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Re: Dinosaurs burrowed to keep warm

Denver Fowler wrote:

>How many potentially burrowing ornithischians were present in the late

In north america: one.

(Warning: Unadulterated speculation coming up.)

I don't know which one Denver had in mind, but it depends upon how prevalent this behavior was among ornithischians. _Thescelosaurus_ and _Bugenasaura_ were both late Maastrichtian "hypsilophodont"-grade ornithopods. Although these two are outside the _Orodromeus_-_Oryctodromeus_-_Zephyrosaurus_ clade (in Varricchio et al.'s analysis, anyway), burrowing behavior may have first evolved much earlier in Ornithischia, as Dann suggested.

A _Zephyrosaurus_-_Orodromeus_ clade has been found before (e.g., Weishampel and Heinrich, 1992), and was diagnosed by the presence of a jugal boss (unknown in adult _Oryctodromeus_). It is possible that the jugal boss is related to burrowing too (to help push dirt aside?). A jugal boss is also seen in heterodontosaurs and many marginocephalians. Weishampel and Heinrich (1992) proposed that the jugal boss is primitive for the Ornithopoda+Marginocephalia clade (Cerapoda), but was secondarily lost in most ornithopods (including the stiff-tailed iguanodonts). I'm not putting too much stock in this single character, or its association with burrowing; it's all just dodgy speculation on my part.

The thing is, burrowing behavior in _Oryctodromeus_ would probably not have been inferred from anatomical evidence alone. The anatomy of _Oryctodromeus_ was interpreted in light of the taphonomic evidence. The anatomy of the closely related taxa (_Zephyrosaurus_, _Orodromeus_) was then re-interpreted. If other ornithischian taxa turn up in burrows, then other characters might be re-interpreted as burrowing characters too.



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