492 FOSSIL VERTEBRATES/Dinosaurs Figure The diagnostic characters of (A, B) the Ornithischia and (C, D) the Saurischia (A) Left lower jaw of an ornithischian in lateral view, showing predentary bone (red) (B) Pelvis of an ornithischian in left lateral view The shaft of the pubis (red) has rotated backwards so that it lies parallel to the ischium (green) (C) The cervical series of a saurischian in left lateral view The first four postaxial vertebrae (red) are elongate and exceed the axis (green) in length (D) A trunk vertebra of a saurischian in (from left to right) anterior, left lateral, and posterior views Red areas show the accessory articulations in the anterior and pos terior views, while the lateral view shows the large pneumatic fossae separated by thin laminae Figures not to scale development) is unique among digit-bearing vertebrates (Figure 1F) There is a cnemial crest This is a vertical crest of bone that grows out of the anterior face of the proximal end of the tibia, just below the knee joint It supported various lower-leg muscles and seems to be correlated with enhanced running ability No other archosaurs have a cnemial crest, but other terrestrial vertebrates, such as cursorial mammals, (Figure 1G) The astragalus has an ascending process on its anterior edge The astragalus is the larger of the two bones in the proximal row of the tarsus (ankle joint) Dinosaurs have a mesotarsal ankle, where there is a simple straight hinge running between the proximal and distal rows of bones in the tarsus The ascending process is a triangular block of bone that fits into a corresponding notch in the distal tibia, thus preventing any movement in this part of the ankle (Figure 1B) Origin of the Dinosauria The closest known relatives of the dinosaurs are small slender avemetatarsalians from the Middle Triassic of South America (Figure 3A) These were relatively unspecialized and form good models for a dinosaurian precursor Marasuchus is the best represented and most thoroughly studied of these animals It was exceptionally small compared with its dinosaurian cousins, standing no more than 150 mm high at the hip and weighing no more than a few hundred grams It was an agile long-legged carnivorous biped By the end of the Carnian Stage of the Late Triassic we have the first known remains of true dinosaurs They are scattered around the world, but the best specimens come, once again, from South America Though much larger than the earlier avemetatarsalians (Herrerasaurus weighed around 200 kg), the early dinosaurs were neither the largest terrestrial animals of their time nor particularly abundant in terms of numbers of species or individuals Nevertheless, a modest radiation had occurred with carnivores (Eoraptor, Staurikosaurus, and Herrerasaurus), omnivores (Saturnalia), and herbivores (Pisanosaurus) all represented Some palaeontologists have proposed that a small but significant extinction event hit many of the dominant groups of terrestrial vertebrates at the end of the Carnian, leaving many of the large animal niches vacant in the succeeding Norian Stage, but this hypothesis remains controversial For whatever reason, dinosaurs dominated the large-herbivore niches around the globe in the Norian, and smaller carnivorous dinosaurs were common Nevertheless, other tetrapod groups were still significant components of the large terrestrial faunas These groups included the last of the dicynodont synapsids and various non-dinosaurian archosaurs, including the rauisuchians, which were the top predators of the time However, by the beginning of the Jurassic Period these groups had become extinct and the ascendancy of the dinosaurs was complete There may have been another mass extinction at the Triassic– Jurassic boundary, and there is some evidence, from eastern North America, for an extraterrestrial bolide impact at that time Dinosaur Subgroups Ornithischia The Ornithischia are a distinctive group, and there is no doubt that they are clade Even the earliest ornithischians were specialized towards herbivory (although some of the earliest members may well have been somewhat omnivorous), and all subsequent species seem to have been herbivorous Two of the most distinctive derived characters are the arrangement of the pelvic bones (Figure 2B) and the presence of an extra bone, the predentary (Figure 2A), in the lower jaw In ornithischians the symphysis between