Hallo Randman,
When I understand you correctly you are asking, why we don’t see transitionals on the species level. To describe a transition between species we would need to find fossils every 20.000 to 80.000 years, the documentation of evolutionary bursts would require even shorter intervals. This is only possible in environments with a constant and undisturbed sedimentation for millions of years, something we generally find only at the bottom of the deep sea. Consequently nearly all examples of species level transitionals are found by deep sea drilling projects (compare [1]).
So what does this mean for whale transitionals? Relevant is not the sheer number of fossils found, but the number of independent records (see [2] )
The number N of independent records can be estimated by counting the
number of records coming from different geological formations,
representing different temporal epochs in various
geographic states or countries of the world.
Gingrich determined by a survey of the Georef online database this number for archaeocetes (23), mysticeti (34) and for odontoceti (46). Considering that the known temporal range of archaeocetes is about 19.8 million years, it should be obvious that we can’t expect the documentation of transitions on the species level, instead what we would long for - and that’s what we in fact observe - is the documentation of a general lineage, defined in [3] as:
This is a sequence of similar genera or families, linking an older group to a very different younger group. Each step in the sequence consists of some fossils that represent a certain genus or family, and the whole sequence often covers a span of tens of millions of years. A lineage like this shows obvious morphological intermediates for every major structural change, and the fossils occur roughly (but often not exactly) in the expected order. Usually there are still gaps between each of the groups -- few or none of the speciation events are preserved. Sometimes the individual specimens are not thought to be directly ancestral to the next-youngest fossils (i.e., they may be "cousins" or "uncles" rather than "parents"). However, they are assumed to be closely related to the actual ancestor, since they have intermediate morphology compared to the next-oldest and next-youngest "links". The major point of these general lineages is that animals with intermediate morphology existed at the appropriate times, and therefore the transitions from the proposed ancestors are fully plausible.
-Bernd
P.S.
Please note that you’ll have to wait for further posts until Sunday. Thanks for you understanding.
References
[1]
http://www.geocities.com/earthhistory/sle.htm[2]
University of Michigan: File Not Found ( 404 )[3]
Indiana University Bloomington
™ Version 4.2