The synarcual of the little skate: Novel development among the vertebrates

The synarcual is a specialized adaptation of the anterior axial skeleton comprising a putatively fused array of vertebral elements characteristic of jawed vertebrate (gnathostome) clades such as batoid and chimaeroid chondrichthyans, as well as a fossil group known as the placoderms. Placoderms represent the phylogenetically most basal jawed vertebrates and the presence of a synarcual in these and chondrichthyans may suggest a conserved vertebral type for jawed vertebrates, predating the divergence of stem and crown gnathostomes. Alternatively, synarcuals may have evolved independently in these lineages, exhibiting a remarkable case of morphological convergence. We investigated the early development of the cervicothoracic synarcual of an emerging model chondrichthyan, the Little skate Leucoraja erinacea, by combining x-ray computed tomography, classical histology, and a de novo transcriptome assembly for two developmental stages of the skate synarcual and post-synarcual axial skeletal elements. We produced a de novo transcriptome assembly and expression analysis from two developmental stages (28 and 33) of the skate, Leucoraja erinacea, synarcual (Syn) and post-synarcual (Psyn) axial skeletal elements.