Seeing elephants in the wild is a timelessly awe-inspiring expertise. There are solely three dwelling species immediately: the African savannah elephant, African forest elephant, and Asian elephant.
They are the remnants of a as soon as affluent lineage of megaherbivores referred to as proboscideans, whose evolutionary epic spanned 60 million years and a few 200 species. The African continent was the centre stage of this story.
But, till now, it hasn’t been clear how pure choice favoured trendy elephants as the solely tiny surviving department of proboscideans.
A new examine by palaeontologists Juha Saarinen and Adrian Lister offers the solutions. Lister supervised my PhD, which targeted on evolutionary relationships of fossil elephants. Saarinen hosts my present postdoctoral place at the University of Helsinki, as we’re pursuing analysis stemming from these findings.
They targeted on the emergence of elephants’ extremely specialised, multi-plated cheek teeth from the primitive dentition of earlier proboscideans. Their conclusions show that behavioural changes in response to altering environments can produce sustained transformative developments in animals’ adaptive buildings over tens of thousands and thousands of years.
Teething issues
Modern elephants’ diets consist of copious quantities of robust, fibrous, typically low-quality fodder. They’re in a position to eat this fashion as a result of of their teeth: six large grinding teeth per jaw half that erupt and expend sequentially in a horizontal “queue” all through the animal’s life, every successively bigger than the earlier tooth. The grownup molars are as massive as bricks.
Enamel ridges that align longitudinally alongside the tooth shear up the vegetation like a cheese grater as the jaws transfer. These teeth even have excessive crowns that repeatedly erupt into place as the older chewing floor wears away, like lead pushing up in a mechanical pencil. This helps elephants take care of steady tooth attrition from chewing.
The fossil document reveals that, 20 million years in the past, proboscidean teeth regarded utterly totally different. These teeth had rounded cusps aligned in pairs, versatile for chewing up a variety of plant meals in the primeval forests and thickets they inhabited. But they had been ill-suited for tackling giant portions of robust, abrasive vegetation.
Scientists theorised that two main elements drove the evolution of wear-resistant dental options seen in elephants immediately. One was the unfold of grass that might tolerate a cooler, drier and extra seasonal climate than the cosy forest houses ancestral proboscideans occupied. The second was that giant herbivores ingested extra adhesive mud in these new arid environments than they’d earlier than.
Both challenges arose inside the previous 20 million years. But it wasn’t clear precisely how these two elements drove adaptive modifications in herbivore teeth. The idea additionally offered a chicken-and-egg paradox: which took off first, the evolving teeth or the altered habitats?
The proof
Saarinen devised a easy quantitative technique to resolve this conundrum. It’s based on a primary mechanical precept. The approach grazers eat wears the teeth flat, whereas browsers’ teeth have a extra “edged” chewing floor. That means grazing elephants ought to develop angles of put on on their teeth which are wider than these of searching elephants.
To take a look at this concept, Saarinen first measured current elephant teeth from Kenya’s Tsavo East National Park, which confirmed his preliminary postulation. He and Lister then turned to Africa’s wealth of geological and fossil data.
Their tooth put on measurements from fossils revealed that, about 21 million years in the past, the first enhance in dietary uptake of grass was achieved by the primitive “gomphothere”-type proboscideans of Africa. Gomphotheres had been the generalised ancestral inventory from which trendy elephants and their different extinct relations advanced.
They additionally found that these gomphotheres grazed extra as grass cowl expanded throughout their habitats between 21 million and 6 million years in the past, as indicated by plant fossil fragments from the websites the place the gomphothere teeth had been discovered. Yet this was achieved with little structural evolution of their teeth.
This was a increase interval for African proboscidean range. Habitats containing three or 4 proboscidean species had been frequent. Some species adopted grazing to minimise competitors with neighbouring pachyderms that most well-liked softer fodder.
Ultimate survivors
To scrutinise beforehand theorised hyperlinks between climate, vegetation change and elephant evolution, Saarinen and Lister examined data from sediment cores drilled off Africa’s west and east coasts over earlier many years. Those cores are primarily archives, capturing tens of thousands and thousands of years of environmental modifications on the continent. Arid durations are marked by dustier segments of the core.
Statistical testing of the knowledge confirmed that the African setting started to get drastically extra arid between 7 million and 5 million years in the past, and this drove an unlimited enhance in the fee of dental evolution in the lineage that gave rise to mammoths and trendy elephants, in addition to selling the additional unfold of grasslands.
Thus got here proof that the high-crowned, multi-plated molars of elephants advanced primarily to face up to grit, quite than fibre, of their meals. This new dental kind offered such a bonus that, round 3.6 million years in the past, early elephants had outcompeted the final African gomphotheres into extinction.
New analysis avenues
The zenith of this evolutionary ratchet was Palaeoloxodon. It was the largest elephant to evolve, standing over 4 metres tall at the shoulder and weighing 12–15 tonnes. With brick-sized molars packing as much as 21 grating plates, Palaeoloxodon was the undisputed prime megaherbivore of the African savannah between 1.5 million and round 130,000 years in the past, in each its stature and feeding diversifications.
Yet, inside the final million years of our story, when African savannahs turned more and more arid and seasonal simply as they did for the earlier 5 million years, these spectacular grazing specialists vanished. Why?
Upon discussing his findings, Saarinen and I converged on a speculation. Modern African elephants are half the measurement of their prehistoric cousins, with teeth much less specialised in the direction of a grass-dominated food plan. This could also be key for elephants to tolerate the arid shrublands of immediately’s Africa, which have little grass cowl to assist such monumental grass-dependent vegetarians as Palaeoloxodon.
We’re analyzing this as half of our present analysis, hoping to decipher whether or not the encroachment of grasslands by shrubby arid zones drove the extinction of Palaeoloxodon and different spectacular big herbivores in Africa over the previous million years. The story we finally inform could function a stark warning for a way climate change will additional decimate African biodiversity, if it’s left unchecked.
