[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Protoavis and Megalancosaurus - and the ABSRD model of bird origins
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.
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
Iowa State University
Ames IA 50014
Phone: 515 294 9233
Fax: 515 294 3163
Get your FREE download of MSN Explorer at http://explorer.msn.com