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Re: Dinosaurs vs. mammals: a hypothetical scenario



> Of course, the Miocene was warm.

Compared to today. The northern latitudes would still have been
roughly comparable to modern cooler temperate regions. So no serious
glacial buildup or permafrost or 8 feet of snowfall, but still a
goodly amount of snow during the winter across what is now basically
the tundra/taiga zones. That's what I've been led to believe, anyway -
temperate deciduous forests up nort'.

> Phytoliths are not, in fact, harder than enamel. It's not about grass, it's 
> about ground-level feeding and sand in the food; sand _is_ harder than enamel.

Huh. Every source I've ever read on the subject has claimed that it's
the silica content *in* the grass that not only drove the evolution of
derived grazing-adapted molars, but also that it was part of the
defensive adaptations of grass and thus part of what allowed it to be
so successful. Can you recommend a source? If this is new information
that overrides all the other stuff I've seen, I need to know about it
. . .

> There was no global savanna. Roughly, today's steppes/prairies were
> savannas; today's savannas were rainforest.

Well I know. That's just the loose stereotype that gets peddled around
for that period. I was using it as shorthand for what in the fossil
record seems like an unusually high diversity (read: niche saturation)
of grassland-adapted mammals on a global scale.

> You might be confusing speed and endurance here.

I suppose that's possible. It's been a while.

> And for both, the caudifemoralis muscle may be a lot more important than most 
> people used to think.

With the implications being . . . ?


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>Ok., but then we may say that the less cursorial bipedal dinos would
not be more outcompeted than the less cursorial mammalian quadrupeds
of the same size (because also in the Cenozoic you have non-cursorial
quadrupeds living along with cursorial quadrupeds). We should compare
the most cursorial quadrupedal mammals with the most cursorial bipedal
dinosaurs.

In some biomes, I would generally agree. Yet the Miocene saw the
replacement of nearly all the archaic lumberers with much more
fleet-footed forms - even in forests, the traditional stronghold of
the more graviportal-ish critters. Not that there weren't holdouts (or
aren't today, for that matter). But as a general trend, the various
horses, rhinos, tylopods, and pecorans that constituted the greater
share of the large-herbivore biota of the day were pretty well-adapted
to swift and/or efficient locomotion.

Of course, the Miocene was also nearly 18 million years long, and the
story during the Miocene climatic optimum would have been very
different (and more favorable for Campanian dinosaurs). But as per the
OP, I've been aiming my thoughts toward the late Miocene specifically,
and that was in many ways a world of cursors.

>Is there a source stating this? Because I would rather think stride
lenght to be variable among both bipeds and quadrupeds, and can mind
long-legged bipeds having a stride advantage over many quadrupeds with
proportionally shorter legs. In addition, moving two legs instead of
four while running looks to me like more energy-efficient also in
running (because of the same reason you mention, expending energy in
moving two extremities instead of four).

I can't pin down a source right now, but consider: at the same limb
lengths and the same range of flexion, a quadruped will always have a
slight stride advantage purely because one pair of limbs is situated a
distance in front of/behind the other set. A biped, with only one pair
of limbs, has to rely entirely on the reach of those limbs; it cannot
use a pair of forelimbs to reach beyond the flexion of the hindlimbs.
As an extreme example, imagine a regular cheetah and a "bipedal
cheetah" - all the additional stride distance the regular cheetah
gains through extra flexing of the spine and forelimb girdles could
not be matched by the bipedal analogue, which lacks those "tools."

You are of course totally right about absolute limb length being
variable between forms (and ostriches being faster than leopards or
wild pigs is certainly not lost on me). I was intending the comparison
to be between creatures with roughly comparable mass and limb length,
and even then only as a general trend.