A Phylogeny for Heterostraci (stem-gnathostomes)

The armoured jawless fishes (ostracoderms) are major and widespread components of middle Palaeozoic ecosystems. As successive plesia on the gnathostome lineage, they reveal the early sequences of vertebrate evolution, including the assembly of the vertebrate skeleton. This is predicated however, on understanding of their diversity and interrelationships. The largest ostracoderm clade, the Pteraspidimorphi, is often reconstructed as sister taxon to other boney vertebrates yet they lack a phylogenetic framework, in particular the heterostracans. Problematic heterostracans with a tessellate headshield (‘tessellate-basal’ model) are often regarded as the plesiomorphic condition for the clade but no phylogenetic analysis has included these taxa. Here we review the Heterostraci and present their first comprehensive phylogenetic analysis (131 heterostracan taxa and 12 outgroup taxa). Heterostraci and Ordovician Pteraspidimorphi are recovered as sister-group to all other boney jawless vertebrates in parsimony analyses, however, in no instances do we recover a monophyletic Pteraspidimorphi. Tree visualization reveals lack of resolution results from two conflicting solutions for the heterostracan ‘root’. Stratigraphic congruences provides support for the macromeric Ctenaspisdidae as sister taxon to all other Heterostraci rather than the “tesselate-basal” model. The results presented here are the first phylogenetic hypotheses of heterostracan relationships and it is hoped a first step into an accurate interpretation of character evolution and polarity in this crucial episode of vertebrate evolution.

congruences provides support for the macromeric Ctenaspisdidae as sister taxon to all other 23 Heterostraci rather than the "tesselate-basal" model. The results presented here are the first 24 phylogenetic hypotheses of heterostracan relationships and it is hoped a first step into an accurate 25 interpretation of character evolution and polarity in this crucial episode of vertebrate evolution.

INTRODUCTION 29
Relationships of early vertebrates are important to understanding the early events in our own evolutionary history. The armoured jawless vertebrates known as the 'ostracoderms' are key

HETEROSTRACAN ANATOMY 77
The bone of heterostracan dermal skeleton is composed mainly of aspidin, a type of acellular bone rostral and orbital areas.   . 2), whereas in others it is encased within the dorsal shield (Fig. 3). The pineal organ is often 8 smaller plates are regarded as the oral platelets. However, the oral platelets of Psammosteids (e.g.

12
The Psammosteidae includes the stratigraphically youngest heterostracans and contains some of

350
The Amphiaspididae are only found in deposits from the Northwestern Siberian platform and

358
The Traquairaspididae have not previously been included in any phylogenetic analyses. Tarrant

359
(1991) subdivided the traquairaspids into two families (Phialaspididae and Traquairaspididae), and 360 placed them both in the order Traquairaspidiformes. When considering the evolutionary 361 relationships of heterostracans, both Obruchev (1967) and Janvier (1996) (Fig. 4)              ctenaspids. These were united under the Cyathaspidiformes, which is in turn were sister group to 565 the Pteraspidiformes (Psammosteidae + pteraspids). All these were informally labelled the 'higher 566 heterostracans' defined by a branchial plate not pierced by the branchial opening (Janvier, 1996).

567
In a polytomy at the base of the heterostracans are Lepidaspis, Toombsaspis (a member of the 568 Traquairaspididae) and ?Traquairaspis mackenziensis (Fig.4C). These taxa are generally regarded 569 as problematic with unknown affinities.

570
The majority of phylogenetic analyses including heterostracans have been clade specific, or only

RESULTS 1292
Initial parsimony searches using all taxa recovered little strict consensus. Eriptychius was 1293 found to be a wild-card taxon and therefore eliminated for subsequent searches. Furthermore, many psammosteid taxa were taxonomically equivalent which prohibitively increased the number of most parsimonious trees without adding phylogenetic signal. Selected psammosteid taxa were 1296 therefore removed from subsequent searches and replaced with an equivalent composite taxon. achieved a better resolution but still resulted in high number of most parsimonious trees (Fig. 5a).

1332
To distinguish between these two equally parsimonious clusters with very different rooting 1333 solutions for Heterostraci, we applied stratigraphic congruence estimates. All topologies resulting 1334 from the different coding methods have a better fit to stratigraphy than those generated from randomly permutated trees, but trees from cluster 2 have the best fit to stratigraphy in all instances   The Bayesian analysis yields a very unconventional result for heterostracan intra-and inter-   relationship is unlikely to change with the discovery of an articulated specimen. Corvaspis is tessellate specimens would allow a clearer interpretation of their anatomy and would perhaps provide more conclusive answers.

CONCLUSIONS
Tormi Tuuling for the help in acquiring papers. Feedback on earlier version of this work was