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Re: Paradoxically temporal

At 09:20 AM 7/23/98 -0400, Jeff Poling wrote:

>>We have, at present, no fossils of
>>_Eoraptor_ which are old enough to be _Plateosaurus_' ancestor.
>   That doesn't answer my question, though. *Why* do you say that the time
>of appearance in the fossil record is a disqualifier given the vagaries of
>fossilization and the fact that speciation can occur without obliterating
>the ancestral stock.

Ah.  I think I see the confusion here.  A species is, to me, a very precise
(indeed, specific) thing.  It's First Appearance Datum (FAD) and Last
Appearance Datum (LAD) give the whole of the known range of that species.
(In _Eoraptor_, as in most dinosaurs, the FAD and the LAD are the same
thing, as currently known).  _Eoraptor lunensis_ may indeed extend into the
early Late Triassic, the Middle Triassic, or earlier, but without direct
fossil evidence, I cannot demonstrate that *this species* was found then.

Having accepted descent with modification, and operating from a given
phylogenetic scheme or schemes, I can see if the *lineage* containing
_Eoraptor lunesis_ extends back to a certain point.  If you have evidence
that two taxa X and Y are sister groups, and X has an older FAD than Y, you
know (barring time warps or special creation) that the line containing the Y
is at least as old as the line containing X.

But that just concerns the lines of descent.  The diagnosible species _Y
species_ could be a long lived form, which first appeared immediately after
diverging from X.  Or, it could have evolved its own specific traits only
shortly before the oldest known specimen of it.  Without other fossils
within the Y lineage, we can't tell.  As in most cases, it is better to err
on the side of caution, and not assume that specimens diagnosible as _Y
species_ are any older than the currently known FAD until such time as they
are found.

To give an example, if I accept a scheme where the mid-K _Archaeoceratops_
and the latest K _Leptoceratops_ are sister taxa (rather than
ancestor-descendants), then I must accept that the lineage containing
_Leptoceratops_ extends back in time to the mid-Cretaceous.  However,
without sampling fossils of that lineage from earlier intervals, I have no
evidence that the species _Leptoceratops gracilis_ is any older than the
oldest known specimen.

So, to summarize (and the following ignores features on _Eoraptor_ which
indicate that it is too specialized to be a direct prosauropod ancestor):
The fact that all known specimens of _Eoraptor lunensis_ are found at the
same time as prosauropods show that, as we currently know it, _Eoraptor
lunensis_ cannot be the direct ancestor of _Plateosaurus_.  If, however, we
found older specimens of _Eoraptor lunensis_, these COULD be ancestral to
prosauropods (and thus, _Eoraptor lunensis_ would be a prosauropod
ancestor).  Nevertheless, those parts of the population of _Eoraptor_ from
the Ischigualasto Fm. (i.e., the type specimen and all others currently
known) would still be too late to be direct ancestors of prosauropods.

Under the Phylogenetic Species Concept, these earlier specimens would have
to be considered different species than the type, even if they were
morphologically identical.  I, however, don't use the PSC as such.

>Is it for the same reason put forth by Chris Brochu,
>that you also use the phylogenetic species concept

Actually, I don't.  The species concept I use is something more like the
traditional "Morphological" Species Concept (although I'd prefer to use the
"Specific Mate Recognition System" Species Concept, but that really isn't
testable with dead things...).  Actually, Horner's got some interesting
things to say about species and species concepts in "Dinosaur Lives" and

>(which I presume, BTW, is something from phylogenetic taxonomy?)?

(Actually, no: the phylogenetic species concept dates back to Hennig in the
1960s; Phylogenetic taxonomy was suggested in the 1980s, but presented as a
system of principles only in the 1990s).

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661