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A habitat is a space (which includes food, water and shelter) suitable for the survival and reproduction of an organism.

The Hadean Eon lasted from 4.6 to 3.9 billion years ago. This "Rockless Eon" was the time when the Earth's continental and oceanic crusts were solidifying. The name Hadean was coined by the geologist Preston E. Cloud in the 1960s.

Hadrocodium (meaning: "heavy or full head") was a tiny mammalian ancestor about the size of a paperclip. It is the earliest-known animal with such mammal-like features. This shrew-like quadruped had a long tail, a long snout, delicate teeth, three middle ear bones, a powerful jaw hinge, matching upper and lower teeth, a large brain case, and five-toed feet. Hadrocodium was an insectivore (insect-eater) that may have been nocturnal (most active at night). It lived about 195 million years ago. A skull (half an inch (12 millimeters) long) was found in the Lufeng Basin in Yunnan, China, in 1985 (it was only recently determined that it was a new species). The type species is Hadrocodium wui; was named by Zhe-Xi Luo et al.

Hadrosaurids, or duck-billed dinosaurs, were the biggest ornithopods (a type of ornithischian or bird-hipped dinosaurs). They could walk on two or four legs. These plant-eaters lived during the late Cretaceous period. Hadrosaur means 'big or bulky lizard.' Hadrosaurs had a wide, flat, toothless beak, hundreds of cheek teeth and powerful jaws. Their hind legs were large and each limb had four digits. Maiasaura, Edmontosaurus, Hadrosaurus, etc. were hadrosaurs. The hadrosaurs evolved from the iguanodontids.

The hadrosaurs, also known as the duck-billed dinosaurs, were a family of bird-hipped herbivore dinosaurs that appeared in the Late Cretaceous period. They had long powerful hind limbs with three-toed feet and smaller more delicate front legs. Many species were characterised by elaborate head crests.

Hainosaurus (meaning: "Haine (River) lizard") was a huge mosasaur that was about 50 feet (15 m) long. The skull is about 1.5 m long. This is the largest mosasaur yet found. They had sharp teeth and ate fish, turtles, and other marine organisms. Fossils have been found in Europe. Hainosaurus was named by Dollo in 1885.

Hallucigenia was a strange, spiked animal that lived during the Cambrian Period, roughly 500 million years ago (found in Canada's Burgess Shale and in China). Hallucigenia as an onychophoran (a "velvet worm") that had 7 tentacles on its top side which it used to grasp food; it used 7 pairs of spines on the underside for walking. Forty fossils of Hallucigenia have been found.

A hallux, or dewclaw, is a functionless claw that doesn't hit the ground. Some dinosaurs had dewclaws.

(pronounced HALL-tik-oh-SAWR-us) Halticosaurus (meaning: "leaping lizard") was a late Triassic dinosaur from about 222 million years ago. It was a very early dinosaur and its classification is unsure (it is perhaps a theropod). It was a speedy bipedal dinosaur about 17 feet (5 m) long. It had a short neck, very long feet, a long, large head with many sharp teeth. Its feet suggest that it was a theropod (a meat eater) but its hip and vertebrae are more like those of plant eaters. An incomplete skeleton was found in Wuerttemberg, Germany in 1906 and named by paleontologist von Huene.

(pronounced hap-lo-KAN-tho-SAWR-us) Haplocanthosaurus (meaning: "single spine lizard") was a sauropod dinosaur from the late Jurassic Period, about 156-145 million years ago. This plant-eater had a long neck, a long tail, a bulky body and a small head. It was about 70 ft (21 m) long. Partial fossils have been found in Colorado and Wyoming, USA. The type species is H. priscus. It was found by paleontologist John Bell Hatcher in 1901, and named by him in 1903.

(pronounced HAR-pee-MIME-us) Harpymimus ("Harpy [a Greek bird-woman monster] mimic") was a theropod dinosaur, a bipedal meat-eater that lived during the Cretaceous Period, about 119- 97.5 million years ago. It had a beak and 10 - 11 conical teeth in its lower jaw. Its diet is uncertain. It had tapered, three-fingered hands. This extremely fast-running dinosaur had thin, long-shinned legs and a light-weight body. It was about 6.5 feet (2 m) long and may have weighed about 275 pounds (125 kg). This bird-like dinosaur is known from a skull and a few bones found in Mongolia. Harpymimus was named by Barsbold and Perle in 1984. The type species is H. okladnikovi.

The first dinosaur models were made by Benjamin Waterhouse Hawkins of England in 1854. He made and sold plaster-cast dinosaurs through the Ward's catalogue of scientific supplies. His original models included Igauanodon, Hylaeosaurus, Megalosaurus, Plesiosaurus and Ichthyosurus. The first dinosaur used for adult amusement was a life-size model of an Iguanodon (made of concrete by Hawkins) that was used to house a dinner party for scientists (including Richard Owen, who coined the term dinosaur) at a major exhibition in London, England, in 1854. The invitations to the party were sent on fake pterodactyl wings..

Helicoprion was an ancient fish from the Late Paleozoic (it appeared during the Carboniferous period, about 345 million years ago). This predatory fish is known only from fossilized teeth; it probably had a cartilaginous body (which does not fossilized well and is possibly why a fossil skeleton from this fish has not been found). Helicoprion was probably related to sharks or was a shark itself. It had big, flattened teeth that were probably used for crushing shellfish and arthropods. These teeth grew in a series of coiled whorls; new teeth relaced broken and worn-out teeth, but the old ones went inside the jaw. These fossilized teeth have been found worldwide. Classification: Family Edestidae

Sue Hendrickson (December 2, 1949 - ) is a self-taught fossil hunter (specializing in fossil inclusions in amber), marine archaeologist, adventurer and explorer. In South Dakota in 1990, Hendrickson found the remarkable Tyrannosaurus rex fossil that is now known as Sue. This Tyrannosaurus rex fossil is the largest and most complete Tyrannosaurus rex found to date. Sue (the fossil) is now displayed at the Field Museum in Chicago, Illinois, USA.

(pronounced hen-o-dus) Henodus was a marine reptile about 3.25 feet (1 m) long. It was not a dinosaur, but a placodont. It had no teeth, but used its hard, horny beak to get and crush shellfish. It had a box-like head, 4 legs, a short, pointed tail, and a flat body. It was protected from predators, like Ichthyosaurs, by bony plates that formed a shell on its back and stomach. Although it resembled a turtle, it was not closely related to turtles. It lived during the late Triassic Period, when it swam in the Tethys Sea. Fossils have been found in Germany. Classification: Order Placondontia, Family Henodontidae (armored placodonts).

An animal that feeds only on plants, and is specially adapted to be able to extract nutrition from tough foliage.

Herrerasaurus was a late Triassic archosaur from about 230 million years ago. It was a speedy bipedal carnivore, 5 m long and weighing about 300 kg, with a short neck and large head. This was one of the first flesh-eating dinosaurs and is known from several skeletons found in Argentina.

(pronounced HES-per-OR-nis) Hesperornis (meaning: "western bird") was an early, flightless bird that lived during the late Cretaceous Period. This diving bird was about 3 feet (1 m) long and had webbed feet, a long, toothed beak, and strong legs. Although it couldn't fly, it was probably a strong swimmer and likely lived near coastlines and ate fish. Fossils have been found in North America. Hesperornis was named by paleontologist O. Marsh in 1872 from fossils found near the Smoky Hill River in Kansas, USA.

(pronounced hes-PARE-uh-SAWR-us) Hesperosaurus (meaning: "western lizard," and originally called Hesperisaurus) was a primitive stegosaurid dinosaur that had a single row of rounded plates running down its back; it also had four bony spikes (thagomizers) at the end of the tail. This plant-eating dinosaur lived during the late Jurassic Period, roughly 150 million years ago. Fossils have been found in the Morrison Formation, Jackson County, Wyoming, USA, North America. The type species is Hesperisaurus mjosi; it was named by paleontologists K. Carpenter et al. in 2001.

(pronounced HET-er-oh-CROW-knee) heterochrony (meaning: "differeny time") is a evolutionary (genetically determined) change in the timing of developmental events or a change in the growth rate, as compared to the same events in ancestors. For example, the time it takes to grow to adulthood may change over time.

(pronounced HET-er-oh-DON-toh-SAWR-us) Heterodontosaurus (meaning: "different-tooth lizard") was an early ornithischian dinosaur. It was an herbivore with three different types of teeth. This lightly-built plant-eater was up to 4 ft (1.2 m) long and weighed perhaps 10 kg. It lived during the early Jurassic Period, about 208-200 million years ago. The type species is H. tucki. Heterodontosaurus was named by Crompton and Charis in 1962.

(pronounced HET-er-oh-TROFE) A heterotroph (or consumer) is a living thing that eats other living things to survive. It cannot make its own food (unlike plants, which are autotrophs). Animals are heterotrophs.