CT Reconstruction and the Phylogeny of Fossil Myriapods

Abstract

Myriapods represent one of the four major extant subphyla of Euarthropoda. They are diverse and ubiquitous, playing important ecological roles in a range of terrestrial ecosystems across all continents except antarctica. The myriapods are an ancient lineage, with molecular clock models estimating Early Cambrian origins, and a fossil record providing the oldest unequivocal evidence of terrestrial animal life. Despite their deep evolutionary history and present-day abundance, questions remain regarding the phylogenetic placement of several important fossil groups, including a potential aquatic stem-group which could help fill the gap between their Cambrian origins and the appearance of terrestrial myriapods in the Silurian. In this thesis, I utilise x-ray microtomography to reconstruct four Carboniferous fossils of the millipede genus Myriacantherpestes (Archipolypoda: Euphoberiida) for the first time, revealing previously undescribed aspects of their cephalic morphology including: a fully articulated antenna; the gnathochilarium; and the basal articles and gnathal lobes of the mandibles. I then perform morphology- based phylogenetic analysis under both maximum parsimony and Bayesian inference, to test the phylogenetic placement of the following myriapod fossil taxa: Arthropleura, Microdecemplex rolfei (Arthropleuridea); Myriacantherpestes sp. (Archipolypoda: Euphoberiida); Pleurojulida (coded to ordinal level); Amynilyspes fatimae (Amynilyspedida); Schramixerxes gerem, Heterocrania rhyniensis (Euthycarcinoidea). These analyses are subsequently re-run with forced constraints to investigate the effects of molecular hypotheses on the position of euthycarcinoids. The results of these analyses find: Arthropleuridea is placed at the base of Diplopoda, either as a monophyletic sister group to Chilognatha or in a polytomy with Penicillata and Chilognatha; Myriacantherpestes is recovered as a helminthomorph millipede in various positions, highlighting the uncertainty surrounding the monophyly of the superorder Archipolypoda; Pleurojulida is resolved within Helminthomorpha, with some evidence of colobognathan affinities; A. fatimae is repeatedly placed alongside extant Pentazonia; and the euthycarcinoids are recovered as monophyletic, with most analyses supporting the recent hypothesis that they represent an aquatic stem-group of Myriapoda. This is the first study to include the arthropleurideans (Microdecemplex, Arthropleura), Myriacantherpestes (Euphoberiida) and Pleurojulida in a well-sampled morphological phylogeny and represents the first cladistic analysis of euthycarcinoids with the addition of characters recently proposed to support a euthycarcinoid-myriapod alliance. The CT reconstructions and morphological matrix produced during this research could be useful to inform future studies, including a millipede-focused phylogenetic analysis and a myriapod-focused total evidence analysis.

Date of Award	14 Apr 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Russell Garwood (Main Supervisor) & Rob Sansom (Co Supervisor)