For hundreds of thousands of years, a strange band of straight-tusked elephants were stomping across parts of Eurasia and Africa. Some species were small, such as the donkey-sized elephant found on the Mediterranean islands, while other species were among some of the largest land mammals that have ever existed.

The history of these prehistoric elephants, known as Palaeoloxodon, has long been a puzzle for scientists, but a new study has finally sought to get the facts straight. Reported in the journal Quaternary Science Reviews, an international team of scientists has categorized two members of the Palaeoloxodon genus into separate species based on their skull shape and other morphological clues. 

One of the main takeaways from the research was that European Palaeoloxodon belongs to its own single species. It was previously speculated the Palaeoloxodon found in Europe and India were the same species, but it appears they were two separate beasts: the Palaeoloxodon namadicus in India and the Palaeoloxodon antiquus in Europe. According to the research, the confusion was simply a matter of the forehead changing as they reach adulthood. 

"Just like modern elephants, Palaeoloxodon went through six sets of teeth in their lifetimes. This means we can tell the age of any individual with confidence by looking at its fossilised teeth,” Hanwen Zhang, who is based at the University of Bristol’s School of Earth Sciences, said in a statement.

"As I plotted various skull and limb bone measurements for these incredible prehistoric elephants, it became clear that the Indian Palaeoloxodon form a distinct group from the European ones; even in European skulls with quite pronounced crests, the skull roof never becomes as thickened as in the Indian specimens,” added Asier Larramendi, lead author and an independent researcher from Spain.

"This tells us we once had two separate species of these enormous elephants in Europe and India."

The skulls of this genus are often very unusual. One of the largest skulls discovered was around 1.4 meters (4.5 feet) tall and belonged to one of the largest land mammals ever, standing around 4 meters (13 feet) at the shoulder. On the other end of the spectrum, there is also Palaeoloxodon falconeri, a dwarf species that can be found on the Mediterranean island of Sicily that was the size of a donkey. 

The gigantic continental species feature a bulging crest along the top of the skull. The researchers say this feature most likely evolved to provide extra areas of the muscle to attach to the skull as a means to stop the animal from falling on its top-heavy head.

Now that the team has solved the question of the European and Indian species, they hope to move their knowledge and skills onto questions regarding the origin and early evolution of Palaeoloxodon in East Africa.

“Having gotten to the bottom of the antiquus/namadicus problem, it then became apparent that other fossil skull materials found in Asia and East Africa represent distinct, possibly more evolutionarily conservative species of Palaeoloxodon,” continued Hanwen Zhang.