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Re: Dinosaurs vs. mammals: a hypothetical scenario



> Even in forests? Is there a source stating this? (of course, I do not
> require it for just now). In South America at least, there was still
> plenty of non-cursorial beasts in the Miocene, especially among
> xenarthrans, marsupials, toxodonts, etc.

South America was definitely an exception in many ways, I'll give you
that. I have to wonder if borhyaenid/phorusrhacid predator dynamics
drove prey responses rather differently than on continents with
placental predators.

>In North America, you still have slow oreodonts by the time. Rodents, 
>generally speaking, never were too fast either, yet they form, today and 
>likely then, great part of the prey of predators.

Oreodonts had a wide range of adaptations; some were fairly cursorial,
while others were less so, especially the semiaquatic forms (though
they occupy a niche without any direct parallel amongst dinosaurs that
I can see). That's certainly a gray area. Most rodents, on the other
hand, are too small to be relevant to these direct comparisons - their
analogues were, themselves, mammals during the Mesozoic. With some
notable exceptions in South America.

>Velocity is mostly important regarding predation, and I do not see that the 
>Miocene presented faster carnivores than today. Today, most terrestrial 
>carnivores which are not canids, felids or hyenids are not cursorial, and they 
>amount to less than half of the diversity of carnivorous mammals, surpassed in 
>number by slower carnivores as mustelids, ursids, viverrids, procyonids, 
>herpestids, etc.

True, though again, the predators large enough to threaten
non-nestling dinosaurs are the ones most in question; viverrids and
mustelids in the Miocene would have been pertinent to very small
dinosaurs, but for the larger dinosaurian herbivores, it would be
felids, barbourofelids, amphicyonids, hemicyonids, borophagines, and
hyaenids that would have be the major causes for concern. Plus a few
others . . . I'm a little rusty on my timing for the appearance, rise,
drawdown, and disappearance of these groups.

>It seems difficult that in the Miocene, with more forests than today as David 
>indicates, and with African creodonts, South American borhyaenoids, and 
>Holarctic amphycyonids, the pressure for velocity on herbivores was any 
>stronger than today. It seems to me that another advantage for cursoriality is 
>stride increase and thus energy-saving locomotion, not only velocity.

Sure, but if cursoriality is itself a proxy for competitive
superiority in the Miocene (regardless of the specific biomechanical
factors), then only relative degrees of such adaptation in dinosaurs
vs comparably-sized mammals need be compared for our purposes here.

I.e., the substantial cursoriality observed in horses, rhinos, camels,
protoceratids, pronghorns, giraffes, palaeomerycids, deer, and early
bovids during the late Miocene, versus chalicotheres, tapirs, the
South American critters, and a few others, strongly suggests simply
this: that the medium- to large-herbivore-guild prey biomass in the
late Miocene was far more skewed toward cursoriality than were the
rough ecological analogues during the Campanian. That's all I'm
saying. I don't think the average *Orodromeus*-type animal was all
that comparable to something like *Pliohippus* or *Dromomeryx* in
terms of sheer speed. Ornithomimids, perhaps, but not a whole lot else
as far as >1 ton dinosaurs go.

Would that I could quantify that . . .