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RE: Epidendrosaurus and the oviraptorid skull



Hi Jaime,

It is great to discuss the skull of Epidendrosaurus with someone
knowledgeable about the anatomy. Thanks for taking the time to give me
your detailed take on it. A little later I'd like to get your take on Dr.
Xu's hypothesis that scansoriopterygids are a possible sister group to
oviraptorids.

But first the skull of Epidendrosaurus itself.

You wrote: "In the mandible, the posterior dentary is virtually absent,
providing us only with a slender, U-shaped apparatus of paired dentaries
in *Epidendrosaurus ningchengensis*"

I would argue instead that this u-shaped apparatus is actually the entire
mandible. We know this because there is a fused symphysis on one end and
an articular on the other of each ramus of the jaw. The articular is
clearly defined in Zhang et al. figure 1b. Thus there is not only no
missing posterior dentary, the other elements of the jaw are all there
also. The whole mandible is therefore something like 15mm long. CAGS02 can
only be missing about the anterior 3mm of the dentaries.

You also wrote:  "no material is apparent anterior/rostral to the
lachrymal." Yet I see a great deal of broken bone matter above the
displaced (right?) lower jaw in CAGS02 main slab, and it is where the
maxilla should be, and it is as long as the lower jaw. Thus I think we are
missing only about 3mm of anterior maxilla and premaxilla.

The specimens are also interpreted as juveniles, possibly even hatchlings,
and ontogeny might predict a very short rostrum, approaching an
equilateral triangle in shape, as was described for for other hatchling
paravians in:
Bever, G.S. and Norell, M.A. (2009). "The perinate skull of Byronosaurus
(Troodontidae) with observations on the cranial ontogeny of paravian
theropods." American Museum Novitates, 3657: 51 pp.

A short skull is characteristic of oviraptorids, of course.

Six years ago I was working out a clay sculpture of the Epidendrosaurus
skull, and I was using a phylogentic bracket to fill in missing
information. Since Epidendrosaurus was described as a basal avialan, that
really meant I was using Archaeopteryx as a model.

I brought this sculpture to Dr. Xu, when he was staff at my Museum, and he
immediately grabbed an oviraptorid skull, pointing to the complete
temporal arcade and the lacrimals that form a wall anterior to the orbit,
and suggesting that I make the Epidendrosaurus skull far boxier.

I mentioned the weirdly frog - mouthed dentaries and said that they seemed
very divergent from basal birds. He said they were well within the range
of variation for birds, indicating caprimulgids, podargids, Crotophaginae,
and
machaerirhynchids.

Xu recently released this paper:

Xing Xu, QingYu Ma and DongYu Hu (2010)
Pre-Archaeopteryx coelurosaurian dinosaurs and their
implications for understanding avian origins.
Chinese Science Bulletin (advance online publication)
DOI: 10.1007/s11434-010-4150-z

which has one hypothesis that places oviraptorosaurs as the sister group
to scansoriopterygids, and both as closer to birds than Archaeopteryx is.
That would explain why he referred me to the oviraptorosaur skull. He
admits that he has not yet done any quantitative analysis to support this
hypothesis, and that quantitative methods contradict it, but it is an
intriguing, if wholly speculative, possibility.

Let me know what you think.

-Jason