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Re: Cladistic definitions of Dinosauria, Saurischia, Sauropodomorpha

On 9/19/05, Mike Taylor <mike@miketaylor.org.uk> wrote:
> Are there "generally accepted" cladistic definitions of Dinosauria,
> Saurischia and Sauropodomorpha that seem to be winning out?  As an
> example, within Sauropoda, it seems clear that Wilson and Sereno's
> 1998 definition of Diplodocoidea as (_Diplodocus_ not
> _Saltasaurus_) is now used more or less universally.  But I know
> that there are various competing candidate anchors for Dinosauria
> (e.g. _Triceratops_ and _Passer_, _Megalosaurus_ and _Iguanodon_)
> and I wondered which way the wind is now blowing.

This is the original diagnosis:

* Owen 1842 (abridged): "This group ... is characterized by a large
sacrum composed of five ankylosed vertebræ of unusual construction ...
; by broad ... coracoids and long and slender clavicles, whereby
Crocodilian characters of the vertebral column are combined with a
Lacertian type of the pectoral arch; the dental organs also exhibit
the same transitional or annectent characters.... The bones of the
extremities ... are provided with large medullary cavities and with
well-developed and unusual processes, and are terminated by
metacarpal, metatarsal, and phalangeal bones."

This is the original composition:

* Owen 1842: "Of this tribe [Dinosauria] the principal and
best-established genera are the _Megalosaurus_, the _Hylæosaurus_
[sic], and the _Iguanodon_."

These are the published definitions:

* Padian and May 1993: "all descendants of the most recent common
ancestor of birds and _Triceratops_"

* Sereno 1998: "_Triceratops_, Neornithes, their most recent common
ancestor and all descendants"

* Kischlat 2002: "o ancestral comum mais recente entre Megalosaurus
(um saurísquio) e Hylaeosaurus (um ornitísquio), assim como todos os
seus descendentes, incluindo as aves"

English translation: "the most recent common ancestor between [i.e.
"of"] _Megalosaurus_ (a saurischian) and _Hylaeosaurus_ (an
ornithischian), as well as all its descendants, including the birds
[my translation, ably assisted by Babelfish]"

abbreviated formula: "Dinosauria=_Megalosaurus_+_Hylaeosaurus_"

* Clarke 2004: "the clade comprised of the most recent common ancestor
of Owen's (1842) specifiers for his 'Dinosauria' (_Megalosaurus_ and
_Iguanodon_) and all of its descendants"

And here is PhyloCode's comment about what the definition should be like:

* Cantino and de Queiroz, 2003 [Internet only]: "The name _Dinosauria_
was coined by Owen for the taxa _Megalosaurus_, _Iguanodon_, and
_Hylaeosaurus_, and traditionally the taxon designated by that name
has included these and certain other non-volant reptiles. It has not
traditionally included birds. Although birds are now considered part
of the dinosaur clade, the name _Dinosauria_ should not be defined
using any bird species as internal specifiers. Such a definition would
force birds to be dinosaurs, thus trivializing the question of whether
birds are dinosaurs. Instead, internal specifiers should be chosen
from among taxa that have traditionally been considered dinosaurs;
e.g., _Megalosaurus bucklandi_ von Meyer 1832, _Iguanodon
bernissartensis_ Boulenger in Beneden 1881, and _Hylaeosaurus armatus_
Mantell 1833."

Padian in Currie and Padian 1997 mentions a personal communication
from T. R. Holtz, suggesting usage of _Megalosaurus_ and _Iguanodon_
as internal specifiers, but follows priority, settling on the
definition from Padian and May 1993.

Note that all of these definitions yield the same clade under all
published phylogenies of the past couple of decades. Hence, I believe
most researchers don't feel the need to pick one, so there is no clear

I'd also like to note that the disagreement between Kischlat and
Holtz/Clarke over which of Owen's ornithischians to use in the
definitions (_Hylaeosaurus_, the first one listed in Owen 1842, and
_Iguanodon_, the first one published, and better known of the two)
could be easily remedied, without affecting the taxon's application,
by including both of them. This would also pay better homage to Owen,
I think: the last common ancestor of _Megalosaurus bucklandi_ von
Meyer 1832, _Hylaeosaurus armatus_ Mantell 1833,  and _Iguanodon
bernissartensis_ Boulenger in Beneden 1881, plus all of descendants
thereof. Unfortunately, I don't think anything like this has been
published. (Clarke comes close, but failes to mention _Hylaeosaurus_.)

—Mike Keesey

P.S. I don't actually have all of these papers in front of me (just
Kischlat 2002 and Currie and Padian 1997), just notes, so you may want
to double-check the original sources.

P.P.S. I could swear there was an early definition of _Dinosauria_
that had some herrerasaurid[s] (or possibly _Herrerasauridae_ itself)
as an internal specifier, but all I can find is a mention that
Gauthier (1986) states a definition like that, but clearly excludes
_Herrerasauridae_ in his usage. I think the definition I'm thinking of
is from someone else's paper, though ... if it exists....