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Re: [dinosaur] Apatoraptor, new caenagnathid oviraptorosaur from Late Cretaceous of Alberta, Canada
Dawid Mazurek <email@example.com> wrote:
> Also, _Archaeopteryx_ is here closer to oviraptorosaurs than to
> _Velociraptor_. I really enjoy the idea that a
> Middle-Upper Jurassic grade of archaeopterygid 'birds' actually gave rise to
> all other groups of Pennaraptora, most
> of them becoming 2F. The majority of phylogenetic analyses do not follow this
> scenario, but I quess this might be
> because of secondary, reversal 'terrestrialization' of morphology. This would
> be more in line with the stratigraphic
> occurence of main radiations of pennaraptoran ingroups.
This is a very attractive idea - that Middle-Upper Jurassic grade of
archaeopterygid 'birds' actually gave rise to other groups (e.g.,
deinonychosaurs, oviraptorosaurs) via secondary loss of flight and/or
arboreality. But as you say, the phylogenetic analyses currently do
not support this scenario.
However, I don't think secondary reversal to 'terrestrialization' of
morphology is a factor. I know this is a contentious point, and I
don't wish to inflame any passions, but there is very little in the
anatomy of _Archaeopteryx_ to suggest it was arboreal. (By 'arboreal'
I mean spending most or all of its time in trees.) The anatomy of
_Archaeopteryx_ is thorougly terrestrial. The great John Ostrom made
this point decades ago, and he's still correct. Yes, _Archaeopteryx_
may have occasionally entered trees; but even if was capable of
resting on branches (of which I'm skeptical) this does not
automatically make _Archaeopteryx_ arboreal.
Also, there is nothing particularly 'special' about _Archaeopteryx_
compared to other Jurassic pennaraptorans like _Aurornis_,
_Anchiornis_ or _Xiaotingia_. Irrespective of whether these four taxa
belong in the same family-level clade (Archaeopterygidae), they are
all fairly similar morphologically. They can be loosely considered
'archaeopterygian grade'. Not one of these critters looks arboreal.
Certain 'arboreal' features have been discerned in the skeletons of
_Archaeopteryx_ and kin (including certain non-avialan
pennaraptorans). These include a longer and/or more distally
positoned metatarsal I (interpreted as a perching adaptation), as well
as certain claw/ungual curvatures (interpreted as a climbing
adaptation). However, both interpretations are not settled.
Alternative (non-arboreal) hypotheses are equally viable, if not more
so - such as predation. In _Archaeopteryx_ and non-avialan
pennaraptorans the hallux is not reversed. This does not necessarily
refute the idea that these theropods were arboreal - but it is very
inconvenient to the arboreal hypothesis.
(I'm partial to the hypothesis of Fowler &c that the derived hallucal
position was tied to predation, and was only later co-opted for
perching. Note that, throughout theropod evolution, metatarsal I was
sliding down the metatarsus ever since it lost contact with the ankle
in basal neotheropods. So the more distally positioned metatarsal I
in pennaraptorans is the product of a fairly established trend. Birds
took this to the extreme, alongside the retroversion of the digit
[hallux] attached to metatarsal, for anisodactyl perching.)
Theropods began as terrestrial bipeds. The bird clade (Avialae)
includes arboreal bipeds. There is no evidence that theropods ever
passed through a transitional phase of arboreal quadrupeds to link the
two. Rather, the evidence suggests that arboreal bipedal theropods
evolved directly from terrestrial bipedal theropods.
There is little if any support for the view that deinononychosaurs,
oviraptorosaurs or therizinosaurs evolved from arboreal pennaraptorans
that returned to the ground. Instead, as the phylogenetic analyses
suggest, the immediate ancestors of these groups were likewise
terrestrial. Some may have had limited flight capacity... but that's