re: dwarfing

[Liz.Reed@flinders.edu.au (Liz Reed)]

Previous episodes of the "Biggest Dinosaurs" saga.....


> To which  Betty Cunningham replied.
> >but this seems to ignore those island-dwelling dwarf elephants found
> near Southern California.  If they were in this kind of surroundings
> long enough to become dwarfed (genetically) then the population must
> have been viable for quite some time in just such a situation as you
> infer.  Why couldn't this have happened to sauropods?<
>
>    I am not familiar with this elephant.  However, an isolated habitat
> that
> could not support a large elephant might support a smaller one.  If it is
> extinct now, it just supports the thought that isolated habitats result in
> the
> extinction of large animals over many many years.  In addition, did the
> elephants on this island evolve smaller or evolve from a smaller ancestor?

>I thought the dwarf elephants Betty described were actually dwarf Mammoths
>that lasted on the island (which had been part of a larger landmass prior to
>the isolation of the island).  They supposedly survived until about 3000 years
>ago.  I also think that the island in question was further north closer to
>Western Canada and Alaska.

Dann Pigdon wrote:
>"Pygmy" mammoths are known from islands off northern Europe, and
>have been dated to around 4,000 years ago or there abouts. They
>assumedly evolved from much larger ancestors. Not only did they adapt
>to island living, but also mamaged to outlast their larger cousins.
>I beleive from memory that they were never very numerous, and that
>all sorts of diseases are evident in the most recent remains.


The dwarf mammoths being referred to were from Wrangel Island in the high
Arctic (off Siberia). Vegetation changes during the terminal Pleistocene
reduced the range of the wooly mammoth which at that time became extinct.
The population  on Wrangel Island was a small, relict population separated
from the mainland by sea level rise. Their survival through the critical
period was probably due to:
1) grassland and herb vegetations persisted on the island after the
mainland changes, thus favouring the "midget" mammoths.
2) a decrease in body size of about 25-30% occurred in the Wrangel Island
mammoths.

The fossils of these dwarf mammoths have been dated to 3,700 BP. I don't
think any fossils have been found from 11-8ka during the "dwarfing" period.
The population must have become large enough for them to remain viable for
4,000 or so years after their larger relatives bought it. I learned of this
from Dr. Andrei Sher who did all the work on these mammoths and stayed with
us recently in Adelaide following the CAVEPS conference where he'd
presented the results.
Refer to Sher, A. (1997). Environmental turnover and extinction in
Beringida during the Pleistocene termination.  Abstract in the 1997 CAVEPS
abstracts volume.

Why then did the little guys go extinct? Don't ask me, but their prolonged
survival is probably an exception, rather than the rule, although island
dwarfing has been documented in several fossil mammal species. Recently on
Kangaroo Island in South Australia, we found a new dwarf late Pleistocene
Zygomaturus species.

I might be barking up the wrong tree, but maybe changes in range size or
isolation, would result in similar responses in Dinos, especially
herbivorous ones.

Sorry to prattle on so long,
Cheers,
Liz Reed