| Entelodontidae | |
|---|---|
_(Harrison_Formation%2c_Lower_Miocene%3b_Agate_Springs_Fossil_Quarry%2c_Nebraska%2c_USA)_1_(33515247252).jpg) | |
| Complete skeleton ofDaeodon | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Clade: | Cetancodontamorpha |
| Family: | †Entelodontidae Lydekker, 1883 |
| Type genus | |
| †Entelodon | |
| Genera | |
| Synonyms | |
| |
Entelodontidae is anextinctfamily of pig-likeartiodactyls (even-toedungulates) which inhabited theNorthern Hemisphere (Asia,Europe, andNorth America) from the lateEocene[1] to the earlyMioceneepochs, about 38-19 million years ago. Their large heads, low snouts, narrow gait, and proposedomnivorous diet inspires comparisons tosuids (true pigs) andtayassuids (peccaries), and historically they have been considered closely related to these families purely on amorphological basis.[2][3][4] However, studies which combine morphological andmolecular (genetic) data on artiodactyls instead suggest that entelodonts arecetancodontamorphs, more closely related tohippos andcetaceans through their resemblance toPakicetus, than tobasal pigs likeKubanochoerus and other ungulates.[5][6]
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 wasDaeodon, a North American entelodont which could reach an estimated weight of 750 kg (1650 pounds),[2] and a height up to 1.9 m (6.2 ft) tall at the shoulder.Paraentelodon intermedium, a Eurasian species known mostly by the teeth and jaws, was similar in size toDaeodon.[7][4]
Entelodonts had huge heads, ornamented with distinctive bony expansions. Thezygomatic arches (cheekbones) develop hugejugal flanges which project downwards and outwards. Moreover, the underside of thelower 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.[2][3][4]
The snout was narrow and elongated, especially in later species. The cranium was robust, with strong zygomatic andpostorbital arches forming the rim of voluminoustemporal fossae, separated by a sharpsagittal crest.[2][3][4] However, the rear of the skull was also much shorter than the snout, and thebraincase was relatively small. Most of the braincase contributed to largeparanasal sinuses andolfactory bulbs at the front, while thecerebrum was underdeveloped. Large olfactory bulbs are likely indicative of a good sense of smell. Moreover, theorbits (eye sockets) are oriented further forwards than in most artiodactyls, suggesting that entelodonts hadbinocular vision.[3] Compared to other artiodactyls, the jaw was slender at the rear, with a short, triangularcoronoid process which is shifted forwards. Themandibular condyle (jaw joint) is set back and below the level of the tooth row. Themandibular symphysis (chin) was fused, and thepterygoid bones along the middle of the roof of the mouth were connected by a stronginterdigitating suture.[2][3][4]
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Similar to pigs, entelodonts retain a large number of teeth, aplesiomorphic trait approximating the ancestral condition for artiodactyls. They have a typical mammaliandental formula of 3.1.4.3 / 3.1.4.3, meaning that each tooth row has three pairs of robustincisors, a pair of largecanines, four pairs of pointedpremolars, and three pairs of relatively simple and flatmolars. This unreduced, or "complete" dentition is the origin of the family's name, which isGreek for "complete teeth".[3][4]
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).[2][3] The canines have thickenamel and are circular in cross section, unlike most artiodactyls. In older individuals, the tip of the upper canine often heavily worn or even chipped off. Premolars are triangular when seen from the side, with a large and conical maincusp. They are elongated from front-to-back and widely-spaced, taking up a large portion of the tooth row. The molar teeth arebunodont, 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 (themetaconid andprotoconid) may be connected by a horizontal crest and are slightly larger than the rear two cusps.[2]
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 in total, with the middle two forming small, pointedcloven hooves, while the remaining two werevestigial and likely not externally visible. In larger species, abison-like spinal hump supported the weight of the heavy head. The limbs were long, and theradius andulna were fused.[4] Though not fused, themetatarsals (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.[2]
_(Harrison_Formation%2c_Lower_Miocene%3b_Agate_Springs_Fossil_Quarry%2c_Nebraska%2c_USA)_8_(33542317891).jpg)
The wide and tall temporal fossa allowed for a very largetemporalis muscle, which extends from the side of the cranium to the coronoid process of the mandible. Thetemporalis was not only large and strong, but also had a longmoment 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 thetemporalis. The size and orientation of thetemporalis is similar to carnivorans, where it corresponds to a strong and stable scissor-like (orthal) bite.[2]
Though the low jaw joint provided more room for thetemporalis muscle, it also posed a problem for themasseter muscle. Themasseter, which extends from the zygomatic arch to the lower rear corner of the mandible, is a major component of the chewing apparatus in herbivorous artiodactyls. While other artiodactyls added torque to the muscle by raising the jaw joint, entelodonts instead expanded the rear of the jaw downwards, as a deep, curved flange. Moreover, the characteristic jugal flanges of entelodonts were covered with muscle scars on the inside, likely attachment points to strengthen themasseter. Only a few modern mammals have overdeveloped projections on the zygomatic arch, includingxenarthrans,kangaroos, and certainrodents. Like entelodonts, these mammals use their equivalent projections as a means of providing extra space for the attachment of themasseter muscle, and develop robust cranial bars to resist the resulting forces on the skull.[3] Thepterygoideus muscle, which follows a similar path and function to the masseter, also benefited from the deep flange at the back of the jaw.[2]
The function of the mandibular tubercles is not certain, but they may also be related to jaw musculature. They are only clearly correlated with the size of the individual, though a few taxa (Brachyhyops andCypretherium) can be diagnosed by the absence of a specific pair of mandibular tubercles. Generally, the posterior (rear) mandibular tubercles develop later in life than the anterior (front) pair, and none of the tubercles stop growing as the animal develops. The use of the anterior tubercles is unclear; one speculative idea suggests that they served as an attachment point for strong lip muscles in particularly herbivorous entelodonts. The posterior tubercles may provide a link to thedigastricus muscle which helps to open the jaws. Hippos, which have a particularly complex and well-developeddigastricus, occasionally develop a tubercle to support thedigastricus in an equivalent area on the jaw.[3]
The jaw joint of entelodonts was likely more strongly connected than the loose jaws of most other artiodactyls. The mandibular condyle was convex and inserted into a strongly concave facet (glenoid) on the zygomatic arch, which would have restricted front-to-back (propalinal) jaw movement. Nevertheless, the structure of the mandibular condyle itself allowed for a wide range of movement, and the laterally bowed zygomatic arch provided some room for side-to-side (transverse) movement driven by themasseter andpterygoideus. The low, unconstrained jaw joint and short coronoid process may correspond to long muscle fibers. This points to a hinge-like jaw suspension with a very wide gape, similar to some modern carnivorans such asfelids (cats). Based on the shape of the mandibular condyle, the maximum gape possible based on the underlying bones (though not necessarily the widest gape possible in life) was about 109 degrees inArchaeotherium.[2]
Wear facets on entelodont teeth support three-part food processing. First, the incisors and canines bite in a strong orthal motion, grabbing and puncturing food. Then, the food is transferred back to the premolars, which breaks apart tough parts of the food with similar movements. Finally, the food is crushed and ground up by the molars, using a combination of orthal and transverse grinding. This same basic process is seen in modern pigs and peccaries, which have similar dentition. Individuals may have preferred one side of the jaw for chewing, as premolars and molars often show an asymmetrical distribution of wear between the left and right sides of the mouth.[2]
By comparison to pigs and peccaries, entelodonts were almost certainlyomnivorous to an extent. Their teeth and jaw structure would have assisted processing of large and tough food items. Unlike the diverse and fully herbivorouspecoran artiodactyls, entelodonts lack specializations for chopping and shredding grass and other particularly fibrous plants. Instead, entelodonts were probablybrowsers, with roots, nuts, fruits, and branches as their preferred sources of vegetation.[2] A 2022 study found thatEntelodon magnus had an omnivorous diet similar towild boar (Sus scrofa). This conclusion was justified by its pattern of toothmicrowear, run through alinear discriminant analysis calibrated by modern herbivorous and omnivorous mammals. Based on pigs, entelodonts probably had a simple stomach and relied on thecaecum to ferment and digest plant matter. They would have been opportunistic omnivores, capable of digesting a variety of plant and animal matter and moderating their food preferences based on seasonal ability.[8]
The same adaptations useful for processing tough plant material would be equally useful for carrion and bones, which could have been major components of the diet for some entelodonts.[2] Unlike pigs, the youngest juvenile entelodonts had a full set of 32deciduous teeth. The teeth were sharp, slender, and semi-serrated, less suitable for crushing tough food compared to adult entelodonts.[3]
In many entelodonts, the canine teeth acquire rounded wear surfaces at their tips, indicating regular use on hard material such as bones. Similar patterns of canine wear are observed in modern cats, which rely on strong bites administered through their canine teeth when killing prey. In some species the bases of the canines are scoured by smooth grooves, a trait consistent with abrasions from sediment-covered plant material such as roots.[2] These grooves instead could have been produced by stripping long, fibrous vegetation, such as water-rich grape vines.[3]Daeodon is known to have had a distinctive type of "piecrust" tooth wear at the tips of the premolars, with a flatdentine surface surrounded by chippedenamel. This has also been observed in livinghyenas.[2] Few contemporary mammals approached entelodonts in the extent of adaptations consistent with scavenging. Fossils with large scrapes and puncture marks are found throughout entelodont-bearing sites in the AmericanGreat Plains, including a skull ofMerycoidodon with an embedded incisor of the entelodontArchaeotherium.[9]
Entelodonts may have engaged in active predation, though the extent of this behavior is debated. Several species of modern pigs occasionally engage in predation, and even traditional herbivores like camels show dental wear consistent with scavenging.[2] If they did engage in predation, entelodonts would not have been alone: many other contemporary mammals filled apex predator niches, including cat-like saber-toothednimravids,amphicyonids ("bear-dogs"), andhyaenodontidcreodonts.[9] One of the most apparent examples of circumstantial evidence for predation is a fossil found in theWhite River Formation ofWyoming, representing acache of partial skeletons and other remains of the early camelidPoebrotherium. The carcasses were covered with large punctures on the skull, neck, and the transition from thethoracic tolumbarvertebrae, which have been attributed to predation and scavenging byArchaeotherium.[10][11]Entelodon's tooth microwear showed no overlap with the modernbrown bear (Ursus arctos), and it probably did not actively hunt large mammals as part of its normal diet.[8]
The jaw structure and estimated musculature hold numerous lines of evidence indicating that entelodonts could open their mouths unusually wide.[2] This trait may have been useful in hunting or feeding on carrion, but similar adaptations have also been linked to competitive behaviors in herbivores. Hippos, a related group with similar adaptations, are aggressive herbivores which can open their jaws up to 150 degrees and display enlarged canines in order to intimidate rivals. Male hippos engage in head-to-head "yawning" and jaw-wrestling contests, while females attack by approaching from the side and slamming their head into the opponent's body.[12] The wide gape and low skulls of entelodonts would have assisted biting competitions, which are supported by fossil evidence. Large bite marks, including healed punctures, are common on skulls of various American entelodonts. These wounds are concentrated above the sinuses, and are only found on adult specimens. One could easily draw comparisons between these bite marks and the wide range ofintraspecific competition over mates or territories in modern artiodactyls.[3][9] Snout biting in particular is a common competitive behavior among malecamels, another group of "primitive" artiodactyls.[2][3] Ribcage injuries have been attributed to intraspecies aggression inArchaeotherium.[13] One possible function for the anterior tubercles is as a support for toughened skin, which would have acted as a buffer or display feature during competitive behavior.[3]
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 wasEntelodon magnus, a European species which was named by French paleontologistAuguste 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 asElotherium around the same time. Pomel's volume was likely published in 1846 or 1847, albeit with surviving reprints dating to 1848.Entelodon andElotherium 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 useEntelodon for the purpose of clarity, even though it would not take priority under strict rules of nomenclature. The confusion of priority betweenEntelodon andElotherium is reflected in the name of their corresponding family.Edward Richard Alston coined the name Elotheriidae in 1878, whileRichard Lydekker used the name Entelodontidae in 1883. As withEntelodon, nearly all paleontologists prefer Entelodontidae when referring to the family.[3][4]
Following the confusion betweenEntelodon andElotherium, 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 intoArchaeotherium andDaeodon. 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 theCarnegie 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).[3][4]
The first described entelodonts were described in conjunction withRichard 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.[14] 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).[15][3]
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, andhelohyids were frequently associated with entelodonts, sometimes even as potential ancestors. Later, thesuperfamilyEntelodontoidea was named to encompass Entelodontidae and their supposed closest extinct relatives. In modern studies, Entelodontidae is generally considered the only family within Entelodontoidea.[3][4]
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 ofSuina is still in use, as a major artiodactyl suborder encompassingTayassuidae (peccaries) andSuidae (pigs). Earlycladisticphylogenetic analyses of artiodactyls placed Entelodontidae as thesister taxon to a Tayassuidae + Suidaeclade. This seemed to justify the frequent morphological comparisons between entelodonts and pigs.[3][4]
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.[6] Cladistic analysis of the position of whales in relation to artiodactyls andmesonychians changes radically depending on whether the giant enigmatic mammalAndrewsarchus is included, and it has been suggested thatAndrewsarchus is in fact an entelodont or close relative.[5][16][6]
Many former genera of entelodonts have been synonymized. For example, some authors have synonymizedDinohyus withDaeodon shoshonensis, a species described from fragmentary material by Cope.[4]
In popular media, entelodonts are sometimes nicknamedhell pigs orterminator pigs.[20]
Entelodonts appear in the third episode of the popularBBC documentaryWalking with Beasts, where, in the program, the narrator always refers to the creatures as "entelodonts" rather than a more specific genus, such asEntelodon. The same creatures appear in another BBC production, the2001 remake ofThe Lost World.
Entelodonts were also the main focus of episode 4 ofNational Geographic Channel's showPrehistoric Predators in an episode titled "Killer Pig". The episode featured a number of claims unproven or disproven by science, such asArchaeotherium (identified as "entelodont") being the top predator of the AmericanBadlands, and evolving directly into the even largerDaeodon (called "Dinohyus" in the episode).
