Episode 13 Field Guide: Following in the Footsteps of Dinosaurs

When we think of paleontologists, we think of people hunkered down with bones, teeth, and shells studying the preserved body parts of dead organisms. But animals leave behind more than just their skeletons. As they walk they can leave behind footprints, as they eat they can leave behind bite marks, and as they finish eating they leave behind…well…what comes from the behind. The study of the traces of past behavior is called ichnology and Dr. Tony Martin (@Ichnologist) and author of Dinosaurs Without Bones is with us to reveal all the amazing insights a paleontologist can learn from the fossil record, even when there aren’t any bones! He takes us through a wandering heard of sauropod dinosaurs, into the burrow of the dinosaur Oryctodromeus, and across the mudflats of an amphibian-dominated Alabama. These animals may be extinct, but their traces bring them back to thundering life in this episode of Past Time!

The easiest way to listen to the episode is to subscribe to Past Time in iTunes at this link. If you would rather download the audio follow this link or stream the episode here. Click below to see images of trackways and for more information on ichnology! If you have a favorite spot for finding footprints, leave your recommendation in the comments section!

PALEONTOLOGY ON TRACK

People have been studying the tracks and traces of animals for a long, long time. Hundreds of thousands of years ago, the family’s next protein jolt from a zebra/deer/tapir/kangaroo steak was dependent on the tracking skills of the group. Even today, using a careful, observational eye, trackers can learn a lot about their prey from the direction, depth, and distance of tracks, the size of their burrows, and the freshness of scat. Trackers also watch for traces of creatures that might want to make make them into prey! 

The careful study of footprints and traces left behind by organisms is called “ichnology”, Greek for “The study of footprints.” Because organisms have been leaving behind signs for their existence since life first got moving, ichnologists aren’t limited to studying the behavior of organisms that are alive today. In fact, a whole branch of paleontology is dedicated to ichnology. 

Two-Medicine-Selfie
Dr. Tony Marin of Emory University takes a break from fossil tracking in the Two Medicine formation to grin.

Our expert in this episode, Dr. Tony Martin from Emory University in Georgia, has been studying ancient tracks and signs for most of his career. Unlike most paleontologists, he doesn’t spend a whole lot of time with bones, teeth, and shells. Instead he tracks ancient creatures across ocean bottoms and stream beds that have solidified into solid rock, using trails to solve the mysteries of extinct lifestyles. To do this he draws on his knowledge of geology, anatomy, physics, ethology (the study of animal behavior), and more!

An individual dinosaur or snail will only leave one skeleton or shell behind, but it will leave behind dozens of types of traces as it feeds, sleeps, nests, and poops. To decipher all the strange marking left in ancient sand and mud, Tony Martin spends a lot of time hiking through modern environments studying the modern behavior of animals so he can apply observations of modern behavior to extinct animals’ traces. One of the reasons we talked to him in this episode it because he just published a fantastic new book on dinosaur traces, called “Dinosaurs without Bones” but he is also the authored “Life Traces of the Georgia Coast” a book that focuses entirely on track makers that are still living. To be a good paleoichnologist, Dr. Martin spends a lot of time jumping from the world of living animals to the extinct and right back.

Dinosaurs without Bones is a great new book for any curious dino-oriented reader by by Tony Martin.
Dinosaurs without Bones is a great new book for any curious dino-oriented reader by by Tony Martin.

WALKING WITH DINOSAURS

Dinosaur bones can tell us a lot about their size and relationships to each other, but there are a lot questions about dinosaurs that can be difficult to sort out from their skeletons: Did the herbivores move in herds? How did the carnivores hunt? Did they drag their tails when they walked? 

Footprints have the answers.

The same processes that preserve layers of rock forming the sedimentary record of Earth history preserve the impressions animals leave behind. Chemical changes in surface and groundwater may erode bone, but the footprints often stay preserved in the rock. Many trackway sites don’t have a scrap of bone, but preserve the record of a herd of massive sauropod dinosaurs traveling together along a muddy floodplain, amphibians cruising ancient Alabama, or even our first upright ancestors strolling ancient Tanzania:

Trackways

THE SCAT MAN

In the episode, we get into the less…delicate side of ichnology. Tracks aren’t the only signs left behind by animals. Feces/dung/poop/scat/leavings/stool reveal important insights into the diet, size, and internal anatomy of animals, living and extinct. When poop fossilizes it’s called a coprolite (which literally means “dung stone”). The species of plants favored by different herbivores can be determined by studying the pollen, stems, and leaves that aren’t quite digested in a coprolite. Because one animal was able to eat everything in its stool, coprolites can help paleontologists piece together the floral diversity of the ecosystem. Dung is also mana from heaven to many smaller species looking for nutrients that bigger animals can’t fully process. Dung beetles famously nest on rolls of poop, providing their larva with plenty of secondary nutrients form the dung ball they hatch on to. Other insects like flies and caterpillars will stop by a feces feast and sometimes evidence of their behavior is fossilized with the coprolite, giving paleoentymologists insight into invertebrate diversity in the past!

CoprolitesFinal

DIGGING DINOSAURS

One slightly frustrating aspect of ichnology is it can be tough to tie a specific extinct animal to a specific trace. If there are multiple medium-sized, carnivorous dinosaurs living in an ecosystem, a medium-sized, carnivorous dinosaur footprint (think of a large bird print) is tough to link to one species. So, the Holy Grail of paleontology is a trackway or trace fossil found along with the animal that made it. In 2007 Dr. Martin and a team of paleontologists working in Montana documented a small, herbivorous dinosaur skeleton. Even better, the largest skeleton was associated with two smaller skeletons of the same species. Maybe mom and babies? Then they looked a little wider around the skeleton and noticed the rock encasing the skeletons was a little different than the rock blanketing the landscape. They traced the border between the two rock types, and were saw they had discovered a dinosaur burrow! The den was large enough for the small dinosaurs, called Oryctodromeus cubibularis (meaning “Clawed runner living in a burrow”). In case there was any doubt that this was a burrow and not just a naturally occurring, filled-in hole, Dr. Martin documented smaller burrows made off the larger burrows. He knew that burrows like the holes dug by burrowing gopher tortoises are often exploited by smaller, digging animals. Why dig the whole thing yourself when you can mooch some vertical distance down from a larger stronger animal? The burrow of Oryctodromeus had insect burrows shooting off of it, and even a burrow large enough for a mammal to make a home. This evidence convinced Dr. Martin beyond doubt that they were looking at evidence of dinosaurs moving some dirt rather than a weird geological event that happened to capture three little dinosaurs.

Oryctodromeusfinal

Dr. Martin is a passionate footprint evangelist and his exploits can be followed at his Twitter handle @Ichnologist. If you have any questions for him about a weird divot in a rock, we’re sure he will help you sort it out! If you have any other questions about the podcast or our experts shoot us a message, too, or leave a comment!

FURTHER READING FOR EVERYONE

Dinosaurs Without Bones: Dinosaur lives revealed by the trace fossils by Anthony Martin, PhD. Pegasus Books. 368 pages. In his latest book from Pegasus Books, Dr. Martin explores the science of ancient biology through tracks and traces. He highlights some of the great places to watch dinosaurs walk around the world and some of the unexpected insights that have been gleaned from prints, droppings, and burrows. You will never look at a footprint the same way again.    Life Traces of the Georgia Coast: Unseen lives of the Georgia Barrier Islands by Anthony Martin. Indiana University Press. 692 pages. At the end of the episode we point out how you can be a paleontologist without ever looking at a fossil. If you train your eye to read stories left behind by modern animals, you will ready to figure out dinosaur tracks when you finally make it to a Mesozoic outcrop. In this book Dr. Martin documents the tracks and traces found on the beaches of Georgia. His insights can help you understand the trails and tracks along many beaches, rivers, and streams.

FURTHER READING FOR A TECHNICAL AUDIENCE

The article published by Dr. Martin and colleagues describing Oryctodromeus and its burrow:

David J. Varricchio, Anthony J. Martin, and Yoshihiro Katsura. 2007. First trace and body fossil evidence of a burrowing, denning dinosaur. Proceedings of the Royal Society: Biological Sciences. Vol. 274 no. 1616. doi:10.1098/rspb.2006.0443. 

An ichnotaxon (animal name based on a trace fossil) named Grallator, probably the footprint of a carnivorous dinosaur.

Orodromeus-skull

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