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Re: Microraptor preyed on birds (official paper in PNAS)
On every one of his points, Mickey is strictly correct. And yet...
I don't think we can just look at the stomach contents in isolation.
By itself, the presence of an arboreal bird or a scansorial mammal in
the stomach of _Microraptor_ would seem to be piss-weak evidence for
arboreal hunting. I agree. But the stomach contents should be viewed
in the context of previous work on _Microraptor_:
(1) The pes shows modifications that have been associated with
scansorial or even arboreal behavior (e.g., Xu et al., 2000). These
modifications include the elongation of the penultimate phalanges and
the more distal position of the hallux.
(2) The feathers of _Microraptor_ are elaborately modified for aerial
locomotion - and arguably only suitable for descents from high places.
Admittedly, in terms of scansorial/arboreal adaptations, the pedal
modifications are hardly compelling. The quantitative and qualitative
analyses of Dececchi and Larsson (2011) explicitly rejected
arboreality in _Microraptor_ (and archaeopterygids too). This study
focused on the lack of branch-grasping adaptations and limited limb
joint mobility in _Microraptor_, which makes it extremely unlikely
that this theropod could move about within the tree-crown, or descend
tree trunks. However, the sort of trunk-scaling envisioned by
O'Connor &c (Fig. 3) does not entail branch-grasping. And if the
wings of _Microraptor_ were used for parachuting/gliding to the
ground, trunk-climbing descents would not be necessary.
I've often wondered if the aerodynamic plumage of _Microraptor_,
_Archaeopteryx_ , _Anchiornis_ etc was to compensate for the extremely
rudimentary scansorial/arboreal abilities. And the only reason why
these theropods entered trees at all was for opportunistic feeding
(e.g., catching arboreal prey). I certainly disagree with the
statement by O'Connor et al. (2011) that "_M. gui_ was spending a
substantial amount of time in the trees." But I think we can build a
circumstantial case that _Microraptor_ occasionally climbed trees.
On Wed, Nov 23, 2011 at 1:28 AM, Mickey Mortimer
> Thanks to Tim Williams for a copy of the paper. I can now finally say that
> nothing in the paper changes my opinion. The only quote which argues the
> bird was killed instead of scavenged is-
> "What is preserved of the ingested enantiornithine skeleton is still in
> articulation with the feet located in the proximal end of the stomach,
> suggesting that the meal was not scavenged, but captured and swallowed nearly
> whole and proximal end first, as in most living predatory birds."
> First, the "skeleton still in articulation" is a partial wing and two feet
> lying parallel to each other. Sure they're positioned right if we insert the
> rest of the body between them, but it could also be chance. Yet that doesn't
> even matter since O'Connor et al. never even bring up the possibility that
> scavenged prey can be articulated and eaten head first as well. Plenty of
> birds die while whole, and even more die with two legs and a wing still
> connected. Nor is there any reason it wouldn't be eaten head first if found
> dead. But that doesn't even matter either, since we all know living arboreal
> birds often spend time and are killed on the ground. O'Connor et al.'s only
> statement defending Microraptor killing it in a tree is-
> "The predation of an arboreal enantiornithine suggests Microraptor hunted in
> an arboreal environment (Fig. 3)."
> Wow. That's.... completely untrue. Sure it might have killed the bird in a
> tree, but it might have killed it on the ground, or eaten it after finding it
> dead on the ground, or dead hanging from a tree branch for all we know.
> What's especially amusing is that O'Connor et al. note two other cases of
> fossil diets. One is a mammal skeleton found in another Microraptor, as
> described by Larsson et al. (2010)-
> "The morphology of the foot is most similar to Eomaia and Sinodelphys,
> although this specimen lacks the level of arboreal adaptations seen in those
> taxa. The foot is relatively long, with a shortened first metatarsal and
> elongate phalanges possessing a phalangeal ratio of around 1. The preserved
> unguals are moderately recurved, the phalanges are straight and the ratio of
> proximal to distal phalanges does not indicate a dedicated arboreal lifestyle
> but suggests the animal was most likely scansorial."
> The other is an enantiornithine tibiotarsus found in an ichthyosaur as
> described by Kear et al. (2003). Maybe it was scavenged, but maybe the
> ichthyosaur killed it. Sharks kill birds today after all. O'Connor et al.
> argue the bird's incompleteness indicates scavenging, but the turtles
> preserved in that ichthyosaur are also incomplete.
> So here we have a Microraptor that ate a non-arboreal animal, and a
> definitely non-arboreal animal that ate an enantiornithine. If these two
> cases don't eliminate the importance of O'Connor et al.'s specimen in regard
> to Microraptor's arboreality, what would? And yet we get statements like-
> "This new specimen indicates that M. gui fed on arboreal birds, lending
> further support to interpretations that M. gui was spending a substantial
> amount of time in the trees."
> "Further, because Jehol enantiornithines were distinctly arboreal, in
> contrast to their cursorial ornithurine counterparts, this fossil suggests
> that Microraptor hunted in trees thereby supporting inferences that this
> taxon was also an arborealist, and provides further support for the
> arboreality of basal dromaeosaurids."
> My basic issue with this paper isn't that it's wrong, since maybe Microraptor
> was arboreal and maybe this specimen did kill that bird in a tree. It's that
> the paper doesn't even try to support the various arguments it takes to get
> to that point. Where's the data showing scavenged birds are usually
> disarticulated? Where's the data showing modern predatory birds usually
> don't swallow carcasses head first? Where's the data showing arboreal birds
> are usually killed in trees? Nowhere. It's a neat specimen, but to infer
> anything more than "Microraptor sometimes ate at least partially articulated
> enantiornithines head first" is story-telling instead of science, at least at
> the level of O'Connor et al.'s analysis.
> Taxonomy-wise, O'Connor et al. refer the specimen to M. gui based on "its
> large size (relative to Microraptor zhaoianus), the proportions of its manual
> digits, curvature of the pubis, and slight bowing of the tibia", which were
> all convincingly shown to be problematic by Senter et al. (2004). They also
> refer the bird to Cathayornithiformes, a group which has never been supported
> by an analysis or defended by synapomorphies.
> Finally, I do want to end on a positive note and praise Brian Choo's
> excellent restoration.
> Mickey Mortimer
>> Date: Tue, 22 Nov 2011 17:13:31 +1100
>> From: firstname.lastname@example.org
>> To: email@example.com
>> Subject: Re: Microraptor preyed on birds (official paper in PNAS)
>> Mickey Mortimer <firstname.lastname@example.org> wrote:
>> > Well, since it's not open access I can't say anything about the paper
>> > itself (though I could access it for 2 days for the low low price of $10),
>> > but the
>> > Discover article implies the bird wasn't scavenged because it's
>> > articulated and facing headfirst. This doesn't address the obvious
>> > argument made
>> > on the DML that the enantiornithine could have been caught on the ground,
>> > since that's the case for many modern predators attacking arboreal
>> > birds. Indeed, I don't see why a predator wouldn't eat a scavenged bird
>> > the same way it eats a recently killed bird, since the animal still goes
>> > down
>> > the throat more smoothly head first. So while I still technically reserve
>> > final judgement until I read the paper, I stand by my earlier statement
>> > that it's
>> > impossible to infer arboreality from one Microraptor eating one arboreal
>> > bird.
>> I'm prepared to cut the authors some slack here. Here we have a small
>> theropod (_Microraptor_) that previous studies have inferred to be at
>> last partly arboreal. And what turns up in the stomach of one of
>> these theropods? An arboreal bird.
>> Yeah, there is no reason to assume that the little bird was
>> necessarily in a tree when the _Microraptor_ snapped it up. Modern
>> specialist perching birds often forage on the ground. _Microraptor_
>> might have pounced on the bird while it was on the ground. Or the
>> bird might have already been dead when _Microraptor_ gobbled it up.
>> But the behavior depicted for _Microraptor_ in Fig. 3 is at least
>> consistent with the skeletal (and integumental) anatomy of this
>> theropod: the hindlimbs 'walk' up the tree, the hands grip the tree on
>> either side to provide support, and the jaws catch the prey.
>> _Microraptor_'s 'wings' could then be deployed to return the animal
>> to the ground. No perching (or roosting) is required - so
>> _Microraptor_ doesn't require a perching pes or prehensile manus.