Why Eat a Rock? Gastroliths (‘Stomach Stones’) as Trace Fossils

During the past 30 years or so, trace fossils – such as tracks, burrows, nests, toothmarks, coprolites (fossil feces), and so on – have been steadily gaining more respect and recognition in paleontology. I would like to think this is a direct result of my dedicated “ichnoevangelism,” in which I have attempted to convert The Great Unwashed who fail to see the paleontological truth inherent to trace fossils, but the world of paleontology is much bigger than the Church of Ichnology (so far). Nonetheless, despite this overall increase in enlightenment, one type of trace fossil still causes some recalcitrant non-believers to turn gazes elsewhere and ignore their scientific worth. Those would be gastroliths, otherwise known as “stomach stones.”

Gastroliths extracted from the interior of a Cretaceous elasmosaur (a large, long-necked marine reptile) in central Queensland, Australia. Specimens are at the Stonehouse Museum and collected by “Dinosaur” Dick Suter in Boulia, Queensland. I know, they just look like a bunch of rocks, not trace fossils. Please read on, and prepare to learn.

A Trace Fossil by Any Other Name

Seemingly everyone – but especially five-year-olds – can enthusiastically rattle off the genus or species name of their favorite dinosaur or other prehistoric animal. “Tyrannosaurus rex!” “Triceratops!” “Dimetrodon!” But I will bet a six-pack of my favorite adult beverage that 99% of people – kids and adults alike - cannot name a single trace fossil in the same way. And even if they did, could you imagine anyone applying the same vigor and gusto reserved for body fossils as they shout out the names of trace fossils? “Ophiomorpha nodosa!” “Eubrontes! “Celliforma!”

Wow, look! It’s Thalassinoides in Early Cretaceous rocks of Victoria, Australia! I absolutely love that trace fossil! Wait, come back – where are you going? Was it something I said?

Who Made the Three-Toed Dinosaur Track?

In paleontology, occasionally a scientific hypothesis rules for several decades or more and enters the public realm, becoming part of popular lore. Nonetheless, science is always changing, which means that what we took for granted as a “true” story can be upended in a way that surprises everyone, perhaps even the paleontologists doing the revising. Such is the situation with the “giant-stalking-theropod-dinosaur-causing-a-dinosaur-stampede” story of the Lark Quarry dinosaur tracksite in Queensland (Australia). This tale has been reigning for more than 30 years and is known worldwide by paleontologists and laypeople alike, but now faces a makeover in the light of new evidence.

Here are some dinosaur tracks from Queensland, Australia. The dinosaurs that made them had three toes on their rear feet, and walked only on those two feet (bipedally). So were these from theropod dinosaurs or ornithopod dinosaurs? Don’t know? Then I guess you’ll have to read more, won’t you? Display is at the Museum of Tropical Queensland in Townsville, Queensland (Australia). Note the stylish sunglasses (lower right) for scale.

The Case of the Mistaken Dinosaur

Like all stories in paleontology, that of Lark Quarry – a world-famous dinosaur tracksite in Queensland, Australia - starts in the geologic past; in this instance, during the Cretaceous Period, about 98 million years ago. The cast of this tale consisted of about 150 dinosaurs, played by:

• One species of a large herbivorous dinosaur (an ornithopod), who was a mere bit actor, making a cameo appearance before the main act.

• Two species of small dinosaurs – one a theropod and the other an ornithopod – who made up most of the cast; and

• The star of the show, a tyrannosaur-sized theropod, who made a grand entrance toward its climatic finish.

The story is filled with dramatic flourishes, of a tranquil scene shattered by brutal carnivory, and of a quiet Cretaceous lakeshore becoming a killing ground. The former lakeshore, though, left no bodies, only tracks. So we have to reconstruct what happened there by using the oldest science known to humankind, ichnology: the study of modern and fossil traces.

A small sample of the 3,300 dinosaur tracks of Lark Quarry in Queensland, Australia, showing evidence for three species of dinosaurs that either walked or ran along a Cretaceous lakeshore 98 million years ago. Who made these tracks,  what behaviors do they represent, and how were the trackmakers’s behaviors related to one another? That is the mystery – and now the controversy - of Lark Quarry.