Sunday, January 22, 2017

Entelodonts: Killer Jaws with Cloven Hooves

When we think of ungulates or hoofed mammals, the image which comes to mind is usually that of a plant-eating animal with a relatively small head and grinding teeth. This highly generalized description accurately describes most ungulates alive today. However, there once existed a group of fossil ungulates whose members possessed massive heads with jaws and teeth adapted for tearing flesh and crushing bones. These were the entelodonts (Entelodontidae).

Entelodonts were medium-sized to large artiodactyls that  lived in the Northern Hemisphere from the late Eocene to the middle Miocene. Certain aspects of their morphology have led scientists to traditionally think of these animals as close relatives of pigs and peccaries. As a result, entelodonts have often been informally referred to as “killer pigs”, “hell pigs” or “terminator pigs” among other misnomers. Recent molecular studies, however, have shown that entelodonts were actually terrestrial relatives of semiaquatic anthracotheres, hippos, and fully aquatic whales. 

Archaeotherium skull on display at the Buffalo Museum of Science. Wiki.

The earliest known entelodont, Eoentelodon yunanense, is known from China by about 38mya during the late Eocene. The family reached its peak diversity during the Oligocene and could be found throughout every continent in the Northern Hemisphere. The last known entelodont, the giant North American species Daeodon shoshonensis, became extinct during the early Miocene about 19mya.

The name entelodont, derived from the Greek words enteles and odontos, literally means “Perfect Tooth”, referencing the complete dental array of 44 teeth. Overall, entelodont dental morphology suggests that these animals were highly predatory in habitus but retained the ability to ingest plant matter (mesocarnivorous). The anterior dentition is adapted for piercing, gripping, and pulling. The well-developed incisors are pointed and are either rounded or sub-triangular in cross-section, ideal for puncture-and-tear feeding. The canines are very large, sturdy, and are serrated along their posterior edges when unworn. The large premolars are triangular in lateral profile, well-suited to gripping relatively large prey items. The robust molars have bunodont cusps for grinding and crushing.

Skull of Archaeotherium mortoni at the Paleontology Museum of Zurich.
Entelodonts were equipped with a complete dentition which was adapted for
gripping and crushing. Wiki.

A living entelodont would have appeared rather strange to a modern observer, in many ways resembling a large carnivorous peccary. Like peccaries, their elongated jaws were able to open incredibly wide thanks to a posteriorly-orientated mandibular fossa (jaw joint). The skull itself was very large in proportion to the body; comprising up to a quarter to one-third of the head-and-body length depending on the species. The snout is characteristically long and relatively narrow, although earlier genera like Eoentelodon and Brachyhyops had relatively shorter skulls than their later relatives. The posterior region of the skull then broadens drastically behind the eyes because of exceptionally broad with wide zygomatic arches and well-developed sagittal crests, indicating the presence of huge jaw muscles that would generate tremendous bite forces.
Reconstructed head of Archaeotherium mortoni depicted yawning
to demonstrate the exceptionally wide gape.

Perhaps the most unique aspect of the entelodont anatomy are the prominent bony growths which protrude from the skull and mandible; wing-like flanges flare out laterally from the zygomatic arches and two pairs of knobby protuberances extend from the bottom of the lower jaw. These structures are sexually dimorphic, being much larger in mature males and more modest in females and juveniles. In life, these structures were likely covered by thickened skin and would have served to shield vital parts head, such as the eyes and ears, from bite wounds during fights with other individuals of their species. Similar structures may be found on the faces of modern pigs of the genera Potamochoerus (bushpigs) and Phacochoerus (warthogs), although these projections are purely made of soft tissue with no bony core.
Reconstructed faces of male (left) and female (right) Archaeotherium mortoni.
Entelodonts possessed characteristic bony protrusions on their skulls and
mandibles, which are notably larger and broader in mature male specimens.

The only modern mammals which possess facial growths analagous to those
of entelodonts are certain types of pigs, such as this male Common Warthog
(Phacochoerus africanus). Unlike entelodonts, these "warts" are made
entirely of soft tissue with no internal bony structure. However, these are
sexually dimorphic features which act as shields protecting the eyes from
damage during intraspecific fights. Wiki.

Although entelodonts ranged in size from that of a wolf to that of a bison, all retain the same general physical characteristics. Their necks were short and robust. Elevated neural spines on the thoracic vertebrae provided attachment sites for the large muscles and ligaments  to support the weight of their massive heads, as well as providing the strength needed to subdue and transport prey items. The forelimbs were typically longer and more robust than the hindlimbs, placing the center of gravity over the shoulders. Such body proportions may be seen among modern bone-cracking hyenas and is associated with the ability to travel considerable distances while carrying large sections of carcasses. Entelodonts were built to run very swiftly, but they were not particularly specialized for cursoriality. Rather, they were probably best suited to travel at sustained, moderate speeds for extended periods of time. The limb bones were relatively long and slender with the radius and ulna fusing into a single bone in advanced species. The lateral toes were reduced to tiny splints of bone, leaving only the two central digits on each foot to bare the weight of the body.

Mounted skeleton of Daeodon shoshonensis, formerly Dinohyus hollandi. Common
physical features shared among all members of the Entelodontidae include a
proportionally large head, short neck, elevated thoracic spines, compact body,
slender yet sturdy limbs with didactyl, unguligrade feet. Wiki.

Life restoration of Daeodon shoshonensis, the last and largest of the entelodonts.
This species lived in North America during the early Miocene and is known to
have fed upon many of the larger mammals with which it coexisted.

Entelodonts would have filled ecological niches which are today occupied by such animals as hyenas and bears. As noted above, entelodonts have dental adaptations which enabled them to consume both animal and plant matter. However, evidence of the predatory nature of these animals is particularly well-represented in the fossil record: 

  • Bite marks and tooth fragments from Archaeotherium and Daeodon have been identified on the bones of contemporary camels, oreodonts, chalicotheres, and rhinos. 
  • Mass accumulations of prey species with tell-tale tooth impressions show that entelodonts kept meat caches, a behavior which is common among modern predatory animals. Excess food is transported to a relatively secure location and stored there to be consumed later, similar to how we might leave our leftovers in the refrigerator. 
  • A fossil trackway from Toadstool Geologic Park seems to depict Archaeotherium mortoni hunting the cow-sized rhino Subhyracodon occidentalis

Entelodonts differ from other artiodactyls in having eyes that are more anteriorly-oriented, enabling greater depth perception and stereoscopic vision; a condition which can be found among most terrestrial mammalian predators. Furthermore, the olfactory lobes of the brain were exceptionally well-developed, indicating that the sense of smell was very important to these animals. The following animation, generated by WitmerLab at Ohio University, shows a 3D rendering of the skull and brain endocast of Arcaeotherium.

References & Further Reading
Spaulding M, O’Leary MA, Gatesy J (2009). “Relationships of Cetacea (Artiodactyla) among mammals: increased taxon sampling alters interpretations of key fossils and character evolution”. PLoS ONE 4(9): e7062 <Full Article>

Vislobokova IA (2008). "The oldest representative of Entelodontoidea (Artiodactyla, Suiformes) from the Middle Eocene of Khaichin Ula II, Mongolia, and some evolutionary features of this superfamily". Paleontological Journal: 643–654 <Abstract>

Sundell KA. Taphonomy of a Multiple Poebrotherium kill site - an Archaeotherium meat cache <>

Benton RC, Terry DO, Evanoff E, McDonald HG. 2015. The White River Badlands: Geology and Paleontology. Indiana University Press. Bloomington, Indiana <Book>

Prothero DR & Foss SE (eds). 2007. The Evolution of Artiodactyls. Johns Hopkins University Press. Baltimore, Maryland <Books>

Sunday, January 1, 2017

Cetaceans: Hoofed Mammals Turned Masters of the Oceans

Cetaceans (whales, dolphins, and porpoises) are marine mammals which descended from artiodactyls during the early Eocene, their closest modern being hippos (Hippopotamidae). This discovery has led some authors to combine the Cetacea and Artiodactyla into a single common order known as “Cetartiodactyla”. Cetaceans themselves are the most diverse and the most specialized of all marine mammals, represented today by 13 families with almost 90 species divided between them. They range in size from the 1 meter long, 50kg Hector’s Dolphin (Cephalorhynchus hectori) to the 30m long, 190,000kg Blue Whale (Balaenoptera musculus), the largest mammal known to have ever lived.

Representatives of each of the three cetacean lineages: the late Eocene
Basilosaurus cetoides and the modern Orca (Orcinus orca) and
Humpback Whale (Megaptera novaeangliae).

Molecular evidence has shown that cetaceans are part of the whippomorph clade within the Artiodactyla; a subgroup which includes hippos along with the extinct anthracotheres and entelodonts. The origin of these animals appears to have been centered around southern Asia along the coast of the ancient Tethys Sea: a shallow seaway which once separated Asia and Africa before the former islands of India and Arabia joined the greater Eurasian landmass during the early Oligocene. The first identifiable cetaceans are present in this region by the early Eocene, most likely having descended from a family of semiaquatic artiodactyls called the Raoellidae.

Archaeoceti (ancient whales)- The first whales belong to the clade archaeoceti, a group which lasted from the early Eocene to the early Oligocene. Contained within this group are the families are the Pakicetidae, Ambulocetidae, Remingtonocetidae, and Protocetidae, all of whom are sometimes informally referred to as the “walking whales”. The reason for this is because these aquatic animals retained well-developed limbs with webbed toes which still enabled terrestrial locomotion, albeit somewhat clumsy. These early whales would have been ecologically similar to modern seals and sea lions: spending most of their lives in the water while returning to land only to rest and breed. Members of the family Basilosauridae, the family from which all modern whales descend, were fully aquatic animals that possessed key adaptations retained by all modern cetaceans including streamlined bodies, forelimbs which had become modified into flippers, highly reduced hindlimbs, and powerful tails which ended with a horizontal fluke for propulsion. The archaeocete transition from land to sea has been beautifully illustrated in this brief animation by the Museum of New Zealand Te Papa Tongarewa, embedded here: 

Odontoceti (toothed whales)- Of the three cetacean suborders, the Odontoceti is the most diverse: comprising 72 of the 85 modern species. Included within this group are the modern dolphins (Delphinidae), porpoises (Phocoenidae), beaked whales (Ziphiidae), sperm whales (Physeteridae), and narwhals (Monodontidae), along with several species of freshwater dolphins in their own distinct families. As their common name suggests, the primary feature that distinguishes these whales from their filter-feeding relatives (see below) is the retention of teeth as adults. Another characteristic of this group is a type of sonar known as echolocation. At its simplest (see the embedded video below for a more detailed explanation), echolocation in odontocetes occurs when repetitive clicking sounds produced in the airway are amplified by a special organ in the forehead known as the “melon”, producing a focused beam of sound. These outgoing sound waves bounce off distant objects and the resulting echoes are then received through complex fatty tissues along the lower jaw en route to the middle ear, producing an accurate image of distant objects. Echolocation has been employed by toothed whales at least since the early Oligocene and may have originated as a means to locate prey buried in sediment, in murky water, or in other areas of low visibility. 

Mysticeti (baleen whales)- The Mysticeti contains the largest of all cetaceans and, indeed, the largest of all mammals. Modern families include the Eschrichtiidae (gray whales), Balaenidae (right whales), Cetotheriidae (pygmy baleen whales), and Balaenopteridae (rorqual whales). Mysticetes are unique among the cetaceans in that they feed by filtering small prey animals in bulk from the surrounding water using baleen plates: dual rows of keratin bristles that protrude down from either side of the palate, resembling a pair of massive brushes. When feeding, these whales take in large volumes of water that contains clusters of an intended prey item. The water is then pushed out the sides of the partially opened mouth by pressing the tongue against the palate, leaving the prey trapped against the baleen plates to be swallowed. The earliest mysticetes, such as the early Oligocene Fucaia buelli, have been suggested to have employed suction-feeding as a precursor to the aforementioned filter-feeding. Suction-feeding involves the rapid expanding of the oral cavity, creating a pressure difference between the inside of the mouth and the surrounding water, engulfing nearby prey items. This technique is used by modern beaked whales, which are mostly toothless. All cetaceans are generally highly vocal animals, but mysticetes are known for producing complex vocalizations known as "songs", especially by males during breeding season. These songs vary in complexity from one species to another, may be heard for hundreds of kilometers away, and tend to be region specific. The following video provides better insight into the anatomy and function of whale song.

Skull of a modern Gray Whale (Eschrichtius robustus) with
attached baleen plates along with a schematic illustration
of the filter-feeding process of baleen whales.

Defining Characteristics
Cetaceans are by far the most specialized of all aquatic mammals. The skeletal proportions of the walking whales suggest that they had a swimming style similar to that of modern otters and beavers. They propelled themselves using shallow undulations of their lower back and tail combined with powerful thrusts of their hindlimbs, a swimming stroke that is employed to great effect by modern otters and beavers. The basilosaurids were the first whales to become fully aquatic and utilize tail-powered swimming involving vertical undulations of the posterior half of the body with the cartilaginous, horizontal fluke providing thrust. The numerous adaptations which have enabled cetaceans to dominate the oceans include: 
  • Strengthening of the lumbar (lower back) and caudal (tail) vertebrae, with their associated muscles powering the vertical tail stroke. The cervical (neck) vertebrae, meanwhile, are reduced or fused to gain stability.
  • Ribs that are bound by flexible cartilage to allow the ribcage to collapse under water pressure as they dive. The lungs are able to collapse along with the ribcage, maintaining a balance of internal and external pressure and preventing them from rupturing.
  • Sealable nostrils which have migrated from the tip of the snout to the forehead, known as the “blowhole”. This enables cetaceans to breathe without needing to cease swimming or raise their heads completely above the surface.
  • Streamlining of the body achieved through the reduction and eventual loss of the hindlimbs, loss of fur and external ear lobes, and genitalia concealed within the body cavity.
  • Forelimbs have become modified into rigid flippers involved in steering and stabilization while swimming. In addition, many whales possess dorsal fins which further aid in stabilization.
  • An atrophied pelvis which is no longer connected to the vertebral column since it is no longer needed to support body weight. Instead, this structure serves to support for the genitalia.
  • Smooth, hairless skin with an underlying layer of blubber for insulation. This fat layer can double as an emergency energy reserve during periods of fasting.
  • Loss of the sense of smell and the development of an enhanced sense of hearing.

Skeletons of the archaeocete whales Dorudon atrox (A & B) and Maiacetus innus (C & D).
Basilosaurid whales such as Dorudon were the first whales to employ the tail-powered
swimming method used by all modern whales. Notable skeletal adaptations include reduced
cervical vertebrae, rigid forelimbs, reduced hindlimbs, a pelvis which is detatached from the
vertebral column, and strengthened lumbar and tail vertebrae. Walking whales like Maiacetus 
still had well-developed limbs capable of terrestrial locomotion and swam in a
manner similar to that of modern otters. Figure 1 from Gingerich et al. 2009.

Like their terrestrial ancestors, archaeocetes possessed a complete, heterodont* dentition which included conical incisors and canines adapted for gripping prey. Behind these were serrated, triangular premolars and molars which sliced prey into manageable pieces. This suggests that, like pinnipeds and otters, archaeocetes may have carried large prey to the surface of the water to be eaten as demonstrated here. Smaller prey items would have simply been swallowed whole. Early odontocetes, such as the squalodonts (shark-toothed dolphins), retained the gripping-slicing dentition of the ancestral archaeocetes although these had begun to add extra teeth. Later toothed whales became fully homodont* with many species developing a significantly increased dentition: some modern dolphins may have as many as 100 to 200 conical teeth lining their jaws in contrast to the 44-tooth maximum seen among archaeocetes and most other mammals. Members of the families Monodontidae and Ziphiidae have taken the opposite route, instead vastly reducing the number of teeth until many are almost toothless, although the males may have tusks which are used to combat other males. Early mysticetes also retained heterodont dentition, although these teeth atrophied over time as they were gradually replaced by baleen plates.

Skulls of the archaeocete, Dorudon atrox (top), and the modern odontocete,
Orcinus orca (bottom). All archaeocetes and the early members of Odontoceti
and Mysticeti possessed complete, heterodont denition adapted for piercing and
slicing. More derived odontocetes developed homodont dentition strictly adapted
for piercing.

  • Archaeoceti (ancient whales)
    • Raoellidae
    • Pakicetidae
    • Ambulocetidae
    • Remingtonocetidae
    • Protocetidae
    • Basilosauridae
  • Odontoceti (toothed whales)
    • Squalodontidae (shark-toothed dolphins)
    • Delphinidae (dolphins)
    • Monodontidae (narwhals)
    • Phocoenidae (porpoises)
    • Iniidae
    • Pontoporiidae
    • Platanistidae
    • Lipotidae
    • Physeteridae (sperm whales)
    • Ziphidae (beaked whales)
  • Mysteceti (baleen whales)
    • Aetiocetidae
    • Mammalodontidae
    • Eschrichtiidae (gray whales)
    • Balaenidae (right whales)
    • Cetotheriidae (pygmy baleen whales)
    • Balaenopteridae (rorqual whales)

·       Glossary*
Astragalus: the ankle bone which connects the foot to the rest of the leg.
Heterodont: possessing multiple tooth morphologies.
Homodont: possessing a single tooth morphology.

References & Further Reading
Park T, Fitzgerald EMG, Evans AR (2016). “Ultrasonic hearing and echolocation in the earliest toothed whales”. Biology Letters 12: 20160060 <Full Article>

Marx FG, Tsai C, Fordyce RE (2015) "A new early Oligocene toothed ‘baleen’ whale (Mysticeti: Aetiocetidae) from western North America: one of the oldest and the smallest". Royal Society Open Science 2: 150476 <Full Article>

Uhen MD, Pyenson ND, Devries TJ, Urbina M, Renne PR (2011). "New middle Eocene whales from the Pisco Basin of Peru". Journal of Paleontology 85(5): 955-969 <Full Article>

Gingerich PD, ul-Haq M, von Koenigswald W, Sanders WJ, Smith BH, Zalmout IS (2009). "New protocetid whale from the middle Eocene of Pakistan: birth on land, precocial development, and sexual dimorphism". PLoS ONE 4(2): e4366 <Full Article>

Spaulding M, O’Leary MA, Gatesy J (2009). “Relationships of Cetacea (Artiodactyla) among mammals: increased taxon sampling alters interpretations of key fossils and character evolution”. PLoS ONE 4(9): e7062 <Full Article>

Thewissen JGM, Cooper LN, Clementz MT, Bajpai S, Tiwari BN (2007). "Whales originated from aquatic artiodactyls in the Eocene epoch of India". Nature 450: 1190-1194 <Abstract>

Price SA, Bininda-Emonds O, Gittleman JL (2005). "A complete phylogeny of the whales, dolphins and even-toed hoofed mammals (Cetartiodactyla)". Biological Reviews 80(3): 445-73 <Full Article>

Gatesy J, Milinkovitch M, Waddell V, Stanhope M (1999). “Stability of cladistics relationships between Cetacea and higher-level artiodactyl taxa”. Systematic Biology 48(1): 6-20 <Full Article>

Montgelard C, Catzeflis FM, Douzery E (1997). “Phylogenetic relationships of artiodactyls and cetaceans as deduced from the comparison of cytochrome b and 12S rRNA mitochondrial sequences”. Molecular Biology and Evolution 14(5): 550-559 <Full Article>

Gatesy J, Hayashi C, Cronin MA, Arctander P (1996). "Evidence from milk casein genes that cetaceans are close relatives of hippopotamid artiodactyls" Molecular Biology and Evolution 13(7): 954-963 <Full Article>

Graur D, Higgins DG (1994). “Molecular evidence for the inclusion of cetaceans within the order Artiodactyla”. Molecular Biology and Evolution: 357-364 <Full Article>

Artiodactyla: Even-toed Ungulates

Artiodactyla is the order of mammals which includes modern pigs, peccaries, hippos, deer, antelope, giraffes, and cattle. Members of this group are characterized by having limbs in which the main weight of the body is equally distributed through the third and fourth digits of each foot. As a result, these animals have a tendency to reduce the number of functional toes from five to just four or two. For this reason, artiodactyls are commonly referred to as the “even-toed ungulates”. Perissodactyls, the “odd-toed ungulates”, differ in that the main weight of the body is distributed through the third digit with the number of functional toes being reducd down to three or one. The name Artiodactyla was coined by the renowned English scientist Sir Richard Owen in 1848: derived from the Greek words artios, meaning “even”, and daktulos, meaning “finger”. Cetaceans (whales, dolphins, and porpoises) descended from early artiodactyls during the Eocene and are close relatives of hippos, leading some authors to combine the Cetacea and Artiodactyla into a common order known as “Cetartiodactyla”.

Illustration showing the foot anatomy of modern artiodactyls: (1) the foot with four
functional toes as demonstrated by the hippo, (2) the more common cloven-hoofed foot
in which there are always have two functional toes with reduced or absent lateral
toes, and (3) the padded foot which is found only among camels. In all cases,
the central third and fourth digits support the main body weight and the first
digits are always absent, resulting in an even number of toes.

The first artiodactyls are known to have lived in Europe during the early Eocene, a time when perissodactyls were the dominant terrestrial herbivores. The oldest known artiodactyl, Diacodexis (Dichoubunidae), was a rabbit-sized animal with bunodont dentition. Already, its limbs anatomy was highly modified for running with specializations that would be inherited by all later artiodactyls. The first cetaceans appear in the fossil record shortly thereafter, but these highly derived animals will be covered in a separate blog post. Disregarding their aquatic relatives, artiodactyls are among the most successful order of mammals, having diversified in the past 10 million years into a wide array of families, subfamilies, tribes, and genera, native to every continent except Antarctica and Australia. There are 10 modern families with 240 species between them. The order is subdivided into four suborders: Whippomorpha, Suina, Tylopoda, and Ruminata.

Whippomorpha- Whippomorpha is a group which contains the earliest artiodactyls and cetaceans. Traditionally, the families Hippopotamidae (hippos), Anthracotheriidae (anthracotheres), and Entelodontidae (entelodonts) were been placed within the Suina for morphological reasons. However, a 2009 study of artiodactyl phylogeny has since found that these three families, together with the middle Eocene predator Andrewsarchus mongoliensis, are much more closely related to Cetacea than they are to other artiodactyls. Barring the cetaceans, members of this group are the least derived among the artiodactyls with a history dating back to the early Eocene.

Representatives of the suborder Whippomorpha. This group
includes hippos (top-left), cetaceans (top-right), entelodonts
(bottom-left), and the middle Eocene predator Andrewsarchus
(bottom-right) among others.

Suina- The Suina, or “pig-like artiodactyls”, contains the modern families Suidae (pigs) and Tayassuidae (peccaries). These animals are characterized by having relatively robust heads with specialized nasals which support a distinctive snout which ends in a disc-shaped nose. Their bodies are typically robust, and cursorial adaptations are minimal. These animals first originated in the Northern Hemisphere during the early Oligocene. Members of this group may be either hypocarnivores or specialized herbivores.

Representatives of the suborder Suina: pigs (left) and peccaries (right).

Tylopoda- The Camelidae (camels) are the sole modern representatives of the group Tylopoda, a group which branched away from other artiodactyls during the middle Eocene. Extinct members of this group include the endemic European families Xiphodontidae and Cainotheriidae and the endemic North American families Protoceratidae and Merycoidodontidae. The latter family were once erroneously placed within the Suina due to their common, yet inaccurate description of “ruminating hogs”.

Representatives of the suborder Tylopoda. This group includes modern camels
(top-left), and the extinct protoceratids (top-right), oreodonts (bottom-left),
and cainotheres (bottom-right).

Ruminata- At present, the Ruminata (ruminants) are the most diverse group of ungulates, comprising nearly 200 modern species divided among the families Tragulidae (chevrotains), Moschidae (musk deer), Cervidae (deer), Antilocapridae (pronghorns), Giraffidae (giraffes), and Bovidae (antelopes and cattle). Extinct families include Hypertragulidae, Gelocidae, and Palaeomerycidae. The word ruminant is derived from the Latin word ruminare, meaning to chew over again, describing the unique method in which these animals digest their food. Rumination is a multi-step process which involves a complex, four-chambered stomach: 
  • First, food is consumed and stored in the first and largest chamber (rumen) where it mixes with saliva and microorganisms which begin to breakdown the tough cell walls of the plants.
  • After feeding, this stored food will gradually be regurgitated and rechewed over a period of time to allow further breakdown of plant fibers, as demonstrated here
  • The "cud" is then reswallowed and transferred to the second chamber (reticulum) where smaller, unwanted debris is trapped.
  • Food is then passed to the third chamber (omasum) where any large particles are broken up and moisture is absorbed.
  • Finally, the food travels to the fourth chamber (abomasum), or the "true stomach", where it is exposed to digestive enzymes and acids before it enters the small intestine. 
Click here for a more detailed description of the anatomy and function of the ruminant digestive tract. Through this process, ruminants are able to 1) quickly consume large quantities of food at a given time to be digested at a later period and 2) to extract nutrients from their food more efficiently than competing herbivores. 

Illustration showing the basic structure and organization
of the multi-chambered ruminant stomach. Wiki.

Most ruminants lack upper incisors, instead having a toughened pad which occludes with the lower incisors. The cuboid and navicular bones of the hindfoot have fused together, forming the cubonavicular bone. Additionally, many ruminant families have convergently evolved paired head projections used for species identification, gender identification, and combat (horns, antlers, pronghorns, or ossicones). The first true ruminants appear in the fossil record at the end of the Eocene and underwent a gradual increase in diversity from the Miocene onward.

Representatives of the suborder Ruminata: pronghorns (top-left), deer (top-middle),
giraffes (top-right), chevrotains (bottom-left), musk deer (bottom-middle), and bovids

Defining Characteristics
All artiodactyls (including the earliest cetaceans) share a double-hinged astragalus* that features two symmetrical, pulley-shaped surfaces. This innovation has provided significant advantages for cursoriality: it restricts movement of the foot to the vertical plane and allows leverage to be altered as required. This adaptation is similar to the mechanism possessed by perissodactyls, albeit somewhat more sophisticated. Artiodactyls also share with perissodactyls a reduction or loss of the clavicle (collar bone) which allows them to maximize their stride length while running. The first digit (the equivalent of our thumbs and big toes) is absent in all species. Most artiodactyls have “cloven hooves”; having unguligrade feet in which the weight-bearing third and fourth digits are enlarged while the lateral digits are reduced or absent depending. Among modern species, only hippos have functional lateral digits that actively play a role in locomotion. Camels have a unique foot morphology in which the toes splay out and the body weight is supported by fatty pads, making them effectively digitigrade.

Astragali from the early whale Pakicetus (left), and a modern pig (middle)
and deer (right). Note that both ends of the astragali have a pulley-shaped
articular surface, a unique and defining feature of the order Artiodactyla.

  • Whippomorpha
    • Dichoubunidae: early Eocene to late Oligocene
    • Entelodontidae (entelodonts): late Eocene to middle Miocene
    • Anthracotheriidae: middle Eocene to late Pliocene
    • Hippopotamidae (hippos): middle Miocene to present
  • Suina
    •  Suidae (pigs): early Oligocene to present
    • Tayassuidae (peccaries): late Oligocene to present
  • Tylopoda
    • Protoceratidae: middle Eocene to early Pliocene
    • Camelidae (camels): middle Eocene to present
    • Xiphodontidae: late Eocene to late Oligocene
    • Cainotheriidae: Eocene to early Miocene
  • Ruminata
    • Hypertragulidae: middle Eocene to middle Miocene
    • Tragulidae (chevrotains): Oligocene to present
    • Gelocidae: Eocene to late Miocene
    • Moschidae (musk deer): early Miocene to present
    • Palaeomerycidae: Eocene to Miocene
    • Cervidae (deer): early Oligocene to present
    • Giraffidae (giraffes): early Miocene to present
    • Antilocapridae (pronghorns): early Miocene to present
    • Bovidae (antelopes): early Miocene to present

Astragalus: the ankle bone which connects the foot to the rest of the leg.

References & Further Reading
Spaulding M, O’Leary MA, Gatesy J (2009). “Relationships of Cetacea (Artiodactyla) among mammals: increased taxon sampling alters interpretations of key fossils and character evolution”. PLoS ONE 4(9): e7062 <Full Article>

Thewissen JGM, Cooper LN, Clementz MT, Bajpai S, Tiwari BN (2007). "Whales originated from aquatic artiodactyls in the Eocene epoch of India". Nature 450: 1190-1194 <Abstract>

Price SA, Bininda-Emonds O, Gittleman JL (2005). "A complete phylogeny of the whales, dolphins and even-toed hoofed mammals (Cetartiodactyla)". Biological Reviews 80(3): 445-73 <Full Article>

Gatesy J, Milinkovitch M, Waddell V, Stanhope M (1999). “Stability of cladistics relationships between Cetacea and higher-level artiodactyl taxa”. Systematic Biology 48(1): 6-20 <Full Article>

Montgelard C, Catzeflis FM, Douzery E (1997). “Phylogenetic relationships of artiodactyls and cetaceans as deduced from the comparison of cytochrome b and 12S rRNA mitochondrial sequences”. Molecular Biology and Evolution 14(5): 550-559 <Full Article>

Gatesy J, Hayashi C, Cronin MA, Arctander P (1996). "Evidence from milk casein genes that cetaceans are close relatives of hippopotamid artiodactyls" Molecular Biology and Evolution 13(7): 954-963 <Full Article>

Graur D, Higgins DG (1994). “Molecular evidence for the inclusion of cetaceans within the order Artiodactyla”. Molecular Biology and Evolution: 357-364 <Full Article>

Rose KD (ed). 2006. The Beginning of the Age of Mammals. The John Hopkins University Press, Baltimore, Maryland:243-257 <Book>

Macdonald DW (ed). 2006. The Princeton Encyclopedia of Mammals. Princeton University Press, Princeton, New Jersey <Book>

Augusti J, Anton M (2002). Mammoths, Sabertooths, and Hominids: 65 Million Years of Mammalian Evolution in Europe. Columbia University Press, New York: 36-37 <Book>