The early evolution of jawed vertebrates (gnathostomes), with a special focus on sensory systems and the application of Bayesian phylogenetic methods

Author: Ben King

King, Ben, 2018 The early evolution of jawed vertebrates (gnathostomes), with a special focus on sensory systems and the application of Bayesian phylogenetic methods, Flinders University, College of Science and Engineering

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The jawed vertebrates, or gnathostomes, are a highly diverse group that have conquered almost every habitat on earth. Indisputable articulated gnathostome fossils first appear in the Silurian Period, following which the group underwent a major radiation. Modern gnathostomes include osteichthyans (bony fishes, including tetrapods) and chondrichthyans (cartilaginous fishes i.e. sharks, rays and ratfishes). Extinct gnathostome groups include placoderms (“armoured fish”) and acanthodians (“spiny sharks”). In this thesis, three main approaches are taken to study the early evolution of gnathostomes. First, studies of sensory systems, particularly electroreception, provide insights into ecological function. Second, phylogenetic studies aim to reveal sequences of character change and rates of morphological evolution. Third, descriptions of new fossils of key taxa provide new character information and ecological interpretations.

Electroreception, the ability to detect electric fields, is phylogenetically widespread in vertebrates, suggesting it may be an ancestral feature. However, the deep evolutionary history of electrosensory systems is largely unknown. In this thesis, I utilise computed tomography (CT) scans and digital segmentation to study putative electroreceptors in early vertebrate fossils. I conclude that there is currently no evidence for electroreception in placoderms. However, the “pore-group” pits on the snouts of some early osteichthyans (which are particularly developed in early lungfishes) are shown to have several features consistent with identification as electroreceptors.

The phylogenetic relationships of early gnathostomes are a major source of debate. Current hypotheses favour a scenario in which the placoderms are paraphyletic with respect to other gnathostomes. In this thesis, tip-dated (or morphological clock) Bayesian methods, which model morphological evolution, speciation, extinction and fossil sampling, are for the first time applied to the problem of early vertebrate phylogeny. These methods strongly support placoderm monophyly, despite morphological evidence being essentially equal for paraphyly and monophyly.

Fossils of the placoderm Brindabellaspis have been central to discussions of early gnathostome phylogeny. New material of Brindabellaspis reveals a bizarre elongate rostrum supported by a thin anterior expansion of the braincase. The dorsal surface has a midline sensory canal, which CT scans reveal to be equivalent to a transverse canal in other placoderms that has doubled back and fused on itself. The new specimens highlight morphological disparity in early gnathostomes.

The skull of the osteichthyan “Ligulalepis” revealed an unexpected combination of features when first described. I describe a second specimen, together with substantial new information from CT scans. “Ligulalepis” has an unusual combination of morphological features, including a placoderm-like pituitary vein, a chondrichthyan-like labyrinth. Presence of a lateral cranial canal is variable between specimens.

The effect of Tip-dated Bayesian methods on the recovery of evolutionary relationships is relatively unexplored. Here I show that these methods favour trees with better stratigraphic fit, which have a higher prior probability. These prior probabilities are particularly influential in parts of phylogenetic trees with weak character evidence. As applied to early osteichthyan phylogeny, tip-dated methods favour an actinopterygian position for “Ligulalepis” over a stem osteichthyan position.

Keywords: early vertebrates, fossils, vertebrates, Bayesian, phyogenetics, tip-dating, electroreception, placoderms

Subject: Biology thesis

Thesis type: Doctor of Philosophy
Completed: 2018
School: College of Science and Engineering
Supervisor: John Long