SEATTLE — Tyrannosaurus rex may have had small arms, but it was no pushover.

This fierce dinosaur is known for its giant head, powerful jaws and overall fearsome appearance — except for those comical-looking arms. But the roughly meter-long limbs weren’t just vestigial reminders of a longer-armed past, paleontologist Steven Stanley of the University of Hawaii at Manoa said October 23 at the Geological Society of America’s annual meeting. Instead, the limbs were well-adapted for vicious slashing at close quarters, he argued.
T. rex ancestors had longer arms that the dinos used for grasping. But at some point, T. rex and other tyrannosaurs began to use their giant jaws for grasping instead, and the limbs eventually atrophied. Many people have hypothesized that the shrunken arms were, at best, used for mating or perhaps pushing the animal up off the ground; at worst, they were completely functionless.

But Stanley noted that the arms were quite strong, with robust bones that could sustain the impact of slashing. Each arm ended in two sharp claws about 10 centimeters long. Two claws give more slashing power than three, because each one can apply heavier pressure. Furthermore, the edges of the claws are beveled and sharp like those of a bear rather than flat like the grasping claws of an eagle. Those traits support the slasher hypothesis, Stanley concluded.

Many scientists aren’t convinced. While an interesting idea, it’s still unlikely that an adult T. rex would have used its arms as a primary weapon, says vertebrate paleontologist Thomas Holtz of the University of Maryland in College Park, who was not involved in the study. Although strong, the arm of a fully grown T. rex would barely reach past its chest, greatly reducing its potential strike zone. But a T. rex’s arms grew more slowly than its body, so younger dinos would have had proportionally longer arms. It’s possible the juveniles might have found them useful for slashing prey, he says.

At the beginning of 2017, parents and pediatricians got new peanut guidelines that, for most kids, are very pro-peanut. My colleague and fellow mom Meghan Rosen wrote about the recommendations, issued from the National Institute of Allergy and Infectious Diseases.

This “let them eat nuts” advice is based in part on a large and unusually clear dataset from a study that looked at babies at high risk of developing an allergy to peanuts. In the study, some of the children were regularly fed peanut-containing foods until their fifth birthdays. The others avoided any food with peanuts. By the end of the study, the kids who regularly ate peanut-containing food were way less likely to have a peanut allergy than the kids who had avoided the nut, the researchers found.
In a nutshell, parents of low-risk babies (infants without an egg allergy or severe eczema) should feel free to put peanut-containing food in the rotation as soon as their babies are ready for solid foods, around 4 to 6 months of age.

Whole peanuts and peanut butter are both choking hazards and shouldn’t be fed to babies. Instead, peanut butter (or peanut flour or peanut butter powder) can be mixed into breast milk, formula, fruit, yogurt or purees.

Babies with severe eczema or who are allergic to eggs ought to be seen by an allergist who can help guide the introduction of peanuts to the diet. Those appointments may reveal that some babies are in fact already allergic to peanuts. For those kids, peanuts may need to be avoided altogether.

When the recommendations were released, health officials were optimistic that the advice would lead to a reduction in peanut allergy in kids, given the study that found an early peanut introduction curbed this allergy. But that intro may not be happening as early or often as officials had hoped.
Bryce Hoffman, an allergist in New York City, suspected that the guidelines weren’t being used. Anecdotally, he and his colleagues hadn’t seen many infants coming into his allergy office with referrals for peanut allergy testing.

To get an idea of whether the guidelines were being applied (or not), Hoffman and his colleagues surveyed pediatricians about their advice to parents on peanuts. The results, which he presented October 30 at the annual meeting of the American College of Allergy, Asthma and Immunology, were discouraging.

Of the 79 pediatricians who responded, 30, or 38 percent, were not using the new guidelines in their practice. What’s more, 61, or 77 percent, of the pediatricians recommended high-risk patients eat peanuts later than ages 4 to 6 months, instead of sending those children to an allergist first for testing. Close to half (44 percent) of the pediatricians said they didn’t routinely test high-risk patients for allergy or send them to an allergist for testing.

This reluctance to test poses a problem for infants who “may have a dangerous anaphylactic reaction if given peanut,” Hoffman says. Testing can help clear up whether these infants should strictly avoid peanuts or be exposed to the nut first in an allergist’s office.

Peanuts got a bad rap for a long time, so it’s not surprising that parents and pediatricians might not jump at the chance to get peanut-containing food on the menu. The survey suggested that pediatricians aren’t familiar enough with the guidelines and, what’s more, are often too rushed to delve into the details during checkups. But for the new guidelines to do any good, they need to be used.

NASA’s New Horizons mission needs a catchier nickname for its next destination. The bar isn’t exactly high.

On New Year’s Day 2019, the spacecraft will fly by the tiny Kuiper Belt world that bears the official designation of (486958) 2014 MU69. NASA announced Monday that it is asking the public for an easier-to-remember nickname. The SETI Institute is hosting the contest.

As with similar crowdsourced naming campaigns, the name options vary widely. Current candidates range from Mjölnir (the hammer of the Norse god Thor) to Z’ha’dum (a planet from Babylon 5) to Peanut, Almond and Cashew — multiple name options may be necessary if the object is a binary pair. Whatever the object is named, it will be the most distant solar system body ever visited.
NASA will submit a formal name (or names) to the International Astronomical Union after the flyby, based on whether MU69 turns out to be a single body, binary pair or other system.

While anyone is welcome to submit a name or vote on existing options, SETI must approve any options before they appear on the ballot. So the odds don’t look good for Planet McPlanetface.

The naming campaign will close at 3 p.m. EST on December 1. The winner will be announced in early January.

Immigrants, they get the job done — eventually. Among dwarf mongooses, it takes newcomers a bit to settle into a pack. But once these immigrants become established residents, everyone in the pack profits, researchers from the University of Bristol in England report online December 4 in Current Biology.

Dwarf mongooses (Helogale parvula) live in groups of around 10, with a pecking order. The alphas — a top male and female — get breeding priority, while the others help with such group activities as babysitting and guard duty. But the road to the top of the social hierarchy is linear and sometimes crowded. So some individuals skip out on the group they were born into to find one with fewer members of their sex with which to compete —“effectively ‘skipping the queue,’” says ecologist Julie Kern.
Kern and her colleague Andrew Radford tracked mongoose immigration among nine packs at Sorabi Rock Lodge Reserve in Limpopo, South Africa. The researchers focused on guard duty, in which sentinels watch for predators and warn foragers digging for food.

Dwarf mongoose packs gain about one member a year. Among pack animals, higher group numbers are thought to come with the benefit of better access to shared social information like the approach of prowling predators. But upon arrival, new individuals are less likely to pitch in and serve as sentinels, Kern and Radford found. One possible reason: Immigrants lose weight during their transition from one pack to another and may not have the energy required for guard duty.
Pack residents don’t exactly put out a welcome mat for strangers, either. On the rare occasions when newcomers take a guard shift, residents tend to ignore their warning calls. Newbies may be seen as less reliable guards, or packs may have signature alarm calls that immigrants must learn. But after five months, these immigrants have come far. “Given time to recuperate following dispersal and a period of integration,” Kern says, “they contribute equally to their new group.”

Science came out of the lab and touched people’s lives in some awe-inspiring and alarming ways in 2017. Science enthusiasts gathered to celebrate a total solar eclipse, but also to march on behalf of evidence-based policy making. Meanwhile, deadly natural disasters revealed the strengths and limitations of science. Here’s a closer look at some of the top science events of the year.

Great American Eclipse
On August 21, many Americans witnessed their first total solar eclipse, dubbed the “Great American Eclipse.” Its path of totality stretched across the United States, passing through 14 states — with other states seeing a partial eclipse. This was the first total solar eclipse visible from the mainland United States since 1979, and the first to pass from coast to coast since 1918 (SN: 8/20/16, p. 14).
As people donned protective glasses to watch, scientists used telescopes, spectrometers, radio receivers and even cameras aboard balloons and research jets in hopes of answering lingering questions about the sun, Earth’s atmosphere and the solar system. One of the biggest: Why is the solar atmosphere so much hotter than the sun’s surface (SN Online: 8/20/17)? Data collected during the event may soon provide new insights.

March for Science
On April 22, Earth Day, more than 1 million people in over 600 cities around the world marched to defend science’s role in society. Called the first-ever March for Science, the main event was in Washington, D.C. Featured speakers included Denis Hayes, coordinator of the first Earth Day in 1970, and science advocate Bill Nye (SN Online: 4/22/17). Attendees advocated for government funding for scientific research and acceptance of the scientific evidence on climate change.

The march came on the heels of the Trump administration’s first budget proposal, released in March, which called for cutting federal science spending in fiscal year 2018 (SN: 4/15/17, p. 15). Some scientists worried that being involved with the march painted science in a partisan light, but others said science has always been political since scientists are people with their own values and opinions (SN Online: 4/19/17).

Climate deal announcement
On June 1, President Donald Trump announced that the United States would pull out of the Paris climate accord (SN Online: 6/1/17) — an agreement the United States and nearly 200 other countries signed in 2015 pledging to curb greenhouse gas emissions to combat global warming. With the announcement, Trump made good on one of his campaign promises. He said during a news conference that the agreement “is less about the climate and more about other countries gaining a financial advantage over the United States.”

Nicaragua and Syria signed on to the agreement in late 2017. A withdrawal from the United States would leave it as the only United Nations–recognized country to reject the global pact. President Trump left the door open for the United States to stay in the climate deal under revised terms. A U.S. climate assessment released in November by 13 federal agencies said it is “extremely likely” that humans are driving warming on Earth (SN Online: 11/3/17). Whether that report — the final version of which is due to be released in 2018 — will have an impact on U.S. involvement in the global accord remains to be seen.

North Korea nuclear test
On September 3, North Korea reported testing a hydrogen bomb, its sixth confirmed nuclear detonation, within a mountain at Punggye-ri. That test, along with the launch of intercontinental ballistic missiles this year, increased hostilities between North Korea and other nations, raising fears of nuclear war. As a result of these tests, the United Nations Security Council passed a resolution strengthening sanctions against North Korea to discourage the country from more nuclear testing.

As the international community waits to see what’s next, scientists continue to study the seismic waves that result from underground explosions in North Korea. These studies can help reveal the location, depth and strength of a blast (SN: 8/5/17, p. 18).

Natural disasters
The 2017 Atlantic hurricane season saw hurricanes Harvey, Irma and Maria devastate areas of Texas, Florida and the Caribbean. More than 200 people died from these three massive storms, and preliminary estimates of damage are as high as hundreds of billions of dollars. The National Oceanic and Atmospheric Administration had predicted that the 2017 season could be extreme, thanks to above-normal sea surface temperatures. The storms offered scientists an opportunity to test new technologies that might save lives by improving forecasting (SN Online: 9/21/17) and by determining the severity of flooding in affected regions (SN Online: 9/12/17).

In addition to these deadly storms, two major earthquakes rocked Mexico in September, killing more than 400 people. More than 500 died when a magnitude 7.3 earthquake shook Iran and Iraq in November. And wildfires raged across the western United States in late summer and fall. In California, fires spread quickly thanks to record summer heat and high winds. At least 40 people died and many more were hospitalized in California’s October fires. Rising global temperatures and worsening droughts are making wildfire seasons worldwide last longer on average than in the past, researchers have found (SN Online: 7/15/15).

Not so long ago, the lives of sea turtles were largely a mystery. From the time that hatchlings left the beaches where they were born to waddle into the ocean until females returned to lay their eggs, no one really knew where the turtles went or what they did.

Then researchers started attaching satellite trackers to young turtles. And that’s when scientists discovered that the turtles aren’t just passive ocean drifters; they actively swim at least some of the time.
Now scientists have used tracking technology to get some clues about where South Atlantic loggerhead turtles go. And it turns out that those turtles are traveling to some unexpected places.

Katherine Mansfield, a marine scientist and turtle biologist at the University of Central Florida in Orlando, and colleagues put 19 solar-powered satellite tags on young (less than a year old), lab-reared loggerhead sea turtles. The turtles were then let loose into the ocean off the coast of Brazil at various times during the hatching season, between November 2011 and April 2012.

The tags get applied to the turtles in several steps. Turtle shells are made of keratin, like your fingernails, and this flakes off and changes shape as a turtle grows. Mansfield’s team had figured out, thanks to a handy tip from a manicurist, that a base layer of manicure acrylic deals with the flaking. And then some strips of neoprene along with aquarium silicone attach the tag to the shell. With all that prep, the tag can stay on for months. The tags transmit while a turtle is at the water’s surface. A loss of the signal indicates that either the tag has fallen off and sunken into the water, “or something ate the turtle,” Mansfield says.
The trackers revealed that not all Brazilian loggerhead sea turtles stay in the South Atlantic. Turtles released in the early- to mid-hatching season stay in southern waters. But then the off-coast currents change direction, which brings later-season turtles north, across the equator. Their trajectories could take them as far as the Caribbean, the Gulf of Mexico or even farther north, which would explain genetic evidence of mixing between southern and northern loggerhead populations. And it may help to make the species, which is endangered, more resilient in the face of environmental and human threats, the researchers conclude December 6 in the Proceedings of the Royal Society B.

But, Mansfield cautions, “these are just a handful of satellite tracks for a handful of turtles off the coast of Brazil.” She and other scientists “are just starting to build a story” about what happens to these turtles out in the ocean. “There’s still so much we don’t know,” she says.

Mansfield hopes the tracking data will help researchers figure out where the young turtles can be found out in the open ocean so scientists can catch, tag and track wild turtles. And there’s a need for even tinier tags that can be attached to newly hatched turtles to see exactly where they go and how many actually survive those first vulnerable weeks and months at sea. Eventually, Mansfield would like to have enough data to make comparisons between sea turtle species.

“The more we’re tracking, the more we’re studying them, we’re starting to realize [the turtles] behave differently than we’ve historically assumed,” Mansfield says.

It’s like having “an elephant stand on your thumb.”

That’s how deep-sea physiologist and ecologist Mackenzie Gerringer describes the pressure squeezing down on the deepest known living fish, some 8 kilometers down. What may help these small, pale Mariana snailfish survive elephantine squashing, says Gerringer of the University of Washington’s Friday Harbor Labs, is a body bulked up, especially at the rump, with a watery goo.

The snailfish family gets its nickname from the way some shallow-water species in thundering tides grip a rock with a little suction cup on the belly and curl up. “Quite cute,” Gerringer says, and maybe, if you squint, somewhat like a snail.
She and colleagues discovered the deepest fish in 2014 in the western Pacific Ocean’s Mariana Trench and described the newly named Psuedoliparis swirei November 28 in Zootaxa. To catch specimens, Gerringer and colleagues turned to extreme trapping. They weighted a boxy, mesh-sided trap with steel plates to sink it. It took about four hours to fall to the bottom.

The scientists baited traps with mackerel, which snailfish don’t eat. But the fish do eat the underwater amphipods that mob a mackerel feast. Remotely related to harmless garden pill bugs, trench amphipods clean mackerel to the bones, Gerringer says. “I certainly wouldn’t swallow a live amphipod after seeing what they can do.” A snailfish, however, has a second set of jaws at the back of its throat that render crustaceans safe to swallow.

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For animals that live in such extreme pressures and temperatures (1° or 2° Celsius), snailfish don’t “look very robust … or very armored,” she says. “You can actually see the brain through the skull.”

Skimping on dense muscles and bones may improve snailfish buoyancy and save energy. These fish also lack air pockets that give a little lift to some other fishes, but that would get squashed to nothing so far down. Instead, the snailfish have inner deposits of a watery goo, more buoyant than muscles and bones and less compressible than air.

The goo also may aid swimming efficiency by offering a cheap shape improvement, Gerringer and colleagues proposed December 6 in Royal Society Open Science. To test the idea, she 3-D printed and motorized a robo-snailfish. Easier than catching a real one, Gerringer says.

A latex sleeve around the robot tail let her add or subtract water as an approximation of the gelatinous tissue. With an empty sleeve, the wide fish body pinches in to a thin tail, inviting vortices that cause drag. With this abrupt narrowing, robo-swimming proved a struggle. Filling the tail-sleeve to create a tapering rear let the robo-snailfish swim faster.

This goo is cheap tissue to grow, Gerringer says. It’s mostly water, one thing a fish living underneath eight kilometers of ocean has in abundance.

OXON HILL, Md. — Fast radio bursts could come from a turbulent home. At least one source of these bright, brief blasts of radio energy may be a young neutron star assisted by a nearby massive black hole, new research suggests.

“The biggest mystery around fast radio bursts is how such powerful and short-duration bursts are emitted,” says astronomer Daniele Michilli of the University of Amsterdam. The latest observations, reported online January 10 in Nature and at a meeting of the American Astronomical Society, suggest the bursts are coming from an environment with an unusually strong magnetic field. That field leaves a signature mark on the radio waves, twisting them into spirals, Michilli and his colleagues report.
Only a few fast radio bursts have ever been detected, and most appear as one-off events. Few known processes in the universe can explain them. But one burst, FRB 121102, has been seen repeating over the past decade or so (SN Online: 12/21/16). That repetition let astronomers follow up on the burst, and track it to a dwarf galaxy some 2.5 billion light-years away (SN: 2/4/17, p. 10).

Now, Michilli and his colleagues have used the Arecibo radio telescope in Puerto Rico to show that the burst’s source is embedded in an extremely strong magnetic field, 200 times stronger than the average magnetic field in the Milky Way.

The team measured the radio waves from 16 distinct bursts over three two-hour observational runs spanning several months. The bursts were exceptionally brief, the shortest lasting just 30 microseconds. That means that whatever emitted it must be just 10 kilometers wide, Michilli says.

“To emit a short burst you need a small region,” he says. “Therefore compact objects such as neutron stars are strongly favored by this result.”
The team also analyzed the radio waves in a new way, revealing that what looked like individual bursts were actually composed of many smaller sub-bursts, says astronomer Andrew Seymour of the Universities Space Research Association at Arecibo. That complicates the picture even further. The sub-bursts might be intrinsic to the object that creates them, or they might be the result of the waves passing through blobs of plasma, he says.

Finally, the observations showed that the waves were polarized, all oriented in the same direction. But something had twisted the waves, forcing them to rotate in corkscrews on their way from the dwarf galaxy to Earth. Follow-up observations with the Green Bank Telescope in West Virginia confirmed the twists were really there.

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The only phenomenon that is known to create such a rotation is a strong magnetic field, Michilli says. There are two main hypotheses for the bursts’ behavior. One is that they are from a young, energetic neutron star called a magnetar that’s sitting inside a shell of magnetized gas, which the magnetar itself expelled in a supernova explosion. The magnetar emits radio waves, and the shell makes them rotate.

“If you have young magnetars that have just been born in supernova explosions, only a few decades old, they could be very bursty objects, have very violent youths, and that could give rise to repeating fast radio bursts,” says astronomer Brian Metzger of Columbia University, who was not involved in the new study.

But Michilli points out that in order to drive such strong magnetic fields, the supernova remnant would have to be a million times brighter than even the brightest remnant in the Milky Way, the Crab nebula (SN: 1/1/11, p. 11). Instead, the bursts could come from a young neutron star orbiting the dwarf galaxy’s dominant black hole, which probably has between 10,000 and 1 million times the mass of the sun, he says.

Such large black holes are already known to have strong magnetic fields and to make polarized light rotate. And a neutron star nestling up next to a black hole is a plausible setup: There’s one orbiting the supermassive black hole at the center of the Milky Way. Although this neutron star’s radio waves don’t come in brief bright bursts, they are also twisted, the researchers say.

If not for that neutron star, “this would seem very contrived to me,” Metzger says. “That combines two unlikely things.”

More exotic explanations remain possible, too, Michilli’s team says.

“The joke is there are far more theories than there are observed bursts,” said coauthor Jason Hessels of the University of Amsterdam in a news conference January 10. “In the coming weeks we expect that very creative theorists will come up with explanations for our observations we haven’t thought of yet.”

Questions remain about whether all fast radio bursts, including the ones that don’t repeat, come from such exciting neighborhoods. “We cannot say yet if there are two classes with different properties, or if it’s one class of fast radio bursts and they just happen to be seen in different configurations,” Michilli says.

It’s also still unknown whether any other bursts have twisted waves at high frequencies — the smoking gun for strong magnetic fields. Measuring the rotation of the waves in FRB 121102 required hacking Arecibo with new hardware that let it detect higher frequencies than before. “We weren’t able to do that until recently,” Seymour says. “I stayed up on Christmas evening [2016] and made these observations, and luckily it paid off.” Maybe other fast radio bursts that Arecibo observed didn’t show the same rotation signature because the telescope wasn’t ready to measure it yet.

Hessels thinks “the prospects are quite good” for figuring out what fast radio bursts are in the near future. Several new radio observatories around the world are due to come online in the next few years. “These are going to be FRB factories,” Hessels said. He expects to find other repeating bursts, if they exist. “Then we can see if this repeating source is really a complete oddball, or part of a distribution of sources.”

Lab-grade flight tracking has gone wild, creating a broad new way of studying some of the flashiest of natural acrobats, wild hummingbirds.

One of the findings: Bigger hummingbird species don’t seem handicapped by their size when it comes to agility. A battleship may not be as maneuverable as a kayak, but in a study of 25 species, larger hummingbirds outdid smaller species at revving or braking while turning. Measurements revealed these species have more muscle capacity and their wings tended to be proportionately larger for their body size than smaller species. Those boosts could help explain how these species could be so agile despite their size, researchers report in the Feb. 9 Science.
Adapting a high-speed camera array and real-time tracking software to perform in field conditions let the researchers analyze more than 200 wild birds swerving and pivoting naturally. With over 330,000 bird maneuvers recorded, the researchers could compare the agility of the different species. It’s the first comparative study of natural flight moves in wild birds, says coauthor Roslyn Dakin, who is based in Ottawa with the Smithsonian Conservation Biology Institute.

“What makes this research a clear advance is the methods they used,” says Christopher J. Clark of the University of California, Riverside. His hummingbird studies have revealed how the birds’ feathers squeal during flight (SN: 4/4/15, p. 5), but he was not involved in the new research.

In the experimental setup, four cameras film a temporary flight chamber in the field. Customized computer software allows the team to track birds in 3-D as they explore the space in any way they choose.
The project began almost a decade ago when coauthor Paolo Segre adapted and then hauled the flight-tracking system to Ecuador, Costa Rica and Peru — great places to find hummingbirds with different wing shapes and body sizes, but hard on equipment. At the time, he needed five computers, sometimes running just on solar panels and a generator in the Amazon. “We were in a thatched hut,” accessible only by several hours’ boat ride, he says. “Monkeys poked their heads in.” Since then, computers have improved, and one machine is enough to run the software.

The new paper uses some of this hard-won data to focus on two kinds of turning maneuvers — a simple flying arc and a pitch-and-roll move that Dakin calls “turning on a dime.” That involves the bird slanting its body and then pivoting in place.

Birds’ agility did not appear to be affected in field sites at higher elevations. In theory, less oxygen and lower air pressure should make athletic flying tougher. An earlier study by the same researchers found that Anna’s hummingbirds (Calypte anna) accelerated more slowly and had other performance falloffs at altitudes higher than the birds’ home range. Yet birds that call those higher elevations home had adapted to the conditions.

Studying how hummingbirds, or even birds in general, maneuver has been hard to do. Previous approaches to understanding bird motion were limited to capturing data on animals performing set tasks. “A ballerina has a number of different moves,” Clark says, but “so far we’ve just studied individual moves.” Now researchers are starting “to put together the entire dance.”

Fossilized footprints from an iguana-like reptile provide what could be the earliest evidence of a lizard running on two legs.

The 29 exceptionally well-preserved lizard tracks, found in a slab of rock from an abandoned quarry in Hadong County, South Korea, include back feet with curved digits and front feet with a slightly longer third digit. The back footprints outnumber the front ones, and digit impressions are more pronounced than those of the balls of the feet. The lizard’s stride length also increases across the slab.
That’s what you’d expect to see in a transition from moseying along on four legs to scampering on two, says Yuong-Nam Lee, a paleontologist at Seoul National University who first came across the slab back in 2004. A closer examination two years ago revealed the telltale tracks.

Lee and his colleagues attribute the tracks to a previously unknown lizard ichnospecies, that is a species defined solely by trace evidence of its existence, rather than bones or tissue. Lee and his colleagues have dubbed the possible perpetrator Sauripes hadongensis and linked it to an order that includes today’s iguanas and chameleons in the Feb. 15 Scientific Reports.
Bipedal running certainly would have come in handy when escaping predatory pterosaurs some 110 million to 128 million years ago, the age of the rock slab. Lizard tracks are pretty rare in the fossil record, due to the reptiles’ lightweight bodies and penchant for habitats that don’t make great fossils. Though tracks appear in older fossils from the Triassic Epoch, 200 million to 250 million years ago, those prints belong to more primitive lizardlike reptiles. The new find edges out another set from the same region as the oldest true lizard tracks in the world by a few million years, the researchers say.
Plenty of modern lizards use two legs to scurry around. Some studies have linked similarities in ancient lizard bone structure to bipedal locomotion, but it is unclear exactly when lizards developed bipedalism. Lee’s team argues that these tracks represent the earliest and only direct evidence of bipedal running in an ancient lizard.

Martin Lockley, a paleontologist at the University of Colorado Denver who studies ancient animal tracks, points to alternative explanations. S. hadongensis might have trampled over front prints with its back feet, obscuring them and giving the appearance of two-legged running. Preservation can vary between back and front footprints. And the stride lengths aren’t quite as long as what Lockley says he’d expect to see in running. “Running or ‘leaping’ lizards make for a good story, but I am skeptical based on the evidence,” he adds.

So it may take the discovery of more fossilized lizard prints to determine whether S. hadongensis’ tracks truly represent running on two legs rather than simply scurrying on four.