Machairodontinae
Machairodontinae, a type of ancient sword, and ὀδούς is an extinct subfamily of carnivoran mammals of the cat family Felidae, representing the earliest diverging major branch of the family.
Machairodonts varied in size from comparable to lynxes to exceeding that of lions. The Machairodontinae contain many of the extinct predators commonly known as "saber-toothed cats", including those with greatly elongated upper maxillary canines, such as the famed genus Smilodon and Megantereon, though the degree of elongation was variable, and in some machairodontines like Dinofelis the length of the upper canines was much more modest. Sometimes, other carnivorous mammals with elongated teeth are also called saber-toothed cats, although they do not belong to the felids. Besides the machairodonts, other saber-toothed predators also arose in the nimravids, barbourofelids, machaeroidines, hyaenodonts and even in two groups of metatherians. Unlike living big cats, which generally clamp the muzzle or throat of prey to asphyxiate them, saber-toothed machairodontines are thought to have killed prey using a bite to the neck once immobilised, using their neck muscles to drive the saber teeth into the throat while the lower jaw served as an anchor, causing rapid death via blood loss.
Likely originating in Eurasia during the Middle Miocene, they eventually spread to every continent except Australia and Antarctica. Machairodonts were the dominant group of cats and large mammalian predators across Afro-Eurasia and North America during the late Miocene and Early Pliocene, a time when the ancestors of living cats were mostly small sized. Machairodonts began to decline from the end of the Miocene onwards, which accelerated during the Pleistocene, perhaps as a result of environmental change and consequential changes in prey abundance, competition with large living cat lineages such as the pantherins as well as possibly archaic humans. The last species belonging to the genera Smilodon and Homotherium became extinct along with many other large mammals around 12-10,000 years ago as part of the end-Pleistocene extinction event, following human arrival to the Americas at the end of the Late Pleistocene.
Evolution
Family Felidae
Based on mitochondrial DNA sequences extracted from fossils, machairodontines diverged from the ancestors of living cats around 20 million years ago, with the last surviving machairodont genera Homotherium and Smilodon estimated to have diverged from each other about 18 million years ago.The Machairodontinae originated in the middle Miocene of Europe. 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 machairodont 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 that also bore long sabre-teeth.
Traditionally, three different tribes of machairodontines were recognized, the Smilodontini with typical dirk-toothed forms, such as Megantereon and Smilodon, the Machairodontini or Homotherini with scimitar-toothed cats, such as Machairodus or Homotherium, and the Metailurini, containing genera such as Dinofelis and Metailurus. However, some in the past have regrouped the Metailurini within the other felid subfamily, the Felinae, along with all modern cats. A 2022 phylogenetic study suggests a polyphyletic relationship between Metailurini and Smilodontini, with the genus Paramachairodus being ancestral to both groups.
The name 'saber-toothed tigers' is misleading. Machairodonts were not in the same subfamily as tigers, there is no evidence that they had tiger-like coat patterns, and this broad group of animals 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.
Classification
Phylogeny
Phylogeny after Werdelin and Flink :Composite cladogram following Lautenschlager et al. :Evolutionary history and origin of phenotype
Until the recent discovery of the Late Miocene fossil depository known as Batallones-1 in the 1990s, specimens of Smilodontini and Homotheriini ancestors were rare and fragmentary, so the evolutionary history of the saber-toothed phenotype, a phenotype affecting craniomandibular, cervical forelimb and forelimb anatomy, was largely unknown. Prior to the excavation of Batallones-1, the predominating hypothesis was that the highly derived saber-toothed phenotype arose rapidly through pleiotropic evolution. Batollnes-1 unearthed new specimens of Promegantereon ogygia, a Smilodontini ancestor, and Machairodus aphanistus, a Homotheriini ancestor, shedding light on evolutionary history.. The leopard-sized P. ogygia inhabited Spain.The current hypothesis for the evolution of the saber-toothed phenotype, made possible by Batollnes-1, is that this phenotype arose gradually over time through mosaic evolution. Although the exact cause is uncertain, current findings have supported the hypothesis that a need for the rapid killing of prey was the principal pressure driving the development of the phenotype over evolutionary time. As indicated by high instances of broken teeth, the biotic environment of saber-toothed cats was one marked by intense competition.
Broken teeth indicate the frequency at which teeth contact bone. Increased teeth-bone contact suggests either increased consumption of carcasses, rapid consumption of prey, or increased aggression over kills – all three of which point to decreased prey availability, heightening competition between predators. Such a competitive environment would favor the faster killing of prey, because if prey is taken away before consumption the energetic cost of capturing that prey is not reimbursed, and, if this occurs often enough in the lifetime of a predator, death by exhaustion or starvation would result. The earliest adaptations improving the speed at which prey was killed are present in the skull and mandible of P. ogygia and of M. aphanistus, and in the cervical vertebrae and forelimb of P. ogygia. They provide further morphological evidence for the importance of speed in the evolution of the saber-toothed phenotype.
Machariodont diversity declined from the end of the Miocene onwards, especially during the Pleistocene, by the Late Pleistocene, only two genera of machairodonts remained, Smilodon, and the distantly related Homotherium, both largely confined to the Americas. These two genera became extinct around 13,000-10,000 years ago as part of the wave of extinctions of most large animals across the Americas.
Fossil remains
Skull
The most studied section of the machairodont group is the skull, and specifically the teeth. With a large range of genera, good fossil representation, comparable modern relatives, diversity within the group, and a good understanding of the ecosystems inhabited, the machairodont subfamily provides one of the best means of research for the analysis of hypercarnivores, specialization, and the relationships between predator and prey.Image:Saber_tooth_jaws,_La_Brea_tarpits_museum,_LA.jpg|thumb|left|Undersides of the skulls of two Smilodon, the skull on the left being in the process of replacing the canines
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.
Carnivores reduced the number of their teeth as they specialized in eating meat instead of grinding plant or insect matter. Cats have the fewest teeth of any carnivore group, and machairodonts reduce the number even further. Most machairodonts retain six incisors, two canines, and six premolars in each jaw, with two molars in the upper jaw only. Some genera, such as Smilodon, bear only eight premolars with one fewer on the mandible, leaving only four large premolars on the mandible along with two stunted canines and six stout incisors. The canines are curved back smoothly, and serrations are present, but are minor and wear away with age, leaving most middle-aged machairodonts with no serrations. Hints in the bones such as these help paleontologists to estimate the age of an individual for population studies of an animal long extinct.
Longer canines necessitate a larger gape. A lion with a gape of 95° 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 similar teeth by convergent evolution, needed a way to change their skulls to accommodate the canines in several ways.
The main inhibitors of a large gape for mammals are the temporalis and masseter muscles at the back of the jaw. These muscles have the capacity to be powerful and undergo a great degree of modification for ranging bite forces, but are not very elastic due to their thickness, placement, and strength. To open the mouth wider, these species needed to make the muscles smaller and change their shape. The first step in this was to reduce the coronoid process. The masseter, and especially the temporalis, muscles insert on this jutting strip of bone, so reduction of this process meant the reduction of the muscles. Less mass for each muscle allowed greater elasticity and less resistance to a wide gape. Changing the shape of the temporalis muscle in this respect created a greater distance between the origin and insertion, so that the muscle became longer and more compact, which is generally a more suitable format for this type of stretching. This reduction led to a weaker bite.
The skulls of machairodonts suggests another change in the shape of the temporalis muscle. The main constraint to opening the jaws is that the temporalis muscle will tear if it is stretched past a critical degree around the glenoid process when the mouth is opened. In modern felids, the occipital bone extends backward, but the temporalis muscles that attach to this surface are strained when opening the jaw wide as the muscle is wrapped around the glenoid process. To reduce the stretch of the temporalis muscle around the immovable process, machairodonts evolved a skull with a more vertical occipital bone. The domestic cat has a gape of 80°, while a lion has a gape of 91°. In Smilodon, the gape is 128°, and the angle between the ramus of the mandible and the occipital bone is 100°. This angle is the major limiting factor of the gape, and reducing the angle of the occipital bone relative to the palate of the mouth, as seen in Smilodon, allowed the gape to increase further. Had the occipital bone not been stretched towards the palate, and closer to perpendicular, the gape would theoretically be less, at roughly 113°.
The skulls of many sabre-tooth predators, including machairodonts, 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, while the muzzle is shorter. These changes help to compensate for an increased gape. Machairodonts also had reduced bottom canines, maintaining the distance between those in the upper and lower jaws.