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Re: Segnosauria vs. Therizinosauria



On Thu, 6 Apr 2000, Jaime A. Headden wrote:

>   Segnosauria is a historic taxon including
> *Segnosaurus* and *Erlikosaurus* and the unnamed taxon
> (at the time) *Enigmosaurus* (Perle and Barsbold,
> 1981). Therizinosauria, while implied, doesn't stand
> up to the criteria of formal taxon until it's actually
> used. This has only been done colloquially, and as
> such, doesn't validate a consideration by both ICZN or
> PT standards, if I've read my material right (but I do
> not have the 1966 or 1999 ICZN, so... and nor have I
> read all the de Quieroz and Gauthier stuff).

ICZN is not concerned with this issue, since neither Segnosauria nor
Therizinosauria are familial, generic, or specific ranks. In the matter of
familial ranks, ICZN would favor any "Therizinosaur-" taxon over any
"Segnosaur-" taxon, since Therizinosauridae was named before
Segnosauridae.

I'm not sure whether PT "rules" (there is no governing body for PT)
require a that the taxon name be stated formally. My copy of the
Encyclopedia is on loan, but I think the the name "Therizinosauria" is
stated in the section (in fact I think it may be the section's heading).
My memory could, of course, be faulty.
 
>   Russell's read of "therizinosaurs" from the
> _Encyclopedia of Dinosaurs_ probably stems from
> Russell and Dong, 1993, and by implication is probably
> his colloquial use of Therizinosauroidea, a grouping,
> as detailed by Tom Holtz, was made in Barsbold and
> Maryanska, 1990, in _The Dinosauria_, so is likely to
> imply that group and assemblage, plus *Alxasaurus*, so
> "therizinosaurs" is Alxa + all other "therizinosaurs"
> like Segno, Therizino, and Erliko.

a) If he meant "therizinosauroid", why would he say "therizinosaur"?

b) Therizinosauroidea is a node-based taxon. Russell's definition of
therizinosaurs is stem-based (and more inclusive).
 
>   Segnosauria still stands as the most valid and
> oldest grouping of taxa including a higher relative
> rank (pardon the term) than Segnosauridae, or by
> inclusion, Therizinosauridae. However, a formal
> taxonomical, anatomical, or historical study has yet
> to be performed on the taxa involved, and even
> *Enigmosaurus* involves material not pictured,
> including some vertebrae and ribs, and was never
> pictured or well illustrated, so definite works needs
> to be done on the taxa involved. Thus far, only
> *Erlikosaurus* and the refered PIN arm and claws have
> been properly or even close to being fully described,
> and as such are the dependant specimens for
> comparison. I used *Erlikosaurus* and will continue to
> use it as the anchor, at least informally, until a
> proper description of the type and two refered
> specimens of *Segnosaurus* are properly described, and
> the type of *Therizinosaurus* is shifted to more
> diagnostic material, like one of the PIN specimens.

IMHO it is always a bad idea not to use eponymous genera as
anchors. Consider the unfortunate example of Ornithosuchia, which no
longer contains _Ornithosuchus_. Consider Sereno's definition of
Ceratosauria anchored by _Coelophysis_ -- by some phylogenies
_Ceratosaurus_ would not be a ceratosaur!

In this case, it is so far uncontested that using _Erlikosaurus_,
_Segnosaurus_, or _Therizinosaurus_ as the anchor would result in the same
taxon. If this is so, why *not* use _Therizinosaurus_? And if it isn't
so, then we're stuck with a taxon named after a genus which does not
include that genus!

Allowing the definition to shift to _Therizinosaurus_ in such a case goes
against the idea of PT, which is to have stable definitions.

>   Dong's Segnosaurischia is a junior synonym of
> Segnosauria, by priority; and because a segnosaur
> taxon (*Beipiaosaurus*) falls outside
> Therizinosauroidea, and it most certainly clades there
> (Xu et al. 1999), Segnosauria can be used to reflect
> this clade, and another appropriate stem can be used
> once a definite matrix or consensus turns up the more
> likely position of segnosaurs compared to ornithomimes
> or ovis

If you are proposing a node-based definition for Segnosauria
({_Beipiaosaurus_ + _Segnosaurus_}), then that would be fine -- the clade
is unnamed. However, at present it would have exactly the same membership
as Therizinosauria (a stem-based taxon), so it would not become a useful
taxon until a therizinosaur more basal than _Beipiaosaurus_ is found.

-- T. Michael Keesey .................................. <tmk@dinosauricon.com>
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