IMAGINE THE worst nightmare of a typical dinosaur palaeontologist: to wake up one day and find all of their precious body fossils gone. No mounted skeletons, no bones at field sites or in plaster jackets or museum drawers.
Grown men and women would cry, children would wail and museum directors everywhere, out of sheer desperation, would be forced to consider including more displays of fossil mammals, or (worst of all), fossil invertebrates and plants. The horror, the horror.
On the other hand, dinosaur ichnologists – those palaeontologists who study dinosaur trace fossils, such as tracks, nests, and faeces – would wake up, smile, and say, “We told you so!”, then roll over and go back to sleep.
They would suddenly find their social status elevated in the world of dinosaur palaeontology, more properly feted as demi-gods or goddesses and oracles of palaeontological wisdom, filling gaps in our knowledge about dinosaurs, and especially dinosaur behaviour.
Of course, dinosaur ichnologists do not need fantasy scenarios to know they make significant contributions to our understanding of dinosaurs. Nonetheless, ichnologists’ discoveries and studies are sometimes overshadowed by the exploits of their osteocentric colleagues.
So just to put their work into perspective, perhaps the best question to ask should be based on the opposite premise of vanishing bones. What would we NOT know about dinosaurs if we did not have their trace fossils?
Let’s start with dinosaur tracks, the most common of dinosaur trace fossils. These are in many instances preserved abundantly in rocks devoid of dinosaur bones. Tracks can thus inform us of a dinosaur presence, letting us know where they were and when, and without having to search for fragmentary body parts.
For example, Early Cretaceous rocks of Western Australia hold thousands of dinosaur tracks, including some of the largest in the world that were probably made by gigantic sauropods called titanosaurs. Are there any bones in those same rocks? Nope.
Tracks also directly reflect dinosaur behaviour without having to resort to reconstructing incomplete skeletons, using complicated computer models, or making your toy dinosaurs fight each another. For example, from tracks, we can make estimates of how fast dinosaurs moved, where they stopped, sat down, ran, limped, swam, or simply changed their minds. For social behaviours, tracks can reveal whether some plant-eating dinosaurs moved in herds, or whether predatory dinosaurs walked together in packs.
What about dinosaur nesting and raising of young? Thanks to trace fossils, we know that at least a few dinosaurs, such as the Late Cretaceous theropod Troodon from Montana (USA) and Late Cretaceous sauropods in Argentina, built rimmed ground nests to accommodate and protect their eggs. (A nest like this makes an early appearance in the Walking with Dinosaurs 3D trailer, so look out for that.)
Another dinosaur, the mid-Cretaceous Oryctodromeus, dug burrows and probably used these as dens for raising their young; skeletons of an adult and two juveniles were found in one such burrow. Such intimate details of dinosaur ‘family values’ would be much tougher to figure out without the help of trace fossils like nests and burrows.
Most people – but especially children – also know about one of the most important of dinosaur trace fossils: coprolites, or fossil faeces. If it were not for dinosaur coprolites, such as those from Late Cretaceous rocks in Montana (USA) or Alberta (Canada), we might have never thought about hadrosaurs eating rotten wood, or confirmed that tyrannosaurs ate young hadrosaurs.
We also would not have known about the oldest grasses in the geological record, recovered from Late Cretaceous coprolites of India that were probably made by sauropods. Also related to dinosaur diets are gastroliths (‘stomach stones’), rocks that some dinosaurs swallowed and might have used to help digest their food.
Dinosaur trace fossils were even left on dinosaur bones, such as toothmarks on bones, scratch marks on teeth caused by silica-rich plants, and damage inflicted by one dinosaur on another’s body. For instance, in the trailer for Walking With Dinosaurs 3D, the Pachyrhinosaurus “hero” knocks heads with a rival Pachyrhinosaurus. Such conflict likely happened with Triceratops, in which their horns pierced head shields and left permanent marks, as well as with tyrannosaurs that bit each other’s faces.
What would we not know about dinosaurs without their trace fossils? Quite a lot, so thank an ichnologist today. Nonetheless, dinosaur palaeontologists work best when they work together, so through combining their studies of dinosaur body fossils with dinosaur trace fossils, they can paint more wondrous and fulfilling pictures of dinosaur lives.
- Photographs of Early Jurassic large theropod dinosaur tracks, southwestern Utah, used with permission of Tony Martin.