Parexus (Watson, 1937)

Brazeau, Martin D. & Friedman, Matt, 2014, The characters of Palaeozoic jawed vertebrates, Zoological Journal of the Linnean Society 170 (4), pp. 779-821 : 812-813

Parexus

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alternative hypotheses concerning the placement of acanthodians. The first of these places most acanthodians on the chondrichthyan stem, with Acanthodes and its closest relatives representing members of the osteichthyan total group ( Fig. 13A ). The second hypothesis places all acanthodians as stem chondrichthyans ( Fig. 13B ). This latter hypothesis enjoys support from a number of dermal and endoskeletal characters, as well as some braincase characters in Acanthodes newly recognized by Davis et al. (2012) and evidence for the homology of placoderm and osteichthyan macromeric skull conditions described by Zhu et al. (2013). These alternatives differ from one another by only two steps under our synapomorphy scheme, and we regard this as insufficient to distinguish between them at present.

We have emphasized the importance of phylogenetic background assumptions in argumentation about the systematic significance of characters underwriting this set of tree topologies. This explicitness renders our proposals open to empirical refutation in the hope that this work will promote future research focused on testing these distributions in a methodologically consistent manner. We expect that many of these proposals will succumb to refutation, consistent with the current level of uncertainty in the study of early gnathostome phylogenetics. Because of this, we have not only emphasized our current preferred solution, but have explored a number of alternatives that could easily supplant our proposal with further study and new fossil data.

Figures

Figure 13. Summary cladograms of hypotheses of phylogenetic placements argued in this paper. A, cladogram depicting acanthodian genera distributed on the chondrichthyan and osteichthyan stems. B, cladogram depicting acanthodians restricted to chondrichthyan stem, but left unresolved. Character transformation labels at internal nodes correspond to those in the text. Numbers in parentheses reflect ambiguities that are resolved to their most inclusive level (i.e. ‘accelerated transformation’) and could have more restricted distributions.