HAAST'S EAGLE

Haast's Eagle (Harpagornis moorei) was a species of massive eagle that once lived on the South Island of New Zealand. The species was the largest eagle known to have existed. Its prey consisted mainly of gigantic flightless birds that were unable to defend themselves from the striking force and speed of these eagles, which at times reached 80 km/h (50 mph). The Eagle's massive size may have been an evolutionary response to the size of its prey, as both would have been much smaller when they first came to the island, and would have grown larger over time due to lack of competition (see Island gigantism). The Haast's Eagle became extinct around the year 1400, when its major food sources, the moa, were hunted to extinction by Maori living on the island and much of its dense-forest habitat was cleared.

DNA analysis has shown that this raptor is related most closely to the much smaller Little Eagle as well as the Booted Eagle (both of these two species were recently reclassified as belonging to the genus Aquila ) and not, as previously thought, to the large Wedge-tailed Eagle.Thus, Harpagornis moorei may be reclassified as Aquila moorei, pending confirmation. H. moorei may have diverged from these smaller eagles as recently as 700,000 to 1.8 million years ago. Its increase in weight by ten to fifteen times over that period is the greatest and quickest evolutionary increase in weight of any known vertebrate. This was made possible in part by the presence of large prey and the absence of competition from other large predatorsHaast's Eagle was first classified by Julius von Haast in the 1870s who named it Harpagornis moorei after George Henry Moore, the owner of the Glenmark Estate where bones of the bird had been found.

The genus name is a compound crassis word of the Greek word "harpax", meaning 'grappling hook', and the Greek "ornis", meaning 'bird'.

Haast's Eagles were the largest known true raptors, slightly larger even than the largest living vultures. Female eagles are significantly larger than males. Females of the Haast species are believed to have weighed 10–15 kg (22–33 lb) and males 9–12 kg (20–26 lb). They had a relatively short wingspan, measuring roughly 2.6–3 m (8 ft 6 in–9 ft 10 in). This wingspan is similar to that of some extant eagles (the wingspan now reported in large specimens of Golden Eagles and Steller's Sea Eagles). Even the largest extant eagles, however, are about forty percent smaller in body size than the size of Haast's Eagles.

Short wings may have aided Haast's Eagles when hunting in the dense scrubland and forests of New Zealand. Haast's Eagle sometimes is portrayed incorrectly as having evolved toward flightlessness, but this is not so; rather it represents a departure from the mode of its ancestors' soaring flight, toward higher wing loading. Two of the largest extant eagles, the Harpy Eagle and the Philippine Eagle, also have similarly reduced relative wing-length in adaptation to forest-dwelling.

The strong legs and massive flight muscles of these eagles would have enabled the birds to take off with a jumping start from the ground, despite their great weight. The tail was almost certainly long, up to 50 cm (20 inches) in female specimens, and very broad. This characteristic would compensate for the reduction in wing area by providing additional lift. Total length is estimated to have been up to 1.4 m (4 ft 7 in) in females, with a standing height of approximately 90 cm (2 ft 11 in) tall or perhaps slightly greater.

Haast's Eagles preyed on large, flightless bird species, including the moa, which was up to fifteen times the weight of the eagle. It is estimated to have attacked at speeds up to 80 km/h (50 mph), often seizing its prey's pelvis with the talons of one foot and killing with a blow to the head or neck with the other.[citation needed] Its size and weight indicate a bodily striking force equivalent to a cinder block falling from the top of an eight-story building. Its large beak also could be used to rip into the internal organs of its prey and death then would have been caused by blood loss.[citation needed] In the absence of other large predators or scavengers, a Haast's Eagle easily could have monopolised a single large kill over a number of days

Early human settlers in New Zealand (the Māori arrived around the year 1280) preyed heavily on large flightless birds, including all moa species, eventually hunting them to extinction. The loss of its natural prey caused the Haast's Eagle to become extinct as well around the year 1400, when the last of its natural food sources were depleted.

A noted explorer, Charles Edward Douglas, claims in his journals that he had an encounter with two raptors of immense size in Landsborough River valley (probably during the 1870s), and that he shot and ate them. These birds might have been a last remnant of the species, but some might argue that there had not been suitable prey for a population of Haast's Eagle to maintain itself for about five hundred years before that date,[citation needed] and 19th century Māori lore was adamant that the pouakai was a bird not seen in living memory. Still, Douglas' observations on wildlife generally are trustworthy; a more probable explanation, given that the alleged three-metre wingspan described by Douglas is likely to have been a rough estimate, is that the birds were Eyles' Harriers. This was the largest known harrier (the size of a small eagle) — and a generalist predator — and although it also is assumed to have become extinct in prehistoric times, its dietary habits alone make it a more likely candidate for late survival.

Until recent human colonisation that introduced rodents and cats, the only mammals found on the islands of New Zealand were three species of bat, one of which recently has become extinct. Free from terrestrial mammalian competition and predatory threat, birds occupied or dominated all major niches in the New Zealand animal ecology because there were no threats to their eggs and chicks by small terrestrial animals. Moa were grazers, functionally similar to deer or cattle in other habitats, and Haast's Eagles were the hunters who filled the same niche as top-niche mammalian predators, such as tigers or lions.

GORGONOPS

 Gorgonops is an extinct genus of therapsid which lived about 255-250 million years ago, during the latest part of the Permian Period. It was a typical representative of the suborder Gorgonopsia, the dominant predators of their day, which in the largest forms grew to over four metres long. Gorgonops possessed highly developed, 12-cm long "sabre" canine teeth, similar to those found in sabre-toothed cats in the Cenozoic. Arguments have even been made for therapsids of its time being endothermic, though no strong evidence exists either way. All of the gorgonopsids are believed to have died out in the Permian extinction.
Gorgonops itself was a medium to large-sized representative of the group, with a skull length of twenty to thirty-five centimetres, depending on the species. It ranged from 1.5 to 3 metres long from nose to tail. Gorgonops derived superior speed from long legs held beneath its body.
The holotype of the type species, Gorgonops torvus, was in 1876 one of the first therapsids described, by Richard Owen, who also coined the name "Dinosauria" on the basis of the first known dinosaur fossils. It was also used as the type for which Richard Lydekker described the family in 1890. Five years later, in 1895, Harry Govier Seeley used this genus to establish the group as a whole. In later years, a large number of further species and genera were designated, but some of these turned out to be synonyms

The following list of species follows the list provided Sigogneau-Russell, 1989. The type species is listed first, then others in alphabetical order.

Since the publication of the work cited above, the dating of the Karoo Basin (Beaufort Group) has been revised. According to Smith and Keyser 1995, Gorgonops is known from the Tropidostoma and most of the Cistecephalus Assemblage Zones, and this has been taken into account when compiling the following list.
Gorgonops torvus (Owen, 1876) The type species. The holotype is an incomplete and flattened skull found at Mildenhalls, Fort Beaufort, South Africa. A number of other specimens have been found since, all from the Tropidostoma and/or Cistecephalus Assemblage Zone(s). This was a medium-sized therapsid, with a skull about twenty cm in length. It is distinguished from other species by a longer snout, and other details of the bones of the skull. Originally considered rather simple, it is actually (according to Sigogneau-Russell) a rather specialised member of the group.
Gorgonops longifrons (Haughton 1915) A large specimen known from an incomplete and flattened skull about thirty-five cm long. Orbit larger and snout longer than G. whaitsi, from which it may have descended. Beaufort West, Tropidostoma/Cistecephalus Assemblage Zone. Synonyms: Gorgonognathus longifrons Haughton 1915

Gorgonops whaitsi (Broom, 1912) Larger than G. torvus, with the rear of the skull wider, and other details of proportion. Originally the type species of Scymnognathus. Despite being known from a large number of specimens from the Karoo Basin, Beaufort West (Tropidostoma/Cistecephalus Assemblage Zone), the species remains poorly known. Watson and Romer placed Gorgonops and Scymnognathus in two different families, while Sigogneau-Russell 1989 placed the two species in the same genus, and considers G. whaitsi a more primitive (less derived) form. Synonyms: Scymnognathus whaitsi Broom, 1912
The following are more problematic species (either synonyms or uncertain placement):
Gorgonops? dixeyi (Haughton, 1926) A large, incomplete and flattened skull, from Chiweta Beds, Nyassaland. Placement uncertain. Probably Low Cistecephalus Assemblage Zone equivalent (= middle of the Wuchiapingian Stage). See Jacobs et al. 2005 for more on this species discussion on its age. Synonyms: Chiwetasaurus dixeyi Haughton, 1926
Gorgonops? kaiseri (Broili & Schroeder, 1934). A large (estimated total length about 35 cm long), incomplete skull, with a high snout and narrower in the rear than other species, from the "High Tapinocephalus zone" (i.e. earlier than the other species, most probably Pristerognathus Assemblage Zone) Synonyms: Pachyrhinos kaiseri Broili & Schroeder, 1934
Gorgonops? eupachygnathus (Watson, 1921) A flattened, incomplete, medium-sized skull, probably a juvenile of either G. torvus or G. whaitsi Synonyms: Scymnosuchus whaitsi Watson, 1921, Scymnognathus whaitsi (Watson, 1921), Leptotrachelus eupachygnathus WATSON, 1921, Leptotracheliscops eupachygnathus (Watson, 1921)

ENTELODONTS:THE TERMINATOR PIGS

Entelodonts, sometimes nicknamed hell pigs or terminator pigs,are an extinct family of pig-like omnivores endemic to forests and plains of North America, Europe, and Asia from the middle Eocene to early Miocene epochs (37.2—16.3 mya), existing for approximately 20.9 million years.

Entelodontidae was named by Richard Lydekker and assigned to Nonruminantia by Gregory (1910). Then assigned to Artiodactyla by Lucas et al.[4] (1998); and to Entelodontoidea by Carroll (1988) and Boisserie et al. (2005). While entelodonts have long been classified as members of the Suina, Spaulding et al. have found them to be closer to whales and hippos than to pigs.

[edit]ntelodonts are an extinct group of rather pig-like omnivorous mammals with bulky bodies but short, slender legs, and long muzzles. The largest were the North American Daeodon shoshonensis, the Entelodon and the Eurasian Paraentelodon intermedium, standing up to 2.1 metres (6.9 ft) tall at-shoulder, with brains the size of an orange.

A single specimen was recorded by M. Mendoza, C. M. Janis, and P. Palmqvist for body mass and was estimated to have a weight of 421 kg (930 lb).

Entelodonts had a full set of teeth, including large canines, heavy incisors, and relatively simple, yet powerful, molars. These features suggest an omnivorous diet, similar to that of modern pigs. Like many other artiodactyls, they had cloven hooves, with two toes touching the ground, and the remaining two being vestigial.

The most distinguishing characteristic of the animals, however, would have been the heavy, bony lumps on either side of their heads which are similar to a warthog's. Some of these may have been attachment points for powerful jaw muscles, but some were larger in males, suggesting that they may also have had a role in competition for mates.Entelodonts lived in the forests and plains where they were the apex predators of North America's Early Miocene and Oligocene, consuming carrion and live animals and rounding off their diet with plants and tubers. They would have hunted large animals, like the cow-sized artiodactyl Eporeodon major and the sheep-sized cameloid Poebrotherium wilsoni, dispatching them with a blow from their jaws. Some fossil remains of these other animals have been found with the bite marks of entelodonts on them. Like modern day pigs, they were omnivores, eating both meat and plants, but their adaptations show a bias towards live prey and carrion. They were most likely opportunists, mainly eating live animals, but not sneering at carrion and roots and tubers in times of drought. Some entelodonts even exhibited cacheing behavior, as an Archaeotherium's cache has been discovered, made up of the remains of several early camels.

Entelodonts appear in the third episode of the popular BBC documentary Walking with Beasts, where in the program, the narrator always refers to the creatures as "Entelodonts," rather than a more specific genus, such as Entelodon or Archaeotherium. The same creatures appear in another BBC production the 2001 remake of The Lost World. Entelodonts were also the main focus of Episode 4 of National Geographic Channels show Prehistoric Predators in an episode titled Killer Pig. The episode featured Archaeotherium (identified as "Entelodont") as being the top predator of the American Badlands, and how it evolved into the even larger Daeodon (aka "Dinohyus").

MEGALODON:THE KING OF THE OCEANS

Megalodon is an extinct species of shark that lived approximately 28 to 1.5 million years ago, during the Cenozoic Era (late Oligocene to early Pleistocene).
The taxonomic assignment of C. megalodon has been debated for nearly a century, and is still under dispute. The two major interpretations are Carcharodon megalodon (under family Lamnidae) or Carcharocles megalodon (under family Otodontidae). Consequently, the scientific name of this species is commonly abbreviated C. megalodon in the literature.
C. megalodon is regarded as one of the largest and most powerful predators in vertebrate history, and likely had a profound impact on the structure of marine communities.Fossil remains suggest that this giant shark reached a maximum length of 15.9–20.3 metres (52–67 ft), and also affirm that it had a cosmopolitan distribution.Scientists suggest that C. megalodon looked like a stockier version of the great white shark, Carcharodon carcharias.
According to Renaissance accounts, gigantic, triangular fossil teeth often found embedded in rocky formations were once believed to be the petrified tongues, or glossopetrae, of dragons and snakes. This interpretation was corrected in 1667 by Danish naturalist Nicolaus Steno, who recognized them as shark teeth, and famously produced a depiction of a shark's head bearing such teeth. He described his findings in the book The Head of a Shark Dissected, which also contained an illustration of a C. megalodon tooth.

The most common megalodon fossils are its teeth. Diagnostic characteristics include: triangular shape,robust structure, large size, fine serrations,and visible v-shaped neck.Megalodon teeth can measure over 180 millimetres (7.1 in) in slant height or diagonal length, and are the largest in size of any known shark species

Megalodon fossils have been excavated from many parts of the world, including Europe, Africa and both North and South America, as well as Puerto Rico, Cuba, Jamaica, Australia, New Zealand, Japan, Malta, Grenadines and India. Megalodon teeth have been excavated from regions far away from continental lands, such as the Mariana Trench in the Pacific Ocean.
The earliest megalodon remains were reported from late Oligocene strata, circa 28 million years old.Although fossils are mostly absent in strata extending beyond the Tertiary boundary, they have been reported from subsequent Pleistocene strata. It is believed that megalodon became extinct in the Pleistocene, probably about 1.5 million years ago.

Length


Reconstruction by Bashford Dean in 1909, with fossil teeth assembled from various localities.
The first attempt to reconstruct a megalodon jaw was made by Bashford Dean in 1909. From the dimensions of this jaw reconstruction, it was hypothesized that megalodon could have approached 30 metres (98 ft).Better knowledge of dentition and more accurate muscle structures, led to a rectified version of Dean's jaw model about 70 percent of its original size and to a size consistent with modern findings. To resolve such errors, scientists, aided by new fossil discoveries of megalodon and improved knowledge of its closest living analogue's anatomy, introduced more quantitative methods for estimating its size based on the statistical relationships between the tooth sizes and body lengths. Some methods are mentioned below.

Enamel height

In 1973, Hawaiian ichthyologist John E. Randall used a plotted graph to demonstrate a relationship between the enamel height (the vertical distance of the blade from the base of the enamel portion of the tooth to its tip) of the largest tooth in the upper jaw of the great white shark and its total length.[26][27] Randall extrapolated this method to estimate megalodon's total length. Randall cited two megalodon teeth in his work, specimen number 10356 at the American Museum of Natural History and specimen number 25730 at the United States National Museum, which had enamel heights of 115 millimetres (4.5 in) and 117.5 millimetres (4.63 in) respectively.These teeth yielded a corresponding total length of about 13 metres (43 ft).In 1991, Richard Ellis and John E. McCosker claimed that tooth enamel height does not necessarily increase in proportion to the animal's total length.

Largest anterior tooth height

In 1996, after scrutiny of 73 great white shark specimens, Michael D. Gottfried, Leonard Compagno and S. Curtis Bowman proposed a linear relationship between the height of the largest upper anterior tooth and total length in the great white shark. The proposed relationship is: total length in metres = − (0.096) × [UA maximum height (mm)]-(0.22). Gottfried and colleagues then extrapolated their technique to megalodon. The biggest megalodon tooth in the possession of this team was an upper second anterior specimen, whose maximum height was 168 millimetres (6.6 in). This tooth had been discovered by Compagno in 1993. It yielded an estimated total length of 15.9 metres (52 ft).Rumors of larger megalodon teeth persisted at the time. The maximum tooth height for this method is measured as a vertical line from the tip of the crown to the bottom of the lobes of the root, parallel to the long axis of the tooth. In layman's terms, the maximum height of the tooth is its slant height.

Root width

In 2002, shark researcher Clifford Jeremiah proposed that total length was proportional to the root width of an upper anterior tooth. He claimed that for every 1 centimetre (0.39 in) of width, there is approximately 4.5 feet (1.4 m) of the shark. Jeremiah pointed out that the jaw perimeter of a shark is directly proportional to its total length, with the width of the roots of the largest teeth being a proxy for estimating jaw perimeter. The largest tooth in the possession of Jeremiah had a root width of about 12 centimetres (4.7 in), which yielded 16.5 metres (54 ft) total length. Ward asserted that this method is based on a sound principle that works well with most large sharks.

Crown height

In 2002, paleontologist Kenshu Shimada of DePaul University proposed a linear relationship between tooth crown height and total length in great white sharks after conducting anatomical analysis of several specimens.This relationship is expressed as: total length in centimetres = a + bx, where a is a constant, b is the slope of the line and x is the crown height of tooth in millimetres. This relationship allowed any tooth to be used for the estimate. The crown height was measured as maximum vertical enameloid height on the labial side. Shimada pointed out that previously proposed methods were based on weaker evaluation of dental homology, and that the growth rate between the crown and root is not isometric, which he considered in his model. Furthermore, this relationship could be used to predict the total length of sharks that are morphologically similar to the great white shark, such as megalodon. Using this model, the upper anterior tooth (with maximum height of 168 millimetres (6.6 in)) possessed by Gottfried and colleagues corresponded to a total length of 15.1 metres (50 ft).In 2010, shark researchers Catalina Pimiento, Dana J. Ehret, Bruce J. MacFadden and Gordon Hubbell estimated the total length of megalodon on the basis of Shimada's method. Among the specimens found in the Gatun Formation of Panama, specimen number 237956 yielded a total length of 16.8 metres (55 ft).

Dentition and jaw mechanics

Reconstruction showing the position of the replacement teeth.
A team of Japanese scientists, T. Uyeno, O. Sakamoto, and H. Sekine, discovered and excavated partial remains of a megalodon, with a nearly complete associated set of its teeth, from Saitama, Japan in 1989.Another nearly complete associated megalodon dentition was excavated from Yorktown Formations of Lee Creek, North Carolina in the United States and served as the basis of a jaw reconstruction of megalodon at the American Museum of Natural History in New York City.These associated tooth sets solved the mystery of how many teeth would be in the jaws of the megalodon in each row. As a result, highly accurate jaw reconstructions became possible. More associated megalodon dentitions were found in later years. Based on these discoveries, scientists S. Applegate and L. Espinosa published an artificial dental formula (representation of dentition of an animal with respect to types of teeth and their arrangement within the animal's jaw) for megalodon in 1996. Most accurate modern megalodon jaw reconstructions are based on this dental formula.

SARCOSUCHUS:THE SUPERCROC

 Sarcosuchus, commonly called SuperCroc, is an extinct genus of crocodyliform and distant relative of the crocodile that lived 112 million years ago. It dates from the early Cretaceous Period of what is now Africa and South America and is one of the largest crocodile-like reptiles that ever lived. It was almost twice as long as the modern saltwater crocodile and weighed approximately 8 to 10 tonnes.

Until recently, all that was known of the genus was a few fossilised teeth and armour scutes, which were discovered in the Sahara Desert by the French paleontologist Albert-Félix de Lapparent, in the 1940s or 1950s. He called it the "Aoulef crocodile". However, in 1997 and 2000, American paleontologist Paul Sereno discovered half a dozen new specimens, including one with about half the skeleton intact and most of the spine. All of the other giant crocodyliformes are known only from a few partial skulls, so which is actually the biggest is an open question.When fully mature, Sarcosuchus is believed to have been as long as a city bus (11.2–12.2 metres or 37–40 ft) and weighed up to 8 tonnes (8.75 tons).[1] The largest living crocodyliform, the saltwater crocodile, is less than two-thirds of that length (6.3 meters or 20.6 ft is the longest confirmed individual) and a small fraction of the weight (1,200 kg, or 1.3 tons).

The very largest Sarcosuchus is believed to have been the oldest. Osteoderm growth rings taken from an 80% grown individual (based on comparison to largest individual found) suggest that Sarcosuchus kept growing throughout its entire 50–60 year average life span. True crocodiles grow at a rapid rate, reaching their adult size in about a decade, then growing more slowly afterward.

Its skull alone was as big as a human adult (1.78 m, or 5 ft 10 inches). The upper jaw overlapped the lower jaw, creating an overbite. The jaws were relatively narrow (especially in juveniles). The snout comprises about 75% of the skull's length.

The huge jaw contained 132 thick teeth; Larsson said they were like "railroad spikes".[citation needed] Instead of being narrow and adapted for slashing like the teeth of some land-dwelling carnivores, the teeth were conical, adapted for grabbing and holding, more like those of true crocodilians, which normally dispatch prey simply by holding them underwater until they drown. Sarcosuchus could probably exert a force of 80 kN (18,000 lbf) with its jaw, making it very unlikely that prey could escape.

It had a row of bony plates or osteoderms, running down its back, the largest of which were 1 m (3 ft) long. The scutes served as armour and may have helped support its great mass, but also restricted its flexibility.

Sarcosuchus also had a strange depression at the end of its snout. Called a bulla, it has been compared to the ghara seen in gharials. Unlike the ghara, though, the bulla is present in all Sarcosuchus skulls that have been found so far. This suggests it was not a sexually selected characteristic; only the male gharial has a ghara. The purpose of this structure remains enigmatic. Sereno and others asked various reptile researchers what their thoughts on this bulla were. Opinions ranged from it being an olfactory enhancer to being connected to a vocalization device.

The eye sockets of Sarcosuchus rotated upwards and were somewhat telescoped.This suggests that the animal probably spent most of its time with the majority of its body submerged, watching the shore for prey.

It seems likely that it ate the large fish and turtles of the Cretaceous. As the overhanging jaw and stout teeth are designed for grabbing and crushing, its primary prey may have been large animals and smaller dinosaurs, which it ambushed, dragged into the water, crushed, drowned and then tore apart.

It may have come into conflict with Suchomimus, an 11 m (36 ft) theropod dinosaur with a gharial-like snout, whose fossils were found in the same geological formation as Sarcosuchus. According to Sereno,[citation needed] "because the ancient animal was so large, it could easily handle huge dinosaurs, including the massive long-necked, small-headed sauropods that were common in that African region".

Other crocodilian biologists are skeptical of the animal's "giant killing" capabilities.[citation needed] The long, thin snout of Sarcosuchus was very similar to the thin snouts of the modern gharial, the false gharial and the slender-snouted crocodile, all of which are nearly exclusive fish-eaters and incapable of tackling large prey. This can be contrasted to both the modern Nile crocodile and the extinct Deinosuchus, both of which exhibit very broad, heavy skulls, suitable for dealing with large prey. This, coupled with the abundance of large, lobe-finned fish in its environment, leads many to suggest that, far from being a dinosaur killer, Sarcosuchus was simply a large piscivore, a scaled-up version of the modern gharial.

However, while the snout of juvenile Sarcosuchus strongly resembled modern narrow-snouted crocodiles in width, it expands dramatically in mature individuals.While still comparatively narrower than the snout of a Nile crocodile, the snout is still much wider than the snouts of crocodilians like the gharial. In addition, the teeth do not interlock, like those of mostly piscivorous crocodilians. This suggests that, like the Nile crocodile, it may have complemented a primarily fish diet with terrestrial animals, at least upon maturity.

It is pertinent to note, though, that the lobe-finned fish that shared the waters with Sarcosuchus were often in excess of 1.8 m (6 ft) long and 90 kg (200 lb) in weight.[citation needed] This raises the possibility of those adaptations, which seem to indicate large or moderate-sized terrestrial prey, may instead have been adaptations for dealing with exceptionally large fish (many species of which possessed a layer of osteoderms, for protection).

110 million years ago, in the Early Cretaceous, the Sahara was still a great tropical plain, dotted with lakes and crossed by rivers and streams that were lined with vegetation. Based on the number of fossils discovered, the aquatic Sarcosuchus was probably plentiful in these warm, shallow, freshwater habitats.

Unlike modern true crocodiles, which are very similar in size and shape to one another and tend to live in different areas; Sarcosuchus was just one of many crocodyliformes, of different sizes and shapes, all living in the same area.[5] Four other species of extinct crocodyliformes were also discovered in the same rock formation along with the Sarcosuchus, including a dwarf crocodile with a tiny, 8 cm (3 in) long skull.[5] They filled a diverse variety of ecological niches, instead of competing with each other for resources.

The Sarcosuchus remains are from several individuals and include a spine (vertebrae), limb bones, hip bones (a pelvic girdle), the bony armored plates that ran down its back (scutes) and more than a half-dozen skulls. Many crocodyliform skulls are thick and heavy.[citation needed] They tend to be found more frequently than the rest of the body.[citation needed] This is quite a contrast with dinosaurs, whose relatively fragile skulls rarely become part of the fossil record.The osteoderms of ancient reptiles have been used to determine age (Erickson & Brochu, 1999). Since they retain growth rings, like those found in trees (most other bones "suffer" remodeling with age, which destroys former growth rings), it is theoretically possible to count the age of the individual that the bones belonged to. One 80% grown specimen was discovered with 40 rings, indicating that it had lived for 40 years. This form of growth rate calculation has been somewhat controversial. Others (Schwimmer, 2002) have criticised this form of growth measurement, as annular rings are harder to determine in a creature that lives in an environment that is without extreme seasonality, such as the Mesozoic.

No skeleton was complete enough to measure directly, so the maximum length estimate was calculated by measuring the largest skull and comparing it to modern crocodiles. In modern crocodiles, the skull and body are the same proportion regardless of age or sex.The primary difference is that species with a long snout have larger heads in proportion to their bodies than species with relatively broad snouts. The length of Sarcosuchus was the average of the expected length of the narrow-snouted gharial and the intermediate-snouted saltwater crocodile, while the mass was the expected mass of the latter. Sereno also measured living crocodilians in India and Costa Rica and used that data in his analysis.

As part of a National Geographic Society Special, Greg Erickson of Florida State University, Kent Vliet of the University of Florida and Kristopher Lapping of Northern Arizona University provoked American alligators, at the St. Augustine Alligator Farm and Zoological Park in Florida, into biting a bar studded with piezoelectric sensors. The largest alligator they tested was able to exert a force of 9.45 kN (2,125 lbf). By comparing the force exerted by more than 60 animals, they were able to determine that the force exerted was proportional to the size of the animal, which allowed an estimate of the biting power of the Sarcosuchus. This force was calculated to be 80 kN (18,000 lbf). 

The fossils were discovered in Gadoufaoua, Niger in the Ténéré Desert, which is part of the Sahara. The first Sarcosuchus teeth and scutes were recovered by the French paleontologist Albert-Félix de Lapparent, in the 1940s or 1950s. It was 1964, however, before a skull was discovered by geologists and brought to the attention of paleontologist Philippe Taquet. He shipped it to Paris, where it was examined by France de Broin. Together, they formally named and described the species, in 1966, before returning the specimen to Niger.

In 1966, de Lapparent's niece France de Broin and Taquet named the creature Sarcosuchus imperator, which is dervived from sarco (meaning "flesh"), suchus (meaning "crocodile") and imperator (Latin, meaning "emperor"), i.e., the flesh crocodile emperor.[5] The holotype specimen is MNN 604.

The next major expedition was Paul Sereno's trip, in 1997 and the follow-up trip in 2000. He recovered partial skeletons, numerous skulls and 20 tons of assorted other fossils from the deposits of the Elrhaz Formation, which has been dated as late Aptian or early Albian stages of the late Cretaceous. It took about a year to prepare the Sarcosuchus remains. The discovery was then published on October 25, 2001, in the scientific journal Science by Paul C. Sereno of the University of Chicago and National Geographic's Explorer-in-Residence, Hans C. E. Larsson from Yale University and the University of Toronto (formerly a student at the University of Chicago), Christian Sidor of the New York College of Osteopathic Medicine in Old Westbury, New York and Boubé Gado of the Institut de Recherches en Sciences Humaines in Niamey, Niger.

Additional fossil material has been found from the area of Nalut in northwestern Libya. These fossils are from the Cabao Formation, which is likely to be Hauterivian to Barremian in age. Other vertebrates from the Cabao Formation include the hybodont shark Priohybodus, an abelisaurid, a baryonichine spinosaurid, Spinosaurus, and a large sauropod.

HYENODON:THE RAZOR-JAWS

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 hyenas.

The average weight of adult or subadult H. horridus, the largest North American species, is 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

SMILODON :THE SABER-TOOTH CAT

 Smilodon ( /ˈsmaɪlədɒn/), often called a saber-toothed cat or incorrectly a saber-toothed tiger, is an extinct genus of machairodonts. This saber-toothed cat was endemic to North and South America, living during the Pleistocene epoch (2.5 mya—10,000 years ago).

The nickname "saber-tooth" refers to the extreme length of their maxillary canines. Despite the colloquial name "saber-toothed tiger", Smilodon is not closely related to the tiger (or any other living felid); the latter belongs to subfamily Pantherinae, whereas Smilodon belongs to subfamily Machairodontinae. The name Smilodon comes from Greek: σμίλη, (smilē), "carving knife" + ὀδoύς (odoús), "tooth", or in the genitive: ὀδoύς, ὀδόντος, odóntosThe genus Smilodon was described by the Danish naturalist and palaeontologist 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. Genetic evidence suggests the members of the genus diverged from the main lineage of modern cats (subfamily Felinae) around 14-18 million years ago.

Smilodon gracilis, 2.5 million-500,000 years ago; the smallest and earliest species, estimated at 55 to 100 kg (120 to 220 lb) in weight 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. Matching the largest living cat, the Siberian tiger in size, it ranged from 160 to 280 kg (350 to 620 lb).[7] Sometimes two additional species are recognized, Smilodon californicus and Smilodon floridanus, but usually they are considered to be subspecies of Smilodon fatalis.

Smilodon populator, 1 million-10,000 years ago; occurred in the eastern parts of South America and it was perhaps the largest known felid with a body mass range of 220 to 360 kg (490 to 790 lb) possessing a massive chest and front legs, It was more than 1.40 m (55 in) high at the shoulder, 2.6 m (100 in) long on average and had a 30 cm (12 in) tail. Its upper canines reached 30 cm (12 in) and protruded up to 17 cm (6.7 in) out of the upper jaw.

The species of Smilodon were among the largest felids ever to live; the heaviest specimens of the massively built carnivore S. populator may have exceeded 400 kg (880 lb).

A fully-grown Smilodon weighed approximately 55 to 360 kg (120 to 790 lb), depending on species. It had a short tail, powerful legs, muscular neck and long canines. Smilodon was more robustly built than any modern cat, comparable to a bear. The lumbar region of the back was proportionally short, and the lower limbs were shortened relative to the upper limbs in comparison with modern pantherine cats, suggesting that Smilodon was not built for speed.

The largest species, the South American S. populator, had higher shoulders than hips and a back that sloped downwards, superficially recalling the shape of a hyena, in contrast to the level-backed appearance of S. fatalis, which was more like that of modern cats. However, while its front limbs were relatively long, their proportions were extremely robust and the forearm was shorter relative to the upper arm bone than in modern big cats, and proportionally even shorter than in S. fatalis. This indicates these front limbs were designed for power rather than fast running, and that S. populator would have had immense strength in its forequarters.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 saber-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.

Restoration by Charles R. Knight

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.A later study concurred, finding that the forelimbs of Smilodon, more than those of any modern felid, were capable of minimizing the struggles of prey and positioning them for a quick kill with the predator's canines without them fracturing.

Smilodon became extinct at the end of the Pleistocene around 10,000 BC, a time which saw the extinction of many other large herbivorous and carnivorous mammals.

Prehistoric humans, who reached North America at the same time and are known to have hunted many of the species that disappeared, are often viewed as responsible for this extinction wave. Others have suggested that the end of the ice age caused the extinction. As the ice sheets retreated there would have been changing vegetation patterns. Grasslands expanded. The summers became more extreme and parts of North America began to dry out. However, this hypothesis does not explain why Smilodon and its ancestors as well as other megafaunal species successfully survived many previous interglacials, and then fairly suddenly died out over the entire contiguous land area of North and South America.

TORVOSAURUS :THE JURASSIC KING

Torvosaurus is a genus of large theropod dinosaur that lived during the Late Jurassic period (mid-late Kimmeridgian stage in North America, early Tithonian stage in Europe). The nameTorvosaurus means "savage lizard" and is derived from the Latin torvus ("savage") and the Greek σαυρος/sauros ("lizard").

 TORVOSAURS UP
ALLOSAURS DOWN 

Fossilized remains of Torvosaurus have been found in North America and Portugal. Torvosaurus reached 9 to 11 meters (30 to 36 ft) in length and an estimated weight of about 2 metric tons (2.2 tons), which made it the largest carnivore of its time, except for Epanterias (which may have been just a big Allosaurus) andSaurophaganax.

It was first discovered by James A. Jensen and Kenneth Stadtman in the rocks of the Morrison Formation at the Dry Mesa Quarry, Colorado in 1972. The genus and the type species T. tanneri were named and described in 1979 by Peter M. Galton and Jensen. The type specimen from Colorado was further described by Brooks Britt, and the Portuguese specimen by O. Mateus and M.T. Antunes.The size of Torvosaurus is not definitely known because only incomplete material has been found, but it was a large theropod. North American Torvosaurus material is estimated as belonging to an individual about 9.0 meters long (29 ft), with a weight of about 1.95 metric tons (2.15 tons).Material from Portugal indicates even larger animals. In 2006 a nearly complete maxilla found in Portugal was assigned to Torvosaurus tanneri. It measured 63 cm in length (2.13 ft), significantly larger than the 47 cm (1.54 ft) long maxilla of the American specimen (total skull length 118 cm [3.87 ft]). Based on this, a skull length of 158 cm (5.18 ft) was estimated for the Portuguese specimen. This is comparable to the largest T. rex skulls, and makes Torvosaurus the largest known Jurassic theropod (surpassing Saurophaganax/Allosaurus maximus and Edmarka), and among the largest of all theropods. A partial femur from another individual belongs to an animal estimated as 11 m (36 ft) long.Torvosaurus is related to the earlier Megalosaurus but is seemingly more advanced. Its classification is still uncertain but its family Megalosauridae is most commonly held as a basal branch of Tetanurae, less derived than carnosaurs or coelurosaurs, and likely related to the spinosaurids.

KELENKEN :THE TERROR BIRD

Kelenken is an extinct genus of giant flightless predatory birds of the family Phorusrhacidae or "terror birds".

These birds lived in the Middle Miocene, some 15 million years ago, in Argentina along with Argentavis. With a skull 28 inches (71 cm) long (including its 18 inch, 45.7 cm beak), it had the largest head of any known bird. The tarsometatarsus was about 45 cm long .Kelenken was one of the largest carnivorous birds of all time, possibly reaching 2.28 to 3 meters (7.5 to 10 feet) tall and weighing around 500 pounds (230 kg).It is not entirely clear how Kelenken captured and killed its prey. As a large flightless carnivore, Kelenken likely chased down and killed its prey with several bone-shattering blows from its massive beak. Another possibility is that it may have picked up its prey item, then proceeded to shake it vigorously in order break its back. It is also possible that Kelenken may have been a scavenger, driving off other animals with its impressive size, and ferocious looks.

DUNKLEOSTEUS

Dunkleosteus (From "(David) Dunkle" + osteus (οστεος, Greek: bone)) is a genus of prehistoric fish, one of the largest arthrodire placoderms ever to have lived, existing during the Late Devonian period, about 380-360 million years ago.This hunter, measuring up to 10 metres (33 ft) and weighing 3.6 tonnes (4.0 short tons), was a hypercarnivorous apex predator. Few other placoderms, save, perhaps, its contemporary, Titanichthys, rivaled Dunkleosteus in size.Dunkleosteus is a member of the pachyosteomorph arthrodires, and is more specifically usually placed in the family Dinichthyidae, a family composed mostly of large, carnivorous arthrodires like Gorgonichthys. Anderson (2009) suggests that because of its primitive jaw structure Dunkleosteus should be placed outside the family Dinichthyidae, perhaps close to the base of the clade Pachyosteomorpha, near Eastmanosteus, but this idea has yet to be tested.New studies have revealed several features in both its food and biomechanics as its ecology and physiology. Placodermi first appeared in the Silurian, and the group became extinct during the transition from the Devonian to the Carboniferous, leaving no descendants. The class lasted barely 50 million years, in comparison to the 400 million year long history of sharks.

In recent decades, Dunkleosteus has achieved recognition in popular culture, with a large number of specimens on display, and notable appearances in entertainment media. Numerous fossils of some species have been found in North America,Poland, Belgium and Morocco.Due to its heavily armoured nature, Dunkleosteus was likely a relatively slow, but powerful, swimmer. It is thought to have dwelled in diverse zones of inshore waters. Fossilization tends to have preserved only the especially armoured frontal sections of specimens, and thus it is uncertain what exactly the hind sections of this ancient fish were like. As such, the reconstructions of the hindquarters are often based on smaller arthrodires, such as Coccosteus, that had hind sections preserved.

The most famous specimens of Dunkleosteus are displayed at the Cleveland Museum of Natural History. Others are displayed at the American Museum of Natural History and in the Queensland Museum in Brisbane, Queensland.Instead of teeth, Dunkleosteus possessed two pairs of sharp bony plates which formed a beak-like structure. After studying a biomechanical model of the fish's jaws, scientists at the Field Museum of Natural History and the University of Chicago concluded that Dunkleosteus had the second most powerful bite of any fish (the giant Megalodon being the strongest). Dunkleosteus could concentrate a pressure of up to 8,000 pounds per square inch (55 MPa) at the tip of its mouth, placing Dunkleosteus in the same league as Tyrannosaurus rex and modern crocodiles as having the most powerful known bite.Dunkleosteus could open its mouth in one-fiftieth of a second, which would have caused a powerful suction that pulled the prey into its mouth,a food-capture technique used by many fish today.

ORNITHOCHEIRUS: THE T-REX OF THE SKIES

Ornithocheirus (from Greek "ορνις", meaning bird, and "χειρ", meaning hand) was a pterosaur from the Cretaceous period of Europe and South America. Based on poor fossil material, the genus has caused enduring problems of zoological nomenclature.

Fossil remains currently classified as Ornithocheirus have been recovered mainly from the Cambridge Greensand of England, dating to the beginning of the Albian stage of the late Cretaceous period, about 110 million years ago.Additional fossils from the Santana Formationof Brazil, dating to 112-108 million years ago, have been classified as species of Ornithocheirus.The original material of Ornithocheirus simus, recovered from England, indicates a mid-sized species with a wing span of 2.5 m (8 ft). Referred specimens attributed to Ornithocheirus simus(alternately called Criorhynchus simus) can reach 5 m (16 ft). Tropeognathus mesembrinus is also usually considered a part of the Ornithocheirus genus as O. mesembrinus, and reached over 6 m (20 ft) in wingspan.Both O. simus and O. mesembrinus bore distinctive convex "keeled" crests on their snouts.The upper crests arose from the snout tip and extended back to the nostril. An additional, smaller crest projected down from the lower jaw at the symphysis ("chin" area). While many ornithocheirids had a small, rounded bony crest projecting from the back of the skull, this was particularly large and well-developed in Ornithocheirus.Unlike the related Anhanguera and Coloborhynchus, which had an expanded rosette of teeth at the jaw tips, Ornithocheirus had straight jaws that narrowed toward the tip. Also unlike related pterosaurs, the teeth of Ornithocheirus were mostly vertical, rather than set at an outward-pointing angle. They also had fewer teeth than related species.The type specimen of Ornithocheirus simus is represented only by a broken piece of the upper jaw tip. While it does preserve several characteristic features of Ornithocheirus, it is nearly identical to comparable bones in o. mesembrinus, making clear distinction between these two species impossible.During the 19th century, in England many fragmentary pterosaur fossils were found in the Cambridge Greensand, a layer from the early Cretaceous, that had originated as a sandy seabed. Decomposing pterosaur cadavers, floating on the sea surface, had gradually lost individual bones that sank to the bottom of the sea. Water currents then moved the bones around, eroding and polishing them, until they were at last covered by more sand and fossilised. Even the largest of these remains were damaged and difficult to interpret. They had been assigned to the genus Pterodactylus, as was common for any pterosaur species described in the early and middle 19th century.

A DYNASTY OF TYRANTS: DASPLETOSAURUS

Daspletosaurus meaning "frightful lizard) is a genus of tyrannosaurid theropod dinosaur that lived in western North America between 77 and 74 million years ago, during the Late Cretaceous Period. Fossils of the only named species (D. torosus) were found in Alberta, although other possible species from Alberta and Montana await description. Including these undescribed species, Daspletosaurus is the most species-rich genus of tyrannosaur.Daspletosaurus is closely related to the much larger and more recent Tyrannosaurus. Like most known tyrannosaurids, it was a multi-ton bipedal predator equipped with dozens of large, sharp teeth. Daspletosaurus had the small forelimbs typical of tyrannosaurids, although they were proportionately longer than in other genera.As an apex predator, aspletosaurus was at the top of the food chain, probably preying on large dinosaurs like the ceratopsid Centrosaurus and the hadrosaur Hypacrosaurus. In some areas, Daspletosaurus coexisted with another tyrannosaurid,Gorgosaurus, though there is some evidence of niche differentiation between the two. While Daspletosaurus fossils are rarer than other tyrannosaurids, the available specimens allow some analysis of the biology of these animals, including social behavior, diet and life history.While very large by the standard of modern predators, Daspletosaurus was not the largest tyrannosaurid. Adults could reach a length of 8–9 meters (26–30 ft) from snout to tail.Mass estimates have centered around 2.5 tonnes (2.75 short tons), but have ranged between 1.8 tonnes (2 tons) and 3.8 tonnes (4.1 tons).Daspletosaurus had a massive skull that could reach more than 1 meter (3.3 ft) in length. The bones were heavily constructed and some, including the nasal bones on top of the snout, were fused for strength. Large fenestrae (openings) in the skull reduced its weight. An adult Daspletosaurus was armed with about six dozen teeth that were very long but oval in cross section rather than blade-like. Unlike its other teeth, those in the premaxilla at the end of the upper jaw had a D-shaped cross section, an example of heterodonty always seen in tyrannosaurids. Unique skull features included the rough outer surface of the maxilla (upper jaw bone) and the pronounced crests around the eyes on the lacrimal,postorbital, and jugal bones. The orbit (eye socket) was a tall oval, somewhere in between the circular shape seen in Gorgosaurus and the 'keyhole' shape ofTyrannosaurus.The type specimen of Daspletosaurus torosus (CMN 8506) is a partial skeleton including the skull, the shoulder, a forelimb, the pelvis, a femur and all of the vertebrae from the neck, torso and hip, as well as the first eleven tail vertebrae. It was discovered in 1921 by Charles Mortram Sternberg, who thought it was a new species ofGorgosaurus. It was not until 1970 that the specimen was fully described by Dale Russell, who made it the type of a new genus, Daspletosaurus, from the Greek stems δασπλητo-/daspleto- ('frightful') and σαυρος/sauros('lizard'). The type species is D. torosus, which is Latin for 'muscular' or 'brawny.' Aside from the type, there is only one other well-known specimen, a complete skeleton discovered in 2001. Both specimens were recovered from the Oldman Formation in the Judith River Group of Alberta. A specimen from the younger Horseshoe Canyon Formation in Alberta has been reassigned to Albertosaurus sarcophagus. The Oldman Formation was deposited during the middle Campanian stage of the Late Cretaceous, from about 77 to 76 Ma (million years ago).

A DYNASTY OF TYRANTS: GORGONOSAURUS

Gorgosaurus meaning "fierce lizard" is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period, between about 76.5 and 75 million years ago. Fossil remains have been found in the Canadian province of Alberta and possibly the U.S. state of Montana. Paleontologists recognize only the type species, G. libratus, although other species have been erroneously referred to the genus.Like most known tyrannosaurids, Gorgosaurus was a bipedal predator weighing more than a metric ton as an adult; dozens of large, sharp teeth lined its jaws, while its two-fingered forelimbs were comparatively small. Gorgosaurus was most closely related to Albertosaurus, and more distantly related to the larger Tyrannosaurus. Gorgosaurus and Albertosaurus are extremely similar, distinguished mainly by subtle differences in the teeth and skull bones. Some experts consider G. libratusto be a species of Albertosaurus; this would make Gorgosaurus a junior synonym of that genus.Gorgosaurus lived in a lush floodplain environment along the edge of an inland sea. An apex predator, it was at the top of the food chain, preying upon abundant ceratopsids and hadrosaurs. In some areas, Gorgosaurus coexisted with another tyrannosaurid, Daspletosaurus. Though these animals were roughly the same size, there is some evidence of niche differentiation between the two. Gorgosaurus is the best-represented tyrannosaurid in the fossil record, known from dozens of specimens. These plentiful remains have allowed scientists to investigate its ontogeny, life history and other aspects of its biology.Gorgosaurus was smaller than Tyrannosaurus or Tarbosaurus, closer in size toAlbertosaurus and Daspletosaurus. Adults reached 8 or 9 meters (26 to 30 ft) from snout to tail.Paleontologists have estimated full-grown adults to weigh more than 2.4 tonnes (2.7 short tons). The largest known skull measures 99 centimeters (39 in) long, just slightly smaller than that of Daspletosaurus.As in other tyrannosaurids, the skull was large compared to its body size, although chambers within the skull bones and large openings (fenestrae) between bones reduced its weight. Albertosaurus and Gorgosaurus share proportionally longer and lower skulls than Daspletosaurus and other tyrannosaurids. The end of the snout was blunt, and the nasal and parietal bones were fused along the midline of the skull, as in all other members of the family. The eye socket was circular rather than oval or keyhole-shaped as in other tyrannosaurid genera. A tall crest rose from the lacrimal bone in front of each eye, similar to Albertosaurus and Daspletosaurus. Differences in the shape of bones surrounding the brain set Gorgosaurusapart from Albertosaurus.

A DYNASTY OF TYRANTS: ALECTROSAURUS

Alectrosaurus   is a genus of tyrannosaurid theropod dinosaur from the Late Cretaceous Period of Inner Mongolia. It was a bipedal carnivore with a body shape similar to its much larger relative,Tyrannosaurus rex. Alectrosaurus was much smaller though, most likely less than five meters (17 ft) long.

The generic name Alectrosaurus can also be translated as "alone lizard," and is derived from the Greek words alektros("unmarried") and sauros ("lizard"). At the time of its discovery, it was unlike any other Asian carnivore known. There is one named species (A. olseni), which is named in honor of George Olsen, who discovered the first specimens in 1923 on the thirdAmerican Museum of Natural History expedition to Mongolia. Both genus and species were named by American paleontologist Charles Gilmore in 1933. In a 2001 study conducted by Bruce Rothschild and other paleontologists, 23 foot bones referred to Alectrosaurus were examined for signs of stress fracture, but none were found.The holotype (AMNH 6554), or name-bearing specimen, of Alectrosaurus is a hind limb discovered in the Iren Dabasu Formation of the Inner Mongolia Autonomous Region (Nei Mongol Zizhiqu) of the People's Republic of China. Forelimb material in the type specimen was later found to be misidentified therizinosauroid remains.The age of this geologic formation is not clear, but is commonly cited as the Campanian stage of the Late Cretaceous Period, about 83 to 74 million years ago.

More material, including comparable hind limb material as well as skull and shoulder elements, has been referred to Alectrosaurus. These fossils were found in the Bayan Shireh Formation of Outer Mongolia, a formation which is also of uncertain age. It may possibly extend into the early Campanian, but recent estimates suggest it was deposited from Cenomanian through Santonian times. Iren Dabasu and Bayan Shireh dinosaur faunas are similar, but van Itterbeecka et al. claimed that the Iren Dabasu is probably Campanian-Maastrichtian in age and possibly correlated with the Nemegt Formation, so it is not surprising that a species ofAlectrosaurus would be found there.

Furthermore, several more partial skeletons may have been found in both Inner and Outer Mongolia. These remain undescribed as of early 2007.Alectrosaurus is undoubtedly a tyrannosauroid, but due to its fragmentary nature, there is presently very little confidence in restoring its relationships with other tyrannosauroids and many recent cladistic analyses have omitted it altogether. One study recovered Alectrosaurus at no less than eight equally parsimonious positions in a tyrannosauroid cladogram.Some paleontologists have considered Alectrosaurus olseni to be a species of Albertosaurus.