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Re: Dinosaur diversity

Nathan Myhrvold wrote:
> All it really has to be be for the argument is "mosasaur-free".  A lake
> connected to the ocean by a river with waterfalls etc. which would
> prevent large aquatic reptiles from entering is a sufficient condition.
> The Mesozoic was long enough, and the world big enough that at least
> some aquatic environments that lacked aquatic reptiles are likely to
> have existed for say a million year span.   Of these, some may have been
> too mineralized to support life, or otherwise unsuitable.  But still, it
> seems pretty likely that a hospitable environment did occur, although it
> is possible that none did.

Granted that such a thing would have prevented mosasaurs, which were
strictly _marine_ animals, from getting in.  Would it also have
prevented crocodiles or turtles?  Crocs seem to have been extremely
widespread in the Mesozoic, much more so than today.  And crocs and
their relatives are primarily freshwater animals today.  As for turtles,
there are some _huge_ predatory turtles around -- snappers, softshells,
etc.  At least some types of turtles even use swarm attacks.

> Islands in which major groups were missing seem even more likely - i.e.
> islands without dinosaurs, but with reptiles, mammals or birds, should
> be reasonably common.  The advance and retreat of the inland seaways
> must have isolated some ecosystems for a million years or so.  Some of
> these, by luck would have an idiosyncratic distribution of species.
> Plate tectonics should have been churning up just as many island arcs as
> it does today.  The same mechanisms for getting species to those new
> islands must have existed.
> I will grant that it may be hard to find fossilized remains of such an
> isolated ecosystem.   It certainly would be fascinating if we did.
> Part of the reason to speculate about such systems is that they might
> illustrate .  As an example, most isolated island systems are mammal
> free, so giant reptiles and birds fill the niches.  There are even
> islands with giant insects (St. Helena, New Zealand).  Why does it seem
> harder to get mammals to these places than everything else?   Why didn't
> either mammals or dinosaurs compete in marine environments?

In general, island ecosystems have lots of species of animals that can
easily get there, and few species of animals that can't get there.  I
know that seems so obvious as to be trite, but I think it's important to
keep in mind.  Birds, bats, and pterosaurs could fly.  Amphibians and
reptiles can remain torpid on vegetable rafts for weeks or months.  But
mammals, with their high metabolism, have a lot of difficulty surviving
such long journeys without food.  In general, island-dwelling mammals
seem to be those that can fly (bats) or swim (seals) to the islands
under their own power.  Small rodents like rats do seem to turn up
fairly often, but then rats are generally vegetarian and have low food
requirements because of their size, so they could make it on a raft that
served as both ship and sustenance.  

Further, once a land-dwelling animal reaches an island it usually finds
a food base that can only support animals that are small,
low-metabolism, or both.  There are few islands that could support a
herd of sauropods, for example.  And if large herbivores can't survive,
then neither can large carnivores.  So I think the apparent lack of
dinosaurs on islands is not all that difficult to explain.

-- JSW