ADREWSARCHUS: THE TERROR OF THE EOCENE

Andrewsarchus mongoliensis
, was a mammal that lived during the Eocene epoch, roughly between 45 and 36 million years ago. It had a long snout with large, sharp teeth and flat cheek teeth that may have been used to crush bones. Because Andrewsarchus is only known from a single skull, whether it was an active predator or a large scavenger is open to debate, as is its exact time range.
Andrewsarchus is named for the famous explorer and fossil hunter Roy Chapman Andrews. It was discovered in June 1923 by Kan Chuen Pao, a member of Andrews' expedition, at a site in the Gobi Desert in Mongolia known as Irdin Manha [variants: Erdeni-Mandal and Erdenemandal ('jeweled mandala')] on the third Asiatic expedition that was led by Andrews and sponsored by the American Museum of Natural History. The skull is now on display at the American Museum of Natural History in New York; the lower jaw was not found. It was classified in the clade Mesonychia due to the similarity in structure between its teeth and skull with those of other mesonychid species known from complete skeleton, however, much of this was based only on Osborn's original publication, and more recent studies have found it to have no special mesonychid affinities, instead grouping with various artiodactyl clades. Indeed one study (Spaulding et al.) has not only found them to be closer to entelodonts, but as kin to Cetancodonta in their Cetacodontamorpha.

The appearance and behavioral patterns of Andrewsarchus are virtually unknown and have been topics of debate among paleontologists ever since it was first discovered. All that is known about Andrewsarchus comes chiefly from the single meter-long skull found in Late Eocene sediments in what is now Mongolia. New theories indicate that the teeth of Andrewsarchus may have been blunt and uncharacteristic of predators. Its diet could have been more omnivorous than carnivorous, consisting of carrion, bones, rooted plants, or mollusks rather than freshly killed meat. As a scavenger, Andrewsarchus may have gained access to freshly killed carcasses by using its formidable size to scare away other smaller predators and scavengers. Until more fossil evidence that may provide insight into these areas of uncertainty is uncovered any reconstructions remain highly speculative.

Andrewsarchus possessed some of the strongest jaws ever evolved in a land mammal, able to bite through large bones if needed. To judge from its immense jaws, and the coastal location of the fossils, Andrewsarchus may have fed on beached primitive whales, shellfish and hard-shelled turtles, and contemporary large mammals at various periods during its existence. Toward the end of the Eocene very large mammals (such as the brontotheres) had evolved in the region of Central Asia.

Despite the enormous jaws and very sturdy teeth, Andrewsarchus did not have teeth adapted for the carnassial shear. Judging by its size, the animal most likely fed on large animals such as the extinct brontotheres, which were among the largest herbivorous mammals at the time, possibly both hunting them, and scavenging already dead carcasses.\
 If plant material was also eaten, Andrewsarchus would have had a lifestyle similar to entelodonts.
Due to the food requirements of Andrewsarchus, sources of large animals are thought to have been present in Central Asia during the Eocene, most likely on a year-round basis. When the Indian subcontinent collided with Asia during the Late Eocene/Early Oligocene, this event caused the uplift of the Himalaya mountains while closing off the eastern Tethys Ocean, thus changing weather patterns, and caused Central Asia to dry out, ultimately resulting in a dramatic faunal turnover. It is suggested that Andrewsarchus became extinct due to this orogeny.

DIRE WOLF: THE KING OF THE PACK

The dire wolf (Canis dirus "fearsome dog") is an extinct carnivorous mammal of the genus Canis related to the smaller extant gray wolf. It was most common in North America and South America from the Irvingtonian stage to the Rancholabrean stage of the Pleistocene epoch, living 1.80 Ma—10,000 years ago, persisting for approximately 1.79 million years.

Dire wolves' overpowering bite, 129% of the force of the modern gray wolf, could hold and subdue their prey. As inferred from their large bodies and carnivorous teeth, they often took on large prey or megafauna, made possible by traveling in packs. Dire wolves were not specialized hunters—they fed on whatever megafauna was abundant.
The recovery of material belonging to over 4,000 individuals at the La Brea Tar Pits suggests that dire wolves were social animals living in large groups


Whether the dire wolf originated in North America versus South America is the subject of controversy. Most paleontologists lean toward a North American origin for three reasons: first, more potential progenitors are present in the middle Pleistocene of North America; second, distribution of C. dirus is much better represented in North America, with 136 sites versus only three localities in South America; and last, C. dirus appears earlier in the fossil record in North America than South America. A North American origin implies that C. dirus migrated into South America from North and Central America.


The fossil record suggests that the genus Canis diverged from the small, foxlike Leptocyon in North America sometime in the Late Miocene epoch 9 to 10 million years (Ma) ago, along with two other genera, Urocyon, and Vulpes. Canids soon spread to Asia and Europe (8 Ma BP) and became the ancestors of modern wolves, jackals, foxes, and the raccoon dog.
By 3–5 Ma BP, canids had spread to Africa (Early Pliocene) and South America (Late Pliocene). Their invasion of South America as part of the Great American Interchange was enabled by the formation of the Isthmus of Panama 3 Ma ago.
Over the next nine million years, extensive development and diversification of the North American wolves took place by the Middle Pleistocene. Canis armbrusteri appeared, possibly from C. chihliensis in Asia. There is good evidence that the dire wolf evolved from C. armbrusteri, with the two taxa sharing in the open plains and grasslands of what is now the central United States.
C. dirus eventually displaced C. armbrusteri, with the latter's final range shrinking to what is now the southeastern U.S., more specifically Florida. While this occurred, C. dirus expanded its range to include that of C. armbrusteri and moved into Central America and South America, appearing in the Late Pleistocene fossil record in northwestern South America.
Two subspecies of the dire wolf are known to have inhabited what is now the United States. C. dirus guildayi was smaller and ranged west of the Rocky Mountains. C. dirus dirus was larger and ranged east of the Rockies.
Although it was closely related to the gray wolf and other sister species, C. dirus is not the direct ancestor of any modern species. Unlike the gray wolf, which is of Eurasian origin, the dire wolf evolved on the North American continent, along with the coyote. The dire wolf co-existed with the gray wolf in North America for about 100,000 years.

The dire wolf's teeth were similar to the gray wolf's, only slightly larger, pointing to a hypercarnivorous to mesocarnivorous activity. Paleontologist R.M. Nowak states the dietary characteristics are primarily carnivorous, as well as partially omnivorous.
The slicing teeth (P4, the carnassial) on the upper jaw of C. dirus are larger than those of the gray wolf, but those on the lower jaw are similar. The temporalis of the dire wolf could generate more force than seen in modern gray wolves, suggesting stronger killing bites.

Many paleontologists have proposed that the dire wolf may have used its relatively large teeth to crush bone, an idea supported by the frequency of large amounts of wear on the crowns of their fossilized teeth. The upper carnassial had much larger blade than that of the gray wolf, indicating greater slicing ability. It had a longer temporal fossa and broader zygomatic arches, indicating the presence of a large temporalis muscle capable of generating slightly more force than a gray wolf's. However, other scientists have noted the dorsoventral and labiolingual force profiles are indistinguishable from those of other canids, such as coyotes and African wild dogs, indicating a similar diet.
Dire wolf teeth lacked the craniodental adaptations of habitual bonecrushers such as hyenas and borophagines. The dorsoventrally weak symphyseal region indicates it killed in a manner similar to its modern relatives, by delivering a series of shallow bites, strongly indicating pack hunting behaviour. However, the incidence of broken postcarnassial molars is much higher than in fossil gray wolves, indicating the species was probably less adapted to bone crushing than the gray wolf.

The dire wolf was one of the abundant Pleistocene megafauna—a wide variety of large mammals that lived during the Pleistocene. Approximately 10,000 years ago the dire wolf became extinct along with most other North American megafauna.[20] This timing can only be approximated because very few radiocarbon dates are directly associated with the dire wolf.
During the Late Pleistocene (300,000 years ago) the gray wolf (C. lupus) crossed into North America on the Bering Strait land bridge and competed with the dire wolf. Starting about 16,000 years ago, coinciding with the end of the last glacial period and the arrival of humans in North America, most of the large mammals upon which the dire wolf depended for prey began to die out (possibly as a result of climate and/or human-induced changes as suggested in a 2008 National Geographic Channel documentary.
Slower than the other wolf species on the continent at the time, primarily the gray wolf and red wolf, the dire wolf could not hunt the swifter species that remained and was forced to subsist by scavenging. By approximately 10,000 years ago, the large mammals and the dire wolf were extinct. In order to fully understand the extinction of C. dirus, many more dire wolf specimens must be directly dated. In addition to this, more information must be gathered on the factors that affected its biogeographical range and population size, including competition, interactions with predators and prey, its physical environment, as well as how all of its competitors and prey responded to the event of time; thus, the timing of extinction of megafauna that closely interacted with C. dirus must be determined.

TASMANIAN TIGER

The thylacine was the largest known carnivorous marsupial of modern times. It is commonly known as the Tasmanian tiger (because of its striped back) or the Tasmanian wolf. Native to continental Australia, Tasmania and New Guinea, it is thought to have become extinct in the 20th century. It was the last extant member of its family, Thylacinidae, although several related species have been found in the fossil record dating back to the early Miocene.
The thylacine had become extremely rare or extinct on the Australian mainland before European settlement of the continent, but it survived on the island of Tasmania along with several other endemic species, including the Tasmanian devil. Intensive hunting encouraged by bounties is generally blamed for its extinction, but other contributing factors may have been disease, the introduction of dogs, and human encroachment into its habitat. Despite its official classification as extinct, sightings are still reported, though none has been conclusively proven.
Like the tigers and wolves of the Northern Hemisphere, from which it obtained two of its common names, the thylacine was an apex predator. As a marsupial, it was not closely related to these placental mammals, but because of convergent evolution it displayed the same general form and adaptations. Its closest living relative is thought to be either the Tasmanian devil or numbat. The thylacine was one of only two marsupials to have a pouch in both sexes (the other being the water opossum). The male thylacine had a pouch that acted as a protective sheath, covering the male's external reproductive organs while he ran through thick brush. It has been described as a formidable predator because of its ability to survive and hunt prey in extremely sparsely populated areas.

The modern thylacine first appeared about 4 million years ago. Species of the family Thylacinidae date back to the beginning of the Miocene; since the early 1990s, at least seven fossil species have been uncovered at Riversleigh, part of Lawn Hill National Park in northwest Queensland.[9][10] Dickson's Thylacine (Nimbacinus dicksoni) is the oldest of the seven discovered fossil species, dating back to 23 million years ago. This thylacinid was much smaller than its more recent relatives. The largest species, the powerful thylacine (Thylacinus potens) which grew to the size of a wolf, was the only species to survive into the late MioceneIn late Pleistocene and early Holocene times, the modern thylacine was widespread (although never numerous) throughout Australia and New Guinea.

An example of convergent evolution, the thylacine showed many similarities to the members of the dog family, Canidae, of the Northern Hemisphere: sharp teeth, powerful jaws, raised heels and the same general body form. Since the thylacine filled the same ecological niche in Australia as the dog family did elsewhere it developed many of the same features. Despite this, it is unrelated to any of the Northern Hemisphere predators.

Descriptions of the thylacine vary, as evidence is restricted to preserved joey specimens, fossil records, skins and skeletal remains, black and white photographs and film of the animal in captivity, and accounts from the field.

The thylacine resembled a large, short-haired dog with a stiff tail which smoothly extended from the body in a way similar to that of a kangaroo. Many European settlers drew direct comparisons with the hyena, because of its unusual stance and general demeanour. Its yellow-brown coat featured 13 to 21 distinctive dark stripes across its back, rump and the base of its tail, which earned the animal the nickname, "Tiger". The stripes were more marked in younger specimens, fading as the animal got older.One of the stripes extended down the outside of the rear thigh. Its body hair was dense and soft, up to 15 mm (0.6 in) in length; in juveniles the tip of the tail had a crest. Its rounded, erect ears were about 8 cm (3.1 in) long and covered with short fur. Colouration varied from light fawn to a dark brown; the belly was cream-coloured.

The mature thylacine ranged from 100 to 130 cm (39 to 51 in) long, plus a tail of around 50 to 65 cm (20 to 26 in).The largest measured specimen was 290 cm (9.5 ft) from nose to tail. Adults stood about 60 cm (24 in) at the shoulder and weighed 20 to 30 kg (40 to 70 lb).There was slight sexual dimorphism with the males being larger than females on average.

The female thylacine had a pouch with four teats, but unlike many other marsupials, the pouch opened to the rear of its body. Males had a scrotal pouch, unique amongst the Australian marsupials, into which they could withdraw their scrotal sac

The thylacine was able to open its jaws to an unusual extent: up to 120 degrees. This capability can be seen in part in David Fleay's short black-and-white film sequence of a captive thylacine from 1933. The jaws were muscular but weak and had 46 teeth.

The thylacine's footprint is easy to distinguish from those of native and introduced species.

Thylacine footprints could be distinguished from other native or introduced animals; unlike foxes, cats, dogs, wombats or Tasmanian devils, thylacines had a very large rear pad and four obvious front pads, arranged in almost a straight line. The hindfeet were similar to the forefeet but had four digits rather than five. Their claws were non-retractable.

The early scientific studies suggested it possessed an acute sense of smell which enabled it to track prey,but analysis of its brain structure revealed that its olfactory bulbs were not well developed. It is likely to have relied on sight and sound when hunting instead. Some observers described it having a strong and distinctive smell, others described a faint, clean, animal odour, and some no odour at all. It is possible that the thylacine, like its relative, the Tasmanian devil, gave off an odour when agitated.

The thylacine was noted as having a stiff and somewhat awkward gait, making it unable to run at high speed. It could also perform a bipedal hop, in a fashion similar to a kangaroo—demonstrated at various times by captive specimens.Guiler speculates that this was used as an accelerated form of motion when the animal became alarmed. The animal was also able to balance on its hind legs and stand upright for brief periods.

Observers of the animal in the wild and in captivity noted that it would growl and hiss when agitated, often accompanied by a threat-yawn. During hunting it would emit a series of rapidly repeated guttural cough-like barks (described as "yip-yap", "cay-yip" or "hop-hop-hop"), probably for communication between the family pack members. It also had a long whining cry, probably for identification at distance, and a low snuffling noise used for communication between family members.

The thylacine was exclusively carnivorous. Its stomach was muscular with an ability to distend to allow the animal to eat large amounts of food at one time, probably an adaptation to compensate for long periods when hunting was unsuccessful and food scarce.Analysis of the skeletal frame and observations of it in captivity suggest that it preferred to single out a target animal and pursue that animal until it was exhausted. Some studies conclude that the animal may have hunted in small family groups, with the main group herding prey in the general direction of an individual waiting in ambush. Trappers reported it as an ambush predator.

Little is known of the thylacine's diet and feeding behaviour. Prey is believed to have included kangaroos, wallabies and wombats, birds and small animals such as potoroos and possums. One prey animal may have been the once common Tasmanian emu. The emu was a large, flightless bird which shared the habitat of the thylacine and was hunted to extinction around 1850, possibly coinciding with the decline in thylacine numbers. Both dingoes and foxes have been noted to hunt the emu on the mainland. European settlers believed the thylacine to prey upon farmers' sheep and poultry.Throughout the twentieth century, the thylacine was often characterised as primarily a blood drinker; according to Robert Paddle, the story's popularity seems to have originated from a single second-hand account heard by Geoffrey Smith in a shepherd's hut. In captivity, thylacines were fed a wide variety of foods, including dead rabbits and wallabies as well as beef, mutton, horse, and occasionally poultry. Tasmania's leading naturalist Michael Sharland published an article in 1957 stating that a captive thylacine refused to eat either dead wallaby flesh or to kill and eat a live wallaby offered to it, but "ultimately it was persuaded to eat by having the smell of blood from a freshly killed wallaby put before its nose."

 A 2011 study by the University of New South Wales using advanced computer modelling indicated that the thylacine had surprisingly feeble jaws. Animals usually take prey close to their own body size, but an adult thylacine of around 30 kilograms (66 lb) was found to be incapable of handling prey much larger than 5 kilograms (11 lb). Researchers believe thylacines only ate small animals such as bandicoots and possums, putting them into direct competition with the Tasmanian devil and the Tiger Quoll. Such specialisation probably made the thylacine susceptible to small disturbances to the ecosystem.

SEBECUS: THE GOD CROCODILE

Sebecus (meaning "Sebek" in Latin) is an extinct genus of sebecid crocodylomorph from the Eocene of South America. Fossils have been found in Patagonia. Like other sebecosuchians, it was entirely terrestrial and carnivorous. The genus is currently represented by a single species, the type S. icaeorhinus. Several other species have been referred to Sebecus, but were later reclassified as their own genera.

The name Sebecus is a Latinisation of Sebek (also called Sobek), the crocodile god of ancient Egypt. Sebek was considered an alternative to the Greek χάμψαι, or "champsa" in crocodilian nomenclature (the Greek historian Herodotus claimed that champsa was the Egyptian word for crocodile).The specific name icaeorhinus of the type species is derived from the Greek words εικαίοs and ρύγχος. Εικαίοs means "random" or "not according to plan" and ρύγχος means "snout", in reference to the animal's unusually deep snout

Named by American paleontologist George Gaylord Simpson in 1937, Sebecus was one of the first known sebecosuchians. Simpson described the type species, S. icaeorhinus, from a fragmented skull and lower jaw. The specimen was discovered by the American Museum of Natural History's First Scarritt Expedition to Patagonia.[2] Teeth had been known since 1906 when Argentine paleontologist Florentino Ameghino associated them with carnivorous dinosaurs. The more complete material found by Simpson firmly established that the new animal was a crocodyliform. Although Simpson's fossil was considered one of the best finds of the expedition, Simpson described the genus only briefly in 1937. He noted its unusual ziphodont dentition in which the teeth were laterally compressed and serrated. Simpson was preparing a more detailed monograph on the genus, but entered the United States Army before its completion. Another American paleontologist, Edwin Harris Colbert, completed Simpson's work, thoroughly describing the genus and placing it in a new family, Sebecidae. Colbert placed Sebecus and the Cretaceous baurusuchid Baurusuchus (also from South America) in the suborder Sebecosuchia (erected by Simpson for all ziphodont crocodylomorphs), as both had deep snouts and ziphodont teeth.

In 1965, American paleontologist Wann Langston, Jr. named a second species, S. huilensis, from the Miocene La Venta Formation in Columbia.S. huilensis was named on the basis of skull fragments.The deposits are Friasian in age (about 15 million years old), extending the range of the genus into the Neogene by around 40 million years. In 1977, remains were described from the Miocene of Peru.

A third species of Sebecus, S. querejazus, was named in 1991 from the early Paleocene Santa Lucia Formation in Bolivia. This extended the range of Sebecus back to the beginning of the Paleogene, soon after the Cretaceous–Paleogene extinction event. In 1993, Gasparini et al. described Sebecus carajazus. This was not a fourth species but a misspelling, or lapsus calami, of Sebecus querejazus.

A 2007 study of sebecids reclassified several species. The two species S. huilensis and S. querejazus were given their own genera, Zulmasuchus and Langstonia, respectively. Langstonia huilensis, named after Langston, was distinguished from Sebecus by its narrower snout and widely spaced teeth. Zulmasuchus querejazus, named after Zulma Gasparini, one of the authors of the study, differs from Sebecus in its wider snout..

The postcranial skeleton of S. icaeorhinus was virtually unknown until Pol et al. (2012) described postcranial remains of several individuals of this species, including a partially articulated specimen MPEF-PV 1776 with anterior region of the dentary (allowing the identification of this individual as representing S. icaeorhinus) and most of the postcranial skeleton preserved. Estimates of total body length and mass of MPEF-PV 1776 vary from 2.2 to 3.1 m, and from 52.2 to 113.5 kg, respectively. The postcranial skeleton of Sebecus provides additional evidence of its terrestriality. Its limbs, especially femora, were proportionally longer than limbs of living crocodilians; the shoulder-to-hip length of its body can be estimated at 2.3 times the length of the femur - similar to another, unrelated terrestrial crocodylomorph, Pristichampsus, while American alligators have proportionally shorter femora.

Unusual among crocodyliforms, Sebecus has a deep, narrow snout. The nares, or nostrils, open anteriorly at the tip of the snout. While most crocodilians have flat skulls that are raised near the eyes and postorbital region behind the eyes, the skull of Sebecus is essentially level. The great depth of the snout makes most of the length of its upper margin level with the margin of the orbits, or eye sockets. The supratemporal fenestrae, two holes on the skull table, are relatively small.

Top view of a cast of the skull of Sebecus icaeorhinus (AMNH 3160)

Laterally compressed, or ziphodont teeth, are characteristic of Sebecus and other sebecosuchians. Although the teeth vary in size, they are homodont, having a similar shape throughout the jaw. At the tips of the upper and lower jaws, the teeth are rounder in cross-section. The fourth dentary tooth is raised in the lower jaw to form an effective canine. The foremost teeth of the lower jaw are much smaller and lower than the fourth tooth. At the tip of the jaw the first dentary tooth is procumbent, or directed forward. The teeth of the upper and lower jaws form an alternate pattern to allow the jaw to close tightly. A notch is present between the maxilla and premaxilla bones of the upper jaw, accommodating the fourth dentary tooth when the jaw is closed. The procumbent first dentary teeth fit between the first and second premaxillary teeth. This close fit allows the serrated edges of the teeth shear with one another.

The articulation between the articular and quadrate bones at the jaw joint is well developed. Along with the broad downturned "wings" formed by the pterygoid and ectopterygoid bones at the bottom of the skull, this articulation restricts the jaw to up-and-down movement. The jaw movement and close shearing of the teeth suggest that Sebecus was carnivorous. Its compressed, blade-like teeth would have been well-suited for cutting meat. In contrast, living crocodilians have circular, widely spaced teeth and usually consume their food in large pieces. Sebecus likely consumed food in a manner more similar to theropod dinosaurs than living crocodilians. In particular, the teeth of tyrannosaurids bear the closest resemblance to those of Sebecus..Both animals have serrated teeth with rounded projections called denticles, and sharp clefts between the denticles called diaphyses. These diaphyses compress meat fibers between the serrations and rip them apart. Ultrastructural analyses using electron microscopes have revealed microwear scratches on the teeth that are suggestive of this form of cutting.

A cast of the skull roof and jaws of Sebecus icaeorhinus (AMNH 3160)

Colbert's monograph on Sebecus included a description of the brain, Eustachian tubes, and jaw musculature. Details of these soft tissues were inferred from characteristics of the skull and endocasts, or molds of its interior. The deep snout of Sebecus makes the shape of its brain somewhat different from those of living crocodiles, although its structure is the same. The olfactory bulb is elongate and makes up a significant portion of the brain. The cerebrum is narrow and long in comparison to crocodilians, and tapers toward the olfactory bulb. The temporal lobes are somewhat smaller than those of living crocodilians. Colbert interpreted the smaller cerebrum of Sebecus as a sign of primitiveness, with an evolutionary trend toward larger brain size in crocodilians. Hans C. E. Larsson performed a 2001 study of the endocranial anatomy of the dinosaur Carcharodontosaurus saharicus, comparing the ratio of its cerebrum to its total brain volume to the ratios of other prehistoric reptiles. Larsson found that Sebecus and Allosaurus fragilis had similar ratios to C. saharicus, falling within the 95% confidence range characterizing living reptile species.

 The Eustachian tubes, passages that connect the middle ear with the pharynx, are very complex in crocodilians. Unlike those of other vertebrates, the tubes are made of several interconnected branches. This branching is fully developed in Sebecus, and probably appeared much earlier in crocodylomorphs.

The jaw muscles of Sebecus were likely similar to those of living crocodilians, but the distinctively deep skull of Sebecus indicates that the muscles were longer. In Sebecus, the shape of the skull and jaws provides more room for adductor muscles, or muscles that close the jaws. The supratemporal fenestrae at the top of the skull are relatively wide, allowing for the passage of large muscles. In crocodilians, the depressor muscle that opens the jaws originates near the top of the skull and inserts into a projection at the back of the jaw called the retroarticular process. Living crocodilians have a straight retroarticular process at the back of the jaw and a low point of origin for the depressor muscle. Sebecus, with its deeper skull, has a higher point of origin for the depressor, but the retroarticular process curves upward to make the length of the depressor about the same as it is in crocodilians. Like living crocodilians, the depressor muscle of Sebecus was relatively underdeveloped. Therefore, while the closure of the jaws would have been very strong, the ability to open the jaws would be much weaker.

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.