Tuesday, November 20, 2012

Early archosaur hypothesis

Here is some background on the idea that an early archosaur (eg. Longisquama,could be the ancestor of birds. The pterosaur to bird theory is much more credible.

James and Pourtless:
"The   “early ­archosaur  hypothesis”
states that the origin of birds is more likely to be
among early archosaurs than among the theropod
(e.g., Tarsitano and hecht 1980; feduccia
and wild 1993; welman 1995; feduccia 1999, 2002;
Czerkas and yuan 2002; Czerkas et al. 2002; Martin
2004; feduccia et al. 2005, 2007). As presently un­
derstood, this hypothesis includes the propositions
that most maniraptorans are flying and flightless
lineages within Aves (as in figs.  1B and 3f)  and
that they are, in fact, not theropod dinosaurs (Czer­
kas et al. 2002; feduccia 2002; Martin 2004; feduc­
cia et al. 2005, 2007). According to this alternative,
the  Theropoda  as  presently  constituted  are  not
monophyletic. Aves, including various manirap­
torans, is not nested inside Theropoda. Similarities
between nonmaniraptoran theropods and birds are
accounted for by homoplasy."
I would suggest they are also accounted for by symplesiomorphy .

"The best ­studied current candidate for a potential archosaurian ancestor or sister taxon is
Longisquama. Sharov (1970) noted similarities to birds in the skeleton and integument of
Longisquama and sug­ gested that it may be close to avian ancestry. Jones et al. (2000, 2001)
described the birdlike osteologi­ cal characters and the featherlike morphology of the
integumentary appendages of Longisquama, but the latter was disputed by Prum et al. (2001) and
Unwin and Benton (2001). Unwin and Ben­ ton (2001) and Senter (2004) questioned the status of
Longisquama as an archosaur, but its antorbital fenestra (Jones et al. 2000, 2001; Martin 2004; f.
James and J. Pourtless pers. obs.; see Table 1) sup­ports the classification of Sharov (1970) and Jones
et al. (2000, 2001). Martin (2004) elaborated on the osteological similarities between Longisquama and
birds in dentition, characters of the skull, and the presence of a boomerang­shaped furcula similar
to that of basal birds. Unfortunately, the pelvic girdle and hind limb are not known. Longisquama
is best considered a basal archosaur of uncertain affinity (see fig. 2)".
In addition to the results obtained through use
of Kishino-Hasegawa tests, we also recovered a
clade of maniraptorans, birds, and the basal archosaur Longisquama, though we note that it was only weakly supported (Figs. 9 and 12). These
results nevertheless support the possibility of a
sister-group [ancestral] relationship between Longisquama
and Aves (inclusive of some maniraptorans). In
addition, birds and maniraptorans were never
unambiguously associated with nonmaniraptoran theropods in any of our trees (Figs. 9–13)." 

Temporal range: Middle or Late Triassic
Longisquama means "long scales"; the specific name insignis refers to its small size. The Longisquama holotype is notable for a number of long structures that appear to grow from its skin. These structures have been interpreted as either primitive feathers suggesting Longisquama is a close relative of birds, or as feather-like structures that have evolved independently and do not indicate a close relationship with birds. Longisquama has been used in a heavily publicized debate on of the origin of birds. To some, Longisquama is the gliding, cold-blooded, protobird predicted by Gerhard Heilmann's hypothetical "Proavis" in 1927, and it proves that birds are not dinosaurs. The current opinion is that Longisquama is an ambiguous diapsid and has no bearing on the origin of birds.

Ellenberger and DeVillalta (1974) and Ellenberger (1978, 1993) considered Cosesaurus a bird ancestor, and as such Ellenberger interpreted many aspects of Cosesaurus as proto-avian.

Here is an analysis by Peters of cosesaurus:

Monday, November 19, 2012

The origin of pterosaurs (2)

From David Peters:
Click to enlarge:

Cosesaurus is a genus of  prolacertiform archosauromorph reptile. It is known from a single, hand-sized fossil from the middle Triassic period of Spain. The fossil is a perfect impression of a largely articulated and complete specimen preserving soft parts, including an adhering jellyfish, but no bones remain in these impressions.
In 1977, Ellenberger proposed that Cosesaurus was an ancestor of modern birds.[1] That publication followed the description of the bird-like theropod,Deinonychus, but it appeared long before the theropod ancestry of birds had been widely accepted. In his large and highly detailed treatise, Ellenberger interpreted the following traits in the fossil: a strap-like scapula, a furcula(wishbone), a keeled sternum, beak-like jaws, a retroverted pubis and tailfeathers. Some of these interpretations have not been supported by subsequent research. Padian and Chiappe (1998) regarded Cosesaurus as a member of the archosauromorph clade Prolacertiformes.[2]

Ellenberger and DeVillalta (1974) and Ellenberger (1978, 1993) considered Cosesaurus a bird ancestor, and as such Ellenberger interpreted many aspects of Cosesaurus as proto-avian.

Sharovipteryx ("Sharov's wing", known until 1981 as Podopteryx, "foot wing"), was an early gliding reptile, from the middle-late Triassic period (230-225 million years ago). Fossils have been found from the Madygen Formation ofKyrgyzstan along with the unusual reptile Longisquama. It was approximately eight inches (20 cm) long, with an extremely long tail, and weighed about 7.5 grams. It may have been closely related — or perhaps even ancestral — to pterosaurs,[1] although this remains controversial. Unlike pterosaurs, its main flight membrane was stretched between long back legs rather than its very short front limbs.If Sharovipteryx was a relative of pterosaurs, then its membrane may have stretched to its front legs, or it may have had a separate membrane joined to its front limbs alone. A secondary membrane is visible between the thighs and the trunk. Front wing membranes have not been seen; Peters (2006) has claimed to have traced the fingers[2] and that they show similarities to Cosesaurusand Longisquama and to a lesser extent, pterosaurs.

Note that David Peters no longer considers Cosesaurus to be a member of Archosauromorpha.

Sunday, November 18, 2012

The origin of pterosaurs (1)

To this point I have not posted about the origin of pterosaurs. I will do that now in a series of posts.
I will work on the basic idea that pterosaurs stem from the archosauromorpha.
And I will build upon the work of David Peters who has developed (in amazing detail) the case that pterosaurs developed from gliding creatures like Sharovipteryx and Longisquama..

Here are some helpful references:



Friday, November 9, 2012

Pterosaur Launch


CHARLOTTE, N.C. — It's a bird … It's a plane … It's a plane-size bird! If humans had lived 67 million years ago in what is now Texas, they would've had a hard time missing the giant flying pterosaur calledQuetzalcoatlus, which was the size of an F-16 fighter jet.  The biggest animal ever to fly in the history of the world, this pterosaur dominated the sky with its 34-foot (10 meters) wingspan.
Fossils of the creature have been found in Big Bend National Park, in an area that was heavily forested in the late Cretaceous.   But this presents a puzzle: How did it fly? The region lacked the cliffs that make flight for such large birds easy to conceive.  
A new computer simulation has the answer: These beasts used downward-sloping areas, at the edges of lakes and river valleys, as prehistoric runways to gather enough speed and power to take off, according to a study presented Wednesday (Nov. 7) here at the annual meeting of the Geological Society of America.
First the animal would start running on all fours, Texas Tech University scientist Sankar Chatterjee, a co-author of the study, told LiveScience. Then it would shift to its back legs, unfurl its wings and begin flapping. Once it generated enough power and speed, it finally would hop and take to the air, said Chatterjee, who along with his colleagues created avideo simulation of this pterosaur taking flight.
Chatterjee said the flight and landing of Quetzalcoatlus probably looked like that of an albatross or the Kori bustard, the heaviest modern-day bird capable of flight.[Avian Ancestors: Dinosaurs That Learned to Fly]

Wednesday, November 7, 2012

Walking Pterosaurs

A question occurred to me concerning pterosaurs walking.
They could be walking either with their arms held off the ground or with their hands being placed on the ground (walking on all fours).
Would the footprint of the feet be the same in both cases?

Let's look at this diagram:

It would seem possible that the plantigrade footprint is an artifact of all-fours walking.
It could well have been digitigrade when the arms were held off the ground.
This would explain the plantigrade footprints of a pterosaur found (based on an instance of all-fours walking). And explain the digitigrade footprints of birds which never walk on all fours.

* Foot posture in a primitive pterosaurJ. M. Clark1, J. A. Hopson2, R. Hern├índez R.3, D. E. Fastovsky4 & M. Montellano3