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RE: Lack of Running Giant Theropod Tracks
I think we need to be careful no to turn this into one big argument from
analogy. Analogies are useful to a degree, but it can be very difficult to
determine how flawed an analogy actually is. For example, while it is true that
an ostrich can run faster than its chicks, or even a roadrunner, that does not
automatically mean that was the case for giant theropods, because we don't have
any extant 6 ton bird legged bipeds to look at today. While it is true that as
certain animals "scale up", they may actually *increase* in speed such as with
small cats and lions, one should not ignore the fact that scaling issues will
start to become a problem at some point; the question is where that point is.
Insects are a great example of this: large cockroaches or probably faster than
smaller roaches, but I doubt a hercules beetle would win many bug races.
Theropods may have been able to remain quite fast as they increase in size,
especially since bone is highly resistant to compressive forces, but I'm sure,
as Hutchinson and his colleague's work suggests, it eventually gets to a point
where not enough muscle can be mounted on the skeleton in any functional way.
So I imagine there would be somewhat of a bell curve in performance, and we are
only seeing half of that bell with most vertebrates alive today.
Unfortunately, as the experts have pointed out, there are a huge number of
unknowns with these animals, so we are left with a range between 15 to 25 mph.
The reason I lean to the extreme high end of that estimate is because we are
dealing with a predator that has to chase down and kill other animals. As I
have already stated, I find the argument that their prey was even slower
extremely unconvincing, as it assumes that these animals needed only to hunt
the largest animals in their ecosystem, which were animals of comparable size
and power. I think these were animals designed to run, not just because it
would be cool, but because the same research on these scaling issues suggests
that the smaller dinosaurs were fast. I suspect that giant theropods hit a
"wall" of predatorial effectiveness due to a decline in speed, and this may in
fact be why the only giant predatory theropods slightly larger than T. rex were
(probably) pack hunting sauropod hunters and a very large piscivorous
Spinosaurus. Any slower than 25-30 mph, and I think these giant predator's prey
options would have become severely limited. This may be why hardrosaurs such as
Shantungosaurus could grow to twice the mass of a T. rex: they didn't have to
chase animals smaller and more agile than themselves. Because of this, I like
to look at this problem from more of a "design" standpoint. What plausible limb
and muscle design would have allowed for the highest speeds possible for T.
rex? Because we are not talking about a grazing animal here; this is an animal
that had to chase other creatures down and kill them.
> Date: Tue, 30 Nov 2010 17:05:02 +0000
> From: firstname.lastname@example.org
> To: email@example.com
> CC: firstname.lastname@example.org; email@example.com; firstname.lastname@example.org
> Subject: Re: Lack of Running Giant Theropod Tracks
> On 30 November 2010 16:58, Erik Boehm wrote:
> > It certainly is plausible that adult forms were considerably more bulky
> > than the juveniles, and thus slower, and this would be more likely if they
> > relied on size and not speed to survive as adults.
> Plausible, yes. But I'm still interested in the question I asked a
> day or two ago: does anyone know ANY extant tetrapod in which the
> juveniles can run faster than the adults? I can't think of any --
> which of course doesn't mean there aren't any, hence the question.
> If we can't, between us, come up with counter-examples, then I think
> we'd need to say the most likely explanation is that evolution
> optimises for adult speed rather than juvenile speed, and that in the
> absence of actual evidence to the contrary, the same was likely true
> of those other tetrapods, tje dinosaurs -- tyrannosaurs and sauropods
> (One of the recurring lessons for me over that past few years has been
> that "what makes sense" doesn't always correspond well with "what
> animals actually do" -- as in, for example, the matter of habitual
> neck posture.)