New evidence has been provided confirming previous compelling geological findings that today’s continents were once linked in one giant land mass. The evidence has come through the discovery that two fossils, one from South Africa and the other from Brazil, were cousins.
The discovery of a Brazilian plant-eating herbivore fossil in 2008 prompted a restudy of the South African cousin of the same size and with a remarkably similar skull discovered 10 years earlier. These two species from Gondwana – the ancient super continent formed by now separated southern continents such as Africa and South America – show features in their skull and teeth that indicate they were closely related.
Close examination of the two skulls, identified as four-legged or tetrapod animals that date back to a time before dinosaurs existed, revealed two further astonishing facts. The first is that 270 million years ago they were already capable of chewing their food like modern ruminants such as cattle, sheep, goats and deer.
The fossils, which date from what is known as the Middle Permian period, also show that the plant-eating tetrapods had developed two specialisations that they used in combat – a feature typical of today’s cows and deer.
And the most fascinating aspect of all is that these not too distant cousins were found more than 8000 kilometres apart on different modern day continents.
The deers of yesteryear
Living mammals have a rich history documented by fossils going back 300 million years. Ancestral lineages of mammals were included in a group known as therapsids that flourished during the Permian, which predated the age of dinosaurs, and are exquisitely documented in the Karoo Basin of South Africa.
Plant-eating animals are now far more diverse and abundant than carnivores, a trend that began during the Permian. A particular group called anomodonts can best be described as the “Permian deers”. Besides being plant-eating and the most abundant lineage of the Permian, anomodonts were extremely variable in size. They were also very different in their shapes, particularly the earliest members of the group.
The Brazilian fossil had some unexpected features for a herbivore. Three stand out. The first is that it had occluding teeth that allowed them to chew, or masticate, food – a feature that is a landmark of today’s mammals.
The second is that it had a long outsized blade-like canine (~120 mm long). This shows, for the first time, the presence of saber-tooth in herbivores mammals around 270 million years ago. Saber teeth are found in some great carnivores from the past such as the gorgonopsians or the Smilodon sabre-toothed cat, and other Ice Age cats.
But carnivores do not need to chew their food, so that the Brazilian anomodont had several occluding teeth proved that it was a dedicated herbivore after all.
But the surprises did not end there. Tiarajudens eccentricus, the Brazilian species, show teeth that are commonly located at the margin of the mouth, positioned on a bone of the palate. The novelty is that no other therapsid was known to possess teeth in this bone. In fact no other therapsids are known to have complex, molar-like teeth (molariforms) in the roof of their mouths.
After additional cleaning of the bones of the fossil found in South Africa, called Anomocephalus africanus, it was found it also had molariforms in the palate, identical to those of Tiarajudens. The South African fossil has a complete mandible and its teeth are in contact with the palate, confirming the occlusion between upper and lower teeth. The only apparent difference between the two fossils is the absence of blade-like canines in the African species.
The skull of these cousins are nearly the same size – between 210 and 220 mm. They show a domed profile with a very short snout, large orbits, and temporal opening for chewing muscles about the same size or slightly larger than the eye socket.
The long canine in the Brazilian species is represented in a few living deer such as the Asian water-deer, musk-deer, and muntjacs. In all these cases the enlarged canines are used in male-male visual displays during fighting. The long canine in Tiarajudens eccentricus is being interpreted as an indication of its use in a similar way, representing the oldest evidence of use of canine in a herbivore for male-male fight.