Here we use synchrotron tomography to characterise dental vasculature in the oldest known tooth-bearing

sharks, Leonodus carlsi Mader, 1986 and Celtiberina maderi Wang, 1993. Three dimensional reconstruction

of the vascular system and microstructure of both taxa revealed a complex and dense network of canals, including

horizontal, ascending and secondary bifurcated canals, as well as histological features consistent with

an osteodont histotype. However, L. carlsi and C. maderi also exhibit significant morphological differences,

showing Leonodus a typical diplodont tooth morphology with a linguo-labially elongated base, that contrast

with Celtiberina’s teeth that show a single conical cusp curved lingually with a week developed flat base mesio-

distally extended, perhaps reflecting distant relationship. These data are compatible with a pre-Devonian

diversification of the two main tooth types traditionally recognised in Palaeozoic sharks (i.e., “cladodont” vs

“diplodont”). Finally, our data demonstrate that existing dental classification schemes based on styles of vascularisation

are over-simplified, especially when Palaeozoic taxa are considered.