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Bits of DNA versus whole skeletons
By questioning whether there are 100 characters showing an
ancestor-descendent relationaship among theropods and birds, Pretzman exposes
what appears to be a distressing lack of knowledge about the skeletal
morphology of the archosaurs he is studying. Bone by bone, in every region of
the skeleton, the two groups share an intimate derived morphology not
observed in any other tetrapod group. If someone sat down and wrote them all
out the number of shared characters would easily be in the hundreds. This is
a basic problem with using skeletal characteristics to study phylogentics.
There are so many potential characters, and so many taxa to assess, that one
soon gets into a serious data gathering problem.
If birds and theropods are not closely related, then it is the most
extraordinary example of convergence ever, in which virtually every bone
somehow became almost identical in the two groups. The firm assertion of the
relationship is not, as Pretzman argues, an example of arbitrary prejudgment,
but an inescapable conclusion based on overwhelming data.
Abundant data based on hundreds of skeletal characters from hundreds of
skeletons shows that the divergence between synapsids (pelycosaurs,
therapsids, mammals) and diapsids (lizards, crocs, pterosaurs, dinos, birds,
etc) occurred in the late Paleozoic, and that theropods and birds split in
the mid-Mesozoic. The few characters cited by Pretzman are of the gross,
superficial sort last cited in serious circles (and Gardiner is not to be
taken seriously) in the 1800s.
Those attempting to use DNA as a magic way of determining phylogeny are in
danger of failing to see the forest for the trees. DNA is a simple
quaddigital computer subject to convergence, reversals, etc. like anything
else. One also suspects that Pretzman is picking up DNA contaminents of a
mammalian sort in his dinosaur bones. That orginal, unaltered dinosaur DNA is
common in fossils has yet to be fully established to the best of my
knowledge. Those who wish to discern the relationships of amniotes would do
well to educate themselves about the well preserved characters they so airily