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Re: Floating allosaurs?? (sort of...)



Heinz Peter Bredow wrote:

> 
> Just some thoughts and very, very rough calculations,
> 
> "Island hopping" needs time, really much time. You need a population of
> allosaurs which is large enough to survive diseases and other catastrophies
> on an isolated island. In other words, you need a gene pool which is large
> and variable enough to ensure survival of at least some animals.
> 
>   Numbers I have read in popular science reports range from 500 to 2000
> animals for a species to survive over time.  But now there's a report in
> Nature (No. 409, p. 303, 2001) about a small herd of cattle in England which
> is inbred for at least 300 years. Today it numbers 49 animals which are
> genetically very similar but healthy. A classical case of a genetic
> bottleneck. With such a homogeneous gene pool a deadly disease would
> certainly wipe out the whole herd.

A similar case seems to have happened with the Wollemi pine. The last
surviving remnant population are genetically almost identicle.
Inbreeding seems to be a problem only if there are nasty recessive genes
in the population. If the animals are genetically very robust, then
barring mutation (which is probably a quite rare event, at least within
the coding parts of the DNA that matter most to survival), they should
be able to survive quite a bit on inbreeding. Over time I suspect that
isolated populations either become genetically robust or become extinct.
In which case, a species specially adapted to island living might
require less genetic variety (or none at all).

Compare the "giant" tiger snakes that live on islands in Bass Strait in
the south of Australia with their mainland (or Tasmanian) cousins.
They've probably been isolated for at least 8000 years or so, and are
still going strong. Put a similar sized population of mainland tiger
snakes in the same situation and they'd probably die out. There isn't
much selective pressure to survive a restricted gene pool when there's
plenty of genetic variability.

> SNIP

>   IMHO "island hopping" for a species to reach another mainland is something
> for smaller theropods and small or medium sized herbivorous dinosaurs,
> because of the need to have a large enough population. Big sized theropods
> and  herbivorous dinosaurs were more in need of land bridges to reach another
> mainland, think about Tyrannosaurs.

Both Asian and African elephants have been known to swim for large
distances. Certainly they managed to populate many of the islands in
south east Asia. The last populations of mammoth existed until about
5000 years ago on islands. Most species of proboscidean that stayed put
on islands tended to develop dwarf forms (even the last mammoths),
however those that did (and do) "island hop" are better off with large
body sizes. It enables them to embark on longer journeys, and it would
seem to me that a large animal would be more stable in the water than a
smaller one. I suspect they are less prone to hyperthermia as well.

-- 
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Dann Pigdon                   Australian Dinosaurs:
GIS Archaeologist           http://dannsdinosaurs.terrashare.com
Melbourne, Australia        http://www.alphalink.com.au/~dannj/
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