evolution in the fossil record: migration, stratigraphic sequence, etc.

Getting back to some older messages-I've been busy with wrapping up the semester and tending a new baby.

Although seasonal migrations are rather difficult to identify in the fossil record (there are some examples of what seems to be a herd/flock/etc. killed by a catastrophe during some sort of migration; perhaps more definitive are regular variations in stable isotope ratios within the skeleton of an individual that reflect periodic movement from one region to another), I think what the original posting had in mind was the case of an organism that evolves in one place and then spreads to another, perhaps dying out in the first area only to recolonize it later. Thus, presence of putative ancestor and descendant in two layers does not necessarily mean that the intermediate would be in the intervening layers in the same geographical region. Failure to recognize this underlies one antievolutionary attack on the horse evolutionary series.

I have not specifically gone out looking for an intermediate form. However, I do know of plenty of examples in things with good fossil records, such as shallow marine mollusks or microfossils. There are also many good transitions known among vertebrates; the greater patchiness of terrestrial fossils and the extensive post-mortem falling apart are compensated for by the greater study they have received.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL 35487-0345 USA
    bivalve@mail.davidson.alumlink.com

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa
Received on Tue Dec 21 16:02:30 2004