Can we ever recover ancestors? (Was: Fwd: Are dinosaurs really reptiles? (2))

[Mike Taylor <mike@indexdata.com>]

David Marjanovic writes:
 > You know how matter consists almost entirely, _way_ over 99 %, of
 > vacuum, with an atomic nucleus here and there, and with a few
 > electrons thinly smeared all over the place? That's a good metaphor
 > for the fossil record.  The fossil record doesn't have a terribly
 > strong systematic bias (within terrestrial tetrapods, that is!!!)
 > -- I've published on this --, but it still consists mostly of
 > absence. _Fossils are extremely rare._ Or rather, fossiliferous
 > sites are extremely rare and much, much poorer than you seem to
 > imagine.
 > 
 > It follows logically that we will more or less _never_ find a
 > direct ancestor of any known species of terrestrial tetrapod older
 > than Miocene perhaps. (To really find ancestors, you have to look
 > at Pliocene _diatoms_ and the like, in other words, at rocks that
 > consist entirely of fossils.)

Actually, I'm not sure that's true.  We do have some strata that are
unusually well sampled, and where we can be fairly confident that we
have found most of the large-bodied taxa to be found (because we keep
finding more specimens of the same taxa).  In those cases, when we
pretty much know what taxa have existed, we can be much more confident
about guessing ancestor-descendant relationships.

The example that springs to mind is the stratigraphic sequence of
centrosaurines in Horner's book _Dinosaur Lives_.  I'm not saying that
it's a slam-dunk, but I would at least be slow to dismiss the
possibility that they really do constitute an ancestor-descendent
chain.

The problem is not so much that we can never find an ancestor; it's
that we can never know when we _have_ found it.

 _/|_    ___________________________________________________________________
/o ) \/  Mike Taylor    <mike@indexdata.com>    http://www.miketaylor.org.uk
)_v__/\  "(12+144+20 + 3*(4^(1/2))) / 7 + 5*11 == 9^2+0" -- unattributed
         arithmetic limerick, from /usr/games/fortune.