Perhistoric Birds

                                 Titanis

 Titanis is a large extinct flightless carnivorous bird of the family Phorusrhacidae, endemic to North America during the Blancan stage of the Pliocene living 4.9—1.8 Ma, and died out during the Gelasian Age of the earliest Pleistocene, existing approximately 3.1 million years.

 The generic name, Titanis, refers to the titans, Ancient Greek gods that preceded the Twelve Olympians, in allusion to the bird's size. The specific name, T. walleri, honors the holotype's collector, Benjamin I. Waller.

  It lived approximately 5-2 million years ago (early Pliocene to early Pleistocene) in North America. Fossil evidence has been found in Gilchrist County, Florida dating from 3.0 to 2.9 million years.
From circumstantial evidence (i.e., bone fractures), it has been suggested that the species did not become extinct until 15,000 years ago,  but more precise dating by McFadden and colleagues refutes such a late date; all known Titanis fossils appear to be at least 2 million years old.Titanis was part of the group of giant flightless birds called the Phorusrhacidae, which are nicknamed "terror birds", and has been thought to represent the youngest species of the lineage (recently, a significantly younger South American example has been reported ). The Phorusrhacidae originated in South America; Titanis is the only known member of the branch of the group that migrated out of that continent during the Great American Interchange. It is estimated that Titanis could run at speeds of 65 km/h. It was 2.5 metres (8 ft 2 in) tall and weighed approximately 150 kilograms (330 lb), but with large variance (perhaps indicating strong sexual dimorphism). Though its skull has not been found, it most probably would have been large, with a huge, axe-like beak, like its relatives.
The wings were small and could not have been used for flight. The wing bones articulated in an unusual joint-like structure, suggesting the digits could flex to some degree. It also had a relatively rigid wrist, which would not have allowed the hand to fold back against the arm to the same degree as other birds. This led one scientist, R.M. Chandler, to suggest that the wings may have supported some type of clawed, mobile hand similar to the hands of non-avian theropod dinosaurs, such as the deinonychosaurs (also popularly known as "raptors"). However, it was later pointed out that this wing joint is not in fact unique, and is present in seriamas (modern members of the same bird group to which Titanis belonged), which do not have any specialized grasping hands.
Overall, Titanis was very similar to the South American Phorusrhacos and Devincenzia, its closest relatives. However, it differs from these in having a shorter, thicker neck, a bulkier head, and an overall more heavily-built bodily structure. Little is known of its body structure, but it seems to have been less wide-footed than Devincenzia, with a proportionally much stronger middle toe. (Onactornis is now considered a junior synonym of Devincenzia).

Dinosaurs

                            Tyrannosaurus Rex

 

                                
    

Tyrannosaurus meaning "tyrant lizard", from Greek τυράννος (tyrannos, "tyrant") and σαύρος' (sauros, "lizard")) is a genus of theropod dinosaur. The species Tyrannosaurus rex (rex meaning "king" in Latin), commonly abbreviated to T. rex, is a fixture in popular culture. It lived throughout what is now western North America, with a much wider range than other tyrannosaurids. Fossils are found in a variety of rock formations dating to the Maastrichtian age of the upper Cretaceous Period, 67 to 65.5 million years ago. It was among the last non-avian dinosaurs to exist prior to the Cretaceous–Tertiary extinction event.

Like other tyrannosaurids, Tyrannosaurus was a bipedal carnivore with a massive skull balanced by a long, heavy tail. Relative to the large and powerful hindlimbs, Tyrannosaurus forelimbs were small, though unusually powerful for their size, and bore two clawed digits. Although other theropods rivaled or exceeded Tyrannosaurus rex in size, it was the largest known tyrannosaurid and one of the largest known land predators, measuring up to 12.8 m (42 ft) in length,up to 4 metres (13 ft) tall at the hips, and up to 6.8 metric tons (7.5 short tons) in weight. By far the largest carnivore in its environment, Tyrannosaurus rex may have been an apex predator, preying upon hadrosaurs and ceratopsians, although some experts have suggested it was primarily a scavenger. The debate over Tyrannosaurus as apex predator or scavenger is among the longest running in paleontology.

More than 30 specimens of Tyrannosaurus rex have been identified, some of which are nearly complete skeletons. Soft tissue and proteins have been reported in at least one of these specimens. The abundance of fossil material has allowed significant research into many aspects of its biology, including life history and biomechanics. The feeding habits, physiology and potential speed of Tyrannosaurus rex are a few subjects of debate. Its taxonomy is also controversial, with some scientists considering Tarbosaurus bataar from Asia to represent a second species of Tyrannosaurus and others maintaining Tarbosaurus as a separate genus. Several other genera of North American tyrannosaurids have also been synonymized with Tyrannosaurus. Tyrannosaurus rex was one of the largest land carnivores of all time; the largest complete specimen, FMNH PR2081 ("Sue"), measured 12.8 metres (42 ft) long, and was 4.0 metres (13.1 ft) tall at the hips. Mass estimates have varied widely over the years, from more than 7.2 metric tons (7.9 short tons),to less than 4.5 metric tons (5.0 short tons), with most modern estimates ranging between 5.4 and 6.8 metric tons (6.0 and 7.5 short tons). Packard et al. (2009) tested dinosaur mass estimation procedures on elephants and concluded that dinosaur estimations are flawed and produce over-estimations; thus, the weight of Tyrannosaurus could be much less than usually estimated.

Although Tyrannosaurus rex was larger than the well known Jurassic theropod Allosaurus, it was slightly smaller than some other Cretaceous carnivores, such as Spinosaurus and Giganotosaurus.

The neck of Tyrannosaurus rex formed a natural S-shaped curve like that of other theropods, but was short and muscular to support the massive head. The forelimbs had only two clawed fingers, along with an additional small metacarpal representing the remnant of a third digit. In contrast the hind limbs were among the longest in proportion to body size of any theropod. The tail was heavy and long, sometimes containing over forty vertebrae, in order to balance the massive head and torso. To compensate for the immense bulk of the animal, many bones throughout the skeleton were hollow, reducing its weight without significant loss of strength.

The largest known Tyrannosaurus rex skulls measure up to 5 feet (1.5 m) in length.Large fenestrae (openings) in the skull reduced weight and provided areas for muscle attachment, as in all carnivorous theropods. But in other respects Tyrannosaurus’ skull was significantly different from those of large non-tyrannosauroid theropods. It was extremely wide at the rear but had a narrow snout, allowing unusually good binocular vision.The skull bones were massive and the nasals and some other bones were fused, preventing movement between them; but many were pneumatized (contained a "honeycomb" of tiny air spaces) which may have made the bones more flexible as well as lighter. These and other skull-strengthening features are part of the tyrannosaurid trend towards an increasingly powerful bite, which easily surpassed that of all non-tyrannosaurids. The tip of the upper jaw was U-shaped (most non-tyrannosauroid carnivores had V-shaped upper jaws), which increased the amount of tissue and bone a tyrannosaur could rip out with one bite, although it also increased the stresses on the front teeth. The teeth of Tyrannosaurus rex displayed marked heterodonty (differences in shape). The premaxillary teeth at the front of the upper jaw were closely packed, D-shaped in cross-section, had reinforcing ridges on the rear surface, were incisiform (their tips were chisel-like blades) and curved backwards. The D-shaped cross-section, reinforcing ridges and backwards curve reduced the risk that the teeth would snap when Tyrannosaurus bit and pulled. The remaining teeth were robust, like "lethal bananas" rather than daggers; more widely spaced and also had reinforcing ridges. Those in the upper jaw were larger than those in all but the rear of the lower jaw. The largest found so far is estimated to have been 30 centimetres (12 in) long including the root when the animal was alive, making it the largest tooth of any carnivorous dinosaur yet found. Tyrannosaurus is the type genus of the superfamily Tyrannosauroidea, the family Tyrannosauridae, and the subfamily Tyrannosaurinae; in other words it is the standard by which paleontologists decide whether to include other species in the same group. Other members of the tyrannosaurine subfamily include the North American Daspletosaurus and the Asian Tarbosaurus,both of which have occasionally been synonymized with Tyrannosaurus Tyrannosaurids were once commonly thought to be descendants of earlier large theropods such as megalosaurs and carnosaurs, although more recently they were reclassified with the generally smaller coelurosaurs.

 The identification of several specimens as juvenile Tyrannosaurus rex has allowed scientists to document ontogenetic changes in the species, estimate the lifespan, and determine how quickly the animals would have grown. The smallest known individual (LACM 28471, the "Jordan theropod") is estimated to have weighed only 30 kg (66 lb), while the largest, such as FMNH PR2081 ("Sue") most likely weighed over 5,400 kg (12,000 lb). Histologic analysis of Tyrannosaurus rex bones showed LACM 28471 had aged only 2 years when it died, while "Sue" was 28 years old, an age which may have been close to the maximum for the species. Histology has also allowed the age of other specimens to be determined. Growth curves can be developed when the ages of different specimens are plotted on a graph along with their mass. A Tyrannosaurus rex growth curve is S-shaped, with juveniles remaining under 1,800 kg (4,000 lb) until approximately 14 years of age, when body size began to increase dramatically. During this rapid growth phase, a young Tyrannosaurus rex would gain an average of 600 kg (1,300 lb) a year for the next four years. At 18 years of age, the curve plateaus again, indicating that growth slowed dramatically. For example, only 600 kg (1,300 lb) separated the 28-year-old "Sue" from a 22-year-old Canadian specimen (RTMP 81.12.1). Another recent histological study performed by different workers corroborates these results, finding that rapid growth began to slow at around 16 years of age. This sudden change in growth rate may indicate physical maturity, a hypothesis which is supported by the discovery of medullary tissue in the femur of a 16 to 20-year-old Tyrannosaurus rex from Montana (MOR 1125, also known as "B-rex"). Medullary tissue is found only in female birds during ovulation, indicating that "B-rex" was of reproductive age. Further study indicates an age of 18 for this specimen. Other tyrannosaurids exhibit extremely similar growth curves, although with lower growth rates corresponding to their lower adult sizes. Over half of the known Tyrannosaurus rex specimens appear to have died within six years of reaching sexual maturity, a pattern which is also seen in other tyrannosaurs and in some large, long-lived birds and mammals today. These species are characterized by high infant mortality rates, followed by relatively low mortality among juveniles. Mortality increases again following sexual maturity, partly due to the stresses of reproduction. One study suggests that the rarity of juvenile Tyrannosaurus rex fossils is due in part to low juvenile mortality rates; the animals were not dying in large numbers at these ages, and so were not often fossilized. However, this rarity may also be due to the incompleteness of the fossil record or to the bias of fossil collectors towards larger, more spectacular specimens. As the number of specimens increased, scientists began to analyze the variation between individuals and discovered what appeared to be two distinct body types, or morphs, similar to some other theropod species. As one of these morphs was more solidly built, it was termed the 'robust' morph while the other was termed 'gracile'. Several morphological differences associated with the two morphs were used to analyze sexual dimorphism in Tyrannosaurus rex, with the 'robust' morph usually suggested to be female. For example, the pelvis of several 'robust' specimens seemed to be wider, perhaps to allow the passage of eggs. It was also thought that the 'robust' morphology correlated with a reduced chevron on the first tail vertebra, also ostensibly to allow eggs to pass out of the reproductive tract, as had been erroneously reported for crocodiles in recent years, evidence for sexual dimorphism has been weakened. A 2005 study reported that previous claims of sexual dimorphism in crocodile chevron anatomy were in error, casting doubt on the existence of similar dimorphism between Tyrannosaurus rex genders. A full-sized chevron was discovered on the first tail vertebra of "Sue", an extremely robust individual, indicating that this feature could not be used to differentiate the two morphs anyway. As Tyrannosaurus rex specimens have been found from Saskatchewan to New Mexico, differences between individuals may be indicative of geographic variation rather than sexual dimorphism. The differences could also be age-related, with 'robust' individuals being older animals.

Only a single Tyrannosaurus rex specimen has been conclusively shown to belong to a specific gender. Examination of "B-rex" demonstrated the preservation of soft tissue within several bones. Some of this tissue has been identified as a medullary tissue, a specialized tissue grown only in modern birds as a source of calcium for the production of eggshell during ovulation. As only female birds lay eggs, medullary tissue is only found naturally in females, although males are capable of producing it when injected with female reproductive hormones like estrogen. This strongly suggests that "B-rex" was female, and that she died during ovulation.Recent research has shown that medullary tissue is never found in crocodiles, which are thought to be the closest living relatives of dinosaurs, aside from birds. The shared presence of medullary tissue in birds and theropod dinosaurs is further evidence of the close evolutionary relationship between the two.

Prehistoric Whales

                                Rodhocetus

                        

Rodhocetus is one of several extinct whale genera that possess land mammal characteristics, thus demonstrating the evolutionary transition from land to sea.

The first species to be discovered (Rhodocetus kasrani) exhibited such features as a large pelvis fused to the vertebrae, hind legs, and differentiated teeth. Of a recently discovered species (Rodhocetus balochistanensis), the ankle bones were recovered, further strengthening the already well-founded link to artiodactyls, and weakening the link to mesonychids. Rodhocetus balochistanensis is in fact believed to demonstrate a direct evolutionary link to artiodactyls (modern examples of which include hippopotamuses, now believed to be the closest cousin species of the cetaceans). The structure of the ankle bones of this species, the trochlea, is double-spooled. This trait is only known in artiodactyls, as all other mammalian orders have a single-spooled trochlea. This matches studies of the genetic relations between whales and other animals. Previous fossil-based hypotheses that whales were directly descended from mesonychids have been largely overturned.
The ear bones of Rodhocetus are already very whale-like, though the swimming style is very different. Rodhocetus is more obviously aquatic than earlier known species (e.g. Ambulocetus) and had large, paddling hind feet to propel it through the water. It also had a strong tail which may have helped to act as a rudder. Many suggest that Rodhocetus may have swum like a modern otter, but through a principal components analysis done in 2003, Philip Gingerich demonstrated that its limb proportions were closer to that of the Russian desman.
  The first fossils of this species were found in Balochistan Province, Pakistan in 2001 by Philip Gingerich. Dating from about 47 million years ago, they are one of a series of recent discoveries, including the pakicetids, which have thrown considerable light on the previously mysterious evolutionary origin of whales.

                   

                                    

Prehistoric Whales

                            Ambulocetus

    Ambulocetus (or the "'walking whale'") was an early cetacean that could walk as well as swim. It lived during early Eocene some 50-49 million years ago. It is a transitional fossil that shows how whales evolved from land-living mammals. The Ambulocetus fossils were found in Pakistan by anthropologist Johannes Thewissen. When the animal was alive, Pakistan was a coastal region of India, which was then a large island in the Indian Ocean.

 

  Having the appearance of a 3 meter (10-foot) long mammalian crocodile, it was clearly amphibious, as its back legs are better adapted for swimming than for walking on land, and it probably swam by undulating its back vertically, as otters and whales do. It has been speculated that Ambulocetids hunted like crocodiles, lurking in the shallows to snatch unsuspecting prey. Chemical analysis of its teeth shows that it was able to move between salt and fresh water. Ambulocetus did not have external ears. To detect prey on land, they may have lowered their heads to the ground and felt for vibrations. Scientists consider Ambulocetus to be an early whale because it shares underwater adaptations with them: it had an adaptation in the nose that enabled it to swallow underwater, and its periotic bones had a structure like those of whales, enabling it to hear well underwater. In addition, its teeth are similar to those of early cetaceans.

 

                                                     

 

 

Prehistoric Whales

                                                   

                   

                         Pakicetus     

                

                                   

Pakicetus is a genus of extinct terrestrial carnivorous mammal of the family Pakicetidae

-which was endemic to Pakistan from the Eocene (55.8 ± 0.2—40 ± 0.1 Ma). Pakicetus existed for approximately 15.8 million years. Many paleontologists regard it as a close relative to the direct ancestors of modern day whales.

 Pakicetus was assigned to Protocetidae by Gingerich and Russell (1981), Carroll (1988) and Benton (1993). Then to Pakicetinae by Gingerich and Russell (1990) and McKenna and Bell (1997); and to Pakicetidae by Thewissen and Hussain (1998), Thewissen et al. (2001), Thewissen et al. (2001), Geisler and Sanders (2003), McLeod and Barnes (2008) and Uhen (2010).

 The first fossils were uncovered in Pakistan, hence their name. The strata of western Pakistan where the fossils were found was then the coastal region of the Tethys Sea. The first fossil found of the creature consisted of an incomplete skull with a skull cap and a broken mandible with some teeth. It was thought to be from a mesonychid, but Gingerich and Russell recognized it as an early cetacean from characteristic features of the inner ear, found only in cetaceans: the large auditory bulla is formed from the ectotympanic bone only. This suggests that it is a transitional species between extinct land mammals and modern cetaceans. It was restorated on the cover of Science as a semiaquatic, somewhat crocodile like mammal, diving after fish.

 Somewhat more complete skeletal remains were discovered in 2001, prompting the view that Pakicetus was primarily a land animal about the size of a wolf, and very similar in form to the related mesonychids. In 2001, J. G. M. Thewissen and colleagues wrote that "Pakicetids were terrestrial mammals, no more amphibious than a tapir."
However, in 2009 Thewissen et al argued that "the orbits ... of these cetaceans were located close together on top of the skull, as is common in aquatic animals that live in water but look at emerged objects. Just like Indohyus, limb bones of pakicetids are osteosclerotic, also suggestive of aquatic habitat"

        

Prehistoric Mammals

                             Hyaenodon

Hyaenodon ("hyaena-toothed") is an extinct genus of Hyaenodonts, a group of carnivorous creodonts of the family Hyaenodontidae endemic to all continents except South America, Australia and Antarctica, living from 42—15.9 mya, existing for approximately 26.1 million years. Some species of this genus were amongst the largest terrestrial carnivorous mammals of their time; others were only of the size of a marten. Hyaenodon was one of the latest genera of the Hyaenodonts and is known from the Late Eocene to Early Miocene. Remains of many species are known from North America, Europe, Asia and Africa (In 1993 42 species were distinguished). Typical of early carnivorous mammals, the Hyaenodon had a very massive skull but only a small brain. It had a long skull with a narrow snout - much larger in relation to the length of the skull than in canine carnivores, for instance. Its neck was shorter than its skull, while its body was long and robust and terminated in a long tail. Despite the name, these creatures are not related to hyenasThe average weight of adult or subadult H. horridus, the largest North American species, are estimated to about 40 kilograms (88 lb) and may not have exceeded 60 kilograms (130 lb). H. gigas, the largest Hyaenodon species was much larger, being 500 kilograms (1,100 lb) and around 10 feet. H. crucians from the early Oligocene of North America is estimated to only 10 to 25 kilograms (22 to 55 lb). H. microdon and H. mustelinus from the late Eocene of North America were even smaller and weighed probably about 5 kilograms (11 lb).
 In North America the last Hyaenodon disappeared along with species like H. brevirostris in the late Oligocene. In Europe they had already vanished earlier in the Oligocene. From the Miocene in Africa there are three species (H. andrewsi, H. matthewi and H. pilgrimi) known, but none of these reached the dimensions of Asian species like H. gigas and H. weilini

Prehistoric Birds

                            Gastornis

 Gastornis is an extinct genus of large flightless bird that lived during the late Paleocene and Eocene epochs of the Cenozoic. It was named in 1855, after Gaston Planté, who had discovered the first fossils in Argile Plastique formation deposits at Meudon near Paris (France). At that time, Planté (described as a "studious young man full of zeal" was at the start of his academic career, and his remarkable discovery was soon to be overshadowed by his subsequent achievements in physics.
In the 1870s, the famous American paleontologist Edward Drinker Cope discovered another, more complete set of fossils in North America, and named them Diatryma ( daɪ.əˈtraɪmə dy-ə-try-mə ),from Ancient Greek διάτρημα, diatrema, meaning "canoe"
The fossil remains of these birds have been found in western-central Europe (England, Belgium, France and Germany). Gastornis parisiensis measured on average 1.75 meter (5.7 ft) tall, but large individuals grew up to 2 meter (6.6 ft) tall. The Gastornis had a remarkably huge beak with a slightly hooked top, which was taken as evidence suggesting that it was carnivorous. Gastornis had large powerful legs, with large, taloned feet, which also were considered in support of the theory that it was a predator.

 The plumage of Gastornis is unknown; it is generally depicted with a hair-like covering as in ratites, but this is conjectural. Some fibrous strands recovered from a Green River Formation deposit at Roan Creek, Colorado were initially believed to represent Gastornis feathers and named Diatryma filifera. Subsequent examination showed that they were actually not feathers at all but plant fibers or similar

 The skull of Gastornis remained unknown except for nondescript fragments, and several bones assigned to it were those of other animals. Thus, the European bird was long reconstructed as a sort of gigantic crane-like ornithuran, very different from the North American species. Eventually this was sorted out, and only then it was realized that Gastornis and Diatryma were so much alike to make many scientists today consider the latter a junior synonym of the former pending a comprehensive review. Consequently the correct scientific name is Gastornis. In fact, this similarity was recognized as early as 1884 by Elliott Coues, but his reasoning was initially discounted and subsequently ignored until the late 20th century. The following species are accepted today:

•   Gastornis were variously considered allied with diverse birds, such as waterfowl, ratites or waders. Their highly apomorphic anatomy makes reliable assignment to any one group of birds difficult, in particular since no particularly close relatives survive today. In modern times, they were placed with the "Gruiformes" assemblage, which includes cranes. But in the 21st century, these birds are most often considered to be Galloanseres, or "fowls", in the same family as chickens and waterfowl. Quite ironically, the original assessment of Hébert - who perceived similarities with the Anseriformes in the original tibia - thus would be far more correct than any later placement. Incidentally, since the Galloanseres are known to originate in the Cretaceous already, it is comfortably explained how such a gigantic bird could be around less than 10 million years after the non-avian dinosaurs became extinct. The following species are accepted today: Gastornis parisiensis Hébert, 1855 - the type species

Late Paleocene - Early Eocene of WC Europe

Synonyms: Gastornis edwardsii Lemoine, 1878; G. klaasseni Newton, 1885; G. pariensis (lapsus)

• Gastornis russeli L.Martin, 1992

Late Paleocene of Berru, France

• Gastornis sarasini (Schaub, 1929)

Early Eocene - middle Eocene of WC Europe

• Gastornis giganteus Cope, 1876

Early -? middle Eocene of SC North America

Synonyms: Barornis regens Marsh, 1894; Omorhamphus storchii Sinclair, 1928; O. storchi Wetmore, 1931 (unjustified emendation) Omorhamphus storchii was described based on fossils from the Lower Eocene of Wyoming. The species was named in honor of T. C. von Storch, who found the fossils remains in Princeton 1927 Expedition.. It was synonymized subjectively with Gastornis gigantea by Brodkorb (1967) and Witmer and Rose (1991).

Prehistoric Mammals

                           Smilodon   

.  Smilodon, often called a saber-toothed cat or (incorrectly) saber-toothed tiger, is an extinct genus of machairodonts. This sabre-toothed cat was endemic to North America and South America, living from near the beginning through the very end of the Pleistocene epoch (2.5 mya—10,000 years ago).
  The nickname "sabre-tooth" refers to the extreme length of their maxillary canines. Despite the colloquial name "sabre-toothed tiger", Smilodon is not a tiger; the latter belongs to subfamily Pantherinae, whereas Smilodon belongs to subfamily Machairodontinae. The name Smilodon comes from Greek: σμίλη, smilē, "chisel" and  Greek ὀ δoύς (ho doús), "the tooth", Genitive: δoύς, δόντος,dóntos.
   The genus Smilodon was described by the Danish naturalist and paleontologist Peter Wilhelm Lund in 1841. He found the fossils of Smilodon populator in caves near the small town of Lagoa Santa, in the state of Minas Gerais, Brazil.
A number of Smilodon species have been described, but today usually only three are recognized.

•    Smilodon gracilis, 2.5 million-500,000 years ago; the smallest and earliest species (estimated to have been only 55 to 100 kg (120 to 220 lb)) was the successor of Megantereon in North America, from which it probably evolved. The other Smilodon species probably derived from this species.
•    Smilodon fatalis, 1.6 million-10,000 years ago, replaced Smilodon gracilis in North America and invaded western South America as part of the Great American Interchange. In size it was between Smilodon gracilis and Smilodon populator, and about the same as the largest surviving cat, the Siberian Tiger. This species was about 1 m high at the shoulder and is estimated to have ranged from 160 to 280 kg (350 to 620 lb). Sometimes two additional species are recognized, Smilodon californicus and Smilodon floridanus, but usually they are considered to be subspecies of Smilodon fatalis.

•    Smilodon populator ("the devastating Smilodon"), 1 million-10,000 years ago; occurred in the eastern parts of South America and was the largest species of all machairodonts. It was much larger than its cousins, S. fatalis and S. gracilis, possessing a massive chest and front legs, and is the largest known variety of sabre-toothed cat. It was more than 1.22 m (48 in) high at the shoulder, 2.6 m (100 in) long on average and had a 30 cm (12 in) tail. With an estimated weight of 360 to 470 kg (790 to 1,000 lb), it was among the heaviest known felids. Its upper canines reached 30 cm (12 in) and protruded up to 17 cm (6.7 in) out of the upper jaw. Genetic evidence suggests that Smilodon populator and other members of the genus diverged from the main lineage of modern cats (subfamily Felinae) around 14-18 million years ago.

Smilodon populator is also known from the famous cave site of Ultima Esperanza, with well-preserved remains retaining endogenous DNA.

Smilodon probably preyed on a wide variety of large game including bison, tapirs, deer, American camels, horses and ground sloths. As it is known for the sabre-toothed cat Homotherium, Smilodon might have also killed juvenile mastodons and mammoths. Smilodon may also have attacked prehistoric humans, although this is not known for certain. The La Brea tar pits in California trapped hundreds of Smilodon in the tar, possibly as they tried to feed on mammoths already trapped. The Natural History Museum of Los Angeles County has many of their complete skeletons.

Modern big cats kill mainly by crushing the windpipe of their victims, which may take a few minutes. Smilodon’s jaw muscles were probably too weak for this and its long canines and fragile skull would have been vulnerable to snapping in a prolonged struggle or when biting a running prey. Research in 2007 concluded that Smilodon most likely used its great upper-body strength to wrestle prey to the ground, where its long canines could deliver a deep stabbing bite to the throat which would generally cut through the jugular vein and/or the trachea and thus kill the prey very quickly. The leaders of this study also commented to scientific journalists that this technique may have made Smilodon a more efficient killer of large prey than modern lions or tigers, but also made it more dependent on the supply of large animals. This highly-specialized hunting style may have contributed to its extinction, as Smilodon’s cumbersome build and over-sized canines would have made it less efficient at killing smaller, faster prey if the ecosystem changed for any reason.
Research upon which African carnivores respond to playback of animals in distress has been used to analyses the finds of animal species and their numbers at the La Brea tar pits. Such playbacks find animal distress calls such as would come from an animal trapped in the tar pit would attract pack hunters such as lions and spotted hyenas, not lone hunters. Given the carnivores found at tar pits were predominately Smilodon and the social dire wolf, this suggests that the former like the latter was also a social animal. One expert, who found the study convincing, further speculated that if that was the case, then Smilodon's exaggerated canine teeth might have been used more for social or sexual signaling than hunting. ^{[Full citation needed]} However, the lack of sexual dimorphism in the canine teeth makes this unlikely.
Smilodon is most famous for its relatively long canines, which are the longest found in the sabre-toothed cats, at about 28 cm (11 in) long in the largest species Smilodon populator.
They did not use their long teeth particularly for hunting in risk of breaking them; only when the prey is totally subdued did they use their teeth to simultaneously sever the blood supply and strangle the wind pipe, instantly killing their prey. The 20 cm sabres were fragile and could not have bitten into bone.^{[citation needed]}
Despite being more powerfully built than other large cats, Smilodon actually had a weaker bite. Modern big cats have more pronounced zygomatic arches, while Smilodon had smaller zygomatic arches which restricted the thickness and therefore power of the temporalis muscles, and thus reduced Smilodon’s bite force. Analysis of its narrow jaws indicates that it could produce a bite only a third as strong as that of a lion. There seems to a be a general rule that the sabre-toothed cats with the largest canines had proportionally weaker bites. However, analyses of canine bending strength (the ability of the canine teeth to resist bending forces without breaking) and bite forces indicate that sabre-toothed cats' teeth were stronger relative to the bite force than those of modern "big cats". In addition, Smilodon could open its jaws 120 degrees, whereas the lion's gape is limited to 60 degrees.
It has been suggested that Smilodon's smaller temporalis muscles (controlling much of the bite force) were not used in the killing of prey; rather, Smilodon stretched its jaws around the throat and pressed its canines into the prey with the use of its immense neck and forelimb muscles. The penetration was the result of the neck flexors instead of the jaw muscles, according to this hypothesis.

Prehistoric Mammals

                             Brontotherium                     

 Brontotherium ('thunder beast') is an extinct genus of prehistoric odd-toed ungulate mammal of the family Brontotheriidae, an extinct group of rhinoceros-like browsers related to horses. The genus was found in North America during the Late Eocene Many Brontotherium remains have been found in South Dakota and Nebraska. In the past, specimens exposed by severe rainstorms were found by Native Americans of the Sioux tribe. The Sioux believed these creatures produced thunderstorms when running over the clouds, and called them 'thunder horses'. Many of the skeletons found by the Sioux belonged to herds which were killed by volcanic eruptions of the Rocky Mountains, which were volcanically active at the time. According to Mihlbachler et al. 2004, the species within genus Brontotherium were merged with the genus Megacerops, though not all authorities agree on this .Brontotherium stood about 2.5 meters (8.2 ft) tall at the shoulders and resembled a large rhinoceros, possessing a Y-shaped horn-like protrusion on its nose, with blunt ends. Brontotherium's dorsal above the shoulders had extra long spines to support the huge neck muscles needed to carry the heavy skull. Possibly, Brontotherium had fleshy lips and a long tongue, perfect for carefully selecting food. The shape of its teeth suggests that it preferred food such as soft stems and leaves, rather than tough vegetation

Prehistoric Mammals

                                                                                                                                                                      Megantereon 

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Megantereon was an ancient machairodontine saber-toothed cat that lived in North America, Eurasia, and Africa. It may be the ancestor of Smilodon Fossil range: Fossil fragments have been found in Africa, Eurasia, and North America. The oldest confirmed records of Megantereon are known from the Pliocene of North America and are dated to about 4.5 Million years. About 3-3.5 Million years ago it is firmly recorded also from Africa, about 2.5 to 2 Million years ago also from Asia. In Europe the oldest remains are known from Les Etouaries (France), a site which is now dated to less than 2.5 Million years old. Therefore a North American origin of Megantereon has been suggested. However, recent findings of fragmentary fossils from Kenya and Chad, which are dated to about 5.7 and 7 Million years, respectively, are probably from Megantereon. If these identifications are right, they would represent the oldest Megantereon fossils in the world. The new findings therefore indicate an origin of Megantereon in the Late Miocene of Africa.
At the end of the Pliocene it evolved into the larger Smilodon in North America, while it survived in the Old World until the middle Pleistocene. The youngest remains of Megantereon from east Africa are about 1.5 million years old. In southern Africa the genus is recorded from Elandsfontein, a site dated to around 700,000-400,000 years old. Remains from Untermaßfeld show that Megantereon lived until 900,000 years ago in Europe. In Asia it may have survived until 500,000 years ago, as it is recorded together with Homo erectus at the famous site of Zho-Khou-Dien in China. The only full skeleton was found in Senéze, France.
Morphology
Megantereon was built like a modern jaguar or somewhat heavier. It had stocky forelimbs with the lower half of these forelimbs lion-sized. It had large neck muscles designed to deliver a powerful bite. The elongated upper canines were protected by flanges at the mandible. Mauricio Anton's reconstruction in The big cats and their fossil relatives depicts the full specimen found at Seneze in France as 72 centimeters (28 in) at the shoulder. The largest specimens with an estimated body weight of 90–150 kilograms (200–330 lb) (average 120 kilograms (260 lb)) are known from India. Medium sized species of Megantereon are known from other parts of Eurasia and the Pliocene of North America. The smallest species from Africa and the lower Pleistocene of Europe have been estimated to only 60–70 kilograms (130–150 lb). However, other sources estimated Megantereon from the European lower Pleistocene at 100–160 kilograms (220–350 lb).
Hunting technique
In Europe, Megantereon may have preyed on larger artiodactyls, horses or the young of pachyderms like rhinos and elephants. It is unlikely that Megantereon simply bit its prey as the long, sabre-teeth that Smilodon is famed for are not strong enough to leave buried inside a struggling prey animal: the teeth would break off. It is possible that they bit their prey and then allowed it to bleed to death, but then they would have to protect that animal from other predators and thus their tactic for killing remains uncertain. It is now generally thought that Megantereon, like other saber-toothed cats, used its long saber teeth to deliver a killing throat bite, severing most of the major nerves and blood vessels. While the teeth would still risk damage, the prey animal would be killed quickly enough that any struggles would be feeble at best.