On April 19th, 2018 we learned about

Fossilized, cavity-ridden teeth reveal bears’ long history of eating high-sugar foods to prepare for hibernation

Skeletal fossils help describe the shape of an ancient creature, as they were literally part of the animal’s anatomy. Trace fossils record an interaction of some sort, such when a dinosaur stepped into some soft mud to make a footprint. In some cases, paleontologists get lucky, and find both types of evidence at once, as with a collection of cavity-pocked bear teeth found On Ellesmere Island in Canada.

The 3.5 million-year-old teeth not only describe the dental anatomy of two Protarctos abstrusus, but they show evidence of how their behavior ate holes in their teeth. Eliminating speculation about what events lead to the small bears’ dental decay, the fossils were found alongside an ancient collection of raspberry, blueberry, lingonberry and crowberry plants. Like 44 percent of modern bears, it seems that a love of sugar-filled foods caused a fair amount of damage to these animals’ teeth while they were alive.

Bulking up for winter since bears began

The evidence suggests that this kind of sugary, high-calorie diet was readily available millions of years ago, but it certainly isn’t present any longer. The area is now a polar desert, but in the Pliocene it was 20 degrees warmer, and able to support a forest full of trees, primitive horses, beavers and deer. However, the winters were cold enough to freeze, which is probably why P. abstrusus was looking to bulk up on berries— all that sugar would have helped the bear pack on fat to help it hibernate when food was scarce.

If those berry-binges truly were part of the bears’ preparations for winter, it would be the earliest known example of this behavior. While this species is not thought to have been a direct ancestor of modern species like the black bear, the fact that it hibernated to survive northern winters shows that this strategy must have developed very early on in bears’ evolution.

Source: Primitive fossil bear with a sweet tooth identified from Canada’s High Arctic, Canadian Museum of Nature

On April 12th, 2018 we learned about

Disregarded fossils and footprints help disprove dicynodonts’ assumed extinction

Around 70 years ago, two sets of fossilized footprints were found in South Africa. One was clearly a Triassic era dinosaur, but the other… didn’t make sense. The five-toed prints most closely resembled the feet of a dicynodont— a group of stocky, beaked-and-tusked herbivores that were widely believed to have gone extinct at the start of the Triassic period. While both sets of prints were acknowledged to contemporaneous of each other, the confounding dicynodont prints were basically ignored. The puzzling prints are finally getting explained though, largely thanks to modern paleontologists and an eccentric librarian from the 19th century.

Discovered but not formally described

Alfred “Gogga” Brown was employed as a librarian, postmaster and postman, but his real vocation was as a paleontologist. The self-taught scientist is credited with the discovery of 21 species of dinosaurs, reptiles and fishes, although he only published on paper on his work. Working on his own, Brown managed to collect a set of fossils in his adopted South Africa in the 1870s. They were never properly examined and described, as Brown had difficulty getting any paleontologists in Europe to take much interest in his work. As such, this batch of mystery fossils remained in storage at the Natural History Museum in Vienna, presumed to be fragments of a Triassic dinosaur.

Fortunately, Brown’s fossils have now been given a closer look and a new species name to boot. Pentasaurus goggai, named for Brown’s “Gogga” nickname, was found to be a close match for the mysterious footprints found decades ago. The animal was clearly a dicynodont synapsid, and as such is more closely related to mammals than dinosaurs or reptiles. They were widespread in their heyday, living all around the world until (mostly) going extinct at the end of the Permian period. However, this combination of trace fossil footprints and recovered skeletal fossils helps prove that some species managed to survive alongside the dinosaurs that were coming to dominate the planet’s terrestrial ecosystems.

Surviving in secret?

Strangely, Pentasaurus goggai is not be the only dicynodont to cause confusion by not going extinct when it was supposed to. In 1915, fossils of a dicynodont were found in Queensland, Australia, although their identity was controversial because they were found in Cretaceous period rock layers. That’s around 60 million years past these creatures’ supposed extinction, prompting vigorous tests in 2003. Taken together, it’s becoming increasingly clear that dicynodonts lived much longer than paleontologists once believed, although how they managed to hang on as their ecosystems were upended by new species isn’t totally clear yet. Maybe the next answer is still in museum’s collection somewhere.

Source: 60-year-old paleontological mystery of a 'phantom' dicynodont by North Carolina Museum of Natural Sciences, Science Daily

On April 5th, 2018 we learned about

Four-eyed lizard fossil helps explain the evolution of “extra” eyes in vertebrates

At first glance, the skull of a Saniwa ensidens doesn’t look terribly different than other monitor lizards. Even though this species lived 48 million years ago, it would have closely resembled monitor lizards alive today. A closer look would reveal two important features though— S. ensidens had two tiny holes in its head that were there for its third and fourth eyes. While that sets a record for ocular organs in a jawed vertebrate, the most important part of these extra peepers is how they differed from each other, a fact that may help explain skull-top eyes in animals alive today.

Two evolutionary tracks for third eyes

As unfamiliar as it may sound, plenty of vertebrate animals sport a third eye on top of their head (with lampreys also having four.) They’re not eyeballs in the way we’re used to, but are light-sensitive anatomy known as parietal eyes, growing from either the parapineal or pineal glands through the top of an animal’s head. Fishes and frogs generally have pineal organs, while lizards (and lampreys) have parapineal organs. Mammals and birds lack these third eyes, raising questions about how this anatomy ended up in some terrestrial species but not others.

One possible explanation was that lizards had basically kept and modified the extra eye they inherited from fish and amphibians. It’s not clear why mammals and birds would have given this up (hair and feathers in the way?) but this explanation also doesn’t explain why lizards’ parapineal organs closely line up with distant relatives like lampreys. Fortunately, CT scans of S. ensidens were able to capture enough detail in skull anatomy to reveal that this extinct lizard had both the fish and lizard versions of these eyes, clearly demonstrating that the two variations developed in parallel to each other, rather than in succession.

The view from the top of one’s head

There are still some questions about the utility of parietal eyes. The organs are formed when an embryo’s developing brain comes in contact with a patch of skin. That contact triggers a cascade of activity in the affected cells, creating any one of an animal’s three eyes. So far, pineal and parapineal eyes have been linked to endocrine functions (such as circadian rhythm management) and navigation. Some vertebrates seem to be able to detect the polarization of light with their third eye, then use that information to better orient themselves in their local environment.

Source: A four-eyed lizard offers a new view of eyesight’s evolution in vertebrates by Jim Shelton, Yale News

On March 29th, 2018 we learned about

Horns and frills helped ceratopsians find mates more than they dissuaded contact with other species

As far as anatomy goes, pointy things are almost always used for poking other animals. As a soft, relatively blunt human being, it’s hard to see spines, teeth or claws as being good for anything but stabbing and slicing. However, some of the world’s most impressive spikes, such as those extending from the face of a Triceratops, probably didn’t do much puncturing, and were instead grown for their visual appeal. With a lack of battle damage in fossils plus an impressive amount of variety in the headgear of each ceratopsian species, the question isn’t if horns were meant to be eye-catching, but exactly what message was being delivered.

Were spikes signs for other ceratopsians?

One possible explanation has been that each species’ unique combination of horns, knobs, spikes and frills helped them sort themselves out when mingling with other ceratopsians. A Triceratops, for instance, might have recognized that a Nedoceratops didn’t have the same horn proportions, and thus wasn’t worth trying to mate with. This would allow confused individuals to avoid hybrid offspring that would either be unviable or at least incapable of further reproduction.

New analysis of a spectrum of species has cast doubt on this idea though. After comparing anatomy from different locations and time periods, researcher found no evidence that the need to identify other species made any difference to these animals’ evolution. If looking different than other ceratopsians helped these dinosaurs reproduce more successfully, then you’d expect to see more differentiation between species that lived near each other. Instead, having other ceratopsians as neighbors didn’t result in fancier frills or horns. Those ornaments were apparently meant to draw a different kind of attention.

Frills to impress closer kin

A more likely scenario is that horns and frills were meant to impress members of each dinosaur’s own species. Like antlers on a deer, large horns and frills would essentially advertise that a particular individual was healthy and strong, and probably worth mating with. While there’s no sign of (most) ceratopsians showing clear sexually dimorphic traits, like antlers that only grow on male deer, researchers still found patterns in the examined skulls to suggest that horns were meant to impress one another. The rate at which horns and frills evolved suggest that having the right headgear was a big mating advantage, and that that helped lead to the diversity of anatomy in these dinosaurs’ skulls.

That idea is based on the lack of battle damage we see in fossilized ceratopsian horns, as well as the crazy amount of anatomical variety seen between species. From that perspective, each dinosaur apparently wanted its own unique set of spikes to stand out, although recent analysis has found that that’s not actually the case.

Source: Dinosaur frills and horns did not evolve for species recognition by Queen Mary, University of London, Phys.org

On March 15th, 2018 we learned about

Scans show that Archaeopteryx’s arm bones were able to flap like a pheasant

For over 100 years, the biggest point of fascination on Archaeopteryx fossils wasn’t the animal’s bones, but its feathers. When first discovered in the 1860s, people were understandably fixated on the impressions of the specimens long feathers left in the rock around the skeleton. They appeared to be long and rigid like a modern bird’s feathers, right down to tiny, interlocking barbules that would give each feather more strength. On the other hand, Archaeopteryx’s skeleton seemed to contradict this bird-like anatomy, as its long tail and toothed mouth aren’t found in any modern avians, and its breast bone lacked the large keel that modern birds use to attach powerful chest muscles needed for flapping. To dig in a little deeper, the latest study of Archaeopteryx looked inside the animal’s bones, and found that they probably could fly like a bird, but only those birds that stay close to the ground.

X-ray scanning for signs of strength

With the exterior of Archaeopteryx’s fossil having been extensively documented, researchers opted to look at the inner structure of each bone in the European Synchrotron Radiation Facility. The powerful x-rays would let them look at delicate structures inside these 150 million-year-old fossils with amazing resolution without needing to damage them in the process. The goal was to measure the arm bones’ torsional resistance, which is how well they would stand up to being twisted when used in flight. Since modern birds that do a lot of continuous flying have higher torsional resistance than birds that don’t, this measurement could be used as another way to assess how flight-ready Archaeopteryx was, regardless of feathers or breast bones. To make sure they weren’t missing a larger pattern, the bones were also compared to crocodilians and pterosaurs as well.

To nobody’s surprise, Archaeopteryx didn’t soar like an eagle, or even a Quetzalcoatlus. However, its arms did appear to handle more than just crawling around, most closely resembling birds like quails and pheasants that are known for short bursts of flight, usually to avoid danger. The x-ray scanning also revealed a large number of blood vessels in Archaeopteryx’s skeleton, a trait associated with high growth rates and metabolism. This would indicate that while the dinosaur wasn’t a bird itself, it probably grew and moved like one.

Not a fully-fledged flyer— yet

This still doesn’t make Archaeopteryx the world’s first bird, or even a bird ancestor. Other species have been found with feathers, even predating Archaeopteryx. We also don’t believe that Archaeopteryx was part of the raptor lineage that eventually developed into modern birds, and instead was a case of convergent evolution. In this case, that convergence would be the capacity for short, evasive flight, which makes sense as avoiding predators has been found to be the most likely reason any species develops wings in the first place. The one catch is how Archaeopteryx would ever get off the ground in the first place. Until evidence of something like a breast keel made from cartilage can confirm its flapping strength, we’re still not sure how well the animal could defy gravity to get itself airborne.

Source: This Famous Dinosaur Could Fly— But Unlike Anything Alive Today by Michael Greshko, National Geographic

On March 15th, 2018 we learned about

Burning coal was likely the key component of the world’s worst extinction event

As dramatic as a good asteroid strike can be, giant falling space rocks aren’t the only thing that has wiped out life on Earth. The mass extinction that ended the Age of Dinosaurs was actually the fifth time nearly everything died. Before the first dinosaur was ever born, an extinction event known as “The Great Dying” took place, a horrific series of events that choked, poisoned or burned multitudes of animals on both the land and in the seas. 70 percent of terrestrial vertebrates and 90 percent of sea life went extinct during this time 252 million years ago, with the devastation taking at least 10 million years to show signs of recovery. While many of the terrible details about how things died have previously been discovered, research out of Utah is helping piece together what started all this destruction in the first place.

Indirect effects of eruptions

With no sign of an asteroid strike in sight, researchers have been looking for other events that might have knocked the world’s climate and atmosphere so far out of balance that it became toxic for most creatures to breathe. There’s evidence that massive volcanic eruptions took place in Asia around the end of the Permian period, but they predated the fossil records of the Great Dying by 300,000 years. Furthermore, analysis of rock layers from Utah don’t show signs of direct volcanic impact at that time— instead of the metals like nickel that you’d expect to  be carried from underground by a volcano’s magma, deposits from the end of the Permian have extra mercury, lead and carbon-12, all of which are associated with burning coal.

The picture that then emerged was one where volcanic eruptions were a trigger for The Great Dying, but not the exact cause, as their ash wasn’t influential enough to reach around the world, such as to what is now Utah. Instead, the erupting lava seems to have hit and ignited massive coal beds that were originally deposited in Asia in the Carboniferous period. As that coal burned, it spread around the world, setting off the bigger chain of events that led to mass extinctions.

From coal to corrosion

The fallout from the burning coal might be enough to make a prehistoric therapsid dream of asteroid strikes. The soot from the coal led to severe changes in the planet’s climates, raising temperatures, and acidity, of the oceans. As the oceans warmed, barium levels indicate that more methane was released from the sea floor, trapping even more heat in the atmosphere. After all this, an abundance of pyrite that was formed at this time suggests that the oceans became depleted of oxygen, naturally leading to more dead marine animals. Those deaths were so abundant that the bacteria that set to work consuming corpses released an immense amount of hydrogen sulfide gas (H2S), bringing us to what happened to the poor creatures living on land.

Hydrogen sulfide gas is toxic in large doses, but more importantly can react with moisture in the air to form acidic sulfur dioxide (SO2). So as bacteria tried to clean up the oceans, their waste led to acid rain that started killing plant life on land. Between the toxic, burning atmosphere and a lack of plants, the food chain understandably would have collapsed, taking both herbivores and the carnivores that ate them with it.

Current costs of burning coal

The scariest part of all this is probably just how mundane the idea of burning coal seems today. Thanks to industrialization, we don’t even need the help of a volcano to burn massive amounts of the stuff around the world. Thankfully, air quality legislation has managed to take steps to reign in acid rain, so we’re not corroding our forests into pulp right now. However, the seas do seem to be starting to relive some of the Great Dying, as temperatures and pH levels have been rising in various patches of the ocean. Thankfully, unlike a volcano or asteroid strike, there’s more we can actually do to head off The Great Dying II, because that’s definitely a sequel nobody wants to ever see.

Source: Burning coal may have caused Earth’s worst mass extinction by Dana Nuccitelli, The Guardian

On March 8th, 2018 we learned about

Reptiles’ detachable tails first evolved, and died out, with Permian era lizards

As a general rule, you don’t want cracks in your bones. Then again, you probably don’t want to lose major portions of your anatomy either, and yet lizards apparently love this survival strategy to have had it evolve at least twice in their history. Known as autotomy, this seemingly unpleasant strategy is actually a way that prey animals can stay safe, as their lost tail can confuse or distract a predator long enough for the rest of the lizard (or crab or even mouse!) escape to safety.

Autotomy was thought to have first evolved in Lepidosaurs, or non-crocodile or turtle reptiles, around 70 million years ago. However, fossils indicate that captorhinids, a group of small reptiles from the Permian period 251 million years ago were also dropping their tails to avoid danger. These small omnivores and herbivores weren’t much of a match for larger predators of their time, but they managed to escape being eaten enough to spread across the lost supercontinent of Pangea. Captorhinus and its relatives seem to have been the only reptiles making use of autotomy at the time, and it may have been the critical component that let them dominate their ecological niche. On the other hand, since Captorhinus effectively had a monopoly on autotomy in the Permian period, when they went extinct they apparently took their pre-cracked tails with them.

Best way to break one’s tail bones

The cracks in question aren’t the long, skinny fissures you’d imagine in a fractured bone. They look more like small perforations, similar to the holes between paper towels or toilet paper squares. They worked the same way, as they basically created a point where stress on a yanked tail would be concentrated, ensuring the tail would break at exactly that point as cleanly as possible. Since jettisoning one’s tail is such a high-stakes maneuver, modern lizards with this kind of “intravertebral autotomy,” give themselves a bit more control with muscles that can contract to help break the bone. That’s followed-up by muscle contractions around the caudal artery to minimize bleeding. With no soft tissue preserved, we can’t say for sure if captorhinids also had this kind of control with their version of autotomy, but controlling bleeding seems like an useful adaptation in any era.

Many Captorhinus fossils have been found, allowing researchers to see how an individual’s age changed their bones. The intravertebral autotomy-enabling cracks were more pronounced in young animals than in adults, which makes sense since younger lizards would be at greater risk of predation (similar to just about every animal on Earth).

My third-grader asked: Does it hurt to lose a tail? What if it doesn’t tear off right, like when you rip a paper towel wrong?

There’s no information about lizards appearing to be in pain after dropping their tails, which makes sense from the perspective of survival. The animal’s goal is to escape, and being gripped by pain would probably just be a distraction in the critical moment they’re trying to avoid being eaten. That said, not every break is clean— pet owners are advised that if their lizard’s dropped tail is accompanied by heavy or continuous bleeding, something has gone wrong and the animal will need veterinary care.

Even when things go smoothly, caudal autotomy is still a taxing course of action. The lizard will have to invest more energy in healing and growing a new tail, while at the same time suffering from reduced balance, a loss of fat storage and lacking an escape route if a predator attacks too soon.

Source: Ancient reptile Captorhinus could detach its tail to escape predator's grasp by University of Toronto, Science Daily

On March 1st, 2018 we learned about

Researchers suspect that fish’s first legs evolved to avoid being trapped by tides

Why would any fish want to leave the water? Life, we’ve been told, is so much better down where it’s wetter, and yet humans and every other land-dwelling animal with a backbone are proof that some of our ancestors were highly motivated to get out of the water. One possible answer to that conundrum is that our ancestors didn’t purposely leave the ocean. Instead, what if they were forced to adapt the land after the ocean kept leaving them?

Limbs to cope with lunar influences

This isn’t to say that the oceans dried up 400 million years ago, requiring fish to quickly grow lungs and legs. However, anyone who’s spent enough time on the beach has seen the temporary version of that scenario take place multiple times per day in the form of tides. As the Moon orbits the Earth, it’s gravitational influence on any particular location changes, significantly changing water levels across the ocean. Those varying water levels then fill and drain tide pools along the shore, which we know demands adaptations from the various animals that live in them. It’s possible then that if those conditions were exaggerated further, fish trapped in shallow tide pools would be forced to deal with being cut off from the larger, safer ocean in a bigger way. Any fish that could cope with this isolation, possibly by dragging themselves across exposed land back to the water, would then increase their chances of survival and reproduction, eventually spawning creatures with increasingly robust limbs.

The advantages of being slightly amphibious make sense, but it may be hard to understand how the tides could sufficiently isolate these species to trigger such pronounced changes in anatomy. The answer to that may be the Moon itself. 400 million years ago, when these transitions were taking place, the Moon was around 10 percent closer to the Earth than it is now, since it’s in the slow process of flying away from our planet. As such, its gravitational influence on the ocean would have been even more pronounced than it is today, making high and low tide dramatically different conditions.

Searching for ancient shorelines

While we can be confident about the math behind the Moon’s distance and gravitational influence on the ancient ocean, it’s harder to test its effect on transitional fish species. With no fish clearly captured mid-crawl in the fossil record, researchers are instead looking for patterns in where transitional fossils are found. The first step was to map where fossils from proto-limbed fish like Melanognathus have been discovered, such as in Europe, Canada and Ireland. Those locations were then compared to maps of the world from 400 million years ago, before plate tectonics shifted all the continents into their current positions. The resulting maps lined up very well, and even predicted that more lobe-finned fish are likely buried in other ex-shores, like Syria and Afghanistan.

This correlation is not definitive though. There are other hypotheses about what pressures pushed sea life onto shore, none of which disprove the other. If more specimens can be found, they’ll hopefully shed more light on the just how big a role being stuck on a beach played in pulling vertebrates out of the ocean.

Source: Strong tides may have pushed ancient fish to evolve limbs by Katherine Kornei, Science

On February 22nd, 2018 we learned about

Understanding why so many armored ankylosaurs were found buried on their backs

As jumbled, crunched and scattered fossils make clear, dinosaurs were not buried in an orderly fashion. Bones are often missing, broken or just separated from a body enough to give paleontologists a challenging puzzle to put them back in the same configuration and posture that they had in life. Some of the strange, contorted poses we find skeletons in actually make a lot of sense, such as the bent necks seen on many theropods. In other cases, there’s more of a mystery to be solved, such as why the hulking, armored dinosaurs known as ankylosaurs somehow managed to always get themselves buried upside-down.

The first step to unraveling this mystery to was to confirm if it really existed in the first place. Researchers needed to make sure that a couple of memorable anecdotes about upside-down ankylosaurs hadn’t skewed people’s perception of how common this kind of interment really was. So a short survey of ankylosaur excavations was conducted, looking at both the fossils and field notes concerning 36 different specimens. Sure enough, 26 of those dinosaurs were found laid out on their backs, showing that these animals were unusually likely to be buried upside-down.

Fatal falls

Four hypotheses were developed then tested to see which seemed to be the most plausible explanation for this flipped dinosaurs. Well, they were mostly tested– one suggestion was that these particular dinosaurs were simply unusually clumsy, and did a great job of rolling onto their backs with no way to right themselves. There wasn’t really any evidence to support this idea, and it seemed unlikely that multiple species would have survived as long as they did if something so trivial could lead to their deaths. So option one was essentially discounted from the start.

Pushed by predators

The second hypothesis was that the upside-down ankylosaurs had been flipped by hungry predators. Since their backs were so well protected by bony plates and bumps, called osteoderms, this model suggested that predators could only eat the ankylosaurs’ relatively soft bellies. This kind of activity would presumably be a bit destructive though, and only one specimen out of the 26 was found to have a tooth mark on it. Unless predators were eating ankylosaurs with a very sharp straw, the fossils just didn’t carry signs of damage from other dinosaurs.

Turned over by tummies

If the ankylosaurs were apparently dying peacefully, the focus then looked at the process of decomposition itself. As a body begins to break down, microbes in the stomach can cause it to inflate with gas, leading to bloating in the abdominal cavity. With such a tough, rigid back, a swollen abdomen might push the dead animals off their feet, rolling them over before they were buried and later fossilized. Without any trace fossils of bloated bellies documenting this kind of deformation, researchers looked for analogs in living animals to see if this kind of decomposition was possible in the first place.

While rhinos are large, tough herbivores, they don’t have the right body structure to act as a proxy for ankylosaur death. Instead, researchers looked to armored armadillos, which were said to experience bloating and flipping after they died. Nearly 200 fresh carcasses were located and examined, but none of them were about to roll over as they rotted. A few were even brought to researchers own backyards to make sure they weren’t bothered by scavengers, but their stomachs just didn’t seem to be up to the task of being a post-mortem pneumatic jack.

Whirled in the waves

The final hypothesis involved bloating, but only in a much more mobilized corpse. This model required that the flipped ankylosaur be floating in water, making its top-heavy body much easier to rotate. Armadillos were spared the indignity of being thrown in the water, as researchers tested this idea with computer simulations to see if the weight distribution of various ankylosaur species was at all amenable to rolling over in a lake, river or ocean.

In these simulations, the ankylosaurs finally ended up on their backs. Dinosaurs from the nodosaurid family flipped at the slightest provocation in the water, as even a one-degree rotation could turn them over. Other ankylosaurs, like Ankylosaurus, were a bit less likely to capsize, but some bloating and a good nudge from a wave or predator could still leave them upside-down. What’s more, the sand and silt found at the bottom of many bodies of water would then be excellent material for fossilizing the body once it sank, further increasing the odds that we’d find flipped fossils millions of years later.

Source: Most ankylosaurs were fossilized belly up. Now, scientists think they know why by Matt Warren, Science

On February 15th, 2018 we learned about

Fossil footprints suggest that lizards have long been able to run on only two legs

110 million years ago, a small lizard near the coast of what is now South Korea was being pursued by a pterosaur. To better evade the predator’s attack, the lizard appears to have reared up, sprinting on its back legs across the moist sand. This upright-sprinting not only gave the lizard a boost in speed, but probably helped the lizard’s maneuverability, making it that much harder for the pterosaur to catch.

My third grader asked: Wait, the lizard ran upright? Like a human?!

Well, not exactly like a human, as lizards’ legs are angled out to the sides of their bodies, rather than under them. Looking at the 50 species of living lizards that can also pull off this maneuver, they don’t look a whole lot like humans as they run. In some cases it’s almost supernatural looking, as when the basilisk or “Jesus Lizard” (Basiliscus basiliscus) sprints across the surface of water.

My third grader said: Oh, like Dash from The Incredibles.

Ok, they’re not quite that fast, but the lizards that run on their hind legs today to move pretty quickly. That speed may even help explain how they started running this way to begin with— one hypothesis is that as a lizard with smaller front legs picks up speed, their center of gravity effectively moves back to their hips. They can then tip up over their hips, making turning easier compared to running on all four legs. However, that uses a lot of energy for a cold-blooded animal, which is probably why they’d save it for critical moments instead of becoming bipedal all the time.

Scientists aren’t actually sure how, or when, this behavior got started. Small lizards don’t get fossilized as often as bigger animals, so we don’t have tons of examples of every intermediate form that would hint at changes in the reptiles’ gait. Fossilized footprints aren’t common either, and these particular tracks aren’t only the first record of a bipedal lizard’s movement, but of any lizard at all.

My four-year-old asked: How are they sure the tracks are from the back feet only?

The tracks are clear enough to preserve specific details about the feet that made them, right down to toe and claw placement. There are a couple of prints that appear to be from the lizard’s front feet towards one end of the trackway, which help contrast from the back feet as well, particularly in their size. Combined with the distance between each foot, it all matches what might be expected when a small lizard was making a quick dash on its back legs.

Of course, it’s hard to be absolutely certain about something like a footprint. There is always a chance that the lizard was walking on all four limbs, but that the smaller front legs just weren’t making impressions in the soil. There’s not a lot to suggest that that’s the case, but it’s good to keep in mind that footprints alone aren’t giving us the whole story.

My third grader asked: So are we sure it was a lizard? How do they know it was being chased by a pterosaur?

The feet do match the anatomy of a lizard, and there’s no doubt that this type of animal was alive 110 million years ago. What motivated this particular critter to take off running is much more speculative— Yuong-Nam Lee, the paleontologist who discovered the tracks, was originally searching in those rocks for pterosaur prints. At first, he barely paid attention to the lizard prints once he realized they weren’t from a pterosaur. So while pterosaurs likely lived in the vicinity of the sprinting lizard at some point, there’s no direct evidence that these tracks were made to avoid being eaten.

Source: Lizards ran bipedally 110 million years ago by Hang-Jae Lee, Yuong-Nam Lee, Anthony R. Fiorillo & Junchang Lü, Nature