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Re: polyphyletic Alvarezsauria assemblage
Alvarezsaurids show a number of bizarre characters which
appear to unite them. One of them is the caudally expanded,
anteriorly tapered iliac blade, seen in Alvarezsaurus and the
Mongolian material, nothing else looks like it in the Theropoda.
Another is the strongly procoelous, ball-and-socket caudals which are
anything but common in theropods, and the digital proportions are
just bizarre: the second and fourth digits are more robust than the
third which is almost unique, other than having evolved convergently
in jerboas and Borogovia (I checked out the possibility that
Borogovia was an alvarezsaur, however the laterally compressed second
metatarsal and the well-developed medial condyle of the astragalus
indicate that it is in fact troodontid).
Its also worth noting that fusion of the carpals is
notoriously variable. Protarchaeopteryx for example has distal
carpals 1 and 2, unfused, capping metacarpals 1 and 2. The holotype
of _Oviraptor philoceratops_ preserves a full fusion of the carpals
and metacarpals, forming a carpometacarpus, which has not been
reported in other oviraptorids. So should we now decide that
Oviraptoridae is really polyphyletic and Oviraptor goes with
Mononykus, Avimimus, and pygostylians, other "oviraptoridae" are more
primitive, and Protarchaeopteryx is a basal form related to
therizinosaurs, all based on this "key character" (all the other
evidence being homoplastic noise)?
I've been looking at the characters that supposedly unite
Alvarezsauria, and all I can see is a bunch of homoplasic "noise".
The first two (small cervical epipophyses and neural spines) seem
to be very subject to convergence and found in other groups.
Procoelous caudals and sacrals pop up here and there in all kinds of
archosauromorphs (incl. the bird Patagopteryx), and they grade into
slightly procoelous-to-amphiplatyan forms in a variety of
neotheropods. I find nothing convincing in this list of so-called
synapomorphies. The similarities appear to be pretty superficial
I'm not clear, how is this process different from the work of
those who would say "I've been looking at the characters which
supposedly unite birds and dinosaurs, and all I can see is a bunch of
homoplasic 'noise'," and then argue that each supporting character is
highly homoplastic or easy to evolve convergently, so it can be
conveniently dismissed without an analysis?
The problem with manually picking and choosing characters is
we can get absolutely anything we want if we have the will to ignore
the counterevidence. Take a set of animals, find characteristics that
unite them, and call those "key characters" or "good characters",
and everything else can be dismissed as homoplasy. And lo and behold:
all the evidence supports my hypothesis, except forthat evidence