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causes of K-T ferns
David Marjanovic mentioned a fern spore spike in New Zealand, which,
together with iridium etc., is putative evidence of the global effects of a
K-T asteroid spike.
In fact, fern blooms can result from a variety of causes. They cover
areas disturbed by volcanic eruptions. Volcanism associated with the
Laramide Revolution occurred worldwide around K-T time. By burning away the
usual plant cover, lavas provide opportunities for the proliferation of
resilient ferns. Wolbach noted evidence of carbon concentration, or fire, in
the New Zealand K-T layer. Inasmuch as New Zealand is far from Chicxulub
(even if it did occur 65 Ma.) Volcanism may provide a better explanation for
local fires and subsequent fern blooms (but not dinosaur extinction,
directly, considering the benign nature of Deccan eruptions.)
Of course, volcanism occurred in America too, and may explain local
ferns, iridium, etc. Recent research, however, suggests another alternative
to an impact as a cause of fern proliferation: overgrazing.
An expanded deer population has so stressed the flora that edible, or
preferred, vegetation has been replaced by ferns over wide areas. Entire
forests have been seriously affected, not only by constant eating of
seedlings, but by the tendency of ferns to smother tree seeds. Could this
have had a parallel around K-T time?
Given the opportunity, Tyrannosaurus may have extirpated unprepared
foreign prey, but it probably could not have wiped out the highly "escalated"
defenders of America such as Triceratops. It is likely, however, that T. rex
originally unbalanced the Lancian ecosystem by extirpating weaker (and
slower) taxa e.g. lambeosaurs. Bakker characterized the uneven Lancian fauna
as moribund. The proliferation of tough chasmosaurines could have greatly
taxed their preferred vegetation. A breaking point may have been reached
around K-T time if many T. rexes did wander into other paleoenvironments,
reducing local predation pressure and causing Triceratops, etc. polulations
and food demands to increase even more. Or perhaps a wetter climate toward
the end favored re-colonization of grazed areas by ferns more than previously.
But the main point here is that an impact is not necessarily implied
by a fern spore spike. At least two alternative causes have been confirmed
by direct observation. An alternative source of iridium--volcanism--has also
been confirmed, whereas the link between such evidence and impacts is
probably mostly conjectural.