Re: the understanding of earthworms and molluscus?

Dear friend,

Interesting question, or rather two questions. The first answer depends on whom you ask. If you ask a geologist, the answer will be quite simple. Worms have soft bodies. These bodies are not as well preserved as fossils. This would mean that we have fewer samples of the ancient history or worms. Some fossilized worm burrows have been found.

If you ask a biologist, they may tell you that there is sufficient fossil evidence to show that the pieces are all in place. A specialist may go into great detail about the earthworm’s origins. Actually, quite a lot is known.

Worms have been around since the Pre-Cambrian Era, or about six hundred million years, give or take. Worms are so simple, and nature has used this shape early on. Worm-shaped (vermiform) creatures have been found in the fossil record during the Cambrian, but evidence exists they were around earlier.

Once worm-shaped creatures were in existence, the next big advancement was the development of a digestive tube, also called a coelom (see-lum). Once vermiform coelomates (worm-shaped critters with a mouth and hind-end) were on the scene, we have achieved worm-ness. From there it’s only a short jump to the segmented worms, also known as annelids. The night crawler is one of these.

But that wasn’t your final question. Your question was regarding how the lack of information influences the study of evolutionary biology. Well, as with all branches of science, the more data we have, the more accurate our understanding. As we have had an incomplete record, we filled in the gaps with logical hypotheses. As more information is discovered, either it supports our original idea, or the idea gets changed to incorporate the new evidence.

Science has a working idea of how animal life evolved. There seems to be a logical and natural progression that accounts for all of the animals, including earthworms. At the moment, the perceived lack of evidence in the fossil record is not troubling at all, and the evidence we do have fits into the overall evolutionary model. As the palentology department at UC Berkeley puts it,

The fact that some transitional fossils are not preserved does not disprove evolution. Evolutionary biologists do not expect that all transitional forms will be found and realize that many species leave no fossils at all. Lots of organisms don’t fossilize well and the environmental conditions for forming good fossils are not that common. So, science actually predicts that for many evolutionary changes there will be gaps in the record(Misconceptions in Evolution Website at The University of California Museum of Paleontology, Berkeley).

There is always the possibility that we could find new and unexpected evidence though. If this happens, our current ideas get modified to fit the new evidence. That’s the beauty of science; a good theory can shift to become a better one.

References:
UC Berkeley Museum of Paleontology
Tim Pipes, CSU Sacramento Biogeography paper
Science Oct 2 1998: 80-83.
Yale University Invertebrate paleontology gallery
And everything else used as a link in this answer.