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Background extinctions are those extinctions that occur continually throughout time. These extinctions are caused by small changes in climate or habitat, depleted resources, competition, and other changes that require adaptation and flexibility. Most extinctions (perhaps up to 95 per cent of all extinctions) occur as background extinctions.

Bacteria are one-celled, microscopic organisms that live all over the world. They are important in the decay of organic material and in the fixing of nitrogen.

(pronounced BACK-tra-SAWR-us) Bactrosaurus or Bactrasaurus (which means 'club-spined lizard') was a large, plant-eating, duck-billed dinosaur (a lambeosaurid hadrosaurine), up to about 20 feet (6 m) long, weighing roughly 1500 kg. Its femur (thigh bone was 80 cm long. About six fragmentary Bactrosaurus fossils were found in Mongolia and China. It lived during the late-Cretaceous Period, about 97-85 million years ago and was named by C.W. Gilmore in 1933. The type species is B. johnsoni.

Baculites (which means "walking stick rock") was a genus of ammonite that lived during the late Cretaceous Period. It was heteromorphic: in its juvenile stages it had a coiled shell; as an adult, it was straight-shelled, tube-like, and about 2 m long (but some later forms had a cork-screw shape). Baculites was a wide-spread marine cephalopod mollusk that may have lived in colonies on the sea floor. Baculites species are very widespread, so they are used as an index fossil. Baculites was named by the French paleontologist Alcide Dessalines d'Orbigny in 1850.

Badlands are barren, severely eroded places on Earth where the soft rock layers are sculpted into beautiful forms. These exposed rock layers are often wonderful places in which to find fossils. They're called badlands because the land is useless for farming and many other human purposes.

(pronounced BAG-uh-CER-uh-TOPS) Bagaceratops (meaning: "small horned face") was a ceratopsian dinosaur about 3 feet (1 m) long. Bagaceratops weighed rouhghly 65 pounds (30 kg). It was a quadrupedal plant-eater with a short neck frill and a small horn on its snout. It lived in what is now Mongolia, China during the late Cretaceous Period, about 70 million years ago. It was described by Halszaka Osmólska in 1975. The type species is B. rozhdestvenskyi.

(pronounced BAG-ah-RAH-tahn) Bagaraatan (meaning: "small hunter" ) was a speedy, bipedal, meat-eating dinosaur (an early coelurosaur). This theropod was about 11.5 feet (3.5 m) long and dates from the late Cretaceous Period. An incomplete skeleton was found in Mongolia and was named by paleontologist Osmolska in 1996. The type species is B. ostromi.

(pronounced bah-hah-REE-ya-SAWR-us) Bahariasaurus (meaning: "oasis lizard") was a theropod dinosaur (perhaps a carnosaur) about 20-40 feet (6-12 m) long, weighing about 4 tonnes. It was a bipedal meat-eater from what is now Egypt. It lived during the Cretaceous Period, about 109 to 95 million years ago. It was named by Stromer in 1934. The type species is B. ingens. The only known fossil was destroyed in World War II.

Robert Bakker (1945- ) is a US paleontologist and dinosaur artist who revolutionized our view of dinosaurs in the late 1960's, drawing them as active animals standing upright and not dragging their tails. He named: Chassternbergia (1988), Denversaurus (1988), Drinker (1990, with others), Edmarka (1992, with others), and Nanotyrannus (with others, 1988).

Baluchitherium (now called Indricotherium) is a large, extinct, hornless rhinoceros. It was one of the largest land mammals. Adults were about 26 feet (8 m) long, 18 feet (5.5 m) tall, and weighed about 17 - 18 tons (16 tonnes). The skull was 4.25 feet (1.3 m) long. This herbivore ate leaves and twigs from the tops of trees. It had four teeth; two tusk-like front teeth in the top jaw, pointing down and two on the bottom pointing forwards. This extinct ungulate (hoofed mammal) had three toes on each foot and lived from the Oligocene to the early Miocene in central Asia (Pakistan, Mongolia and China). Classification: Order Perissodactyla (odd-toed ungulates), Family Hyrachyidae (odd-toed ungulates between tapirs and rhinos).

(pronounced BAM-be-RAP-tor) Bambiraptor (also known as Bambi) was a juvenile coelurosaur, an advanced theropod (meat-eating dinosaur). This small predator was originally thought to be a juvenile Velociraptor or perhaps Saurornitholestes langstoni. It was a small, fast, meat-eating dromaeosaurid dinosaur from the late Cretaceous Period. Bambiraptor was about 3 feet (1 m) long and may have weighed about 7 pounds (3 kg). It had a deadly, sickle-shaped toe claw on the second toe of its foot. Bambiraptor was found in the upper Two Medicine Formation, Montana, USA. The nearly complete, fossilized skeleton was found in a dinosaur bonebed, near a Maiasaura skull. Bambiraptor was named by Burnham, Derstler, Currie, Bakker, Zhou, and Ostrom, 2000. The type species is Bambiraptor feinbergi.

(pronounced buh-RAH-pah-SAWR-us) Barapasaurus (meaning: "big-legged lizard") was a sauropod (a long-necked, long-tailed, small-headed, short-legged giant). It was an herbivore, a plant-eater that was about 60 feet (20 m) long, weighing roughly 48400 kg. The femur (thigh bone) is 5.5 ft (1.70 m) long. Barapasaurus lived during the early Jurassic Period, about 208 to 188 million years ago. 6 partial skeletons have been found in Southern India's Godavari Valley, but no skulls or feet have been found. It ay belong to the family vulcanodintidae, but this is unsure. Barapasaurus was named by Jain, Kutty, Roy-Chowdhury and Chatterjee in 1975. The type species is B. tagorei.

Barbourofelis (meaning: "Barbour's cat") was a carnivorous (meat-eating) mammal, an early cat with long incisors - it was not a dinosaur. This lion-sized cat was the last of the family Nimravidae (false-saber-tooth cats) and lived during the late Miocene, about 15-7 million years ago. Fossils have been found in North America, Turkey, and Spain. Classification: Class Mammalia (mammals), Order Carnivora, Superfamily Feloidea (cats, mongooses), Family Nimravidae (false-saber-tooth cats, early cats), Genus Barbourofelis.

(pronounced BAR-oh-SAWR-us) Barosaurus (meaning: "heavy lizard") was a diplodocid sauropod (a long-necked, long-tailed, small-headed, short-legged giant). It was an herbivore, a plant-eater. It was huge and slow-moving, perhaps over 60 to 88 feet (20-27 m) long, weighing roughly 40000 kg. Its femur (thigh bone) was 8.2 ft (2.52 m) long. Its primary defense against predators was its size. Barosaurus lived during the late Jurassic Period, about 156 to 145 million years ago. Its fossils have been found in western North America and East Africa. Barosaurus was named by paleontologist Othniel C. Marsh in 1890. The type species is B. lentus.

Rinchen Barsbold is a Mongolian paleontologist. He named: Adasaurus (1983), Ansermimus (1988), Conchoraptor (1986), the family Enigmosauridae (1983), Enigmosaurus (with A. Perle, 1983), Gallimimus (with H. Osmólska and E. Roniewicz, 1972), Garudimimus and the family Garudimimidae (1981), Harpymimus and the family Harpymimidae (with A. Perle, 1984), Ingenia (1981), the family Ingeniidae (1986), the family Oviraptoridae (1976), and the suborder Segnosauria (with A. Perle, 1980). Barsboldia (Maryanska et Osmolska, 1981) was named to honor Rinchen Barsbold.

(pronounced bars-BOWL-dee-ah) Barsboldia (named for Rinchen Barsbold) was a duck-billed dinosaur (a lambeosaurine hadrosaur) about 30 feet (10 m) long, weighing roughly 6500 kg. It was a quadrupedal plant-eater with a hollow crest and tall spines on its back vertebrae (which may have formed a fin on its back). It lived in what is now Mongolia during the late Cretaceous Period, about 70 million years ago. The type species is B. sicinskii. It was named by Maryanska and Osmolska in 1981. This is a doubtful genus since it is so poorly known.

A large 9 metre long theropod from the early Cretaceous period. With a long crocodile-like head and relatively long arms with huge curved claws on the thumbs, Baryonyx may have been primarily a fish-eater. It is known from a relatively well-preserved skeleton found in a quarry in Surrey, England by an amateur fossil hunter in 1983.

Basilosaurus was an Archaeoceti whale, a primitve, extinct whale from the Eocene epoch, 50 million of years ago.

(pronounced buh-SUE-TOE-don) Basutodon (meaning: "Basuto [former name of Lesotho, South Africa] tooth") is a dubious genus of reptile that may or may not be dinosaurian. Fossil teeth that date from the Triassic period were found in South Africa. Basutodon was named by paleontologist von Huene in 1932. The type species is B. ferox.

Bavarisaurus was a small, fast-moving, ancient lizard (it was not a dinosaur) that lived during the Jurassic period. A partially digested, fossilized skeleton of a Bavarisaurus was found inside a Compsognathus (a small, meat-eating dinosaur) fossil in Bavaria, Germany.

Becklespinax (meaning: "Beckles' spine") was a theropod dinosaur (a tetanurid) about 15 feet (5 m) long. It was a bipedal meat-eater with spines along its back. It lived in what is now England during the early Cretaceous Period, about 125 million years ago. It is only known from three elongated vertebrae and teeth found by Samuel H. Beckles in the 1850's. It was named by paleontologist Olshevsky in 1991.