A 270 million-year-old pre-dinosaur species had sabre-teeth that males used to fight off competitors.
Tiarajudens eccentricus was discovered four years ago in Brazil and a full description of its skull, skeleton and teeth has now been published in the journal Royal Society Open Science.
The mammal ancestor baffled scientists when it was first uncovered as it had massive protruding sabre-tooth canines and occluding postcanine teeth – yet it was herbivorous.
Researchers from the Evolutionary Studies Institute at the University of the Witwatersrand said sabre-teeth belong to the large predatory Permian reptiles and the famous big cats from the Ice Age.
Tiarajudens eccentricus' discovery allowed the researchers to reanalyse the South African species Anomocephalus africanus, which was discovered a decade earlier – both species have similar features suggesting they were closely related, but the latter lacked sabre-teeth.
Another group of mammals living at the same time were also found to have extremely thick bones in their foreheads – interpreted as being used in head-butting combat. This is seen in many deer species today.
These factors combined led the researchers to speculate as to what they were used for – and said they were likely used in combat between males.
"The outsized, blade-like caniniforms of the herbivorous Tiarajudens allow several non-exclusive ecological interpretations, among which we favour intraspecific display or combat. This behaviour was an alternative to the head-butting practised by the contemporary dinocephalians."
Lead author Juan Carlos Cisneros said: "It is incredible to think that features found in deer such as the water deer, musk deer and muntjacs today were already represented 270 million years ago.
"Fossils are always surprising us. Now they show us unexpectedly that 270 million years ago two forms of interspecific combat represented in deer today, were already present in the forerunners of mammals."