From the above-noted url link in the first post of this thread:
A Bit Of Historical Background
When The Origin Of Species was first published, the fossil record was poorly known. At that time, the complaint about the lack of transitional fossils bridging the major vertebrate taxa was perfectly reasonable. Opponents of Darwin's theory of common descent (the theory that evolution has occurred; not to be confused with the separate theory that evolution occurs specifically by natural selection) were justifiably skeptical of such ideas as birds being related to reptiles. The discovery of Archeopteryx only two years after the publication of The Origin of Species was seen a stunning triumph for Darwin's theory of common descent. Archeopteryx has been called the single most important natural history specimen ever found, "comparable to the Rosetta Stone" (Alan Feduccia, in "The Age Of Birds"). O.C. Marsh's groundbreaking study of the evolution of horses was another dramatic example of transitional fossils, this time demonstrating a whole sequence of transitions within a single family. Within a few decades after the Origin, these and other fossils, along with many other sources of evidence (such as developmental biology and biogeography) had convinced the majority of educated people that evolution had occurred, and that organisms are related to each other by common descent.
Since then, many more transitional fossils have been found, as sketched out in this FAQ. Typically, the only people who still demand to see transitional fossils are either unaware of the currently known fossil record (often due to the shoddy and very dated arguments presented in current creationist articles) or are unwilling to believe it for some reason.
What Does The Fossil Record Show Us Now?
I think the most noticeable aspects of the vertebrate fossil record, those which must be explained by any good model of the development of life on earth, are:
- A remarkable temporal pattern of fossil morphology, with "an obvious tendency for successively higher and more recent fossil assemblages to resemble modern floras and faunas ever more closely" (Gingerich, 1985) and with animal groups appearing in a certain unmistakable order. For example, primitive fish appear first, amphibians later, then reptiles, then primitive mammals, then (for example) legged whales, then legless whales. This temporal- morphological correlation is very striking, and appears to point overwhelmingly toward an origin of all vertebrates from a common ancestor.
- Numerous "chains of genera" that appear to link early, primitive genera with much more recent, radically different genera (e.g. reptile- mammal transition, hyenids, horses, elephants), and through which major morphological changes can be traced. Even for the spottiest gaps, there are a few isolated intermediates that show how two apparently very different groups could, in fact, be related to each other (ex. Archeopteryx, linking reptiles to birds).
- Many known species-to-species transitions (primarily known for the relatively recent Cenozoic mammals), often crossing genus lines and occasionally family lines, and often resulting in substantial adaptive changes.
- A large number of gaps. This is perhaps the aspect that is easiest to explain, since for stratigraphic reasons alone there must always be gaps. In fact, no current evolutionary model predicts or requires a complete fossil record, and no one expects that the fossil record will ever be even close to complete. As a rule of thumb, however, creationists think the gaps show fundamental biological discontinuities, while evolutionary biologists think they are the inevitable result of chance fossilizations, chance discoveries, and immigration events.