166-million-year-old fossil sheds new light on salamanders

dr. Marc Jones, Prof Roger Benson and Prof Susan Evans explain how a discovery in Scotland turns what scientists thought they knew about salamander evolution on its head.

A version of this article was originally published by The conversation (CC BY-ND 4.0)

In the fairytale landscape of the Isle of Skye off the northwest coast of Scotland, the skull of one of the oldest salamanders ever discovered was unearthed from Jurassic limestone. But it would be decades before scientists had the technology and funding to assemble the salamander.

Part of the skeleton was collected in the early 1970s when paleontologists Michael Waldman and Robert Savage noted that black bone was visible on the hard gray rock surface, suggesting a fossil trapped within. They collected it realizing that it could be something important.

Although parts of the fossil were subsequently uncovered, it was too little to warrant 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 have been foundincluding salamanders. Roger Benson examined the block collected in the 1970s. He realized that the fractured surface matched a specimen he collected in 2016.

Most bones collected during field trips are not immediately studied. Getting money for fieldwork is hard, but getting funding to study the fossils you collect is even harder. It’s not uncommon for them to go unstudied for decades.

X-ray micro-CT scanning revealed that the rock contained the remains of a new fossil salamander species: Mamorerpeton wakeic. At 166 million years old, it is one of the oldest known salamanders and documents one of the earliest known stages of their evolution.

Salamander fossils are rare. For the whole Jurassic period (201-145m years ago) less than 20 species are found. On the other hand, we know 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.

Finally recognition

Paleontologists had the first clues to an extinct salamander species 30 years ago, when parts of fossilized spine and jawbones were found near Oxford in England.

However, it was largely ignored by the scientific community in favor of research into the Karaurus salamander from Kazakhstan in the Middle Jurassic period. Until now, the Karaurus has often been treated as the common ancestor of modern salamanders.

Mamorerpeton’s fossil bones are still preserved in hard rock. Until we used X-ray micro-CT scanning, we weren’t sure about the content. Most blocks were collected without knowing exactly what was inside. A fossil block recovered in 2016 turned out to be the other half of a specimen collected from the same site more than 40 years earlier.

Most of the skeleton has been preserved, including the skull and tail. Bones turn into digital models is painstaking work, but it allowed us to create an (unprocessed) three-dimensional model of the skull, which is unprecedented for a fossil salamander.

Often fossils are collected during field trips, but not studied for years due to lack of time or expertise. For the 1971 specimen, the edges of some bones were visible, but removing the bones would have been very difficult. Mechanical removal could have damaged them, but X-ray micro-CT scanning showed us the bones clearly.

What we learned

U.S new analysis places the new species Marmorerpeton in the extinct group Karauridae. Members of this group all have skull bones with a crocodile-like ornament and have bony projections behind the eye. The new species is named after the late Prof David Wakea leading US authority on salamander evolution.

The broad skull, deep tail and limbs with unfinished ends indicate that Marmorerpeton had an aquatic lifestyle similar to the living hellbender salamander of North America (crypto branch) and the giant salamander of China and Japan (Andrias). They probably fed on insects using suction feedand laid eggs that were fertilized externally.

Salamanders are generally either aquatic (such as Siren), on land (such as Plethodont) or start out as aquatic and become terrestrial in adulthood (such as triturus). It’s possible that the earliest salamanders were all aquatic, but not enough fossils have been found to be sure.

Our study shakes up what scientists thought they knew about salamander evolution.

Our analysis suggests several Jurassic and Cretaceous fossils of China (such as: Chunerpeton), once thought to be early members of modern salamander groups, are not closely related to living salamanders. Previous studies relied too much on Karaurus (the Archeopteryx of salamanders), from the late Jurassic of Kazakhstan.

Salamanders today

Salamanders are crucial to science. Scientists have studied salamanders looking for clues to understand skeletal development, regeneration of limbs and organs and toxicology in all vertebrates, but humans themselves know surprisingly little about salamanders. Many people think that salamanders are a type of lizard and do not know how diverse they are.

Today there are more than 750 species, spread across the northern continents. There are eel-like shapes that live underwater caves, swimming herbivores with beaksand little salamanders on the land that climb trees with their tails or use chameleon tongues to catch prey. Several species exhibit parental care, such as: nest preparation and nest monitoring.

The UK has three types of salamander. They all live in the water as juveniles (salamanders) and are established on land as adults. They return to the water to breed. Salamanders are important for food webs. Many of them eat a lot of insects and they are prey for many animals and even some plants. Unfortunately, many species are threatened by loss of habitat.

The fossil sites in the Middle Jurassic of Skye are important worldwide. fossils of lizard-like reptiles, early lizards, crocodylomorphs, turtles, pterosaurs, mammalian forms and long neck dinosaurs are all found there.

Through Dr Marc Jones, Prof Roger Benson and Prof Susan Evans

dr. Marc Jones is a postdoctoral research associate at University College London, studying the evolution of salamanders. Prof Roger Benson is professor of paleobiology at the University of Oxford. Prof Susan Evans is Professor of Vertebrate Morphology and Paleontology at the University College London.

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