I happened to catch a great tweet by Michael Shermer. Apparently there was some back and forth about some fossils with Carl Zimmer. Anyway, Don Prothero commented about it and this is a great quote for a variety of reasons.
— Michael Shermer (@michaelshermer) January 5, 2016
Not the least of which is that it deftly refutes the creationist claims that using index fossils to date rock layers is circular reasoning. Their logic (which is wrong) goes like this: Fossils are x millions of years old because they are in this rock layer. That rock layer is x millions of years old because of the fossils in it.
As Prothero explains so clearly, that is not how index fossils work. In fact, he specifically states that if you use age to define the fossils, you can’t use them for dating the rock. He even mentions that is circular reasoning.
This is just the latest in another long line creationist misrepresentation.
Here’s how index fossils work.
First, you need a type of fossil that is common, widely distributed, and easily identified at the species level. There are several groups that meet these criteria; ammonites, trilobites, corals, for example.
Next, you need a location where the fossils are between two layers that can be dated. For example, one might find a layer of ocean limestone between two layers of igneous rock (best) or even have a layer of igneous rock in the middle (as might happen when an island formed.
So, we have one place where we have distinct fossils and a time period that those fossils must exist around. Then we can look in other places that don’t have that easily dated layer of igneous rock. If we find the same kind of rock with the same fossils (and we’ve established that they are good index fossils), then we know the age (within a few million years) of that layer of rock.
That’s pretty much all there is to it, at a macro level. But just being able to determine what is a good index fossil is not an easy task. As I mentioned, trilobites are excellent ones. It’s an entire class of organisms that lived for more than 270 million years. The first fossil trilobites are from the early Cambrian and the group disappears at the Permian extinction event, right before the dinosaurs. The class contains some 17,000 species. Each species has multiple unique markers that identify it and each species lived for a relatively narrow time frame.
Just doing a quick search online, I found dozens of peer-reviewed research that discuss the status of various trilobite species and their use or non-use as index fossils. Most of these include a discussion of how the age of rock was determined by radiometric dating (see a series of articles on radiometric dating here and here).