Dating Fossils for the Dummy Geology Student

We aren't quite sure how the site that calls itself HowStuffWorks.com works, but our staffers suspect that the site may have started out as an eHow-style content farm that subsequently tried to recast itself as an authority on everything (not unlike WiseGEEK.com). You can (usually) tell at which stage in the site's evolution a post was written: if there's a byline, it's meant to be authoritative; if written by a "contributor," it's from the eHow-style era. Unfortunately, some of the long and involved posts with bylines are... less than authoritative. Here's one of them, Tracy V. Wilson and her take on "How Do Scientists Determine the Age of Dinosaur Bones?"

The short answer, Tracy, is that scientists don't directly determine the age of bones or any other fossil. Instead, they infer the age of the bones from the age of the rock that contains them. A geologist would say that we cannot know the absolute age of the bones, only their relative age. That is an important process, one that seems lost on creationists, the sort of people who deny the theory of evolution (see right).

Wilson pumped out more than 800 words on the subject, including a topical topic sentence about a recent scientific discovery. Unfortunately, well over half her post is a primer on radiometric dating. For the most part, it's accurate: simplistic, but accurate. However, 660 words into the article, we "learn" that you can't date most fossils.

"So to determine the age of sedimentary rock layers, researchers first have to find neighboring layers of Earth that include igneous rock, such as volcanic ash. These layers are like bookends -- they give a beginning and an end to the period of time when the sedimentary rock formed. By using radiometric dating to determine the age of igneous brackets, researchers can accurately determine the age of the sedimentary layers between them."

Like so many science articles written by freelancers with English degrees, Tracy's post drives right up to the gate of the answer... and then doesn't go in. The remaining paragraph of Wilson's post babbles about the age of the earth and the oldest known rocks; but says jack about fossils. So here's what Tracy left out:

Fossils aren't known by their personal names but by genus and species: instead of "Sue," the fossil is a Tyrannosaurus rex. Even if there aren't ash layers in the rocks near Sue, there are in rocks near other T. rex fossils: that's how we know the range of ages of all T. rex fossils. There are also other species present in the same layer as Sue, species with slightly different age ranges. It is the overlap in known ages of all fossil species present that is used to bracket the age of the bones in question; not "neighboring layers... that include igneous rock." Wilson blew her chance to educate the public by blathering for hundreds of words about using radiometric dating to determine absolute age and then giving short shrift to determining relative ages. In other words, she blew it – and, ultimately, didn't answer the question. Here's her reward: Tracy's our Dumbass of the Day (again).

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