From the creepiest Cambrian critters to massive marine reptiles, wonderfully weird sea creatures have inhabited our oceans for over half a billion years. We’ve put together a list of 25 of the strangest ancient sea monsters ever to have lived, all of which went extinct long before humans came along.

The only reason we know that these evolutionary marvels existed is because some left behind fossilized remains in rocks. Modern researchers are still interpreting these fossils and making fresh discoveries all the time, so be sure to keep up with the latest Live Science fossil news.

Plesiosaurs

An artist’s depiction of a short-necked plesiosaur attacking a juvenile long-necked plesiosaur. (Image credit: Mark Garlick/Science Photo Library via Getty Images)

Plesiosaurs were a group of marine reptiles with boat-like bodies and four flippers. There were long-necked plesiosaurs (think ancient Loch Ness monster ) and short-necked plesiosaurs (imagine a Loch Ness monster with a short neck and a massive head). Plesiosaurs lived from the Triassic period (251.9 million to 201.4 million years ago) until they went extinct alongside the non-avian dinosaurs at the end of the Cretaceous period (145 million to 66 million years ago). They lived across the world’s oceans.

“Not only were these animals odd compared to things that we have alive today, but they were also globally distributed and very, very diverse,” Michael Caldwell , a vertebrate paleontologist at the University of Alberta in Canada, told Live Science.

Related: Newfound ‘snaky croc-face’ sea monster unearthed in Wyoming

Tanystropheus hydroides

The long neck of Tanystropheus hydroides may have helped the species sneak up on ocean prey. (Image credit: Spiekman et al., Current Biology (2020))

Michael Caldwell Social Links Navigation Professor Michael Caldwell is a professor in the departments of Biological Sciences and Earth & Atmospheric Sciences at the University of Alberta. His research career has broadly focussed on marine reptile evolution, and includes studies on mosasaurs, dolichosaurs, ichthyosaurs, plesiosaurs and extinct snakes.

Tanystropheus hydroides lived in the Tethys Sea off the ancient supercontinent Pangaea , when all of the continents were joined together, during the Triassic period around 242 million years ago. Researchers identified these ancient marine reptiles from bizarre fossils located on what is now the border between Switzerland and Italy. They had weird, broomstick-like necks that stretched to 10 feet (3 meters) in length — three times the length of their torsos.

“Like [long-necked] plesiosaurs, tanystropheids have small heads on the front and these tiny, weird little bodies way behind this gigantic neck,” Caldwell said. “They are ungainly and awkward.”

Helicoprion

An illustration of two Helicoprion individuals. (Image credit: HYPERSPHERE/SCIENCE PHOTO LIBRARY via Getty Images)

Helicoprion, or the ” buzz saw sharks ,” was a group of shark-like fish with a spiral jaw that made their teeth resemble the edge of a buzz saw. They inhabited Earth’s oceans from the Devonian period (419.2 million to 358.9 million years ago) to the Triassic period, according to the Australian Museum . Fossil records indicate that these fish grew up to around 25 feet (7.7 m) long, making them 5 feet (1.5 m) longer than the largest known modern great white sharks (Carcharodon carcharias).

Habelia optata

An artistic reconstruction of the tiny sea predator Habelia optata. (Image credit: Joanna Liang/Royal Ontario Museum)

Habelia optata was more of a mini monster, with a body length of up to 1.6 inches (4.1 centimeters). These tiny sea predators had helmet-like heads and creepy mouth appendages for catching and ripping apart their prey. H. optata fossils can be found in British Columbia, Canada, and date back around 505 million years to the Cambrian period (538.8 million to 485.4 million years ago), according to the Royal Ontario Museum .

Lyrarapax unguispinus

An artist’s rendering shows a baby (foreground) and adult Lyrarapax unguispinus hunting the Cambrian seas like the creepy predators they were. (Image credit: Science China Press)

The Cambrian period also saw the reign of a claw-faced sea monster that was totally unlike anything swimming in our oceans today. Lyrarapax unguispinus was one of many bizarre arthropods that lived during the Cambrian period, but even for its time, this species was strange. It grew up to 3.2 feet (1 m) long and had a claw-shaped appendage on the front of its head to grasp prey. This killer arthropod was one of the world’s first apex predators.

Mosasaurs

An illustration of two mosasaurs fighting for territory. (Image credit: Mohamad Haghani/Stocktrek Images via Getty Images)

Mosasaurs may not be the strangest animals on this list, but they are certainly worthy of the name “sea monster.” Before they fell to the same fate as the nonavian dinosaurs, this group of marine reptiles roamed the world’s oceans, chowing down on almost anything that moved, including other mosasaurs. A 2014 study in the journal Proceedings of the Zoological Institute RAS estimated that the mosasaur Mosasaurus hoffmanni grew to be around 56 feet (17 m) long.

Related: This ancient sea monster could do the breaststroke

Placodonts

An illustration of a Placodus species from the placodontid family. (Image credit: Corey Ford/Stocktrek Images via Getty Images)

Placodonts were an order of turtle-like Triassic marine reptiles that lived in what is now Europe, the Middle East and China. Caldwell told Live Science that placodonts “had incredibly bad buck teeth that they could have picked apples through a picket fence with.” They used their front teeth to pluck shells and mollusks off reefs or the ocean floor, and they had flat crushing plates at the backs of their mouths for munching.

Sea scorpions

An illustration of a Eurypterid on the seafloor. (Image credit: Aunt_Spray via Getty Images)

Sea scorpions, or eurypterids, were a group of ocean-dwelling arthropods that resembled modern-day scorpions. What made them strange? Well, some were enormous compared with scorpions living today. For example, one eurypterid fossil found in New York is estimated to have come from a sea scorpion larger than a human. Members of this group could exceed 8 feet (2.5 m) in length, according to the Yale Peabody Museum of Natural History in Connecticut. Sea scorpions terrorized the seas for more than 200 million years, until they went extinct at the end of the Permian period (298.9 million to 251.9 million years ago).

Saccorhytus coronarius

A 3D digital model of Saccorhytus coronarius. (Image credit: Philip Donoghue et al)

Saccorhytus coronarius was essentially a wrinkly sac with no anus . These weirdos lived during the Cambrian period around 500 million years ago and are known from microfossils discovered in China. The Minion-like creatures may have spent their days catching prey in seafloor sediment, but researchers’ understanding of the animals’ lives is limited. They are believed to be related to penis worms and mud dragons.

Ichthyosaurs

A 3D science rendering of ichthyosaurs in the Stenopterygius genus. (Image credit: Dotted Yeti via Shutterstock)

Try to picture a reptilian version of a dolphin, and you won’t be far off the appearance of an ichthyosaur . This diverse group of pointed-nose predators evolved to have dolphin- or fish-like bodies, but they looked far more menacing. Ichthyosaurs evolved around 250 million years ago and went extinct around 90 million years ago. While there were ichthyosaur species as small as 1 foot (0.3 m) long, the group was home to several giants in the late Triassic period. In 2018, researchers estimated that a fossilized jawbone from the U.K. belonged to an ichthyosaur that was more than 85 feet (26 m) long, which is nearly the size of a blue whale (Balaenoptera musculus).

Tully monsters

An illustration of a Tully monster (Tullimonstrum gregarium). (Image credit: Stocktrek Images/Getty Images)

The Tully monster (Tullimonstrum gregarium) was a soft-bodied species with primitive eyes on stalks and a long, thin appendage that ended in a claw-like feature. These mysterious creatures were so strange that researchers today have had trouble agreeing on the animals’ place on the tree of life. Whatever they were, these monsters hunted in marine coastal environments 300 million years ago and are found only in fossils from Illinois, according to the Illinois State Museum .

Related: The mysterious ‘Tully monster’ just got more mysterious

Odontochelys semitestacea

An illustration of two members of Odontochelys semitestacea. (Image credit: Marlene Hill Donnelley, Field Museum)

Odontochelys semitestacea swam in the Triassic coastal waters of what is now China 220 million years ago. The species was one of the first known turtles , but it looked very different from its modern relatives.

“These most ancient turtles have got the chest piece, or the plastron, but they don’t have the carapace on the back,” Caldwell said. “So, here we have early versions of turtles that are lacking the turtle shell, the carapace, and are still toothed.”

Typhloesus wellsi

An artistic representation of the “alien goldfish” Typhloesus wellsi hunting prey. (Image credit: Drawing by Joschua Knüppe © Royal Ontario Museum.)

Typhloesus wellsi left behind such strange fossils that Simon Conway Morris , an emeritus professor of paleobiology at the University of Cambridge in the U.K., gave them the nickname “alien goldfish” in a 2005 article published in the journal Astronomy & Geophysics . Morris joked that they could have been brought to Earth by a visiting intergalactic commodore who grew tired of keeping them as pets and dumped them here during the Carboniferous period (358.9 million to 298.9 million years ago). The species shot a toothy “tongue” out of its gut to catch prey and may have been an early gastropod.

Basilosaurus

An illustration of Basilosaurus. (Image credit: Sebastian Kaulitzki/Science Photo Library via Getty Images)

Basilosaurus swam through the ocean like a giant sea serpent from 37.8 million to 33.9 million years ago, with a slender body that stretched up to 59 feet (18 m) in length. The name Basilosaurus translates to “king lizard” because the researchers who named it mistook the gigantic life-form for a marine reptile, like a mosasaur or ichthyosaur. But the species wasn’t a serpent or a lizard; it was a mammal, and a relative of modern whales, according to the University of Michigan’s Museum of Paleontology .

Fanjingshania renovata

A reconstruction of Fanjingshania renovata. (Image credit: ZHANG Heming)

This shark-like fish was heralded as being unlike any vertebrate ever discovered when it was unveiled in 2022. Covered in spiny fins with teeth-like scales and bony armor, Fanjingshania renovata is somewhere between a bony fish and a shark on the fish family tree. It lived in what is now southern China during the Silurian period (443.8 million to 419.2 million years ago).

Opabinia regalis

An illustration of Opabinia regalis. (Image credit: Nobumichi Tamura/Stocktrek Images via Getty Images)

When paleontologist Harry Blackmore Whittington presented an early reconstruction of Opabinia regalis to a meeting of fellow paleontologists in 1972, everyone in the room laughed, according to the Royal Ontario Museum . Another small, British Columbian beasty from the middle Cambrian, O. regalis had five eyes and claws on its long, flexible snout to catch prey. The species swam through ancient oceans around 505 million years ago using lateral lobes and a tail fan to steer.

Related: The ‘weirdest wonder’ of evolution had an even weirder cousin, new study finds

Archelon ischyros

An illustration of Archelon, the largest turtle ever to have lived. (Image credit: Sciepro/Science Photo Library via Getty Images.)

There’s nothing particularly strange about the sea turtles we see today, but what if they were bigger — like, much bigger? That would be a little odd, right? Turn back the clock 65 million years, and the ocean featured 15-foot-long (4.6 m) supersize turtles named Archelon ischyros. They would have dwarfed the biggest turtles alive today — leatherback turtles (Dermochelys coriacea), which max out at around 5.9 feet (1.8 m) long.

Megalodon

A computer-generated image of a megalodon with its mouth open. (Image credit: Gil Cohiba/Shutterstock)

Megalodon (Otodus megalodon) was another supersize version of a modern animal. Fossilized teeth suggest that megalodon, which reigned over ocean ecosystems between around 23 million and 2.6 million years ago, was at least three times longer than a modern great white shark, and the biggest shark on record. The beast’s exact size is disputed in scientific circles, but it could have been up to 60 feet (18 m), or even 80 feet (24 m), long. This shark was so big, it could have devoured a modern orca (Orcinus orca) in just a few bites.

Titanokorys gainesi

Titanokorys gaines viewed from underneath. (Image credit: Illustration by Lars Fields, copyright Royal Ontario Museum)

Titanokorys gainesi may have been only 2 feet (0.6 m) long, but it was one of the largest predators during the Cambrian period. The early arthropod swam across the ocean floor, hoovering up prey like a Roomba and devouring it with a toothy, circular mouth. Half-a-billion-year-old fossils from British Columbia reveal that the creature’s helmeted head was disproportionately large, making up around two-thirds of its total body length.

Websteroprion armstrongi

Head of a living marine worm (Eunice aphroditois), photographed in Indonesia. (Image credit: WaterFrame/Alamy Stock Photo)

Websteroprion armstrongi was a mighty worm of the Devonian period and extinct relative of modern marine worms. The carnivore dwarfed its fellow ancient worms, with an estimated body length of up to 6.6 feet (2 m). It was so large, in fact, that when researchers described the species from Canadian fossils in 2017, it immediately became the largest marine jawed worm on record. And if a giant worm weren’t already metal enough, the researchers named its genus Websteroprion after death-metal guitarist Alex Webster from the band Cannibal Corpse.

Related: Giant worms terrorized the ancient seafloor from hidden death traps

Dunkleosteus terrelli

An illustration of the Devonian-period fish Dunkleosteus. (Image credit: MR1805 via Getty Images)

Dunkleosteus terrelli, or “Dunk” for short, was a bus-size armored fish that lived during the Devonian period. When researchers started discovering Dunk skulls in Cleveland 150 years ago, they estimated that the creature was 30 feet (9.1 m) long. However, a 2023 study published in the journal Diversity found that the creatures were actually more like 13 feet (4 m) long, but super chunky . D. terrelli was a superpredator, with blade-like jaws for slicing through any animal it could digest.

Nothosaurs

A 3D illustration of a nothosaur. (Image credit: Warpaintcobra via Getty Images)

A 2014 study published in the journal Scientific Reports described a nothosaur species, Nothosaurus zhangi, that had a 26-inch-long (65 cm) lower jaw and an estimated total body length of up to 23 feet (7 m). These predators propelled themselves through the water with their forelimbs and snatched prey with fang-like teeth. N. zhangi lived around 245 million years ago in what is now southwestern China.

Dolichosaurs

A fossilized skeleton of Dolichosaurus. (Image credit: The History Collection/Alamy Stock Photo)

Dolichosaurs were slender, serpent-like lizards with small limbs that snaked through the water, chasing prey. They lived during the Cretaceaous period and were discovered in English fossils in the mid 19th century. Caldwell said the largest dolichosaurs he encountered in the fossil record were only around 2 feet long, but their necks were longer than those of modern lizards, and they contained many more cervical vertebrae. “They had this fiendishly long neck, which is bizarre among lizards,” Caldwell said.

Diplocaulus magnicornis

An illustration of Diplocaulus. (Image credit: Dottedhippo via Getty Images)

Diplocaulus magnicornis stands out among even the strangest creatures of the ancient aquatics because of its boomerang-shaped skull. Researchers aren’t sure why this amphibian evolved such a bizarre head, but it probably played a role in how the species swam. D. magnicornis lived about 275 million years ago, during the Permian period, according to the American Museum of Natural History . The fossils left behind by this species are found in modern-day Texas.

Shell-dwelling penis worms

An illustration of a Cambrian penis worm inhabiting a hyolith shell. (Image credit: Zhang Xiguang)

Monitor lizards, also known in Australia as goannas, are some of the most iconic reptiles on the continent. Their lineage not only survived the mass extinction that ended the reign of non-avian dinosaurs, but also gave rise to the largest living lizards on Earth.

Today, these formidable creatures pace through forests and scrublands, flicking their tongues as they go.

A new study published in the Zoological Journal of the Linnean Society looks beneath their skin. For the first time, it reveals hidden bone structures that may hold the key to the evolutionary success of goannas in Australia.

An essential organ

The skin is an organ essential for survival. In some animals, it includes a layer of bone plates embedded among the skin tissue. Think of the armour-like plates in crocodiles or armadillos: these are osteoderms.

Their size ranges from microscopic to massive, with the back plates of the stegosaurus as the most impressive example.

We have only just started to understand these enigmatic structures. Osteoderms can be found in animal lineages that diverged up to 380 million years ago. This means these bone plates would have evolved independently, just like active flight did in birds, pterosaurs and bats.

But what is their purpose? While the advantage of flight is undisputed, the case is not as clear for osteoderms.

The most obvious potential would be for defence – protecting the animal from injuries. However, osteoderms may serve a far broader purpose.

In crocodiles, for example, they help with heat regulation, play a part in movement, and even supply calcium during egg-laying. It is the interplay of these poorly understood functions that has long made it difficult to pinpoint how and why osteoderms evolved.

A cutting-edge technique

To help resolve this enigma, we had to go back to the beginning.

Surprisingly, to date science has not even agreed on which species have osteoderms. Therefore, we assembled an international team of specialists to carry out the first large-scale study of osteoderms in lizards and snakes.

We studied specimens from scientific collections at institutions such as the Florida Museum of Natural History, the Natural History Museum in Berlin, and Museums Victoria.

However, we soon learnt that this came with challenges. Firstly, the presence of osteoderms can vary dramatically between individuals of the same species. Secondly, there is no guarantee that osteoderms are sufficiently preserved in all specimens.

Most importantly, they are buried deep within skin tissue and invisible to the naked eye. Traditionally, finding them meant destroying the specimen.

Instead, we turned to micro-computed tomography (micro-CT), an imaging technique similar to a medical CT scan, but with much higher resolution. This allowed us to study even the tiniest anatomical structures while keeping our specimens intact.

Roy Ebel

Using computer-generated 3D models, we then digitally explored the bodies of lizards and snakes from all parts of the world. Incorporating data from prior literature, we processed almost 2,000 such samples in our search for osteoderms.

To illustrate our results, we devised a technique called radiodensity heatmapping, which visually highlights the locations of bone structures in the body.

For the first time, we now have a comprehensive catalogue showing where to find osteoderms in a large and diverse group; this will inform future studies.

Roy Ebel

Not just anatomical curiosity

What we found was unexpected. It was thought only a small number of lizard families had osteoderms. However, we encountered them nearly twice as often as anticipated.

In fact, our results show nearly half of all lizards have osteoderms in one form or another.

Our most astonishing finding concerned goannas. Scientists have been studying monitor lizards for more than 200 years. They were long thought to lack osteoderms, except in rare cases such as the Komodo dragon.

So we were all the more surprised when we discovered previously undocumented osteoderms in 29 Australo-Papuan species, increasing their overall known prevalence five times.

Roy Ebel

This isn’t just an anatomical curiosity. Now that we know Australian goannas have osteoderms, it opens up an exciting new avenue for further studies. This is because goannas have an interesting biogeographic history: when they first arrived in Australia about 20 million years ago, they had to adapt to a new, harsh environment.

If osteoderms in goannas showed up around this time – possibly owing to new challenges from their environment – we’d gain crucial insights into the function and evolution of these enigmatic bone structures.

Not only may we just have found the key to an untold chapter in the goanna story, our findings may also improve our understanding of the forces of evolution that shaped Australia’s unique reptiles as we know them today.

Roy Ebel receives funding from the Australian Government’s Research Training Program.

This article was originally published on The Conversation. Read the original article.

Scientists uncover hidden bone structures in the skin of Australian monitor lizards

Rosenberg goanna (Varanus rosenbergi WAM R95408) with osteoderms and endoskeleton revealed in the left half. Credit: Roy Ebel

Beneath the scales of Australia’s iconic monitor lizards (commonly known as goannas), scientists have discovered an unexpected secret: a hidden layer of bony skin structures known as osteoderms. These structures, which have been long overlooked, may hold the key to understanding how these ancient reptiles not only survived but thrived in one of the world’s harshest environments.

The findings, published today in the Zoological Journal of the Linnean Society, mark the first large-scale global study of osteoderms in lizards and snakes. The international collaboration brought together researchers from Australia, Europe and the United States, who used cutting-edge micro-CT scanning to examine nearly 2,000 reptile specimens from major museum collections including those held at Museums Victoria’s Research Institute.

“We were astonished to find osteoderms in 29 Australo-Papuan monitor lizard species that had never been documented before,” said Roy Ebel, lead author and researcher at Museums Victoria Research Institute and the Australian National University. “It’s a fivefold increase in known cases among goannas.”

Osteoderms are most commonly known from crocodiles, armadillos, and even some dinosaurs like Stegosaurus. But their function has remained something of an evolutionary mystery. While they may provide protection, scientists now suspect they may also support heat regulation, mobility and calcium storage during reproduction.

This new research reveals that osteoderms are far more widespread in lizards than previously thought, occurring in nearly half of all lizard species worldwide—an 85% increase on earlier estimates.

At the heart of this discovery lies the power of museum collections. Scientific institutions like Museums Victoria Research Institute play a critical role in preserving biodiversity over time, enabling researchers to study species long after they are collected.

Many of the specimens used in this study were decades old, and in some cases over 120 years old, but advances in imaging technology enabled scientists to uncover new insights without harming the original material. These collections are not just archives, they’re active tools for scientific discovery.

“What’s so exciting about this finding is that it reshapes what we thought we knew about reptile evolution,” said Dr. Jane Melville, Museums Victoria Research Institute Senior Curator of Terrestrial Vertebrates. “It suggests that these skin bones may have evolved in response to environmental pressures as lizards adapted to Australia’s challenging landscapes.”

In this green tree monitor (Varanus prasinus UF 71411, MorphoSource), radiodensity heatmapping shows bone tissue, including osteoderms, in yellow to red. Credit: Roy Ebel

Until now, the presence of osteoderms in monitor lizards was considered rare and mostly confined to the famed Komodo dragon. But the discovery of their widespread presence across Australo-Papuan goannas opens up new questions about how these lizards adapted, survived and diversified across the continent.

This landmark study not only tells a new chapter in the story of Australia’s goannas, it provides a powerful new dataset for exploring how skin, structure, and survival have intertwined across millions of years of evolution.

Beneath the scales of Australia’s iconic monitor lizards (commonly known as goannas), scientists have discovered an unexpected secret: a hidden layer of bony skin structures known as osteoderms. These structures, which have been long overlooked, may hold the key to understanding how these ancient reptiles not only survived but thrived in one of the world’s harshest environments.

The findings, published on July 21 in the prestigious Zoological Journal of the Linnean Society, mark the first large-scale global study of osteoderms in lizards and snakes. The international collaboration brought together researchers from Australia, Europe and the United States, who used cutting-edge micro-CT scanning to examine nearly 2,000 reptile specimens from major museum collections including those held at Museums Victoria’s Research Institute.

‘We were astonished to find osteoderms in 29 Australo-Papuan monitor lizard species that had never been documented before,’ said Roy Ebel, lead author and researcher at Museums Victoria Research Institute and the Australian National University. ‘It’s a fivefold increase in known cases among goannas.’

Osteoderms are most commonly known from crocodiles, armadillos, and even some dinosaurs like Stegosaurus. But their function has remained something of an evolutionary mystery. While they may provide protection, scientists now suspect they may also support heat regulation, mobility and calcium storage during reproduction.

This new research reveals that osteoderms are far more widespread in lizards than previously thought, occurring in nearly half of all lizard species worldwide – an 85% increase on earlier estimates.

At the heart of this discovery lies the power of museum collections. Scientific institutions like Museums Victoria Research Institute play a critical role in preserving biodiversity through time, enabling researchers to study species long after they were collected. Many of the specimens used in this study were decades, and in some cases over 120 years old, but advances in imaging technology enabled scientists to uncover new insights without harming the original material. These collections are not just archives, they’re active tools for scientific discovery.

‘What’s so exciting about this finding is that it reshapes what we thought we knew about reptile evolution,’ said Dr Jane Melville, Museums Victoria Research Institute Senior Curator of Terrestrial Vertebrates. ‘It suggests that these skin bones may have evolved in response to environmental pressures as lizards adapted to Australia’s challenging landscapes.’

Until now, the presence of osteoderms in monitor lizards was considered rare and mostly confined to the famed Komodo dragon. But the discovery of their widespread presence across Australo-Papuan goannas opens up new questions about how these lizards adapted, survived and diversified across the continent.

This landmark study not only tells a new chapter in the story of Australia’s goannas, it provides a powerful new dataset for exploring how skin, structure, and survival have intertwined across millions of years of evolution.

Hump-backed and slow, chameleons sway like leaves as they creep along branches. Their coned, protruding eyes scan their environment, searching for prey. They can move and focus each eye independently, looking two ways at once, or locking both eyes onto an insect before they strike with a sticky, missile-like tongue. The world must look very different to a chameleon.

Chameleon lives are oriented around their excellent vision, which they use not only to target prey and avoid predators, but to read covert signals written on the bodies of other chameleons. We’re still learning exactly how the chameleon has such an impressive command of color, but have long observed chameleons changing the color and skin patterns to respond to their environment and to express their moods in sophisticated displays.

In 2014, researchers at the University of Geneva reported that in addition to mobile pigments, chameleons also had shards of guanine nanocrystals embedded below their skin. When excited, chameleons change the spacing of these crystals, which affects how light moves through them, and in turn adjusts the color produced by the pigments. It’s like each chameleon has a prism under its skin.

These masters of disguise have been keeping another secret from us: they glow. Late last year, David Prötzel, a PhD student and lead author of a paper with colleagues at a slew of German and Dutch institutions, described how chameleons are on a very exclusive list of land-dwelling vertebrates – including some frogs, marine turtles, and parrots – that have natural fluorescence abilities. But true to form, chameleons found a way to glow that is unique: they are the first known animals that use their bones to glow.

For chameleons, beauty is more than skin deep.

To reveal their glowing patterns, Prötzel and his colleagues photographed living lizards in their habitats and recently preserved specimens in museums under UV light. Chameleons are flamboyant even in the dark, illuminated by pinpricks of deep blue and purple light, which traces the complex architecture of their skull – the ridges and sharp edges, crests, bumps, and peaks. The sheer intricacy of the patterns puzzled the scientists, until they peeled back the layers of the chameleon’s anatomy with micro-computed tomography, a technique that create 3D X-ray images of the skeleton on the microscopic level.

A chameleon’s body is ornamented with bumps, called tubercules, that are actually projections of their skeleton. They look impressive, and are generally more extravagant in rowdy males than in females. By matching these bumps with the glowing pattern, the team concluded that the bony tubercles were responsible.

Bone has been known to naturally fluoresce under UV light since the 1960s, and forensic researchers have used this phenomenon for decades. But the reason chameleons glow under blacklight and we do not is that the tubercules push aside all but the topmost layer of skin, including the dermis that contains the color-producing chromatophores and melanophores. This leaves a kind of “window” of skin over the bone, nearly transparent and thin enough to see its natural glow. This layer of skin also gives the glow its color, acting as a filter that nudges the fluorescence toward the blue end of the color spectrum. Chameleons are the first animals known to co-opt this natural property of bone as part of their coloration.

Looming over the finding is another question: why do they glow?

Asking why animals look, work, and act the way that they do is perhaps the central question in evolutionary biology. To approach it, scientists often take a few minutes to take the lay of the land and try to identify common patterns across many species. One unusual phenomenon – say, a glowing lizard – in one or two species is a curiosity. Seeing it appear in multiple species and across families suggests that the trait means something evolutionarily interesting to it.

Natural history collections like those at the Zoologische Staatssammlung München (in Munich, Germany), the Museo Regionale di Scienze Naturali (Torino, Italy), and the Senckenberg Museum (Frankfurt, Germany), are ideal for these sort of broad, evolutionary questions, because they have hundreds of specimens, often including rare or extinct species, waiting to be poked and prodded by researchers.

Digging deep into the reptiles collections, Prötzel and his team surveyed the fluorescent ability of over 50 species of chameleons native to Madagascar and parts of Africa, a total of more than 300 individual specimens. They discovered that eight of 12 groups of closely-related species, called genera, displayed at least some bone-based fluorescence. Two out of three ain’t bad, especially for evolutionary biology.

So bone-based fluorescence is widespread among chameleon lineages. The tricky part was rationalizing why chameleons, already so good at silently expressing their feelings through colour, needed to add some glitter and shine to the conversation. Perhaps, the researcher suggested, this bone-based fluorescence is a clandestine extension of the chameleon’s visual communication system, already highly developed.

Having a secondary signaling system actually makes a lot of sense – it would mean that chameleon visual communication doesn’t go silent after dark, and that they can continue to communicate with each other far into the night. The bone glow has another advantage, too: many animals, including humans, can’t see UV light as chameleons can. They are effectively blowing dogwhistles at each other, sending messages only other chameleons can hear.

Interestingly, the glow is typically more prominent in chameleons that live in shady, humid, forest habitats than those in open, dry grasslands. The physics of UV light work in favor of forest-dwelling chameleons: light with a short wavelength, like UV, scatters more in humid environments. This means that there is proportionally more UV light under the trees compared to open environments, and therefore chameleons under a canopy will shine brighter than those in a field. Perhaps the supercharged glow caused by the humid forest canopy has shaped the evolution of forest chameleons, giving them more intricate displays because they get more bang for their buck.

What exactly the chameleons are saying to each other is still a mystery. One hypothesis is that it’s all about impressing the girls, since male chameleons typically have more intricate tubercules, and the brightness of the glow may help male chameleons attract interested ladies. Even if we can’t see with a chameleon’s eyes, the scenario is fun to imagine. Pretend you’re a beautiful lady chameleon, and a suitor shuffles onto your branch – and promptly lights up the forest. Like, he literally glows. Who could resist?

Fluorescence is common in the marine environment – think of deep sea anglerfish or jellyfish– and we are quickly learning it’s much more common than we thought on land. It brings into question what else we may be missing because we weren’t looking hard enough, or under the right conditions. Who know’s what other secrets Mother Nature has written out in invisible ink, waiting for just the right lighting?