Cross a crocodile and a fish…
The two new species, named Tutusius umlambo in honour of Archbishop Emeritus Desmond Tutu, and Umzantsia amazana to affirm its South African origin, are the only Devonian tetrapods ever discovered in Africa and the only ones ever found in what was once within the Antarctic Circle. They are a remarkable 70 million years older than any other four-legged vertebrates ever found in Africa.Dr Robert Gess of the Albany Museum in Grahamstown has discovered fossils of two unique tetrapods that will force a significant rewrite of the story of life in Africa during the Devonian Period, 360 million years ago.
The significance of this discovery lies in the place where they were found. When the four legged vertebrates paddled around the shallow lagoon, in what is now part of Waterloo Farm just outside Grahamstown, it was much closer to the South Pole than it is now.
Until this major discovery, palaeontologists had only found Devonian tetrapod fossils in areas that lay within the tropics during the Devonian period. These were the first four-legged animals, from whom all later four-legged animals, including amphibians reptiles, birds and mammals are descended. Scientists therefore had good reason to believe that stem tetrapods evolved, and lived only in parts of our planet between 30 degrees north and south of the equator. At first these creatures probably still lived mainly in water but having developed legs and advanced air breathing they were later able to colonise the land.
Scientists assumed, based on the known fossil record, that vertebrates moved from water onto land (terrestrialisation) in the tropics and therefore attempted to understand the causes of this major evolutionary step by studying conditions prevalent in tropical water bodies.
Palaeontologists focusing on how certain fishes evolved the abilities to inhabit the land will now have to take into consideration the two tetrapods and their surroundings discovered on Waterloo Farm.
The approximately metre-long Tutusius and the somewhat smaller Umzantsia are both incomplete. Tutusius is represented by a single bone from the shoulder girdle, whereas Umzantsia is known from a greater number of bones – but they both appear similar to previously known Devonian tetrapods.
Gess says that when the tetrapods were alive, “they would have resembled a cross between a crocodile and a fish, with a crocodile-like head, stubby legs, and a tail with a fish-like fin”.
The evolution of tetrapods from fishes during the Devonian period was a key event in our distant ancestry. During the Devonian Period, tetrapods were largely aquatic so we can imagine that most of the time they swam or walked around in shallow parts of the estuary that was to become part of Waterloo farm.
Occasionally they may have left the water either to escape from a predator or to chase down some prey. It is also possible that they would have sought isolated pools where they could protect their young.
Their gradual transition to becoming land animals is pivotal to our existence because the basic tetrapod body plan – a spine with four limbs is the same design that has carried through to all four-legged vertebrates today. Indeed virtually every bone in our bodies can be matched back to one in these early creatures.
The descendants of stem tetrapods include all amphibians, reptiles, birds and mammals alive today. In some cases the front limbs of the animals have evolved even further so that the front legs of all birds and bats have been modified to form wings while human front legs are no longer used for walking. All us tetrapods are descended from the largely aquatic tetrapods that are found in the Devonian period, which already had four legs but which still had a finned tail.
An astonishing discovery: ‘I’ll never forget that afternoon’
Dr Rob Gess had always hoped to find the remains of a Devonian tetrapod at Waterloo Farm even though the text books had warned him that this was an unlikely prospect. Yet one day he was splitting shale with Chris Harris, one of his students, when he found the cleithrum of Tutusius.
He describes the moment, “I just knew that this was what I’d spent years looking for. I went all quiet and then abandoned what I was doing and went to fetch the literature just to double check. I’ll never forget that afternoon!”
Gess wanted further confirmation that he had correctly identified the fossil so he sent photos to international colleagues including Professor Per Ahlberg in Sweden, one of the world leaders in the study of Devonian tetrapods.
Ahlberg was thrilled with the images, “The moment I saw Rob’s first photo of the shoulder girdle of Tutusius, which he sent me as an email attachment, I knew he’d got this right”. Ahlberg has spent many years studying the shoulder girdles of the earliest tetrapods so he “knew that the shape of this particular bone, the cleithrum, is absolutely characteristic”.
He said that when Rob Gess found the cleithrum of the second tetrapod Umzantsia – which is quite different and perhaps even more primitive than that of Tutusius – “we both began to realize what a remarkable discovery Rob had made.
“When we were finally able to work on the material together in Grahamstown, we quickly discovered even more bones, turning this into one of the most significant tetrapod discoveries of recent decades. I am thrilled to have been invited onto this project.”
Gess and Ahlberg have written about the tetrapods in an academic article which is being published internationally today in Science magazine.
Dr Rob Gess is South Africa’s leading researcher on Devonian estuarine ecosystems and early vertebrates (ancient fish and early tetrapods). The driving force behind research on Waterloo Farm he has excavated thousands of specimens from the black shale which have so far allowed for the description and naming of 20 species new to science and the reconstruction of an entire estuarine ecosystem. He is a member of the DST-NRF Centre of Excellence in Palaeosciences (which is based at the University of the Witwatersrand) who together with the Millennium Trust, support his research at the Albany Museum in Grahamstown. He lectures in the Rhodes University Geology Department.
Professor Per Ahlberg is the head of the Evolution and Development research programme at Uppsala University in Sweden. He is one of the world leaders in the study of Devonian tetrapods: alone or in collaboration with various co-authors, he has described the earliest known tetrapod body fossils and footprints as well as several new tetrapod genera (Elginerpeton, Ventastega, Sinostega, Ymeria, Occidens) that greatly expand our understanding of these earliest four-legged animals. He also investigates other areas of early vertebrate evolution, such as the origin of jaws.