But if you ever play a word-association game and the words “brilliantly modern” are referred to a concept associated with evolution and island biogeography, it is best to just answer “Darwin.” Yes, that’s right, Charles Darwin thought of flighted birds taking both seeds and small invertebrates to new homes, and repeating these actions over many generations. He even set up a few experiments to test this idea, none of which involved using GPS-enabled devices, satellites, computers, lasers, 3-D printers, or emergency medical holograms. For one experiment, he simply dipped duck feet into a pool of water with aquatic snails, watched them crawl onto the feet, then took the duck feet out of the water to see how long the snails stayed attached. In short, he just observed, questioned, tested, observed more, and then wrote a carefully worded conclusion based on the preceding. From such methods, he not only devised some of the most important tenets of modern evolutionary theory, but also explained how flowering plants, freshwater invertebrates, and marine invertebrates were able to travel to faraway places through the power of birds.
What Darwin did not know, though, was that through his study of birds and their long-distance movement of seeds and animals, he was also studying the ichnology of dinosaurs. In the late 19th century, Darwin was aware of the few Jurassic and Cretaceous non-avian dinosaurs that had been discovered in the United Kingdom and elsewhere, as well as the Late Jurassic Archaeopteryx from Germany. But he did not know, nor could he have suspected at the time, that birds are dinosaurs.
Interestingly, fossil theropod and ornithopod tracks were already known during Darwin’s lifetime, but their makers had been misidentified. For instance, in 1845 he corresponded with ichnologist Edward Hitchcock of Amherst College in Massachusetts, who surmised that Late Triassic and Early Jurassic dinosaur tracks from the Connecticut River Valley were oversized bird tracks. In that letter to Hitchcock, in which Darwin refers to these dinosaur tracks as “footsteps,” he mused about the meaning of these tracks with relation to birds:
In my opinion these footsteps (with which subject your name is certain to go down to long future posterity) make one of the most curious discoveries of the present century & highly important in its several bearings. How sincerely I wish that you may live to discover some of the bones belonging to these gigantic birds: how eminently interesting it would be (to) know whether their structure branches off towards the Amphibia, as I am led to imagine that you have sometimes suspected.
Had both Hitchcock and Darwin lived to today, they would have been disappointed to learn that this wistful hope of finding bones of those “gigantic birds” (dinosaurs, as it were) would have been largely unfulfilled. As of this writing, despite continued discoveries of theropod and ornithopod tracks, almost no dinosaur bones are known from the Connecticut River Valley. Furthermore, body fossils of birds are unknown from rocks older than the Late Jurassic anywhere in the world.
Nonetheless, even though Darwin and Hitchcock missed the connection between dinosaur tracks, the evolution of birds, and biogeography, they each in their own way linked these seemingly disparate subjects. What Hitchcock got right, but without knowing it, was how the close resemblance of theropod tracks to those of modern birds did indeed reflect their evolutionary relatedness. What Darwin also got right was how birds changed the face of our modern ecosystems. Darwin had also very likely seen tracks of rheas (Rhea pennata and R. pennata) during his travels in South America, which closely resembled the fossil tracks Hitchcock described. Most important, though, both of their initial studies cleared intellectual trails, on which subsequent generations of biologists and paleontologists, likewise making careful observations of tracks and other traces, led to broader and more eclectic views of how the earth’s surface has been and will continue to be shaped by the traces of dinosaur behavior.
Regardless of how quickly our technological tools might serve us in such endeavors, the preceding methods and concepts can be done and understood by using the greatest scientific tools we already possess—our senses and cognition.
Dinosaurs Without Bones: Trace Fossils of the Future
So now that we know that we are living with dinosaur traces, both ancient and modern, we can also better appreciate the role of individual dinosaur trace fossils in our understanding of these iconic animals. But what then to do with this ichnologically bestowed enlightenment, a newly realized super-power that allows for recognizing how dinosaur trace fossils superbly augment and oftentimes surpass the paleontological worth of dinosaur bones in interpreting dinosaur behavior?
Obviously, we must apply what we’ve learned about dinosaur trace fossils of our paleontological past to those of the future. With that goal in mind, here are words of advice and predictions, given with full awareness that my own knowledge of dinosaur trace fossils, much like a landscape occupied by dozens of small burrowing ornithopods, is full of holes.
We’ll find more dinosaur trace fossils, and they’ll be different. One of the most excitedly received dinosaur books in recent years was All Yesterdays (2012), coauthored by paleontologists Darren Naish and C.M. Kosemen and co-illustrated by paleoartists John Conway and Scott Harman. Although only 100 pages long, and with its illustrations taking up more space than its words, its inventive artistic renderings and descriptions of unexpected behaviors in dinosaurs—such as tree-climbing ceratopsians and mud-wallowing sauropods—wowed many of its fans. (The book’s subtitle very easily could have been Dinosaurs Gone Weird.) That book’s popularity then led to a sequel published in late 2013, titled All Your Yesterdays, bearing contributions by a variety of artists in which they portrayed yet more unconventional dinosaur behaviors. This sort of cooperation between paleontologists and visual artists—including those who create computer-generated imagery—points toward a potential fountain of creativity that can expand our perception of dinosaurs as real, living animals.
The main point of these books was to promote thinking a little differently about dinosaur behaviors. In that spirit, I will now take it a step further in this book and ask us to think differently about trace fossils that could be made by different behaviors and different dinosaurs. Furthermore, because dinosaur trace fossils are so much more common than their bones in most places, finding odd trace fossils should be easier than finding bones of odd dinosaurs. Sure, the usual theropod, ornithopod, and sauropod tracks will continue to be discovered nearly every day. Yet I have a more ambitious wish list of trace fossils, some of which I sincerely hope will be uncovered by ichnologically adept paleontologists in upcoming years. These are a few of my favorite things:
Ceratopsian and pachycephalosaur trackways that confirm these big-headed dinosaurs really did smash into one another with their bony accouterments. Or, more interesting, show that they did not, and instead got along famously and that those big heads and accompanying horns and shields were there for some other purpose.
More examples of dinosaur urolites. After all, they all had to go some time.
Clear, definitive dinosaur regurgitalites, ideally with partially digested food directly associated with a probable regurgitator nearby.
Long, continuous trackways made by really big theropods. I’m talking about trackways made by 7–9 ton Cretaceous theropods like Spinosaurus or Carcharodontosaurus of northern Africa, Gigantosaurus of South America, or Tyrannosaurus of North America. Such trackways would give us much more information on how these dinosaurs really moved, while also reeking of awesomeness.
Sauropod trackways that demonstrate whether or not these animals traveled in family structures like modern elephants, with mixtures of young and old.
More detailed studies of microwear on ornithopod teeth, illuminating how these dinosaurs ate, and in some instances what they ate.
Dinosaur toothmarks preserved in unusual substrates, like molluscan shells or eggshells, and not just bones.
Closer looks at broken and healed lower limb bones in theropods to see if these were the marks of cranky stegosaurs and angry ankylosaurs who objected to unwanted attention fro
m those theropods. Likewise, more healed tail spikes and clubs of these dinosaurs, further showing how they disciplined their oppressors.
Dig marks in fossil termite or ant nests that match the dimensions of therizinosaur hands or the limbs of other unusual dinosaurs.
More dinosaur ground nests and nesting grounds. So few of these are known as trace fossils, yet more must be out there, waiting to be recognized.
Burrows made by small theropods or other dinosaurs. Why restrict all of the underground fun to small ornithopods?
More gastroliths, whether in or out of their dinosaur hosts: who really had them, who really lacked them?
Enterolites or coprolites that show clear evidence of insectivory or omnivory in dinosaurs. They weren’t just all “meat eaters” or “plant eaters” (how boring).
Dinosaur sex traces. (Need I say more?)
Although paleontologists may find none of these, or only stumble upon a few of them, if any are found we should all celebrate their success with alacrity. What is also gratifying to know is that some of the people who find these may not even be professional paleontologists, but people who happened to be looking in the right place at the right time and with the right search image in mind. Paleontology, archaeology, and astronomy are among the few sciences in which amateurs regularly make important contributions through their numerous eyes on the ground (or on the stars). Let the games begin.
Nevertheless, we should describe and interpret these trace fossils with care. One of the best facets of the fossil record is that it gets better every day, and trace fossils are a big part of this daily improvement. Yet of the dinosaur trace fossils found, many either remain undescribed or (far worse) have been under-described, without enough meaningful attention to detail to be faithful records of dinosaur behavior. If dinosaur trace fossils are treated like curios that are simply named “track,” “toothmark,” or “coprolite,” for instance, and linked to a possible dinosaur, they will become as dead as their makers. Technology will certainly play a part in better describing trace fossils. For example, 3-D images and 3-D printers of dinosaur tracks have already allowed researchers on other sides of the globe to work cooperatively, or to add to one another’s observations.
Remember that dinosaurs are still making traces. With 10,000 species of extant birds, all of which are making traces, we have no shortage of modern analogs for dinosaur behaviors. Granted, these dinosaurs are limited to avian theropods, but their tracks, nests, burrows, gastroliths, feces, and other traces supply many opportunities to think about how dinosaurs both avian and non-avian might have behaved, and whether they made similar traces. Furthermore, although crocodilians are only distant relatives of dinosaurs, these big archosaurs are wonderful examples of big scaly four-legged toothy tracemakers that fit as tracemaking surrogates for a few dinosaurs and their Mesozoic relatives.
Become ichnologically imaginative. As one might have intimated from the wish list given above, I am encouraging everyone interested in dinosaurs to speculate about behaviors dinosaurs may have done and the traces that resulted from these behaviors. In the spirit of leading by example, I’ve done a few flights of reasonable fancy in this respect throughout the book, with the hope that more people will do the same. As a result, I fully expect dinosaur enthusiasts, armed with a heightened awareness of trace fossils and their importance in our understanding of dinosaurs, to make and share a surfeit of their own creative traces, both whimsical and serious.
So as we all make our own traces in our everyday lives, also imagine which ones will outlast us all. Will any of our traces also survive longer than the tracks, nests, burrows, and toothmarks of dinosaurs? Or will dinosaur trace fossils somehow surpass our own meager ichnological record? They already have had a 230-million-year head start, and with thousands of living descendants sharing the earth now, the odds are stacked against us. But no matter. This renewed appreciation for dinosaurs is now permanent, and one that recognizes another dimension of them beyond their bones. Place your hand on a dinosaur track, and you connect with the breathing essence of its maker, leaving your own fingerprints on it: life traces intersecting through time.
Notes
CHAPTER 1: SLEUTHING DINOSAUR
p. 10 “It turns out a good number of Triceratops head shields, which are composed of paired parietal and squamosal bones, bear deformities in the squamosals.” Farke, A.A., Wolff, E.D.S., and Tanke, D.H. 2009. Evidence of combat in Triceratops. PLoS One, 4(1): e4252. doi:10.1371/ journal.pone.0004252.
p. 10 “Ceratopsians, a group of dinosaurs that includes Triceratops and related horned dinosaurs, also made tracks, which are preserved in Cretaceous rocks from about 70 million years ago… .” Ceratopsian tracks are still quite rare, but a few are known from Colorado, Utah, and Alaska: (1) Lockley, M.G., and Hunt, A.P. 1995. Ceratopsid tracks and associated ichnofauna from the Laramie Formation (Upper Cretaceous: Maastrichtian) of Colorado. Journal of Vertebrate Paleontology, 15: 592-614; (2) Milner, A.C., Vice, B.S., Harris, J.D., and Lockley, M.G. 2006. Dinosaur tracks from the Upper Cretaceous Iron Springs Formation, Iron County, Utah. In Lucas, S.G. and Sullivan, R.M. (editors). 2006 Late Cretaceous vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin, 35: 105-113.
p. 10 “In the Cretaceous Period, about 95 million years ago and on a lake-shore in what is now Queensland, Australia, nearly a hundred small, two-legged dinosaurs ran in the same direction and at high speed.” Although this interpretation has been disputed in the past few years (see Chapter 3), the original study of these tracks is: Thulborn, R.A., and Wade, M., 1979. Dinosaur stampede in the Cretaceous of Queensland. Lethaia 12: 275-279.
p. 11 “One grouping of swim tracks, made by seven separate theropods, is preserved in Early Cretaceous rocks (110 million years ago) of Spain. Many more swim tracks are in Early Jurassic rocks from about 190 million years ago in southwestern Utah.” Two articles on dinosaur swim tracks came out in quick succession in 2006 and 2007: (1) Ezquerra, R., Doublet, S., Costeur, L., Galton, P.M., and Pérez-Lorente, F., 2007. Were non-avian theropod dinosaurs able to swim? Supportive evidence from an Early Cretaceous trackway, Cameros Basin (La Rioja, Spain). Geology, 35, 507-510. (2) Milner, A.R.C., Lockley, M.R., and Kirkland, J.I. 2006. A large collection of well-preserved theropod dinosaur swim tracks from the Moenave Formation, St. George, Utah. In Harris, J.D., et al. (editors), The Triassic-Jurassic Terrestrial Transition. New Mexico Museum of Natural History and Science Bulletin 37: 315-328.
p. 11 “In 2007, I helped two other paleontologists document the first known burrowing dinosaur (Oryctodromeus cubicularis, a small ornithopod) from Cretaceous rocks (95 million years old) in Montana.” Varricchio, D.J., Martin, A.J., and Katsura, Y. 2007. First trace and body fossil evidence of a burrowing, denning dinosaur. Proceedings of the Royal Society of London, B, 274: 1361-1368.
p. 12 “Two years later, I interpreted similar burrows in older Cretaceous rocks (105 million years old) of Victoria, Australia.” Martin, A.J., 2009. Dinosaur burrows in the Otway Group (Albian) of Victoria, Australia, and their relation to Cretaceous polar environments. Cretaceous Research, 30: 1223-1237.
p. 12 “A different type of digging by other dinosaurs also has been inspired by unusual trace fossils found in Late Cretaceous rocks (about 75 million years ago) of Utah in 2010.” Simpson, E.L., Hilbert-Wolf, H.L., Wizevich, M.C., Tindall, S.E., Fasinski, B.R., Storm, L.P., and Needle, M.D. 2010. Predatory digging behavior by dinosaurs. Geology, 38: 699-702.
p. 12 “These nests also contained clutches of paired eggs, which were arranged vertically in the nests by one or both of the parents after egg-laying.” (1) Varricchio, D.J., Jackson, F., Borkowski, J., and Horner, J.R. 1997. Nest and egg clutches of the dinosaur Troodon formosus and the evolution of avian reproductive traits. Nature, 385: 247-250. (2) Varricchio, D.J., Jackson, F., and Trueman, C.N. 1999. A nesting trace with eggs for the Cretaceous theropod dinosaur Troodon formosus. Journal of Vertebrate Paleontology, 19: 91-100.
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nbsp; p. 12 “Similarly, a spectacular find of Late Cretaceous nests in Argentina from 70 to 80 million years ago and attributed to gigantic sauropods called titanosaurs shows that dinosaurs other than Troodon made ring-like enclosures for their eggs.” (1) Chiappe, L.M., Coria, R.A., Dingus, L., Jackson, F., Chinsamy, A., and Fox, M. 1998. Sauropod dinosaur (2) Chiappe, L.M., Schmitt, J.G., Jackson, F., Dingus, L., and Grellet-Tinner, G. 2004. Nest structure for sauropods: sedimentary criteria for recognition of dinosaur nesting traces. Palaios, 19: 89-95.
p. 13 “What has surprised paleontologists in recent years, though, is the realization that a few theropods, a group of dinosaurs once assumed to have been exclusively carnivorous, also have these ‘stomach stones.’” Wings, O. 2007. A review of gastrolith function with implications for fossil vertebrates and a revised classification. Acta Palaeontologica Polonica, 52: 1-16.
p. 13 “Although no one has yet found gastroliths directly associated with Struthiomimus, some of its relatives, collectively called ornithomimids (‘ostrich mimics’), do have them.” Kobayashi, Y., Lu, J.-C., Dong, Z.-M., Barsbold, R., Azuma, Y., and Tomida, Y. 1999. Herbivorous diet in an ornithomimid dinosaur. Nature, 402: 480.
p. 13 “This is backed by healed toothmarks caused by a large predatory theropod preserved in a few bones of Edmontosaurus, including at least one with a smoking gun (or tooth, as it were) linking it to Tyrannosaurus or its close relatives.” Carpenter, K. 2000. Evidence of predatory behavior by carnivorous dinosaurs. Gaia, 15: 135-144.
p. 14 “Triceratops bones also bear toothmarks that could only have been made by tyrannosaurs, including those that mark the front of the face… .” At the time of this writing, this research had only been reported through an abstract, so hopefully a paper is out now: Fowler, D., Scannella, J., Goodwin, M., and Horner, J. 2012. How to eat a Triceratops: large sample of toothmarks provides new insight into the feeding behavior of Tyrannosaurus. Journal of Vertebrate Paleontology [Supplement to 3], Society of Vertebrate Program and Abstracts, October 2012, p. 60.
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