In the know: Ammonites
Named after the Egyptian ram-horned god Ammon, ammonites were prehistoric, squid-like creatures that lived inside coil-shaped shells. They became extinct, together with the dinosaurs, about 65 million years ago.
Ammonites belong to a group of predators known as cephalopods, which includes their living relatives—the octopus, squid, cuttlefish and nautilus. They had sharp, beaklike jaws inside a ring of tentacles that extended from their shells to snare prey such as small fish and crustaceans.
They first appeared about 240 million years ago, though they descended from small, straight-shelled cephalopods known as bacrites from about 415 million years ago. They began life as tiny planktonic creatures less than one millimeter in diameter, but quickly assumed a strong protective outer shell.
Although ammonites only lived in the outer chamber, they constantly built a new shell as they grew. Evidence suggests that they gained in size rapidly, with females growing up to 400 percent larger than males, presumably to make room to lay eggs.
They moved by expelling water through a funnel-like opening to propel themselves in the opposite direction. They spent most of their life in warm, shallow waters and typically lived for two years, although some species survived longer.
Among the most abundant fossils today, ammonites were prolific breeders, and lived in schools. Fossilized shells are usually, but not always, beautiful spirals. Scientists use the various shapes and sizes of ammonite shells that appeared and disappeared through the ages to date other fossils.
Many factors contribute in the production of ammonite fossils, which may be traces or impressions on rocks, but being covered in silt or sand plays a big role as the shells of the ammonites that had sunk to the seabed become protected from stormy seas and damage.
Over the years, the ammonite shells are covered in layers of sand and silt until they decompose. As the climate changes and the ocean recedes, the seabed turns into dry land and the void left by the shells are replaced by rock-like substitutes of the original creature. The compressed seabed also turns to shale or rock and becomes exposed to erosion and other elements until the fossilized ammonites are exposed to be discovered. Almi Ilagan-Atienza,Inquirer Research
Sources: http://animals.nationalgeographic.com/animals/prehistoric/ammonites/, http://www.discoveringfossils.co.uk/
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