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Straight-tusked elephants were some of the largest land mammals ever to have existed, but no one has been quite sure just how many of the now-extinct species existed throughout Africa, Europe and Asia during the last million years.
By studying the shape of the skulls, researchers have reassessed the animals' evolutionary history.
The straight-tusked elephants, which belonged to the genus Palaeoloxodon, were a widespread group that lived during the Pleistocene (2.5 million to 11,700 years ago). But despite being well known from the fossil record, the exact number of species and their evolutionary relationships have remained something of a mystery.
Steven Zhang is a PhD student at the Museum who has been working with colleagues from Italy and Spain to resolve this confusion, by looking at the shapes of the animals' skulls.
These ancient elephants were unusual in that their skulls contain a feature not seen in any other species. Running around their head was a crest of bone jutting forward, like a headband. It is thought that this helped muscles to support their enormous heads. More importantly, the size and robustness of the crest varies on skulls from across Africa and Eurasia.
By looking that this, Steven and his colleagues determined that at least four separate species of Palaeoloxodon evolved between one million and 400,000 years ago across Africa, Europe and Asia, and there may have been two more species.
The results are published in Quaternary Science Reviews.
It's hard to understate just how huge straight-tusked elephants were.
Today, the largest land mammal is the African elephant, which stands around 3.3 metres at the shoulder and weighs up to seven tonnes.
Some straight-tusked elephants, however, may have reached up to 4.5 metres tall and could have tipped the scales at over 14 tonnes. This would make them possibly the largest land mammal ever to have existed, similar in size to the gigantic giraffe-like rhinos that lived in central Asia around 25 million years ago.
'The surviving elephants today are kiddies compared to some of the really gigantic extinct species,' says Steven. 'An adult straight-tusked elephant would easily have been able to rest its chin on the back of a bull African savannah elephant.'
Originating on the African continent, ancient DNA now suggests that Palaeoloxodon is most closely related to the living African elephants. Over the last million years the ancient elephants spread as far east as Japan, across Europe and as far north as the UK. In fact, remains of straight-tusked elephants have even been found beneath Trafalgar Square in London.
While those that inhabited the mainland remained massive, some animals made it onto a number of islands scattered throughout the Mediterranean. Isolated from the rest of the Palaeoloxodon populations on small islands, these elephants underwent what is known as island dwarfism to become miniature species of elephant.
The animals would have been living in a world vastly different to today. When they dispersed out of Africa, Palaeoloxodon would have encountered an environment in sway to the huge ice sheets that capped the northern hemisphere, and a continent already populated with elephants more familiar to the cooler climate, mammoths.
'It's really interesting,' explains Steven, 'because in Africa Palaeoloxodon is one of the classic indicator fossils of arid savannahs with very few trees, like what we see in east Africa today.
'But we know from tooth wear that once they got to Europe, they seem to have changed diet from grazing animals into browsing ones. This was probably to avoid competition with grazing mammoths, as mammoths migrated into Eurasia at least two million years before Palaeoloxodon did.'
The changes in diet were only one aspect that altered as the straight-tusked elephants moved north.
As Steven and his colleagues determined, there were also changes in the shape of their skulls around the large crest of bone. As the elephants spread eastwards this crest became larger and more pronounced, likely as a result of the increasing size of their heads.
They were able to show that the ancestor species, Palaeoloxodon recki, dispersed from Africa into the Near East roughly 780,000 years ago. From here they spread into India to become Palaeoloxodon namadicus, which was the largest species of straight-tusked elephant. They then colonised the Japanese archipelago, evolving into Palaeoloxodon naumanni by 400,000 years ago.
In Europe, some thought that two separate species existed, while others suggested that the differences seen were simply variation found within a single species. The data collected by the team show that mainland Europe was indeed home to just a single species of straight-tusked elephant, Palaeoloxodon antiquus, which itself gave rise to the number of dwarf species found on the islands of Crete, Cyprus, Sicily and Malta.
What's more, the team raised the possibility that there may have been at least another two species that lived in Eurasia: one in China and the other in Turkmenistan and India.
The straight-tusked elephants survived until at least 21,000 years ago before going extinct. While this means that they significantly overlapped in time with modern humans - one Palaeolithic site in Italy shows that people butchered these animals - whether humans actively hunted them is less certain.
Their demise is more likely linked to the dramatic changes in the environment that occurred throughout the Pleistocene, as the huge northern ice sheets expanded and contracted. This led to the expansion of the dry steppes which favoured the mammoths, and the contraction the of open woodlands which benefitted the straight-tusked elephants.
However, it's clear that these giants dominated the landscape for hundreds of thousands of years before they disappeared.