 |
| Dr Robert Gess from Grahamstown and Prof Per Ahlberg from Sweden |
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 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.
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
focussing 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.
