Dinosaurs Without Bones Read Online Free PDF Page A

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 movementand 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 themost astonishing recently found in Cretaceous rocks of Alaska. These four-toed feet mayrepresent 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