With jaws like these, who needs big arms? A new analysis suggests dinosaurs like Tyrannosaurus rex had shrunken forelimbs because their massive, powerful heads became their primary tool for killing large prey, rendering their arms redundant. It is an evolutionary approach that five different lineages of large theropod dinosaurs took independently.

Researchers are well aware that a number of large, predatory theropods followed a trend towards bigger bodies, bigger heads and smaller, shorter arms over time. But it wasn’t known why this pattern repeated across multiple predatory dinosaur families, scattered across the globe and separated by many millions of years, says at University College London. There was also little understanding of how the bones in their ever-heftier skulls changed as their arms became proportionally smaller.

“This paper tackles one of the big evolutionary questions in theropod dinosaurs,” says at the University of Bristol, UK, who wasn’t involved with the research.

Scherer and his colleagues compiled data on the proportions of the forelimbs and skulls of 85 theropod species, along with body-mass data. This allowed them to calculate a ratio between the skull dimensions and forelimb lengths, quantifying just how small the arms were compared with the head. The researchers then compared this ratio with other measurements of the dinosaurs’ bodies, along with a measure of the skulls’ strength based on factors such as bite force and skull rigidity.

The team found that skull durability was associated with smaller arms, regardless of where the species sat in the theropod evolutionary tree. “If it’s a predatory theropod and has a very robust skull, it will most likely have relatively small forelimbs,” says Scherer.

The researchers found this head-arm divergence evolved independently in five theropod groups: tyrannosaurids, the short-snouted abelisaurids, the knife-toothed carcharodontosaurids, ceratosaurids and megalosaurids. This evolutionary pattern hadn’t been identified in the last two groups until this study, points out at the Beipiao Pterosaur Museum of China, who wasn’t involved in the research. This shows how hidden evolutionary signals can be revealed when traits are quantified in this way, she says.

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The findings provide clues as to why the dinosaurs’ arms kept shrinking. These predators’ increasing skull strength and body size coincided with the rising mass of their quarries. The theropods evolved huge, sturdy skulls for subduing their large, difficult-to-control prey. Their heads were clearly doing the majority of the work, says Scherer, reducing the need for strong, grappling arms.

“Nature doesn’t like to have everything all at once,” he says. A big, powerful head plus strong forelimbs would require a lot of energy to maintain.

This creates a trade-off between jaws and claws. Other theropods like the megaraptorans and spinosaurs were also very large predators, but they took the opposite route to dinosaurs like T. rex, couplingÌęlong arms with slender skulls.

Rowe is curious about the mechanical function of the jaw-centric theropods’ arms, even in their shortened state. “Yes, tyrannosaurs had tiny, vestigial arms, but that does not necessarily mean they were completely useless,” he says.

He adds that the study emphasises the evolutionary diversity of dinosaurs. “It reminds me of why I fell in love with dinosaurs in the first place,” he says. “They were some of the most innovative and successful animals to ever exist.”

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Traces of a bacterium related to syphilis have been found in a bone from a person who lived in the mountains of Colombia over 5000 years ago.

The discovery shows that this group of corkscrew-shaped bacteria was infecting humans thousands of years earlier than previously thought, before the rise of intensive agriculture, which many researchers consider a catalyst for the spread of pathogens.

Today, three subspecies of the bacterium Treponema pallidum cause the diseases syphilis, bejel and yaws. The deep history of these ailments is murky, and researchers have debated where diseases like syphilis arose and how they became widespread. Ancient bacterial DNA and markers of infection on skeletal remains lend us some clues, but these are rare and can be ambiguous.

So, when researchers studying the ancient DNA of 5500-year-old human remains in the BogotĂĄ savannah detected the genome of Treponema pallidum in a human leg bone sample, it was a surprise.

“This finding was completely unexpected, because the individual studied had no skeletal evidence of a Treponema infection,” says at the University of California, Santa Cruz.

It is widely thought that many common diseases started to affect humanity after the dawn of intensive agriculture, when people began living in denser communities. But this individual lived in a very different context, where small hunter-gatherer groups travelled frequently and were in close contact with wildlife.

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“Our results can tell us a lot about the long-term evolutionary history of [this bacterium] by revealing a long-standing association with human populations,” says at the University of Lausanne in Switzerland.

When Broomandkhoshbacht, Bozzi and their colleagues compared the ancient genome to those of other T. pallidum bacteria, they found it was part of a completely different lineage from any known modern relatives. This indicates that, millennia ago, ancient relatives of syphilis had already diversified in the Americas and were infecting humans, and the team’s analysis suggests they had many of the same genetic features that make today’s strains harmful.

The findings point to an early presence of these pathogens in the Americas, but it is also possible that they have been infecting humans for even longer across the world.

at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, notes that the ancient strain might belong to an elusive, “missing” pathogen: Treponema carateum, which causes a skin disease called pinta. The bacterium is only known from physical descriptions, not genetics.

at the University of Zurich, Switzerland, wonders what additional ancient genomes can tell us. “Were there perhaps many extinct lineages and perhaps different diseases caused by these pathogens in the past?” she says.

For Bozzi, understanding how pathogens evolve to cause diseases like syphilis and yaws is a crucial step in finding the genetic quirks that allow pathogens to infect new hosts and make their associated illnesses more dangerous.

More than a dozen mammal species shimmer and glint purple and green, like precious opals. Their fur is iridescent, meaning its colour appears to change depending on the animal’s orientation relative to the viewer. The effect is similar to an oil slick’s colourful sheen, or the metallic dazzle of hummingbird feathers – and it’s more common among mammals than biologists thought.

at Ghent University in Belgium was studying colour in mammals using specimens at the Royal Museum for Central Africa, also in Belgium, when she noticed an electric blue glint on the fur of a tropical vlei rat (Otomys tropicalis).

“I immediately headed back to the office to see if it was documented anywhere, because everything I’d read up to then had been telling me [mammal] iridescence is only found in golden moles,” says Dobson. Golden moles are African burrowing mammals more closely related to aardvarks and elephants than true moles, their name derived from their tinsel-like hairs.

Dobson searched through the scientific literature and found casual mentions of other mammal species’ shiny fur dating as far back as the 1890s. Investigating further, she and her colleagues examined the fur on specimens of mammals that were associated with anecdotal reports of iridescence – or that were closely related to species with such reports. They used a light microscope to shine light on the fur at differing angles and analysed the wavelengths – and therefore the colours – of the light reflected off the hairs.

The analysis revealed that golden moles aren’t uniquely shimmery. An additional 14 mammal species have iridescent coats, including 10 rodents and the giant otter shrew (Potamogale velox), a semiaquatic predator that is neither otter nor shrew. Six of these species had never previously been considered iridescent in the scientific literature.

“It’s really nice to see such a detailed description of a previously under-described – and often undescribed – phenomenon,” says at Northeastern Illinois University in the US, who wasn’t involved with the study.

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When the researchers looked at the hairs of these iridescent mammals under a high-powered microscope, they found they were all unusually smooth, each made of a series of sub-parallel layers that were more highly compressed than the layers inside typical mammalian hair.

“Each layer bends light in slightly different ways and creates the shifting colours,” says Dobson.

The researchers think the shininess is probably a by-product of other hair-based adaptations. Most of these mammals are burrowing or swimming species, and the hairs’ smooth surfaces may help keep the fur clean in dirty or wet environments.

It is, however, possible the iridescence serves a function, perhaps for visual communication, , says Dobson. The new findings open the door to explore that possibility.

There could be plenty of new opportunities to do just that. In 2024, a from Indonesia was described as having somewhat shiny fur. Dobson suspects there may be even more iridescent mammals out there, still unrecognised.

“Especially when you consider there’s about 2500 rodent species, and we looked at a tiny fraction of them.”

About 13 million years ago in a vast South American wetland, colossal predators clashed.ÌęThe fossilised bone from an enormous flightless bird found in Colombia shows tooth marks made by a giant caiman.

at the University of the Andes in Colombia and his colleagues were studying crocodile fossils in a museum collection when they realised one of the bones didn’t fit. It turned out to belong to a phorusrhacid bird – a group also known as the “terror birds”. These top predators had hatchet-shaped beaks and powerful legs with sharp claws on their toes. The fossilised bone came from the lower leg of a 2.5-metre-tall species, possibly one of the largest types of terror bird yet discovered.

But this predator may have met a grisly end. The bone, originally discovered in Colombia’s Tatacoa desert region by local palaeontologist CĂ©sar Perdomo, was scarred with four deep divots: teeth marks.

Link and his team wanted to know what beast dared wrap its jaws around such an intimidating predator. So they scanned the surface of the fossil to generate a digital model of the tooth marks and compared them with the teeth of ancient predators from the region. The culprit likely wasn’t a mammal.

“There’s no evidence of gnawing and the marks are rounded and in [a] line, more similar to those inflicted by crocodiles and caimans,” says Link.

The terror bird lived at a time when northern South America was dominated by the Pebas system, a massive network of wetlands interspersed with tropical forests and grasslands. The flooded ecosystem hosted a great diversity of crocodilians, and the team found a match for the teeth marks in one of them: a giant caiman called Purussaurus neivensis. Link estimates the reptile would have been about 4.5 metres long.

“Terror birds were undoubtedly at the top of the food chain,” says Link. “But this evidence shows us that they could also fall as prey of large caimans when approaching large water bodies. Maybe they went there to look for prey or [were] moving across this complex ecosystem.”

The team notes they can’t rule out the possibility the bird was already dead when the caiman found it, and the tooth marks are evidence of scavenging. There are no signs of bone healing around the tooth marks. So either way, the bird didn’t survive the encounter.

“These kinds of [tooth] traces are more common than people think,” says at the National University of La Plata in Argentina.

In a study published last year, she and a colleague on a much smaller and older terror bird fossil – roughly 43 million years old – from Argentina. The markings suggest an ancient carnivorous marsupial fed on that bird. Since those traces were also on the lower leg, Hospitaleche wonders if that part of the terror bird body was a vulnerable place for predators to chomp and grip their prey.

“[Bite marks] provide us with these amazing little snapshots into life in the past,” says at the University of Tennessee.

When studying ancient environments, there is a tendency to attempt to precisely categorise extinct organisms within particular ecological roles, she says. However, food webs can be complex.

“This is an animal that was living in the water and doing things in the water, this is an animal that was living up on land and doing things upon land, and never the two shall meet,” says Drumheller. “But of course, nature is always messier than our nice, little, neat boxes.”

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