From “Half-Billion-Year-Old Predator Tracked: Multi-Legged Creature Ruled the Seas” (ScienceDaily, Nov. 8, 2011), we learn:
Researchers from the University of Saskatchewan and Royal Ontario Museum (ROM) have followed fossilized footprints to a multi-legged predator that ruled the seas of the Cambrian period about half a billion years ago.
Tracks, trackways, burrows, bite marks, feces etc. are called trace fossils. They don’t provide a physical look at the animal but they tell us it lived at a certain place and time, and paleontologists can learn some things about an animal before they actually find it. Here, there was already a suspect, though not found on the scene.
In this case, size of the tracks and the number of legs needed to make them left only one suspect: Tegopelte gigas. This caterpillar-like animal sported a smooth, soft shell on its back and 33 pairs of legs beneath. One of the largest arthropods of its time, it could reach up to 30 cm in length.
By analyzing both the fossilized remains of Tegopelte and the trackways, the researchers were able to reconstruct how this animal would have moved. The creature was capable of skimming rapidly across the seafloor, with legs touching the sediment only briefly, supporting the view that Tegopelte was a large and active top carnivore.
If they find a fossil surrounded by shelly detritus, call it a smokin’ gun.