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Re: Dinosaurs vs. mammals: a hypothetical scenario

Mmm . . . pure versus debating.

I think the realistic conclusion is ecological chaos. Assuming that
disease and parasites from both don't decimate the other, that modern
plants don't incude terrible allergies for the Mesozoic contenders,
and that stuff like oxygen content in the atmosphere isn't too
pertinent, it's still true that neither set has behaviors adapted to
dealing with the other. That would leave the result open mainly to
chance - if your adaptations happen to work well in the new reality,
you succeed, but other than sheer happenstance it's more likely that
the new conditions (including how stressed any given ecosystem becomes
just from the initial influx of calorie-consuming biomass).

By the Miocene, a lot of large mammals had become pretty fleet of
foot, so the majority of them aren't going to be viable prey for most
large dinosaurian predators, at least in open habitats. Forests might
be another story. I'd expect smaller, leaner types of tyrannosaurid to
do fairly well out in the open on a pure speed basis, but camouflage
and the possible lack of cooperative hunting would be serious limiting
factors. The larger dromaeosaurs may do all right in forested
environs, while abelisaurs . . . well, know one quite knows what the
hell abelisaurs were doing in the first place, so that's harder to
even guess at.

Sauropods and ankylosaurians are not going to do well in any
environment where trees lose their leaves for the winter *and* it's
cold during winter. Hadrosaurs and ceratopsians might be able to
process woody material well enough to eke out an existence up into the
warm-temperate zone, where winters have snow but are fairly mild, but
past that the lack of insulation is damning. In the reverse, however,
the "inhale and ferment" strategy of sauropods and ankylosaurians
might prove better at bypassing the high silica content of grass than
the erodible dental batteries of hadrosaurs and ceratopsians. All of
the big herbivores have the advantage of being immune to virtually any
mammalian predator as adults.

Small, adaptable omnivores with insulation, like oviraptorosaurs,
could conceivably do well in regions with sufficient cover to let them
hide from hemicyonids and other mammalian predators fast enough to
overrun them. Ornithomimids might be one of the few smaller-dinosaur
clades inherently fast enough to thrive in a Miocene "global savanna"
situation. Smaller ornithischians (most hypsiloids, pachycephalosaurs,
etc) are going to have a rough time. Compared to the various horses
and ruminants dominating the planet, they're slow during escapes
(quadrupeds > bipeds in speed at a given size, usually, especially
when we're talking cursorial vs non-cursoiral), they lack highly
derived silica-grinding tooth batteries and/or digestive tricks, and
they lack serious insulation, so short of hibernation they're hosed
when it comes to dealing with snowy winters.

My overall gut reaction is that hadrosaurs and various maniraptorans
have the best opportunities to succeed, followed by the faster members
of the medium-sized theropod guilds, then *maybe* sauropods,
ankylosaurs, and large ceratopsians, and then everything else has
chances of slim to none.