Science

Surprisingly, palaeontologist Hendrik Nowak at the University of Nottingham, UK, doesn’t see it that way. He points to fossil pollen from other sites that also suggests “little or only short-lived disruption” from the end-Permian event. In fact, Nowak argues that the impact was so minimal that – for plants, at least – there simply was no mass extinction then.

This conclusion is controversial. Nevertheless, studies on two other major groups of organisms – insects and four-limbed land animals – echo the findings in plants. The emerging picture means Nowak isn’t the only palaeontologist questioning whether the impact of the end-Permian mass extinction was as colossal as we thought. Spencer Lucas at the New Mexico Museum of Natural History & Science goes even further – he suspects life on land has never experienced a mass extinction. “I think that you’ve got a better chance of beating a big extinction if you’re on land than you do if you’re in the sea,” he says. This revolutionary rethink could rewrite the history of life on Earth. It would upend the idea that the continents have witnessed five mass extinctions – and it even has implications for how we frame the current human-induced biodiversity crisis.

The most famous victims of a mass extinction are the dinosaurs that died out around 66 million years ago, but much of what we know about such events comes from studying marine life. Indeed, the idea that Earth has experienced five mass extinctions came from a 1982 analysis of the marine fossil record. Two palaeontologists, the late David Raup and Jack Sepkoski, tracked changes in marine biodiversity over the past half a billion years and noticed that the record was punctuated by five crashes. These were at the end of the Ordovician (445 million years ago), the late Devonian (372 million years ago), the end-Permian (252 million years ago), the end-Triassic (201 million years ago) and the end-Cretaceous periods, the latter being when most dinosaurs went extinct. These events came to be known as the big five.

It remains beyond doubt that these mass extinctions devastated ocean life, but – dinosaurs notwithstanding – it wasn’t initially clear that they had also rippled through ecosystems on land. Mike Benton at the University of Bristol, UK, recalls that textbooks from the late 1980s “stated quite categorically” that there was little evidence of an end-Permian mass extinction of four-limbed animals, or tetrapods, that lived on land. Modern tetrapods include all reptiles, amphibians, birds and mammals. That, says Benton, was largely down to a lack of data. It is relatively easy for a dead marine organism to be buried in mud and begin the fossilisation process, whereas land organisms are less likely to become fossils.

There are, however, a few sites that do capture a reasonably good fossil record of life on land during mass extinction events. Over the past 30 years, researchers have spent countless hours collecting and analysing tetrapod remains from such sites. A clear picture has emerged, says Benton: there were tetrapod mass extinctions on land to match those in the sea. This makes sense, given that the big five were driven primarily by a combination of rapid climate change and massive environmental upheaval, triggered by things like asteroid impacts and volcanic activity. “There are considerable feedbacks between the land and the oceans,” says Benton. Runaway global warming, for instance, puts stress on both marine and terrestrial life. As such, he argues it is difficult to imagine a mass extinction affecting only one of the two realms.

So clear is this connection that many researchers now make it central to their understanding of these dramatic events. “Mass extinctions happen everywhere, all at once, on land and in the sea,” says Paul Wignall at the University of Leeds, UK. Nevertheless, some have begun to express doubts and Lucas is prominent among them. In a 2017 paper, he examined the claim that there was an end-Permian mass extinction of land-based tetrapods. There were extinctions, he concluded, but fewer than 20 genera disappeared – hardly evidence of a catastrophic loss of diversity, given that there must have been many hundreds or even thousands of tetrapod genera at the time. “There was no big extinction of tetrapods on land at the end-Permian,” he says.

Since then, Lucas has taken a critical look at the rest of the big five. In a review published in 2021, he concluded that land-based tetrapods were barely affected by any of them. “I think that there’s a lot of hyperbole involved in this,” he says. “It’s a big deal that the non-avian dinosaurs go extinct at the end of the Cretaceous. That said, I don’t think it’s really a mass extinction.” He points out that plenty of other large, land-living tetrapods, including the crocodilians, survived. And, of course, we now know that one group of dinosaurs – the birds – didn’t go extinct, nor did the mammals. Lucas argues that tetrapods on land are in a better position to avoid extinction because air has a lower viscosity than water, which makes migrating to new regions following the deterioration of the local environment energetically less costly for land animals than for their marine counterparts.

Unsurprisingly, the claim that there has never been a mass extinction of land-based tetrapods has faced pushback. For instance, Benton maintains that the group did face a massive die-off at the end-Permian, with the disappearance of major branches including the sabre-toothed gorgonopsians – but that the extinction occurred over 1 million years. He says Lucas has “missed the bigger picture” by zooming in on the very end of that protracted extinction. Wignall is another critic. “I think it would be fair to say that Lucas’s viewpoint is not mainstream,” he says.

However, Lucas is not a lone voice in questioning the big five mass-extinction paradigm. Beyond the tussle over tetrapods, researchers who focus on other major groups of land-based organisms are coming to similar conclusions.

Take insects, of which there are millions of species today. In 2021, Sandra Schachat, now at the University of Hawaiʻi at Mānoa and Conrad Labandeira at the National Museum of Natural History in Washington DC assessed the fossil record of insects and concluded that the tiny animals seem never to have suffered a mass extinction. This doesn’t mean they have had a crisis-free existence. Most notably, insect communities changed dramatically near the end-Permian, says Schachat. Important groups, including the dragonfly-like Palaeodictyoptera, vanished. Others, such as the Hemipteroidea – which includes the true bugs – rose to dominance. But crucially, she says, we have no idea how these changes came about because the insect fossil record is extremely patchy, with a gap of about 20 million years near the end-Permian. Over such a long period, insect communities can change gradually, but drastically, through evolution by natural selection alone. “When the fossil record is so incomplete that your best snapshots of a group of organisms come tens of millions of years apart, you’re going to expect to see big changes, with or without a mass extinction,” says Schachat.

Researchers including Wignall argue that it makes the most sense to tie the insect community changes near the end-Permian to a mass extinction. That is a possibility, says Schachat, but it ignores an important point: insect species are impressively well equipped to survive tough times. In their 2021 paper, she and Labandeira pointed out that insects occur in vast numbers and have short generation times. This means natural selection can proceed exceptionally quickly, helping insect species adapt to rapidly changing conditions. Moreover, faced with an acute crisis, individual insects can enter a period of dormancy called diapause until conditions improve.

Arguably, some marine invertebrates have similar features. But the fact that they live in the ocean may leave them more vulnerable to extinction, according to Schachat and Labandeira. Most notably, changes in atmospheric oxygen and carbon dioxide levels can trigger ocean stagnation, leading to the death of marine creatures through asphyxiation. Organisms on land don’t face that problem. “We see tremendous changes in marine communities that correlate with drops in atmospheric oxygen, and then if we look at the record on land, we don’t see anything like that,” says Schachat.

The fossil record of land plants also fails to conform to the big five narrative. In 2013, Borja Cascales-Miñana, now at the University of Lille in France, and Christopher Cleal, now at the University of Bristol, took a close look at the record and concluded that plant mass extinctions are surprisingly rare. For instance, no family of vascular plants, a group that includes things like ferns and conifers, died out during the supposed fifth mass extinction at the end of the Cretaceous. In fact, they concluded, only one of the big five – the end-Permian – coincided with a mass extinction of plants. And as this year’s study of the South Taodonggou fossils makes clear, even that is now questioned by some.

“It’s pretty evident if you look at the fossil record from a broad perspective that something happened: the terrestrial flora changed quite a bit,” says Nowak. For instance, forests dominated by the genus Glossopteris vanished in the end-Permian. “But can you call that a mass extinction?” In a 2019 study, Nowak and his colleagues argued that you can’t. They concluded that this event affected some plants, including ferns, but had little impact on others. Conifers even appear to have increased in diversity. Cascales-Miñana and Cleal stand by their finding of an end-Permian plant mass extinction, pointing out that Nowak’s team focused largely on pollen and spores, which are released in the billions by one tree, and so could create an impression of many plants even amid a large decline. “If you are counting spores, you are not counting plants,” says Cascales-Miñana. But this controversy shouldn’t detract from the broader message, which is that plants weren’t badly affected by most of the big five mass extinctions. “I think that idea is pretty well accepted among palaeobotanists,” says Cleal.

Again, the lack of mass extinctions among plants probably comes down to biology. They have several strategies for withstanding disaster. The most significant, says Cleal, is that they can survive for decades or even centuries as seeds and spores. “Imagine shooting all the elephants in the world: 10 years later, there are still no elephants,” he says. “Now imagine cutting down all the oak trees in the world: 10 years later, there are the beginnings of new oak forests because the acorns germinated.”

The fact that plants were largely unaffected by most – and potentially all – of the big five extinctions leads to an intriguing philosophical question, one that was first posed by Cascales-Miñana and Cleal in their 2013 paper. Should we label an event a “mass extinction” if it only affects a limited set of organisms and has little impact on other major groups? Lucas, for one, thinks we shouldn’t. “How would you create a mass extinction on land?” he asks. “You would kick the floor out from under the food pyramid, take out the plants. But wait a second: the plants aren’t going extinct at these events. Then how does the animal community collapse?”

The growing uncertainty about what counts as a mass extinction has implications for the way we think about the biodiversity crisis unfolding today because of human activities. Many researchers have begun labelling it Earth’s sixth mass extinction, but, for life on land, it may arguably be the first. However, either label may not be warranted. For instance, Schachat and Labandeira argued in their 2021 paper that we need to see the disappearance of entire branches of the insect evolutionary tree to declare that a mass extinction of these animals has begun. They are undoubtedly experiencing catastrophic losses of abundance and biomass right now. “[But] there are no indications that we are anywhere near a crisis of this severity,” the pair wrote.

It may seem unwise to question the sixth mass extinction idea – especially as it is framed as a rallying call to urgently conserve biodiversity, says John Wiens at the University of Arizona. Nevertheless, he thinks conservationists would benefit from abandoning this rhetoric. “Many people are now saying: we’ve got to stop the sixth mass extinction. But, actually, that is not an ambitious or even an urgent conservation goal,” he says.

To understand why, we need to go back to basics. Surprisingly, there is no precise definition of the term “mass extinction”, but there is a general consensus that these events see the loss of at least 75 per cent of species over the course of several thousand years to around 2 million years. Meanwhile, estimates indicate that over the past 500 years, less than 0.1 per cent of known species have become extinct. In a paper published earlier this year, Wiens and his colleague Kristen Saban at Harvard University point out that these figures suggest avoiding the sixth mass extinction will be a breeze. “We could lose half the species on the planet over the next 3000 years and still say, ‘Yeah, we did it! We prevented the sixth mass extinction,’” says Wiens. Targeting such an easily achievable goal risks doing more harm than good. If we really want to conserve biodiversity, we should aim to prevent human-induced extinctions rising to 0.2 per cent, not 75 per cent or even 50 per cent, he says.

No doubt, the debate about whether life on land has experienced five mass extinctions, one or even none will continue. Whatever palaeontologists conclude, that doesn’t change the urgent need to address our current crisis, says Wiens. “It’s popular right now to talk of a sixth mass extinction. But it’s just the wrong way to think about it.”

• Lost to science for more than a century, the Omiltemi cottontail rabbit has been confirmed by scientists to be alive and hopping in southern Mexico.
• The species was rediscovered via interviews with local communities and footage from camera traps intended to photograph jaguars.
• Sierra Madre del Sur in the state of Guerrero is the only place in the world where the Omiltemi cottontail is known to exist.
• Satellite data show continued forest loss within its known range, while hunting for food by local communities remains another threat to the species.

Footage captured in 2024 of a small rabbit hopping about in front of a camera trap had scientists baffled. The juvenile, with gray-brown fur and a black tail, didn’t resemble any known species in the Sierra Madre del Sur, in the southern Mexican state of Guerrero.

Biologist Fernando Ruiz-Gutiérrez anxiously searched his records and consulted with colleagues to confirm his hypothesis. A few kilometers away, ecologist José Alberto Almazán-Catalán had the answer: having captured an adult specimen years earlier and conducted a series of studies, he now had irrefutable proof that the Omiltemi cottontail rabbit (Sylvilagus insonus), believed to be extinct for the past 120 years, was still alive.

The last time scientists knowingly encountered the Omiltemi cottontail was in 1904, when U.S. naturalist Edward William Nelson described it for the first time. Habitat loss, poaching and subsistence hunting have been the biggest threats to the species throughout its existence, which is why it took more than a century to rediscover the elusive rabbit, hidden in the forest.

“It was very exciting to pin down an animal that we not only believed to be extinct but that also has an almost mythical quality, because the furs we have in Mexico are not as precise as we would like since they were not taken by a mammalogist but donated by campesinos [small-scale farmers],” says Almazán-Catalán, president of the Institute for the Management and Conservation of Biodiversity (INMACOB), a Mexican NGO.

“We really weren’t sure this rabbit existed. It could’ve been an anomaly. Finding it was alive and that there were healthy populations was a great relief,” says Almazán-Catalán, who led a five-year investigation to discover the continued existence of the species.

The Omiltemi cottontail rabbit is endemic to the state of Guerrero. Its previously known distribution was restricted to the area around the village of Omiltemi, in the municipality of Chilpancingo. An elusive species with nocturnal habits and low population densities, it’s classified as endangered by Semarnat, Mexico’s environment ministry, and very little is known about its distribution, ecology and biology.

Ruiz-Gutiérrez, who led a team that helped rediscover the Omiltemi cottontail, calls it the most endangered rabbit in the world, saying the species was even considered to be an example of modern-day extinction.

Members of a research team walk through a pine-oak forest, one of the habitats in which they carried out monitoring activities. Image courtesy of Wild Felids Conservation México.

Like other rabbits, Omiltemi cottontails live in warrens, and primarily inhabit coniferous forests, but can also be found in some deciduous forests, at elevations ranging from 7,000-10,000 feet (about 2,100-3,000 meters).

“They are rich and generally very diverse territories, with a wide variety of plants and animals. This mountainous region is well preserved, with forests that haven’t been disturbed for many years, which still have primary forest cover and haven’t been greatly impacted by human activity,” Ruiz-Gutiérrez says.

“We’ve found areas with beautiful, pristine rivers, with an impressive quantity of crystal-clear water, with areas of very dense forest which are difficult to access and other areas where there is human intervention, but not very often. It’s in these places that we’ve had sightings of the Omiltemi rabbit,” Ruiz-Gutiérrez says.

The mountains of Guerrero are made up of rugged and varied landscapes that support a rich diversity of species. Image courtesy of Wild Felids Conservation México.

The Omiltemi cottontail is reddish-brown in color, and its body and ears are smaller than those of other rabbits that inhabit the area. But Almazán-Catalán says that the most obvious feature that distinguishes the species from the closely related Mexican cottontail (S. cunicularius), which inhabits the same area, is its small black tail.

“It’s difficult to see it in a camera trap, but with a good photo you might be able to recognize that it’s a different rabbit,” Almazán-Catalán says. “The first clear photo of the Omiltemi cottontail rabbit is from 2009, taken by photographer Stephen John Davies, from the United Kingdom, who came to Puerto del Gallo and created a controversy [about the rabbit’s continued existence] on the iNaturalistMX website, which is where the first debate about what animal it was took place.”

Photograph of an Omiltemi cottontail rabbit taken in 2009 by photographer Stephen John Davies, in Guerrero. Image courtesy of Stephen John Davies/iNaturalistMX.

Field crew members install and test a camera trap. Image courtesy of Wild Felids Conservation México.

The rediscovery of the Omiltemi cottontail after more than a century was accidental. Ruiz-Gutiérrez and his team had been setting up camera traps along wildlife trails to monitor jaguars (Panthera onca) in the central portion of the Sierra Madre del Sur as part of the National Jaguar Census (Cenjaguar).

“We use the camera trap to identify the presence of jaguars, but also any associated fauna, that is, its potential prey and other felines with which they cohabit,” Ruiz-Gutiérrez says. “From this we can generate estimates of species richness and biodiversity in general, and, in particular, population aspects of the jaguar in this region, which are then extrapolated to a state and national level, giving us the data we use for Cenjaguar.”

And then, one day in May 2024, a mysterious young rabbit appeared in front of a camera trap set up in the forest near the village of Jaleaca de Catalán.

“It really caught our attention because, when you compare it to other animals in footage from the same camera, the rabbit is tiny. We got a photo of a squirrel just opposite the place where the rabbit was and, when we put them next to each other, they were the same size,” Ruiz-Gutiérrez says. “So we began to talk about it seriously with Dr. Gerardo Ceballos, director of Cenjaguar; I sent him all the evidence and with it we were able to ascertain that it really was the mythical Omiltemi cottontail rabbit, and we began to get excited about this important discovery.”

A composite image showing a squirrel and the Omiltemi cottontail rabbit. Image courtesy of Wild Felids Conservation México.

When Ruiz-Gutiérrez and his team went over archive camera-trap footage from the past 11 years, they found additional photos and videos of Omiltemi cottontails captured in the municipalities of Atoyac, Chilpancingo and Técpan de Galeana. This footage indicates the distribution of the species extends 111 kilometers (69 miles) beyond what was previously known, from Omiltemi to the Técpan de Galeana mountains.

“We were delighted to be able to rediscover the presence of this species, to confirm that it’s still alive and continues to have small populations in the Guerrero mountains,” Ruiz-Gutiérrez says. “We need to redouble our efforts to conserve it in the medium and long term, working with communities on conservation strategies that will help us to protect it so that it won’t yet disappear from the face of the Earth.”

By analyzing their camera-trap footage more closely, Ruiz-Gutiérrez and his team were able to glean information about the behavior of the Omiltemi cottontail. For example, 68% of sightings were at night, indicating the species may be largely nocturnal.

Most images captured were of single individuals, suggesting the species is mostly solitary. There were sightings of juvenile individuals, or kits, in May and December, which indicates breeding activity may take place twice a year.

An Omiltemi cottontail rabbit foraging with its back to the camera trap. Image courtesy of Wild Felids Conservation México.

While camera-trap footage indicate the distribution of the Omiltemi cottontail is broader than previously expected, the threats facing the species — habitat loss driven by fires and agricultural clearing — still persist, and little is known about the impact of hunting by local communities, according to the research team.

Portions of the rabbit’s range fall within several protected areas, including the new Sierra Tecuani Biosphere Reserve. However, satellite data from Global Forest Watch show ongoing forest loss in the reserve, and there were “no special conservation measures to protect the species” as of January 2025, according to conservation NGO Re:wild.

Community technician Pascual Ramírez and Wild Felids Conservation México biologist Gricell Villegas install a camera trap. Image courtesy of Wild Felids Conservation México.

“It’s a huge opportunity for us to be able to contribute to the conservation of such an enigmatic species,” Ruiz-Gutiérrez says. “All this work has been thanks to conservation of the jaguar, which was the flagship species that opened the door to us beginning to study and protect it, but most of the conservation work we can do must be done together with the ejidos [community-managed farmland] and communities.”

The Omiltemi cottontail is the 13th species rediscovered as part of Re:wild’s “Search for Lost Species,” a project aiming to find and protect plant, animal and fungi species that have been lost to science for years but not yet been declared extinct.

When Almazán-Catalán and his team began searching for the rabbit in 2019 in the forests of Chilpancingo, they didn’t find any sign of the species. So from 2020-2022, they refocused their search to high-altitude coniferous forests. There, in the Filo Mayor region, they consulted with local communities where they suspected the Omiltemi cottontail and another rabbit species were hunted for food by community members.

“The campesinos had three specimens of S. insonus in their possession and gave them to us for scientific purposes when we explained why we wanted the fur and their tissues,” Almazán-Catalán says. “They had the animals for personal consumption and donated them to the investigation. The community weren’t to blame; they simply didn’t know it was Sylvilagus insonus.”

• The Wild Felids Conservation México technical team and the Jaleaca de Catalán community team, who participated in the fieldwork that led to the rediscovery of the Omiltemi cottontail rabbit. Image courtesy of Wild Felids Conservation México.

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When the research team compared the communities’ rabbits with the furs available in collections and descriptions in the scientific literature, their morphological characteristics matched those of the Omiltemi cottontail rabbit first described by Nelson in 1904.

“This finding suggests that we still have a lot of fieldwork to do because although we think we have all the species documented, this really isn’t the case. We need to get more people involved, more specialists in this area,” Almazán-Catalán says. “It was a joy to behold this small animal and to discover that it’s there, alive, that it’s still hopping about, and we hope that there will be many more sightings and that it will continue to inhabit this region of the Guerrero mountains.”

Banner image: Omiltemi cottontail rabbit foraging in front of a camera trap. Image courtesy of Wild Felids Conservation México.

This story was first published here in Spanish on April 5, 2025.

Sihao Cheng at the Institute for Advanced Study in Princeton, New Jersey, and his colleagues first detected the object, known as 2017 OF201, as a bright spot in an astronomical image database from the Victor M. Blanco Telescope in Chile.

2017 OF201 is about 700 kilometres across – big enough to qualify as a dwarf planet like Pluto, which has a diameter about three times as big. The object is currently about 90.5 astronomical units (AU) away from us, or roughly 90 times as far from Earth as the sun is.

Because 2017 OF201’s average orbit around the sun is greater than that of Neptune, it is what’s known as a trans-Neptunian object (TNO). It passes through the Kuiper belt, a disc of icy objects in the outer solar system beyond the orbit of Neptune.

The researchers looked back over 19 observations, taken over seven years by the Canada France Hawaii Telescope, to determine that the closest 2017 OF201 gets to the sun – its perihelion – is 44.5 AU, which is similar to Pluto’s orbit. The furthest it gets from the sun is 1600 AU, way outside the solar system.

This far-flung orbit may be the result of an encounter with a giant planet, which ejected the candidate dwarf planet out of the solar system, say the researchers.

“It’s a really cool discovery,” says Kevin Napier at the University of Michigan. The object would go so far outside the solar system that it could be interacting with other stars in the galaxy just as strongly as it interacts with some of the planets in our solar system, he says.

The orbits of many extreme TNOs seem to cluster in a specific orientation. This has been interpreted as evidence that the solar system contains a ninth planet hidden in the Oort cloud, a vast cloud of icy rocks encircling the solar system. The idea is that Planet 9’s gravity pushes the TNOs into their specific orbits.

But the orbit of 2017 OF201 doesn’t fit this pattern. “This object is definitely an outlier to the observed clustering,” says team member Eritas Yang at Princeton University.

Cheng and his colleagues also modelled simulations of the object’s orbit, and how it might interact with Planet 9. “In the one with Planet X, the object gets ejected after a couple of hundred million years, and without Planet X, it stays,” says Napier. “Certainly, this is not evidence in favour of Planet 9.”

But until there is more data, the case isn’t closed, says Cheng. “I hope Planet 9 still exists, because that’ll be more interesting.”

The candidate dwarf planet takes roughly 25,000 years to complete an orbit, which means it spends only about 1 per cent of its time close enough to Earth for us to detect it. “These things are really hard to find because they’re faint, and their orbits are so long and skinny that you can only see them when they’re really close to the sun, and then they immediately head right back out and they’re invisible to us again,” says Napier.

That means there might be hundreds of such objects out there. The Vera C. Rubin Observatory, due to go online later this year, will look deeper into space and will potentially detect many more objects like this, which should tell us more about them – and whether Planet 9 actually exists.