From head to tail: 3D imaging the whole body morphology of the stem gnathostome Anglaspis heintzi

Stem-gnathostomes are pivotal in reconstructing the evolutionary history of vertebrates but three-dimensional data of their body anatomies are currently lacking. Heterostracans, which constitute one of the earliest-branching lineages of stem-gnathostomes, present an ideal group to fill this gap as they are more commonly and completely preserved than other ostracoderm groups. Here, we reconstruct the first three-dimensional whole body morphology of the cyathaspid heterostracan Anglaspis heintzi, using X-ray microtomography. The oral apparatus is composed of three oral plate pairs, one pair of lateral oral plates, and bounded by a pair of lateral plates. The trunk is formed of a series of heavily ornamented trunk, dorsal ridge, ventral ridge and ventrolateral scales leading to the ‘hypocercal’ tail fin composed of numerous tail scales. Our retrodeformed reconstruction reveals a rigid oral apparatus incapable of substantial movement, resulting in a short wide gape to the mouth. This oral morphology precludes all previously proposed heterostracan feeding modes apart from suspension feeding. These findings contribute to the emerging consensus that heterostracans and jawless vertebrates more broadly possessed a greater diversity of feeding and swimming ecologies than previously thought.