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RE: No Mesozoic rainforests?



> I may be a layperson, but this sounds completely
> nonsensical. The Mesozoic was hot enough for
> rainforest conditions to be fairly widespread, or at
> least exist. Granted, the vegetation composition of
> Mesozoic rainforests may have differed from that of
> their Cenozoic counterparts (more conifers, presence
> of pre-K/T flora families), but you have to be
> pulling my leg to tell me that warm, humid forests
> just suddently appeared for the first time in a
> measly million or so years after the K/T.

The ITC is a consequence of planetary physics, so an
equatorial rainbelt can be expected for any time and
area where there is no rainshadow by mountains to the
west, and especially where there is an ocean to the
west (The ITC is characerized by westerly winds, but
generally updrafts dominate, hence the rain). The
places to check would be for example southern
Venezuela and the northern coast of the Gulf of
Guinea. I do not think sufficient fieldwork has been
done in the latter area, while there may be some in
the former. The Atlantic was narrow enough 65 mya to
act, climate-wise, more like an epicontinental straits
(certainly it was to narrow for today's system of
currents to exist there and then).

Ethiopia might also be a candidate location; Africa
was a bit more S than today and apparently rotated
somewhat clockwise in respect to its present
placement; thus in E Africa the ITC would be well
north of where it is today. But mountains may have
interfered in Venezuela (the Andean uplift had begun
at the N end) and perhaps in Ethiopia too.

As much as 10 years ago it was possible to predict a
climate technically suitable for rainforest in these
areas: http://www.whoi.edu/ccod/aoc_fig9_upchurch.jpg

Presumably it has been updated since, but the
underlying climatology is not that difficult. You need
to know where the general conditions were on the
greenhouse-icehouse spectrum, and the location of
major mountian ranges, and with that a crude climate
estimate is already possible.

-----

Two issues: While the existence of an equatorial
rainbelt is as good as given, it is less clear whether
there was a vegetation that would be considered
"rainforest" - indeed there is no major gymnosperm
component in present-day equatorial rainforest. But I
don't know whether it was displaced or whether
gymnosperms have always been outcompeted by something
else under such conditions (E.g. _Metasequoia_ seems
to have been prominent in *temperate* rainforest)*.
Also, preservational conditions may have prevented
sufficient fossils to form, so as opposed to places
like Liaoning or Messel absence of evidence is really
nothing but straightforward absence of evidence here,
for the time being.

-----

Today the Fabaceae (and to a somewhat lesser extent
Fabales in general) are very significant in tropical
rainforests. They are a lineage of eurosid clade I,
which also contains the Fagales for which there is
apparently considerable indication of a Late
Cretaceous pollen record. Consequently, some early
Fabales seem to have been around by the Maastrichtian
also.

OTOH, the eurosids I are a mixed bunch with no clear
pattern of growth form, and the Fabales themselves are
too. So it is not clear whether the basal Fabales were
herbaceous plants or trees; it is fairly certain that
the tree habitus evolved at least very early after the
divergence of the Fabaceae lineage, but I am very much
in doubt that this was in the Mesozoic already.

A similar reasoning might be possible for other
lineages common in tropical rainforests today (I just
happen to know it for the Fabaceae because I checked
out some eco studies on them lately, and these days
such information can be examined for phylogenetic and
biogeographic signal). Of course this is no substitute
for a fossil record, or even just something to rest a
hypothesis on, but it allows one to line up
*candidate* plants with a higher-than-average
probability of occurring in any Maastrichtian tropical
rainforests as may have been around.

But I would suppose that the "seed fern" grade and not
the very limited angiosperm diversity was actually the
closest ecological equivalent of today's angiosperms
in their role as dominant floral components in the
inner Tropics. A "seedfern rainshrub" is by no means
the same as a rainforest.

-----

Plus, the megaherbivores then were certainly much more
"mega" than in post-Mesozoic times, and megaherbivores
are limiting factor for forest in general. Rather less
due to grazing than due to trampling perhaps - time to
sexual maturity is a crucial factor. A rainforest
canopy tree would have a generation time of 10 years
or more, and half the time at least it would still be
vulnerable to being kicked over by a sauropod. Most
herbaceous or shrubby plants would reach sexual
maturity in 2-3 years at most, all the while being
able to achieve higher population density.

-----

So while the climate of the Maastrichtian would
certainly have allowed for tropical rainforest - even
more so than today, in all probability, due to a wider
ITC (in the wide sense, i.e. the actual convergence
line as well as the outer monsoon belt) - I'd rather
doubt that the floral composition would have allowed
for much that even remotely resembled tropical
rainforest as we know it.

A kind of subtropical montane rainforest, with
gymnosperms, treeferns and Fagales, would have been
possible in the northern Andes for example. But such
forest would probably be limited in distribution, both
locally and on a global scale.


Regards,

Eike


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