490 FOSSIL VERTEBRATES/Dinosaurs Olson EC (1991) An eryopoid (Amphibia: Labyrinthodon tia) from the Fort Sill fissures, Lower Permian, Okla homa Journal of Vertebrate Paleontology 11: 130 132 Reisz RR (1977) Petrolacosaurus, the oldest known diapsid reptile Science 196: 1091 1093 Reisz RR (1997) The origin and early evolutionary history of amniotes Trends in Ecology and Evolution 12: 218 222 Reisz RR, Heaton MJ, and Pynn BR (1982) Vertebrate fauna of Late Pennsylvanian Rock Lake Shale near Gar nett, Kansas: Pelycosauria Journal of Paleontology 56: 741 750 Reisz R (1972) Pelycosaurian reptiles from the Middle Pennsylvanian of North America Bulletin of MCZ 144: 27 61 Reisz RR (1986) Pelycosauria vol 17 A Handbuch der Palaă oherpetologie Wellenhofer P (ed.) Stuttgart, New York: Gustav Fischer Verlag, 102 Reisz RR, Berman DS, and Scott D (1992) The cranial anatomy and relationships of Secodontosaurus, an un usual mammal like reptile (Synpasida: Sphenacodonti dae) from the early Permian of Texas Zoological Journal of the Linnean Society 104: 127 184 Reisz RR and Modesto SP (1996) Archerpeton anthracos from the Joggins Formation of Nova Scotia: a microsaur, not a reptile Canadian Journal of Earth Sciences 33: 703 709 Romer AS and Price LW (1940) Review of the Pelycosauria Geologica Society of America Special Papers 28: 538 Dinosaurs A M Yates, University of the Witwatersrand, Johannesburg, South Africa ß 2005, Published by Elsevier Ltd Introduction Dinosaurs were the dominant terrestrial vertebrates of the Mesozoic Era All large terrestrial animals from the end of the Norian Stage (Late Triassic) to the close of the Cretaceous were dinosaurs This is not to say that any large Mesozoic animal is automatically referred to as a dinosaur Membership of the group can be diagnosed by the presence of numerous evolutionary novelties (Figure 1), which indicate that dinosaurs are a natural evolutionary group or clade On the evolutionary tree of terrestrial vertebrates, dinosaurs nest deeply within the Reptilia Within the reptiles they belong to the Diapsida (reptiles with two openings, or fenestrae, for jaw muscles in the temporal regions of their skulls), within the diapsids they belong to the Archosauria (diapsids with an additional fenestra in the skull behind the nostril and in front of the orbit and that lay calcite-shelled eggs), and within the archosaurs they belong to the Avemetatarsalia (archosaurs with a simple hinge-like, or mesotarsal, ankle joint and elongate bunched metatarsal bones in the foot) The Dinosauria contains two subgroups (Figure 2), the Ornithischia (‘bird-hipped’ dinosaurs) and the Saurischia (‘lizard-hipped’ dinosaurs) These subgroups were long thought to have evolved from different reptiles, but modern cladistic work has shown that they almost certainly form a clade that excludes all other archosaurian groups Diagnostic Characters of the Dinosauria A great number of derived characteristics have been proposed for the diagnosis of the Dinosauria However, many have since been shown to be present in other archosaurs (e.g an s-shaped neck) or to be independent acquisitions in different dinosaur groups (e.g three or more sacral vertebrae) The following list includes the characters that have been well documented and are likely to remain diagnostic of the dinosaur clade Loss of the postfrontal bone from the skull roof In other archosaurs there is a postfrontal bone between the frontal and postorbital bones A contribution of the frontal bone to the depressed area that surrounds the upper temporal fenestra In life this fossa would have served as an attachment site for jaw closing muscles In other archosaurs the frontal bone is excluded from this fossa Loss of the lateral sheet of the squamosal bone that covers the dorsal head of the quadrate bone The dorsal end of the reptilian quadrate bone (which forms the upper part of the jaw joint) forms a smooth, rounded head that fits into a socket under the squamosal bone In other archosaurs a lateral sheet of the squamosal bone obscures the quadrate head in lateral view but in dinosaurs the quadrate head is plainly visible The sternum consists of paired ossified plates In early tetrapods the sternum was composed of a single median interclavicle situated behind the paired clavicles (‘collar bones’ in humans) The interclavicle was lost early in the history of archosaurs, while the clavicles were strongly reduced in size