Re: Osteichthyes ( was: Re: The Lost Dinosaurs Of Egypt ... and insects)

["David Marjanovic" <david.marjanovic@gmx.at>]

----- Original Message -----
From: "Steve Brusatte" <dinoland@lycos.com>
Sent: Thursday, June 06, 2002 2:19 AM

> On Wed, 5 Jun 2002 17:26:22
>  David Marjanovic wrote:
> >...and some
> >new trees get Placodermi and Osteichthyes as sister groups...
>
> Refs...please. :-)

Zhu Min, Yu Xiaobo & Per E. Ahlberg: A primitive sarcopterygian fish with an
eyestalk, Nature 410, 81 -- 84 (1 March 2001, not 1 April!!!)

For a pretty long summary try the lower half of
http://phylocode.miketaylor.org.uk/archive/messages/2001-06-0010.html. :-)
But also consider the December 2001 issue of JVP -- *Onychodus* is not a
coelacanth but the sister group to *Psarolepis*, and as an extremely basal
chondrichthyan braincase shows, Osteichthyes is left with no more than 3
synapomorphies, the presence of premaxilla, maxilla and cleithrum -- all 3
are dermal bones, and chondrichthyans don't have any dermal bones, so they
can't have those 3; not to mention that who knows which placoderm bones
might be homologous to them. I haven't seen an alternative arrangement so
far, though.

> I'm really lacking knowledge when it comes to "fish," but I know of only
one study that has placed placoderms and osteichthyans as possible sister
groups (Janiver, P.E. Early Vertebrates. Oxford University Press, 1996).

Didn't know that one. :-)

> Zhu et al., in a Nature article describing the weird Devonian fish
_Psarolepis_, actually found Osteichthyes united by no synapomorphy, but
supported only by homoplasies.  They offered a real tentative cladogram
showing placoderms as the sister taxon to chondrichthyans+ acanthodians and
osteichthyans.

So they got Osteichthyes monophyletic again?

> But, this analysis was preliminary, especially considering that
_Psarolepis_ is basically a mix of traditional placoderm, sarcopterygian,
and actinopterygian characters.  So, it's difficult to deduce the polarity
of certain characters,

Well... some time ago it was thought that having an eyestalk diagnoses
Chondrichthyes, or (Placodermi + Chondrichthyes). Instead it diagnoses (at
least?) Gnathostomata, and both basal actinopterygians and sarcopterygians
have eyestalks. :-)