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RE: The Effects of Dromaeosaur Predation
Scott Selberg wrote:
> Jonas Weselake-George's request for ornithopod forelimb movement got me
> thinking about an old theory of mine. It has always struck me that
> considering the build and posture of Iguanodon, the spiked thumb would not be
> a very effective weapon against a large theropod like an
> allosaurid. However, given the proper mobility in the shoulder and elbow, the
> spikes would be very effective for peeling a dromaeosaur off of the
> animals flanks. Obviously there are many other factors involved, but I've
> always found it interesting that right when large dromaeosaurs become
> common, allosaurs decline and large herbivores with unprotected flanks are
> largely replaced by herbivores like armoured titanosaurs,
> nodosaurs, and perhaps iguanodontids that were designed for flank protection.
> Also, when tyrannosaurs came to the fore and dromaeosaurs are
> somewhat relegated to small game hunting, large herbivores with relatively
> unprotected flanks again come to the forefront. Again, I realize that
> there are many other factors involved and other things going on at that time,
> but I think that the effects of the radical new approach that
> dromaeosaurs brought to predation may have had a much bigger effect on
> herbivore evolution than is generally considered.
Overall, the idea of thumb-spikes being wielded to fend off flank attacks, as
well as the idea that dromaeosaurs helped drive stegosaurs to extinction, are
both intuitively attractive. In fact, at least one of those ideas might have
actually have appeared in the literature (but don't quote me on that!).
However, a few extra things should perhaps be taken into account. First of
all, I'm not sure we can confidently say that "when tyrannosaurs came to the
fore and dromaeosaurs are somewhat relegated to small game hunting".
_Velociraptor_, as one of the later dromaeosaurs, was an exceedingly formidable
predator - especially if it was willing and able to tackle an adult
_Protoceratops_. (I know there are differing interpretations of the
_Velociraptor_-_Protoceratops_ association, but the view that the former was
actively attacking the latter does seem to be the most plausible.) Among Late
Cretaceous dromaeosaurs, _Velociraptor_ was probably not exceptional in terms
preference for large(r) prey.
Secondly, one of the best defenses against predators is large body size. Many
(but by no means all) titanosaurs seem to have adopted this approach, given
that this group churned out some truly enormous forms (_Puertasaurus_,
_Argentinosaurus_, etc). As you mention, many titanosaurs show body armor,
which was another method of defense, especially given that it was the smaller
titanosaurs that tend to be armored (especially _Saltasaurus_ and co).
Iguanodontians also attained large body size, with some hadrosaurs being truly
enormous (with some easily outweighing many contemporary sauropods). Certain
"iguanodontids" (using the term for the grade of iguanodontian ornithopods
leading up to hadrosaurs) were also impressive in size, with things like
_Lurdusaurus_ and _Lanzhousaurus_ being especially large and bulky, much like
hadrosaurs. Some ankylosaurs were also enormous (up to ~ 10 m for
_Ankylosaurus_). Ankylosaurs (including nodosaurs) were around since the mid
their evolution really went into overdrive in the Cretaceous. So in general,
ankylosaurs were a lot more successful than the stegosaurs, and the ability to
defend against predator attacks may have had something to do with the relative
success of the two groups.
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