Cook Acta Palaeontologica Polonica. Alcheringa: An Australasian Journal of Palaeontology. Ogg and L. Hinnov The Geologic Time Scale Bulletin of the Geological Society of Denmark. Vandenberghe; F. Hilgen; R. Speijer The Geologic Time Scale : — Geologija in Slovenian. Catalogue of the fossil fishes in the British Museum Natural History. British Museum. Journal of Paleontology. Imprimerie de Petitpierre et Prince.
Recherches sur les poissons fossiles in French. Museum of Comparative Zoology. Geological Survey of Belgium. Palaeontographica in German. Retrieved 31 March Palaeovertebrata in French. Moody; P. Sutcliffe Cretaceous Research. Palaeontographica Abteilung A in French. Ebersole; Dana J. Ehret Journal of Vertebrate Paleontology. Fossils of the Harrana Fauna and the Adjacent Areas.
Retrieved 14 May Bibcode : PLoSO.. The primary use of the caudal fin is to provide a "push" while the shark swims. The upper lobe of the caudal fin produces the most amount of the push, and usually forces the shark downwards. The pectoral fins and the shape of the body like an airfoil work together to counter this force. The strong, non-lunate caudal fin in most benthic shark species allows the shark to swim close to the seabed such as the nurse shark.
However, the fastest swimming sharks such as the mako sharks tend to have lunate-shaped crescent-shaped caudal fins. Sharks have keen olfactory sense organs in the short duct between the front and back nasal openings. They can detect blood from miles away: as little as one part per million of blood in sea water may be enough. Sharks have the ability to determine the direction of a given scent based on the timing of scent detection in each nostril.
They are more attracted to the chemicals found in the intestines of many species, and as a result often linger near or in sewage outfalls. Some species, such as nurse sharks , have external barbels that greatly increase their ability to sense prey. Shark eyes are similar to the eyes of other vertebrates , including similar lenses , corneas and retinas. Their eyesight is well adapted to the marine environment. They can contract and dilate their pupils , like humans, something no teleost fish can do.
A tissue behind the retina reflects light back, thereby increasing sight in darker waters. Sharks have tiny holes all over the shark's snout, especially between the eye and the tip of the snout. In them are nerve receptors which are called the ampullae of Lorenzini.
Animals in the water give off electricity: every time an animal's heart beats or it moves, tiny currents of electricity are made. These tiny electric currents make signals that travel through water and get sensed. Sharks may use this sense when they catch their prey, even more than they use their sight. Although it is hard to test sharks' hearing, they may have a sharp sense of hearing and can possibly hear prey many miles away. The lateral line detects changes in water pressure.
It is open to the environment by a line of pores. This and the sound-detecting organs are grouped together as the 'acoustico-lateralis system', because they have a common origin. In bony fish and tetrapods the external opening into the inner ear has been lost. This system is found in other fish as well.
It detects motion or vibrations in water. Just a few million years ago, a giant shark called Megalodon swam in the seas. It was 18 meters long, twice as long as the closely-related great white shark, and it ate whales. Megalodon died out 1. Much of what we understand about prehistoric sharks comes from the study of their fossils. While sharks have skeletons made of soft cartilage that can fall apart before fossilizing, their teeth are harder and easily fossilized. Prehistoric sharks, like their modern descendants, would grow and shed many thousands of teeth over their lifetime.
For this reason shark teeth are one of the most common fossils. Pups are born with a full set of teeth, and are capable of taking care of themselves.
Once born, they quickly swim away from their mothers, who sometimes feed on the pups. Litters vary from one or two pups great white shark to one hundred pups blue shark and whale shark. Some sharks are oviparous , laying their eggs in the water.
Shark eggs sometimes called "mermaid's purses" are covered by a tough, leathery membrane. Most sharks are ovoviviparous , meaning the eggs hatch inside the female's body, with the babies developing within the mother, but there is no placenta to nourish the pups. Instead the young feed on the egg's yolk.
The pups eat any unfertilized eggs and sometimes each other. Very few pups in a litter survive until birth due to this form of sibling cannibalism. Great white sharks , mako sharks , nurse sharks , tiger sharks , and sand tiger sharks give birth this way.
Vector shark Silhouettes. Tags: shark , underwater, great, reef, animal, sea, water, fish, mouth, ocean, danger, teeth , dangerous, isolated, shark s, diving, great white shark. Set Of shark Vectors.
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