ZoomDinosaurs.com Dinosaur and Paleontology Dictionary

THAGOMIZER Thagomizers are the pairs of long, sideways-pointing tail spikes located at the end of Stegosaurus' tail. The word thagomizer is taken from a Far Side cartoon (written by Gary Larson) in which a caveman character is explaining in a lecture that the spiky tail of a Stegosaurus is called the thagomizer, named for the late Thag Simmons.

THALASSOMEDON Thalassomedon (meaning "lord of the seas") was a plesiosauroid plesiosaur (not a dinosaur, but an extinct marine reptile from the Mesozoic Era that lived in the open oceans and breathed air). It had a long neck (with 63 vertebrae), a long snout, long, sharp teeth (up to 5 cm long), a short, pointed tail and four flippers. This reptile was about 40 feet (12 m) long; its skull was about 18.7 inches (47 cm) long. Fossils have been found in the USA. The type species, Thalassomedon haningtoni (named by Welles in 1943), is pictured above at the Denver Museum of Nature and Science (photo courtesy of Jim Puckett); this late Cretaceous (95 my) specimen was found in 1939, in Graneros shale in Baca County, Colorado, USA.

THALATTOSAURS Thalattosaurs (meaning "sea lizards"), also known as Thallatosaurians, were large lizard-like marine reptiles (roughly 6-13 ft = 2-4 m long) from the middle to late Triassic period, over 200 million years ago. These primitive diapsids (neodiapsids and primitive archosauromorphs) were distantly related to the dinosaurs. They had a long body, a long tail and four short limbs (that may have had webbed feet, were not paddle-like). The nostrils were towards the middle of the snout, not at the tip. Fossils of this obscure reptile have been found in western North America and Italy.

THALATTOSUCHIAN Thalattosuchians (meaning "sea crocodiles"), were large marine crocodilians that lived from the early Jurasic period to the early Cretaceous period. They were well-adapted to sea life, having four padle-like limbs, a long tail with a tail fin and a long snout. It probably ate fish. Geosaurus was a thalattosuchian.

THECODONT Thecodonts were socket-toothed reptiles, archosaurs that lived dring the Triassic period. They may have been the ancestors of dinosaurs, birds, pterosaurs, and crocodilians. Chasmosaurus, pictured above, was a primitive thecodont that lived during the early Triassic period, about 250 million years ago.

THECODONTOSAURUS (pronounced THEE-co-DONT-oh-SAWR-us) Thecodontosaurus (meaning "socket-toothed lizard") was a very early dinosaur from the late Triassic period. Thecodontosaurus ate plants and was about 7 feet (2.1 m) long. It had a small head, large thumb claws, long legs, a relatively short neck, short arms than legs, and a long tail. It could probably walk on two or four legs, perhaps grazing and walking on all fours, but running on two legs. It had blunt teeth with serrated edges. It had four toes on each leg and five fingers on each hand. Fossils have been found in Britain, which was probably dry and desert-like when Thecodontosaurus lived. Thecodontosaurus is classified as a saurischian (a "lizard-hipped" dinosaur), a sauropodomorpha (usually quadrupedal herbivores), a prosauropod (an early, dead-end branch of the sauropodamorphas), and an Anchisaurid (the earliest prosauropods).

THEORY A theory is a proposed explanation for a phenomenon.

THERAPSID Therapsids are a clade of advanced synapsid animals from the late Permian period. They had teeth that were differentiated into post-canines and incisors. Dominant among the early therapsids were the large dinocephalians. The clade of therapsids includes the mammals, some close relatives, and their recent common ancestors. Some therapsids include the Eotitanosuchids, herbivorous dicynodonts (like Boreogomphodon, Lystrosaurus, Kannemeyeria, Estemmenosuchus, and Ischigualastia), and the cynodonts (like Cynognathus, Probainognathus, and Thrinaxodon).

THERIZINOSAUR (pronounced THER-uh-ZEEN-oh-sawrs) Therizinosaurs (meaning "scythe lizards") were a family of strange theropod dinosaurs that had toothless beaks and four toes on each foot. Some Therizinosaurs include Beipiaosaurus (the most primitive therizinosaur), Therizinosaurus, Segnosaurus, Nanshiungosaurus, Erlikosaurus, Enigmosaurus, Alxasaurus, and others.

THERIZINOSAURUS (pronounced THER-uh-ZEEN-oh-SAWR-us) Therizinosaurus (meaning "scythe lizard") was a huge, unusual theropod dinosaur known only from fossilized arms with three enormous, sickle-like claws found in the southern Gobi Desert in 1948. When it was found, it was thought to be a giant turtle, not a dinosaur. Its classification is uncertain. The huge arms were 8 feet (2.45 m) long and the claws were 2 feet (60 cm) long. Some paleontologists have speculated that these claws may have been used to rip open giant termite nests, making Therizinosaurus an insectivore (an insect-eater), but most think that it was an herbivore. Therizinosaurus lived during the late Cretaceous period (roughly 77-69 million years ago). Therizinosaurus was named by the Russian paleontologist E. A. Maleev in 1954.

THEROPOD (pronounced THER-uh-pod) Theropods comprise a suborder of saurischian dinosaurs. They were fast-moving, bipedal carnivores (meat-eaters) with grasping hands and clawed digits. Theropods lived from the mid-Triassic period until the end of the Cretaceous period. Theropod means "beast-footed." The term theropod was first used in 1881.

THEROSAURUS Therosaurus is an invalid name for Iguanodon, a plant-eating dinosaur with thumb spikes. It was about 33 feet (10 m) long and lived during the early Cretaceous period, roughly 130 to 110 million years ago.

THESCELOSAURUS (pronounced THES-ke-loh-SAWR-us) Thescelosaurus (meaning "Marvelous lizard") was a bipedal, plant-eating dinosaur whose fossilized, four-chambered heart was found, perhaps indicating that it was warm-blooded. It dates from the late Cretaceous period, about 77 to 65 million years ago. It had a small head, a bulky body, a long, pointed tail and short arms. It was a thescelosaurid or hypsilophodontid, and an ornithischian dinosaur about 12 feet (3.5 m) long and 3 feet (0.9 m) tall at the hips. Thescelosaurus could probably run at about 30 mph (50 km/hr) for an extended time. Fossils of Thescelosaurus, including skin impressions, have been found in western North America. Thescelosaurus was named by paleontologist Gilmore in 1913. The type species is T. neglectus.

THESPESIUS (pronounced thes-PEES-ee-us) Thespesius (meaning "mighty one") was a plant-eating dinosaur. This hadrosaur (duck-billed dinosaur) lived during the late Cretaceous period, roughly 65 million years ago. Fossil were found in Nebraska and South Dakota, USA, North America. Only a few vertebrae (back bones) and a foot bone were found. The type species is T. occidentalis; it was named in 1856 by Joseph Leidy. Thespesius is a nomen dubium.

THRINAXODON (pronounced thrin-AX-ah-DON) Thrinaxodon was a cat-sized cynodont, an ancestor of the mammals, but not a mammal itself. Thrinaxodon was not a dinosaur. This therapsid lived during the late Triassic period, roughly 240 million years ago. Small holes in the snout bones suggest that it may have had whiskers, a mammalian characteristic. Its teeth were also similar to those of mammals. The skull was about 1.5 inches (3.8 cm) long. Fossils have been found in South Africa.

THYLACOLEO (pronounced thy-la-ka-LEE-oh) Thylacoleo (meaning "pouched lion") was an ancient meat-eating, lion-like marsupial from Australia. This pouched, predatory mammal lived from the Late-Oligocene to the Pleistocene, roughly a million year ago. Thylacoleo had a powerful jaws and legs, a muscular tail, and slashing, retractable claws. It had two front teeth (like its kangaroo relatives) plus huge, shearing premolars (for cutting meat efficiently). Fossil of this "marsupial lion" have been found in Australia. Many specimen have been found were found. The type species is T. carnifex; it was named in 1859 by the paleontologist Richard Owen.

THYREOPHORA (pronounced thii-RE-ah-FOR-ah) Thyreophorans (meaning "shield bearers") were armored, plated, and/or spiked ornithischian dinosaurs that lived from the early Jurassic to the late Cretaceous period. These quadrupedal (four-legged) plant-eaters included Scutellosaurus, Scelidosaurus, ankylosaurs and stegosaurs.

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