Why cat-like creatures vanished from North America for 6 million years

By Chelsea Whyte

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’t 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’t always like this. “These animals made their home here as early as 35 million years ago, when this part of Oregon was covered in dense jungle,” says Nick Famoso, a palaeontologist at the John Day Fossil Beds National Monument in Oregon. “It was such a subtropical land that bananas grew here. We’ve collected their fossilised seeds.”

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, seven of the 10 known nimravid genera inhabited North America 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 named by US palaeontologist Edward Drinker Cope in the late 1800s. 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. “They 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,” says Famoso. Cats also have retractable claws and a tail that helps with balance. “All cat-like things tend to have those three structures,” he says. “True cats do, and nimravids have them, too.”

However, by 1880, Cope had noted that some nimravid features didn’t match up with what is expected in cats. Certain structures of their inner ears and teeth, as well as passages for their nerves and blood vessels, differed from those of felines, says Paul Barrett, a palaeontologist at the University of Oregon. What’s 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. “There has been this quibbling back and forth,” says Barrett. “Nimravids have gone from being cats to being their own family and back again.” Finally, in the 1980s, phylogenetic analysis – which examines evolutionary connections of species – solved the issue. “You throw all the characteristics into a computer model and see what shakes out,” he says. “And nimravids have been shown to be their own family.” They aren’t 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 – found in what is now Wyoming, North Dakota and South Dakota – stood about a metre high, with the look of a long-bodied leopard. Its name translates to “true sabre”. Nimravus, or “ancestral hunter”, 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, a type of lynx that is about twice the size of a house cat, 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 “beard tooth”, together with the two earliest nimravids found in North America – Hoplophoneus, whose name translates to “armed murderer”, and Dinictis, the “terrible cat”. 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 revisions in taxonomic analysis of old ones. That raises the question of whether it is simply an anomaly. Perhaps nimravids persisted, but we haven’t 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’t 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’t yet know it. “As long as we keep museum collections properly maintained, they are clues. It’s like a fingerprint from a cold case that maybe should have been analysed,” says Ashley Poust at the San Diego Natural History Museum in California. Indeed, he and his colleagues have recently reanalysed one specimen that appears to push back the origins of nimravids in North America. “It was just mislabelled in the collection here,” he says.

“The ‘Cat Gap’ that followed the nimravids’ demise lasted 6.5 million years”

Nevertheless, the consensus is that the Cat Gap is real, that new discoveries may shrink it but they won’t close it altogether. “Now 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,” 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 5000 cubic kilometres of material. For comparison, the 1980 eruption of Mount St Helens in Washington expelled 2.5 cubic kilometres of debris.

The energy of the La Garita eruption may have been partly due to its silica-rich magma. “The higher the silica, the more explosive an eruption can be,” says Peter Lipman at the US Geological Survey, 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. “Everything would have been killed by the heat of the ash alone for at least 150 kilometres beyond La Garita,” he says. “Beyond that, ash that went up higher in the atmosphere would certainly reduce sunlight and temperatures for a year or two.”

“It would have been devastating for the flora and fauna,” says Barrett. And nimravids were no exception: some of the best-preserved specimens come from sites rich in ash layers. “Some did bite it. But the nimravids seemed to persist through these cataclysmic events,” 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’t explain why they went extinct altogether.

If volcanism didn’t 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. “Prey species at the time were going extinct, so that is probably related to why the predators followed soon after,” says Barrett. Nimravids were at a disadvantage when attempting to adapt. They had evolved to be hypercarnivorous – meaning most of their diet was meat – with blade-like teeth towards the front of their mouths used for stabbing, 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. “They’re like horrible scissors,” says Poust. As they slide past one another, the bottom teeth grind against the top and hone them to a point. “From the moment they stop drinking milk to the moment they die, they need to use that tool,” he says.

The perils of hypercarnivory

Such specialisation often leads to an evolutionary dead end, and if hypercarnivory caused the extinction of nimaravids, they wouldn’t have been the only ancient animals to succumb to an over-reliance on meat-eating. It also played a part in the demise of several species of wild dogs in North America around 11,000 years ago. Even today, a set of more general-purpose teeth has been key to the survival of various large predators. “If you’re a black bear, you can eat almost anything. You can eat garbage. That’s part of why they do a better job of dealing with living near big cities and today’s tigers often don’t,” 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. “One of the last nimravids of the Oligocene, Dinaelurus, seems to have a similar morphology to what we see in cheetahs today,” says Barrett. Its skull is tilted in a similar way. “It 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’s quickly coming towards you,” 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’t 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. Sea levels dropped as glaciers grew, 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, around 5 million years ago, the barbourofelids died out. However, Pseudaelurus persisted and is thought to be the common ancestor of everything from North America’s mountain lions to bobcats and even the fluffballs currently occupying the best spots on many sofas.

More on these topics: