Dinosaurs Without Bones Read Online Free PDF Page B

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, lieswith 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