[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
RE: Dinosaur extinction
> From: email@example.com [mailto:firstname.lastname@example.org]On Behalf Of
> In a message dated 2/11/01 3:07:01 PM, email@example.com writes:
> << David's views would
> seem to infer that in answering questions about animal evolution
> through time the results of molecular phylogenies on extant taxa have
> primacy over geological and palaeontological data. >>
> That it does, but stratigraphy trumps everything, inclucing
> cladistics and
> molecular phylogenies.
Ummm, no. No, it doesn't.
Some of the factors that keep the stratigraphic record from being The
Literal And Absolute Truth:
1) Nondeposition: At any moment in the history of the Earth, huge tracts of
land are in nondepositional environments. These environments may be actual
erosional environments, or environments not supporting preservation of
particular hard part mineralogies (e.g., bones in rainforests), or simply
regions lacking either a sediment source or a basin. Thus, any organisms
living in these regions have zero chance of becoming fossilized and thus
leaving a fossil record.
2) Erosion: Big chunks of Earth History are forever lost to us, due to
subsequent erosion. Thus, even if there was deposition and subsequent
fossilization of various organisms that were endemic to that region, all
record of them could be destroyed for all eternity should those sedimentary
units be eroded.
3) Burial beyond current retreival: on the flip side of two, should the
entire range of a particular extinct taxon be subsequently buried but not
exposed, we would not be able to know anything about that taxon. For
example, any taxa which were endemic to the (then-emergent) Kerguelen
Plateau of the Cretaceous now exist as fossils underneath the deep sea.
Until the rise of oceanographic macrofossil surveys (calling SeaQuest DSV!),
the only fossils that can be recovered from this landmass are those that can
fit inside deep sea drilling cores. Now consider how much of the evolution
of lemurs we would know about if all Madagascar were submerged before human
4) The chancy nature of fossilization: The chance of any individual being
preserved is extremely small. A lot of the biomass is kept in circulation:
it is eaten up or decayed and becomes part of a new organisms ("The Circle
of Life..."). Only that fraction that happens to get out of circulation has
a chance of being fossilized, and that only if it is in the right
environment at the right time.
5) The chancy nature of fossil discovery: Finding fossils is a crap-shoot to
a degree. Even when you know the sediments you are working in, there are
always new things being discovered. As my favorite example, the Morrison
Formation has been worked for many thousands upon thousands of person-hours
since the 1870s, and yet good ankylosaur fossils were not recovered until
very, very recently. Prior to those discoveries, a "literal" reading of the
stratigraphic record would say that there were no ankylosaurs in North
America until the Early Cretaceous, and yet now we have two species in the
6) The differential in preservation potential for different critters: not as
much an issue with dinosaurs, which are all made of the same stuff, but
important for many broader issues. Taxa with hardparts have a vastly
greater potential for being preserved than those known only from softparts.
But for a few spectacular sites (Konservat-Lagerstatten) lots of groups
would have zero fossil record. For example, EVERY SINGLE fossil lamprey and
hagfish is from the Francis Creek Shale of northeastern Illinois. Had that
site been eroded out by a glacial advance, or had it not been exposed, a
literal reading of the stratigraphic record would say that lampreys and
hagfish both appeared simultaneously within human history. Similar records
exists for various softbodied invertebrates (ctenophores, for instance).
In sort, stratigraphic "literalists" seem to show a lack of appreciation for
the details of stratigraphy, sedimentology, taphonomy, and fossilization.
Thomas R. Holtz, Jr.
Department of Geology Director, Earth, Life & Time Program
University of Maryland College Park Scholars
College Park, MD 20742
Phone: 301-405-4084 Email: firstname.lastname@example.org
Fax (Geol): 301-314-9661 Fax (CPS-ELT): 301-405-0796