[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: low latitude paleoflora



Tracy Ford Wrote:

>> Also, Cantrill (1998) suggests that
>> the presence of Lophosoria cupulatus indicates that the MAT was at least
>> 8degreesC in the southern High Latitudes (that's above freezeing right?)
>> in the southern high latitudes. 

To add more information...

Mean annual temperature is just that - the average for the entire year.
It is often abbreviated MAAT (Mean Annual Air Temperature), so I don't
think it takes into account wind-chill factors. Also, the climate isn't
expected to stay bang on the average figure all year. It would have been
warmer in summer, colder in winter. A year-long average of 8 degrees
would probably indicate periods of below zero in the winter, but not for
extended periods of time.

The plants from Victoria have been used to suggest a MAAT of around 10
degrees celsius. The oxygen isotope studies suggest a MAAT of -2 to +5
degrees celsius. The studies on the numerous cryturbation structures
(studied by many international experts in geology and modern permafrost
conditions) suggest an MAAT of between -6 and +3 degrees celsius. The
absence of sand- or ice-wedges in some of these cryoturbation sites
suggests it didn't get colder than -6 degrees celsius for any extended
period of time (at least, not long enough for those structures to form).

> The current knowledge of the unquestionable
> presence of Osmundaceae, Cyatheaceae, Lophosoriaceae, Marattiaceae and
> the
> possible presence of Schizaeaceae, Lygodiaceae and Polypodiaceae allow
> us to
> recognize ferns as a diverse group in the Early Cretaceous plant
> communities
> of the South Shetlands Islands. According to the present requirements of
> those families, some of which are restricted to montane forests, tropical
> and subtropical, the climate was mild enough YEAR around, so that these
> ferns could reach an important ecological role in the community, which is
> reflected in the fossil record.

Many studies have shown that the ancestors of some modern plants were
capable of withstanding a wider range of conditions than their modern
counterparts (cycads for example, from the studies by Frakes). We can't
rule out that modern plants may have experienced a genetic bottle neck
since then, due to mass extinction episodes.

...and longrich@alumni.princeton.edu replied:

> The possibility also exists that
> in the past when these families were more diverse, members may have
> existed which were capable of tolerating temperate, subalpine and
> subarctic conditions. Plants are pretty adaptable. There are I think a
> couple species of prickly pear and one pincushion cactus in Alberta for
> example; they survive temperatures down to forty below.

Bingo!

And a more recent "Dropstone" reference is:

Frakes, Alley & Deynoux 1995 "Early Cretaceous Ice Rafting and Climate
Zonation in Australia" International Geology Review 37:567-583

-- 
________________________________________________________________

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