An experimental morphological investigation into the hydrodynamics and locomotion of the Palaeozoic jawless vertebrates Poraspis, Errivaspis and Ateleaspis.

Ostracoderms represent a crucial phase of vertebrate evolution during which many of the characters that are now taken as typical vertebrate features appeared for the first time. While recent work has gone a long way to clarifying relationships and patterns of character acquisition, understanding of the ecology and biomechanics of these early fishes has lagged behind. Very little is known, for example, about how they swam. This thesis consists of two investigations, a new reconstruction of the caudal fin of Errivaspis waynensis, and an experimental investigation into the hydrodynamic characteristics of three ostracoderm species, Poraspis polaris, Errivaspis waynensis and Ateleaspis tesselata using models mounted in a wind tunnel. Errivaspis waynensis has previously been reconstructed with an elongate ventral tail lobe or “hypocercal” tail, based on a number of mouldic specimens held at the Natural History Museum London. These specimens were examined and it was concluded that traditional reconstructions of an extended ventral lobe are not upheld by the material and that the previously interpreted extreme hypocercal morphology is an artefact of missing material in the specimens. A new reconstruction for the caudal fin of Errivaspis is proposed with the ventral margin slightly longer than the dorsal margin. The experimental data from the wind tunnel tests suggested that Ateleaspis was more suited to a benthic lifestyle than Poraspis or Errivaspis, being able to utilize ground effect to swim very economically while close to the bottom and having a morphology that would permit it to turn rapidly in the vertical plane. Poraspis was found to have a morphology adapted to minimising drag for greater hydrodynamic efficiency and executing rapid manoeuvres in mid water column. Errivaspis was found to have a morphology with adaptations for both benthic and mid water swimming suggesting that it was a generalist.