Message 2001-06-0127: Re: Crown groups mainstream?

Sat, 23 Jun 2001 17:00:42 -0700 (PDT)

[Previous by date - Crown groups mainstream?]
[Next by date - Re: Crown groups mainstream?]
[Previous by subject - Re: Crown groups (no longer long)]
[Next by subject - Re: Crown groups mainstream?]

Date: Sat, 23 Jun 2001 17:00:42 -0700 (PDT)
From: "Jaime A. Headden" <>
Subject: Re: Crown groups mainstream?

Ken Kinman ( wrote:) wrote:

<Most mammalogists still seem to think that a Mammalia which is
cladistically anchored on a monotreme (which still have a
miserable fossil record) is a dumb idea.


I would very HIGHLY recommend that PhyloCode workers NOT define
Mammalia as a crown group, because there is a good chance it
will end up excluding multituberculates, and it will certainly
exclude sinoconodonts, morganucodonts, and docodonts (all of
which have the well-known mammalian jaw and three ear

  Yuk. Poor attitudes about how some groups have less
specialized ancestors that must somehow be included because
certain classic features are present -- or not, thus excluded --
are in bad taste, in my opinion. The establishment of the taxon
Mammalia to include only the smallest group possible including
the most recent ancestor of *Ornithorhynchus* [monotreme],
*Macropus* [marsupial], or *Canis* [placental] is based on the
ability to recognize groups on well-established membership,
without fossil taxa literally screwing the tree up. That a
fossil's possition may rotate or compeletely squash the proposed
phylogeny is anchored on that taxon, this disrupts what is
generally perceived of as a stable group. So friggin' what if
multis are not member of Mammalia? They can still be mammals. So
may anything grouped in Mammaliaformes, which is the stem
opposing the cynodonts. If you want to base the name on an
apomorphy or two, like three middle ear bones (formed from the
jaw, and thus the same character as the dentary-only lower jaw)
then go ahead, and see if it survives criticism. Not that I'm
arguing against it, but just such a group is bound to be
unstable, and would get ripped to shreds the first moment a
sphenacodont or less closely related amniote (or more basally!)
is found with the same condition. Otherwise, we are left with
bounding taxa in terms of their associate taxa, and outgroups,
not what they bear.

<I think it is pretty obvious a crown group Aves has been
rejected already, and who knows what might happen to a crown
group Archosauria,>

  Do you possibly think that birds are not bound within the
group defined as being closer to crocodilians than to snakes,
lizards, and the like? Otherwise, Archosauria is a real entity,
defined on birds and crocs.

  I have it on good authority (a little bird told me) that Aves
being used in tendem with Neornithes will be used to group the
Cretaceous/Jurassic fossil birds and the living ones. Thus,
hesperornithiforms & ichthyornithiforms become birds by
extension, and the name Aves has a broader concept than
Neornithes, which is the present broad use of the living
membership. Anyways, a crown-group is a node and it stable by
its content, now matter how "far" these constituent taxa may be
from eachother, intervening taxa ranging in the hundreds or not.

<because pterosaurs may not be the only group that gets ejected
from that taxon.>

  Pterosaurs are irrelevant to Archosauria -- they cannot
comprise the definition of a crown-group, as they are extinct.
As well, Dave Peters quite clearly espouses and convinces the
relationship of pterosaurs as basal to archosaurs, as
archosauriforms closer to lizards than crocs (lepidosauriforms),
as members of Archosauromorpha, and which becomes a senior
subjective synonym of Ornithodira by content.

<It is perhaps still the best hope that some large crown group
within amniotes will become mainstream, but I wouldn't even bet
on that one.>

  Amniota = mammals and reptiles. Mainstream in the sense you've
used above.

<Crown groups seem to cause more problems than they solve, and
if I was a paleontologist I think I would dislike them even

  Exactly what problem has a crown-group caused for you?

Jaime A. Headden

  Where the Wind Comes Sweeping Down the Pampas!!!!

Do You Yahoo!?
Get personalized email addresses from Yahoo! Mail


Feedback to <> is welcome!