Sparassodonts

1. Borhyaena 2. Protodidelphis 3. Arctodictis 4. Prothylacinus 5. Dukecynus
Sparassodonta is an extinct order of carnivorous metatherian mammals native to South America. They were once considered to be true marsupials, but are now thought to be either a sister taxon to them,or considerably distantly related, part of a separate clade of Gondwanan metatherians.A number of these mammalian predators closely resemble placental predators that evolved separately on other continents, and are cited frequently as examples of convergent evolution.
Sparassodonts can be divided into six major groups; basal sparassodonts (earliest Paleocene-late Miocene), species that cannot be easily assigned to any of the other sparassodont groups and whose teeth often exhibit adaptations for omnivory; hathliacynids (late Oligocene-early Pliocene/late Pliocene), which range from a marten to a thylacine in size, and have long, fox-like snouts and teeth strongly adapted for carnivory; basal borhyaenoids (middle Eocene-late Miocene), borhyaenoids that cannot be easily classified into the families Borhyaenidae, Thylacosmilidae, or Proborhyaenidae and vary in size and shape; borhyaenids (early-late Miocene), the sparassodont group most specialized for running, but not as much as living carnivorans or even thylacines; proborhyaenids (middle Eocene-late Oligocene), robust, wolverine-like forms with ever-growing upper and lower canines; and thylacosmilids (early Miocene-late Pliocene), another terrestrially specialized group with ever-growing saber-like upper canines.With the exception of some basal sparassodonts, all members of this group were hypercarnivorous.Based on studies of the postcranial skeleton, it appears as though most sparassodonts were scansorial (adapted for climbing), although terrestrial adaptations evolved in Lycopsis longirostrus, borhyaenids, proborhyaenids, and thylacosmilids.

After the middle Miocene, sparassodonts began to slowly decline in diversity. Basal borhyaenoids are last known from the early late Miocene (Pseudolycopsis cabrerai and Lycopsis viverensis), and after this time were at least partially replaced by large-bodied basal sparassodonts such as Stylocynus. It has been suggested that this shift in dominance was due to the more omnivorous habits of basal sparassodonts, which may have been better able to exploit the more seasonal climates of South America during the late Neogene.Borhyaenids are last known from the latest Miocene, though only fragmentary remains of this group are known from this period.Later remains assigned to this group have since been reidentified as thylacosmilids or procyonids. By the Pliocene, only two families of sparassodonts remained in South America, the Hathliacynidae and the Thylacosmilidae. Pliocene hathliacynid remains are rare, and it is possible that these animals may have competed with the large carnivorous didelphids such as Lutreolina that appeared around this time.Hathliacynids are last definitively known from the early Pliocene, though it has been suggested that the genus Borhyaenidium may have survived into the late Pliocene.

The thylacosmilids, on the other hand, were more successful and abundant, being some of the only large mammalian carnivores in South America during the Pliocene, and only died out at the end of the epoch. It is still not certain why Sparassodonta declined in diversity and became extinct during the late Cenozoic, but it appears as though competition from eutherian carnivorans was not a factor, as the placental analogues of sparassodonts (dogs, weasels, and saber-toothed cats) did not enter South America until the middle Pleistocene, several million years after their sparassodont counterparts became extinct.The overall decline in sparassodont diversity from the late Miocene to the end of the Pliocene may be linked to the cooling climates following the middle Miocene climatic optimum and the onset of the Pleistocene ice ages.


The largest carnivorous marsupial were the Proborhyaena gigantea

Proborhyaenidae is an extinct family of metatherians mammals of the order Sparassodonta, which lived in South America since the Eocene until the Oligocene. Sometimes it have been included as a subfamily of their relatives borhyaenids (as Proborhyaeninae). Members of this family were carnivores, usually large, being among the largest representatives of the sparassodonts and carnivore metatherians; the species Proborhyaena gigantea could reach 500 kg, a size comparable to the bears.
It was a carnivore with a peculiar teeth: its fangs were projected forward and also would not stop growing. This is complemented with robust skull, 60 cm long, and a powerful and muscular neck.
Although some experts believe that usually, this prehistoric predator rather than hunt, began to steal prey from other predators (size permitting). And thanks to the wear of the teeth, experts have concluded that it was dedicated to crushing bones to eat: from animals hunted either carrion than it could take. The denture reminds Tasmanian devils now living today.
Proborhyaenid remains have been found in western Bolivia, Uruguay, southern Brazil, and the provinces of Mendoza, Salta, and Chubut, in Argentina.
Most proborhyaenids had a robust, hyena-like skull, although one species, Callistoe vincei, had an elongate, narrow skull more reminiscent of a thylacine The teeth were strongly specialized as carnassials for eating meat, and in Arminiheringia rotated throughout the animal"s life to maintain a continuous shearing blade on the tooth. Preserved specimens of their canines lack enamel; in life, the enamel may have been very thin or restricted to the tooth tips. In the genus Arminiheringia the lower canines are protruding. Proborhyaenids can be distinguished from other sparassodonts by their grooved upper and lower canines, which grew continuously throughout the animals" lives like rodentincisors. Bond and Pascual (1983) argued that proborhyaenid canines stopped growing in late adulthood based on a specimen from Mendoza Province, Argentina, but the proborhyaenid identity of this specimen is disputed. The presence of open-rooted upper canines in thylacosmilids has led to the suggestion that proborhyaenids are closely related to, or even ancestral to, this group, but this is still controversial.

Thylacosmilus  (Marsupial Saber-Tooth)

Thylacosmilus had long, saber-like upper canines and short, blunt, peg-like lower canines. The incisors were missing altogether and the other teeth were severely reduced, but, as distinct from machairodonts, their number was complete. It is estimated to have weighed around 150 kilograms. Thylacosmilus went extinct during the Pliocene, a timeframe closely corresponding to the arrival of the saber-toothed cats Smilodon  and Homotherium from North America in the Great American Interchange. As such out-competition from the anatomically similar Smilodon appears to have driven the extinction, though this is unusual among other South American extinctions during the period.
With its oversized front teeth Thylacosmilus appears to be very similar to sabre toothed cats like Smilodon. The key difference between these two types of animal is that the sabre toothed cats were placental mammals, which means that young developed while connected to a placenta via an umbilical cord while remaining inside the mother’s body. Marsupials like Thylacosmilus however have a pouch which young (called neonates) are birthed into at an early stage of development where they remain until they are ready to be upwardly mobile themselves. This means that while it looked similar to a sabre toothed cat, Thylacosmilus was in the same group as modern kangaroos and the extinct Thylacine (better known as the Tasmanian Tiger).
The most striking feature of Thylacosmilus are the teeth, even though they actually only share superficial similarity to those of the sabre toothed cats. One key difference is that the teeth constantly grew throughout the life of Thylacosmilus, meaning that if they were not worn down they would keep on growing larger. Teeth may have been worn down by wear form contact with the bones of prey. The mandible also had two large bone growths referred to as flanges that ran alongside both teeth so they had extra support and protection when the jaw was closed. The jaws could also open to a very wide angle, something that would allow the large teeth to be used efficiently. The price of this wide opening is that Thylacosmilus would have had a reduced bite force. However, this is not a drawback for a predator that relies upon driving through the points of just two teeth to kill its prey, as focusing the force upon just two points would have increased the impact pressure meaning that strong jaw muscles would not have been necessary for an effective bite.
Thylacosmilus may have used the oversized teeth in a similar fashion to the better known sabre toothed cats. This could see Thylacosmilus ambushing prey by either staying low in the vegetation or leaping onto it from above. During these surprise attacks Thylacosmilus could inflict a deep bite to a vulnerable area like the neck, something that could sever arteries that resulted in rapid blood loss. In this Thylacosmilus is usually depicted as a lone hunter, although multiple Thylacosmilus hunting together is not an outright impossibility. Mothers would have hunted with offspring still inside their pouch and may have even been joined by them for a period of time when the young were approaching adulthood.
Ultimately it may have been contact with the anatomically similar Smilodon that brought about the downfall of Thylacosmilus. This contact was a product of the Great American Interchange, an event triggered by the joining of North and South America which allowed animals that were once isolated from one another to intermix and spread into new areas. Smilodon was larger and more powerful than Thylacosmilus, and probably had a near identical killing technique and prey preference. When two predators of the same prey animals are in competition to one another it is usually the weaker and less well adapted of the two that disappears.
Climate change is another strong factor as the overall climate where Thylacosmilus is most well-known was starting to become drier as Thylacosmilus began to decline and disappear. While climate usually has little direct impact upon predators, it does have a bigger effect upon prey. The changing types of prey available (as well as increased competition from new predators) may have meant that Thylacosmilus was no longer the efficient killer that it had been in the past. The attrition of living in a world that was becoming harder to live in could have ultimately brought the downfall and extinction of Thylacosmilus.
It is worth remembering that while Thylacosmilus was certainly a formidable hunter, the top predators of South America during the existence of the genus would have actually been large phorusrhacids, more popularly known as ‘terror birds’.
Besides Proborhyaenadae, Thylacosmilus and the similarly sized marsupial lion(Thylacoleo carnifex) were the largest carnivorous metatherians.

 

The marsupial lion (Thylacoleo carnifex)

The marsupial lion is the largest meat-eating mammal known to have ever existed in Australia, and one of the largest marsupial carnivores from anywhere in the world (although see Thylacosmilus and Borhyaena). Individuals ranged up to around 75 cm (30 in) high at the shoulder and about 150 cm (59 in) from head to tail. Measurements taken from a number of specimens show they averaged 101 to 130 kg (223 to 287 lb) in weight, although individuals as heavy as 164 kg (362 lb) might not have been uncommon. This would make it quite comparable to female lions and tigers in general size.
The animal was extremely robust with powerfully built jaws and very strong fore limbs. It possessed retractable claws, a unique trait among marsupials. This would have allowed the claws to remain sharp by protecting them from being worn down on hard surfaces. The claws were well-suited to securing prey and for climbing trees. The first digits ("thumbs") on each hand were semiopposable and bore an enlarged claw. Palaeontologists believe this would have been used to grapple its intended prey, as well as providing it with a sure footing on tree trunks and branches. The hind feet had four functional toes, the first digit being much reduced in size, but possessing a roughened pad similar to that of possums, which may have assisted with climbing. It is unclear whether the marsupial lion exhibited syndactyly (fused second and third toes) like other diprotodonts.
The marsupial lion"s hindquarters were also well-developed, although to a lesser extent than the front of the animal. Remains of the animal show it had a relatively thick and strong tail and the vertebrae possessed chevrons on their undersides where the tail would have contacted the ground. These would have served to protect critical elements such as nerves and blood vessels if the animal used its tail to support itself when on its hind legs, much like present day kangaroos do. Taking this stance would free up its fore limbs to tackle or slash at its intended victim.
The ancestors of thylacoleonids are believed to have been herbivores, which is unusual for carnivores but cranial features and arboreal characteristics suggest that thylacoleonids share a common ancestor with wombats. While other continents were sharing many of their predators amongst themselves, as they were connected by land, Australia "s isolation caused many of its normally docile herbivorous species to turn carnivorous.  Possum-like features were once thought to indicate that the marsupial lion"s evolutionary path was from a phalangeriform ancestor, however, scientists agree that more prominent features suggest a vombatiform ancestory; today represented by Lasiorhinus krefftii, Lasiorhinus latifrons, Phascolarctos cinereus, and Vombatus ursinus.
The marsupial lion was a highly specialised carnivore, as is reflected in its dentition. Like other diprotodonts, it possessed enlarged incisors on both the upper (maxillae) and lower (mandibles) jaws. These teeth (the lower in particular) were shaped much more like the pointed canine teeth of animals such as dogs and cats than those of kangaroos. The most unusual feature of the creature"s dentition were the huge, blade-like carnassialpremolars on either side of its jaws. The top and bottom carnassials worked together like shears and would have been very effective at slicing off chunks of flesh from carcasses and cutting through bone.
The jaw muscle of the marsupial lion was exceptionally large for its size, giving it an extremely powerful bite. Biometric calculations show, considering size, it had the strongest bite of any known mammal, living or extinct; a 101-kg individual would have had a bite comparable to that of a 250-kg African lion. Using 3D modeling based on X-ray computed tomography scans, marsupial lions were found to be unable to use the prolonged, suffocating bite typical of living big cats. They instead had an extremely efficient and unique bite; the incisors would have been used to stab at and pierce the flesh of their prey while the more specialised carnassials crushed the windpipe, severed the spinal cord, and lacerated the major blood vessels such as the carotid artery and jugular vein. Compared to an African lion which may take 15 minutes to kill a large catch, the marsupial lion could kill a large animal in less than a minute. The skull was so specialized for big game, it was very inefficient at catching smaller animals, which possibly contributed to its extinction.
The marsupial lion"s limb proportions and muscle mass distribution indicate that, although it was a powerful animal, it was not a particularly fast runner. Paleontologists conjecture that it was an ambush predator, either sneaking up and then leaping upon its prey, or dropping down on it from overhanging tree branches. That fits with the stripes: camouflage of the kind you need for stalking and hiding in a largely forested habitat (like tigers) rather than chasing across open spaces (like lions). It is thought to have hunted large animals such as the enormous Diprotodon and giant browsingkangaroos like Sthenurus and Procoptodon, and competed with other predatory animals such as the giant monitor lizard, megalania, and terrestrial crocodiles such as Quinkana. The marsupial lion may have cached kills in trees in a manner similar to the modern leopard. Like many predators, it was probably also an opportunistic scavenger, feeding on carrion and driving off less powerful predators from their kills.
CT scans of a well-preserved skull have allowed scientists to study internal structures and create a brain endocast showing the surface features of the animal"s brain. The parietal lobes, visual cortex, and olfactory bulbs of the cerebrum were enlarged, indicating the marsupial lion had good senses of hearing, sight, and smell, as might be expected of an active predator. Also, a pair of blind canals within the nasal cavity were probably associated with detecting pheromones as in the Tasmanian devil. This indicates it most likely had seasonal mating habits and would "sniff out" a mate when in season.
Numerous fossil discoveries indicate the marsupial lion was distributed across much of the Australiancontinent. A large proportion of its environment would have been similar to the southern third of Australia today, semiarid, open scrub and woodland punctuated by waterholes and water courses.
It would have coexisted with many of the so-called Australian megafauna such as the previously mentioned Diprotodon, giant kangaroos, and Megalania, as well as giant wallabies like Protemnodon, the giant wombatPhascolonus, and the thunderbird Genyornis.
Australia"s Pleistocene megafauna would have been the prey for the agile T. carnifex, who was especially adapted for hunting large animals, but was not particularly suited to catching smaller prey. The relatively quick reduction in the numbers of its primary food source around 40,000 to 50,000 years ago probably led to the decline and eventual extinction of the marsupial lion. The arrival of  humans in Australia and the use of fire-stick farming precipitated their decline. The extinction of T. carnifex makes Australia unique from the other continents because no substantial, apex mammalian predators have replaced the marsupial lions after their disappearance.
The marsupial lion is classified in the order Diprotodontia along with many other well-known marsupials such as kangaroos, possums, and the koala. It is further classified in its own family, the Thylacoleonidae, of which three genera and 11 species are recognised, all extinct. The term marsupial lion (lower case) is often applied to other members of this family. Distinct possum-like characteristics led Thylacoleo to be regarded as members of Phalangeroidea for a few decades. Though a few authors continued to hint at phalangeroid affinities for thylacoleonids as recently as the 1990s, cranial and other characters have generally led to their inclusion within vombatiformes, and as stem-members of the wombat lineage. Marsupial lions and other ecologically and morphologically diverse vombatiforms were once represented by over 60 species of carnivorous, herbivorous, terrestrial and arboreal forms ranging in size from 3 kg to 2.5 metric tons. Only two families represented by four herbivorous species (koalas and three species of wombat) have survived into modern times and are considered the marsupial lion"s closest living relatives. 
Studies research suggest that most of mega fauna animals can survive and adapt to extreme weatherclimate change, therefore it is unlikely to be the cause of the extinction of the T.carnifex. With that being said, scientists are now investigating the next possible cause which is the arrival of humans to Australia and how their actions could be the cause of the extinction of the T. carnifex.
When humans came to Australia, they probably burnt the landscape of Australia to enhance their survival. Changing the ecosystem of the once grassy woodland to desert shrub, the humans" actions may result in the extinction of plants that large and small animals feed off of. As a result, humans reduced the number of prey that big carnivores depend on (carnivores are sensitive to prey abundance). If humans reduce the number of potential prey below a threshold then large predators like the T. carnifex will likely starve to death.
Keep in mind that the T. carnifex is an ambush predator, therefore by burning down the grassy woodlands it would be difficult for the marsupial lion to catch prey that is faster than itself.