Fossil forensics: Two ancient lizards, two amber-encased mysteries

Two ancient lizards, entombed long ago in amber. The first one comes from a known fossil locality, but its identity is an enigma; the second has been identified, but where it came from remains unknown. These are the dual mysteries that two teams of palaeontologists are working to solve using the latest sleuthing techniques.

Mysterious identity

The first lizard appears as a scaly outline inside its amber capsule. While its skin is preserved, most of its insides have been lost to time, leaving behind the transparent shape of the tiny reptile, like a golden ghost from the ancient past. But its identity – where it fits on the lizard family tree – is a challenge to determine.

This is the lizard specimen currently being studied by the Texas team. Incredibly, its skin has preserved while most of its insides have not, leaving a beautiful lizard-outline. Image: Ru Smith

The fossil comes from the famous Hukawng Valley of Myanmar, home to 99-million-year-old Burmese amber, the same site that has recently produced such gems as an ancient baby bird, a feathered dinosaur tail and an alien-looking insect. The find was purchased by a collector named Ru Smith, who in turn lent it to Juan Daza of Sam Houston State University in Texas. Using a high-resolution CT-scanner, Daza and his colleagues were able to get a close-up glimpse at the lizard's body.

While the spine, skull and ribs are gone, the lizard's limb bones are preserved in incredible detail, from the hooked toe claws to its tiny kneecaps. This particular preservation is very unusual, which makes the fossil fascinating for its uniqueness, but also frustrating, since identifying the animal requires comparison to other species.

"This is almost detective work because the specimen preserved different portions [of the body]," said Daza. Palaeontologists often use the skulls of fossil lizards to identify them, for example, and this amber find is missing that important piece.

Read more: A feathered dinosaur tail preserved in amber

"[A]mber can be pretty selective, it seems, about what elements get preserved and what doesn't," said Kelsey Jenkins, a graduate student at Sam Houston, who presented research on the fossil at the Society of Vertebrate Paleontology meeting last month. "That's kind of the fun challenge of this fossil. Since so few elements are preserved with little that is easy to characterise and identify, figuring out what kind of lizard – skink, gecko, or something else – is a puzzle."

Comparing this specimen with other fossil lizards is even more challenging considering how rare these animals are in the fossil record. Tiny, delicate creatures don't often last long enough to become preserved, and fossilisation is rare in the kinds of environments they frequent, such as tropical forests. Only very few lizards from this time period have been found encased in exceptional pieces of amber like this one.

"[Small] vertebrates can often be an overlooked component of paleontological sites as they may seem less charismatic than larger fossils," Jenkins explained, "but really small animals can act as good environmental indicators."

The team suspects this lizard might be a skink, but research is still underway. More examination, and more comparison, will tell.

Mysterious origins

The second fossil has quite a different story. For the last few decades, it's lived in the collections of the Queen's University Natural History Museum in Ontario, Canada. While it was occasionally brought out for students to see, just like so many amber specimens, it had no associated data, meaning no one knew where it came from or how old it was.

The lizard being studied by the Canadian team. It's well preserved inside the amber, but X-ray scans were needed to get more detail. Image: Corey Lablans

Recently, however, researchers were able to bring the specimen to a new X-ray microscopy scanner at McGill University in Montreal, providing a high-tech look at the lizard for the first time.

"Our palaeontology professor, he's been teaching this course for a good 20, maybe 30, years, and he brings in this amber sample at the start of every course," said undergraduate student Ellen Handyside while presenting research on the fossil at the same conference. "This year, he got to say, 'We're finally doing something with it!' which was really exciting."

With so much of the skin and skeleton preserved in this case, identification was pretty straightforward: it's a gecko, an ancient species with long toes, curved claws and seemingly no sticky foot pads (although these might simply not have been preserved).

A closer look, via X-ray microscopy scanning, of the Canadian team's lizard specimen, preserved in exquisite skeletal detail. Image: Rui Tahara and Hans Larsson

The big mystery with this lizard is what time and place it called home. Tracking a fossil back to its original provenance is tough, often impossible, but this research team thinks they might be able to do it by analysing the chemical composition of the amber.

The chemistry of the amber matches what is expected from a warm, tropical environment, likely of the Neogene Period (less than 25 million years ago), which helps narrow down the specimen's age and habitat. "However, specifying that to Columbia or Malaysia or Mexico or the Dominican, that's where the trouble really is," Handyside said.

Read more: This amber find is an alien in the insect world

Zeroing in on the lizard's home requires comparing the chemistry of amber in different regions. So far, the researchers have found that this lizard's amber is a poor match for Baltic amber, and a better match for the Dominican, for example, but more data is still needed before they can pinpoint a fitting location.

It's a case of prehistoric forensics work, with scientists trying to trace the chemical evidence to the scene of the victim's death. And just like criminal investigators, they might also look to bugs for help: the same amber piece that trapped this lizard also captured an ancient spider, which might offer more clues to the habitat in which the amber formed.

Both reptilian mysteries are subjects of ongoing research, and when all is said and done, the two lizards' stories may become yet more intertwined: Handyside expressed her team's interest in reaching out to Daza for assistance. Perhaps a collaborative effort can help put these riddles to rest, eventually.