Protoavis and Megalancosaurus - and the ABSRD model of bird origins

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

Rob Gay wrote:

> It is my opinion that _Protoavis_ is not composed of
> coelophysoid material. The premaxilla looks slightly coelophysoid, as do
> some of the cervicals. However, they are still quite distinct.

A better candidate for the identity of the skull and cervical vertebrae of 
_Protoavis_ may be the drepanosaurids.  Silvio Renesto recently re-described 
_Megalancosaurus_.  He regarded _Megalancosaurus_ as an arboreal creature 
(but not a glider) belonging to the Drepanosauridae.  In the past, 
_Megalancosaurus_ has been linked to the origin of birds, notably by 
Feduccia, Olson, and Ruben, among others, who propose that birds evolved 
from small arboreal diapsids in the Triassic.  What is obvious is that 
Feduccia and others will consider almost any Triassic diapsid as possibly 
ancestral to birds, as long as it is (a) small in size, (b) shows arboreal 
adaptations, and (c) isn?t a dinosaur.  In the absence of a clearly-defined 
proposed ancestor or sister taxon, I call Feduccia's "theory" on bird 
origins ?Anything But a Small Running Dinosaur? (ABSRD for short).

As the ABSRD theory (it sure ain't a hypothesis) supports a Triassic origin 
for the Aves, the presence of a fairly advanced bird in the Norian 
(Chatterjee regards _Protoavis_ as a bird more derived than _Archaeopteryx) 
is congruent with ABSRD.  This relegates _Archaeopteryx_ to a primitive 
relict taxon for its Late Jurassic (Tithonian) time.  Chatterjee, however, 
supports the origin of birds from theropods; Feduccia does not.

However, _Protavis_ may be no more closely related to birds than is 
_Megalancosaurus_.  Renesto found some impressive similarities between 
_Megalancosaurus_ and _Protoavis_...

Renesto (1999): ?_Megalancosaurus_ and _Protoavis_ skull [sic] are 
superficially very similar, being narrow and pointed anteriorly with an 
inflated the postorbital region [sic]; the lower jaw is also similar in 
shape, with a ventrally bent anterior portion.?

The cervical vertebrae are also strikingly similar between _Protoavis_ and 
_Megalancosaurus_ (based on specimens TTM 9201 and MPUM 6008, respectively), 
not only in the presence of hypapophyses, but in the narrow, elongate, and 
procoelous centra; prezygapophyses terminating as convex surfaces; and low 
neural spines.  (Renesto's illustrations also show the uncanny similarity.)  
The cervicals are also around the same size: 10-11 mm long on MPUM 6008, and 
10-12mm long in the smaller _Protoavis_ specimen (TTM 9201).  Renesto points 
out that drepanosaurid remains are also known from the Chinle Formation and 
Newark Supergroup of Late Triassic North America.

Might (as many have suspected) _Protoavis_ be a chimera?  If so, the type 
material may include at least some drepanosaurid material.  The 
drepanosaurids, by the way, were basal archosauromorphs, not too far from 
the prolacertids.  The Prolacertiformes (= Protorosauria - could someone set 
me straight if these two groups aren't the same) have spawned another 
alleged proavian in the past: _Cosesaurus aviceps_.  This little 
archosauromorph (redescribed as a juvenile _Macrocnemus_, the last I heard) 
was also once regarded as an ancestral avian on account of its rather 
bird-like head and long limbs.

In pushing the ABSRD model, Ruben and Feduccia have been quite keen on 
linking _Megalancosaurus_ to the origin of birds.  In 1993, Feduccia and 
Wild put forward a trio of ?avimorph thecodonts? ? _Megalancosaurus_, 
_Longisquama_, _Scleromochlus_ ? as somehow being connected to bird origins. 
 Feduccia found that these genera combined showed a number of bird-like 
characters, although Feduccia did not demonstrate how these genera were 
related to each other.  By the way, this paper (Feduccia and Wild, 1993) is 
the first example I?ve ever seen of morphologically disparate genera being 
linked not by common ancestry, but by an alleged common descendent (birds).

As pointed out by Renesto (2000), the skull and neck of _Megalancosaurus_ do 
show bird-like features (skull triangular in profile, inflated posterior 
region of braincase, hypapophyses on the underside of cervical vertebrae).  
But the postcranium is distinctly un-bird-like, and more like that of a 
modern chameleon.  The paper also gives a purpose for the hypapophyses on 
the underside of the cervicals (I?m sure many on this list knew what they?re 
for, but I didn?t): ?The hypapophyses ? allowed the insertion of a well 
developed longus colli, for the extension of the neck.?  In addition to the 
tall neural spines of the anterior dorsal vertebrae, for attachment of a 
strong musculature, ??these features permitted the neck of _Megalancosaurus_ 
to be both retracted and suddenly projected forward, as in some projectile 
feeders."  Or small carnivores - darting the jaws forward to sieze small 
prey.  Hypapophyses on the neck vertebrae are also seen in crocodilians, 
varanids (monitors) and coelurosaurian theropods.


Tim


References

Feduccia, A and Wild, R. (1993).  Birdlike characters in the Triassic 
archosaur Megalancosaurus.  Naturwissenschaften. 80 (12): 564-566.  
(Incidentally, one of the worst papers that I have ever read).

Silvio, R. (2000).  Bird-like head on a chameleon body: New specimens of the 
enigmatic diapsid reptile Megalancosaurus from the Late Triassic of Northern 
Italy.  Rivista Italiana di Paleontologia e Stratigrafia. 106 (2): 157-180.



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

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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