Machairodontinae

This article is about an extinct group of cats called Machairodontinae. It is different from the similarly named Machaeroidinae, a group of extinct meat-eating mammals. "Sabercat" also leads here, and for other uses you can see Sabercat (disambiguation). "Saber-toothed cat" is sometimes used for other animals too, so for those see Saber-toothed predator.

Machairodontinae gets its name from two Ancient Greek words: μάχαιρα (mákhaira), meaning a type of ancient sword, and ὀδούς (odoús), meaning "tooth." It is an extinct group of carnivoran mammals in the cat family Felidae. It was the first major branch of this family to split off from the others.

Machairodonts ranged in size from about as big as lynxes to bigger than lions. Many of the extinct predators known as "saber-toothed cats" belong to this group, including the famous Smilodon and Megantereon, which had very long upper maxillary canines. The length of these teeth varied. In some machairodonts, such as Dinofelis, the upper canines were much shorter. Other saber-toothed predators included gorgonopsians, nimravids, machaeroidines, and thylacosmilids. Unlike living big cats, which usually clamp the muzzle or throat of their prey to asphyxiate it, machairodonts are thought to have bitten the neck of their prey. They used their neck muscles to press the long teeth into the prey, while the lower jaw helped hold it in place.

Machairodonts likely began in Eurasia during the Middle Miocene. They eventually spread to every continent except Australia and Antarctica. During the late Miocene and Early Pliocene, they were the most common group of cats and large meat-eating mammals across Afro-Eurasia and North America. At that time, the ancestors of living cats were mostly small. Machairodonts began to decline from the end of the Miocene, and this decline sped up during the Pleistocene. This may have been caused by changes in the environment and in prey animals. Competition with large living cats, such as the pantherins, and possibly archaic humans, may also have played a part.

The last species of Smilodon and Homotherium died out around 12,000 to 10,000 years ago. They became extinct along with many other large mammals during the end-Pleistocene extinction event. This happened after human arrival to the Americas at the end of the Late Pleistocene.

Evolution

Based on mitochondrial DNA from fossils, the machairodonts split from the ancestors of today's cats about 20 million years ago. The group began in Europe during the middle Miocene. The early cat Pseudaelurus quadridentatus had longer upper teeth and is thought to be near the base of their family tree. The oldest known machairodont is Miomachairodus, from Africa and Turkey. Until the late Miocene, they lived alongside barbourofelids, large meat-eating animals that also had long sabre-teeth. Scientists have usually grouped these cats into three tribes. The Smilodontini had dagger-like teeth, such as Megantereon and Smilodon. The Machairodontini, also called Homotheriini, had curved, scimitar-like teeth, such as Machairodus and Homotherium. The Metailurini include Dinofelis and Metailurus.

The name "saber-toothed tigers" is misleading. These animals were not part of the same group as tigers. There is no evidence they had tiger-like stripes, and they did not all live or hunt the same way as the modern tiger. A DNA analysis published in 2005 confirmed that machairodontines split off early from the ancestors of modern cats. A 2022 study suggests the Metailurini and Smilodontini did not come from one single group, with Paramachairodus being an ancestor of both. Fossils found at Batallones-1 in the 1990s gave scientists new clues about how saber teeth developed. The saber-toothed look is now thought to have grown slowly through mosaic evolution, rather than quickly through pleiotropic evolution. The main pressure was probably the need to catch prey quickly. Over time, these cats declined. By the Late Pleistocene, only Smilodon and Homotherium remained, mostly in the Americas. Both became extinct about 13,000 to 10,000 years ago, as part of the wave of extinctions of most large animals across the Americas.

Fossil remains

Scientists have studied the skulls and teeth of machairodonts more than any other part of their bodies. There were two main types. Dirk-toothed cats had long, narrow upper canines and sturdy bodies. Scimitar-toothed cats had broader, shorter upper canines and slim bodies with longer legs. The cats with longer teeth often had a bony flange on their lower mandible. One genus, Xenosmilus, had the heavy limbs of a dirk-toothed cat and the thick canines of a scimitar-toothed cat.

Over time, these cats stopped needing as many teeth. Cats have fewer teeth than any other carnivore group, and machairodonts had even fewer. Their long canines needed the mouth to open wider, so their skulls and jaw muscles changed. The temporalis and masseter muscles became smaller, and the occipital bone became more upright. This let them open their mouths wider than a domestic cat or a lion can, but their bite was weaker. Machairodonts and other animals with similar teeth, which developed through convergent evolution, show these changes. Clues in the bones also help paleontologists estimate how old an animal was, which is useful for population studies.

Dirk-toothed machairodonts, such as Smilodon, Megantereon, and Paramachairodus, were strong and sturdy. They were not built for long chases. Their ribcages and shoulder bones were well developed to support strong muscles. Their tails became shorter from the earliest forms to the most advanced ones, and their bodies looked more like modern bears than modern cats. Scimitar-toothed machairodonts, grouped in Machairodontini, Homotherini, and Metailurini, were much more varied. The homotherines were long-legged and lean. Homotherium serum may have been built for running long distances, much like the living spotted hyena. The metailurines looked more like modern pantherines and ranged in size from a small cheetah to a small lion.

In 2020, the mummified remains of a baby cub of the species Homotherium latidens were found in Yakutia, Russia. It is believed to be the first of its kind ever found. Scientists were surprised by the color and softness of its fur.

Derived anatomy and diet

Machairodonts with long canines, such as Smilodon and Megantereon, had weak jaws. Computer models comparing lion and Smilodon skulls suggested that Smilodon would have had trouble holding onto struggling prey, because its lower jaw could break under strong pressure. Using only its jaw muscles, Smilodon would have had a bite about one-third as strong as a lion's. Strong neck muscles pushed its head down, though, which helped drive its canines into prey. Fossilised bones sometimes keep proteins from the animals a predator ate. These showed that Smilodon mainly hunted bison and horses, and sometimes ground sloths and mammoths. Homotherium often hunted young mammoths and other grazers, such as pronghorn antelope and bighorn sheep, when mammoths were not available. Studies published in 2022 by Xenosmilus suggested that machairodonts could strip meat from a carcass efficiently and ate bone to a degree similar to modern lions.

American paleontologist George Miller suggested several features for the soft tissues of Smilodon. He proposed that its ears looked lower because of a higher sagittal crest, but modern cats have similar ear positions even when their crests are large. Fossils cannot show the shape, position, or colour of the pinnae, or outer ears. Miller also suggested a pug-like nose, but scientists criticised this idea because lions have strongly pulled-back nasal bones, yet their rhinarium, or external nose, is no more pulled back than a tiger's. Miller's idea of longer lips is disputed by scientists, though artists still use it often. It appears in many reconstructions, with lips that look like the jowls of large dogs. Studies suggest that Homotherium had lips and gums that hid its upper canines, while Smilodon's canines stayed partly exposed. Comparisons of the hyoid bones show that Smilodon could possibly have roared like lions, but a 2023 study suggested that its shape was closer to that of purring cats.

Social behavior

Main article: Smilodon § Social behavior

Scientists have asked whether Smilodon lived in groups or alone. A 2009 study compared modern animals in reserves in South Africa and Tanzania with fossils from California's La Brea tar pits. Long ago, animals got stuck in the tar, and their calls drew in predators, which also got stuck. The study assumed that solitary predators would avoid these calls, while social animals such as lions would come closer. The fossils matched social animals, so the study concluded that Smilodon was most likely social. At Friesenhahn Cave in Texas, the remains of almost 400 young mammoths were found with skeletons of Homotherium. Some scientists think Homotherium groups hunted young mammoths and brought them into caves to eat. Its good night vision may have helped it hunt in the dark arctic regions. Not all experts agree that a cat could drag a mammoth calf weighing about 180 kilograms (400 pounds) into a cave without damaging its teeth.

Fossils of Machairodus also hint at group living. One study of M. aphanistus found animals with broken jaws and other injuries that had healed, and the authors suggested it lived in groups. Amphimachairodus hezhengensis also shows signs of healed injuries. One older animal died from a serious nasal infection, so its skull can be understood in different ways. Many Smilodon fossils from La Brea show signs of injuries from hunting. Some animals lived for years with weak legs, which suggests that other members of their group helped them survive. A 2021 study found a young Smilodon with dysplasia that lived to adulthood. The authors argued this would not be possible without a social group. However, the question is still debated. Smilodon had a fairly small brain, which may suggest it could not cooperate in complex ways. Some experts think solitary cats can heal from serious wounds using stored body reserves. Others point out that the dire wolf, which seems intelligent, was common at La Brea, so intelligence may not explain how animals avoided the tar pits. Solitary leopards often die from serious injuries before they can heal, while lions survive them by living in prides.

Functionality of the sabers

Scientists have suggested that machairodonts may have used their long teeth to stab prey. The animal would grab its prey and swing its head down. People once thought the teeth worked like a knife. However, teeth made of enamel could break against bone. The mandible also limited how wide the animal could open its mouth, so scientists now reject this idea. The long teeth may instead have been shaped by mate selection, like a lion's mane, and used to show off and to show social status. Many machairodonts were probably scavengers. They found dead animals by smell and hearing, or took kills from dire wolves or short-faced bears. Many modern cats do this too. The many Smilodon skeletons found in the La Brea Tar Pits in California support this idea. Still, the teeth were built for cutting flesh, not crunching bone. The carnassials of a modern cheetah are similar.

The most common view is that machairodonts killed by biting the throat. Modern cats use a throat clamp to squeeze the windpipe and stop their prey from breathing. One problem with this idea is that spilled blood could attract other predators. Another idea, called "bite and compress," was based on the earlier cats Pseudaelurus and Proailurus. In 1985, William Akersten suggested that a group of machairodonts could hold down a prey animal while one bit into its abdominal cavity. Scientists doubt this, since a kick from a large ungulate could injure the cat. In 2004, an experiment used mechanical jaws on a cow carcass. It found that the belly was too wide for the canines to pierce the skin. The model's jaw moved differently from a real machairodont's, so the results are still uncertain. Scientists also still do not know what role machairodonts played in their environment.