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RE: therizinosaurs - how did it happen?
Tom Holtz wrote:
> As Lindsay Zanno showed at SVP last year, there is a fairly good chance
> that the shift from a strictly carnivorous diet in coelurosaurs may
> have been a single event shared by the common ancestor of
> ornithomimosaurs, therizinosaurs, alvarezsaurs, oviraptorosaurs, and
> eumaniraptorans (with hypercarnivorous eumaniraptorans being
> reversals). If so, and given the presence of the ?Early Jurassic?
> Eshanosaurus, this origin would have been in a fairly transitional
> period of dinosaur history. (Of course, the age of Eshanosaurus is
> debatable; however, given the presence of Late Jurassic eumaniraptorans
> [and maybe even Middle Jurassic ones], the divergence among
> maniraptoriforms was certainly a Jurassic event). So the
> non-carnivorous coelurosaur radiation may have occurred with the other
> major dinosaur radiations of the Early and Middle Jurassic.
Barrett (2009; Paleontol. doi: 10.1111/j.1475-4983.2009.00887.x) presents a
convincing case for why _Eshanosaurus_ is more likely to be a therizinosaur
rather than a sauropodomorph. However, while Barrett's paper convinced me that
_Eshanosaurus_ was probably not a sauropodomorph (or an ornithischian!), IMHO
it was less convincing in showing that it was a therizinosaur. So a third
possibility (also in line with Barrett's evidence) is that _Eshanosaurus_ is a
theropod, but not a therizinosaur.
If the shift from a strictly carnivorous diet in coelurosaurs was indeed a
single event shared by the common ancestor of ornithomimosaurs, therizinosaurs,
alvarezsaurs, oviraptorosaurs, and eumaniraptorans, then _Eshanosaurus_ may lie
close to this shift. In other words, the therizinosaur-like characters of
_Eshanosaurus_ are plesiomorphic, and precede this non-carnivorous coelurosaur
radiation suggested by Tom (above).
Yet other possibility is that _Eshanosaurus_ is a non-maniraptoriform
coelurosaur that evolved adaptations for herbivory independently. Considering
the trophic diversity of theropods, this shift from carnivory to herbivory may
have occurred several times. In addition to the herbivorous non-avian
coelurosaur groups, we now also have _Limusaurus_ (ceratosaur) representing
another lineage of herbivorous theropod.