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Re: Dinosaurs vs. mammals: a hypothetical scenario



>As far as I know, most dinosaur bipeds had cursorial proportions for
their size.

I was a bit ambiguous there. I meant *in cases* where cursorial
adaptations are clearly lacking in one and present in the other. What
I had in mind, in the context of that paragraph, was things like
pachycephalosaurs, small ceratopsians, and perhaps some of the
stumpier small ornithopods. Those critters weren't lumbering tortoises
by any means, but compared with a *Miohippus,* gazelle-camel, or
dromomerycid, those sorts of dinosaurs don't seem notably well-adapted
for sustained fast running in open terrain. Especially since
quadrupedal running is generally more energy-efficient than bipedal
running due largely to superior stride length, if I'm not mistaken
(although at the opposite end of the spectrum, bipedality is more
energy-efficient for walking because fewer limbs are expending
energy).

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>Look what happens when even small mammals like rats or mice are introduced to 
>islands where birds have no experience of mammalian carnivores - bird numbers 
>tend to plummet dramatically. Introduce a single cat to a small island and it 
>can wipe out an entirely bird species in short order. The dinosaurian 
>behavioural repertoir would most likely not have included appropriate 
>responses to large mammals, so they may have been in a similar situation to 
>that of modern birds that evolved on mammal-free islands.

I have to disagree, simply because in cases like those you cite, it's
almost always a matter of mammalian predator = ground predator. Birds
on those islands are vulnerable because they co-evolved with few to no
non-aerial predators *period,* not because they didn't co-evolve with
mammals specifically. Snakes can decimate them as or more efficiently
than pigs or carnivorans.

Like ostriches and other modern ground-nesting birds, ground-nesting
dinosaurs surely had adaptations to cope with the numerous small
predators - in their case coelurosaurs - that like to poke around such
nests, and against a three-ton hadrosaur I can't imagine a hyena or
even a bear being grossly more threatening than a good-sized troodont
or larger dromaeosaur. The behavior would likely be similar enough to
offer a similar degree of deterrence.

For smaller dinosaurs, cryptic nests or nests within sheltered
cavities, etc would probably have been just as effective as for small
mammals and ground-nesting birds today, although obviously quite a few
of them did take the opposite tack and opt for strength in numbers at
large breeding colonies. And that hypothetical interplay is one of the
more interesting questions here . . . come to think of it, finding a
good nesting site in thick angiosperm-dominated woodlands or open
grassland might be challenging for dinosaurs unacquainted with those
biomes.