| [Thread Prev] | [Thread Next] | [Thread Index] | [Date Prev] | [Date Next] | [Date Index] |
>Nevertheless,
>statements such as "the dinosaurs went extinct at the K-T boundary" imply
>that neither the dinosaurs nor any of their direct descendents are to be
>found in Tertiary sediments. This is simply false. The problem is of
>importance to extinction studies because one cannot talk of the
>"extinction" of higher taxonomic groups unless one has a unambiguous
>definition of such groups. While there are many ways to define groups of
>organisms, definitions that refer to common ancestrty have tradiationally
>been preferred. By this definition, the dinosaurs did not go extinct.
Yes, Norm, it's certainly a matter of definition. But you _cannot_ call on
traditional preferences in support of the method of classification you are
vouching for. Traditional preferences in biological classification were
rather to classify like with like, based on inherited likenesses, and to
keep unlikes apart. Cladism has changed all that: taxa are now commonly
defined on their inferred evolutionary origins irrespective of their
history after this origin. Whatever happens to a group after its
origination is of no concern to its classification - 'it's just an
autapomorphy'.
The result of this is statements such as:
>there is a closer
>relationship between birds and T. rex than there is between T. rex and
>Triceratops horribilis, both of which are Maastrichtian dinosaurs.
Is there? Your statement is equivalent to saying that there is a closer
relationship between me and my great-great-great-great-great grandfather
than between him and his childless twin brother. If you follow the lineages
of T. rex and Tri. horribilis back to the points where they had just split
from their last common ancestor, you will find two groups of reptiles that
are siblings, for all practical purposes indistinguishable from each other
(let's say one of them has developed an autapomorphy, just to keep the
discussion on a cladistically acceptable footing). Yet cladistic
classification would force you to conclude that one of them is closer
related to a hummingbird than to its sibling species. It would also force
you to conclude that one was an ornitischian and the other a saurischian.
Genealogies are great for a number of purposes, but when used as the
exclusive basis for classification they produce a number of artifacts that
defy some of the 'traditional' purposes of biological classification, such
as grouping like with like. 'Pseudoextinctions' may be as real as 'real
extinctions' and the dinosaurs may yet be dead.
'Chirp, I say chirp!' (Opus Penguin, alleged dinosaur, Bloom County)
Stefan Bengtson _/ _/ _/_/_/ _/ _/
Department of Palaeozoology _/_/ _/ _/ _/ _/_/ _/_/
Swedish Museum of Natural History _/ _/ _/ _/ _/ _/ _/ _/ _/
Box 50007 _/ _/ _/ _/_/_/ _/ _/ _/
S-104 05 Stockholm _/ _/_/ _/ _/ _/ _/
Sweden _/ _/ _/ _/ _/ _/
tel. +46-8 666 42 20
+46-18 54 99 06 (home)
fax +46-8 666 41 84
e-mail Stefan.Bengtson@nrm.se
Partial index: