''Achaenodon'' is an extinct

artiodactyl The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

, possibly belonging to the eloiids. It lived in the Mid-To-Upper

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

(about 43-39 million years ago) and its fossil remains have been found in

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

Description

This animal was large and looked vaguely like a

wild boar The wild boar (''Sus scrofa''), also known as the wild swine, common wild pig, Eurasian wild pig, or simply wild pig, is a suid native to much of Eurasia and North Africa, and has been introduced to the Americas and Oceania. The species is ...

, but the size of an

American black bear The American black bear (''Ursus americanus''), also called simply a black bear or sometimes a baribal, is a medium-sized bear endemic to North America. It is the continent's smallest and most widely distributed bear species. American black bear ...

(''Ursus americanus''): it was about two meters long and weighed about 285 kilograms. Achaenodon's snout was very short and sturdy, with large sideways expanded

zygomatic arches In anatomy, the zygomatic arch, or cheek bone, is a part of the skull formed by the zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the opening of the ear) and the temporal process of the zygomati ...

, making the skull extremely wide. There was a very high

sagittal crest A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and reptilian skulls, among others. The presence of this ridge of bone indicates that there are exceptiona ...

, which connected later to an expanded

nuchal crest The nuchal lines are four curved lines on the external surface of the occipital bone: * The upper, often faintly marked, is named the highest nuchal line, but is sometimes referred to as the Mempin line or linea suprema, and it attaches to the epi ...

; these two structures, together with the large cheekbone arches, indicate that the musculature of the jaws was extremely powerful. ''Achaenodon''s teeth were characterized by large curved

canines Canine may refer to: Zoology and anatomy * a dog-like Canid animal in the subfamily Caninae ** ''Canis'', a genus including dogs, wolves, coyotes, and jackals ** Dog, the domestic dog * Canine tooth, in mammalian oral anatomy People with the surn ...

and large post-dumping teeth with a

bunodont The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...

structure. The dental formula was typical of the

artiodactyls The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...

with three incisors, a canine, four premolars and three molars; the first lower premolar was present in juveniles and, growing up, was expelled due to the growth of the canine. The upper molars were square in shape and equipped with four large conical cusps, surrounded by sturdy precingulation and postcingulation and extraordinarily thickened enamel. One particular species (A. frendi) still possessed protoconule and hypoconus, which disappeared in the other achenodonts. Achaenodon''s front legs were short and the hand had four fingers. The hind legs were slightly longer.

Classification

First described in 1893 by

Edward Drinker Cope Edward Drinker Cope (July 28, 1840 – April 12, 1897) was an American zoologist, paleontologist, comparative anatomist, herpetologist, and ichthyologist. Born to a wealthy Quaker family, Cope distinguished himself as a child prodigy interested ...

, ''Achaenodon'' is best known for fossil remains from the Wyoming Middle

. The type species is ''Achaenodon insolens'', but other species (''A. robustus'', ''A. uintense'', ''A. frendi'') are also known, from

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

Oregon Oregon () is a U.S. state, state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington (state), Washington, while the Snake River delineates much of it ...

Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to it ...

, and

Wyoming Wyoming () is a U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the south ...

. The affinities of ''Achaenodon'' are unclear: for a long time this animal has been classified among the entelodontids, the so-called "terror pigs" typical of the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

and

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

; this classification was mainly due to the large size of ''Achaenodon'' and the similarities in the particularly massive teeth. In fact, the similarities between ''Achaenodon'' and entelodonts were mainly due to evolutionary convergence and the development of characteristics due to the large size It is likely, however, that ''Achaenodon'' was a highly specialized representative of the heloiids (

Helohyidae Helohyidae were a group of artiodactyl mammals. They were most prominent in the mid-to-upper Eocene (~50 to 39 million years ago). Description Helohyidae share vague similarities to present-day pigs, though were slimmer in build. They possesse ...

), a group of primitive artiodactyls typical of the North American and Asian Eocene, usually much smaller in size. ''Achaenodon'', if it had actually been a giant eloiid, would also have been the last representative of the family (except the Mexican ''Simojovelhyus'', dating back to the Upper Oligocene). An animal closely related to ''Achaenodon'' is ''Parahyus'', of smaller size. Other studies have indicated that Achaenodon could be a baseline representative of the

Cetancodontamorpha Cetancodontamorpha is a total clade of artiodactyls defined, according to Spaulding ''et al''., as Whippomorpha "plus all extinct taxa more closely related to extant members of Whippomorpha than to any other living species". Attempts have been m ...

group.

Paleoecology

The large size of ''Achaenodon'' made it a difficult target for middle Eocene carnivores. ''Achaenodon'' was the first large artiodactyl, and remained the only artiodactyl to be part of the megafauna throughout the Eocene; even after the disappearance of this animal, there were no other large artiodactyls for at least 5 million years, with the appearance of ''

Archaeotherium ''Archaeotherium'' ( grc, αρχαιοθήριον, meaning "ancient beast") is an extinct genus of entelodont artiodactyl endemic to North America during the Eocene and Oligocene epochs (35—28 mya), existing for approximately . ''Archaeo ...

''.

References

{{Taxonbar, from=Q11904091 Helohyids Prehistoric even-toed ungulate genera Clarno Formation