclosed in lacunae in a matrix of cartilaginous nature [as supported by positive Alcian blue staining (Figures 4F,H)], although the cells are flattened ( Figures 4E' ,G'). Moreover, no cells can be observed fully embedded in the mineralized cartilaginous matrix, either at the hatching or juvenile stage and the mineralized layer appears discontinuous (Figures 4G' ,H), supporting the presence of aligned tesserae as described in other holocephalan species (Pears et al, 2020).…”

“…Because chondrocytes are not embedded in the mineralized matrix of the poorly developed tesserae of E. spinax, C. crepidater, and H. colliei, we could not detect Liesegang lines. However, Liesegang lines have been described in the thin tesserae of other holocephalan species (Pears et al, 2020;Seidel et al, 2020), defining these tesserae as sites of globular mineralization. Previous studies also showed that fully developed tesserae first appear only as subperichondrial focal mineralization sites (Enault et al, 2015;Seidel et al, 2016).…”

“…More recent studies have reassessed the diversity of poorly described features of mineralization in cartilaginous fishes (Eames et al, 2007;Enault et al, 2015;Seidel et al, 2017Seidel et al, , 2020Atake et al, 2019;Debiais-Thibaud, 2019;Smith et al, 2019;Chaumel et al, 2020;Pears et al, 2020), raising new questions on the origin and evolution of mineralized tissues in this clade and therefore in vertebrates. Comparative studies of skeletal tissues in non-tetrapods have uncovered a wide range of skeletal tissues that had remained unknown from the sole study of tetrapods (discussed by Huysseune, 2009 andHall andWitten, 2019).…”

“…Extant chondrichthyans include three major clades. Holocephalans are divided into the three extant families Callorhinchidae, Chimaeridae, and Rhinochimaeridae and have long been considered to have a non-mineralized cartilaginous endoskeleton but are now recognized to have tesserae (Finarelli and Coates, 2014;Maisey et al, 2020;Pears et al, 2020;Seidel et al, 2020). Sister to holocephalans, elasmobranchs include selachians (sharks) that are grouped into Galeomorphii (orders: Carcharhiniformes, Heterodontiformes, Lamniformes, and Orectolobiformes) and Squalomorphii (orders: Hexanchiformes, Pristiophoriformes, Squaliformes, Squatiniformes, and the family Echinorhinidae) and batoids (rays, guitarfishes, skates, and sawfishes) that are divided into Myliobatiformes, Rhinopristiformes, Rajiformes, and Torpediniformes (Naylor et al, 2012;Ebert et al, 2013;Last et al, 2016).…”