Natural Science Forum / Biology / Paleontology / December 2005

True, but that doesn't really change the point.  The structural
material -- bone -- in vertebrates was originally used externally,
as a carapace, but ended up being used internally, as an endoskeleton.
It's possible something similar could have happened with our
hypothetical large land arthropods.  With land animals I kind of
think there *would* have needed to be a "migration", with internal
projections originally used as muscle attachment points gradually
taking over structural function.  All speculation of course, but
I don't see any reason it couldn't have happened that way.

You are thinking notochord. The first boney material was in the skin. In the
boney headed fish it stayed there to some degree. If we were descended from
them our heads would look rather different.

I would rephrase that slightly to say that T-Rex was as fleet footed
as it *could* be.  In "Predatory Dinosaurs of the World" Gregory
Paul argued that "if an animal looks fast, it's fast," and then
went on to explain why T-Rex "looks fast" to him.  I thought his
argument was persuasive, but it doesn't really answer the question
"how fast?"  The cheetah (and to a lesser extent other big cats)
is highly optimized for speed, because its lifestyle is such that
the faster it can run, the better.

But that doesn't mean it's ever going to hit 120mph, however big
a selective advantage that would bring!  Likewise I think that
T-Rex was also optimized for speed, and that the faster it could
run the more food it could catch.  But that still doesn't answer
the question.  For all we know with an animal that size evolution
may hit the wall at 15mph!  (Still faster than a lot of people of
course, and remember that the animals it was chasing would also be
large, and thus would face similar constraints).  But it doesn't
do to underestimate Mother Nature, or imagine we can anticipate
all of her clever devices, so my guess would be a top speed in the
20-25mph range, or 30mph at an absolute upper limit.  (Note that
Paul considered 30mph to be the lower limit!).

<snipped>

This "fourth power factor" formed one of the main arguments in support
of a new theory of dinosaur extinction hatched in a pub over several
drinks during a conference a few years. It goes like this:

As animals increase in size, the amount of muscle mass increases
exponentially. For mammals it peaks out at around 10 tons. So large
sauropod dinosaurs, some of which have been estimated as weighing 80
tons (though I suspect that such estimates were *also* the result of
late-night drinking sessions) were finding a way around this limit.

Feeding is no particular problem. The long neck provides a large area
of grazing, so they can feed from a large volume of vegetation with
very little effort. Walking can be slow and stately, never putting much
dynamic load on the joints. The problem arises (as it so often does)
with sex.

When a male sauropod is humping a female sauropod, his weight has to be
supported by her hind legs, and if they are already loaded to close to
their limit. So either the male is much smaller than the female or he
has a long, steerable penis which can be inserted from a side-to-side
stance. (Having a "bit on the side", as we say in England). However,
such speculations are unsupported by any evidence, which leaves us with
a third option: large sauropods were all female, and reproduced
parthogenically. This idea is supported by some evidence - turkeys
occasionally do this, and they are, after all, related.

This is fine, so long as there are no major changes in environment.

We now have to take thermal control into account.

Large sauropods were passive homotherms, and because of their enormous
size could maintain a very stable body temperature, responding only
very slowly to changes in the temperature of the environment. However,
although such changes were slow, they were hard for the animals to
control. They could not increase metabolic rates during cold weather by
increased activity (remember the "fourth power" factor reducing their
gait to a slow and stately progression), so if the season turned cold,
they would need to migrate to warmer climates to compensate.

Similarly, during hot seasons they would find it very hard to lose
heat, and would need to head for cooler climes during the warmer part
of the year. So they became locked into a seasonal migration between
tropical and temperate latitudes simply in order to maintain a constant
body temperature. Large herds of giant sauropods moving across a
landscape and eating every scrap of vegetation has a profound effect on
the ecology.

This is where we have to consider the ecology of large sauropods.

Sauropods had, as we now know, bony plates and osteoderms on their
backs, and a rough, rugged skin. This would have provided the nooks and
crevices needed for small plants to grow, these would in turn attract
insects and other invertebrate feeders, and thes would attract the
small mammals and pterosaurs feeding on them. So the sauropod was the
centre of a whole ecological system in much the same way as an oak tree
is today.

Now plate tectonics comes into play.

During much of the Mesozoic, the continents were arranged so that the
migration routes from the tropics to the north and south was relatively
easy, and involved no water crossings. Towards the end of the
Cretaceous however, the continents fragmented, and these routes became
barred by stretches of deep water. So the sauropods, unable to evolve
into new forms to overcome this problem because they were both
parthogenic and long-lived, died out, and their demise brought a whole
slew of ecological systems crashing down with them, bringing radical
changes to climate and rainfall patterns, and triggering a mass
extinction event.

Curiously, this speculation was witnessed by a BBC researcher working
on an proposal for "Walking with Dinosaurs". A small detail - the
pterosaurs nesting on the backs of the sauropods - actually made it
into the final version.

RF