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All About Sharks!
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Dermal denticles are small, hard, tooth-like structures, also called placoid scales. They have the same structure as a tooth, having an outer layer of enamel, dentine and a central pulp cavity. Sharks' teeth and skin are composed of this type of scale.
Diapsids include all the reptiles (except turtles) and birds. They are distinguished by having two holes in the rear upper part of their skulls and two holes behind the eyes.
A dichotomous key is a method for determining the identity of sometihing (like a butterfly, a plant, or a rock) by going through a series of choices that lead the user to the correct name of the item. At each step of the process, the user is given two choices; each alternative leads to another questions until the identification is completed. For example, a question in a dichotomous key for trees might be something like, "Does it have flat or needle-like leaves?" Dichotomous means "divided in two parts".
Dinichthys (meaning "terrible fish"), was a family of ancient, meat-eating fishes. Dunkleosteus was the biggest member of this family. It was a heavily armored primitive fish from the Late Devonian period, about 360 million years ago. This top predator was up to 11.5 ft (3.5 m) long and had large, scissor-like cutting jaws with serrated, razor-sharp bones, but no teeth. Its skull was was over 2 feet (65 cm) long. It had a jointed neck, an eel-like tail, a scale-less body, and hinged body shields. Dinichthys were not sharks but placoderms with a shark-like tail. They may have eaten sharks. Fossils have been found in Morocco, Africa, Poland, Belgium, and the USA.
Dinosaurs were land-dwelling reptiles that walked with an erect stance during the Mesozoic Era. Their unique hip structure caused their legs to stick out from under their bodies, and not sprawl out from the side (like other reptiles). They are extinct.
Dogfish are the most common shark. They congregate in schools and migrate. They range in size from tiny to enormous and are found worldwide.
Dolphins are small cetaceans that have a long, beaklike snout, a falcate (sickle-shaped) dorsal fin, and conical teeth. They are Odontoceti (toothed whales).
Dorsal means of, pertaining to, or near an animal's back or upper surface of the body.
The dorsal fin is a fin on an animal's back.
The dorsal ridge is a raised line or hump on the back of a some whales (instead of a dorsal fin).
Dunkleosteus, the biggest member of the family Dinichthyidae ("terrible fishes"), was a heavily armored primitive fish from the Late Devonian period, about 360 million years ago. This top predator was up to 11.5 ft (3.5 m) long and had large, scissor-like cutting jaws with serrated, razor-sharp bones, but no teeth. Its skull was was over 2 feet (65 cm) long. It had a jointed neck, an eel-like tail, a scale-less body, and hinged body shields. Dunkleosteus was not a shark but a placoderm with a shark-like tail. It may have eaten sharks. Fossils have been found in Morocco, Africa, Poland, Belgium, China, and the USA.
The Dusky Shark (Carcharhinus obscurus) is a requiem shark that is also known as the bay shark, the shovelnose, the lazy-gray, and the black whaler. It is charcoal colored on top and white with copper-colored markings below. It has a faint pale stripe along its sides. Dusky sharks average about 10 ft (310 cm) long; females are slightly larger than males. The largest one found was about 13 ft (4 m) long. It lives at all levels of the ocean and from the shoreline to out at sea. It is found in warm temperate and tropical waters worldwide. It migrates to cooler waters during warm weather, probably for reproduction and feeding. This shark is often seen following ships and is hunted as a game fish. Overfishing is diminishing the numbers of this shark. Few Dusky shark attacks are documented, but it is considered dangerous. It normally eats small fish (like sardines), large fish (like tuna), flatfish, other sharks, and eels. Females are mature when they are 9 ft (2.8 m) long and have litters of about 10 pups which are roughly 3 ft (95 cm) long each. There is a 16 month gestation period. Classification: Order Carcharhiniformes, Family Carcharhinidae (requiem sharks), Genus Carcharhinus, Species obscurus.
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