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:

No comments:

Post a Comment