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Re: Cladistic definitions of Dinosauria, Saurischia, Sauropodomorpha



On 9/19/05, Phil Bigelow <bigelowp@juno.com> wrote:
> 
> On Mon, 19 Sep 2005 12:25:19 -0700 "T. Michael Keesey" <keesey@gmail.com>
> writes:
>
> IMO, cladistic definitions should recognize and embrace both change and
> advancement in the science (provided that there is preexisting and
> abundant empirical evidence to do so).  Ostrom's ground breaking
> character trait work and Gauthier's early work on the cladistics of the
> group should be formalized into the definition of Dinosauria, which in my
> opinion only further compliments Owen's original name for the group.
> Both Owen and Padian and May win.  Tradition is preserved (to some
> extent), and yet change is also recognized and respected.

Change can be recognized by placing _Aves_ within _Dinosauria_. There
is no call for *mandating* that it be within _Dinosauria_, especially
when it was the *last* major taxon to be transplanted into it, however
successfully? Why pick a specifier that's only been included for the
past 20 years when there are perfectly good ones that have been
consistently included for over 160 years?

> Congruency of an idea over time has its good points, no doubt about that.
>  But should that be one of the prime factors in defining a clade?
> Shouldn't the communication of new knowledge also be a factor?

No! Then we'd have to change our definitions every time new data was discovered.

_Cetacea_ has been recently found by some studies to lie within
_Artiodactyla_. If this becomes an established hypothesis, would you
ever advocate using _Balaena_ as a specifier for _Artiodactyla_?

Definitions are supposed to be stable (at least eventually); diagnoses
and composition lists are what may vary.

> But by leaving birds out of the definition, we are only placating
> Feduccia, Martin, Olsen, and a couple other hold-outs (some of whom show
> ignorance of the available character data set).  The rest of the world
> has moved on.

I don't see it as placating them; I see it as honoring Owen and the
long tradition of _Dinosauria_ as a taxon. I also see it as sensible,
since, as noted above, birds have only recently been considered
dinosaurs.

> <sigh>Okay, how about a compromise definition:
> 
> Dinosauria = (Gallus gallus + Iguanodon)

What's wrong with acknowledging that birds are descended from the last
common ancestor of _Megalosaurus bucklandii_, _Hylaeosaurus armatus_,
and _Iguanodon bernissartensis_? That reflects the history of usage
and the history of discoveries.

(And why _Gallus gallus_? Others have used _Passer domesticus_ [type
species of the largest avian order] or _Vultur gryphus_ [first avian
listed by Linnaeus].)

—Mike Keesey