Geology is a science of untangling mysteries written in the rocks, but dating fossilized spider footprints is a special sort of challenge. Stratigraphic principles and careful reasoning can identify these particular footprints as 190 million years old, created by a prehistoric spider out for a stroll.

Fossilized spider footprints from 190 million years ago. Image credit: National Park Service

Last weekend, I shared a mystery fossil with you, asking you what creature formed a set of footprints from 190 million years ago. In the subsequent discussion,

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how we knew how old the footprints were.

. The full story is more complicated and far cooler.

After seeing our question, the Dinosaur National Monument palaeontologist Dan Chure got in touch with the full story, explaining:

The answer to the question of the age is that to the southwest margin of the great sand sea known as the Navajo-Nugget-Aztec Sandstone [that includes the spider tracks], the sand sea approaches volcanic highlands and there are radiometric dates from sediments shed from those highlands that inter-tongue with the eolian sands and they give an isotopic date of ~190 million years.

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Translating from geology-jargon, that means that once upon a time, Dinosaur National Park was a massive extent of sand, like White Sands desert in New Mexico. This sand sea nudged right up against a volcano. As that volcano eroded, the sediments joined the sand in distinct alternating layers, encompassing the dune containing the tiny spider footprints. Those volcanic sediments can be directed dated through the radioactive decay of the isotopes from the time of formation (eruption and cooling), marking them as approximately 190 million years old.

The principle of superposition is really simple: old things are on the bottom, and young things are on top. If you looked at a vertical section of the area, the bottom and oldest layer is the Chinle Formation, the Nugget (with spider-footprints) is in the middle, and the top and youngest layer is marine sediments. From radiometric dating, the Chinle Formation is from the late Triassic, which is 201 to 235 million years ago. From a combination of radiometric dating and biostraigraphy, where marine invertebrate fossils in the sediments are unique to a particular time period, the marine sediments are mid-Jurassic, so from 163 to 174 million years ago. By superposition, the Nugget formed somewhere between those ages — younger than 201 million years old, but older than 174 million years old. This age is consistent with the date from the volcanic sediments.

What about the Triassic-Jurassic boundary?

The Triassic-Jurassic boundary may well lie within the vast sandpile of the Nugget-Navajo-Aztec but there are no volcanic ashes and no age diagnostic fossils to ascertain where that might be (yet). Geologists and palaeontologists continue fieldwork and maybe such a touchstone will turn up. It would be of great scientific interest to pin point that boundary!

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Thank you to the social media team at Dinosaur National Park for getting back to us! Between this, and helping us resolve the expansion-date mixup (and correcting their website!), I get the feeling that the National Park Service loves you just as much as I love them.