Sharks

Hello marine lovers! In this post, you will find out more about sharks, and please do not be afraid when you read this! Sharks are a type of fish with a full cartilaginous skeleton and a streamlined body. They respire with the use of five to seven gill slits. Sharks have a covering of dermal denticles that protect their skin from damage and parasites and improve fluid dynamics, and they also have replaceable teeth. Sharks range in size from the small dwarf lanternshark, Etmopterus perryi, a deep sea species of only 17 centimetres in length, to the whale shark, Rhincodon typus, the largest fish, which grows to a length of approximately 12 metres 39 ft and which, like baleen whales, feeds only on plankton, squid, and small fish through filter feeding. The bull shark, Carcharhinus leucas, is the best known of several species to swim in both salt, freshwater and in deltas.Most sharks live for 20 to 30 years, while the spiny dogfish and the whale shark lives a record lifespan of more than 100 years. Until the 16th century, sharks were known to mariners as "sea dogs". According to the OED the name "shark" first came into use after Sir John Hawkins' sailors exhibited one in London in 1569 and used the word to refer to the large sharks of the Caribbean Sea, and later as a general term for all sharks. Evidence for the existence of sharks extends back over 450–420 million years, into the Ordovician period, before land vertebrates existed and before many plants had colonised the continents. All that has been recovered from the first sharks are some scales. The oldest shark teeth are from 400 million years ago. The first sharks looked very different from modern sharks. The majority of the modern sharks can be traced back to around 100 million years ago!
Mostly only the fossilized teeth of sharks are found, although often in large numbers. In some cases pieces of the internal skeleton or even complete fossilized sharks have been discovered. Estimates suggest that over a span of a few years a shark may grow tens of thousands of teeth, which explains the abundance of fossils. As the teeth consist of calcium phosphate, an apatite, they are easily fossilized.
Instead of bones, sharks have cartilagenous skeletons, with a bone-like layer broken up into thousands of isolated apatite prisms. When a shark dies, the decomposing skeleton breaks up and the apatite prisms scatter. Complete shark skeletons are only preserved when rapid burial in bottom sediments occurs.
Among the most ancient and primitive sharks is Cladoselache, from about 370 million years ago, which has been found within the Paleozoic strata of Ohio, Kentucky and Tennessee. At this point in the Earth's history these rocks made up the soft sediment of the bottom of a large, shallow ocean, which stretched across much of North America. Cladoselache was only about 1 m long with stiff triangular fins and slender jaws. Its teeth had several pointed cusps, which would have been worn down by use. From the number of teeth found in any one place it is most likely that Cladoselache did not replace its teeth as regularly as modern sharks. Its caudal fins had a similar shape to the great white sharks and the pelagic shortfin and longfin makos. The discovery of whole fish found tail first in their stomachs suggest that they were fast swimmers with great agility.
From about 300 to 150 million years ago, most fossil sharks can be assigned to one of two groups. One of these, the Acanthodii, was almost exclusive to freshwater environments. By the time this group became extinct. About 220 million years ago, they had achieved worldwide distribution. The other group, the hybodonts, appeared about 320 million years ago and was mostly found in the oceans, but also in freshwater.
Modern sharks began to appear about 100 million years ago. Fossil mackerel shark teeth occurred in the Lower Cretaceous. One of the most recent families of sharks that evolved is the hammerhead sharks (family Sphyrnidae), which emerged in Eocene. The oldest white shark teeth date from 60 to 65 million years ago, around the time of the extinction of the dinosaurs. In early white shark evolution there are at least two lineages: one with coarsely serrated teeth that probably gave rise to the modern great white shark, and another with finely serrated teeth and a tendency to attain gigantic proportions. This group includes the extinct Megalodon, Carcharodon megalodon, which like most extinct sharks is only known from its teeth and a few vertebrae. This shark could grow to more than 16 metres long and is recognized as the biggest known carnivorous fish to have ever existed. Fossil records reveal that this shark preyed upon whales and other large marine mammals.
It is believed that the immense size of predatory sharks such as the great white may have arisen from the extinction of giant marine reptiles, such as the mosasaurs and the diversification of mammals. It is known that at the same time these sharks were evolving some early mammalian groups evolved into aquatic forms. Certainly, wherever the teeth of large sharks have been found, there has also been an abundance of marine mammal bones, including seals, porpoises and whales. These bones frequently show signs of shark attack. There are hypotheses that suggest that large sharks evolved to better take advantage of larger prey. Studies on the behaviour of sharks have only recently been carried out leading to little information on the subject, although this is changing. The classic view of the shark is that of a solitary hunter, ranging the oceans in search of food; however, this is only true for a few species, with most living far more sedentary, benthic lives. Even solitary sharks meet for breeding or on rich hunting grounds, which may lead them to cover thousands of miles in a year. Migration patterns in sharks may be even more complex than in birds, with many sharks covering entire ocean basins.
Some sharks can be highly social, remaining in large schools, sometimes up to over 100individuals of scalloped hammerheads congregating around seamounts and islands e.g. in the Gulf of California. Cross-species social hierarchies exist with oceanic whitetip sharks dominating silky sharks of comparable size when feeding.
When approached too closely some sharks will perform a threat display to warn off the prospective predators. This usually consists of exaggerated swimming movements, and can vary in intensity according to the level of threat. So as you see marine lovers, you might notice that I have posted mostly about marine animals, because I truly, truly hope that we, as people, will feel like we have a duty to treat them well. So long!
~~~~~~Megan~~~~~~