Heterostraci systematics, phylogenetics and macroevolution: investigating evolutionary patterns of extinct jawless vertebrates

Heterostracans are extinct armoured jawless vertebrates and are hypothesized to be positioned as sister group to all other boney vertebrates - they are therefore fundamental to understanding early vertebrate evolution. Despite being taxonomically the largest clade of stem gnathostomes (jawed vertebrates), they lack the evolutionary framework required to further explore marcoevolutionary patterns and transitions. This thesis therefore aims to address one over arching theme: understanding the evolution and diversity of the Heterostraci and by extension, that of early vertebrates. This was first explored by creating novel phylogenies for 1) the Pteraspidiformes, 2) ‘higher heterostracans’ and lastly 3) Heterostraci as a whole. To achieve this I explored the inclusion of quantitative characters and implied versus equally weighted characters in reconstructing their evolutionary relationships. Stratigraphic congruence indices were also employed to assess tree topology arising from different treatments of characters. Evolutionary relationships arising from all three phylogenies reveal the Heterostraci form a clade, however, phylogenetic analyses including representatives of all major clades belonging to total group Gnathostomata did not recover a monophyletic Pteraspidimorphi. Instead the pteraspidimorphs were placed in a polytomy as the deepest branching boney vertebrates. Investigation into the fossil record of heterostracans reveals a cryptic early history, with their marcoevolutionary patterns tied to abiotic drivers (i.e. palaeosea-level). Direct biotic interactions between heterostracans and jawed vertebrates, in the form of bite marks, was also collated and assessed. This revealed new evidence of predation of heterostracans, however, this was not linked to any one group of jawed vertebrates. Instead heterostracan assemblages (and their constituent jawed vertebrates) were found to be statistically more similar to each other than those without predation traces, indicating certain taxa were likely acting as predators in the Silurian-Devonian seas.