Descriptionary

ichthyosaur Greek term for "fish lizard," a dol­phinlike reptile that lived during the Mesozoic.

iguanodontid from the late Jurassic to the Creta­ceous, very large plant eaters with beaks and thumb spikes, including iguanodon and camptosaurus, that eventually evolved into the hadrosaurs.

iridium layer a heavy element seen in meteorites and found in an unusually high concentration in a widespread geological deposit known as the K-T layer, dated from 65 million years ago. It is believed to have been dispersed around the earth by the aster­oid impact that killed the dinosaurs.

Jurassic period the time period from 208 to 146 million years ago, when many dinosaurs thrived.

K-T extinction short for the Cretaceous-Tertiary extinction that killed off the dinosaurs 65 million years ago.

marginocephalian any Cretaceous plant eater with horns, beak, or thick and bony skull.

megaraptor a 26-foot (8-m) bipedal meat eater characterized by a very long, sicklelike claw on the side of each foot.

mesozoic era a broad expanse of geological time that encompasses the Cretaceous, Jurassic, and Trias- sic, from 251 million to 65 million years ago, when the dinosaurs lived.

microraptor among the smallest of the dinosaurs, a dromaeosaurid that grew from 1.5 to 2.5 feet (0.5­0.8 m) long and sported feathered wings on its arms, tail, and legs and probably glided from tree to tree. Thought to be a possible forerunner of birds, it lived during the early Cretaceous.

mosasaur a marine reptile, growing up to 57 feet (17.5 m) long; living during the Cretaceous, it was the deadliest marine predator of the period.

multiple impact theory the theory held by some scientists that more than one asteroid or chunk of asteroid struck the earth 65 million years ago and killed off the dinosaurs. See Alvarez extinction theory, Chicxulub, Shiva.

omnivore a dinosaur that ate both plants and meat.

ornithischian a family of plant-eating dinosaurs with hip structures similar to birds, including tricer- atops, stegosaurus, ankylosaurs, and others. They are not, however, the ancestors of birds.

ornithopod from the Jurassic to the Cretaceous, any two- or four-legged plant eater without body armor.

oviraptor a theropod with a beaked, birdlike head and long fingers.

pachycephalosaurid a bipedal plant eater with an extraordinarily thick or bony head that lived in the Cretaceous period. Some, like pachycephalosaurus, had skulls 10 inches (25 cm) thick. Also known as a bonehead.

paleontologist a scientist who gathers fossils in the field and studies them to learn more about living organ­isms, such as dinosaurs, from past geological periods.

paleontology the study of living organisms and fos­sils from past geological periods.

parasaurolophus a beaked or duck-billed plant eater with a long crest that lived during the Creta­ceous.

pterodactyl see pterosaur.

pterosaur closely related to the dinosaurs, a group of flying, fish-eating reptiles that lived from the Trias- sic to the end of the Jurassic period. Hollow-boned, with membranelike wings, they ranged in size from a few inches to as large as 40 feet (12 m). Pterodactyls are probably the best-known pterosaurs.

quetzalcoatlus a reptile closely related to the dino­saurs; a massive pterosaur with a 40-foot (12-m) wingspan that lived during the Cretaceous.

saurischian an order of dinosaurs that were the ances­tors of birds, but with hip structures similar to lizards. Sauropods and theropods were both saurischians.

sauropod any large, four-legged plant eater with a long neck and tail, ranging in size from 7 feet (2 m) to more than 100 feet (30 m) long.

scute a bony or horny plate or scale, as found on the skin of crocodiles, which many dinosaurs may have had.

seismosaurus "quake lizard," a massive diplodocid dinosaur that grew as long as 170 feet (52 m) and lived during the late Jurassic. It had an extremely long neck, which it used to peer into and forage along the edges of thick woods, especially useful when the rest of its body was simply too big to pass through.

shantungosaurus the largest of all the duck-billed dinosaurs, or hadrosaurs, growing up to 48 feet (15 m) in length. It lived during the late Cretaceous.

Shiva located under the Arabian Sea off India, a massive crater, stretching 370 miles (600 km) across, 280 miles (450 km) wide, and 7.5 miles (12 km) deep, possibly created by an asteroid or meteoroid 65 million years ago, which may have, along with the impact at Chicxulub in the Yucatan, brought on the extinction of the dinosaurs. See Alvarez extinction theory, Chicxulub, multiple impact theory.