Exploring the three-dimensional vasculature of dermal hard tissues in thyestiid osteostracans using synchrotron radiation microtomography

Osteostracans are stem-gnathostomes with bony tissues that represent an important link between living jawed and jawless vertebrates, the latter of which lack skeletal hard tissues. In this study, a number of skeletal microremains from a diverse group of osteostracans called thyestiids have been investigated using propagation phase-contrast X-ray synchrotron microtomography. This enabled detailed reconstructions of their three-dimensional vasculature. The dermal skeleton of thyestiids is divided into three layers that vary in proportion between taxa, similar to gnathostomes and other osteostracans. The basal layer is confirmed as an acellular bony tissue composed of fiber bundles that contains narrow, vertical canals and/or large cavities. Bundles bend around smaller canals but form jagged edges around larger cavities, which suggests delayed mineralization. The presumed vascular mesh canals of the deep middle layer are radially arranged in some taxa, but more irregular or even tree-like in others, suggesting a relation to how the elements developed and grew. A number of mesh canals ascend toward the superficial layer where they supply the subepidermal vascular plexus, or create pore fields that open to the surface in some taxa. These ascending mesh canals most likely expanded and formed a network of canals underneath more extensive pore fields. Aestiaspis viitaensis has a simple upper canal system above these fields, while Tremataspis mammillata and Tr. milleri scales have a polygonal network of upper mesh canals above extensive perforated septa. These systems are most likely homologous, but their relation to intra- and inter-areal canals described in other osteostracans remains ambiguous.