In the adventurous landscape of the Isle of Skye off the north-west coast of Scotland, the skull of one of the oldest salamanders ever discovered to date was excavated from Jurassic limestone. But it would be decades before scientists had the technology and funding to piece the salamander together.
Part of the skeleton was collected in the early 1970s when paleontologists Michael Waldman and Robert Savage noticed black bone exposed on the hard gray rock surface, suggesting a fossil locked inside. They put it together and realized that it could be something important. Although parts of the fossil were subsequently uncovered, it was too little to justify a detailed study. Therefore, the fossil remained in the rock and unstudied for another 45 years.
Excursions to the site started again in 2004 and several fossils, including salamanders, were found. Roger Benson examined the block collected in the 1970s. He realized that the damaged surface matched a specimen he had collected in 2016.
Most bones collected on study trips are not examined immediately. It is difficult to get money for fieldwork, but it is even harder to secure funding to study the fossils you collect. It is not uncommon for them to be left unexplored for decades.
X-ray microCT scan revealed that the stone held the remains of a new fossil species of salamander: Mamorerpeton wakei. With its 166 million years, it is one of the oldest known salamanders, and it documents one of the earliest known stages of their development.
Salamander fossils are rare. Throughout the Jurassic period (201-145 million years ago), fewer than 20 species have been found. In contrast, we know of more than 450 dinosaur species. Salamanders are harder to find because they are small and delicate – but this lack of knowledge may also be due to a lack of scientific attention.
Paleontologists had the first hints of evidence of an extinct salamander species 30 years ago when parts of fossilized backbone and jawbones were found near Oxford in England. However, it was largely ignored by the scientific community in favor of research in Karaurus salamanders from Kazakhstan in the Middle Jurassic period. Until now Karaurus was often treated as the common ancestor of modern salamanders.
That Mammorpeton fossil bones are still preserved inside hard rock. Until we used X-ray microCT scanning, we were not sure about the contents. Most blocks were assembled without knowing exactly what was inside them. A fossil block found in 2016 turned out to be the other half of a specimen collected more than 40 years earlier from the same site.
Most of the skeleton was preserved, including the skull and tail. Transforming bones into digital models is painstaking work, but it allowed us to make a (unbroken) three-dimensional model of the skull, which is unprecedented for a fossil salamander.
Fossils are often collected on excursions, but they are not studied for many years due to lack of time or expertise. For the 1971 test, the edges of some bones were visible, but it would have been very difficult to remove the bones. Mechanical removal could have damaged them, but X-ray microCT scanning allowed us to see the bones clearly.
What we learned
Our new analysis sets the new species Marble marble inside the extinct group Karauridae. Members of this group all have skull bones with a crocodile-like ornament and have bone protrusions behind the eye. The new species is named after the late Professor David Wake, a leading American authority on salamander evolution.
The broad skull, deep tail and limbs with unfinished ends indicate Marble marble had an aquatic lifestyle similar to that of the living Hellbender Salamander in North America (Kryptobranchus) and the giant salamander in China and Japan (Andrias). They probably fed on insects by suction feeding, and laid eggs that were fertilized externally.
Salamanders are generally either aquatic (such as Siren), land-based (such as Pletodon) or begin as aquatic and become land-based in adulthood (such as Triturus). It is possible that the earliest salamanders were all aquatic, but not enough fossils have been found to be safe.
Our study shakes up what scientists thought they knew about salamander evolution. Our analysis suggests several fossils from the Jurassic and Cretaceous in China (such as Chunerpeton), once considered early members of modern salamander groups, is not closely related to live salamanders. Previous studies relied too much on Karaurus (that Archeopteryx of salamanders), from the late Jurassic of Kazakhstan.
Salamanders are essential to science. Researchers have studied salamanders to find clues to understand skeletal development, limb and organ regeneration, and toxicology in all vertebrates, but people know surprisingly little about salamanders themselves. Many people think that salamanders are a type of lizard and are unaware of how different they are.
There are more than 750 species alive today scattered across the northern continents. There are eel-like forms that live in flooded caves, swimming beak herbivores and small land-based salamanders that climb trees using their tails or use chameleon-like tongues to catch prey. Several species exhibit parental care such as nest preparation and nest guard.
The UK has three species of salamanders. All live in the water as young (salamanders) and are land-based as adults. They return to the water to breed. Salamanders are important for food tissue. Many of them eat lots of insects and they are prey for many animals and even some plants. Unfortunately, many species are threatened by habitat loss.
The fossil sites from Middle Jurassic in Skye are globally important. Fossils of lizard-like reptiles, early lizards, crocodylomorphs, turtles, pterosaurs, mammalian forms, and long-necked dinosaurs have all been found there.