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Aachenosaurus is a fossil that was originally thought to be jaw fragments from a duckbilled dinosaur (a hadrosaur), but the fossil turned out to be petrified wood. Aachenosaurus was named by the scientist Abbey G. Smets in 1888 (the type species was called A. multidens). The fossil's name means "Aachen lizard," named for the Aachenian deposits of Moresnet (which was a territory in Belgium) where the fossils were found.

Abavornis (which means ""great-great-grandfather bird") was a primitive bird that lived during the late Cretaceous period, about 85 million years ago. Fossils were found in the Bissekty Formation, Uzbekistan. The type species is A. bonaparti. Abavornis was named by Panteleev in 1998.

(pronounced ay-bel-uh-SAWR-us) Abelisaurus (meaning "Abel's lizard") was a primitive theropod (a bipedal, meat-eating dinosaur) that was roughly 21 to 26 feet (6.5 to 7.9 m) long, weighing roughly 1500 kg. It lived during the late Cretaceous period, 75-70 million years ago, in what is now Patagonia, Argentina. It is known from a 35 inch (90 cm) long incomplete skull named in honor of Roberto Abel (director of the Argentinian Museum of Natural Science) and was named by paleontologists J. F. Bonaparte and F. E. Novas in 1985. The type species is A. comahuensis.

(pronounced uh-BRICK-tuh-SAWR-us) Abrictosaurus (meaning "awake lizard") was a heterodontosaurid (an ornithischian), a bipedal, plant-eating, long-tailed dinosaur that was roughly 4.6 feet (1.2 m) long and weighed about 43 kg. It had high-crowned teeth and lived during the early Jurassic period. Unlike other heterodontosaurids, it lacked canine-like teeth on its lower jaw. It is known from a skeleton and skull found in Lesotho, S. Africa and was named by paleontologist J. A. Hopson in 1975.

(pronounced AB-roh-SAWR-us) Abrosaurus (meaning "delicate or gentle [skull] lizard") was a sauropod, a long-necked, plant-eating dinosaur. It had a bulky body, column-like legs, and a small head with a nasal crest; the nostrils were almost above the eyes. It lived during the early Cretaceous period. Abrosaurus was named by paleontologist Ou in 1986. It is known from fossils found in China. The type species is A. dongpoi. Very little is known about this dinosaur.

Absolute age dating seeks to determine the exact time that an organism lived (or another event occurred). Compare with relative age dating.

Acanthodians were the earliest jawed vertebrates. These early fish (Class acanthodii) lived from the Ordovician to the Carboniferous period. Although most Acanthodians were small, averaging roughly 5-6 inches (13-15 cm) long, some were much larger (for example, the genus Xylacanthus, known from its huge jaws, is thought to have been perhaps 3 feet (1 m) long). Some Acanthodians may have been primitive shark-like fish.

(pronounced uh-CAN-thuh-FOE-lis) Acanthopholis (meaning "spiny scales or scutes") was an armored, quadrupedal, plant-eating Ankylosaur dinosaur from the early Cretaceous period (115 million-91 million years ago). Its armor was rows of oval plates set into its skin, plus it had 9 inch (24 cm) spikes jutting out of its neck and shoulder area along the spine. It was about 15 feet long (4 m) and weighed roughly 380 kg. The type species is A. horridus (but Acanthopholis is a nomen dubium - a dubious species).

Acanthostega (which means "spine plate") was an early tetrapod that lived during the Devonian period (roughly 360 million years ago). This aquatic animal was about 60 cm (2 ft) long (includign its long tail). Its front limbs (and perhaps the rear limbs also) had 8 digits; it had internal gills and lungs. Protective oval-shaped scutes protected its belly. Fossils of this river-dweller have been found in East Greenland. The type species is Acanthostega gunneri, found in 1933 and named in 1952 by Erik Jarvik. Additional specimens were found in 1970 (again in East Greenland) by Jennie Clack and Per Ahlberg.

The acetabulum is the hip socket.

(pronounced ah-key-LOH-uh-SAWR-us) Achelousaurus, aka Achelosaurus (meaning "Achelous' [a river god who lost his horns] lizard") was a Ceratopsid dinosaur from the late Cretaceous period. This hornless, frilled, quadrupedal plant-eater was about 20 ft (6 m) long. It had a nasal boss (a large, rounded knob of bone) on its snout and had two long spikes on the rear of its frill. Achelosaurus seems to be an intermediate between Einiosaurus (which had a similar frill) and Pachyrhinosaurus (which had a similar nasal boss). A partial skeleton was found in Montana, USA. The type species is A. horneri. Achelousaurus was named by Sampson in 1995.

Acid rain is polluted and harmful to the environment. Acid rain has a low pH. Acid rain may have been a component of the K-T extinction.

Acrocanthosaurus is known from incomplete skeletons and teeth. This large carnivorous dinosaur walked on two long, powerful legs. Acrocanthosaurus is famous for the tall spines on its back, and tail. The spines on some large individuals were as much as 2 feet (60 centimeters) tall, which gave this dinosaur a powerful looking profile. The tall vertebral spines on the neck, back and tail, served as attachment points for muscles. And may have been useful in grabbing, holding, or dismembering its prey.