190 million years ago, something scampered across the desert sands that today straddle Colorado and Utah. The footprints were fossilized, recording the trace motions of a creature long gone by. What left these footprints?

The scale bar is in centimeters (2.54 cm = 1 inch). Image credit: National Park Service


This is a photograph from above of the fossilized footprints. Blue arrows point to tiny footprints. Little crescent hills identify the upslope direction (red arrow) versus the direction of the creature's stroll (green arrow).

The traces are located in Dinosaur National Park, which straddles Colorado and Utah. The tracks only contain footprints, with no other marks. This is your last chance to guess the answer in the discussion before I give away the answer below the photo. Any guesses?

Modern sand dunes in Namibia. Image credit: Daniela Borchert

The tracks were made by a tall, long-legged spider!

An ichnologist is a scientist specializing in interpreting trace fossils like these footprints to read the story of the past. It's their job to figure out things like using the crescent-hills to determine slope orientation of a sand dune from a time long past, or the footprint-spacing to calculate stride length. They can look these 190-million-year-old footprints and figure out that this spider had particularly long legs because it left only footprints, with no central print from abdominal drag.


This riddle was posed by the Dinosaur National Monument on Facebook, so check them out for future fossil-mysteries! For more ichnology, consider these trace fossils from Easter.