Creatures that bridged the gap between reptiles and mammals may have been bearing live young 90 million years earlier than we thought.

One of the hallmarks of all mammals apart from monotremes – echidnas and the platypus – is that they give birth to live young. This “viviparity” contrasts with the “oviparity” seen most other vertebrates, in which females lay eggs that hatch outside their bodies.

One long-standing question has been whether creatures known as cynodonts, which emerged around 260 million years ago during the Permian geological period and are directly ancestral to all mammals, laid eggs or gave birth to live young, as no evidence of their reproduction has been discovered either way.

“Whether mammalian ancestors were oviparous or viviparous has been considered a mystery,” says  at Argentina’s National Scientific and Technical Research Council. “We had no clues on how or when one of the most distinctive traits of the majority of present-day mammals, viviparity, evolved in the mammalian lineage.”

This has now changed, thanks to a chance observation made during a postgraduate class at the University of Buenos Aires in Argentina. The students were examining a fossil of Chiniquodon theotonicus, a cynodont that lived 236 million years ago and was unearthed in the Talampaya National Park in north-western Argentina in 2011. Part of the analysis involved using a microscope to examine thin slices taken from one of the ancient animal’s leg bones. At that point, the class noticed a curious growth line inside the bone.

Gaetano and his colleagues investigated further and eventually concluded that this line, analogous to a tree’s growth ring, recorded the moment that the ancient mammal relative went through a growth spurt after its birth. 

The researchers reconstructed how large the leg bone and the animal itself would have been when the growth line formed. They calculated that the animal would have weighed 1.68 kilograms at that point. When the animal died as a full-grown adult, it was just shy of 12 kilograms.

This gave the researchers a ratio of newborn to adult size in the ancient animal. They then compared this figure to similar ratios obtained from a wide variety of modern animals: 1869 mammals, 2619 non-avian reptiles and 782 birds.

“We were amazed to find that ��󾱲Ծ���ܴǻ�Dz���grouped with extant placental mammals,” says Gaetano.

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Egg-laying animal groups produce young that are far smaller than adults when they hatch, he says.

For example, some species of large birds have newly hatched young that are just 0.002 per cent of adult body mass, whereas placental mammals often give birth to young that are over 20 per cent of the size of their parents.

“Only placental mammals produce relatively large newborns, whereas reptile and bird hatchlings are comparatively very small,” says Gaetano. “Hence, we propose that [the] Chiniquodon theotonicus reproductive strategy was most similar to that of placental mammals.”

 at the University of Bristol in the UK says the findings are convincing.

“Further, as they argue, the apparent absence or rarity of actual eggs of any kind in the Permian and Triassic has always been puzzling – the so-called amniote egg gap,” he says.

 at Macquarie University in Sydney says his gut reaction is to be sceptical of the claim of live birth, as we have only indirect evidence. But he is impressed that, in the absence of any direct fossil evidence, the team is still trying to solve the puzzle.

“We may never get any of the kind of direct evidence that solves the
mystery, for example a fossilised mammalian egg or a fossil mother during
pregnancy,” he says. “So it is kind of cool that they’re taking a different approach to trying to find an answer to that question.”

A tiny, long-necked marine predator that lived 245 million years ago is the oldest reptile ever found with an intact digestive system.

The exquisitely preserved fossil, including almost the entire skeleton as well as the stomach, liver and intestines, was found in Yunnan province in China in 2023 and is named Austronaga minuta.

Even blood vessels and the scales of a partially digested fish meal are visible in the remains.

“A fossil this age with such clear preservation of its organs is really a world treasure,” says at Flinders University in Adelaide, Australia, who wasn’t involved in the research.

Austronaga dates back to the Triassic Period, when dinosaurs first evolved, but it isn’t a dinosaur. It belongs to a broader group called archosaurs, which includes dinosaurs, birds and crocodiles, but not other famous marine reptiles like mosasaurs or ichthyosaurs.

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The archosaurs were incredibly diverse in the Triassic and included “all kinds of interesting and strange-looking reptiles”, says at the Natural History Museum Stuttgart, Germany, a member of the team that analysed the fossil. Only later, during the Jurassic and beyond, did they become more restricted, mostly being limited to dinosaurs, including birds, and crocodiles.

This specimen is a mere 60 centimetres in length, but as it is the only known example and the reptile’s age at death is unknown, it is impossible to say whether the species grew larger, says Spiekman.

However, he is confident that it was unlikely to have reached the size of its very long relative Dinocephalosaurus, the “Chinese sea dragon”, which could easily grow to over 4 metres.

“This was certainly a good and agile swimmer, mainly using its tail for swimming, its long front flippers for steering and its long neck to catch fish,” says Spiekman. “A�ܲ��ٰ��DzԲ����� probably would have looked a little like a lizard or a croc trying to be a seal or an otter and likely would have had scaly skin, for example.”

The fossil shows that the basic plan of the reptilian digestive tract has changed little in 245 million years, he says, and the “straight and simple intestines is what we see in most modern predatory reptiles, including those eating fish”.

Although the fossil is of high quality, the internal organs of Austronaga are only preserved in two dimensions. This is unlike those of a 380-million-year-old fossil of an armoured fish that Long and his colleagues , which is regarded as having the oldest known organs of any vertebrate.

“But this is still an incredibly well-preserved fossil,” he says.

A 555-million-year-old worm had a predilection for turning right, possibly indicating the oldest known example of handedness.

Although these worms lacked limbs and so couldn’t be considered left- or right-handed in the way that we understand, the development of a tendency to favour one side over another is evidence of an advanced nervous system.

It remains a feature of free-living mobile life today, but until this discovery, it wasn’t thought to have emerged until the Cambrian Period, which began around 541 million years ago.

at the American Museum of Natural History in New York and his colleagues analysed 100 fossil specimens of a small flatworm-like creature, Spriggina floundersi, collected in South Australia over recent decades.

These animals lived during the Ediacaran Period, when multicellular life first became widespread. It preceded the Cambrian explosion, when animal life diversified dramatically and many groups of animals first appeared.

Spriggina lived in what was, half a billion years ago, a shallow ocean and is thought to have foraged on or close to the seafloor, moving by wriggling to the left or right.

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“We have around 50 specimens of Spriggina that are clearly bent,” says Evans. Twice as many of the fossilised worms are bent to the left than to the right, he says. This means the creature itself bent to the right, as the specimens are mirror-image impressions of the animals, made when storms buried them in sand.

“This appears to be statistically significant and matches what biologists find when they study handedness in different animals today,” says Evans. “Some specimens have multiple bends to both the right and left, suggesting that they all could bend both ways, which makes sense if you don’t want to be stuck moving in a circle.”

While the majority seem to demonstrate right-handedness, it is hard to tell if any were left-handed, he says. “I imagine it’s like taking a picture of 100 people waving with one hand today. You would likely be able to count that more people are waving with their right hand, but you wouldn’t be able to tell who is right- or left-handed.”

Discoveries like this demonstrate that many foundational characteristics that are common to a variety of animals today, such as the ability to move around, bilateral symmetry and handedness, evolved in the Ediacaran, says Evans.

In the Cambrian, organisms built on that foundation to become more complex, for example adding legs to move more efficiently, becoming “less alien and more like the major groups of animals we know today”, says Evans. “This is cool because it suggests that, while the Cambrian was an amazing time in animal evolution, those organisms didn’t just come out of nowhere: they built on the foundations established in the Ediacaran.”

“The presence of handedness in any kind of functional asymmetry, really deep into the fossil record, gives us important and interesting information about how these behaviours have evolved and how deeply in time they emerged,” says at Flinders University in Adelaide, Australia.

The world’s deepest known whale graveyard has been discovered in the southern Indian Ocean at a depth of 7 kilometres. The remains found there include a new species of extinct beaked whale and other fossils that are over 5 million years old.

In early 2023, at the Chinese Academy of Sciences and his colleagues undertook 32 dives in a crewed submersible along 1200 kilometres of the seafloor, in an area known as the Diamantina Zone.

The expedition was part of the Global Hadal Exploration Programme, an effort led by Chinese scientists to explore all the deepest parts of the planet’s oceans, which range from 6000 to 11,000 metres below the surface. At these depths there is no light, and life must survive on what falls from the surface or generate its own energy from chemicals – known as chemosynthesis.

The first whale fossils were found at a depth of 7002 metres in a part of the Diamantina Zone known as the Dordrecht Deep, which is over 1100 kilometres south-west of Perth, Western Australia.

“With the sub’s powerful lighting system, we could see tens of metres around us on the otherwise pitch-dark seafloor,” says Zhou. What they saw was “a little scary, but also incredibly fascinating”, he says.

The researchers estimated there were up to 760 individual whales per square kilometre, including both ancient and recent carcasses, constituting what they have called a “whale necropolis” and a “deep-sea fossil megasite”.

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The recently fallen carcasses, which included a 5-metre-long Antarctic minke whale (Balaenoptera bonaerensis), provide food to a thriving ecosystem of invertebrates – such as bone-eating worms and brittle stars – many thought to be new species, found in densities of up to 2800 individuals per square metre.

“It felt profoundly special,” says Zhou. “We were looking at the final resting place of millions of whales – some over 5 million years old – a deep-time archive of evolution and deep-sea life. It was humbling and awe-inspiring, and we treated the site with the respect it deserves.”

Altogether, the team found 485 active whale-fall and fossil-whale sites during their expedition. Using the submersible’s robotic arms, they collected 43 fossil specimens that were dated to between 120,000 and 5.26 million years old.

Among the younger fossils, most were beaked whales belonging to two living species, Andrews’ beaked whale (Mesoplodon bowdoini) and the strap-toothed whale (Mesoplodon layardii).

So far, the team has formally described one new species, named Pterocetus diamantinae. However, they also collected several fragmentary specimens that may include further species that are unknown to science, says team member at the University of Pisa, Italy.

at the Chinese Academy of Sciences, another team member, says there are a number of factors that mean the whales have been so well preserved. Most of these fossils are beaked whales’ rostra, or snouts. “They’re hyper dense, almost like bone armour, which makes them physically resistant to degradation and less palatable to bone borers,” says Peng.

Only about 0.05 to 0.55  millimetres of sediment has been deposited per thousand years, over the past 5 million years in this region, and many of these bones are coated with ferromanganese oxides, which effectively seals them off from the surrounding environment.

“So it’s really a combination of bone density, slow burial, and mineral coatings that has allowed these bones to escape being eaten for over 5 million years,” says Peng.

The team thinks that a number of factors have led to such a concentration of whale deaths in the Diamantina Zone, including a whale-migration route passing through the area and a V-shaped topography that funnels the carcasses to the trench floor.

at Macquarie University in Sydney, Australia, who was not part of the study, described the find as an “amazing discovery”. “The density of the whale-fall remains is incredible,” he says.

Australia was once home to a second species of koala that lived only in the west of the continent, where it became extinct around 30,000 years ago.

Today, there is only one koala species: Phascolarctos cinereus. It is found almost exclusively in eucalyptus forests in eastern Australia and is threatened by habitat loss, disease, collisions with cars and predation by introduced species.

Numerous koala fossils, aged between 137,000 and 31,000 years old, have been collected in Western Australian caves over the past century. Until now, however, there wasn’t enough material to conclude that the remains were from a different species.

In the past 25 years, more fossils have become available to researchers, including skulls donated by the family of late speleologist Lindsay Hatcher, who discovered numerous ancient remains during his expeditions in caves in the south-west of Western Australia.

“Amongst the donation was a koala skull in very good condition,” says at the Western Australian Museum. “Upon examination of that skull, we noticed differences with modern koalas that got us to start working on the fossil material in the collection.”

To the untrained eye, the new species, named Phascolarctos sulcomaxilliaris, would have been difficult to distinguish from P. cinereus, but there are subtle differences.

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“In short, the Western Australian koalas were same-same but different,” says Travouillon. “They had shorter heads, for sure, and they seem to have less-well-developed chewing muscles than the east-coast koalas. But they simply chewed in a different way by having larger teeth and having a more efficient, shorter jaw to break down the leaves.”

A large groove on the cheek of P. sulcomaxilliaris suggests the animal had a larger muscle attached there that was used to either move a larger lip, with which it perhaps grabbed leaves, or inflate its nostrils to be able to smell leaves over a greater distance. Its skeleton was also less agile, suggesting it spent less time moving between trees.

When the climate dried and Western Australia’s forests disappeared about 30,000 years ago, P. sulcomaxilliaris vanished, along with many other animals that once shared its habitat. “There would have been [Tasmanian] devils, thylacines, giant echidnas, short-faced kangaroos and the giant marsupial Zygomaturus,” says Travouillon.

“Our first peoples in Western Australia would have lived amongst them and they would have been witness to their extinction.”

at the Australian Museum in Sydney says the study makes a “convincing case for the distinctiveness of the Western Australia koalas as a unique species”. “I look forward to seeing if any DNA can be extracted from the fossils,” he says.

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An almost-complete skeleton of a dinosaur that weighed less than a small chicken has provided new insights into the evolution of alvarezsaurs, which are among the smallest dinosaurs that ever lived.

The 95-million-year-old fossil of Alnashetri cerropoliciensis was found at the La Buitrera site in northern Patagonia, Argentina, in 2014.

The first specimen of Alnashetri, found in 2012, was a set of incomplete hindlimb bones, says at the University of Minnesota, who was part of the study on the new fossil. With only fragmentary remains, it was impossible to say more than that it was probably an alvarezsaur. “We were not even sure if it was a juvenile or fully grown,” he says.

“With a whole skeleton, we suddenly had all the information to understand how Alnashetri was similar or differed from other species, and a key to understanding how the unusual anatomy of alvarezsaurs evolved,” says Makovicky.

The new fossil has very long, slender hind limbs and surprisingly long forelimbs that retain three well-developed fingers. Detailed analysis of the fossil bones revealed the dinosaur was an adult and at least 4 years old.

It is estimated to have weighed only 700 grams when it was alive. “The specimen is truly tiny, smaller than a chicken,” says Mackovicky.

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Alvarezsaurs were once thought to be early ancestors of birds. However, it is now clear that, while Alnashetri might have had some superficial resemblance to a bird, it and all the alvarezsaurs were, in fact, non-avian theropods. “The new discovery certainly underscores this,” says Mackovicky.

Previously, it was thought that all the tiny alvarezsaurs had very short, stout forelimbs with a large thumb but shrunken side digits, and tiny teeth. Palaeontologists thought these anatomical features evolved alongside their shrinking body size because they only ate ants and termites, says Makovicky. “But Alnashetri does not fit that mould – it is among the smaller alvarezsaurs, but neither its teeth nor its forelimbs are reduced, because it represents a much earlier branch on the alvarezsaur evolutionary tree.”

In fact, its forearms are more typical of other theropods rather than a specialist ant-eater, he says. “Alnashetri is tiny but is otherwise built like a more typical theropod – given its small size, it probably ate its fair share of invertebrates, but probably had a wider range of prey.”

That means palaeontologists still don’t fully understand why these dinosaurs became so small. “We’re left with only a vaguer sense that alvarezsaurs were successful at occupying the niches of very small predators,” says Mackovicky.

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Were the mysterious dinosaurs known as spinosaurs excellent swimmers that could dive to catch prey? Or were they “hell herons” that plucked huge fish from shallow waters? Fossils of a new spinosaur species that lived around 1000 kilometres inland should settle the debate, say its discoverers, confirming that it was a wader. “Coup de grâce, as far as I’m concerned,” says at the University of Chicago.

The lifestyle of spinosaurs has been a contentious topic among palaeontologists, due to the animal having a strange combination of features, including a large sail, huge claws, broad feet and crocodile-like jaws. In 2025, the BBC series Walking With Dinosaurs depicted them as aquatic hunters.

In 2019, a local guide took Sereno’s team to a remote desert site in Niger, where they found fragments of jawbones that they later realised belonged to some kind of spinosaur. Because of the covid-19 pandemic and the remote location of the site, it was years before they could return.

On their second trip, Sereno and his colleagues found bones from around 10 individual spinosaurs. Within hours of the first finds, the team realised these spinosaurs had a large crest on top of their skulls, in addition to the characteristic sail along their backs.

“It was a glorious moment because we knew that this was a new spinosaur, something that would have a major impact on how we understand this animal,” says Sereno.

The new species, dubbed Spinosaurus mirabilis, lived around 95 million years ago and grew to about 10 to 14 metres in length, the team estimates – nearly as large as the most famous spinosaur, Spinosaurus aegyptiacus. “I wouldn’t want to be near this animal, because it would finish off a human in about 3 seconds,” says Sereno.

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S. aegyptiacus also had a crest, but that of the new species is much larger – the bony part of the skull crest would have been at least 40 centimetres high on large individuals. Based on comparisons with modern birds with crests, such as the helmeted guineafowl, the team thinks the bone would probably have been covered by a keratinous sheath, making the crest at least 50 centimetres high.

The crest is too delicate to be a weapon of any kind. “Probably it was brightly coloured,” says Sereno. “It is meant to say, ‘I am here; I am healthy.'”

It is thought that the large sails of spinosaurs were also for visual display, he says. “So, these animals are really into display, and the question is, why?”

The answer could be that spinosaurs hunted along rivers where they needed to defend territories. “The prevalence of visual cues in environments like beaches or riversides tends to be exaggerated because it’s there that you can look a mile, obstruction-free, and see your competitor, or your mate, much more easily than [in] a typical land environment,” says Sereno.

Modern waders, such as the great blue heron, are also extremely display-oriented, Sereno says, and other characteristics of spinosaurs also fit the wading hypothesis. When his team plotted a range of animals on a graph based on the relative length of the jaw, neck and hind limbs, spinosaurs came out next to waders like herons.

“It can’t swim well because it’s got this huge sail that makes it very unstable in water. But it can go into 10 feet [3 metres] of water as a full adult,” says Sereno.

Then there’s the fact that it lived far inland, whereas most other spinosaurs have been found nearer to where seas were. No marine predator weighing over a tonne has ever moved into freshwater, says Sereno. There are river porpoises and dolphins, but no river orcas. “And so, I think it’s all playing to the same story, that these animals are mega-heron-type animals.”

“The paper really confirms a lot of the consensus that has been building for these animals,” says at Queen Mary University of London. “They are not super swimmers or deep divers, but much more like a heron or stork, wading into water to catch prey, predominantly fish.”

“I think it’s fairly convincing that this is a new species. If it was just the crest, this could well be variation, but there are differences in the jaws and teeth too,” says Hone.

“At face value, the fact that the legs weren’t particularly short or undermuscled suggests that it was no less able to walk and wade than any other predatory dinosaur,” says at the University of Portsmouth, UK. “This doesn’t bode well for proposals of swimming lifestyles, which are already floundering with issues concerning the stability and propulsion of a swimming Spinosaurus.“

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