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An ancient species of requiem shark has been identified using fossil teeth and named after a University of London and Museum researcher.
Today, requiem sharks are found in warm waters around the globe. Many of the species are migratory and viviparous, meaning they give birth to live young. The group includes several well-known species such as the tiger, blue and blacktip reef sharks, but also a host of many lesser-known species such as the graceful and the nervous sharks.
Requiem sharks can be found in fresh, salt and brackish waters. They look classically 'shark-like', with torpedo-shaped bodies and round eyes, perfect for an active predator.
The new species, Carcharhinus underwoodi, has been identified using its distinctive fossil teeth, found in soft rocks exposed on the coastal cliffs of Madagascar. It lived during the Eocene about 40 million years ago.
David Ward, a Research Associate at the Museum who has been studying the ancient fossils, says, 'It is now the oldest named requiem shark and has been named after Charlie Underwood, a researcher at the Museum who has published on ancient requiem sharks.'
Fossils discovered on the coast of Madagascar show that the warm waters around the island during the Eocene would have been a lot like they are now, expect in one spot that supported a curious ecosystem unlike any around today.
Home not only to numerous species of sharks, the area also hosted an extraordinary number of rays.
David says, 'The environment was probably shallow water, most likely protected within a reef, except that this ecosystem was dominated by eagle rays.
'There is no environment to compare this with today.'
There are a few dozen species of eagle rays swimming around the world's oceans today. These cartilaginous fish can grow to large sizes, with most preferring more open water.
The researchers were able to identify these animals in the fossil record due to the distinctive teeth left behind. Not only can the shape of these teeth help identify species, it also gives an indication of the animals' size.
'These were big eagle rays several metres across,' says David. 'Now that suggests high productivity as while most rays live on the bottom, eagle rays don't.'
Eagle rays are more active predators, tending to feed in open water on small fish around the size of a sardine. But the fossils show that these eagle rays were possibly not the only rays around at the time, as David has also been able to identify the barbs from the tails of stingrays and a single tiny tooth.
David and his colleagues have been revealing what else was swimming in this strange, ancient world. They have managed to identify not only three species of ray, but also seven species of shark. Their results are published in PLoS ONE.
'The lagoon had a broad range of sharks, from fairly small ones that lived on small crustaceans through to rather larger sharks that would normally live in the open ocean on medium- to large-sized fish,' explains David. 'That is slightly contradictory, because usually in shallower inshore waters you tend to get one type of shark predominating.'
Interestingly, it seems that this may not be the first time that C. underwoodi teeth have been collected before.
The researchers think that previously unidentified teeth found in Egypt, Pakistan and the USA may also belong to this new species. This early requiem shark probably had an incredibly wide distribution, not unlike what is seen with some of these species today.
'This is the beginning of the requiem shark radiation,' says David. 'It suggests that the lineage - or the radiation at least - may have originated in tropical waters and that we only know part of its story.
'As soon as you think you know what the oldest one is, someone will find an older one,' adds David, 'but this at the moment is oldest.'
The next stage will be to get more material from the Madagascar deposit and start looking at the smaller fossils that were likely missed by this initial collection. From this, David hopes to build a much more rounded image of what this peculiar environment was like, including not only smaller sharks but also the fish on which they preyed.