Misery makes for strange bedfellows: hostile Earth pushes early Triassic mammal and amphibian to share den

25-06-2013

X-ray imaging at the ESRF has revealed a world-first in palaeontological records of a mammal like reptile, Thrinaxodon, nestled together with a primary aquatic amphibian, Broomistega. The odd couple were fossilised together in the burrow of the mammal more than 250 million years ago. Scientists from the ESRF inspected images of the two skeletons to understand the reason for their cohabitation. The results of this research were published in PLoS ONE on 21 June 2013.

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Two hundred and fifty million years ago, at the end of the Permian, almost all life on Earth was wiped out, with only 10% of plants and animals surviving. This mass extinction is known as the Permo-Triassic extinction, or the Great Dying, and is the most dramatic biological crisis ever to have affected life on Earth.

While the causes behind this mass extinction are difficult to pin-point, theories converge towards a sequence of major catastrophes including the collision of a celestial object with Earth, gigantic volcanic eruptions with the release of toxic elements into the air and water and a global warming making the Earth a very hostile home to the survivors of the extinction.

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Among the sequence of catastrophes believed to have caused the "Great Dying", one of the largest known volcanic events in Earth's geological history spanned a one-million-year period at the Permian-Triassic boundary. This was when the Siberian Traps were formed, covering an area larger than western Europe.

The geological record of South Africa documents that amongst the surviving creatures, mammal-like reptiles (the forerunners of mammals) responded to the harsh climatic conditions by digging burrows. Hundreds of fossilised burrows have been discovered in the strata chronologically following the extinction event, testifying that this key adaptation saved our distant ancestors.

These fossilized burrows are produced when rain or flood events fill the underground shelters with sediment, casting the shape of the tunnel, and entombing every creature remaining inside.  Although these burrows represent potential precious insight into ancient life, they have until now been neglected by investigators due to the fact that the fossils inside are completely enclosed in a sediment cast and invisible to the eye.

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3D imaging of the fossilised creatures. Credit: V. Fernandez (ESRF)

The recent development of X-ray imaging at the ESRF has made it possible to scan the contents of the burrow casts and create 3D images of what they contain without breaking into the cast or damaging the fossils inside. "This type of research is only possible at one place in the world: the ESRF", says Vincent Fernandez, post-doctoral fellow on ESRF's ID19 beamline, and one of the main authors of the paper. "Only here do we combine beam characteristics and size to use propagation on such a large specimen".  The fossilised burrow measures roughly 30 cm x 15 cm.

The images obtained at the ESRF are strikingly clear compared to conventional X-ray scans. Scientists can pick out very fine details on the image which enable them to better reconstruct scenarios and understand life on earth millions of years ago. "More than just the skeleton, the X-ray imaging provides us with clues about the sedimentological context, whether the creatures were drowned or were entombed long after their death, for example", says Vincent.

 

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Comparison of results from a traditional laboratory microtomograph and X-ray images obtained at the ESRF. Credit: ESRF/ V. Fernandez

Earlier this year, several burrows from the collection of the Evolutionary Studies Institute (University of the Witwatersrand, Johannesburg, South Africa) were scanned at the ESRF in collaboration with experts from South Africa, Indiana University's Department of Anthropology (USA) and the Museum of Central Australia. The scans revealed the improbable scene of these two different species lying side by side:  the main occupant of the burrow, a mammal-like reptile Thrinaxodon, with a Broomistega, a primary aquatic amphibian belonging to the extinct group of Temnospondyle. "The discovery that two animals were fossilised together inside the burrow was a really exciting moment", recalls Fernandez. "It was the very first burrow to be scanned here. We knew there was something inside the block, but not what it was. As the imaging progressed, very slowly, line by line, we first identified the head of the mammal. Then, much later, we started to make out small teeth and a second jaw nestled in the shoulder of the first animal. It was amazing to think we'd hit lucky on our first scan!"



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Artist's impression of Broomistega seeking shelter in Thrinaxodon's burrow. ESRF/V. Fernandez.

 From the patterns formed by the sediment and the fact that both skeletons are undisturbed and in pristine condition, it can be assumed that the two animals were buried during a flash flood event, probably while both still alive. Careful analysis of the amphibian skeleton uncovered several consecutive broken ribs, some of them indicating that healing of the fractures was initiated several weeks before death. The absence of tooth marks on the skeleton, common when an animal feeds on another, plus the fact that the skeletons were not dismantled, rules out any predator-prey relationship, including scavenging.

While co-habitation of burrows by different species is known to happen, these situations remain rare and correspond to very specific patterns of interactions. The case of the amphibian and the mammal-like reptile is unexpected because both animals are more or less the same body size, they have a similar diet, and because the amphibian likely could not benefit the main occupant by offering antipredator vigilance.

Using analogies with modern animals and the fact that many mammal-like reptiles have been found in curled-up positions, an obvious resting posture, the most probable explanation for the association is that this mammal-like reptile was in deep torpor, unable to escape the flooding or to evict intruders. This special dormancy in a warm environment, called aestivation, is elicited by a paucity of food and water resources; it is a physiological state in which metabolism is reduced to a minimum to avoid wasting energy during the most critical time of the dry season.

The injured amphibian took advantage of this torpor state of the mammal-like reptile to crawl into the burrow to shelter and rest. Both animals were ultimately trapped and drowned by the flooding event, then miraculously preserved side by side for the last 250 million years.


Reference
Synchrotron Reveals Early Triassic Odd Couple: Injured Amphibian and Aestivating Therapsid Share Burrow, V. Fernandez et al., PLoS ONE 8(6): e64978

 

Text by Kirstin Colvin

Top image: 3d image showing a mammal-like reptile nestled with a primary aquatic amphibian. Credit: ESRF - V. Fernandez.