By Paul Rincon
Science reporter, BBC News
The mammoth skull is estimated to be about 400,000 years old
The "extremely rare" fossilised skull of a steppe mammoth has been unearthed in southern France.
The discovery in the Auvergne region could shed much needed light on the evolution of these mighty beasts.
Many isolated teeth of steppe mammoth have been found, but only a handful of skeletons exist; and in these surviving specimens, the skull is rarely intact.
Palaeontologists Frederic Lacombat and Dick Mol describe this skull specimen as being well preserved.
Mr Lacombat, from Crozatier Museum in nearby Le Puy-en-Velay, and Mr Mol, from the Museum of Natural History in Rotterdam, the Netherlands, said the fossil belongs to a male steppe mammoth (Mammuthus trogontherii) that stood about 3.7m (12ft) tall and lived about 400,000 years ago, during Middle Pleistocene times.
The animal was about 35 years of age when it died, the researchers estimate.
The steppe mammoth is of vital importance for understanding mammoth evolution.
It represents the transitional phase between an ancient species known as the southern mammoth and the more recent woolly mammoth.
But comparatively little is known about this intermediate stage.
"This specimen is of extreme importance because we don't know that much about the Middle Pleistocene," Mr Mol told BBC News.
"Lots of the sediments have been eroded and not so many localities are known where we can find fossils.
Southern mammoth (Mammuthus meridionalis) - lived from 2.6 million years ago to 800,000 years ago, during the Early Pleistocene
Steppe mammoth (Mammuthus trogontherii) - lived from 800,000 years ago to 300,000 years ago, during the Middle Pleistocene
Woolly mammoth ( Mammuthus primigenius) - lived from 300,000 years ago to 4,000 years ago, during the Late Pleistocene and Holocene
He added: "We cannot keep saying that we have the [southern mammoth] at the beginning of the Pleistocene, then we have something which we are not sure about, and finally we have the woolly mammoth [at the end of the Pleistocene].
"We need to find what I call the 'missing link' in mammoth evolution."
The southern mammoth appears to have lived in a savannah environment, and was probably a "browser", feeding on trees and shrubs.
However, the molar teeth of steppe mammoth and woolly mammoth show that these animals were adapted to grazing.
This is thought to represent an adaptation to climate change; as conditions got colder and drier over the Pleistocene period, the savannah disappeared, making way for grassy steppe. Mammoth had to adapt their diets accordingly.
"If they have a complete skull then that would be very valuable," Dr Adrian Lister, a mammoth expert from London's Natural History Museum and University College London, told BBC News.
One of the best preserved examples of a steppe mammoth was excavated in the cliffs of West Runton in Norfolk, UK.
"With West Runton, we have a fabulous skeleton. It has its jaws and teeth, but the whole top part of the skull has gone. And that is usually the case with these fossil elephants," explained Dr Lister.
According to a theory developed by Dr Lister with other researchers, the southern mammoth was once widespread in Eurasia. It then evolved into a cold-adapted form - the steppe mammoth - in eastern Asia, where the climate has been chilly for the last two million years.
However, when Ice Age conditions took hold across the northern hemisphere, the steppe mammoth spread outwards, replacing its predecessor in Europe and Asia.
A similar process may have later led to the emergence of the woolly mammoth. According to Dr Lister, it evolved from the steppe mammoth in north-east Siberia, then expanded its range during an Ice Age, eventually displacing its forerunner the steppe mammoth.
The skull is being transported to a nearby museum
However, Dick Mol takes a different view. He thinks evolutionary changes in the mammoth lineage take place too quickly under this model.
Instead, he favours a model in which Europe is the centre for mammoth evolution.
Two molar teeth belonging to the newly discovered specimen were found in 1986, during the construction of a water pipeline.
Frederic Lacombat was able to trace the site of this discovery and subsequent excavations revealed the skull from which they had come.
The team plans to lift the skull out of the ground and transport it on a truck to Crozatier Museum.