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Description

The Irdin Manha Formation of Inner Mongolia, thought to represent the late Lutetian-early Bartonian (45-40 mya), is mainly known for one (often misrepresented) animal, but its wildlife overall is quite interesting and more importantly, it encapsulates a pivotal chapter in early Paleogene mammal evolution; a faunal turnover following the end of the Paleocene–Eocene Thermal Maximum, the last hurrah of the “old guard” (mesonychids, oxyaenids, pantodonts, uintatheriids, etc.) and the dawn of a new reign of large mammals (entelodonts, nimravids, amynodontids, paraceratheres), as well as being a transitional period for some groups that have been around for a while but only now started getting big (such as hyaenodonts and brontotheres).

Irdin Manha showcases that transition well, housing herbivores such as the bizarre-looking (even by its family’s standards) uintatheriid Gobiatherium, possibly the last of its kind, and the coryphodontid Eudinoceras (though its family clung on into the Priabonian with the rhino-sized Hypercoryphodon), but these relics from a bygone age coexisted with newer faces, such as a diverse number of brontotheres like Protitan, Metatelmatherium, and the 1.5-ton Rhinotitan (albeit still half the size of the Priabonian Megacerops and Embolotherium), and basal midget paraceratheres such as Forstercooperia and Pappaceras, who were the size of goats, but their descendants during the Priabonian and subsequent Oligocene epoch would rank among the largest land mammals ever.

However, it’s the big carnivores of Irdin Manha that truly emphasize what a strange time this was, a time when carnivorous mammals ran on hooves. Throughout the early Paleogene (63-40 mya), the top predators across the Northern Hemisphere were mesonychids and oxyaenids, with the former being emblematic of how alien the oldest mammalian megafauna was. These were predatory ungulates, sporting hoof-like toes and long, thick tails akin to early artiodactyls and perissodactyls from the time, yet they were meat-eaters, otherwise similar in shape to canines, hyenas, and hyaenodonts. The large-bodied Ankalagon emerged around 63-60 mya, not long after the K-Pg extinction, followed shortly by Pachyaena (55-50 mya), whose species ranged from coyote-sized to possibly tiger-sized. One of the more complete and famous taxa is the type genus Mesonyx, representing several wolf-sized predators known from the late Ypresian-early Lutetian of North America (50-45 mya), including the diverse Bridger Formation.

Detailing the penultimate chapter of mesonychid history, Irdin Manha houses a number of taxa (some still undescribed) but the largest of them was Mongolonyx, known from skull and jaw material, and represented by two species; M. dolichognathus and M. robustus. The latter is known from a large skull stretching 20.5 inches, and depending on its proportions, it might have weighed circa 1,000 lb, as much as an Alaskan brown bear, but like its cousins, it likely resembled a giant canine but with primitive hooves instead of claws, a “sheep in wolves clothing”, if you will XD Needless to say, these beasts couldn’t grasp prey with hooves, so they relied entirely on their powerful jaws and large heads, making them essentially nature’s first crack at creating a “dog”, a general predatory body plan that would be replicated by other non-canine mammals down the line, like hyaenodonts, real hyenas and even the marsupial thylacine.

Mesonychids are almost always found together with oxyaenids, like Mesonyx with Patriofelis at Bridger and Pachyaena with Oxyaena and Palaeonictis at Willwood, so fittingly enough, Mongolonyx shared its habitat with the giant Sarkastodon. Speaking of these, if mesonychids were the early Paleogene equivalent of dogs, then oxyaenids would have been the “cats”, another standard predatory body plan that would independently emerge in several mammalian lineages. Like mesonychids, oxyaenids were common across North America and Eurasia, but they were members of Ferae, making them close to cats in the grand scheme of mammalian evolution, but still no closer to them and other carnivorans than they are to pangolins. Sarkastodon mongoliensis was the last and largest of its lineage, but it’s only known from partial skull material, though based on its more complete relatives; the lynx-sized Oxyaena and cougar-sized Patriofelis, it would have been a low-slung and robust beast, similar to a big cat but plantigrade like a bear (so an ambush predator), and sporting bone-crushing teeth akin to a hyena, and if similar in proportions to its cousins, it would have been about the size of a tiger, perhaps weighing up to 700 lb.

And lastly, we have the largest and most famous carnivore from Irdin Manha; Andrewsarchus mongoliensis, a giant meat-eating ungulate that is as famous as it is enigmatic. Its holotype was discovered during the AMNH-funded fossil-hunting expeditions across Central Asia led by Roy Chapman Andrews, after whom the animal is named. Like its two competitors, we haven’t found the body of Andrewsarchus, only a single huge skull, stretching nearly 3 feet in length. Andrewsarchus is considered to be a cetancodontamorph, a group whose only living representatives are whales and hippos, and it also includes entelodonts, which some consider the closest relatives of Andrewsarchus. More recently, Paratriisodon henanensis, known from a mandible, teeth, and pieces of the skull from the similar-aged Lushi Formation of Henan, has been suggested to be a synonym of A. mongoliensis, so we might have its whole head preserved, but still no postcrania.

Adding to this, no study has placed Andrewsarchus in any specific cetancodontamorph family, making it hard to try and phylogenetic bracket how the rest of its body could have looked like, with most reconstructions these days showing it as either an entelodont redux or as a kind of hippo-entelodont hybrid with a long tail. Andrewsarchus likely had an oversized head, much like other cetancodontamorphs from the Eocene; such as entelodonts, basal cetaceans, and anthracotheres (stem-hippos), but even with its family’s bobblehead syndrome in mind, this still would have been a huge mammal, likely the size of a cow. Due to the paucity of its remains, we can only guess how A. mongoliensis might have lived, but like entelodonts, or extant bears, it might have been a generalist who used its sheer size and bulk to steal the kills of smaller predators, including Sarkastodon and Mongolonyx, infrequently having to hunt its own prey because of it.

A carnivorous, terrestrial mutant-hippo, a giant wolf with hooves, and a tiger-wolverine would have made for a strange guild of apex predators, but as the climate continued to change (getting colder and drier), these unique creatures vanished and were replaced by a new cast of characters in both Eurasia and North America. The fossil record of Central Asia shows this transition very well, with Andrewsarchus getting replaced by its entelodont relatives, who during its day were small critters like Brachyhyops but by the upper Priabonian, they had evolved into the much larger Entelodon, including the cow-sized E. dirus, while mesonychids and oxyaenids got replaced by nimravids, proto-cats related to the true felids, and larger-bodied Hyaenodon species such as H. gigas (who grew as big as a large tiger) and H. mongoliensis, and these three groups would continue to dominate Eurasia and North America throughout the Oligocene, and only one mesonychid, Mongolestes, lived alongside them during the Priabonian, before disappearing.

Description

The Irdin Manha Formation of Inner Mongolia, thought to represent the late Lutetian-early Bartonian (45-40 mya), is mainly known for one (often misrepresented) animal, but its wildlife overall is quite interesting and more importantly, it encapsulates a pivotal chapter in early Paleogene mammal evolution; a faunal turnover following the end of the Paleocene–Eocene Thermal Maximum, the last hurrah of the “old guard” (mesonychids, oxyaenids, pantodonts, uintatheriids, etc.) and the dawn of a new reign of large mammals (entelodonts, nimravids, amynodontids, paraceratheres), as well as being a transitional period for some groups that have been around for a while but only now started getting big (such as hyaenodonts and brontotheres).

Irdin Manha showcases that transition well, housing herbivores such as the bizarre-looking (even by its family’s standards) uintatheriid Gobiatherium, possibly the last of its kind, and the coryphodontid Eudinoceras (though its family clung on into the Priabonian with the rhino-sized Hypercoryphodon), but these relics from a bygone age coexisted with newer faces, such as a diverse number of brontotheres like Protitan, Metatelmatherium, and the 1.5-ton Rhinotitan (albeit still half the size of the Priabonian Megacerops and Embolotherium), and basal midget paraceratheres such as Forstercooperia and Pappaceras, who were the size of goats, but their descendants during the Priabonian and subsequent Oligocene epoch would rank among the largest land mammals ever.

However, it’s the big carnivores of Irdin Manha that truly emphasize what a strange time this was, a time when carnivorous mammals ran on hooves. Throughout the early Paleogene (63-40 mya), the top predators across the Northern Hemisphere were mesonychids and oxyaenids, with the former being emblematic of how alien the oldest mammalian megafauna was. These were predatory ungulates, sporting hoof-like toes and long, thick tails akin to early artiodactyls and perissodactyls from the time, yet they were meat-eaters, otherwise similar in shape to canines, hyenas, and hyaenodonts. The large-bodied Ankalagon emerged around 63-60 mya, not long after the K-Pg extinction, followed shortly by Pachyaena (55-50 mya), whose species ranged from coyote-sized to possibly tiger-sized. One of the more complete and famous taxa is the type genus Mesonyx, representing several wolf-sized predators known from the late Ypresian-early Lutetian of North America (50-45 mya), including the diverse Bridger Formation.

Detailing the penultimate chapter of mesonychid history, Irdin Manha houses a number of taxa (some still undescribed) but the largest of them was Mongolonyx, known from skull and jaw material, and represented by two species; M. dolichognathus and M. robustus. The latter is known from a large skull stretching 20.5 inches, and depending on its proportions, it might have weighed circa 1,000 lb, as much as an Alaskan brown bear, but like its cousins, it likely resembled a giant canine but with primitive hooves instead of claws, a “sheep in wolves clothing”, if you will XD Needless to say, these beasts couldn’t grasp prey with hooves, so they relied entirely on their powerful jaws and large heads, making them essentially nature’s first crack at creating a “dog”, a general predatory body plan that would be replicated by other non-canine mammals down the line, like hyaenodonts, real hyenas and even the marsupial thylacine.

Mesonychids are almost always found together with oxyaenids, like Mesonyx with Patriofelis at Bridger and Pachyaena with Oxyaena and Palaeonictis at Willwood, so fittingly enough, Mongolonyx shared its habitat with the giant Sarkastodon. Speaking of these, if mesonychids were the early Paleogene equivalent of dogs, then oxyaenids would have been the “cats”, another standard predatory body plan that would independently emerge in several mammalian lineages. Like mesonychids, oxyaenids were common across North America and Eurasia, but they were members of Ferae, making them close to cats in the grand scheme of mammalian evolution, but still no closer to them and other carnivorans than they are to pangolins. Sarkastodon mongoliensis was the last and largest of its lineage, but it’s only known from partial skull material, though based on its more complete relatives; the lynx-sized Oxyaena and cougar-sized Patriofelis, it would have been a low-slung and robust beast, similar to a big cat but plantigrade like a bear (so an ambush predator), and sporting bone-crushing teeth akin to a hyena, and if similar in proportions to its cousins, it would have been about the size of a tiger, perhaps weighing up to 700 lb.

And lastly, we have the largest and most famous carnivore from Irdin Manha; Andrewsarchus mongoliensis, a giant meat-eating ungulate that is as famous as it is enigmatic. Its holotype was discovered during the AMNH-funded fossil-hunting expeditions across Central Asia led by Roy Chapman Andrews, after whom the animal is named. Like its two competitors, we haven’t found the body of Andrewsarchus, only a single huge skull, stretching nearly 3 feet in length. Andrewsarchus is considered to be a cetancodontamorph, a group whose only living representatives are whales and hippos, and it also includes entelodonts, which some consider the closest relatives of Andrewsarchus. More recently, Paratriisodon henanensis, known from a mandible, teeth, and pieces of the skull from the similar-aged Lushi Formation of Henan, has been suggested to be a synonym of A. mongoliensis, so we might have its whole head preserved, but still no postcrania.

Adding to this, no study has placed Andrewsarchus in any specific cetancodontamorph family, making it hard to try and phylogenetic bracket how the rest of its body could have looked like, with most reconstructions these days showing it as either an entelodont redux or as a kind of hippo-entelodont hybrid with a long tail. Andrewsarchus likely had an oversized head, much like other cetancodontamorphs from the Eocene; such as entelodonts, basal cetaceans, and anthracotheres (stem-hippos), but even with its family’s bobblehead syndrome in mind, this still would have been a huge mammal, likely the size of a cow. Due to the paucity of its remains, we can only guess how A. mongoliensis might have lived, but like entelodonts, or extant bears, it might have been a generalist who used its sheer size and bulk to steal the kills of smaller predators, including Sarkastodon and Mongolonyx, infrequently having to hunt its own prey because of it.

A carnivorous, terrestrial mutant-hippo, a giant wolf with hooves, and a tiger-wolverine would have made for a strange guild of apex predators, but as the climate continued to change (getting colder and drier), these unique creatures vanished and were replaced by a new cast of characters in both Eurasia and North America. The fossil record of Central Asia shows this transition very well, with Andrewsarchus getting replaced by its entelodont relatives, who during its day were small critters like Brachyhyops but by the upper Priabonian, they had evolved into the much larger Entelodon, including the cow-sized E. dirus, while mesonychids and oxyaenids got replaced by nimravids, proto-cats related to the true felids, and larger-bodied Hyaenodon species such as H. gigas (who grew as big as a large tiger) and H. mongoliensis, and these three groups would continue to dominate Eurasia and North America throughout the Oligocene, and only one mesonychid, Mongolestes, lived alongside them during the Priabonian, before disappearing.