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. 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. Here, we describe the development and mineralization of the synarcual in an adult and stage 36 elephant shark embryo ( Callorhinchus milii ). 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. During embryonic ontogeny in holocephalans, cervical vertebrae fuse to form a structure called the synarcual.
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