Re: transitional fossils

From: Dr. David Campbell <>
Date: Tue Dec 06 2005 - 14:06:37 EST

> Thanks. It would be nice, however, to know more about what you mean
> by "clear trends." Do you mean things like ring species or changes
in foraminifera, radiolarians and diatoms? The diatom history clearly
> shows morophological change, but then again, no one would say they
are no long diatoms. I'm looking for something a bit more substantial.

There are some well-documented examples of evolutionary transitions in
foraminifera, e.g, in the post-K/T diversification (Miller had a paper
on this some time ago). My own work has been on mollusks.
Macrocallista albaria in the Pliocene of southeastern North America is
a relatively short and tall bivalve. The modern descendant,
Macrocallista nimbosa, the sunray venus, is much more elongate.
Moderately elongate forms appear rarely in the mid-Pliocene of Florida
but don't get to Virginia until after the largest mid-Pliocene
extinction. The modern ones are even more elongate than that form.

In pteriomorph bivalves, there is a sequence of microstructural
evolution from mother of pearl inner layers to crossed lamellar or
calcitic inner layers. Oysters and scallops independently show these
transitions. Oysters start out with something similar to pearl
oysters, and in fact the very earliest oysters turn out to still have
had mother of pearl inner layers, but calcite in the outer shell.
Since then, calcite has taken over practically the whole shell.

There's also the nice therapsid (mammal-like reptile)-mammal series,
in which the reconfiguration of the legs, differentiation of the
teeth, development of fur, and reconfiguration of the jaw and ear
bones, among other features, gradually proceed from a more reptilian
pattern to a mammalian pattern.

Crocodiles go from small, long-legged, fast moving terrestrial forms
to the long-bodied aquatic organisms of today.

Another species-level transition is found in Chesapecten scallops in
the southeastern U.S. C. jeffersonius has fewer, squarer ribs;
specimens from a rarely preserved layer have intermediate rib number
and shape, and C. madisonius has more, rounder ribs.

The original punctuated equilibrium paper has examples of transitions
in trilobites and land snails, among others.

The fossil record is full of such transitions-these are just some
random examples off the top of my head. However, much of the
historical and commercial impetus for paleontology has been
stratigraphic-recognizing which species occur in which layers. Thsi
puts emphasis on finding differences-intermediate forms are not
particularly helpful for this task. Work on microfossils still
strongly emphasizes this, though there is more of an evolutionary
focus in vertebrate paleontology. Thus, many species have not been
examined in an evolutionary context, yet we still have innumerable
examples of major change traceable in the fossil record. Genetic
evidence shows there are mechanisms for change and for resisting
change. It depends on the evolutionary pressures in a given situation
as to which is more advantageous.

Doubts about non-evolutionary models stem in part from the high
frequency of bad non-evolutionary models. This is unfair to anyone
raising legitimate issues, but is a natural consequence of
encountering vast quantities of malarkey.

Dr. David Campbell
425 Scientific Collections Building
Department of Biological Sciences
Biodiversity and Systematics
University of Alabama, Box 870345
Tuscaloosa AL 35487-0345  USA
Received on Tue Dec 6 14:08:39 2005

This archive was generated by hypermail 2.1.8 : Tue Dec 06 2005 - 14:08:39 EST