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Paradoxically temporal

At 09:59 AM 7/22/98 -0400, Thomas R. Holtz, Jr. wrote:
>C) The chances of any individual, or even individual taxon, becoming buried
>(and a potential fossil) is very rare, as indicated by field and
>experimental taphonomy.  Multiply that by that the probability that
>particual column of sediment becomes lithified, the probability that that
>section is not subsequently metamorphosed or melted, the probability that
>particular section is later exposed in the latter decades of human history,
>and the probability that someone happens by that section to see the fossil
>and recover it (and the futher probability of that someone brining the
>fossil to scientific attention).  This is the reason the fossil record is as
>spotty as it is.  We know only a tiny fraction of the life forms that
>existed in the past: Leithia, muse of paleontology, is very stingy in her
>B) As mentioned above, it is NOT the first _diagnosable_ dinosaur: it occurs
>on exactly the same horizon as _Herrerasaurus_ and _Pisanosaurus_.
>Furthermore, this horizon is not older than dinosaur bearing units in other
>parts of the world.  _Eoraptor_ may indeed be the most *primitive* dinosaur,
>but appears too late in time to be ancestral to _Plateosaurus_: the
>existence of ornithischians, sauropodomorphs, and coelophysoids at the same
>time indicates that the Dinosauria had already diverged into several
>different lines by the time _Eoraptor_ lived.  Morphologically may give us
>the best current glimpse of what the ancestral dinosaur looked like, but it
>is too late in the fossil record to BE the direct ancestor of _Plateosaurus_.

   I've long stated that because of the vagaries of fossilization, and the
fact that there's no evolutionary "requirement" that daughter species
completely displace mother species, the fact that one species appears
alongside, or later than, a purported descendant species does not
in-and-of-itself disqualify the first from being the ancestor of second.

   However, I don't think I've ever seen this conclusion debated.  Maybe
now's a good time?

   Why does the fact that _Eoraptor_ appears later or contemporaneously
with _Plateosaurus_ in-and-of-itself disqualify it from being
_Plateosaurus_'s ancestor?

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