Footprints, swimming scrapes, casts, eggshells, toothmarks, and even dung are the traces left by creatures going about their lives. For every fossilized bone, an ancient creature left behind uncounted traces of the life it lived. Tony Martin explores the world revealed by ichnology in his book, Dinosaurs Without Bones.
Bones tell us about the physical form of a dinosaur, but tracks tell us how they behaved. Trace fossils breathe life into ancient dinosaurs, transforming them into creatures who strutted, swam, dug, ran, ate, fought, swim, and bred. In his book, Martin teaches the principles of taking those tracks and interpreting to unravel the mysteries of dinosaur lives.
Parallel, evenly-spaced sauropod footprints are the traces of a herd's path from the Late Jurassic.
Every footprint raises questions about who-what-when-where-why. Once you pin down a footprint's age, so many questions remain. How big is the footprint, and, by extension, how big was the dinosaur? Do the prints give hints on the age or gender of the absent trace-maker? Was the dinosaur a theropod or a sauropod? How many toes does it have, and are they tipped in nails?
What was the dinosaur's gait? Are the footprints close together, or far apart? How is the weight distributed between the left and right, front and back? Was our long-gone dinosaur favouring an injury? Was the dinosaur a biped or quadruped, travelling on two legs or four? Was the dinosaur engaged in a slow stroll, a lazy waddle, a bouncing trot, or a mad dash?
Martin teaches how to approach these questions and more. While technically applied to dinosaurs, since we know their fearsomely fluffy brethren still roam the world around us, the techniques are equally applicable to modern tracking.
Theropod dinosaur tracks, avian and non-avian: a modern Southern cassowary [left] and an Early jurassic theropod [right] (20 cm scalebar on left, 15 cm scalebar on right).
Martin's voice is welcoming, bringing you into the palaeontology with an introduction to how to identify who made the tracks, and what they were doing when they created the traces. What I love about geoscience is that it's a constant inversion problem, where you have the end consequences and need to figure out the processes that happened to create them. His book opens up the world of ichnology, giving a new perspective on approaching the world to unravel the mysteries of tracks left behind. It's a gift of perspective, one that makes me wish I had a garage where I could leave a box of sand and track the passage of critters in my neighbourhood.
Not every track is a footprint. Every stage of eating reveals yet more opportunities to produce more traces. Teeth can leave traces of dinosaur dentistry in toothmarks scouring bones. For a bit of meta-geology, dinosaurs swallowed rocks, gastroliths in abdominal cavities to help grind up food. We can find these stones within stone, trace fossils within body fossils. But my favourite of the consumption-related trace fossils is the theoretical imagining of what Brachiosaurus's projectile vomit would look like, with impact crater, stream, and the remains of his rejected lunch.
Coprolite produced by a Late Cretaceous hadrosaur, with burrows from dung-beetles feasting while it was still fresh (centimeter scalebar).
If full fossilized dinosaur skeletons are the charismatic megafauna of the paleontological field, where do coprolites fall? Coprolites is the discrete name for quite-literal mineralized shit, ancient dung gives us insight into the food web of prehistoric ecologies. The coprolites are not just a look at what the dinosaurs ate, but also at the ancient bugs who feasted on the not-quite-digested foods.
I love the humour in his constant pokes at the practice of science. He teases that by expanding from bone-fossils to trace fossils, we extend from the mere 200-ish bones per creature, "we now frolic in the land of plenty," with thousands of traces left by that creature throughout his life. Of course, that doesn't always help, since as he wryly observes that for some species we've never identified a footprint because we've never found an intact foot. To be fair, some feet are decidedly weird, like the Iguanodon's downright bizarre paw.
Overall, I'm loving Dinosaurs Without Bones, and if you like learning about new topics in natural sciences, I think you will, too. For a taste of his writing style, you can always check out the archives of his blog, Georgia Life Traces. I've highlighted a few entries from it before discussing the tracks made by legless Easter Peeps, and the variety of prints left by a rampaging Godzilla.
Ichnologist and author Tony Martin is going to be joining us in the discussion, so please line up your questions for him. To get you started, I offer a trace fossil riddle from Lark Quarry. Amongst the three-toed footprints, one large print from a large theropod or ornithopod faces one direction, while a scattering of smaller prints from various smaller theropods or ornithopods all face the other direction. Why?
For more ichnology fun, we've previously used gait-analysis to investigate the industrious hedgehog, and investigated mysterious fossilized footprints from a desert-crawler. All images are from Dinosaurs Without Bones and credit Tony Martin.