[Taxacom] Early evolution of Class Aves (should it be expanded?)

[Kenneth Kinman]

Dear All, 
       Dececchi and Larsson, 2009, is a recent paper which
further demonstrates that Archaeopteryx is not as important to the
origin of birds (Class Aves) as it once was.   As I have argued for a
number of years, any taxon that is a potential "Rosetta Stone" (if there
is just one) for Class Aves would have split off earlier than
Archaeopteryx.         
     They conclude that the least inclusive clade which
includes Archaeopteryx (and also corresponding to a punctuated jump in
flight adaptations) is the clade Eumaniraptora. This would at the very
least add the troodonts, microraptorines and other deinonychosaurs (and
probably mononykiforms and oviraptorosaurs as well) to Class Aves. This
is the kind of expansion of Class Aves that I have strongly advocated
since 2002, and I have been including mononykiforms in Aves since 1994.
  
     The only major question in my mind is whether or not to
include segnosaurs (which possibly clade with oviraptorosaurs) at the
base of this expanded Class Aves. Either way, this expanded Class Aves
is characterized by a number of potential synapomorphies related to
flight, including: (1) laterally-directed shoulder joint; (2) enlarged
forelimb; (3) large bony sternum; (4) well-developed semilunate carpal
in the wrist; and (5) "true" feathers (although the exact meaning or
definition of "true feathers" is frankly the most controversial, and
often the least likely of these to be preserved in fossils).  Another
interesting potential synapomorphy (although not flight related) is
eggshell microstructure.        
     The problem has unfortunately been that strict cladists
(and phylocodists in particular) are instead fixated on defining Aves as
either formally anchored on genus Archaeopteryx, or greatly reduced to a
crown group only (thus excluding Archaeopteryx and a lot of other birds
that clearly had evolved powered flight). Most of them seem very
resistant to applying the name Aves to a more inclusive clade, perhaps
because they have erected new names for many of the clades which might
apply to such an expanded Class Aves.   My own proposed Class Aves
approximates either clade Maniraptora or something intermediate between
it and Eumaniraptora. 
        As I have noted previously, the name Maniraptora
is really inappropriate anyway for a taxon in which most members do not
use their forelimb/wing to grasp prey (although some basal taxa did so
in the Mesozoic). The name Aves is much more appropriate since a great
majority are characterized by powered flight, and many of the others
were no doubt facultative gliders and fliers. Anyway, below is a
citation for the Dececchi and Larsson paper and parts of the abstract.
Note that when they refer to "birds", they appear to be referring to the
clade Aves as formally anchored on genus Archaeopteryx.        
            --------Ken Kinman

Dececchi, T.A., and Larsson, H.C.E. (2009) "Patristic evolutionary rates
suggest a punctuated pattern in forelimb evolution before and after the
origin of birds."  Paleobiology, 35:1-12.     
Parts of the Abstract: "The evolution of powered flight has
traditionally been associated with the origin of birds, the most
successful clade of modern tetrapods, as exemplified by the nearly
10,000 species alive today....  Here we show that the origin of birds is
associated with little or no evolutionary change to the skeletal anatomy
of the forelimb, and thus _Archaeopteryx_ is unlikely to be the 'Rosetta
Stone' for the origin of flight it was once believed to be. Using
comparative statistics and time-series analyses on a data set
constructed from all known forelimb skeletal anatomy of non-avian
theropod dinosaurs and a diverse assemblage of early birds, we
demonstrate three focused peaks of rapid forelimb evolution at
Tetanurae, Eumaniraptora, and Ornithothoraces."