Abstract
Ostracoderms are a paraphyletic group of extinct jawless fishes comprising the gnathostome stem and are fundamental to our understanding of early vertebrate evolution. However, only a handful of these clades have robust phylogenies in place, hindering our interpretation of early vertebrate histories. A new phylogeny is proposed for the Pteraspidiformes – the largest and most-studied clade of heterostracan ostracoderms. Difficulties such as large amounts of missing data and the limited morphological variability within the group have led us to explore different coding strategies such as the inclusion of quantitative data and implied weighting. We present a new comprehensive data set including all described genera of Pteraspidiformes (47 taxa) analysed using discrete characters only, a combination of discrete and continuous characters, and gap-coding strategies (transforming the continuous into discrete characters), along with a Bayesian analysis. Two representatives of the Psammosteidae (Drepanaspis and Psammosteus) are also incorporated within the analysis to elucidate their inclusiveness within the Pteraspidiformes. Well-resolved trees are only achieved under re-weighted (implied weighting) analyses. Here, we show that many ‘classic’ Pteraspidiformes clades hold true under our different coding methods, with the implied weighting of discrete characters and inclusion of continuous characters giving very similar topologies. In all instances, the Psammosteidae are found to belong within the Pteraspidiformes, nested with the Spitsbergen genera Doryaspis, Xylaspis and Woodfjordaspis. Gap coding, however, results in a different tree topology to other analyses, perhaps due to the high sensitivity to missing data. Our results indicate that careful consideration and justifications should be applied to quantitative characters when reconstructing relationships of homoplastic ostracoderms. Superfamily Doryaspidae superfam. nov. is introduced.http://zoobank.org/urn:lsid:zoobank.org:pub:BF5255A1-1417-432B-AC64-BF132A0BCC65
Highlights
Phylogenetic relationships of fossil jawless vertebrates (Agnatha) are imperative to our understanding of vertebrate evolution
The anchipteraspidid taxa Rachiaspis, Anchipteraspis and Ulutitaspis are recovered in a clade sister to all other Pteraspidiformes
The Psammosteidae are recovered within Pteraspidiformes but within the unresolved polytomy
Summary
Phylogenetic relationships of fossil jawless vertebrates (Agnatha) are imperative to our understanding of vertebrate evolution. 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).
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