Ancient animals show how elephants got their tusks

The earliest known tusks have been identified by researchers, giving a better understanding of how many iconic species evolved and why only modern mammals have true tusks. 

Tusks in a group of animals known as the dicynodonts, the closest relative to mammals, were found to have evolved over 200 million years ago. Teeth changed from being static and fused to bone into tusks which grow continuously out of the mouth, allowing for a range of new uses. 

Dr Kenneth Angielczyk, co-author on the study, says, 'Dicynodont tusks can tell us a lot about mammalian tusk evolution in general. This study shows that reduced rates of tooth replacement and a flexible ligament attaching the tooth to the jaw are needed for true tusks to evolve.

'It all ladders up to giving us a better understanding of the tusks we see in mammals today.' 

Their findings, led by American researchers, were published in Proceedings B.

What are dicynodonts?

Living before the time of the dinosaurs, the dicynodonts branched off from the ancestors of mammals around 270 million years ago in the Permian period. They survived for some 69 million years until going extinct at the end of the Triassic 210 million years ago. 

Dr Mike Day, Curator of non-mammalian tetrapods at the Museum and who was not involved with the study, says, 'Dicynodonts were a very successful group of therapsids, the branch of life which includes mammals.

'They are the main herbivores in the late Permian until about 250 million years ago and then begin to decline alongside the rise of the dinosaurs. They're very common generally, especially in Africa.' 

Dicynodont actually means 'two canine teeth', referring to the two large tusks sticking down from their upper jaw. This feature sets them apart from most of their relatives, including our mammalian ancestors, with some dicynodonts also having a keratinous beak not unlike a tortoise. 

'During the course of their evolution, the more derived dicynodonts lost most of their teeth,' Mike says, 'and instead had surfaces that probably would have attached to keratinous sheets in a similar way to turtles. However, they kept their canines which became the tusks.' 

The definition of a tusk, however, is difficult to pin down. A lot of features across the animal kingdom are called tusks, but these can refer to a vast range of features that are made of different materials, grow differently and have different purposes. 

In order to find out where tusks came from, the researchers first had to decide what a tusk actually is. 

After looking at the anatomy and morphology of tusks, the team concluded that a tusk must extend out of the mouth, grow continuously, be bound to the jaw by roots rather than fused together, and be made entirely out of dentine. Dentine is the material that supports regular teeth and is often covered in enamel. 

This definition means that elephants, hyraxes and others to pass muster, but not groups like rodents, which have an enamel ring on their teeth. Enamel helps protect teeth from damage but cannot be replaced. 

Lead author Dr Megan Whitney says, 'If an animal has a reduced amount of tooth replacement and a soft-tissue attachment, an ever-growing tooth allows it to get around the fact that it cannot replace the tooth. 

'Instead the tooth evolves to continuously deposit the same tooth tissues and as the animal continues to deposit the tissue, the tooth begins to move outside of the mouth to become functional.' 

The scientists were then able to use this definition to take a look back through time at the steps needed to make a tusk. 

Cutting their teeth 

Minute slices of fossilised dicynodont teeth allowed the researchers to decipher how a canine tooth becomes a tusk. 

They found that at first, teeth were fused to the bone and could be replaced multiple times. Over time, these teeth became replaceable less often and began to develop teeth roots. The final stage saw teeth become true tusks as they grew continuously and stopped being coated in enamel. 

The researchers believe that the adaptations required make each stage irreversible, driving development towards tusks continually forward. This resulted in the development of tusks occurring at a broadly similar time in a range of dicynodonts. 

'I expected there to be one point in the family tree where all the dicynodonts started having tusks, so I thought it was pretty shocking that we actually see tusks evolve convergently,' says Megan. 'This is a similar story to what we see in elephant evolution in that it mirrors a lot of the patterns that have been studied on how elephants got their tusks.' 

While the situation reflects some evolutionary developments in real life, the researchers hope to use their findings to discover more about what dicynodonts used their tusks for, and how tusks in modern animals have evolved. 

'Their use for predation was ruled out quite early on,' Mike says, 'but apart from that there's a lot of uncertainty over what they were used for. There isn't any evidence of microwear on the tusks that might indicate digging, so there are some suggestions they might be for display, like the modern water deer.' 

Co-author Professor Christian Sidor adds, 'We now have good data on the anatomical changes that needed to happen for dicynodonts to evolve tusks.  

'For other groups, like warthogs or walruses, the jury is still out.'