Early vertebrate evolution

Abstract

Debate over the origin and evolution of vertebrates has occupied biologists and palaeontologists alike for centuries. This debate has been refined by molecular phylogenetics, which has resolved the place of vertebrates among their invertebrate chordate relatives, and that of chordates among their deuterostome relatives. The origin of vertebrates is characterized by wide-ranging genomic, embryologic and phenotypic evolutionary change. Analyses based on living lineages suggest dramatic shifts in the tempo of evolutionary change at the origin of vertebrates and gnathostomes, coincident with whole-genome duplication events. However, the enriched perspective provided by the fossil record demonstrates that these apparent bursts of anatomical evolution and taxic richness are an artefact of the extinction of phylogenetic intermediates whose fossil remains evidence the gradual assembly of crown gnathostome characters in particular. A more refined understanding of the timing, tempo and mode of early vertebrate evolution rests with: (1) better genome assemblies for living cyclostomes; (2) a better understanding of the anatomical characteristics of key fossil groups, especially the anaspids, thelodonts, galeaspids and pituriaspids; (3) tests of the monophyly of traditional groups; and (4) the application of divergence time methods that integrate not just molecular data from living species, but also morphological data and extinct species. The resulting framework will provide for rigorous tests of rates of character evolution and diversification, and of hypotheses of long-term trends in ecological evolution that themselves suffer for lack of quantitative functional tests. The fossil record has been silent on the nature of the transition from jawless vertebrates to the jawed vertebrates that have dominated communities since the middle Palaeozoic. Elucidation of this most formative of episodes likely rests with the overhaul of early vertebrate systematics that we propose, but perhaps more fundamentally with fossil grades that await discovery.

Exploring phylogenetic relationships of Pteraspidiformes heterostracans (stem-gnathostomes) using continuous and discrete characters

“…Heterostraci are taxonomically the largest group of stem-gnathostomes and along with other pteraspidimorphs have been interpreted as the most basal with a bony skeleton (Forey & Janvier 1993;Donoghue et al 2000;Donoghue & Aldridge 2001;Janvier 2001;Donoghue & Keating 2014). In previous analyses they have been recovered as the sister clade to galaeaspids, osteostracans and jawed vertebrates, or as a paraphyletic group leading to these taxa (Donoghue & Smith 2001;Gess et al 2006;Sansom et al 2010;Blom 2012).…”

“…Jawed vertebrate novelties such as acellular and cellular bone, a bony dermoskeleton, a differentiated gut, a mineralized neurocranium and paired pectoral fins, arose first within the jawless ostracoderms (i.e. heterostracans, thelodonts, anaspids, galeaspids and osteostracans) (Donoghue & Keating 2014). Identification of ostracoderm evolutionary dynamics along with their ancestral morphotypes will greatly improve our understanding of early vertebrate histories and thus the circumstances behind their diversification and the rise to dominance of gnathostomes.…”

Histology of the heterostracan dermal skeleton: Insight into the origin of the vertebrate mineralised skeleton

“…Like anaspids, however, the phylogenetic position of thelodonts is poorly resolved and it is currently uncertain whether they fall within the pteraspidomorph 1 jawed vertebrate clade. What is more, some studies have cast doubt on the monophyletic status of thelodonts (Donoghue and Keating, 2014;Donoghue and Smith, 2001).…”

“…1. Phylogenetic relationships of the principal vertebrate groups from Donoghue and Keating (2014). Labelled internal branches refer to acquisition of key skeletal apomorphies.…”

“…Evolutionary distribution of tissue types among total-group gnathostomes. Phylogenetic relationships of pteraspidomorphs based on Janvier (1996), relationships among stem-gnathostomes follow Donoghue and Keating (2014). Jawed vertebrate relations follow Zhu et al (2013).…”