Chimeras (was Re: Protoavis & Drepanosauridae (sensu Renesto, 1999)

["Tim Williams" <twilliams_alpha@hotmail.com>]

Dinogeorge wrote:

> If there were more than one kind
> of animal mixed into the material, one would expect to find things like two
> or more differently shaped humeri, or left and right limb elements that 
> don't
> match, and so forth. Nothing like this was found.

This is not always the case for taxa based on composite specimens 
("chimeras").  In most cases, the chimeric nature (or proposed chimeric 
nature) of the skeleton was not immediately revealed by the presence of the 
same part of the skeleton from one or more different species.  There are 
numerous examples of associated material belonging to more than one species 
of archosaur (or archosauriform) which was originally described as a single 
species.

The pes originally referred to the little dinosauriform _Lewisuchus_ was 
recently re-identified as belonging to a proterochampsid.

Non-herrerasaurid material, mistakenly referred to _Ischisaurus_, was 
originally used to justify its generic separation from _Herrerasaurus_.

The prosauropod _Euskelosaurus_ was once thought to be carnivorous due to 
the inclusion of non-prosauropod jaw material in the description.  This jaw 
material (and a femur, also found among the _Euskelosaurus_ type material) 
is now assigned to the carnivorous archosaur (?dinosaur) _Aliwalia_.

_Technosaurus_ was described on the basis of a mixture of prosauropod and 
ornithischian material.

Some sauropod genera from South America may be based on a combination of 
material from more than one sauropod species.  (This applies in particular 
to many of Huene's titanosaurids; Huene claimed that the material was 
associated, but other workers have raised doubts in this respect.)

It has been proposed that _Tsintaosaurus_ is based on mingled lambeosaurine 
and hadrosaurine material.  (Though I think this suggestion may have fallen 
by the wayside.)

The "squamosal" included in the original description of _Zuniceratops_, as 
we all now know, is referrable to the pelvis a therizinosauroid.

The list goes on... _Alectrosaurus_, _Therizinosaurus_ (possibly), 
_Sanpasaurus_, _Dynamosaurus_, _Monoclonius_, and many other dinosaurian 
taxa for which the original decriptions included jumbled material pertaining 
to more than one species.


> All the elements are of the
> right size and shape to fit together to comprise two individuals of the 
> same
> species, one somewhat smaller than the other.

I think it is worthy of note that Chatterjee himself has changed his mind 
regarding the identity of at least one element of _Protoavis_: an element 
originally identified as a sternal plate was subsequently re-identified as 
coming from the palate (Chatterjee, 1999).  Looking at the plates and 
figures in _Palaeontographica Abt._, the descriptions given in this paper 
may not be the final word on the identity of all these elements.

The lack of available material for comparison (if correctly associated, and 
irrespective of its proposed avian identity, _Protoavis_ appears to be a 
unique taxon) and the fact that many elements are poorly preserved, conspire 
to add a degree of uncertainty to the identifications attached to many of 
the elements.


> Unless
> you are accusing Sankar of deliberate fraud

Not at all.  As I recall, this thread started when a listmember inquired 
(quite legitimately) why _Protoavis_ had been excluded from most discussions 
of bird origins.  In response, it was pointed out that many paleontologists 
were highly skeptical of the avian nature of _Protoavis_.  (In fact, Luis 
Chiappe and John Ostrom have said as much in various publications - Ostrom 
even devoted a short paper to challenging Chatterjee's interpretation of 
_Protoavis_ as a bird).  This led to discussions to the possibly chimeric 
nature of the hypodigm of _Protoavis_.  Perhaps there is genuine bird 
material among the disarticulated elements assigned to _Protoavis_.  The 
skull, however, is strikingly drepanosaurid-like; this may or may not be due 
to convergence.  I cannot see how any of the discussion over the past few 
days could possibly be characterized as a personal attack against a 
scientist as well-respected (and well-liked) as Sankar Chatterjee!


Tim



------------------------------------------------------------

Timothy J. Williams

USDA/ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163

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