Peer Reviewed Articles on Recife and Great White Sharks
Sharks have been around for hundreds of millions of years, actualization in the fossil record before trees even existed. But what did they evolve from, are they 'living fossils', and how did they survive 5 mass extinctions?
Sharks vest to a group of creatures known as cartilaginous fishes, because almost of their skeleton is made from cartilage rather than bone. The but part of their skeleton not made from this soft, flexible tissue is their teeth.
The group includes the more famous animals such as whale sharks and great whites, but also all rays, skates and the little-known chimaeras (too known as ratfish, rabbit fish or ghost sharks).
While often referred to every bit living fossils, sharks have evolved many different guises over the hundreds of millions of years that they take been swimming the oceans.
When did sharks outset appear?
The earliest fossil show for sharks or their ancestors are a few scales dating to 450 million years ago, during the Late Ordovician Period.
Emma Bernard, a curator of fossil fish at the Museum, says, 'Shark-similar scales from the Late Ordovician take been found, but no teeth. If these were from sharks it would advise that the earliest forms could have been toothless. Scientists are still debating if these were true sharks or shark-like animals.'
Chimaeras are cartilaginous fishes, but not technically sharks. Information technology is idea that sharks and chimaeras may have diverged upwards to 420 million years ago. Today, many chimaera species are limited to the deep sea © NOAA/Wikimedia Commons
Analysis of living sharks, rays and chimaeras suggests that by around 420 million years ago, the chimaeras had already split from the rest of the group. Every bit there are no fossils of these animals from this period of time, this is based solely on the DNA and molecular show of mod sharks and chimaeras. Information technology was also around this time that the first plants invaded the land.
Oldest shark teeth
The earliest shark-like teeth nosotros have come from an Early Devonian (410-one thousand thousand-year-old) fossil belonging to an ancient fish called Doliodus problematicus. Described every bit the 'least shark-like shark', it is idea to have risen from within a group of fish known as acanthodians or spiny sharks.
'Acanthodians are not at all shark-like in shape, for case they take diamond-shaped scales and spines in front of all the fins,' says Emma. 'Just they do have a cartilage skeleton, a shark-like skull and jaw, and at least some shark-like teeth, which were oftentimes fused together.'
The first recognisable sharks
Past the middle of the Devonian (380 million years ago), the genus Antarctilamna had appeared, looking more than like eels than sharks. Information technology is about this time that Cladoselache besides evolved. This is the offset group that nosotros would recognise as sharks today, but information technology may well have been role of the chimaera branch, and so technically not a shark. Every bit active predators they had torpedo-shaped bodies, forked tails and dorsal fins.
Cladosleache is the first group to evolve that we would recognise every bit sharks, but they may in fact have been a type of chimaera ©No bu Tamura/Wikimedia Eatables
Golden age of sharks
The Carboniferous Period (which began 359 million years agone) is known as the 'aureate age of sharks'. An extinction event at the stop of the Devonian killed off at to the lowest degree 75% of all species on Earth, including many lineages of fish that once swam the oceans. This allowed sharks to dominate, giving rise to a whole diversity of shapes and forms.
Some of the nigh baroque prehistoric 'sharks' to appear during this time really evolved out of the chimaera lineage. These include Stethacanthus, which had a truly peculiar anvil-shaped fin on its back, Helicoprion with a screw fizz saw-like lesser jaw, and Falcatus, in which the males had a long spine jutting out of the dorsum and over the top of the head.
Mod-day chimaeras are much less diverse and typically live in the deep ocean. Growing upward to 1.5 metres long, they are not actually sharks. Their upper jaw is fused with the skull, and nigh chimaera also have venomous spines.
The origin of sharks' fearsome jaws
The end of the Permian Period (252 million years ago) saw nonetheless another mass extinction upshot, wiping out effectually 96% of all marine life. But a handful of shark lineages persisted.
The bluntnosed sixgill shark belongs to one of the oldest shark lineages, thought to have appeared by the Early Jurassic Period © NOAA Ocean Explorer/Wikimedia Commons
Past the Early Jurassic Period (195 1000000 years agone) the oldest-known group of modern sharks, the Hexanchiformes or sixgill sharks, had evolved. They were followed during the rest of the Jurassic past virtually modern shark groups.
Information technology was at this point that they evolved flexible, protruding jaws, allowing the animals to eat prey bigger than themselves, while likewise evolving the power to swim faster.
Shrinking sharks
At the beginning Cretaceous of Menses (145 million to 66 million years ago) sharks were once once more widely mutual and varied in the ancient seas, before experiencing their fifth mass extinction outcome.
While much of life became extinct during the End-Cretaceous extinction event, including all non-avian dinosaurs, sharks once again persisted.
But they were still affected. Fossil teeth testify that the asteroid strike at the stop of the Cretaceous killed off many of the largest species of shark. Simply the smallest and deep-h2o species that fed primarily on fish survived.
An extinct megalodon tooth compared to that of a modernistic dandy white shark shows just how big these ancient predators were © Trustees of the Natural History Museum, London
Dandy white shark evolution
Sharks before long began to increase in size once again, and connected to evolve larger forms throughout the Palaeogene (66 to 23 1000000 years ago). Information technology was during this time that Otodus obliquus, the ancestor to megalodon (Otodus megalodon), appeared.
O. megalodon is the biggest shark ever to take lived, and scientists consider it 1 of the almost powerful predators to have evolved.
Despite what many might think, megalodon is non related to smashing white sharks. In fact information technology may have been in competition with the great white shark's ancestors, which evolved during the Middle Eocene (45 million years ago) from broad-toothed mako sharks.
Hammerhead shark evolution
The youngest living group of sharks are thought to exist the distinctive hammerhead sharks.
Hammerhead sharks are thought to be the youngest shark group to have evolved, and may appointment dorsum simply 23 million years © wildestanimal/Shutterstock
There are at to the lowest degree eight different species of hammerhead shark, and while fossil teeth evidence suggests that their ancestors may have existed 45 million years ago, molecular information points to a much more than recent appearance during the Neogene (which began 23 million years agone).
The strange shape of their head is thought to mainly help in electroreception (the detection of naturally occurring electric fields or currents) as they hunt for casualty. It may also improve their vision, enhance their swimming and refine their ability to smell.
Since the Cease-Cretaceous mass extinction, sharks take come to dominate the oceans once again, returning to the role of apex predator along with big marine mammals.
Why are fossil shark teeth then common?
The vast bulk of shark fossils plant are teeth. This is down to two master reasons.
Because most of the skeleton of sharks is made from soft cartilage, information technology takes special conditions for this to preserve. The teeth, still, are made from a much tougher material known as dentin, which is harder and denser even than bone. While this enables a powerful bite, it also increases the risk that the teeth will fossilise equally they are less likely to decompose.
The teeth of sharks are the virtually mutual fossils, such as these that once belonged toOtodus obliquus, the ancestor to megalodon © The trustees of the Natural History Museum, London
The other reason is just numbers. Rather than having just a few sets of teeth that last all their life, sharks are continually producing new teeth. As an older one breaks or wears downwardly, information technology simply falls out of the front end of the mouth and onto the sea flooring, as a new tooth takes its place.
Depending on species and diet, over its entire lifetime a shark can produce betwixt 20,000 and twoscore,000 teeth.
This means that there is a much greater risk that a shark tooth will be preserved and turned into a fossil. Not only are the teeth the nearly common function of sharks to be establish, they're one of the most common fossils of whatever organism.
How did sharks survive 5 mass extinction events?
There is no single reason sharks survived all five major extinction events - all had unlike causes and different groups of sharks pulled through each one.
Ane full general theme, however, seems to exist the survival of deep-h2o species and the dietary generalist. It is possible that shark diverseness may also accept played an of import part.
During the 450 million years over which sharks evolved they have survived five mass extinction events © solarseven/Shutterstock
Emma explains, 'I call up information technology is rubber to say that it is partly because sharks are able to exploit dissimilar parts of the water column - from deep, dark oceans to shallow seas, and fifty-fifty river systems. They eat a wide variety of food, such as plankton, fish, crabs, seals and whales. This diversity ways that sharks as a group are more likely to survive if things in the oceans modify.'
Rather than sharks simply being incredibly hardy, it is more likely that their amazing diversity is the key to their success. It's no wonder they've been dominating the body of water for hundreds of millions of years.
Source: https://www.nhm.ac.uk/discover/shark-evolution-a-450-million-year-timeline.html
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