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.