Entelodontidae is an extinct family of pig-like artiodactyls (even-toed ungulates) that inhabited the Northern Hemisphere (Eurasia and North America) from the Middle Eocene to the Early Miocene epochs, about 45 to 19 million years ago. Their large heads, low snouts, narrow gait, and proposed omnivorous diet inspires comparisons to suids (true pigs) and tayassuids (peccaries), and historically they have been considered closely related to these families purely on a morphological basis. However, studies which combine morphological and molecular (genetic) data on artiodactyls instead suggest that entelodonts are cetancodontamorphs, more closely related to hippos and cetaceans through their resemblance to Pakicetus, than to basal pigs like Kubanochoerus and other ungulates.
Classification
Early history The earliest entelodont fossils to be named were described within a short time frame in the 1840s. The first entelodont species known from good fossils was Entelodon magnus, a European species that was named by French paleontologist Auguste Aymard. There is some debate over when Aymard's description was first published; though most authors assumed it was written in 1846, a citation within the article suggests that it was not published until 1848. Auguste Pomel, one of Aymard's contemporaries, described another fossil as Elotherium around the same time. Pomel's volume was likely published in 1846 or 1847, albeit with surviving reprints dating to 1848. Entelodon and Elotherium are almost certainly synonymous, though fossils belonging to the latter name are fragmentary and have been lost, while those of the former were likely described later. Nearly all historical and modern authors prefer to use Entelodon for the purpose of clarity, even though it would not take priority under strict rules of nomenclature. The confusion of priority between Entelodon and Elotherium is reflected in the name of their corresponding family. Edward Richard Alston coined the name Elotheriidae in 1878, while Richard Lydekker used the name Entelodontidae in 1883. As with Entelodon, nearly all paleontologists prefer Entelodontidae when referring to the family.
Following the confusion between Entelodon and Elotherium, entelodont fossils continued to be discovered in Europe. Large entelodonts were also described from North America starting in 1850, though most new genera were eventually lumped into Archaeotherium and Daeodon. By the beginning of the 20th century, entelodont skeletal anatomy was well-understood from the quantity of fossils discovered by that point. In 1909, a massive complete skeleton of "Dinohyus" hollandi (= Daeodon), CM 1594, was described and put on display at the Carnegie Museum of Natural History. As the 20th century continued, Asian entelodonts were discovered (Eoentelodon, Paraentelodon), as well as some of the earliest known members of the family (Eoentelodon, Brachyhyops).
Traditional classification The first described entelodonts were described in conjunction with Richard Owen's recognition of the artiodactyls as a natural group. The earliest sources considered entelodonts to be true pigs, but as further fossils were discovered, it became clear that they had a long evolutionary history separate from pigs. Regardless, entelodonts were universally accepted as examples of "primitive" artiodactyls, with unspecialized bunodont teeth in contrast with the strong adaptations for herbivory present in the more "advanced" ruminants.S. G. Lucas, R. J. Emry, and S. E. Foss. 1998. Taxonomy and distribution of Daeodon, an Oligocene-Miocene entelodont (Mammalia: Artiodactyla) from North America. Proceedings of the Biological Society of Washington 111(2):425-435 Various names were erected to encompass living and extinct bunodont-toothed and non-ruminant artiodactyls, such as "Omnivoria" (Owens, 1858), "Bunodontia" (Lydekker, 1883) and "Nonruminantia" (Gregory, 1910).W. K. Gregory. 1910. The orders of mammals. Bulletin of the American Museum of Natural History 27:1-524
Some authors considered entelodonts to be too "primitive" for comparison to modern bunodont artiodactyls. In these studies, entelodonts were placed in "Palaeodonta", a group shared with various other extinct families. Choeropotamids, cebochoerids, and helohyids were frequently associated with entelodonts, sometimes even as potential ancestors. Later, the superfamily Entelodontoidea was named to encompass Entelodontidae and their supposed closest extinct relatives. In modern studies, Entelodontidae is generally considered the only family within Entelodontoidea.
Many studies argued that entelodonts had close relations to living pigs, peccaries, and hippos. Various groups have been developed and named in reference to a pig-like anatomy, with names such as Suina (Gray, 1868) and Suiformes (Jaeckel, 1911) being emplaced in varying contexts. A restricted definition of Suina is still in use, as a major artiodactyl suborder encompassing Tayassuidae (peccaries) and Suidae (pigs). Early cladistic phylogenetic analyses of artiodactyls placed Entelodontidae as the sister taxon to a Tayassuidae + Suidae clade. This seemed to justify the frequent morphological comparisons between entelodonts and pigs. Proentelodon was described in 2008, and was considered to have been the oldest known entelodont.
Cetancodontomorpha 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. Cladistic analysis of the position of whales in relation to artiodactyls and mesonychians changes radically depending on whether the giant enigmatic mammal Andrewsarchus is included, and it has been suggested that Andrewsarchus is in fact an entelodont or close relative. Many former genera of entelodonts have been synonymized. For example, some authors have synonymized Dinohyus with Daeodon shoshonensis, a species described from fragmentary material by Cope.
Many experts have also questioned the placement of Proentelodon within entelodont, as it had a large and elongated hypoconulid on m3 which was absent in entelodonts.Tsubamoto, T., Saneyoshi, M., Watabe, M., Tsogbataar, K., Mainbayar, B. (2011). The Entelodontid Artiodactyl Fauna from the Eocene Ergilin Dzo Formation of Mongolia with Comments on Brachyhyops and the Khoer Dzan Locality. Paleontological Research, 15(4):258-268 Cladistic analysis by Yu et al. recovered it as a sister taxon to the family.
Description Entelodonts could get quite large, and in many cases are the largest mammals in their respective ecosystems. The largest entelodont known from a complete skeleton was Daeodon, a North American entelodont that could reach an estimated weight of 750 kg (1650 pounds), and a height up to tall at the shoulder. Paraentelodon intermedium, a Eurasian species known mostly by the teeth and jaws, was similar in size to Daeodon.
Skull Entelodonts had huge heads, ornamented with distinctive bony expansions. The zygomatic arches (cheekbones) develop huge jugal flanges, which project downwards and outwards. Moreover, the underside of the lower jaw typically has one or two pairs of knob-like mandibular tubercles. These are not always diagnostic to specific taxa: often the size and presence of tubercles is variable within a single species.
The snout was narrow and elongated, especially in later species. The cranium was robust, with strong zygomatic and postorbital arches forming the rim of voluminous temporal fossae, separated by a sharp sagittal crest. However, the rear of the skull was also much shorter than the snout, and the braincase was relatively small. Most of the braincase contributed to large paranasal sinuses and olfactory bulbs at the front, while the cerebrum was underdeveloped. Large olfactory bulbs are likely indicative of a good sense of smell. Moreover, the orbits (eye sockets) are oriented further forwards than in most artiodactyls, suggesting that entelodonts had binocular vision. Compared to other artiodactyls, the jaw was slender at the rear, with a short, triangular coronoid process which is shifted forwards. The mandibular condyle (jaw joint) is set back and below the level of the tooth row. The mandibular symphysis (chin) was fused, and the pterygoid bones along the middle of the roof of the mouth were connected by a strong interdigitating suture.
Teeth
Similar to pigs, entelodonts retain a large number of teeth, a plesiomorphic trait approximating the ancestral condition for artiodactyls. They have a typical mammalian dental formula of 3.1.4.3 / 3.1.4.3, meaning that each tooth row has three pairs of robust incisors, a pair of large canines, four pairs of pointed premolars, and three pairs of relatively simple and flat molars. This unreduced, or "complete" dentition is the origin of the family's name, which is Greek for "complete teeth".
The incisors are closely packed but do not develop a distinct straight chopping surface. They range from chisel-shaped in some entelodonts (Archaeotherium) to massive and rounded in others (Daeodon). The canines have thick enamel and are circular in cross section, unlike most artiodactyls. In older individuals, the tip of the upper canines are often heavily worn or even chipped off. Premolars are triangular when seen from the side, with a large and conical main cusp. They are elongated from front-to-back and widely-spaced, taking up a large portion of the tooth row. The molar teeth are bunodont, with very low and rounded cusps rather than shearing surfaces. Bunodont teeth are common in other omnivorous mammals, including pigs, bears, and humans. The upper molars have up to six cusps and a low crest (a precingulum) on the front edge of the crown. In all but the earliest entelodonts, the lower molars have only four main cusps. The front two cusps (the metaconid and protoconid) may be connected by a horizontal crest and are slightly larger than the rear two cusps.
Postcranial skeleton
The skeleton is fairly unspecialized in entelodonts. They retain typical artiodactyl skeletal traits, such as a double-pulley ankle joint and paraxonic ("even toed") feet with weight split evenly between the two middle toes. They had four toes on each foot, with the middle two forming small, pointed cloven hooves, while the remaining two were vestigial and likely not externally visible. In larger species, a bison-like spinal hump supported the weight of the heavy head. The limbs were long, and the radius and ulna were fused. Though not fused, the metatarsals (raised foot bones) were long and closely packed. The limb and hoof proportions are consistent with other hoofed animals that run well on open ground but are not built for high speed.
Paleobiology
Jaw movement and musculature
The wide and tall temporal fossa allowed for a very large temporalis muscle, which extends from the side of the cranium to the coronoid process of the mandible. The temporalis was not only large and strong, but also had a long moment arm (and thus higher torque) due to the coronoid process shifting forwards. The reinforced pterygoid, zygomatic, and postorbital areas would have supported the forces generated by the temporalis. The size and orientation of the temporalis is similar to carnivorans, where it corresponds to a strong and stable scissor-like (orthal) bite.