Dinosaurs Without Bones Dinosaur Lives Revealed by Their Trace Fossils

Say "dinosaurs", and most people imagine fossilised bones and spectacular museum displays. But body fossils are not the only remains we have with which to reconstruct dinosaur lives. Nor, and this might sound controversial, are they the most important. Or so argues palaeontologist, geologist, and ichnologist Anthony J. Martin. Ichnology is the study of animal traces, whether modern or fossilised. Most traces are ephemeral and disappear within hours or days, but occasionally some are buried and end up in the fossil record. With tongue firmly planted in cheek, and with more puns than you can shake a T. rex thigh bone at, Martin forays into the rich dinosaur trace fossil record: from footprints, burrows, and nests, to teeth marks and fossil faeces. For all the jokes, and despite having been published in 2014, he raises some really interesting points.

You might indeed ask, why review this book now? Way back in 2018 I reviewed Martin's 2017 book The Evolution Underground on the evolutionary history of burrowing behaviour and was suitably impressed. I vowed to search out his previous book, though I did not intend to wait this long. Having just reviewed Lomax's book Locked in Time, now was the right time to make up for that.

Animals of all sorts leave behind traces wherever they go and dinosaurs were no exception. Footprints are probably the first thing to come to mind and a logical place to start. From the US, to Europe, to South America, they have been found on every continent except Antarctica*. Martin discusses how the number of toes and their orientation can reveal what group of dinosaur left the footprint and how their size can be used to estimate its maker's size. Where multiple footprints form a trackway, their distance can reveal gait and velocity, or whether dinosaurs moved alone or in (family) groups. And with technological advances, we can extract more information from footprints than ever before.

One interesting thing Martin reveals here that I was not aware of, is that most fossil tracks are probably undertracks. That is, the subsurface deformation of the substrate caused by the pressure of the foot. Most tracks likely soon weathered beyond recognition, so unless you find clear skin impressions, the prudent assumption is that you are dealing with undertracks. As he points out: "The decided preservational advantage of this phenomenon is that such tracks were already buried, protecting them from destruction" (p. 33).

But feet can do much more than just walking and Martin examines trace fossil evidence of dinosaur nests and fossil burrows. Beyond traces made by feet, bones can record tooth marks, though care is needed to distinguish attacks on living animals from post-mortem scavenging. In turn, food leaves tell-tale traces of microwear on a tooth's surface. More exotic – and controversial – are gastroliths or stomach stones that can act as digestive aids, though likely not all were swallowed on purpose. And then there is that which comes out at the other end. You might have heard of coprolites (fossil dung), and Lomax introduced me to urolites (fossil traces of urination). But what of enterolites (fossil stomach contents) and cololites (fossil intestinal contents)? Or, my favourite, regurgitalites – which is what you think it is.

For some behaviours we do not have clear evidence, while others are highly unlikely to leave traces in the fossil record. But this does segue nicely into one of the most important themes of this book: search image. At several points, Martin encourages readers and researchers alike to ask themselves: what would the traces left by certain behaviours look like? Take sauropod footprints. Given their size you might think they are hard to miss and yet: "In the early days of dinosaur tracking probably more than one paleontologist or geologist walked by their footprints without a second glance, thinking they were some sort of large erosion-caused features. Once these footprints were correlated with the sizes and shapes of sauropod feet [...] sauropod tracks magically appeared in the search images of paleontologists worldwide" (p. 22). An important point of reference, and one that Martin profiled for his home turf of Georgia in his previous book, are the traces left by animals alive today, specifically birds. Because, as he reminds you towards the end of the book by shortly recapping the evolution of birds, technically speaking birds are living, flying, feathered dinosaurs.

The other thing to note is Martin's writing style. All of the above is served up with a healthy dollop of frequently irreverent humour. Some of it borders on dad-jokes though, so let me give you some tasters. He frequently lampoons his own profession: "[...] even dinosaur-track experts have doubts about the identity of some three-toed dinosaur tracks, especially if a rival dinosaur-track expert identified them" (p. 22), and regarding peer review "[...] the scientists who review journal articles are doing it as unpaid volunteers, finding time to perform this important duty in between all of their other tasks such as teaching, grading, research, walking the dog, or (most heinous of all) sleeping" (p. 76). The rear part of a dinosaur "[...] is properly called an ischial callosity, and not the more appealing term "dinosaur butt"" (p. 39). On front teeth: "If a theropod's potential food item, such as a small ornithopod, was still alive and having issues with a proposal that it should devote its life to feeding a theropod, then the front teeth were the most persuasive tools used by that theropod" (p. 181). And on urolites: "Paleontologists who do such research could be assured of making a big splash with it, while also going against the flow of others' prejudices. Afterwards, they will be flushed with success, and their colleagues pissed off" (p. 247). If the above made you laugh, you will have a blast with this book. I certainly did.

Dinosaurs Without Bones is great popular science: fascinating, thought-provoking, and told with verve and wit. This is an excellent companion book to Lomax's Locked in Time and a very nice introduction to trace fossils and ichnology. My only regret is that I waited so long before finally reading it.

* That was true when this book was published in 2014. When I followed up on this, I found one news item claiming the 2016 find of a dinosaur footprint on Antarctica. However, when reading the actual 2019 paper in the open-access journal Polar Research, the authors are a bit more circumspect, attributing this footprint to "a primitive amniote, procolophonid or therapsid".