''Thylacosmilus'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of saber-toothed

metatheria Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as w ...

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...

s that inhabited

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sou ...

from the

Late Miocene The Late Miocene (also known as Upper Miocene) is a sub-epoch of the Miocene Epoch made up of two stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch, which lasted from 11.63 Ma (million years ago) to 5.333 Ma. The ...

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58epochs. Though ''Thylacosmilus'' looks similar to the " saber-toothed cats", it was not a

felid Felidae () is the family of mammals in the order Carnivora colloquially referred to as cats, and constitutes a clade. A member of this family is also called a felid (). The term "cat" refers both to felids in general and specifically to the dom ...

, like the well-known North American ''

Smilodon ''Smilodon'' is a genus of the extinct machairodont subfamily of the felids. It is one of the most famous prehistoric mammals and the best known saber-toothed cat. Although commonly known as the saber-toothed tiger, it was not closely rela ...

'', but a sparassodont, a group closely related to

marsupial Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in ...

s, and only superficially resembled other saber-toothed mammals due to

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...

. A 2005 study found that the bite forces of ''Thylacosmilus'' and ''Smilodon'' were low, which indicates the killing-techniques of saber-toothed animals differed from those of extant species. Remains of ''Thylacosmilus'' have been found primarily in Catamarca, Entre Ríos, and

La Pampa Province La Pampa () is a sparsely populated province of Argentina, located in the Pampas in the center of the country. Neighboring provinces are from the north clockwise San Luis, Córdoba, Buenos Aires, Río Negro, Neuquén and Mendoza. History I ...

s in northern

Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the List of South American countries by area, second-largest ...

Taxonomy

In 1926, the Marshall Field Paleontological Expeditions collected mammal fossils from the

Ituzaingó Formation The Ituzaingó Formation ( es, Formación Ituzaingó), in older literature also described as Entre Ríos or Entrerriana Formation, is an extensive geological formation of Late Miocene ( Tortonian, or Huayquerian in the SALMA classification) age in ...

of Corral Quemado, in

Catamarca Province Catamarca () is a province of Argentina, located in the northwest of the country. The province had a population of 334,568 as per the , and covers an area of 102,602 km2. Its literacy rate is 95.5%. Neighbouring provinces are (clockwise, fr ...

, northern Argentina. Three specimens were recognized as representing a new type of

, related to the borhyaenids, and were reported to the Paleontological Society of America in 1928, though without being named. In 1933, the American paleontologist

Elmer S. Riggs Elmer Samuel Riggs (January 23, 1869 – March 25, 1963) was an American paleontologist known for his work with the Field Museum of Natural History in Chicago, Illinois. Biography He was born in Trafalgar, Indiana, and moved with his fam ...

named and preliminarily described the new genus ''Thylacosmilus'' based on these specimens, while noting that a full description was being prepared and would be published at a later date. He named two new species in the genus, ''T. atrox'' and ''T. lentis''. The generic name ''Thylacosmilus'' means "pouch knife", while the specific name ''atrox'' means "cruel". Riggs found the genus distinct enough to warrant a new

subfamily In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus Genus ( plural genera ) is a taxonomic rank used in the biological classifica ...

within Borhyaenidae, Thylacosmilinae, and stated it was "one of the most unique flesh-eating mammals of all times". The

holotype specimen A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of sever ...

of ''T. atrox'', FMNH PP14531, was collected by Riggs and an assistant. It consists of a skull with the teeth of the right side entirely preserved as well as the left canine found separate in the

matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** '' The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...

, fragments of the mandibles, and a partial skeleton consisting of a humerus, a broken radius and broken femora, and foot bones. Missing and scattered parts of the skull and mandible were reconstructed and fitted together. Specimen P 14344 was designated as the

paratype In zoology and botany, a paratype is a specimen of an organism that helps define what the scientific name of a species and other taxon actually represents, but it is not the holotype (and in botany is also neither an isotype nor a syntype). O ...

of ''T. atrox'', and consists of the skull, the mandible, seven cervical, two dorsal, two lumbar, and two sacral vertebrae, a femur, a tibia, a fibula, and various foot bones. It was one fourth smaller than the holotype, and may have been a young adult. It was collected by the American paleontologist Robert C. Thorne. The holotype of ''T. lentis'', specimen P 14474, is a partial skull with the teeth of the right side preserved, and is about the same size as the ''T. atrox'' paratype. It was collected a few miles away from the site of the ''T. atrox'' holotype discovery, by the German biologist

Rudolf Stahlecker Rudolf Stahlecker (25 November 1898 in Sternenfels near Pforzheim – 26 October 1977 in Urach) was a German geologist and biology teacher. Biography He studied with the German paleontologist Friedrich von Huene at the University of Tü ...

. These specimens were housed at the

Field Museum of Natural History The Field Museum of Natural History (FMNH), also known as The Field Museum, is a natural history museum in Chicago, Illinois, and is one of the largest such museums in the world. The museum is popular for the size and quality of its educational ...

Chicago (''City in a Garden''); I Will , image_map = , map_caption = Interactive Map of Chicago , coordinates = , coordinates_footnotes = , subdivision_type = List of sovereign states, Count ...

, while the ''T. lentis'' type later became part of the Museum of La Plata collection. In 1934, Riggs fully described the animal, after the fossils had been prepared and compared with other mammals from the same formation and better known borhyaenids from the Santa Cruz Formation. Thylacosmilus atrox holotype skull

More fragmentary ''Thylacosmilus'' specimens have since been discovered. Riggs and the American paleontologist

Bryan Patterson Bryan Patterson (born 10 March 1909 in London; died 1 December 1979 in Chicago) was an American paleontologist at the Field Museum of Natural History in Chicago. Life and career Bryan Patterson was the son of the soldier, engineer and aut ...

reported in 1939 that a canine (MLP 31-XI-12-4) tentatively assigned to ''Achlysictis'' or '' Stylocynus'' by the Argentinian paleontologist Lucas Kraglievich in 1934 belonged to ''Thylacosmilus''. A partial right ramus and front half of a skull (MLP 65_VI 1-29-41.) was collected in 1965. In a 1972 thesis, the Argentinian paleontologist Jorge Zetti suggested that ''T. atrox'' and ''T. lentis'' represented a single species, and the American paleontologist Larry G, Marshall agreed in 1976, stating the features distinguishing the two were of dubious taxonomic value, and probably due to differences in age and sex. He also found it hard to explain how two sympatric species (related species that lived in the same area at the same time) would be virtually identical in their specializations. Marshall also suggested '' Hyaenodonops'' could be cogeneric, though it was impossible to determine from the available specimens.

Evolution

Megantereon and Thylacosmilus skulls compared

Thylacosmilus atlas, axis, and limb bones

Though ''Thylacosmilus'' is one of several predatory mammal genera typically called "

saber-toothed cat Machairodontinae is an extinct subfamily of carnivoran mammals of the family Felidae (true cats). They were found in Asia, Africa, North America, South America, and Europe from the Miocene to the Pleistocene, living from about 16 million ...

s", it was not a

placentalia Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...

n, but a sparassodont, a group closely related to

s, and only superficially resembled other saber-toothed mammals due to

. The term "saber-tooth" refers to an ecomorph consisting of various groups of extinct predatory

synapsid Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...

s (mammals and close relatives), which

convergently evolved Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...

extremely long

maxillary canine In human dentistry, the maxillary canine is the tooth located laterally (away from the midline of the face) from both maxillary lateral incisors of the mouth but mesial (toward the midline of the face) from both maxillary first premolars. Both ...

s, as well as adaptations to the skull and skeleton related to their use. This includes members of Gorgonopsia,

Thylacosmilidae Thylacosmilidae is an extinct family of metatherian predators, related to the modern marsupials, which lived in South America between the Miocene and Pliocene epochs. Like other South American mammalian predators that lived prior to the Grea ...

, Machaeroidinae, Nimravidae,

Barbourofelidae Barbourofelidae is an extinct family of carnivorans of the suborder Feliformia, sometimes known as false saber-toothed cats, that lived in North America, Eurasia and Africa during the Miocene epoch (16.9—9.0 million years ago) and existed for a ...

, and

Machairodontinae Machairodontinae is an extinct subfamily of carnivoran mammals of the family Felidae (true cats). They were found in Asia, Africa, North America, South America, and Europe from the Miocene to the Pleistocene, living from about 16 million unti ...

. The

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

below shows the position of ''Thylacosmilus'' within Sparassodonta, according to Suarez and colleagues, 2015. Thylacosmilus atrox Skeletal Reconstruction

Thylacosmilus atrox Skeletal Reconstruction

Description

Body mass for sparassodonts is difficult to estimate, since these animals have relatively large heads in proportion to their bodies, leading to overestimations, particularly when compared with skulls of modern members of Carnivora, which have different locomotory and functional adaptations, or with those of the recent predatory marsupials, which do not exceed 30 kg of body mass. Recent methods, like Ercoli and Prevosti's (2011) linear regressions on postcranial elements that directly support the body's weight (such as tibiae, humeri and ulnae), comparing ''Thylacosmilus'' to both extinct and modern carnivorans and metatherians, suggest that it weighed between , with one estimate suggesting up to , about the same size as a modern

jaguar The jaguar (''Panthera onca'') is a large cat species and the only living member of the genus ''Panthera'' native to the Americas. With a body length of up to and a weight of up to , it is the largest cat species in the Americas and the th ...

. The differences in weight estimations may be due to the individual size variation of the specimens studied in each analysis, as well as the different samples and methods used. In any case, the weight estimations are consistent for terrestrial species that are generalists or have some degree of cursoriality. A weight in this range would make ''Thylacosmilus'' one of the largest known

carnivorous A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other ...

metatherians.

Skull

''Thylacosmilus'' had large, saber-like canines. The roots of these canines grew throughout the animal's life, growing in an arc up the maxilla and above the orbits. ''Thylacosmilus'' teeth are in many aspects even more specialized than the teeth of other sabertoothed predators. In these animals the predatory function of the "sabres" gave rise to a specialization of the general dentition, in which some teeth were reduced or lost. In ''Thylacosmilus'' the canines are relatively longer and more slender, relatively triangular in cross-section, in contrast with the oval shape of carnivorans' saber-like canines. The function of these large canines was once thought to have apparently even eliminated the need for functional incisors, while carnivorans like ''

'' and ''

Barbourofelis ''Barbourofelis'' is an extinct genus of large, predatory, feliform carnivoran mammals of the family Barbourofelidae (false saber-tooth cats). The genus was endemic to North America and Eurasia during the Miocene until its extinction during ...

'' still have a full set of incisors. However, evidence in the form of wear facets on the internal sides of the lower canines of ''Thylacosmilus'' indicate that the animal did indeed have incisors, though they remain hitherto unknown due to poor fossilization and the fact that no specimen thus far has been preserved with its premaxilla intact. Thylacosmilus atrox skull

In ''Thylacosmilus'' there is also evidence of the reduction of postcanine teeth, which developed only a tearing cusp, as a continuation of the general trend observed in other sparassodonts, which lost many of the grinding surfaces in the premolars and molars. The canines were hypsodont and more anchored in the skull, with more than half of the tooth contained within the

alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit. Uses in anatomy and zoology * Pulmonary alveolus, an air sac in the lungs ** Alveolar cell or pneumocyte ** Alveolar duct ** Alveolar macrophage * M ...

, which were extended over the braincase. They were protected by the large symphyseal flange and they were powered by the highly developed musculature of the neck, which allowed forceful downward and backward movements of the head. The canines had only a thin layer of enamel, just 0.25 mm in its maximum depth at the lateral facets, this depth being consistent down the length of the teeth. The teeth had open roots and grew constantly, which eroded the abrasion marks that are present in the surface of the enamel of other sabertooths, such as ''Smilodon''. The sharp serrations of the canines were maintained by the action of the wear with the lower canines, a process known as thegosis. The convex upper portion of the maxilla is ornamented with extensive furrows and pits. This texturing has been correlated with an extensive network of blood vessels, which may suggest that the upper maxilla was covered by some form of soft tissue which tentatively has been hypothesized as a "horn covering" (keratinous structure).

Postcranial skeleton

Although the postcranial remains of'' Thylacosmilus ''are incomplete, the elements recovered so far allow the examination of characteristics that this animal acquired in convergence with the sabertooth felids. Its cervical vertebrae were very strong and to some extent resembled the vertebrae of

; also the cervical vertebrae have neural apophysis well developed, along with ventral apophysis in some cervicals, an element that is characteristic of other borhyaenoids. The lumbar vertebrae are short and more rigid than in '' Prothylacynus''. The bones of the limbs, like the

humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a r ...

and

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates ...

, are very robust, since they probably had to deal with larger forces than in the modern felids. In particular, the features of the humerus suggest a great development of the pectoral and deltoid muscles, not only required to capture its prey, also to absorb the energy of the impact of the collision with such prey. Thylacosmilus vertebrae

The features of the hindlimb, with a robust femur equipped with a greater trochanter in the lower part, the short

tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it conn ...

and plantigrade feet shows that this animal was not a runner, and probably stalked its prey animals. The hindlimbs also allowed a certain mobility of the hip, and possibly the ability to stand up only with its hindlimbs, like ''Prothylacynus'' and '' Borhyaena''. Contrary to felids,

barbourofelid Barbourofelidae is an extinct family of carnivorans of the suborder Feliformia, sometimes known as false saber-toothed cats, that lived in North America, Eurasia and Africa during the Miocene epoch (16.9—9.0 million years ago) and existed for a ...

s and nimravids, the claws of ''Thylacosmilus'' were not retractable.

Palaeobiology

Diet and feeding

Recent comparative biomechanical analysis have estimated the bite force of ''T. atrox'', starting from maximum gape, at , much weaker than that of a

leopard The leopard (''Panthera pardus'') is one of the five extant species in the genus '' Panthera'', a member of the cat family, Felidae. It occurs in a wide range in sub-Saharan Africa, in some parts of Western and Central Asia, Southern Russia, ...

, suggesting its jaw muscles had an insignificant role in the dispatch of prey. Its skull was similar to that of ''

'' in that it was much better adapted to withstand loads applied by the neck musculature, which, along with evidence for powerful and flexible forelimb musculature and other skeleton adaptations for stability, support the hypothesis that its killing method consisted on immobilization of its prey followed by precisely directed, deep bites into the soft tissue driven by powerful neck muscles. It has been suggested that its specialized predatory lifestyle could be linked to more extensive parental care than in modern marsupial predators, due that the killing technique only could be used by adult individuals with a full development of its peculiar dental anatomy and grasping abilities; it could require some time for young individuals to learn the necessary skills, although there are no clear evidence in the fossils ''Thylacosmilus'', and this kind of cooperative behavior is unknown in modern marsupials. Canines of Smilodon and Thylacosmilus

In 1988 Juan C. Quiroga published a study on the

cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. The cerebral cortex mostly consists of the six-layered neocortex, with just 10% consisting o ...

of two proterotherids and ''Thylacosmilus''. The study examines endocranial casts of two ''Thylacosmilus'' specimens: MLP 35-X-41-1 (from the Montehermosan age in

), which represents a natural cast of the left half of the cranial cavity lacking the anterior part of the olfactory bulbs and the brain hemispheres; and MMP 1443 (from the

Chapadmalalan The Chapadmalalan age is a period of geologic time (4.0–3.0 Ma) within the Pliocene epoch of the Neogene used more specifically with South American Land Mammal Ages. It follows the Montehermosan and precedes the Uquian age. Fossil content Fo ...

age in

Buenos Aires Province Buenos Aires (), officially the Buenos Aires Province (''Provincia de Buenos Aires'' ), is the largest and most populous Argentine province. It takes its name from the city of Buenos Aires, the capital of the country, which used to be part of th ...

), which is a complete, artificial cast that shows some ventral displacement but with the anterior right part of the brain hemisphere and olfactory bulb. Quiroga's analysis showed that the somatic nervous system of ''Thylacosmilus'' represented 27% of the entire cortex, with the visual area representing 18% and the auditory area 7%. The paleocortex was more than 8%. The sulci of the cortex are relatively complex and similar in pattern and number to the modern

diprotodont Diprotodontia (, from Greek "two forward teeth") is the largest extant order of marsupials, with about 155 species, including the kangaroos, wallabies, possums, koala, wombats, and many others. Extinct diprotodonts include the hippopotamus-sized ...

marsupials. Compared with ''

Macropus ''Macropus'' is a marsupial genus in the family Macropodidae. It has two extant species of large terrestrial kangaroos. The term is derived from the Ancient Greek μάκρος, ''makros'' "long" and πους, ''pous'' "foot". Thirteen known ext ...

'' and '' Trichosurus'', ''Thylacosmilus'' had less development of the maxillar area with respect to the mandibular area, and the rhinal fissure is taller than in ''Macropus'' and '' Thylacinus''. This disproportion between the maxillar and mandibular areas, which are roughly similar in marsupials, seems to be a consequence of the extreme development of the neck and mandibular musculature, used in the functioning of the osteodentary anatomy of this animal. However, the area dedicated to the oral-mandible region comprises 42% of the somatic area. The comparison between the endocranial casts of ''Thylacosmilus'' and a proterotherid specimen (possibly a species coevolving with ''Thylacosmilus'' and a potential prey item) indicates that ''Thylacosmilus'' had only half of the encephalization and a quarter of the cortical area, however it has more somatized areas, similar visual areas and less auditory area, which suggests different sensomotoric qualities between both animals. The analysis published by Christine Argot in 2002 about the evolution of predatory borhyaenoids suggests that ''Thylacosmilus'' was a specialized form, which have a limited stereoscopic vision with small eyes, with an overlap of 50-60°, very low compared with modern predators, but the ossified and great auditory bulla and the muscular body would indicate that it could be an ambush predator in open and relatively dry environments, where the sound absorption is lesser than in more humid areas, and the acute hearing could compensate the limited vision. Argot suggested that ''Thylacosmilus'' maybe was a nocturnal hunter, as modern lions. A 2005 study published by Wroe ''et al''. analysing bite-forces with the use of regressions on body mass and applying the model of "dry skull" in which the jaw is modeled as a lever based in relations between the skull dimensions and the jaw muscles, was employed in some extinct and extant placental and metatherian predatory mammals. ''Thylacosmilus atrox'' reached the lowest value in that analysis, just barely surpassed by ''Smilodon fatalis''. The authors concluded that both taxa, with low bite forces and peculiar cranial and postcranial anatomies, had a killing technique to dispatch large bodied prey without a true analogue between modern taxa. An analysis by Goswami ''et al''. in 2010 tested if the metatherian mode of reproduction has produced any constraint in their cranial morphological evolution. Using landmarks in the skulls of several eutherian and metatherian meat-eating lineages, they compared the ecomorphological convergences in these groups. Metatherian lineages, including specialised forms as '' Thylacoleo'' and ''Thylacosmilus,'' showed values in morphospace more similar to

caniniform In mammalian oral anatomy, the canine teeth, also called cuspids, dog teeth, or (in the context of the upper jaw) fangs, eye teeth, vampire teeth, or vampire fangs, are the relatively long, pointed teeth. They can appear more flattened however ...

s than felids, due that even the shortening of the skull and reduction of postcanine teeth are not so drastic as in felids, despite them often being compared to feliform eutherians. The study shows that in any case, metatherians could be so diverse in cranial diversity as its eutherian counterparts, even with very extreme forms as ''Thylacosmilus'' itself and that the metherian development doesn't have any significative role in cranial evolution. A 2020 study found several functional disparities between ''Thylacosmiluss cranial anatomy and that of saber-toothed eutherians that cannot be explained by its metatherian status, such as the lack of a jaw symphysis, subtriangular canines instead of blade-like ones, lack of incisors (that would render feline-like feeding behaviours impossible), weak jaw musculature and unaligned teeth with no evidence of shearing activity, as well as a post-cranial skeleton more akin to that of a bear than a cursorial predator like a cat. This study very tentatively suggests that ''Thylacosmilus'' might have been an intestine specialist that slashed open and sucked up the carcass' entrails. A 2021 statistical analysis conversely concluded that ''Thylacosmilus'' killed in the same manner as other sabre-tooths, because the premaxillary area, the carnassial region, and the nape of ''Smilodon'', ''Homotherium'', ''Barbourofelis'', and ''Thylacosmilus'' are all similarly developed, which they presumed was to, respectively, withstand high bite forces, maximise gape, and strengthen neck-driven head pulling. ''Thylacosmilus'' scored closest to ''Barbourofelis''. An isotope ratio study, using stable isotopes of carbon and oxygen from the

tooth enamel Tooth enamel is one of the four major tissues that make up the tooth in humans and many other animals, including some species of fish. It makes up the normally visible part of the tooth, covering the crown. The other major tissues are dentin, ...

of several mammals from the Pampean region from the Late Miocene to Late Pleistocene, was published by Domingo ''et al''. in 2020 and indicates that the favoured prey of ''Thylacosmilus'' were grazers, mainly notoungulates from open areas. This diet seems to coincide with the expansion of vast grasslands of C4 plants in southern South America and the increasing of aridity and lower temperatures, in the interval between 11-3 million years ago known as ''Edad de las Planicies Australes'' ("Age of the Southern Plains", in Spanish).

Motion

Various studies have been published on the musculature and motion of ''Thylacosmilus''. The analysis made by William Turnbull published in 1976 and 1978 included a reconstruction of the masticatory muscles of ''Thylacosmilus'' modelling them with

plasticine Plasticine is a putty-like modelling material made from calcium salts, petroleum jelly and aliphatic acids. Though originally a brand name for the British version of the product, it is now applied generically in English as a product category ...

over a cast of the skull and following the muscle scars on the surface of the fossil, then making a rubber model of the musculature and calculating the percentage of weight of these muscles compared with recent mammals, he concluded that the muscles involved in jaw closing in this animal were not unusual in size neither in form, compared with modern carnivore mammals, even indicating that they weren't so reduced as in the machairodont felids. Turnbull concluded that in ''Thylacosmilus'' these masticatory muscles was not involved at all in the use of the sabertooth canines, which depended of the large neck muscles and the flexion of the head to be used killing the prey, combining in a sense the stabbing and slashing techniques from " dirk-toothed" and "

scimitar A scimitar ( or ) is a single-edged sword with a convex curved blade associated with Middle Eastern, South Asian, or North African cultures. A European term, ''scimitar'' does not refer to one specific sword type, but an assortment of different ...

" sabertooths. The comparative studies of Argot 2004, indicates that the

basicranium The base of skull, also known as the cranial base or the cranial floor, is the most inferior area of the skull. It is composed of the endocranium and the lower parts of the calvaria. Structure Structures found at the base of the skull are for ...

had rugose crests that served as attachments for the neck flexor muscles, which are associated to the increase of the bite strength. The deltopectoral crest is large, with 60% of the length of the humerus, which is correlated with musculature to manipulate heavy prey. This animal had an absent entepicondylar foramen in the humerus, which is correlated with the reduction of the abduction movement in that bone in cursorial ungulates and carnivores ('' Borhyaena'' also show this condition), although it contrasts with its probable powerful adductor muscles. Although the lumbar vertebrae are not completely known, the two last ones are known and suggest for its vertical neural process that there is not an anticlinal vertebra; probably the muscles of the back (''m. longissimus dorsi'') acted to stabilize the column and contribute to body propulsion, as occurs in ''Smilodon'', which contrast with more flexible backs of the closest relatives of these sabertooth taxa.

Distribution and habitat

Based on studies of its habitat, ''Thylacosmilus'' is believed to have hunted in savanna-like or sparsely forested areas, avoiding the more open plains where it would have faced competition with the more successful and aggressive Phorusrhacids it shared its environment with. Fossils of ''Thylacosmilus'' have been found in the

Huayquerian The Huayquerian ( es, Huayqueriense) age is a period of geologic time (9.0–6.8 Ma) within the Late Miocene epoch of the Neogene, used more specifically within the SALMA classification. It follows the Mayoan The Mayoan ( es, Mayoense) age is a ...

(

) Ituzaingó, Epecuén, and

Cerro Azul Formation The Cerro Azul Formation ( es, Formación Cerro Azul), in the Buenos Aires Province also described as Epecuén Formation, is a geological formation of Late Miocene ( Tortonian, or Huayquerian in the SALMA classification) age in the Colorado Basin ...

s and the Montehermosan (

Early Pliocene Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * ...

) Brochero and Monte Hermoso Formations in Argentina.''Thylacosmilus''
at

Fossilworks Fossilworks is a portal which provides query, download, and analysis tools to facilitate access to the Paleobiology Database, a large relational database assembled by hundreds of paleontologists from around the world. History Fossilworks was crea ...

.org

Extinction

Although older references have often stated that ''Thylacosmilus'' became extinct due to competition with the "more competitive" saber-toothed cat ''Smilodon'' during the

Great American Interchange The Great American Biotic Interchange (commonly abbreviated as GABI), also known as the Great American Interchange and the Great American Faunal Interchange, was an important late Cenozoic paleozoogeographic biotic interchange event in which lan ...

, newer studies have shown this is not the case. ''Thylacosmilus'' died out during the Pliocene (3.6 to 2.58 Ma) whereas saber-toothed cats are not known from South America until the

Middle Pleistocene The Chibanian, widely known by its previous designation of Middle Pleistocene, is an age in the international geologic timescale or a stage in chronostratigraphy, being a division of the Pleistocene Epoch within the ongoing Quaternary Period. Th ...

(781,000 to 126,000 years ago). As a result, the last appearance of ''Thylacosmilus'' is separated from the first appearance of ''Smilodon'' by over one and a half million years.

Notes

References

Bibliography

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External links

{{Taxonbar, from=Q131163 Sparassodonts Tortonian first appearances Piacenzian extinctions Miocene mammals of South America Pliocene mammals of South America Chapadmalalan Montehermosan Huayquerian Neogene Argentina Fossils of Argentina Cerro Azul Formation Ituzaingó Formation Fossil taxa described in 1933 Taxa named by Elmer S. Riggs Prehistoric mammal genera