Re: reason for dropping Owen

["T. Mike Keesey" <tmk@dinosauricon.com>]

On Fri, 7 Sep 2001, David Elliott wrote:

> A hypothetical: a fully articulated _Megalosaurus_ is found - it has all
> the bits and peices used as the type specimen, but it also includes the
> rest of the skeleton, and the rest of the skeleton somehow shows it to be
> clearly not a descendant of the most recent common ancestor of
> _Triceratops_ and modern birds... yet _Megalosaurus_ was one of the two
> animals originally used to define Dinosauria...

rather unlikely....

> I know distaste over the definition has been brought up and discussed
> in-depth before, but what i don't understand is something that those
> discussions seem to have taken as given: why the newer definition is
> accepted in the first place, how come "most recent common ancestor of
> _Triceratops_ and birds" has priority? Do the ICZN rules only apply after
> a certain date, or something similar? (serious question, it sounds like
> im being rhetorical here but really im not) Did Owen not publish in the
> right place? Or was there something in his wording... like an explicit
> definition based on characters and not relationships?

The ICZN only covers taxa of lower ranks -- Familia and all ranks derived
from it, and all ranks below it. Furthermore it does not cover
phylogenetic taxonomy, only Linnaean taxonomy. It does not cover
definitions of any kind, except for "the taxon of rank [whatever]
including type specimen [whatever]".

There is nothing governing zoological Linnaean taxa above familial rank,
or phylogenetic taxa. There is a commission being formed for phylogenetic
taxonomy (PhyloCode -- see http://www.ohiou.edu/phylocode/), but it is
still in germinal stages.

Owen's diagnosis is, I think, at least in part, flawed, based on
characters acquired independently in _Megalosaurus_ and the two
ornithischians (_Hylaeosaurus_ and _Iguanodon_). He did not provide a
phylogenetic definition, since the concept had not yet been invented. All
we have to go on is his content list of three species. (Which, by the way,
does not include all non-neornithean dinosaurs known at the time --
notably missing are _Plateosaurus_ and _Cetiosaurus_, several others.)

Padian and May were the first to provide a phylogenetic definition for
_Dinosauria_. It did not use any of Owen's original three dinosaurian
species. I believe the draft PhyloCode includes this as an example of an
improperly converted taxon, and suggests that it be based on Owen's three
species instead: Clade(_Hylaeosaurus armatus_ + _Iguanodon anglicus_ +
_Megalosaurus bucklandii_). (NOTE: The ICZN appointed _I. bernissartensis_
as the neotype of _Iguanodon_, so that may be a better anchor than the
dubious _I. anglicus_.)

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T. MICHAEL KEESEY
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