Functional aspects of the headshield processes in ostracoderms

Abstract Ostracoderms, Paleozoic jawless stem-gnathostomes, are characterized by distinctive bony shields covering the front of their bodies. These headshields exhibit significant variations in morphology across species, boasting frontal, lateral, and dorsal processes. Ostracoderms represent pivotal intermediaries between modern jawless and jawed vertebrates, so understanding their biology and ecology is crucial for unraveling the selective pressures that shaped the early evolution and diversification of jawed vertebrates, which now dominate vertebrate diversity. This study employs virtual paleontology techniques and phylogenetic comparative methods to explore the hydrodynamic and ecological implications of these processes, focusing on pteraspidomorphs, the most diverse ostracoderm group. The analysis reveals widespread convergence in the arrangement and development of headshield processes. Lateral processes enhance hydrodynamic efficiency and generate lift, while combined lateral and dorsal processes provide stability in rolling, yawing, and pitching. Frontal processes reduce drag in many cases. These findings illuminate the enigmatic roles of ostracoderm headshields, showing how the dimensions and arrangement of their processes are biomechanically linked to a range of functions and ecological roles. Collectively, this highlights the intricate evolutionary pathways of lifestyles and ecologies within stem-gnathostomes, challenging the idea of a unidirectional trend toward more active lifestyles in vertebrate evolution and suggesting diverse ecological roles for ostracoderms.