Re: Gnathostome phylogeny (Paleo Find)

["Ken Kinman" <kinman@hotmail.com>]

    The phylogeny of gnathostomes may well be in for a little bit of 
shaking up, but the nomenclature should be pretty steady.  We will probably 
still have 5 classes of gnathostome fishes, even if the order of 
phylogenetic splitting needs changing.  Most internet sites still use the 
traditional topology, which my coding continues to reflect as follows:
 1  Placodermea
 2  Chondrichthyea
 3  Acanthodea
 4  Actinopterygea
 5  Sarcopterygea (paraphyletic)

     Whether the new evidence will threaten the osteichthyes group (4+5) 
remains to be seen.  However, I think that the teleostomi group (3+4+5) is 
probably in deep trouble, and Teleostomi would become a synonym of 
Gnathostomata if Acanthodea are indeed the most primitive of the five 
classes (as some recent evidence indicates):
 1  Acanthodea
 2  Chondrichthyea
 3  Placodermea
 4  Actinopterygea
 5  Sarcopterygea
      I just hope that someone hasn't cladistically anchored Gnathostomata 
on the placoderms (and not on acanthodeans as well), since that could leave 
chondrichthyeans and acanthodeans outside of a cladistic Gnathostomata.  And 
if they have anchored Teleostomi on acanthodians, it might well become a 
heterodefinitional synonym of Gnathostomata.
            -----  Ken
****************************************
David Marjanovic wrote:
> John G. Maisey & M. Eric Anderson: A primitive chondrichthyan braincase 
> from
> the Early Devonian of South Africa, JVP 21(4), 702 -- 713 (December 2001)
>
> This, the earliest known chondrichthyan braincase, has important 
> consequences for the systematics of Gnathostomata. It displays several 
> features that were so far considered synapomorphies of Osteichthyes (or 
> they + Acanthodii). The list of osteichthyan synapomorphies already being 
> eroded by several basal actinopterygians and sarcopterygians (they all have 
> eyestalks, for example, like basal placoderms and chondrichthyans), only 
> three remain -- the presence of premaxilla, maxilla and cleithrum. As these 
> are dermal bones, their absence in chondrichthyans, which don't have any 
> dermal bones, could easily be secondary. Their absence in placoderms may or 
> may not be due to the usual homologization problems with placoderms.
>    From p. 712: "It is quite remarkable that, in the space of just two 
> years, fossils have been described that affect concepts and 
> characterizations of two major clades (the osteichthyans and
> chondrichthyans) that originated in the 19th Century [sic]. The 
> morphological basis for osteichthyan monophyly is weakened by these 
> discoveries, but in reality it was never strong."
>     "From a developmental perspective, it may be much easier to lose new 
> characters than to gain (or regain) them."


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