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RE: transitional fossils



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Buckaroobwana@aol.com
>
> Greetings,
> Can anyone out there either give me a list or tell me where a list can be
> found of transitional animals in the fossil record. I know there
> are supposed
> to be others besides Archeopteryx, and maybe Sinosauropteryx but
> I'm curious
> about the amphibian-reptile transition and the ape-human
> transtion. How many
> species are there that are defintively transitional? I am aware
> that this is
> somewhat off topic, but this is the only place I know that I can get this
> information. Thanks to whoever responds.

Well, part of the problem here is "definitively transitional."  If by this
you mean "a species that possess a subset of features otherwise known only
Group A, but retains other primitive states that are transformed to the
advanced state in Group B", then yes: the fossil record is chock full of
them.  For a dinosaurian example:

_Pisanosaurus_ is derived relative to non-ornithischians by the presence of
the predentary, among other features, but lacks (probably) the backwards
pointing pubis, shelf off of the back of the skull, rostral bone, frill,
horns, and double rooted teeth;
_Lesothosaurus_ is derived relative to non-ornithischians by the presence of
the predentary and the backwards pointing pubis, but lacks a shelf off of
the back of the skull, rostral bone, frill, horns, and double rooted teeth;
_Psittacosaurus_ is derived relative to non-ornithischians by the presence
of the predentary and the backwards pointing pubis, a shelf off of the back
of the skull, and a rostral bone, but lacks a frill, horns, and double
rooted teeth;
_Archaeoceratops_ is derived derived relative to non-ornithischians by the
presence of the predentary, the backwards pointing pubis, a shelf off of the
back of the skull, a rostral bone, and a frill, but lacks horns and double
rooted teeth;
_Zuniceratops_ is derived relative to non-ornithischians by the presence of
the predentary, the backwards pointing pubis, a shelf off of the back of the
skull,a rostral bone, a frill, and horns, but lacks double rooted teeth;
and _Avaceratops_ is derived relative to non-ornithischians by the presence
of the predentary, the backwards pointing pubis, a shelf off of the back of
the skull, a rostral bone, a frill, horns, and double rooted teeth.

(Note that the above are also in stratigraphic order!)

In this sense, each of the ornithischians above is "transitional" between
the more primitive form and the more derived clade (Genasauria or Ceratopsia
or Neoceratopsia or Ceratopsomorpha or "Ceratopsidae sensu Penkalski and
Dodson") below.  I can think of plenty of others just within Dinosauria.

However, if by "definitively transitional" you mean "we can demonstrate with
absolute certainty an direct ancestor-descendant relationship between fossil
A-fossil B-fossil C- etc.", then no.  Direct ancestor-descendant
relationships are well nigh impossible to demonstrate: there is always the
possibility, for example, that fossil A was the sister species (or even the
sibling species: identical in morphology but different by some
non-morphological cue) of true ancestral fossil Q.  We can propose that
fossil A is the ancestor, but we cannot demonstrate.  It is a possibility
which is certainly falsifiable (for example, if all specimens of fossil A
occur stratigraphically after all specimens of fossil B, then the specimens
represented by fossil A cannot be the population which gave rise to fossil
B).  In some cases (taxa confined to a particular geographic region, where
the stratigraphic and fossil sampling is exceedingly good) you might have a
very good potential set of ancestors and descendants; for the most part,
though, these are scattered much farther apart in time, space, and
phylogeny.

Furthermore, if you mean "transitional between two *major groups of
organisms*": well, then you have fallen into one of the traps of Linnean
taxonomy.  Now I know that some people might say that my example wasn't a
"good" example of transitions: "That's just one type of dinosaur changing
into another type of dinosaur.  I want a REAL transition, between different
*kinds* of things."  By saying "Classes are more significant than Orders" or
"Orders are more significant than Families" or so on, you are suggesting
that these ranks have some metaphysical reality outside of human discourse.
The type of changes from taxon to taxon are the same (natural selection
acting upon populations, pleitropy, etc.), regardless of the taxonomic rank
or the taxon name we give them.

Okay, so this isn't exactly what you wanted.  However, I see by now that
other people have posted the URL for the talk.origins transitional taxon
page.

Hope this helps.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-314-7843