When palaeontologists in France discovered one particularly well preserved salamander fossil over a century ago, they knew it was special because of how much of the body's outer details they could see. What they could not have known is what modern technology just recently taught scientists: the minerals that preserved the skin also preserved the animal's insides, including muscles, nerves, organs and even parts of the salamander's last meal!

Modern scanning technologies have allowed scientists to peer inside the ancient remains, where the fossilisation process captured not just the bones, but many of the tissues as well. Image: Jérémy Tissier

The salamander (it was originally identified as a lizard) is the only known specimen of a species called Phosphotriton sigei, and it's estimated at around 35-40 million years old. The ancient find comes from an area called the "Phosphorites de Quercy", a site in southern France rich in phosphate-containing rocks as well as abundant fossils. The amphibian left behind much of its torso and part of its back legs and tail. It is often called a "mummy" because of the exceptional preservation of its features, clear enough that you can even see its cloaca (the hind orifice).

"Mummy" isn't an ideal way to refer to this kind of fossil, since it conjures up thoughts of meticulously preserved Egyptian pharaohs – whereas this fossil was simply lucky enough to have its soft parts fossilised – but it has become a common term. 

"Obviously, [these] are not genuine mummies, but there is no available name for such exceptional fossils," explained researchers in a previous study of the salamander.

The fossil salamander Phosphotriton sigei, preserved in incredible outer detail (left) and inner detail (middle), including even bits of its last meal (right), as revealed by synchrotron study! Image: Jérémy Tissier

The latest study of Phosphotriton sigei, published in PeerJ by researchers from France and Switzerland, makes use of modern scanning technologies – particularly synchrotron X-Ray microtomography, a great alternative to slicing a fossil into pieces – to peer inside the ancient remains, where the fossilisation process captured not just the bones, but many of the tissues as well.

"Now, 140 years after its discovery, and 35 million years after the animal died, we can finally study it, thanks to modern technology," said researcher Michel Laurin in a press release. "The mummy returns!"

A 3-D reconstruction of the skeleton of Phosphotriton sigei. Video credit: Jérémy Tissier

Within the body of the salamander, researchers spotted the remains of the creature's digestive tract, one lung and a pair of organs by the pelvis that they aren't quite sure how to identify (they suspect these are cloacal glands, though they may be kidneys or testicles). Even more incredibly, the scans revealed preserved segments of the animal's spinal cord within some of its vertebrae, and a bundle of nerves by the hips called the lumbosacral plexus.

It is incredibly rare to find mineralised organs in the fossil record, and these might be the very oldest cases of some of these particular organs preserved in three dimensions!

And there was one more big surprise, too. Within the gut, the team noticed a handful of tiny bones (just a few millimetres long) that once belonged to a small frog. This is exciting because fossilised "last meals" are rare in the first place, but also because salamanders eating frogs is almost unheard of today. According to the researchers, this represents the oldest known example of a frog-eating salamander.

Despite its high level of preservation, identifying this animal is tricky, since relatively little is known about how soft tissues differ between salamanders, but the researchers suspect it is related to a group called Salamandridae, which includes modern-day newts and fire salamanders. If the team is right, this fossil, tens of millions of years old, is the only known case of a species in this group eating a frog!

"This fossil, along with a few others from the same site, is the most incredibly well preserved that I have seen in my entire career," said Laurin.

Other "mummies" known from the same area include snake fossils with still-visible scales, an ancient frog staring across time with its well preserved eyes and beetles whose fossilisation has allowed the study of even their guts and breathing passages.

Other exceptional fossil "mummies" from the "Phosphorites de Quercy" site: the frog Thaumastosaurus (left, from Laloy et al. 2016) and the beetle Onthophilus (right, from Schwermann et al. 2016).

Exactly how these fossils were so impressively preserved, however, is difficult to say because the precise spot where they were unearthed is unknown. The finds were dug up during a time of industrial focus on the phosphate rocks in the area, and their location of discovery was not recorded, leaving their exact age and the specific circumstances of their burial a bit of a mystery.

Despite the uncertainty, these "mummies" were likely buried quickly, and phosphate – the coveted resource that fuelled earlier excavations in the site – infiltrated the bodies, rapidly mineralising the bones along with the skin and organs. Sites of such exceptional preservation are called Konservat-Lagerstätte, and these scientists think this "lost locality" certainly qualifies!

Preservation of soft tissue like this is rare, but the fossil record continues to surprise us. Other recent examples, also revealed by modern scanning technologies, include the discoveries of fossilised fish hearts and bits of dinosaur brain!


Top header image: A fire salamander by Alberto Garcia/Flickr. Researchers suspect the ancient salamander was related to a group called Salamandridae, which includes both modern-day newts and fire salamanders.