Placodermi: Phylogeny and Dendrogram

Placodermi phylogeny

Placoderm phylogeny and the interrelations between the different taxa remain highly controversial because of the high degree of parallelism and the difficulty of telling which are the primitive and which the derived traits. The following are several different cladograms and phylogenetic trees that have been proposed for this group.

Most authors consider the antiarchs and arthrodires closely related because they share a single pair of paranuchal plates. Goujet however places them each in distinct clades. Acanthothoracids are usually considered as primitive rhenanids or sister taxa therof (Miles and Young 1977, Long 1995), but Goujet (1984) considers them a basal assemblage of placoderms. Ptyctodontids are sometimes considered the most primitive taxon (Miles and Young 1977, Gardiner 1984, Long 1995), and have a number of features in common with Chondrichthyes (Long 1995), but also share characters with the petalichthyids (Janvier 1996), and are placed by Goujet (1984) in a sister taxon to arthrodires. Phyllolepids have been considered close to arthrodires, and now seem to be included in that clade, with wuttagoonaspids transitional between the two (Long 1995). The poorly known Stensioella and pseudopetalichthyids are considered basal forms by Denison (1978), and primitive, but not the most basal by Gardiner (1984), but their placoderm status is rejected by Janvier (1996), and we have moved them to the basal gnathostomes. MAK061009, 062110, 111025

Miles and Young 1977:

Denison 1978:

Goujet 1984:

Gardiner 1984

Long 1995:

Janvier 1996's work on Palaeozoic fish remains a definitive guide one and a half decades later, and at the time it came out was hugely influential on the two founding members of Palaeos (ATW and MAK). His phylogeny, based on Goujet 1984, is as follows:

Smith & Johanson 2003, after Goujet 2001

Mikko's Phylogeny (an amateur site like Palaeos, not peer reviewed) provides a general synthesis, incorporating Carroll, 1988 and Goujet 2001 [from Johanson, 2003)

In perhaps the most radical divergence from the standard consensus of gnathostome relationships, Brazeau 2009, makes the Placoderms a paraphyletic (ancestral) grade of early fish. This means that, phylogenetically speaking, Placodermi and (stem/node-based) Gnathostomata become synonymous. However, not all workers in the field accept this interpretation.

Dendrogram

The following dendrogram is something of a compromise, although leaning more heavily on more recent work. No doubt it will be revised with time. Two placements of Acanthodii are given, the traditional one, with a monophyletic placodermi, and the phylogeny suggested by Brazeau 2009, in which case Placodermi becomes an ancestral stem ggroup and synonymous with stem-based (bot not crown group) Gnathostomata. Nevertheless, teh many unique features of placoderms make this a controversial, though intriging, hypothesis. Note - links to Mikko's Phylogeny (MH) need revising; the pages exist but have been moved to different directories - MAK111024)