Figure 1. Hypothesis of vertebrate relations based on Keating & Donoghue17.
Heterostracans, together with the Ordovician aged Arandaspids and Astraspis comprise the clade Pteraspidomorpha, interpreted as the sister group to all other ostracoderms + crown gnathostomes (i.e. all other skeletonising vertebrates). Based on our histological analyses we inferred character evolution using parsimony. Black triangles represent character gain and white triangles represent character loss. Cartilage (1), dermal bone (2), dentine (3), enameloid (4), perichondral bone (5), endochondral bone (6), enamel (7), cell spaces in dermal bone (*).