Scientists from South Africa, Australia and France have
discovered a world first association while scanning a 250 million year old
fossilised burrow from the Karoo Basin of South Africa.
The burrow revealed two unrelated vertebrate animals nestled
together and fossilised after being trapped by a flash flood event. Facing
harsh climatic conditions subsequent to the Permo-Triassic (P-T) mass
extinction, the amphibian Broomistega and the mammal forerunner
Thrinaxodoncohabited in a burrow.
Scanning shows that the amphibian, which was suffering from
broken ribs, crawled into a sleeping mammal's shelter for protection. This
research suggests that short periods of dormancy, called aestivation, in
addition to burrowing behaviour, may have been a crucial adaptation that
allowed mammal ancestors to survive the P-T extinction.
The results of this research resulted in a paper entitled
Synchrotron reveals Early Triassic odd couple: injured amphibian and
aestivating therapsid share burrow and that is published in the scientific
journal, PLoS ONE.
How the research was
The international team of scientists was led by Dr Vincent
Fernandez from Wits University, South Africa and the European Synchrotron
Radiation Facility (ESRF) in Grenoble, France. The other authors from Wits
University include Prof. Bruce Rubidge (Director of the newly formed
Palaeosciences Centre of Excellence at Wits), Dr Fernando Abdala and Dr
Kristian Carlson. Other authors include Dr Della Collins Cook (Indiana
University); Dr Adam Yates (Museum of Central Australia) and Dr. Paul Tafforeau
After many impressive results obtained on fossils,
synchrotron imaging has led to revived interest in the studies of the numerous
fossilised burrows discovered in the Karoo Basin of South Africa and dated to
250 million years ago. The first attempt to investigate one of these
burrow-casts surprisingly revealed a world-first association of two unrelated
The fossil was recovered from sedimentary rock strata in the
Karoo Basin. It dates from 250 million years ago, at the beginning of the
Triassic Period. At that time, the ecosystem was recovering from the
Permo-Triassic mass extinction that wiped out most of life on Earth. In the
Pangea Supercontinent context, what is now South Africa was an enclave in the
southern half called Gondwana. It was the scene of pronounced climatic warming
and increased seasonality marked by monsoonal rainfall.
To survive this harsh
environment, many animals, including mammal-like reptiles (mammal forerunners),
developed a digging behavior, attested by the numerous fossilized burrow casts
discovered in the Karoo Basin. These casts have long been thought to enclose
fossilized remains, triggering interest from palaeontologists. Early this year,
an international group of scientists started to research the contents of these
burrows using X-ray synchrotron computed microtomography.
Two burrow casts were selected from the collection at Wits
to be scanned using the state-of-the-art facility at the European Synchrotron
Radiation Facility (ESRF). Using the unique properties of the X-ray beam which
enables non-destructive probing, the scan of the first burrow started to reveal
the skull of a mammal-like reptile called Thrinaxodon, an animal previously
reported in another burrow.
As the scan progressed, the three-dimensional reconstruction
displayed results beyond expectations: the mammal-like reptile was accompanied
by an amphibian Broomistega, belonging to the extinct group of Temnospondyl.
"While discovering the results we were amazed by the
quality of the images", says lead author Fernandez, "but the real
excitement came when we discovered a second set of teeth completely different
from that of the mammal-like reptile. It was really something else".
Besides the pristine preservation of the two skeletons, the
team focused on the reasons explaining such an unusual co-habitation. Fernandez
explains: "Burrow-sharing by different species exists in the modern world,
but it corresponds to a specific pattern. For example, a small visitor is not
going to disturb the host. A large visitor can be accepted by the host if it
provides some help, like predator vigilance. But neither of these patterns
corresponds to what we have discovered in this fossilized burrow".
The scientists gathered all the information to try to
reconstitute the events that led to this incredible fossil aggregation, testing
scenarios one after another. "It's a fascinating scientific question: what
caused the association of these two organisms in the burrow? One of the more
obvious possibilities is a predator-prey interaction, but we inspected both
skeletons looking for tooth marks or other evidence implying predation,
ultimately finding no support for one having attempted to feed on the
other," says Carlson.
His colleague, Cook, adds that the consecutive broken ribs
resulted from a single, massive trauma. The amphibian clearly survived the
injury for some time because the fractures were healing, but it was surely
quite handicapped. According to Fernandez this Broomistega is the first
complete skeleton of this rare species that has been discovered. "It tells
us that this individual was a juvenile and mostly aquatic at that time of its
life," he says.
The scientists eventually concluded that the amphibian
crawled into the burrow in response to its poor physical condition but was not
evicted by the mammal-like reptile.
Numerous Thrinaxodon specimens have been found in South
Africa, many of them fossilized in a curled-up position. Abdala says: "I
have always been fascinated by the preservation of Thrinaxodon fossils in a
curled-up position that show even tiny bones of the skeleton preserved. It's as
if they were peacefully resting in shelters at the time of death".
The shelters prevented disturbance of the skeletal remains
from scavengers and weathering. "We also think it might reflect a state of
torpor called aestivation in response to aridity and absence of food
resources," Abdala says.
Piecing all the clues together, the team finally elucidated
the enigmatic association, concluding that "the mammal-like reptile,
Thrinaxodon, was most probably aestivating in its burrow, a key adaptation
response together with a burrowing behavior which enabled our distant ancestors
to survive the most dramatic mass extinction event. This state of torpor
explains why the amphibian was not chased out of the burrow," says
Both animals were finally entrapped in the burrow by a
sudden flood and preserved together in the sediments for 250 million years.
Tafforeau says: "Thanks to the unique possibilities for
high quality imaging of fossils developed during the last decade at the ESRF,
these unique specimens remain untouched, protected by their mineral matrix. Who
knows what kind of information we'll be able to obtain from them in the future
and which would have been completely lost if the specimen had been prepared out
of its burrow cast?"