[Taxacom] therizinosaurs and Class Aves

[Kenneth Kinman]

Dear All, 
      In a post here back on May 5th, I stated that I was
still somewhat uncertain whether or not to include segnosaurs (a.k.a.
therizinosaurs) in my expanded Class Aves.  A new paper now indicates
that I was right all along in excluding them (although they still appear
to be a closely related sister group).  Zanno et al., 2009, was
published the day before yesterday.  "A new North American
therizinosaurid and the role of herbivory in 'predatory' dinosaur
evolution." Proc. Roy. Soc. B (published online before print July 15,
2009, doi:10.1098/rspb.2009.1029).           The results of
their new phylogenetic analysis of maniraptoriform theropods is very
relevant to my expansion of Class Aves (to include some of these 'birdy'
dinosaurs). In the abstract they state: "Relative completeness of this
specimen permits a phylogenetic reassessment of Therizinosauria-the
theropod clade exhibiting the most substantial anatomical evidence of
herbivory. In the most comprehensive phylogenetic study of the clade
conducted to date, we recover Therizinosauria as the basalmost
maniraptoran lineage."                First of all, this
result confirms my minority hypothesis that therizinosaurs do NOT
exclusively clade with oviraptorosaurs (in a supposed clade called
Enigmosauria). As I have maintained for many years now, therizinosaurs
are more primitive than oviraptorosaurs. Several years ago (when I was
still on the Dinosaur Mailing List), I argued that therizinosaurs
(segnosaurs) were more primitive in a number of different ways (eggshell
microstructure, wrist morphology, feather structure, etc.), but the
majority strict cladists pointed to more numerous proposed
synapomorphies for their Enigmosauria hypothesis. Alas, they never would
heed my warnings that they need to weight their characters.  Further
evidence for more primitive feather structure in therizinosaurs
continues to accumulate, although I really put more faith in certain
"boney" synapomorphies (in the wrist, shoulder, etc.).  Anyway, I will
continue to treat therizinosaurs as the sister group to my expanded
Class Aves.                
      Zanno et al.'s results can be shown in the following
Hennigian comb:     

...  \____ Ornithomimosauria
....  \____ Ornitholestes 
.....  \____ Therizinosauria (segnosaurs)        ......  \____
Alvarezsauridae 
.......  \____ Oviraptorosauria
........  \____ Avialae ("birds")
.........  \____Dromaeosauridae
..........  \____Troodontidae                
 
      If this phylogenetic topology is indeed correct, I
would classify the basal part of my Class Aves something like this:
           
  CLASS AVES 
       1 Plesion Alvarezsaurus
       B Mononykiformes
       2 Caenagnathiformes
       B Oviraptoriformes
       3 Archaeopterygiformes% 
     _1_ {{Avebrevicauda}} (higher birds)     

        However, I am still not convinced that Plesion
Alvarezsaurus and Mononykiformes form an exclusive clade, and believe
that the latter are probably more derived. I also suspect that
oviraptorosaurs are also more derived than Archaeopterygiformes (in
which I still include Dromaeosauridae and Troodontidae).   Anyway,
once I review other recent papers (especially on the interrelationships
of living Orders of birds), I will be adding some newly-discovered
fossil taxa and recoding my entire classification of Class Aves.
However, the contents of this expanded Aves are still basically
unchanged (and coding changes will reflect any changes in the
phylogenetic topology).                
          --------Cheers, 
                          Ken
Kinman