Machairodontinae

Machairodontinae Temporal range:
Smilodon fatalis fossil at the National Museum of Natural History, Washington, DC
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Carnivora
Family:	Felidae
Subfamily:	†Machairodontinae Gill, 1872
Tribes
†Machairodontini †Smilodontini

Machairodontinae is an extinct carnivorous mammal subfamily of Felidae (true cats) endemic to Asia, Africa, North America, South America, and Europe from the Miocene to Pleistocene living from c. 23 Ma until c. 11,000 years ago.

It contains some of the extinct cats commonly known as "saber-toothed cats", including the famed genus Smilodon as well as other cats with only minor increases in the size and length of their maxillary canines. Sometimes other carnivorous mammals with elongated teeth are also called saber-toothed cats, although they do not belong to the felids. Besides the machairodonts, saber-toothed predators arose in the Nimravidae, the Barbourofelidae, the Creodonta (Machaeroides) and even in a group of Marsupials (Thylacosmilus).

Evolution

Family Felidae

The Machairodontinae originated in the early or middle Miocene of Africa. The early felid Pseudaelurus quadridentatus showed a trend towards elongated upper canines and is believed to be at the base of the machairodontine evolution. The earliest known machairodontid genus is the middle Miocene Miomachairodus from Africa and Turkey. Until the late Miocene machairodontines co-existed at several places together with barbourofelids, archaic large carnivores which also bore long sabreteeth. Traditionally three different tribes of machairodontines were recognized, the Smilodontini with typical dirk-toothed forms like Megantereon and Smilodon, the Machairodontini or Homotherini with scimitar-toothed cats like Machairodus or Homotherium and the Metailurini, containing generea like Dinofelis and Metailurus. However, recently the Metailurini are grouped within another felid subfamily, the Felinae, not into the Machairodontinae. The last machairodontine genera Smilodon and Homotherium disappeared not until the latest Pleistocene, roughly 10.000 years ago in the Americas.

Reconstruction of Megantereon

The name 'saber-toothed tigers' is misleading. Machairodonts were not even in the same subfamily as tigers, there is no evidence that they had tiger-like coat patterns, and this broad group of animals certainly did not all live or hunt in the same manner as the modern tiger. DNA analysis published in 2005 confirmed and clarified cladistic analysis in showing that the Machairodontinae diverged early from the ancestors of modern cats and are not closely related to any living feline species. Sabertooths also coexisted in many places together with conical-toothed cats. In Africa and Eurasia, sabertooths competed with several pantherines and cheetahs until the early or middle Pleistocene. Homotherium survived in Northern Europe even until the late Pleistocene. In the Americas they coexisted together with the cougar, American lion, American cheetah, and jaguar until the latest Pleistocene. Saber-toothed and conical-toothed cats competed with each other for food resources, until the last of the former became extinct. All recent felids have more or less conical-shaped upper canines.

Convergent Evolution

The term saber-toothed cat often refers to a large range of species, from this subfamily of true cats, to the related family, Barbourofelidae, distantly related family Nimravidae, and the very distantly related marsupial Thylacosmilus, identified as the same species by the generally public because they bear long teeth.

The baurofelids, including Sansanosmilus and Barbourofelis, were recently pulled from Nimvaridae into their own family, sister taxon to felidae. It is suggested that they are more closely related to modern cats than any living group of animals. These stout animals were powerful and, especially in Barbourofelis, the canines reach extreme lengths, possibly the longest of all the saber-toothed predators.

The nimravids are very often confused with felids. In fact, they are very distantly related to felines or any other modern carnivore for that matter. They converged in apperance with cats very strongly. The best way to tell the difference is that most nimvarids have flanges, or bony projections beneath their jaws as a kind of shield to the canines. Only the machairodont Megantereon bears prominate flanges. They are generally smaller and the most well known nimvarid Hoplophoneus was the general size of a clouded leopard. Some nimravids, though, such as Dinictis do not have flanges.

Thylacosmilus is the easiest of the saber-toothed predators to tell apart by looking at the skull alone. The tooth roots are extremly prominate and appear as a long raised bump of sorts in front of the eyes. The attachment for the temporalis muscle is greatly reduced and a cross section of the canines reveal a triangular shape. Thylacosmilus was moderatly sized being larger than a leopard and smaller than a lion.

• 
  Thylacosmilus atrox of marsupialian Sparassodonta's Thylacosmilidae.
• 
  Barbourofelis of Barbourofelidae.
• 
  Hoplophoneus of Nimvaridae.
• 
  Dinictis of Nimvaridae.
• 
  Dinictis of Nimvaridae.
• 
  Eusmilus of Nimvaridae.

Morphology

Teeth

Machairodonts are divided into two types: dirk-toothed and scimitar-toothed. Dirk-toothed cats had elongated, narrow upper canines and generally had stocky bodies. Scimitar-toothed cats had broader and shorter upper canines and a typically lithe body form with longer legs. The longer-toothed cats often had a bony flange that extended from their lower mandible. However, one genus, Xenosmilus, known only from two fairly complete fossils, broke this mould, possessing both the stout heavy limbs associated with dirk-toothed cats, and the stout canines of a scimitar-toothed cat.

Increasing the Gape

Longer canines necessitate a larger gape. A lion with a gape of 95 degrees could not bear canines that are nine inches long because they would not be able to have a gap between the lower and upper canines larger than an inch or so, not enough to use for killing. Machairodonts, along with the other groups of animals that acquired the relationship of convergent evolution, needed a way to change their skulls to accommodate the canines.

The largest inhibitor of a large gape for mammals is the muscles of the jaw, the temporalis and masseter muscles. These two large muscles of the head have the capacity to be powerful, but not very elastic due to their thickness. To open the mouth wider, these species needed to change the formation of these muscles. The first step in this was to reduce the coronoid process. The temporalis and masseter muscles fine their insertion point and reduction of this process meant reduction of the large muscles which become, for most modern cats, tissues that restrict the gape. This reduction lead to a bite that is weak, though if the neck muscles were strengthened, the canines could still be used to penetrate flesh.

The second step would be to reduce the bottom canines. The main objective would not be to open the mouth wider, per say, but to maintain the distance between the top and bottom canines. If the top canines increase in length, reducing the bottom canines would increase the distance between the points.

The last step involves the origin of the temporalis muscle. The main constraint with the opening of the jaws is that the temporalis muscle cannot be bent beyond a certain point. In modern felids, the occipital bone extends backward, but the temporalis muscle that attaches to this surface are strained when opening the jaw wide. To reduce the stretch of the temporalis muscle, Machairodonts developed a skull with an occipital bone much more vertical. As seen to the right, the American lion has a gape of 91 degrees, and the angle between the ramus of the mandible and the occipital bone is 124 degrees. In the adjacent Smilodon, the gape is a massive 128 degrees and the angle of between the ramus of the mandible and the occipital bone is 100 degrees. The angle between the occipital bone and the mandible is the major limiting factor of the gape, and by reducing the angle of the occipital bone relative to the pallet of the mouth, shown in Smilodon by thirteen degrees, allowed the gape to increase that much more. Had the occipital bone not been stretched towards the pallet of the mouth, the gape would theoretically be thirteen degrees less at 115 degrees.

Skull Warps

Though they are not particularly closely related, though all mammals, the species acquiring the adaptation of extreme maxillary canines found many of the same ways to deal with the problems saber teeth produce. In 2008, Greg Laden, a graduate of Harvard with a PhD in Archaeology and Biological Anthropology , covered the work of Per Christiansen on the skull shapes of Machairodonts, nimravids, and barbourofelids in his research article Evolution of Skull and Mandible Shape in Cats. These papers studied the skull shape of the carnivores by mapping various landmarks on different skulls and measuring the change in position of these landmarks to compare average values. These values, described as warps, led to an in depth analysis of the change in form of the skull, mainly to compensate for an increased gape.

“Relative warp 2 is primarily related to dorsoventral skull shape, and specimens with lower warp scores have a dorsoventrally much taller and anteroposteriorly more compact skull, ventrally deflected glenoid fossa, greatly curved and anteroventrally compressed and dorsoventrally tall zygomatic arch, elevated facial portion of the skull, and abbreviated mid-section of the skull. They also have enlarged external nares and distinct posterior retraction of the infraorbital foramen, posteroventral deflection of the ventral orbital rim, and slightly smaller and dorsally deflected occipital condyles”.

Per Christiansen, 2008

In laymen’s terms, the skull of these saber-toothed predators are tall from top to bottom and short from front to back. The zygomatic arches are compressed, and the portion of the skull bearing facial features, such as eyes, is higher. The muzzle is shorter.

Hunting techniques

The method by which machairodonts hunted is hotly debated. It was originally thought that they used a 'stabbing' motion with their teeth (dropping their jaws wide open, baring their teeth, and thrashing downward). However, this is now considered unlikely, for the teeth, being so long, were fragile, and a large prey animal thrashing about could easily injure the teeth, which would impair hunting. Some scientists suggest that the cats slashed at the bellies of large animals with their teeth and waited for them to die of blood loss, although the risk of breakage would still be high. When the bite of a dirk toothed cat is matched against the neck of a large ungulate, however, it shows that the bite would sever all arteries and veins, while acting as a clamp around the windpipe in a throat clamp. This method would still have been risky for the teeth if the prey moved too much, but less risky than the stabbing method; with the throat clamp, the cat would be anchored with its incisors and lower canines. Also, if the cats worked in prides, they could have subdued the animal before performing the killing bite.

Genera

Skull of the scimitar-toothed Homotherium serum from Friesenhahn cave, Texas Memorial Museum

Skull of the dirk-toothed Smilodon fatalis.

The tribes and genera of Machairodontines:

• Homotherini
  • Homotherium: early Pliocene to latest Pleistocene; Eurasia, Africa, North America and South America
  • Lokotunjailurus: late Miocene; Africa
  • Xenosmilus: early Pleistocene; North America

• Machairodontini
  • Machairodus: Miocene; Eurasia, Africa and North America
  • Miomachairodus: middle Miocene; North Africa, Turkey

• Metailurini
  • Adelphailurus
  • Dinofelis
  • Metailurus
  • Pontosmilus
  • Stenailurus
  • Therailurus

• Smilodontini
  • Megantereon: early Pliocene to middle Pleistocene; Europe, Asia, Africa and North America
  • Paramachairodus: middle to late Miocene; Europe and Asia
  • Smilodon: late Pliocene to latest Pleistocene, North and South America

The Social Hypothesis

La Brea Tar Pits

The Tar Pits in 1910; there are oil derricks in the background

One of the most abundant sources of Machairodont fossils in one locality is the La Brea tar pits in Los Angeles. This rich fossil bed was at one time a pool of thick tar, or asphalt, covered by water to form a small lake. When animals took a drink, they occasionally wandered into the lake, as seen in modern species to soothe skin or sometimes to relieve themselves of parasites, but their feet were caught in the tar, and with each step to try and free themselves, with one foot pulled up out of the tar, the other three sunk deeper. The pits did not kill an animal immediately. They could remain there for days before they died of starvation or shock. In the meantime, vocalizations and struggling attracted predators to the pits, which got themselves stuck as well. The La Brea tar pits are known as a predator trap for this reason. One stuck bison could attract a multitude of predators before expiring. In the pits, predators outnumber prey nine to one.

La Brea Tar Pits fauna as depicted by Charles R. Knight

The Machairodont Smilodon, with 13,000 specimins from some 2,000 indidiviuals recovered, is one the most abundant fossils in the La Brea tar pits, second to the dire wolf which is represented by 200,000 fossils representing 4,000 individuals. Smilodon was always regarded as a solitary species. The depictions of animals were like vultures to recent carcasses, with lone animals congregating on a kill and fighting over the remains with a gaping show of teeth. The idea that Smilodon lived a solitary life and found a dying animal caught in a tar pit and congregated, one by one, would suggest a very high number of predators in relation to prey.

Tropic Level Ratios

Robert Bakker, author of the Dinosaur Heresies, was one of the first to use the ecological laws of sizes of trophic levels in an ecosystem to support a hypothesis . If his argument that dinosaurs were warm-blooded like their avian relatives, he explains that "warm bloodedness is wasteful--so much energy is spent keeping the body warm. A one hundred pound guard dog (plus puppies) demands one thousand pounds of wet dog food per year for an active outdoor existence. But cold bloodedness is far cheaper. A one hundred pound guard lizard (plus hatchlings) is happy with only one hundred pounds of wet lizard chow per year". So, theoretically, for one thousand pounds of food, you can have one family of warm-blooded predators, or ten families of cold-blooded predators, and by measuring the ratio of dinosaurian predators to prey, it suggest warm-blooded ratios. This sort of ration, whether for high or low metabolisms, is what inhibits there being more numbers in a higher trophic level (predators) than a lower one (prey). In an accurate sample of an ecosystem, nine predators to every prey animal is not a possibility. Something must account for the artificial numbers of nine to one.

A pack of grey wolves hunting an elk in a social manner characteristic of the species

Attracted by Sound

Then, of course, there is the sound factor. An animal who is panicked and dying, such as those trapped in the tar pits, will begin to emit calls, whether calling for members of its own species to aid it or out of frustration without a real purpose. These vocalizations can be heard over a large distance and any neighboring predators will hear the calls of animals in distress and be attracted to the potential easy meal.

A pride of lions subduing a Cape buffalo.

A curious factor with this explanation is that species who are attracted to noise tend to be social. In east Africa, the sounds of the African painted dog, Lycaon pictus, usually indicate excitement, and often this excitement is over a kill. Lions, Panthera leo, are not the daring hunters most people think of them as but instead opportunists who steal more kills from hyenas then they almost hunt for themselves, and hearing these calls that might mean food, they move towards the sounds to investigate the source of excitement. The solitary Panthera pardus, the leopard, is never seen approaching these sources of excitement. Other solitary species, including cats such as the serval and cheetah, do not approach these sounds either. The canids in the region, namely jackals, are semi-social with breeding pairs, but they stay away until only the bare scraps are left.

The most suitable rationale for this attraction is that a species in distress can be heard by all the predators in the area. If every predator that heard the sounds were to come to the source, there would be intense competition between the species. A leopard would be killed, and jackals, badgers, and servals would not only be killed, they’d become part of the menu. Lions, aided by numbers and strength, would be the most powerful in the congregation and other predators who would come would be killed or injured. The only other predator that might match the lions’ strength and numbers would be the hyenid Crocuta, the spotted hyena. Hyenas challenge lions on a regular basis at kills and are often attracted to the same calls of distressed animals. The two social species shoulder their way around with lions usually on top, and all other predators who know they don’t have a shot abandon the area to avoid getting caught up.

This tendency would explain Smilodon well. dire wolves are the other common species in the tar pits, and it would make sense for this species to be social, considering their modern relative, the grey wolf is.

How a break in a bone that heals badly distorts the bone.

Damaged Bones

Evidence of sociability can be seen even in the fossils themselves. When an animal stressed its body beyond its abilities to cope, damage is sure to ensue. Smilodon bones often show fractures and deformities. Muscles tearing from bones are not particularly uncommon in this predator. Hunting large prey can stress an animal to this point. What is curious about these wounds is that they heal. A muscle torn from a bone heals and tears again and heals and tears again, leaving the body to struggle to stop the cycle by laying down thick deposits of calcium, warping the bone into having large lumps of bone jutting from the norm and broken bones heal badly, but refuse and the animal lives on.

In solitary animals, such as the modern cheetah, Acinonyx, a simple sprain is enough to inhibit the predator’s hunting to the point that it will starve to death. Hunting with an able body is hard enough. A broken bone would never heal because the animal would die before the body could repair the damage. Smilodon seemed to have suffered great injuries and survived the healing stage to recover and hunt again. The large genus Machairodus often displays broken canines that are worn due to extensive usage after the break, along with Smilodon.

One specific case of fossil deformity is the observed large pelvic fractures that healed in a subadult Smilodon fossils with extensive myositis ossificans tramatica, immobolizing the juvinile completly until healed months later, and even when healed it would have been crippled terribly. The muscle damage was severe and blood would have pooled beneath the skin in the injury. Durring this extended period of several months, it could not run and walking would have been very tiring and painful. It would have had food brought directly to it, carried fron a kill site to where the injured animal lay. Thid behavior has been observed in lionnesses (Schaller, 1972) for up to nine months.

Dire wolves in the tar pits often show much the same tremendous capacity for healing from injuries and surviving practically crippled on occasion. But the dire wolves were social, and as with many living social species such as grey wolves, an injured pack member was cared for until they recovered, at which point they resumed life with the pack. This logic can be used for Smilodon as well: a social species can care for the members of a familial group, a solitary predator has so such back up plan and would die. The fact that Smilodon survived to hunt and be injured again strongly suggest cooperation.

Rebuke to the Social Hypothesis

The question of sociability is still controversial. A strong support to the traditional concept of a solitary Smilodon is found in its brain. Most social predators, including humans, grey wolves, and lions, have brains that are slightly bigger than their loner relatives. Smilodon had a relatively small brain, suggesting an ability for less complex behaviors, such as hunting in groups and cooperative behaviors. The high numbers of Smilodon as a suggestion for a social nature is often dismissed because the golden eagle, a species still alive today, is solitary and yet is found in the pits in just the same numbers as Smilodon. The social grey wolf and coyote lived in the region, but their fossils in the tar pits are rare.

The broken bones seem still to be a large hole in the argument, though. The best explination for a solitary animal healnig from wounds is explained in the fact that cats build up metabolic reserves that can be used in times of need. The example of a cheetah is often viewed as a poor example due to the fact that the cheetah is a specialized species who is a very fragile predator, even as far as cats go. Larger, more sturdy species of cats, such as lions and leopards, have been observed to recover from severe injuries such as broken jaws and torn muscles.

Notes and references

• Report on Barnett group's study in Current Biology August 9, 2005: Ross Barnett et al.: "Evolution of the extinct Sabretooths and the American cheetah-like cat" in Current Biology, Vol. 15, R589-R590, August 9, 2005