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Re: Floating allosaurs??



Silvio Renesto asked on Monday, 22 Jan 2001

>
> 2) How large should have been the eventual island to permit life to even a
> small population of allosaurs, given the (?low) nutritional value of
> conifers etc. on which the allosaur preys fed upon?


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.

So let's assume a number of 100 Allosaurs may be large enough to survive over 
a longer period. According to the DinosaurIcon an adult Allosaurus fragilis 
may have weighed 1.5t and more. If we assume an average weight of 0.5t per 
Allosaurus due to a higher number of juveniles than adults we get a total 
mass of 50t.

  Back to my first real book about dinosaurs, meaning "Dinosaur Heresies". 
Bakker stated a range of 0.5 to 5% for the predator to prey ratio. If we 
assume a ratio of 10% we get 500t of herbivorous dinosaurs. This would mean 
about 25 adults of Apatosaurus or 125 adults of Edmontosaurus. Numbers would 
be higher of course due to lower weight of juvenile animals.

  But how much place needs a herbivorous dinosaur to get enough food? I don't 
know. It depend's on may factors eg. the size of the island, the metabolism 
of dinosaurs and the island vegetation.

  And there's another point. Isolated populations of animals (e.g. on 
islands) tend to develop smaller forms, which numbers can be greater. So over 
time there wouldn't be a population of Allosaurs as large as we know them.

  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.


Cheers

Heinz Peter Bredow