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Re: At long last! Turner, Makovicky & Norell on dromaeosaurids

Jason Brougham <jaseb@amnh.org> wrote:

> This may be my fault. I may have started this by saying that Incisivosaurus 
> (the most basal known oviraptorosaur) is not very flighty. Then someone 
> compared
> Incisivosaurus to Protarchaeopteryx. What I meant is that Incisivosaurus is 
> big and relatively heavy.

No worries, Jason.  I agree with you that size alone makes
_Incisivosaurus_ very un-"flighty" indeed.  However, it is more than
likely that the aerial behaviors of many basal paravians were very
different we see today among extant gliding and flying vertebrates.
Being a bit on the heavy side might not have been a problem for
maniraptorans - especially if incipient wings were used for
orientational control during controlled descents ("parachuting")
rather than extending the glide path.  Of course, we don't yet know if
_Incisivosaurus_ had an any kind of aerial ability at all, because we
don't know what kind of feathers it had.

> Incisvosaurus' skull is 11cm long, while Sapeornis' skull is 6 cm long (IVPP 
> V13275 and IVPP 13276) and Protarchaeopteryx is 7 cm (NGMC 2125).

The skull of _Incisivosaurus_ (which the original description puts at
about 10 cm) was not much larger than that of  _Microraptor hanqingi_,
which was maybe around 9.5 cm long.  Then again, we can't be sure that
the latter glided either.

> And, yes, that is interesting about Sapeornis moving down the phylogenetic 
> tree in Turner et al. 2012.

Yes, _Sapeornis_ had a shoulder anatomy that was not too different
from that of _Archaeopteryx_ - but a tail that was extremely short,
and fused at the end to form a pygostyle.  _Sapeornis_ shows some
adaptations for perching, especially the large hallux and J-shaped
metatarsal I (so the attached digit would be directed more posteriorly
compared to if the metatarsal I was straight, as in _Archaeopteryx_
and non-avialan theropods).  If _Sapeornis_ spent much of its time in
trees, eating seeds (for which we have direct evidence, with two
specimens each showing a seed-filled crop), it could have used its
wings to glide from tree to tree.  A long tail was perhaps of little
(if any) use for an arboreal bird, so it was drastically shrunk and
fused to form a pygostyle in _Sapeornis_.  The pygostyle could have
supported a rectricial fan, as in much more advanced birds; but it's
more likely that the tail of _Sapeornis_ was aerodynamically
redundant, as in confuciusornithids.

By contrast, _Archaeopteryx_ and _Jeholornis_ have long tails (by
avialan standards), which is consistent with the interpretation that
these basal avialans were predominantly terrestrial.  The long tail
was there to help counterbalance the animal when running on the
ground.  The frond-like arrangement of rectrices served to support the
tail during aerial locomotion (gliding?).  But a better strategy was
simply to truncate the tail skeleton altogether, and get it out of the
way.  But this was difficult for cursorial animals that still had a
fairly cranial center of mass, and hip-based stride generation, and
needed a long tail for balance.  The pygostyle-like structures of
certain non-avialan maniraptorans (_Beipiaosaurus_, _Nomingia_,
_Similicaudipteryx_) were not accompanied by the profound truncation
of the tail we see in _Sapeornis_, confuciusornithids, and more
derived short-tailed birds.

Also, the presence of a crop in _Sapeornis_ means that this feature
may be present in the seed-eating avialan _Jeholornis_, which Turner
&c recover as more derived than _Sapeornis_.