Most of the readers of this blog are intelligent, interested, scientifically literate individuals, but I'm guessing that at least a few of you aren't familiar with one of the nouns in the title. Those of you who do know what a conodont is are probably wondering what it has to do with the others. If you bear with me for a little bit, the connection will be clear shortly. It has to do with fossils, fossilization, and the latest spectacular misunderstanding of those two things at Uncommon Descent.
Conodonts are (or, rather, were) an interesting group of animals. They were around from late in the Cambrian period until the end of the Triassic, and were quite common during most of the period. They're not well known to most people outside of geology because the vast bulk of the evidence we have for them consists of very tiny tooth-like fossils. Most are only a millimeter or two in size, and are very hard to see without a microscope. They've received a lot of attention from paleontologists over the years because they're very useful little critters, particularly for geologists who work in the oil and gas industry. The thing is, for a long time nobody knew just what sort of critters they actually were.
The Russian paleontologist who first described conodonts, C. H. Pander, believed that they were the teeth that were left behind by some sort of primitive fish. That hypothesis was soon joined by many others. In 1988, Sweet noted that, "about the only major invertebrate phyla that have not been suggested as the parental group for the conodonts are the Archaeocyathida ... the Porifera, Bryozoa, Echinodermata, and Hemichordata." Scientists weren't sure what conodonts were because all that they had to work with were the little teeth-like microfossils. That was the only feature that the animals had that fossilized readily, and from 1856 until 1983 the phosphatic elements were all that scientists had to go by.
The first fossil of a whole conodont animal was found in 1983. By 1988, when Sweet wrote his monograph, more than five thousand species of conodont had been described, and five individual conodonts had been found with reasonable soft-body preservation. It wasn't until 2000 - nearly 200 years after Pander first described conodonts - that enough data had been gathered for scientists to do a formal analysis of conodonts and conclude that Pander's initial classification was probably close to correct. The conodont elements aren't actually teeth in the strict sense, but cladistic analysis suggests that the conodonts were, "the most plesiomorphic member of the total group Gnathostomata" (Donoghue et al., 2000). In English, that means they were, in fact, primitive fish.
In many ways, the question of what conodonts were is similar to questions about whether dinosaurs had feathers. In both cases, there was a long period of time when there was a limited amount of circumstantial evidence and a limitless amount of speculation based on the evidence that was available. Feathers, like other soft materials, don't preserve very well. There's been speculation almost since Jurassic Park hit theaters about whether the movie's depiction of Velociraptor as an animal with scales was correct, or if the animal should have been shown with feathers instead. It wasn't until last week that anyone was able to present physical evidence for feathers in that species.
When it comes to things like muscle, internal organs, and soft body coverings, the fossil record is very, very limited. We have few fossils that preserve any of those things not because they didn't exist, but simply because they don't preserve well. Skin and muscle decay quickly, while bone tends to hang around for a lot longer. This means that we need to be very careful about how we interpret fossil evidence for the things that preserve poorly - particularly when it comes to timelines.
That's something that they don't seem to have figured out at Uncommon Descent. In a post there today, the author (idnet.com.au) examines the fossil record for feathers, as reported in a seven-year-old Current Opinions in Genetics and Development paper. The UD author, after examining this record, concludes:
This is a clear example where the predicted fossil evidence is actually reversed. Protarchaeopteryx arose 9my after Archeopteryx. Protofeathers arose 25my after flight feathers. This is like finding a fossil rabbit in pre cambrian strata. The conclusion is of course, Darwinian evolution is obviously true.
(all emphasis in original)
The UD author clearly didn't grasp (or read) what the authors of the paper had to say about the timeline, which is actually funny, because he quoted part of it in the UD post:
One may wonder why the more primitive feathers seem to appear later than complex ones in the fossil record. Well preserved fossils, particularly those of the integument, are very rare and the absence of such examples does not mean that they did not exist. Furthermore, different levels of integument complexity probably co-existed, reflecting inhabitance of different niches. Such diversity still exists today.
(Italicized material was not quoted at UD, for some strange reason.)
It's possible that the UD author didn't grasp this because the language of the scientific paper was too obtuse, so let me try to explain this in simpler terms.
It's not safe to assume that the oldest fossil protofeathers we know of are the same age as the first protofeathers to appear on an animal. We don't have fossil evidence that they showed up earlier than that, but given how rare it is for feathers or skin to be preserved, that's not exactly a surprise. The oldest preserved conodont "teeth" come from the Cambrian, but the oldest preserved soft-body conodont fossil that I know of comes from the Ordovician (Gabbott et al., 1995). Dinosaurs first appear in the fossil record about 230 million years ago, but the only preserved dinosaur heart that I know about is only 66 million years old. It's reasonably safe to assume that dinosaurs had hearts during the Jurassic, and that conodonts had bodies during the Cambrian. It's not safe to assume that protofeathers weren't around before the first fossilized protofeather showed up.
Donoghue, P. C. J., Forey, P. L., & Aldridge, R. J. 2000. Conodont Affinity and Chordate Phylogeny. Biological Reviews. Vol 75. Pp 191-251.
Gabbott, S. E., Aldridge, R. J., & Theron, J. N. 1995. A Giant Conodont With Preserved Muscle Tissue From the Upper Ordovician of South Africa. Nature. Vol 374. Pp 800-803
Sweet, W. C. 1988. The Conodonta: Morphology, Taxonomy, Paleoecology, and Evolutionary History of a Long-Extince Animal Phylum. Oxford. Clarendon Press. 212 p.