> Once one gets to multicellular organisms there is
> very little "upward dynamic. In precisely what way is an amphibian higher
> than a fish or a mammal more complex or higher than a reptile. What is the
> criteria by which we place birds in this, ahem, pecking order? All have
> stomachs, eyes, skin (of some sort) a heart, liver, an oxygen obtaining organ,
> ability to locomote in their respective habitats. This business of an "upward
> dynamic is a holdover from 19th century progressive philosophical systems like
> Hegel, Ficte, Marx etc. From a scientific and information theory point of
> view, I see little reason to say that one organism is higher than another.
Thanks Glenn for this. I've been thinking about this theme recently,
as a correspondent presented me with various views about increasing
complexity in the fossil record. It seems to me that this subject
can be approached at different levels.
One level is to consider the emergence of basic body plans - and to
ask, does the fossil record show any trends here. Stephen Jay
Gould's "Wonderful Life" has much to say on this: the indication is
that the fossil record shows an explosion of basic body plans in the
Cambrian, with extinction being the dominant pattern thereafter
(subsequent new body plans being rather scarce!)
Another level is to consider the different groupings of vertebrates -
as Glenn has above. What is the basis for saying there is
"increasing complexity"?
A third level is to take a group of animals (or plants) and look at
trends in the record. This is what I have attempted to do with the
trilobites - see below.
I've drawn the following mainly from: Babin, C. 1980, "Elements
of Palaeontology", John Wiley & Sons Ltd, Chichester.
Numerous species of trilobites are found in Lower Cambrian rocks.
They are well differentiated from other arthropods, resulting in
problems for those who wish to trace an evolutionary ancestry.
Babin says their ancestry is "obscure" (p.255).
There are about 1500 genera of trilobites known, with a more or
less consistent decline in number of genera from the Upper
Cambrian to the Permian. From the Upper Ordovician, the number
of genera was always less than that in the Lower Cambrian. Thus,
there is no trend of increasing numbers of genera in the fossil
record.
Evolutionary trends have been analysesd as follows:
(a) Paedomorphosis (where juvenile features become dominant in
the ontogenesis of descendants).
(b) Gerontomorphosis (where adult features become dominant in the
ontogenesis of descendants).
Sometimes, these two trends coexist: for example, the
paedomorphic tendency towards a reduction in the number of
thoracic segments has been observed to coexist with the
gerontomorphic tendency towards the enlargement of the rachis.
Overall, there are no dominant trends and Babin writes that the
data "still tends to be difficult to interpret" (p.262).
Whatever these trends are, can they be interpreted in any way as
an increase in complexity? Neither of the reported trends
involves novel structures - only modifications of existing
structures.
Trilobite morphologies are, of course, very diverse. Cambrian
faunas are distinctive, although some of these forms are also
found in Ordovician strata. A number of variants are found in
the base of the Ordovician, but these are localised
stratigraphically. The dominant Ordovician groups have a much
broader range. Above the Ordovician, according to Nield and
Tucker (1985), "there seems to have been little further
development in the overall design of trilobites, with only minor
variations on the basic themes stated in the early Ordovician"
(p.26).
The conclusion: in the context of trilobites, claims of increasing
complexity in the fossil record appear to lack a solid foundation.
Feedback welcome.
Best wishes,
*** From David J. Tyler, CDT Department, Hollings Faculty,
Manchester Metropolitan University, UK.
Telephone: 0161-247-2636 ***