Bill Payne raised two questions
>One of the questions I had while reading the book was a statement by Jonathan Wells (pp 137-138): "First, Darwin maintained that major differences evolve from minor ones. Yet the fossil record shows that all of the major animal groups appeared at approximately the same time, without any fossil evidence that they diverged from a common ancestor. These original groups have since diversified into many subgroups, so the major differences among animals appeared before the minor ones....
>It doesn't seem to me that this paradox can be completely resolved by diffusing the "Cambrian Explosion" back into the pre-Cambrian. Valentine and Erwin made their statement in 1987. Is Wells using outdated information, or does he see a pattern which is not generally acknowledged?<<
Statements quite similar to Well's appear in many standard and realtively current biology textbooks. However, there are a couple of problems with the basic claim. As a whole, it seems as though research on this is progessing at just about the right pace for new discoveries and explanations to appear in the scientific literature about when the old versions become popular in textbooks, the media, etc.
There are two fundamental difficulties with the claim that "all the major groups appeared suddenly, and major differences appeared before minor ones". First is the claim that all the major groups appeared suddenly. What is a major group? In this context, probably a phylum or class, but leaving it undefined makes it a bit confusing. Among phyla, a handful are known from generally accepted Precambrian fossils, including very primitive groups like sponges and cnidarians as well as mollusks. There are also plenty of Precambrian fossils that look like more advanced groups, or traces that suggest their activity, but their taxonomic assignment remains controversial. All of the animal phyla with good fossil records except Endoprocta (aka Bryozoa) are known from the Cambrian, as are an assortment of soft-bodied fossils of other phyla with generally lousy records. However, other soft-bodied phyla are first known from various later points, including several only known in the Rec!
ent. A similar pattern applies to classes, with many of those with hard shells first known from the Cambrian but others appearing at various later dates.
There also is some debate as to what is a phylum. Two aspects of this affect the present question. One is the possible assignment of various fossils to extinct phyla, thus affecting the apparent diversity at different times in the past. Another is disagreement on the proper limits to taxonomic groups, which vary as to what proportion of the extinct forms are considered to belong to any particular phylum, class, etc.
Secondly, the claim that major differences appeared before minor ones is based on circular reasoning, as well as not strictly accurate. Minor differences have been present all along, so that many Cambrian genera have several species, etc. We define major differences based on the fact that the organisms with these differences have been separate a long time. Thus, the early appearance of such differences is no surprise. For example, the debate over the range of disparity in Cambrian organisms reflects this problem. Modern arthropods fall into a few groups with highly consistent numbers and types of head appendages (e.g., insects have one pair of antennae, crustaceans have two, spiders have none). The Cambrian arthropods include many that do not exactly fit any of the modenr basic patterns. However, all are unquestionably arthropods and more or less shrimplike. Today, barnacle larvae fit into a standard crustacean pattern, but some adult barnacles consist of a saclike r!
eproductive structure attached to a parasitic network of tissue inside a crab. This is wildly unlike any self-respecting arthropod, yet within the same subclass as rather ordinary looking crustaceans based on the major features of larval form.
Nevertheless, the Cambrian was at the very least a time of exceptional diversification in taxa with hard skeletons. Several natural explanations exist for this, of varying merit.
>A second question along this same line is whether forms which appear to be transitional are truly so. The book _Of Pandas and People_ shows silhouettes of three skulls: a Tasmanian wolf, a North American wolf, and a dog. The cranial-cavity size increases as you go from the Tasmanian wolf to the dog, suggesting a transitional relationship. However, we know the Tasmanian wolf was a marsupial, while the other two are placental mammals. Convergent evolution can produce forms which look transitional but which we know are not, based upon soft-part anatomy, which of course is rarely fossilized. Is this factor just generally ignored by those inferring evolutionary relationships?<<
If the silhouettes are very accurate, you might notice a distinct angle at the lower hind corner of the lower jaw. This is a distinctive marsupial feature. Many other skeletal features readily distinguish marsupials from placental mammals; in fact, this is what established Cuvier's abilities in comparative osteology: he obtained a fossil skeleton, recognized it as a marsupial, and proved it by removing part of the skeleton to reveal the marsupial bones (which support the pouch). The biogeography would also probably produce some suspicion as well.
Establishing a form as a transitional fossil requires careful examination of the known record of the relevant taxa. It is easiest in groups with particularly good fossil record. For example, in planktonic foraminifera, the record is so good that it may sometimes be more of a problem to decide where one kind stops and another begins than it is to find transitions.
Dr. David Campbell
Saint Mary's College of Maryland
18952 E. Fisher Road
St. Mary's City, MD 20686-3001 USA
firstname.lastname@example.org, 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'. And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!
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