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(pronounced dah-sen-TROO-rus ) Dacentrus (meaning: "very sharp point tail") was an early stegosaurid dinosaur, a well-protected plant-eater that was about 15 feet (4.4 m) long. It had two rows of asymmetrical plates on its back and two rows of long spikes (with sharp edges) running along its lower back and tail. The longest spike found is 1.5 feet (45 cm) long. Like other stegossaurids, it had a small head, a bulky body, a long tail, and short legs. This quadruped ornithischian lived in what is now Europe; incomplete fossils have been found in England, Portugal, and France (D. lennieri). An egg that may have belonged to Dacentrus was found in Alfeizerao, Portugal. Dacentrus lived during the late Jurassic Period, 163-150 million years ago. Dacentrus was named by Lucas in 1902. The type species is D. armatus. Some Dacentrus specimens were originally known as Omosaurus.

(pronounced duh-CHONG-ah-SAWR-us ) Dachongosaurus was a large, primitive sauropodomorph dinosaur. It had a small head, a bulky body, and a long tail. It walked on two or four legs. Incomplete fossils (only vertebrae, limbs, and ribs) were found in China. Dachongosaurus lived during the Jurassic Period, about 208-145 million years ago. Dachongosaurus was named by Chinese paleontologist Chao in 1985. The type species is D. yunnanensis. Dachongosaurus is a nomen nudium, a genus that has not yet been formally described.

(pronounced DAM-ah-lah-SAWR-us ) Damalasaurus was a large, primitive sauropod dinosaur. It had a small head, a bulky body, a long tail, and four short legs. Fossils were found in China. Damalasaurus lived during the middle Jurassic Period, about 188-169 million years ago. Damalasaurus was named by Chinese paleontologist Zhao Xijin in 1983. The type species is D. magnus. Damalasaurus is a nomen nudium, a genus that has not yet been formally described.

Damsel Fly
Delicate, weak, flying insects resembling a small dragon fly. They commonly fly in tandem when mating. The eggs are laid in water and the young have gill-like structures to breathe.

(pronounced dan-DAK-oh-SAWR-us ) Dandakosaurus was a large theropod dinosaur. This bipedal meat-eater was about 10 feet (3 m) long. Fossils were found in Andrak Pradesh, India. Dandakosaurus lived during the early Jurassic Period, about 213-188 million years ago. Dandakosaurus was named by paleontologist Yadagiri in 1982. The type species is D. indicus. Dandakosaurus is a nomen nudium, a genus that has not yet been formally described.

Charles Robert Darwin (February 12, 1809-April 19, 1882) was an English naturalist who revolutionized scientific thought with the theory of evolution and natural selection. After studying at Cambridge University, Darwin served as naturalist on the ship HMS Beagle (captained by Robert Fitzroy) during its five-year voyage around the world traveling west ( December 27, 1831-October 2, 1836). On this trip, Darwin studied a tremendous variety of plant and animal life. Darwin's book called "On the Origin of the Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle of Life" was published in November 24, 1859 (and sold out in one day). This world-shaking book outlined the gradual change in a species from generation to generation through natural selection. Alfred Lord Wallace independently proposed the theory of evolution the same time as Darwin. In 1871, Darwin published "The Descent of Man," which focused on the origins of people. The theory of evolution has been debated since it was proposed, but most scientists accept evolution and natural selection as the method of species formation and the foundation of biology.

(pronounced das-PLEET-oh-SAWR-us) Daspletosaurus (meaning: "frightful lizard") was a tyrannosaurid theropod dinosaur that was 26-33 feet (8-10 m) long and weighed perhaps 2-3.5 tons (2300 kg). It had tiny horns behind its eyes and its arms were a bit longer than those of Tyrannosaurus rex. It may have been an ancestor of Tyrannosaurus rex. This bipedal meat-eater lived in marshes by streams in what is now Alberta, Canada, during the late Cretaceous Period, 76-72 million years ago. It may have preyed upon Triceratops. It was named by D. A. Russell in 1970. The type species is D. torosus.

Dating a fossil is determining when that organism was alive. Paleontologists use many ways of dating individual fossils in geologic time, including stratigraphy, observations of the fluctuations of the Earth's magnetic field, radioisotope-dating, and looking at nearby index fossils.

(pronounced DAH-toe-SAWR-us) Datousaurus was a long-necked, long-tailed, quadrupedal, plant-eating dinosaur from the middle Jurassic Period, about 170 million years ago. This solidly-built sauropod was over 50 ft (15 m) long. Its teeth were spoon shaped, and it had a very solid skull (unlike many sauropods). It had 5 toes on each of its four feet. Datousaurus may be a Cetiosaurid sauropod. Fossils have been found in China. The type species is D. bashanensis. Datousaurus was named by Dong and Tang in 1984.

The Death Star Theory refers to the fact that mass extinctions are periodic, and may be caused by the Earth's passing through a cloud of comets (the Oort cloud) every 26 million years. Some people have hypothesized that there is a yet-to-be-discovered dark star or perhaps a planet (called, appropriately enough, Nemesis) orbiting in the outer reaches of our solar system. This body disrupts the Oort cloud (once every 26 million years), sending comets into the inner parts of the solar system, some of which hit Earth and cause mass extinctions.

(pronounced DINE-oh-KIE-rus) Deinocheirus (meaning: "terrible hand") was a large, long-legged, bipedal, meat-eating, big-eyed, bird-like dinosaur from the late Cretaceous period, about 70 million years ago. This Coelurosaurid Ornithomimosaur (ostrich-mimic) theropod was among the fastest of the dinosaurs, perhaps runing at 40-50 mph. Deinocheirus was about 23-38 ft (7-12 m) long, weighing reoughly 9000 kg. It had a toothless, beaked mouth. An incomplete fossil was found in Mongolia. Only two huge arm bones were found. These arms were 8 ft (2.5m) long and had 10-inch (25.5-cm) long claws. The type species is D. mirificus. Deinocheirus was named by Osmólska and Roniewicz in 1970.

(pronounced DINE-oh-don ) Deinodon (meaning: "terrible tooth") was a meat-eating dinosaur from the Late Cretaceous Period. This Coelurosaurid theropod is only known from a dozen large, fossilized teeth collected by Dr. F. V. Hayden by the Judith River in Montana, USA. Deinodon was named by paleontologist J. Leidy in 1856. Deinodon is a dubious genus [nomen dubium]. It may be the same genus as Gorgosaurus or Albertosaurus.

Deinogalerix (meaning: "terrible hedgehog") was an ancient hedgehog that lived during the middle Mocene period (about 15 million years ago). This insect-eater had hair on its body, unlike modern hedgehogs, which have modified hairs that form spines. Deinogalerix is the largest-known hedgehog; it was about 2 feet (60 cm) long and had jaws about 8 inches (20 cm) long. This long-extinct mammal had a long snout, sharp teeth, short legs, and a long tail. Fossils have been found in caves in Italy. Deinogalerix was named by M. Freudenthal in 1973.

Deinonychosaurs (also called "raptors") were advanced theropod dinosaurs. These fierce predators had a long, sharp, sickle-shaped claw on each foot (on the second toe). Some of the Deinonychosauria included Deinonychus, Utahraptor, Velociraptor, Pyroraptor, Saurornitholestes, etc.

A lightly-built, fast-running, theropod dinosaur about 3 metres long, with blade-like teeth and grasping hands. It had a tail that was stiffened with rod-like structures, that it used for balance, and an enormous sickle-shaped claw on the second toe of each foot. Like Utahraptor it was a dromaeosaur that lived in the Lower Cretaceous period.