麻豆传媒

LOOKING out over an expanse of scrubby sagebrush, it is hard to imagine that the high desert in eastern Oregon was once home to large creatures that resembled sabre-toothed cats. The land here is mostly dry and grassy, punctuated by sharp hills. There isn鈥檛 a lot to crouch behind while waiting to ambush prey, and little in the way of trees to climb or sharpen claws on: in some places, the only sign of plant life is a layer of lichen on the rust-coloured slopes. But it wasn鈥檛 always like this. 鈥淭hese animals made their home here as early as 35 million years ago, when this part of Oregon was covered in dense jungle,鈥� says . 鈥淚t was such a subtropical land that bananas grew here. We鈥檝e collected their fossilised seeds.鈥�

This was part of the territory of the nimravids, ancient beasts also known as false sabre-toothed cats. Fossilised remains indicate that for more than 12 million years, from Florida to New Mexico and up beyond what is now the Canadian border. Then, around 23 million years ago, they disappeared. The trail went cold, and the fossil record suggests that there were no cats on the continent for the next 6.5 million years. What caused them to die off? And what allowed felines to finally populate North America 16.5 million years ago? Palaeontologists have long puzzled over this so-called Cat Gap. Finally, they are finding some answers.

Nimravids were . At first, they were classified as members of the cat family, with whom they share some key traits. One of the characteristic features of cats is that they have teeth specialised for eating meat. 鈥淭hey have knife-blade-looking teeth in the back of the mouth where molars are, and canine teeth up front that are well-adapted for killing things,鈥� says Famoso. Cats also have retractable claws and a tail that helps with balance. 鈥淎ll cat-like things tend to have those three structures,鈥� he says. 鈥淭rue cats do, and nimravids have them, too.鈥�

However, by 1880, Cope had noted that some nimravid features didn鈥檛 match up with what is expected in cats. Certain structures of their , as well as passages for their nerves and blood vessels, differed from those of felines, says . What鈥檚 more, instead of walking on their toes like cats, nimravids had a flat-footed walk like bears. They also had five toes on each back paw, unlike the four found on every feline from lions to house cats.

For two centuries, the question of whether nimravids were cats or merely cat-like remained open. 鈥淭here has been this ,鈥� says Barrett. 鈥淣imravids have gone from being cats to being their own family and back again.鈥� Finally, in the 1980s, phylogenetic analysis 鈥� which examines evolutionary connections of species 鈥� solved the issue. 鈥淵ou throw all the characteristics into a computer model and see what shakes out,鈥� he says. 鈥淎nd nimravids have been shown to be their own family.鈥� They aren鈥檛 felines, but feliforms.

Fearsome grins

Short faces and elongated canine teeth gave these not-quite-cats particularly fearsome grins. Nevertheless, they filled the same role in their ecosystems as modern wildcats do today. The ones living in North America came in a wide range of sizes. Eusmilus 鈥� found in what is now Wyoming, North Dakota and South Dakota 鈥� stood about a metre high, with the look of a long-bodied leopard. Its name translates to . Nimravus, or 鈥渁ncestral hunter鈥�, was about half as tall and ranged throughout western North America to parts of South Dakota. Nanosmilus was the smallest, as its name suggests. It was similar in stature to a modern bobcat, , and its fossils have been found in Nebraska.

Other family members had ranges that extended from the Rocky mountains to the west coast of North America. They include Pogonodon, or 鈥渂eard tooth鈥�, together with the two earliest nimravids found in North America 鈥� Hoplophoneus, whose name translates to 鈥渁rmed murderer鈥�, and Dinictis, the 鈥渢errible cat鈥�. Dinictis first appears 35.5 million years ago and was around until about 23 million years ago, making it one of the last known survivors of the group. The other is Dinaelurus, which is recognised from a single specimen found at the John Day fossil beds.

Then the nimravids disappear. Currently, the Cat Gap is thought to have lasted some 6.5 million years, but the length of this supposed cat-free period has changed over the years with the discovery of new fossils and . That raises the question of whether it is simply an anomaly. Perhaps nimravids persisted, but we haven鈥檛 found their remains. Famoso points out that you need the right environment for fossilisation to occur, and there could have been periods when bones simply weren鈥檛 deposited in rock that has persisted for tens of millions of years. Alternatively, we may have already found fossils that fill the gap, but we don鈥檛 yet know it. 鈥淎s long as we keep museum collections properly maintained, they are clues. It鈥檚 like a fingerprint from a cold case that maybe should have been analysed,鈥� says . Indeed, he and his colleagues have . 鈥淚t was just mislabelled in the collection here,鈥� he says.

鈥淭he 鈥楥at Gap鈥� that followed the nimravids鈥� demise lasted 6.5 million years鈥�

Nevertheless, the consensus is that the Cat Gap is real, that new discoveries may shrink it but they won鈥檛 close it altogether. 鈥淣ow that we have an understanding of the Cat Gap, we can go back and look at the collections to see just how big it really is,鈥� says Poust.

The bigger question is how did nimravids go from prowling far and wide across North America to dying out. One theory is that volcanic activity played a role. Nimravid fossils have been found in abundance just east of the Rocky mountains, where the land under their paws was undergoing major changes during the height of their existence. From about 50 million to 25 million years ago, an ancient tectonic plate called the Farallon plate was spreading under North America. When it met the Pacific plate at the western edge of the continent, the result was explosive. Across what is now Colorado, Utah, Nevada and further south into Mexico, there were dozens of volcanic eruptions. The largest of these occurred around 28 million years ago, creating the La Garita Caldera in Colorado, which is 75 kilometres across at its widest point. Volcanic ash spewed out and blanketed the region with . For comparison, the 1980 eruption of .

The energy of the La Garita eruption may have been partly due to its silica-rich magma. 鈥淭he higher the silica, the more explosive an eruption can be,鈥� says , who discovered and studied the caldera. More silica gives the liquid rock higher viscosity, which can help trap more sulphur and carbon dioxide. Then, when the molten material rises and decompresses, the gases are released and create bubbles that explode. 鈥淓verything would have been killed by the heat of the ash alone for at least 150 kilometres beyond La Garita,鈥� he says. 鈥淏eyond that, ash that went up higher in the atmosphere would certainly reduce sunlight and temperatures for a year or two.鈥�

鈥淚t would have been devastating for the flora and fauna,鈥� says Barrett. And nimravids were no exception: some of the best-preserved specimens come from sites rich in ash layers. 鈥淪ome did bite it. But the nimravids seemed to persist through these cataclysmic events,鈥� he says. Poust also thinks that although the 10-million-year flare-up of volcanic activity may not have been easy for individual nimravids, it doesn鈥檛 explain why they went extinct altogether.

If volcanism didn鈥檛 finish them off, what did? Beginning around 23 million years ago, there was a period of massive cooling and drying. Forests gave way to grasslands, which would have affected the animals that nimravids hunted. 鈥淧rey species at the time were going extinct, so that is probably related to why the predators followed soon after,鈥� says Barrett. Nimravids were at a disadvantage when attempting to adapt. They had evolved to be hypercarnivorous 鈥� meaning most of their diet was meat 鈥� with blade-like teeth towards the front of their mouths , and jaws that allowed them to open their mouths to 90 degrees to better pierce prey. Behind the stabbing canines sat pairs of carnassials: sharp, triangular teeth that fit together like puzzle pieces. 鈥淭hey鈥檙e like horrible scissors,鈥� says Poust. As they slide past one another, the bottom teeth grind against the top and hone them to a point. 鈥淔rom the moment they stop drinking milk to the moment they die, they need to use that tool,鈥� he says.

The perils of hypercarnivory

Such specialisation often leads to an , they wouldn鈥檛 have been the only ancient animals to succumb to an over-reliance on meat-eating. It also played a part in . Even today, a set of more general-purpose teeth has been key to the survival of various large predators. 鈥淚f you鈥檙e a black bear, you can eat almost anything. You can eat garbage. That鈥檚 part of why they do a better job of dealing with living near big cities and today鈥檚 tigers often don鈥檛,鈥� says Poust.

There is some evidence that late nimravids had started to adapt to the changing environment. As dense forests gave way to grasslands, they would have needed to run faster and over longer distances to catch their prey. 鈥淥ne of the last nimravids of the Oligocene, Dinaelurus, seems to have a similar morphology to what we see in cheetahs today,鈥� says Barrett. Its skull is tilted in a similar way. 鈥淚t has a distinct bend, which you find in other animals that are adapted for running at high speeds, because it puts the eyes in a place where you more easily see what鈥檚 quickly coming towards you,鈥� he says. Dinaelurus also has bigger sinus cavities than other nimravids, allowing it to take in more oxygen as it ran.

But even this evolution wasn鈥檛 enough. By 23 million years ago, nimravids were gone from North America. The continent was free of cat-like creatures. Then, around 6.5 million years later, the cooling climate that paved the way for the nimravid extinction gave their feline successors access to the continent. , exposing the Bering land bridge that connected Siberia to Alaska. Across it came Pseudaelurus, a lynx-sized cat that was an agile tree-climber. It flourished in the expanding conifer forests in North America, which were also made possible by plant migrations over the bridge. Another group of cat-like animals called barbourofelids also arrived, and new analysis suggests that they were nimravids originating in Africa.

These felines and feliforms finally brought an end to the Cat Gap. Eventually, , the barbourofelids died out. However, Pseudaelurus persisted and is mountain lions to bobcats and even the fluffballs currently occupying the best spots on many sofas.