Dinosaurs Without Bones

by Anthony J. Martin

  In the simplest cladogram for dinosaurs, theropods are more closely related to prosauropods and sauropods than they are to ornithopods, whereas stegosaurs, ankylosaurs, and nodosaurs are more closely related to one another than they are to marginocephalians. Also, because birds descended from theropod ancestors and thus qualify as dinosaurs, these are included on any self-respecting dinosaur cladogram. But if you want to be even more of a “lumper” with classifying dinosaurs, you could go back to their initial split into saurischians (“lizard-hipped” dinosaurs), which includes all theropods (birds too), prosauropods, sauropods, and ornithischians (“bird-hipped” dinosaurs), which are all ornithopods, thyreophorans, and marginocephalians.

  Dinosaur feet can be roughly correlated with the evolutionary history of dinosaurs, based on the appearance and disappearance of these clades in the fossil record. For example, the hypothetical “first dinosaur,” which would have evolved about 235 mya and was the common ancestor to both saurischians and ornithischians, probably walked on its rear two legs (bipedal) and its feet would have had three prominent toes (digits) pointing forward, one toe off to the side, and all toes tipped with claws. Its front feet, had it also used these for walking (making it quadrupedal), would have had five fingers (also digits), again all pointing forward and with claws. All subsequent dinosaur feet were modified from this basic body plan, whether certain dinosaur lineages stayed bipedal, went to quadrupedal, or used some mixture of the two. As a result, dinosaur tracks made by feet that had more than four digits in the rear are rare—happening only with sauropod tracks—and more than five digits in the front would be really weird. If anything, most dinosaur feet reduced or lost digits throughout their evolutionary histories. Unneeded toes or fingers, which can be evolutionarily expensive, were weeded out by survival and propagation of species, in which it was advantageous to get rid of these over time.

  Just as modern trackers might classify mammal tracks by the number of toes, the same can be done with dinosaurs. For the major evolutionary groups of dinosaurs, the following modes of movement and digit numbers, with only a few exceptions, can be applied to help with identifying their tracks:

  Theropods—rear feet only (bipedal), three digits pointing forward, and sometimes another small one off to one side.

  Prosauropods—rear and front feet (bipedal or quadrupedal), four digits on the rear, four on the front.

  Sauropods—rear and front feet (quadrupedal), five digits on the rear and five on the front (although digits are almost never visible in sauropod front-foot tracks).

  Ornithopods—rear feet only or all four feet (bipedal or quadrupedal); on the rear feet, three digits pointing forward, sometimes another digit off to the side, whereas front feet had five or less.

  Stegosaurs—rear and front feet (quadrupedal), rear feet with three digits and front with five.

  Ankylosaurids and nodosaurids—rear and front feet (quadrupedal), four digits in the front and four or three in the rear; most ankylosaurids (perhaps all) had three.

  Ceratopsians—rear and front feet (quadrupedal), four digits in the rear and five in the front.

  Granted, paleontologists, just like any other scientists, delight in pointing out exceptions. So just to get those distracting thoughts out of the way, here are a few dinosaur-foot oddities to keep in mind:

  Therizinosaurs—These were unusual theropods, so unusual that no self-respecting dinosaur paleontologist discussing them can complete a sentence with these as the subject without also saying “strange,” “odd,” “weird,” “bizarre,” and other synonyms denoting their differences from other theropods. As an example of their unusualness, therizinosaurs, unlike all other theropods, had four toes pointing forward on their rear feet. As of this writing, a few therizinosaur tracks are known, with the most astonishing recently found in Cretaceous rocks of Alaska. These four-toed feet may represent some ancestral condition in theropods that was somehow retained in therizinosaurs well into their evolutionary history.

  Dromaeosaurids—These were bipedal theropods with three toes retained on their rear feet, but only two of those digits contacted the ground. One of the three digits was raised and sported a nasty-looking claw, which evidently was used for gripping and holding down prey, climbing, or slashing. Consequently, dinosaur trackers sometimes do a double-take when they spot dromaeosaurid tracks, which look as if their theropod trackmakers somehow lost a digit. This notion is quickly discarded, though, once those paleontologists realize both feet are missing exactly the same toe, which surely is not a coincidence. Once considered rare, dromaeosaurid tracks are now being discovered in some of the same places and geologic formations as dromaeosaurid bones, helping paleontologists to fill out more complete pictures of their life habits.

  Pachycephalosaurs—these dinosaurs, often nicknamed “bone-headed dinosaurs” because of their thick, bony skulls, share a common ancestor with those other big-headed dinosaurs, ceratopsians. Yet we know nothing about their tracks, because we know nothing about their feet. So far, no pachycephalosaur foot parts have been discovered, meaning that we can only speculate about what their tracks look like. Based on what we do know from their skeletal remains, we think they were mostly bipedal, so their tracks mostly just show alternating right–left rear foot impressions. Such tracks possibly might resemble those of their ceratopsian cousins, which have four toes on the rear foot. If so, that difference would be helpful when distinguishing their tracks from those of ornithopods or theropods.

  So you might get the impression that, armed with digit numbers and knowledge of the basic groups of dinosaurs, identifying their tracks will be oh-so-easy, a leisurely stroll in the park while also attended by servants providing tea, backrubs, and answering your e-mail for you. Once you come back from your saunter down a fantasyland version of how dinosaur ichnology is done (hey, I’ve been there, and visit it often) and are ready to do a little more thinking, here is what else you’ll need to know.

  For instance, you’ve probably already noticed that most theropod and ornithopod tracks are three-toed. Then how do we tell the difference between them, especially if they’re preserved in rocks of the same age and place? This can get tricky, especially if the tracks are poorly preserved. But the easiest way to tell the difference between a theropod track and an ornithopod track is to apply three criteria: (1) look at the length of the foot versus its width; (2) check out the width of its toes; and (3) see whether it has claws that end in points or if they’re a little more blunt. Theropods usually left tracks that are longer than they are wide, with thin toes and sharp claws, whereas ornithopod tracks are typically wider than they are long, with thicker toes and blunt tips. Again, there are exceptions to this generality, and even dinosaur-track experts have doubts about the identity of some three-toed dinosaur tracks, especially if a rival dinosaur-track expert identified them. But with application of these three criteria, their distinction is a little more assured. The only other complicating factor is where theropod and three-toed bird tracks overlap in size and shape, but that’s another story and one far too long to tell here and now.

  Prosauropod tracks are also challenging to identify, partly because they weren’t made for very long, geologically speaking. Prosauropods only lived from the Late Triassic through the Early Jurassic periods, from about 230 to 190 mya. These relatives of sauropods, the largest land animals of all time, developed into the world’s largest herbivores during their geologically brief 40-million-year time span, and were among the first dinosaurs to get around on all fours. However, many prosauropod tracks also show them walking on their rear feet only.

  Sauropod tracks are not so difficult to spot, but are tough to recognize for what they are. 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, though, this oversight was quickly rectified, and sau
ropod tracks magically appeared in the search images of paleontologists worldwide. So where we originally had none, we now frolic in the land of plenty, as sauropod tracks have been found on all continents except for Antarctica, and in rocks ranging from the Late Triassic (230 mya) through the Late Cretaceous periods (65 mya). Other than the extraordinary size of the largest tracks, sauropod footprints are oblong (rear) to crescent-shaped (front), and often form two-by-two diagonal patterns. The best-preserved rear-foot tracks show claws at the end of each digit, too. Despite artistic recreations and skeletal mounts depicting sauropods rearing up on their hind legs, no tracks have yet demonstrated that sauropods did anything more than walk on all fours.

  Stegosaur, ankylosaur, and ceratopsian tracks, like sauropod tracks, were similarly considered rare, but were not identified until just in the past few decades. For example, the first undoubted stegosaur tracks were not found until 1994, in Middle Jurassic (about 170 mya) rocks of England. Now stegosaur tracks are becoming more readily recognized, also having been found in Spain, Portugal, Morocco, and the exotic far-off land of Utah. These tracks have also filled gaps in the known geologic history of stegosaurs, handily augmenting or surpassing their skeletal record in some places. Some of these tracks even include skin impressions, the first known glimpse at the scaly feet of stegosaurs. For ankylosaurs, their tracks weren’t noticed until the 1990s in western Canada. Now their tracks are documented from places as widespread as Bolivia, British Columbia (Canada), Colorado (USA), and elsewhere; oddly, all found thus far are in Early Cretaceous rocks. Ceratopsian tracks, also unknown until the 1990s, are still apparently uncommon, but now that people know what to look for, more of these trace fossils are being discovered each year, too.

  So now you have an overview of the major dinosaur groups, what their tracks look like, and the current state of their record. However, simply answering the question “Who?” should not halt all further inquiry. Here is a small sample of the questions that could be asked of any given dinosaur track:

  How old were these dinosaur trackmakers: hatchlings, juveniles, sub-adults, or adults?

  Can we tell dinosaur genders from their tracks?

  How did dinosaurs move: did they ever do anything more than just walk, such as lope, trot, or gallop?

  Did dinosaurs ever stop to take a break from their daily activities and sit down?

  What did dinosaurs do when encountering a body of water: did they walk around it, or swim across it?

  How about their social lives: were some dinosaurs “rugged individualists” who shunned the company of others, or did they seek out and travel with their own kind, whether in small or large groups?

  What about one-on-one encounters, such as those between predatory dinosaurs and their prey?

  Can we even discern a given dinosaur’s medical history, that is, did it have some injury or other affliction that modified its behavior enough that we can notice its effects?

  On a much grander scale, what do dinosaur tracks tell us about the timing of their origins or demise?

  As you can see from these questions, just identifying what dinosaur made a track is actually a very small part of understanding how dinosaurs behaved. So with this humbling thought in mind, let’s go on to those most exciting facets of divining dinosaurs’ lives from their tracks, starting with their evolutionary origins.

  First Steps of the Dinosaurs: Origination

  When were the first dinosaur tracks pressed fresh into the ground? Naturally, the answer to this question also depends on when the first dinosaur existed, a difficult problem to address. It’s like trying to answer the question “When did we first become human?” But the dinosaur-track one has the decided advantage of lacking all of the anthropocentric baggage accompanying the latter inquiry.

  The current claim for “oldest dinosaur from the fossil record,” a label guaranteed to cause a fight among dinosaur enthusiasts, lies with Eodromeus (“dawn runner”). This dinosaur, which was discovered in Late Triassic (230 mya) rocks of Argentina, was a small bipedal theropod that weighed about the same as a big turkey. Despite its antiquity, Eodromeus is nicely preserved, with about 90% of its skeleton known, including its hind limbs.

  From these bones, we know it had four toes on its rear feet, and three of those toes would have likely left impressions as it walked. Thus we can use its feet as a predictor for what its tracks looked like, or those of its close kin. Other dinosaur fossils from rocks of nearly the same age in Argentina include one other theropod, Herrerasaurus, a basal sauropodomorph, Eoraptor, and a primitive prosauropod, Panphagia. Although Eoraptor was chicken-sized and Herrerasaurus was more like a two-legged German shepherd, they both had three prominent toes on their rear feet, with four total. Unfortunately, the skeleton of Panphagia did not include its foot bones, so we don’t know for sure the forms of its tracks or those of its relatives at the time.

  Using this combination of skeletal data and knowing that evolution often preceded the oldest preserved fossils, paleontologists figure that dinosaurs actually originated at around 235 mya, toward the end of the Middle Triassic Period. Sure enough, three- and four-toed tracks similar to those predicted for primitive dinosaurs are fairly common in some Middle Triassic rocks. Because these tracks were so similar to those of known dinosaur tracks, their discoverers excitedly pronounced them as “dinosaur-like” and hinted that these footprints extended dinosaur lineages to well before their skeletal record. Unfortunately, such claims were thoroughly trounced, flogged, ridiculed, and otherwise treated as unworthy of any encouragement whatsoever. The bulk of this disdain, of course, came from paleontologists who studied dinosaur bones, not tracks. As a result, advocates of trace fossil evidence for dinosaur ancestry were stymied, as they also somehow had to connect tracks to feet foretold—but not yet found—for animals that heralded the arrival of true dinosaurs in the Late Triassic Period.

  Some of this disrespect for all things ichnological was allayed in 2010 when a team of paleontologists, led by Stephen Brussatte, published a paper in which they proposed the oldest “dinosauromorph” tracks from the fossil record. Dinosauromorph refers to the clade Dinosauromorpha, which includes all animals more closely related to dinosaurs than other non-dinosaurs, such as pterosaurs and crocodilians. This means that ancestral dinosauromorph tracks almost look like dinosaur tracks, but not quite: sort of how a primitive human’s tracks would differ from those of a same-sized modern human. Some of these tracks, which were from Early Triassic (about 245 mya) rocks in Poland, precede Eoraptor by more than 15 million years. These paleontologists also pointed out, somewhat indignantly, that “… footprints are often ignored or largely dismissed by workers focusing on body fossils, and are rarely marshaled as evidence in macroevolutionary studies of the dinosaur radiation.” Yes, indeed. Knowing they would face resistance from their body-fossil-focused brethren, they carefully linked the tracks to likely anatomical features of dinosauromorphs. They also pointed out that the oldest probable dinosaur tracks were from the end of the Middle Triassic Period, about 235 mya, which were geologically younger than their tracks, but still before the earliest dinosaur body fossils by about 5 to 7 million years. Dinosaurs later became much more abundant and diverse by the end of the Triassic Period, at about 200 mya. Accordingly, their tracks were common and varied by then, too, and became much larger. Within only about 50 million years, dinosaurs began making the largest footprints of any animals that ever lived.

  These Triassic dinosauromorph tracks point toward what paleontologists call, intriguingly enough, a ghost lineage. A ghost lineage is one for which we have evidence that ancestral members of a clade and their descendants lived at a certain time in the geologic past, but we so far lack corporeal evidence for its existence. Given these dinosauromorph tracks, somehow this phrase seems even more appropriate as we consider these ethereal, disembodied traces as evidence of the first proto-dinosaurs.

  Four Legs Good, Two Legs Better, or Does It Matter? Trackway Patterns and Dinosaur Gaits


  Thus armed with all of this knowledge about dinosaur tracks and how they record when dinosaurs first evolved, it’s tempting to think that you can just identify a dinosaur by looking at one track, state confidently “theropod,” “ornithopod,” “sauropod,” or whatever other dinosaur clade you think it belongs to, identify the geologic age of the rocks hosting this track, take a photograph, and be done with it. But that would be a sad state of affairs, utterly lacking a sense of adventure and curiosity, living in a bland world filled with beige tones and unseasoned instant grits. In other words, you don’t want to do that.

  Instead, you want to know more about how this theropod, ornithopod, sauropod, prosauropod, stegosaur, ankylosaur, or ceratopsian behaved. What was it doing in the place where it left its tracks? When did it arrive on the scene relative to other dinosaurs, insects, or worms living in the same area? Where did it go after it made the tracks? Could its body be nearby, or did it travel a long way before dying? Was it with any others of its species, or looking for love in all the wrong places? How long were these tracks there before they were buried and preserved for us to see them millions of years later? You want to know more. Much, much more.

  To understand dinosaur behavior from their tracks, one must absolutely study sequences of tracks, or trackways. Knowing that most dinosaurs either got around quadrupedally or bipedally, trackways therefore can be expected to show right–left rear foot impressions or a combination of all four feet. However, a few dinosaurs mixed it up, switching from bipedal to quadrupedal and back again, just like how someone can go from walking upright while filled with pride to crawling on hands and knees begging for forgiveness to walking tall again. In a dinosaurian sense, though, a change from a four-legged to a two-legged gait meant that a dinosaur was facultatively bipedal (became bipedal when it wanted) and a normally two-legged dinosaur going on all fours was—you guessed it—facultatively quadrupedal. These changes in which limbs touched the ground were likely related to dinosaurs altering their speed, foraging, or other such behavioral shifts necessitated by daily life.