Unravelling the deep homology of gill slits and the origin of bilaterian body plans

Bilaterians comprise the vast majority of animal diversity and disparity and can be divided into three main clades; the ecdysozoans (moulting animals like arthropods), spiralians (e.g. annelid worms and molluscs) and the deuterostomes (including us and starfish). The extant representatives of these groups are so morphologically divergent that identifying shared derived characters that would have been present in the ancestors of major animal lineages is extremely difficult, obfuscating what features may have been present in the ancestral bilaterian.

While it is well established that ecdysozoans and spiralians are sister taxa, the phylogenetic position of deuterostomes is less certain. Recent work has challenged the traditional view that the deuterostomes are monophyletic, instead inviting the possibility that they are a paraphyletic grade at the base of the Bilateria. If correct, this would imply that the ancestor of all or much of bilaterian diversity was deuterostome-like. 

Deuterostomes are a divergent group of animals, encompassing animals as different as starfish, vertebrates and sea squirts and there are very few morphological characters which unite them to the exclusion of other animal groups. Gill slits are an iconic deuterostome character that are conspicuously present in most aquatic vertebrates (i.e. fish), but are absent in echinoderms (starfish, urchins and relatives) which instead breathe through their porous skeletons. Fossil echinoderms first appear over half a billion years ago during the Cambrian Explosion, revealing a diverse array of extinct forms with body plans unlike their living relatives. Intriguingly, multiple different extinct echinoderm lineages show a variety of often serialised structures which have been interpreted as respiratory and therefore may be homologous to gill slits seen in other deuterostomes. However, there is no consensus about the function of these structures across echinoderms and no consensus about whether they are deeply homologous (i.e. share common ancestry) with each other and/or true gill slits in other deuterostomes, or if they have evolved independently in different lineages, perhaps in response to environmental drivers like oxygen concentration.

This project aims to resolve the nature of these respiratory structures in ancient echinoderms and their relationship to gill slits in other deuterostomes by first characterising the morphology of gill slits in living deuterostomes to establish the criteria for recognising them in fossils. The student will then characterise the spectrum of possible respiratory structures in fossil echinoderms both anatomically and functionally. These different lines of evidence will be used to assess homology and produce a new morphological phylogenetic framework in which the deep homology of these structures can be assessed and the nature of the deuterostome and perhaps bilaterian ancestor revealed. 

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