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

Dinosaur extinction



In response to Kendall's question about the comments of David Penny, this
paper claims a gradual disappearance of small to middle sized taxa--but it
refers only to "unpublished data".  I would like to know how one can make
such an imortant claim without publishing the data.  Maybe they have
published by now, I hope so.  In the meantime, if given another chance, a
good question to ask Dr. Penny, would be: "How do you know small to
mid-size dinos gradually disappeared?"  As far as the other extreme is
concerned, in NA at least, many of the species surviving to the K/T _were_
the largest known for their lineages, e.g., triceratops.

If the diversity of size range is as claimed, a very tidy hypothesis can
be made.  And here it is: mammals and/or birds outcompeted or preyed on
juvenile forms of small to mid-size dinos.  Previous to this, the "lawn
mower" ecology had prevented (particularly) small mammals from becoming
bigger.  Once the small to mid size dinos were gone, size constraints were
relaxed and mammals became bigger (there does seem to be such a relaxation
going on in NA toward the K/T since some Didelphodon and Cimolestes
species are the _largest_ known for their respective lineages).  In
extant
mammals there seems to be a threshold size above which they can cause
misery to the eggs and hatchlings of species whose adults can kill
them.  this
threshold appears to have been reached in late K/T taxa.
Birds are another story: two hypotheses vie for attention.  Birds were the
lucky survivors of an environmentqal ctastrophe; or, they outcompeted
their putative competitors, the enantiornithines.  If the latter scenario
were true, it is quite possible that improvements which led to their
dominance also impacted dino hatchlings.  Indeed, the turnover of
terrestrial taxa may have been a result the burgeoning of a predatory
guild, namely, taxa which can expoit nonconcealed nests.  In any case, the
descendants of these birds and mammals certainly pack a wallop such that
that particular strategy is spectacularly unsuccesful today.  Whether or
not this provides an inference for the K/T, I leave to the biases of each
person.  however, it would be lovely to have evidence of such behavior at
the K/T.  The gradual disappearance of small to mid-size dinos would be
circumstantial evidence worth having, for sure.

Bromham, L., M. J. Phillips, and D. Penny.  1999.  Growing up with
dinosaurs: molecular dates and the mammalian radiation.  Theoretical Review
of Evolutionary Ecology 14:113-118.