How to Build a Dinosaur Read Online Free PDF Page B

particular fossil and say, here, this is the ancestor of the dinosaurs. In fact, evolutionary biologists have changed how they group animals and categorize descent from ancestral forms, since the familiar and easily understood tree-of-life diagram was developed to grace textbooks and magazine articles. Biologists no longer look for “the” ancestor. Instead they concentrate on shared characteristics that define one group and the new characteristics that appear in the course of evolution to define a new group.
    These new traits are called derived characteristics. And the groups are called clades. A diagram of the course of evolution using clades is a cladogram and it is somewhat similar to the old tree. Single-celled organisms are at the start of things, fish appear before amphibians, and both before reptiles. Humans, of course, branch out very late from our mammalian and primate origins.
    But cladograms don’t pin down ancestry to one species or genus. A cladogram shows, for example, that from the vertebrates new clades have evolved that have all the shared characteristics of vertebrates plus some new, derived characteristics. For instance, all vertebrates have a backbone. A clade that evolved from the vertebrates, like the mammals, shares the backbone and vertebrate body plan, with eyes and mouth at the front and a digestive system that goes from front to back, among other details.

    Jack Horner looks over eroded badlands near Jordan, Montana. The boundary that marks the end of the nonavian dinosaurs and the top of the Hell Creek Formation is a dark line about two-thirds of the way up the hill, just left of center.
    But the mammals have derived characteristics like mammary glands and fur that we don’t share with other vertebrates. By looking at more detailed skeletal characters, we can see that the ancestors of the mammals branched off from the reptiles before the evolution of the dinosaurs. In fact, the mammals and the dinosaurs appeared around the same time.
    Scientists agree that all the dinosaurs come from one ancestral source. The dinosaurs themselves are in two groups, the Ornithischia and Saurischia, based on the structure of their hips, but these groups have a common dinosaur ancestor. We can’t say what that specific ancestor is, or what creature immediately preceded the dinosaurs, but we do think it was something like Lagosuchus, a reptile that was less than a meter long and walked on its two hind legs. The first dinosaurs were also bipedal, and the four-legged stance of familiar animals like Triceratops and Brontosaurus (Apatosaurus) evolved later.
    A mass extinction killed the dinosaurs and it may have been a mass extinction that gave them their start. There were two extinctions in the Triassic, one around 245 million years ago, perhaps caused by an asteroid hitting the earth, and one 205 million years ago. After the first, many of the ancestors of mammals and dinosaurs disappeared, leaving some good opportunities. The dinosaurs took over. The mammals stayed in the background for 145 million years until the next mass extinction did away with the dinosaurs and once again offered abundant unfilled ecological niches.
    The time of the dinosaurs’ origin was a good one for land animals in the sense that all the modern-day continents were united into one landmass called Pangaea. These continents had previously shifted and drifted apart in various combinations, and once before had united in a supercontinent called Gondwanaland. That broke up, and so, eventually, did Pangaea. The continents continued to drift as the dinosaurs evolved.
    Dinosaur species appeared and disappeared over the next 140 million years, taking on the numerous forms that fossils have preserved, the gigantic sauropods, like Apatosaurus, carnivores like Allosaurus and T. rex, the plant eating duck-billed dinosaurs, small colonial nesters such as Protoceratops, agile small hunters like Deinonychus and Velociraptor . Along the way, birds