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Re: Ah ha! That's where therizinosaurs came from

Regarding fusion of bones, there is the possibility of the alternative
character states representing different ontogenetic stages. We know
very few specimens to completely reconstruct the ontogeny of most
non-avian (whatever defition of what is avian you want) theropod
species. For example, non tetanuran neotheropods were once considered
to share the apomorphy "skeletal fusion" (some specimens present a
great degree of fusion among bones), yet many non-tetanurans lack the
proposed apomorphies occurring by fusion, and were considered as
juveniles, according to an explicit ontogenetic parsimony analysis in
Tykoski's (2005) thesis.

What then if manual bone fusion at older age is more expanded among
neotheropods? Thus, carpal fusion in Recent birds may just reflect the
faster acquisition of the oldest possible morphology in other
theropods, because of the already known rapidly accomplished growth in
Modern birds. We may test this by searching from correlations between
carpometacarpal fusion, in one side, and bone growth rate plus time
required to acquire the oldest (adult) morphology. The hypothesis to
test would require carpal fusion to be directly correlated with bone
growth rate and inversely correlated with the time required to grow to
the oldest morphology.