- Introduction to Sharks
- Shark Facts and Information
- 10 facts about great white sharks!
- See a Problem?
By the end of the period, 45 families of sharks swam in the seas—and resulted in some strange-looking animals. Males of the extinct species Falcatus falcatus were six-inches long, and each had a strange sword-like appendage growing off of its head. One fossil preserved a pair of these sharks in the act of mating, with the larger female grabbing the male by its head spine. Another strange head appendage has been found on the extinct Stethacanthus , a two-foot shark with an anvil-shaped dorsal fin.
And who could forget Helicoprion , an ancient shark that had a whorl of teeth in its mouth like a buzzsaw. But all good things must come to an end: million years ago the largest extinction event in Earth's history called the Permian-Triassic extinction event wiped out 95 percent of all living species on the planet, including many of these bizarre sharks.
Only a few families of fish—food for large ocean predators like sharks—survived the Permian extinction. But as the seas recovered, so did they. Ray-finned fish began to fill the seas, adapting to different habitats. And with them, their predators evolved too.
Introduction to Sharks
During the Jurassic to million years ago and Cretaceous to 66 million years ago Periods, marine reptiles like ichthyosaurs, mosasaurs and plesiosaurs ruled the seas—along with some sharks. By the mid-Cretaceous, around million years ago, sharks that resemble large, fast-swimming modern sharks started to appear. In , the fossilized remains of the foot meter shark Ptychodus mortoni , which swam the ocean 89 million years ago, were found in Kansas Kansas at that time lay under a vast inland sea.
Only a jaw was found—a very big jaw—lined with hundreds of flat teeth that would have helped it crush shellfish. Thus, despite its size, it was likely a slow-moving, bottom-dwelling shark. Around the same time lived the Ginsu Shark Cretoxyrhina mantelli —a slightly smaller shark, at 20 feet 6 meters long, but much more fearsome. The Ginsu is one of the better-known ancient sharks because paleontologists found a nearly complete fossilized spine for the species, along with very impressive teeth.
They were very sharp, 6 centimeters long, and likely used to kill and eat larger fish prey.
Ginsu teeth have been found embedded in pleisiosaur and mosasaur bones, suggesting that they may have gone after small marine reptiles as well. Another group of sharks known as the crow sharks Squalicorax were smaller, at around one-third the size of the Ginsu. Instead of ruling as fierce predators, crow sharks were likely scavengers that fed upon already-dead animals.
Paleontologists think this because bones of large animals from this period have been found covered with crow shark bite marks. The Cretaceous—Paleogene extinction 65 million years ago wiped out the dinosaurs—but not the sharks. Approximately 80 percent of the shark, ray and skate families survived this extinction event. Some of those that survived are the ancestors of the sharks alive today. In the 65 million years since the Cretaceous-Paleogene extinction, sharks have continued to evolve and become the diverse group of cartilaginous fishes we see today.
Some modern sharks have direct ancestors from before the Cretaceous extinction event. Cow sharks date back to million years ago, while the snake-like frilled sharks have fossils from 95 million years ago. That doesn't mean that these modern animals are identical to their ancient versions; on the contrary, they have certainly undergone evolution and changed over the millions of years of their existence. But paleontologists are fairly certain that our modern sharks are directly related to extinct relatives known to us by fossils.
The lamnoid sharks order Lamniformes —including the great white, mako and thresher sharks, among others—also can trace their lineage into the Cretaceous. But paleontologists don't have a good sense of which ancient sharks species evolved into modern lamnoid sharks. Their ancient ancestors left behind many fossilized teeth, but there isn't an easy way to put them in order without more information provided by fossilized skeletons. One well-known extinct relative of modern lamnoid sharks is the Megalodon Carcharodon megalodon , which was more than 50 feet long with seven-inch teeth and lived 16 million years ago.
It went extinct 1. For many years, some scientists believed that the Megalodon was an ancestor of the great white shark—but great whites are more closely related to ancestors of modern mako sharks. It is likely that the Megalodon and great white sharks even coexisted, with the Megalodon feeding primarily on whales and the great white on seals. One notable feature of sharks is that large filter feeders evolved separately multiple times. Between 65 and 35 million years ago, several sharks evolved away from predation and towards filtering tiny plankton out of the water for sustenance.
An ancestor of the modern-day carpet sharks evolved into the whale sharks Rhincodon typus we see today, while two ancient ancestors of the mackerel sharks evolved into basking sharks Cetorhinus maximus and megamouth sharks Megachasma pelagios. The shark family that evolved most recently is that of hammerhead sharks Sphyrnidae , which first appeared 50 to 35 million years ago.
Sharks are found in waters throughout the world, from shallow water to the deepest parts of the ocean. Some species migrate vast distances, moving between various locations to breed and find the best sources of food. Some of these migrations are fairly easy to track. For example, every winter in Florida, blacktip sharks head from the open ocean to the shore where they mate and breed. Thousands of these sharks migrate at once and come close to shore, making it easy for people to spot them and scientists to study them. But sharks migrating far offshore and traveling individually are more difficult to track.
To make up for this, scientists are using tagging and tracking technologies to learn about their movements. They will often place a computerized tag on the back of a shark that sends information about its GPS location back to the scientists on land. New tagging and tracking technology has also allowed researchers to get a better idea of where the gentle whale sharks go after gathering to feed on plankton off the coast of Central and South America.
Even so, new populations continue to be discovered , showing how much we still have to learn about the biggest of all sharks. Several shark species also migrate between deeper and shallower water every day; these migrations are called diel vertical migrations. The distance of these daily migrations range from 30 to feet tens to hundreds of meters depending on the shark species.
Blue sharks Prionace glauca , for example, spend their nights near the ocean's surface top feet or meters , but will dive down to depths of feet meters —and occasionally deeper to feet meters —and back to the surface throughout the day. One of the biggest changes when moving between depths is the temperature. It's likely that the sharks are willing to put up with such cold temperatures in order to hunt deep-water prey like squids and octopods, and then return to the surface to warm up again.
Other sharks like the lesser-spotted catshark Scyliorhinus canicula spend their days in deeper water 65 feet or 20 meters , but swim to the surface at night —probably to keep warm. Shark lifespans are not well known and vary quite a lot among species. Scientists figure out the age of most species of fish by counting the "rings" on their otoliths tiny calcium carbonate structures in their ears like the rings on a tree. But this isn't so easy for sharks because their otoliths are the size of a grain of sand and are thus very difficult to see. Another method measures the growth of shark vertebrae using similar "rings," but how frequently the rings are laid down varies from species to species, making that method unreliable.
Recently, scientists have been using a new method of determining shark age: by using a radiocarbon timestamp found in the vertebrae of sharks left over from nuclear bomb testing in the s and s.
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For example, the oldest male great white shark was 70 years old , and the oldest female was 40 years old. That is much longer than previous estimates of about 20 years. Similarly, sand tiger sharks Carcharias taurus were found to live up to 40 years , which is 11 years longer than expected. Sharks grow and mature slowly and reproduce only a small number of young in their lifetimes. Unlike most bony fish, they put a lot of effort into producing a small number of highly developed young at birth rather than releasing a large number of eggs that have a high probability of not surviving.
Because of these traits, sharks are particularly susceptible to overfishing. All sharks produce young through internal fertilization. A male shark does not have a penis. He has two claspers on the rear of his underside, attached to his pelvic fins, which he inserts into a female shark to deliver sperm to her eggs. Typically the male will only use one of his claspers at a time, depending on the pair's position although some shark species may use both claspers.
Sometimes they mate side by side, while other times the female will lay upside down. There are also several cases of internal asexual reproduction in sharks, a phenomenon called parthenogenesis. This occurred when a captive female shark isolated from males had a shark pup. There are three different ways that a baby shark can be born once a female shark has a fertilized egg, depending on the species.
Viviparity is when a shark nourishes her growing shark embryo internally and gives birth to a fully-functional live pup. These shark species, like the hammerheads Sphyrnidae , maintain a placental link to the embryo, similar to humans. In aplacental viviparity, also called ovoviviparity, there is no placental link. The most common type of reproduction in sharks, ovoviviparity occurs when the egg hatches while still inside the mother.
Sand tiger sharks Carcharias taurus will actually eat their siblings in the womb.
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Female sand tiger sharks often mate with several different males, producing a litter of shark pups from a number of fathers. Researchers think that the larger sharks will consume their smaller siblings that are not as closely related to prevent competition. Other shark species release an egg case, where the developing embryo gains nutrients from a yolk. This is called oviparity. Typically sharks that live on the seafloor, like the swellshark Cephaloscyllium ventriosum , are oviparous.
They attach their egg case to a rock or other hard surface, or wedge it into a safe spot on a sandy bottom or rocky area. The egg case of most sharks is a leathery transparent brown, with slits on either side that allow water to flow through to replenish oxygen in the sac. The tiny shark moves around to help facilitate the water movement and, once the nutrients from the yolk sac are used up, the small shark makes it way out of the case to fend for itself.
- Learn all about the great white shark!!
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- Facts About Great White Sharks.
You can find a shark that eats just about anything: the whale shark, the biggest fish in the sea, eats only tiny plankton, while the bonnethead shark gets some of its nutrition from seagrass, a type of underwater plant. Tiger sharks have even been found with license plates and nails in their stomachs. But most sharks are carnivorous and eat animals ranging from crustaceans like crabs to squid, fish and marine mammals like seals and sea lions. Some sharks have even been found with giant squid beaks in their stomachs!
Many sharks, however, have developed specific mechanisms that help that capture their prey. Some bottom dwelling sharks like wobbegongs also called carpet sharks hide and ambush their prey, sucking them up with small mouths. Some sharks swallow their prey whole, but others rely on very sharp teeth to break apart food—especially food larger than themselves.
The thresher shark Alopias genus has a long, tapered tail that is slaps into a school of fish to stun them and grab its meal. The whitetip reef shark Triaenodon obesus tends to hunt alone, sometimes chasing its prey into a crack and sealing the exit with its body. Sawsharks , meanwhile, get their name from their saw-like snout that is used to scrape up invertebrates from the seafloor and to stun fish. The cookie-cutter shark Isistius brasiliensis is an especially unusual case.
Although its name makes it seem like a Muppet, this shark is actually a quite intimidating creature that takes large round cookie-cutter shaped bites out of animals such as tuna, whales, dolphins, and seals. They sneak up and suction onto larger animals and twist around to take a bite of flesh using their lower row of sharp teeth and tongue-like basihyal. There are also some large species of sharks that are plankton feeders.
The basking shark, megamouth shark and whale shark all consume the tiny crustaceans. Their teeth are small and they have modifications on their gills that act like sieves to capture the plankton so they can swallow them in large gulps. Large sharks have few natural predators besides other sharks, although some small juvenile sharks are eaten by birds and large fish.enter site
Shark Facts and Information
Sharks are primarily killed by humans both intentionally and unintentionally as bycatch. Because of sharks slow growth and low reproduction rates, the rate at which humans are killing sharks is endangering shark populations and ecosystems throughout the world. It's estimated that million sharks are killed every year by commercial and recreational fisheries. Until recently, fishermen and governments didn't keep very good track of official shark catches. Instead of reporting shark catches by species, they'd report all sharks together or even grouped sharks and rays together.
That makes it difficult to know how many sharks were fished historically. Regardless, today scientists estimate that one-quarter of shark species, along with their ray and chimaera relatives, are threatened with extinction according to the International Union for Conservation of Nature IUCN Red List criteria. Sharks are particularly vulnerable to overfishing.
They grow slowly, reproduce late compared to other fishes, and don't have many offspring at once. Combined, these traits make them slow to replenish their populations when they are fished or otherwise killed at such fast rates. A study comparing sharks and bony fishes found that sharks have twice the extinction risk of bony fishes. Some sharks are caught by fisheries targeting sharks specifically. Not all are caught intentionally, however.
Sharks are often caught as bycatch—which means that, while the fishermen were trying to catch a different kind of fish, they accidentally catch sharks in their nets too. Some bigger open ocean-swimming sharks are caught by longline fisheries aiming for big fish like swordfish or tuna. For example, large shark abundance decreased by 21 percent in the tropical Pacific after industrial fishing began in the s. The 90 percent of elasmobranchs sharks, skates and rays that live near the seafloor are particularly susceptible to fisheries that drag a net across the ocean bottom trawling.
This can change local shark populations dramatically. For example, between and , after shrimping began in the Gulf of Mexico, some populations of shallow water sharks and ray species dropped by up to 99 percent. Such a big change doesn't just affect the sharks, but also their prey and the rest of the ecosystem.
See 'Ecosystem Effects'. Today, fins are the most valuable part of a shark. The targeted shark-fin fisheries around the world are trading the fins of roughly to million sharks every year according to a estimate. Driving this trade is the demand for and consumption of shark fin soup in Asia. Historically shark fin soup was only affordable to the richest people, but as the middle class has grown, it has become a more mainstream menu item.
Some of the shark fins used to make this soup are cut off and sold at market alongside the shark they came from.
10 facts about great white sharks!
But many are cut off of live sharks, which are then thrown back into the ocean to save space on board for the more valuable fins to drown— a practice known as shark finning. This practice is increasingly seen as cruel and wasteful, and around the world regulations are being put into effect to end shark finning.
See 'Shark Protections' below. Sharks can play a large role in their ecosystems, no matter their size. Big predatory sharks require a lot of food. So the removal of too many large sharks can have a ripple effect on the populations of their prey: if you remove the sharks, too many prey are able to survive, and those then compete with one another and other animals for food, shifting the food web.
One of the types of prey that can be greatly affected by shark removal is smaller sharks and rays. Often, large sharks are among the only animals that eat small sharks. And so when large sharks are overfished, researchers sometimes see an increase in smaller shark populations. For example, as large sharks were removed from the coast of New England in the s by fisheries, dogfish catch actually went up five-fold into the late s.
This suggests that dogfish were able to thrive once their predators disappeared. But then, as fisheries went after dogfish at higher rates, their populations dropped in turn. Large sharks also commonly prey upon sea turtles, seabirds and marine mammals; in fact, sharks are some of the few predators of large marine mammals. Because of this, their presence or absence can have a large effect on prey populations. The presence of tiger sharks in Shark Bay, Australia, for example, changes the behavior of sea turtles, dolphins and dugongs , which avoid shark-infested waters even when food is abundant there.
One place where shark numbers have definitely decreased is on coastal coral reefs around the world.
See a Problem?
Healthy coral reefs far from human settlements have many sharks —far more than their top predator counterparts like lions on land. But when humans move in, sharks disappear unless they are protected. A recent study found that in the Pacific islands, shark density is only percent what it would be if no people lived in the area.
Because humans have lived near reefs for so long, it's hard to know what these ecosystems should look like with a healthy number of sharks—and thus what effect the removal of sharks is having. Recent studies of remote uninhabited islands show that top shark predators outnumber their prey , in some cases making up 50 to 80 percent of the biomass on a reef! They are able to maintain this ratio because of the speedy transfer of energy up the food chain. Shark populations have been in trouble for decades due to overfishing. In , the International Union for the Conservation of Nature IUCN Redlist released a report from its Shark Specialist Group that reviewed the status of 64 species of open ocean sharks and rays and found that 32 percent were threatened with extinction.
The report called on governments to increase protections of sharks through science based catch limits, end shark finning and improve monitoring and research, among other recommendations. The law said that fishing vessels could not transport or possess shark fins without the corresponding shark body within miles of U. The fins could be separated from the animal aboard the ship, but the carcass must also be kept on board.
However, there were several loopholes in the legislation that let people transfer fins on non-fishing vessels, and the sale and trade of fins were not addressed. The law also was difficult to enforce. For example, regulators typically make sure fishermen aren't breaking this type of law through a shark fin conversion ratio. Measurements of the weight of shark fins are taken and compared to the weight of the remainder of the sharks; if the fins weigh more than an established ratio, it is presumed that illegal shark finning was taking place. Under the Shark Finning Prohibition Act, the shark fin conversion ratio was 5 percent.
As a result, illegal fishers are sometimes able to fake the fin ratio, leaving some shark bodies behind in the water while fooling regulators. In the Shark Conservation Act was signed into law. Hawaii was the first U. In addition to finning bans in the U. These plans reflect the results of research, population assessments and work with fishermen.
Additionally, two populations of scalloped hammerhead sharks were listed under the U. Endangered Species Act in July , making them the first sharks protected under the law. Reducing the accidental catching of sharks as bycatch has also been an important goal. In California, for example, the banning of nearshore gillnets has reduced shark mortality. Similarly, changes in hook and fishing line design make it easier for sharks to escape and improve their ability to survive after their release when they are caught by mistake. Two seals are perched on a rock.
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