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Temporal range: Early Eocene–present
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Clade: Cetancodontamorpha
Suborder: Whippomorpha
Waddell et al. 1999

Whippomorpha or Cetancodonta is a group of artiodactyls that contains all living cetaceans (whales, dolphins, etc.) and hippopotamuses. [1] All Whippomorphs are descendants of the last common ancestor of Hippopotamus amphibius and Tursiops truncatus . This makes it a crown group. [2] Whippomorpha is a suborder within the order Artiodactyla (even-toed ungulates). The placement of Whippomorpha within Artiodactyla is a matter of some contention, as hippopotamuses were previously considered to be more closely related to Suidae (pigs) and Tayassuidae (peccaries). [3] [4] Most contemporary scientific phylogenetic and morphological research studies link hippopotamuses with cetaceans, and genetic evidence has overwhelmingly supported an evolutionary relationship between Hippopotamidae and Cetacea. [5] Modern Whippomorphs all share a number of behavioural and physiological traits; such as a dense layer of subcutaneous fat and largely hairless bodies. They exhibit amphibious and aquatic behaviors and possess similar auditory structures.


Whippomorpha is a subgroup of Cetancodontamorpha, which also includes the extinct entelodonts and Andrewsarchus .


The name Whippomorpha is a combination of English (wh[ale] + hippo[potamus]) and Greek (μορφή, morphē = form). [2] Some attempts have been made to rename the suborder Cetancodonta, due to the misleading utilization of the suffix -morpha for a crown group, [6] as well as the risk of confusion with the clade Hippomorpha (which consists of equid perissodactyls); [7] however Whippomorpha maintains precedence. [7]



Modern Whippomorphs are widely distributed. Cetaceans can be found in almost all of the world's marine habitats, and some species, like the blue whale and humpback whale, have migratory ranges that comprise nearly the entire ocean. These whales typically migrate on a seasonal basis, moving to warmer waters to give birth and raise young before travelling to cooler waters with more optimal feeding grounds. [8] Other cetacean species have smaller ranges that are concentrated around either tropical or subtropical waters. Some cetaceans live exclusively within a single marine body, such as the narwhal, whose range is limited to the Arctic Ocean. [9]

By comparison, modern hippopotamuses are confined entirely to the African continent. Despite once being widespread across Europe, Africa and Asia [10] [11] Hippos are now considered vulnerable and are limited to the lakes, rivers and wetlands of southern Africa. [12]


A hippopotamus surfacing to breathe Hippopotamus ( Hippopotamus amphibius).jpg
A hippopotamus surfacing to breathe

Both whales and hippos must surface to breathe. This can pose problems for sleeping Whippomorphs. Cetaceans overcome this problem by unihemispheric sleep, meaning they rest one side of their brain at a time, allowing them to swim and surface during rest periods. [13] Hippopotamuses surface to breathe every three to five minutes, a process that is partially subconscious, allowing them to do it whilst sleeping. [14] Both whales and hippos exhibit symbiotic relationships with smaller fish, which they use as cleaning stations, allowing the smaller organisms to feed on parasites that enter the creature's mouth. [15]

Hippos are herbivores; normally their diet consists entirely of short grasses that they graze on. Some hippos have been observed consuming animals such as zebra and even other hippo carcasses. [16] [17] A hippo normally spends up to five hours a day grazing. They normally feed only on land, though occasional consumption of aquatic vegetation has been observed. [14] By contrast, cetaceans are all carnivores, feeding on fish and marine invertebrates, with some species feeding on larger mammals and birds (such as seals and penguins). [18]


All whippomorphs are placental mammals, meaning that embryos are fed by the placenta, which draws nutrients from the mother's body. They are k-selected organisms, producing a limited number of offspring, but with a high rate of survival. [19]

A humpback whale (Megaptera novaeangliae) with her calf HIHWNMS -- Whale And Calf (35991441536).jpg
A humpback whale (Megaptera novaeangliae) with her calf

Hippos reach sexual maturity at six years of age and have a gestation period of approximately eight months. Mating typically occurs in the water. Female hippopotamuses isolate themselves for two weeks prior to giving birth. The birthing process also takes place underwater, meaning calves must swim to the surface in order to breathe for the first time. Hippopotamus calves suckle on land.[ citation needed ]

Cetaceans generally reach sexual maturity around 10 years of age, and have a gestation period of around 12 months. Cetaceans give birth to well-developed calves, like hippopotamuses. When suckling, the mother splashes milk into the calves' mouth, as they have no lips. [18]

Taxonomy and phylogeny

Whippomorpha is a suborder located within the Order Artiodactyla, and the clade Cetancodontamorpha. It contains the clades Hippopotamoidea (ancestors of hippopotamuses) and Cetaceamorpha (ancestors of whales and dolphins). Whippomorpha is considered a sister clade to Ruminantia (which contains cattle, sheep and deer), as well as the extinct Raoellidae. [5] [20] Hippopotamoidea was formerly included to Suiformes with Suidae (pigs) and Tayassuidae (peccaries). [21]

Most of the evidence supporting the Whippomorpha clade is based on molecular or genetic analysis. Early support for the existence of a Cetacea/Hippopotamidae clade originated from analysis of the molecular composition of a blood-clotting protein γ-fibrinogen taken from whales and hippopotamuses. [22] Later studies obtained findings that indicated almost 11,000 orthologous genes between cetaceans and hippopotamuses, in addition to numerous positive indicators of a shared evolutionary history between cetaceans and hippopotamuses. [5] Furthermore, some genetic sequences have been found in both whales and hippopotamuses that are not present in the genomes of other mammals. [23] This would indicate that these groups share ancestry.

Whippomorpha's placement within Artiodactyla can be represented in the following cladogram: [3] [24] [25] [26] [27]


Tylopoda (camels) Cladogram of Cetacea within Artiodactyla (Camelus bactrianus).png


  Suina (pigs) Recherches pour servir a l'histoire naturelle des mammiferes (Pl. 80) (white background).jpg


  Ruminantia (ruminants) Walia ibex illustration white background.png  


  Hippopotamidae (hippopotamuses) Voyage en Abyssinie Plate 2 (white background).jpg

  Cetacea (whales) Bowhead-Whale1 (16273933365).jpg


Cladogram showing Whippomorpha within Artiodactylamorpha: Whippomorpha consists of the clades labeled Hippopotamoidea and Cetaceamorpha. Cladogram of Cetacea within Artiodactyla.png
Cladogram showing Whippomorpha within Artiodactylamorpha: Whippomorpha consists of the clades labeled Hippopotamoidea and Cetaceamorpha.

It is unknown whether the last common ancestor of whales and hippos led an aquatic, semiaquatic/amphibious, or terrestrial lifestyle. Therefore, it is a matter of contention whether the aquatic traits of both hippopotamuses and cetaceans are linked or the product of convergent evolution. Recent findings seem to indicate that the latter is more likely. [5]

Whippomorpha diverged from other Cetartiodactyls approximately 59 Myr, whilst whales diverged from hippos approximately 55 Myr. [5] The first branch contained ancestors of Cetacea; semi-aquatic protowhales such as Pakicetus in the group Archaeoceti, which developed into the exclusively aquatic ancestors of modern cetaceans. [28]

One evolutionarily significant whale ancestor was the raoellid Indohyus , which was a Himalayas-dwelling, digitgrade omnivore roughly the size of a raccoon. It was not an adept swimmer, although it was thought to have spent considerable periods of time wading in shallow water. This would have been assisted by its heavy bones, providing stability. Indohyus was likely to have a diet at least partially based on aquatic foraging. Evidence for this includes the fact that the tooth enamel of Indohyus was considerably less worn than would be expected for an animal with an exclusively terrestrial diet. [20] One of the most crucial facets of the discovery of Indohyus was the presence of a thickened auditory bulla, otherwise known as an involucrum. This discovery was significant as the involucrum was a morphology thought previously to be exclusive to cetaceans, a synapomorphy. This feature irrefutably linked cetaceans to raoellids. [20]

An interpretation of Pakicetus Pakicetus BW.jpg
An interpretation of Pakicetus

It is thought that early whales such as Nalacetus and Pakicetus were restricted to freshwater environments, as modern hippopotamuses are. [28] The later Ambulocetus, was likely to have lived a much more aquatic lifestyle, with shorter legs and paddle-like hands and feet. It also likely represented a transitional organism from freshwater to seawater, as the isotopic analysis of the bones and teeth of Ambulocetus indicate that it inhabited estuaries. [29]

The second branch of Whippomorpha is thought to have developed into the Anthracotheriidae family, who were the putative ancestors of modern hippopotamuses. The sediments in which anthracotheriid fossils have been fossilized indicate that they were at least partially amphibious, whilst the jaw structure of fossils of select species, particularly Anthracotherium , seem to indicate that it was an ancestral form of modern hippopotamuses. [20]

These findings somewhat explain the once confusing paleontological age gap that existed as a major piece of evidence against an evolutionary link between Hippopotamidae and Cetacea. Previously, the oldest known cetacean fossils were approximately 50 Myr, while the earliest known hippopotamus fossils were around 15 Myr. [30] The sum of the fossil knowledge indicates that whales and hippopotamuses developed amphibious and aquatic traits independently from one another, but that the features developed by their shared ancestors created pathways to the development of said adaptations. [31] Thus the large difference in time between the discovery of cetacean and hippopotamid fossils is explained by the fact that hippos simply developed their semi-aquatic adaptations at a much later time than their cetacean cousins. [5]


Top: Skeleton of an adult and calf hippopotamus (Hippopotamus amphibius), Bottom: A blue whale calf (Balaenoptera musculus) Whippomorph skeletons.jpg
Top: Skeleton of an adult and calf hippopotamus (Hippopotamus amphibius), Bottom: A blue whale calf (Balaenoptera musculus)

All members of the suborder Whippomorpha share some anatomical similarities. Hippopotamus stomachs are multi-chambered as with all ruminants; however, they do not regurgitate food. Instead, the hippopotamus stomach contains two preliminary chambers, which acts similarly to a compost bin, allowing foodstuffs to ferment before entering the animal's main stomach. All whale species possess similar stomach structures. Additionally, both animals bear single-lobed lungs (similar to other aquatic mammals), which allow to be filled with air more rapidly. This is a critical adaptation for both amphibious and aquatic organisms, as it reduces the frequency of dangerous trips to the water surface, where such organisms are more vulnerable to predation. [32]

Hippos’ bodies contain a layer of dense fat, reminiscent of a whales’ blubber, and situated between skin and muscle. Hippos and whales both possess thick bones, which aid in rapid descent into water, have minimal hair (to aid in hydrodynamics) [31] and a lack of sweat glands. [33] Webbing is also present between the toes of hippopotamuses; a more land-suitable version of a whale's flippers. [31] Hippos possess unique hind-limb musculature that provides them with powerful propulsion capabilities, rather than fine-tuned control. These features are characteristic of other ungulates. [34]

There is strong resemblance between the dentition of primitive cetaceans and primitive ungulates, which seemingly cements the position of Cetacea within Artiodactyla. [22] In addition, both Cetaceans and Artiodactyls possess two distinct components in their ears, the involucrum and sigmoid process. Similar features are considered responsible for the ability of cetaceans to hear underwater. The skeletons of prehistoric whales also contain uniquely shaped ankle bones, including a double-pulley system found only in even-toed ungulates and crucially not present in odd-toed ungulates. [23]

Both hippos and whales have an unusually large and strangely shaped larynx, which enables the booming calls of whales underwater and the unique noises produced by hippos to communicate while submerged. [32]

Relationship with humans

Whippomorphs have always had complex cultural and social relationships with humans. Modern hippopotamuses have a reputation for extreme aggression. Hippos are incredibly territorial and protective of their young, and are the deadliest mammal in Africa, killing between two and three thousand people a year. [12] Despite this, hippos remain popular zoo animals. Hippos were hunted by ancient humans for food and sport. In Ancient Egypt, hippos were recognized as dangerous inhabitants of the river Nile, and a red hippo was the symbol of the god Set. The biblical Behemoth is thought to be based on or inspired by the hippo.

Modern hippopotamuses face a number of threats from humans. Common hippopotamuses are classed as vulnerable, and are subject to habitat destruction as a result of agriculture, water management, climate change and development of housing and urban areas. [35] Pygmy hippopotamuses are considered endangered, with less than three thousand individuals in the wild. The few surviving pygmy hippopotamuses occupy a much smaller habitat area in Liberia, Sierra Leone and the Ivory Coast. They face threats from mining and quarrying, hunting and logging. [36]

Two monsters of biblical legend: Behemoth (top); thought to be inspired by the hippopotamus, and Leviathan (bottom); thought to be inspired by whales Behemoth-Leviathan.jpg
Two monsters of biblical legend: Behemoth (top); thought to be inspired by the hippopotamus, and Leviathan (bottom); thought to be inspired by whales

Cetaceans have also had an extensive history with humans. The primary threats to modern cetaceans are direct danger (from whaling), and indirect damage to whale habitats (through pollution and overfishing). Commercial shipping, petroleum drilling and coastal development can disrupt cetacean habitats. Thousands of cetaceans are affected by bycatching every year. [37] Some evidence also exists that human-generated sound may account for increases in the rate of cetacean strandings. [38]

Whales were inspirations for many mythical creatures, including the Leviathan, which was interestingly associated with the Behemoth. Dolphins are mentioned in historical literature far more frequently than whales. Stories of dolphins typically include them playing a role in helping shipwrecked sailors or guiding lost ships. In the 20th century, perceptions of whales changed, and now tourism for the purposes of whale-watching has become very popular. Cetaceans are revered for their immense size, intelligent and playful dispositions, displays of speed in water, and contributions to scientific research.

Whales have been kept in captivity by humans for research and entertainment for centuries. Particularly popular are killer whales. Conservation and animal rights organizations have been vehemently opposed to the captivity of these cetaceans. It is common for captive killer whales to display aggression towards other whales and their trainers. Bottlenose dolphins are also popular, due to their friendly behavior. They also fare better in captivity than other cetaceans.

Related Research Articles

<span class="mw-page-title-main">Cetacea</span> Infraorder of mammals

Cetacea is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel themselves through the water with powerful up-and-down movement of their tail which ends in a paddle-like fluke, using their flipper-shaped forelimbs to maneuver.

<span class="mw-page-title-main">Ungulate</span> Group of animals that walk on the tips of their toes or hooves

Ungulates are members of the diverse clade Euungulata which primarily consists of large mammals with hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined to be a polyphyletic and thereby invalid clade based on molecular data. As a result, true ungulates had since been reclassified to the newer clade Euungulata in 2001 within the clade Laurasiatheria while Paenungulata has been reclassified to a distant clade Afrotheria. Living ungulates are divided into two orders: Perissodactyla including equines, rhinoceroses, and tapirs; and Artiodactyla including cattle, antelope, pigs, giraffes, camels, sheep, deer, and hippopotamuses, among others. Cetaceans such as whales, dolphins, and porpoises are also classified as artiodactyls, although they do not have hooves. Most terrestrial ungulates use the hoofed tips of their toes to support their body weight while standing or moving. Two other orders of ungulates, Notoungulata and Litopterna, both native to South America, became extinct at the end of the Pleistocene, around 12,000 years ago.

<span class="mw-page-title-main">Hippopotamus</span> Large semi-aquatic mammal native to sub-Saharan Africa

The hippopotamus, also shortened to hippo, further qualified as the common hippopotamus, Nile hippopotamus, or river hippopotamus, is a large semiaquatic mammal native to sub-Saharan Africa. It is one of only two extant species in the family Hippopotamidae, the other being the pygmy hippopotamus. Its name comes from the ancient Greek for "river horse" (ἱπποπόταμος).

<span class="mw-page-title-main">Artiodactyl</span> Order of mammals

Artiodactyls are placental mammals belonging to the order Artiodactyla. Typically, they are ungulates which bear weight equally on two of their five toes: the third and fourth, often in the form of a hoof. The other three toes are either present, absent, vestigial, or pointing posteriorly. By contrast, most perissodactyls bear weight on an odd number of the five toes. Another difference between the two is that many artiodactyls digest plant cellulose in one or more stomach chambers rather than in their intestine as perissodactyls do. The advent of molecular biology, along with new fossil discoveries, found that cetaceans fall within this taxonomic branch, being most closely related to hippopotamuses. Some modern taxonomists thus apply the name Cetartiodactyla to this group, while others opt to include cetaceans within the existing name of Artiodactyla. Some researchers use "even-toed ungulates" to exclude cetaceans and only include terrestrial artiodactyls, making the term paraphyletic in nature.

<span class="mw-page-title-main">Hippopotamidae</span> Family of mammals

Hippopotamidae is a family of stout, naked-skinned, and semiaquatic artiodactyl mammals, possessing three-chambered stomachs and walking on four toes on each foot. While they resemble pigs physiologically, their closest living relatives are the cetaceans. They are sometimes referred to as hippopotamids.

<span class="mw-page-title-main">Pygmy hippopotamus</span> Small species of hippopotamus from West Africa

The pygmy hippopotamus or pygmy hippo is a small hippopotamid which is native to the forests and swamps of West Africa, primarily in Liberia, with small populations in Sierra Leone, Guinea, and Ivory Coast. It has been extirpated from Nigeria.

<span class="mw-page-title-main">Tylopoda</span> Suborder of mammals

Tylopoda is a suborder of terrestrial herbivorous even-toed ungulates belonging to the order Artiodactyla. They are found in the wild in their native ranges of South America and Asia, while Australian feral camels are introduced. The group has a long fossil history in North America and Eurasia. Tylopoda appeared during the Eocene around 50 million years ago.

<span class="mw-page-title-main">Evolution of cetaceans</span>

The evolution of cetaceans is thought to have begun in the Indian subcontinent from even-toed ungulates (Artiodactyla) 50 million years ago (mya) and to have proceeded over a period of at least 15 million years. Cetaceans are fully aquatic marine mammals belonging to the order Artiodactyla and branched off from other artiodactyls around 50 mya. Cetaceans are thought to have evolved during the Eocene, the second epoch of the present-extending Cenozoic Era. Molecular and morphological analyses suggest Cetacea share a relatively recent closest common ancestor with hippopotami and that they are sister groups. Being mammals, they surface to breathe air; they have 5 finger bones (even-toed) in their fins; they nurse their young; and, despite their fully aquatic life style, they retain many skeletal features from their terrestrial ancestors. Research conducted in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea.

<span class="mw-page-title-main">Mesonychia</span> Extinct taxon of carnivorous ungulates

Mesonychia is an extinct taxon of small- to large-sized carnivorous ungulates related to artiodactyls. Mesonychians first appeared in the early Paleocene, went into a sharp decline at the end of the Eocene, and died out entirely when the last genus, Mongolestes, became extinct in the early Oligocene. In Asia, the record of their history suggests they grew gradually larger and more predatory over time, then shifted to scavenging and bone-crushing lifestyles before the group became extinct.

<i>Pakicetus</i> Genus of ancient whales

Pakicetus is an extinct genus of amphibious cetacean of the family Pakicetidae, which was endemic to Pakistan during the Ypresian period, about 50 million years ago. It was a wolf-like animal, about 1 metre to 2 metres long, and lived in and around water where it ate fish and other small animals. The vast majority of paleontologists regard it as the most basal whale, representing a transitional stage between land mammals and whales. It belongs to the even-toed ungulates with the closest living non-cetacean relative being the hippopotamus.

<span class="mw-page-title-main">Anthracotheriidae</span> Extinct family of mammals

Anthracotheriidae is a paraphyletic family of extinct, hippopotamus-like artiodactyl ungulates related to hippopotamuses and whales. The oldest genus, Elomeryx, first appeared during the middle Eocene in Asia. They thrived in Africa and Eurasia, with a few species ultimately entering North America during the Oligocene. They died out in Europe and Africa during the Miocene, possibly due to a combination of climatic changes and competition with other artiodactyls, including pigs and true hippopotamuses. The youngest genus, Merycopotamus, died out in Asia during the late Pliocene, possibly for the same reasons. The family is named after the first genus discovered, Anthracotherium, which means "coal beast", as the first fossils of it were found in Paleogene-aged coal beds in France. Fossil remains of the anthracothere genus were discovered by the Harvard University and Geological Survey of Pakistan joint research project (Y-GSP) in the well-dated middle and late Miocene deposits of the Pothohar Plateau in northern Pakistan.

<span class="mw-page-title-main">Cetruminantia</span> Taxonomic clade

The Cetruminantia are a clade made up of the Cetancodontamorpha and their closest living relatives, the Ruminantia.


Sinonyx is a genus of extinct, superficially wolf-like mesonychid mammals from the late Paleocene of China. It is within the family Mesonychidae, and cladistic analysis of a skull of Sinonyxjiashanensis identifies its closest relative as Ankalagon. S.jiashanensis was discovered in Anhui province, China, in the Tuijinshan formation.

<i>Indohyus</i> Genus of extinct artiodactyl mammals from Eocene Epoch

Indohyus is an extinct genus of digitigrade even-toed ungulates known from Eocene fossils in Asia. This small chevrotain-like animal found in the Himalayas is one of the earliest known non-cetacean ancestors of whales.

<span class="mw-page-title-main">Raoellidae</span> Family of mammals

The Raoellidae, previously grouped within Helohyidae, are an extinct family of semiaquatic digitigrade artiodactyls in the clade Whippomorpha. Fossils of raoellids are found in Eocene strata of South and Southeast Asia.

<i>Artiocetus</i> Genus of mammals

Artiocetus is an extinct genus of early whales belonging to the family Protocetidae. It was a close relative to Rodhocetus and its tarsals indicate it resembled an artiodactyl.

<span class="mw-page-title-main">Artiofabula</span> Clade of mammals comprising pigs, cows, hippos, and whales, among others

Artiofabula is a clade made up of the Suina and the Cetruminantia. The clade was found in molecular phylogenetic analyses and contradicted traditional relationships based on morphological analyses.

<span class="mw-page-title-main">Cetancodontamorpha</span> Clade of tetrapods

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 made to rename the clade Whippomorpha to Cetancodonta, but the former maintains precedent.

<span class="mw-page-title-main">Hans Thewissen</span> Dutch/American paleontologist

Johannes Gerardus Marie (Hans) Thewissen is a Dutch-American paleontologist known for his significant contributions to the field of whale evolution. Thewissen's fieldwork has led to the discovery of key fossils that have shed light on the transition of whales from land to water, including the discovery of Ambulocetus, Pakicetus, Indohyus, and Kutchicetus. In addition to his work on fossil discoveries, Thewissen also studies modern bowhead and beluga whales in Alaska, focusing on their biology and the implications of this knowledge for management and conservation efforts. His research has been instrumental in deepening our understanding of cetacean evolution and the adaptations that allowed these mammals to transition from terrestrial to fully aquatic lifestyles.

<span class="mw-page-title-main">Ancodonta</span> Infraorder of mammals

Ancodonta is an infraorder of semiaquatic artiodactyl ungulates including modern hippopotamus and all mammals closer to hippos than to cetaceans (whales). Ancodonts first appeared in the Middle Eocene, with some of the earliest representatives found in fossil deposits in Southeast Asia. Throughout their evolutionary history they have occupied different browsing and grazing niches in North America, Eurasia and Africa. The last continent is notable as they were among the first laurasiatherian mammals to have migrated to Africa from Europe, where they competed with the native afrothere herbivores for the same niches. Of the nearly 50 genera that have existed, only two of them are extant – Choeropsis and Hippopotamus. The interrelationships within the ancodonts has been contended. The traditional notion is that there at minimum two families Anthracotheriidae and Hippopotamidae and were merely sister taxa. However many detailed research of the dentition among ancodonts, as well as how some anthracotheres were similar to hippos in appearance, lead the current consensus where Anthracotheriidae is paraphyletic to Hippopotamidae. Among the anthracotheres members of Bothriodontinae are among the closest to the ancestry of hippos, with the Oligocene aged Epirigenys from Lokon, Turkana, Kenya being the sister taxon to hippos. In response of this many similar clade names have been used for this clade.


  1. Joeckel, R. M. (1990). "A functional interpretation of the masticatory system and paleoecology of entelodonts". Paleobiology. 16 (4): 459–482. Bibcode:1990Pbio...16..459J. doi:10.1017/S0094837300010198. S2CID   88949308.
  2. 1 2 Waddell, P. J.; Okada, N.; Hasegawa, M. (1999). "Towards resolving the interordinal relationships of placental mammals". Systematic Biology. 48 (1): 1–5. doi: 10.1093/sysbio/48.1.1 . JSTOR   2585262. PMID   12078634.
  3. 1 2 Beck, Robin M.D.; Bininda-Emonds, Olaf R.P.; Cardillo, Marcel; Liu, Fu-Guo; Purvis, Andy (2006). "A higher-level MRP supertree of placental mammals". BMC Evolutionary Biology. 6: 93. doi: 10.1186/1471-2148-6-93 . PMC   1654192 . PMID   17101039.
  4. Black, Riley. "How Did Whales Evolve?". Smithsonian Magazine. Retrieved 2020-11-04.
  5. 1 2 3 4 5 6 Tsagkogeorga, Georgia; McGowen, Michael R.; Davies, Kalina T. J.; Jarman, Simon; Polanowski, Andrea; Bertelsen, Mads F.; Rossiter, Stephen J. (September 2015). "A phylogenomic analysis of the role and timing of molecular adaptation in the aquatic transition of cetartiodactyl mammals". Royal Society Open Science. 2 (9): 150156. Bibcode:2015RSOS....250156T. doi:10.1098/rsos.150156. ISSN   2054-5703. PMC   4593674 . PMID   26473040.
  6. Spaulding, Michelle; O'Leary, Maureen A.; Gatesy, John (2009-09-23). "Relationships of Cetacea (Artiodactyla) among mammals: increased taxon sampling alters interpretations of key fossils and character evolution". PLOS ONE. 4 (9): e7062. Bibcode:2009PLoSO...4.7062S. doi: 10.1371/journal.pone.0007062 . ISSN   1932-6203. PMC   2740860 . PMID   19774069.
  7. 1 2 Asher, Robert J; Helgen, Kristofer M (2010-04-20). "Nomenclature and placental mammal phylogeny". BMC Evolutionary Biology. 10: 102. doi: 10.1186/1471-2148-10-102 . ISSN   1471-2148. PMC   2865478 . PMID   20406454.
  8. "Whale Habitat - Whale Facts and Information" . Retrieved 2020-11-18.
  9. "Narwhal - Facts, Pictures, Habitat, Behavior, Appearance". animalsadda.com. Retrieved 2020-11-18.
  10. Martino, R.; Pandolfi, L. (2022-07-03). "The Quaternary Hippopotamus records from Italy". Historical Biology. 34 (7): 1146–1156. doi:10.1080/08912963.2021.1965138. ISSN   0891-2963. S2CID   239713930.
  11. Jukar, Advait M.; Patnaik, Rajeev; Chauhan, Parth R.; Li, Hong-Chun; Lin, Jih-Pai (September 2019). "The youngest occurrence of Hexaprotodon Falconer and Cautley, 1836 (Hippopotamidae, Mammalia) from South Asia with a discussion on its extinction". Quaternary International. 528: 130–137. Bibcode:2019QuInt.528..130J. doi:10.1016/j.quaint.2019.01.005. S2CID   133765385.
  12. 1 2 "Hippopotamus Facts and Information!" . Retrieved 2020-11-18.
  13. van Aalderink, Eline (2020-10-22). "Whales sleep with half their brain to avoid drowning". Whale Scientists. Retrieved 2020-11-18.
  14. 1 2 Strauss, Bob. "Discover 10 Essential Hippopotamus Facts". ThoughtCo. Retrieved 2020-11-18.
  15. Jirik, Kate. "LibGuides: Hippopotamus (Hippopotamus amphibius) & Pygmy Hippopotamus (Choerpsis liberiensis) Fact Sheet: Behavior & Ecology". ielc.libguides.com. Retrieved 2020-11-18.
  16. Davies, Ella. "The truth about hippos: herbivore or cannibal?". www.bbc.com. Retrieved 2021-02-28.
  17. Dudley, Joseph P.; Hang'Ombe, Bernard Mudenda; Leendertz, Fabian H.; Dorward, Leejiah J.; Castro, Julio de; Subalusky, Amanda L.; Clauss, Marcus (2016). "Carnivory in the common hippopotamus Hippopotamus amphibius: implications for the ecology and epidemiology of anthrax in African landscapes". Mammal Review. 46 (3): 191–203. doi:10.1111/mam.12056. ISSN   1365-2907.
  18. 1 2 "cetacean | Life Span, Evolution, & Characteristics". Encyclopedia Britannica. Retrieved 2020-11-18.
  19. "What Is A Placental Mammal?". WorldAtlas. 10 April 2018. Retrieved 2020-11-18.
  20. 1 2 3 4 Thewissen, J. G. M.; Cooper, Lisa Noelle; Clementz, Mark T.; Bajpai, Sunil; Tiwari, B. N. (December 2007). "Whales originated from aquatic artiodactyls in the Eocene epoch of India". Nature. 450 (7173): 1190–1194. Bibcode:2007Natur.450.1190T. doi:10.1038/nature06343. ISSN   1476-4687. PMID   18097400. S2CID   4416444.
  21. Geisler, Jonathan H.; Uhen, Mark D. (2005-06-01). "Phylogenetic Relationships of Extinct Cetartiodactyls: Results of Simultaneous Analyses of Molecular, Morphological, and Stratigraphic Data". Journal of Mammalian Evolution. 12 (1): 145–160. doi:10.1007/s10914-005-4963-8. ISSN   1573-7055. S2CID   34683201.
  22. 1 2 Gatesy, J. (May 1997). "More DNA support for a Cetacea/Hippopotamidae clade: the blood-clotting protein gene gamma-fibrinogen". Molecular Biology and Evolution. 14 (5): 537–543. doi: 10.1093/oxfordjournals.molbev.a025790 . ISSN   0737-4038. PMID   9159931.
  23. 1 2 "Hippos and Whales: Unlikely Cousins". Royal Ontario Museum. Retrieved 2020-11-04.
  24. O'Leary, M.A.; Bloch, J.I.; Flynn, J.J.; Gaudin, T.J.; Giallombardo, A.; Giannini, N.P.; Goldberg, S.L.; Kraatz, B.P.; Luo, Z.-X.; Meng, J.; Ni, X.; Novacek, M.J.; Perini, F.A.; Randall, Z.S.; Rougier, G.W.; Sargis, E.J.; Silcox, M.T.; Simmons, N.B.; Spaulding, M.; Velazco, P.M.; Weksler, M.; Wible, J.R.; Cirranello, A.L. (2013). "The Placental Mammal Ancestor and the Post-K-Pg Radiation of Placentals". Science. 339 (6120): 662–667. Bibcode:2013Sci...339..662O. doi:10.1126/science.1229237. hdl: 11336/7302 . PMID   23393258. S2CID   206544776.
  25. Song, S.; Liu, L.; Edwards, S.V.; Wu, S. (2012). "Resolving conflict in eutherian mammal phylogeny using phylogenomics and the multispecies coalescent model". Proceedings of the National Academy of Sciences. 109 (37): 14942–14947. Bibcode:2012PNAS..10914942S. doi: 10.1073/pnas.1211733109 . PMC   3443116 . PMID   22930817.
  26. dos Reis, M.; Inoue, J.; Hasegawa, M.; Asher, R.J.; Donoghue, P.C.J.; Yang, Z. (2012). "Phylogenomic datasets provide both precision and accuracy in estimating the timescale of placental mammal phylogeny". Proceedings of the Royal Society B: Biological Sciences. 279 (1742): 3491–3500. doi: 10.1098/rspb.2012.0683 . PMC   3396900 . PMID   22628470.
  27. Upham, N.S.; Esselstyn, J.A.; Jetz, W. (2019). "Inferring the mammal tree: Species-level sets of phylogenies for questions in ecology, evolution, and conservation". PLOS Biology. 17 (12): e3000494. doi: 10.1371/journal.pbio.3000494 . PMC   6892540 . PMID   31800571.(see e.g. Fig S10)
  28. 1 2 Boisserie, Jean-Renaud; Lihoreau, Fabrice; Brunet, Michel (2005-02-01). "The position of Hippopotamidae within Cetartiodactyla". Proceedings of the National Academy of Sciences of the United States of America. 102 (5): 1537–1541. Bibcode:2005PNAS..102.1537B. doi: 10.1073/pnas.0409518102 . ISSN   0027-8424. PMC   547867 . PMID   15677331.
  29. "The evolution of whales". Understanding Evolution. Retrieved 14 October 2020.
  30. "Fossil reveals hippos related to whales". www.abc.net.au. AFP. 2015-02-25. Retrieved 2020-11-04.
  31. 1 2 3 "A Tale of Two Entities: Whales and Hippos | National Center for Science Education". ncse.ngo. Retrieved 2020-11-18.
  32. 1 2 twowheelednomad (2017-07-22). "Are Whales Like Hippos?". Juneau Whale Watching Tours and Excursions - Juneau, AK. Retrieved 2020-11-04.
  33. "Relationship Between the Hippopotamus & the Whale". animals.mom.com. Retrieved 2020-11-04.
  34. Fisher, Rebecca E.; Scott, Kathleen M.; Adrian, Brent (2010). "Hind limb myology of the common hippopotamus, Hippopotamus amphibius (Artiodactyla: Hippopotamidae)". Zoological Journal of the Linnean Society. 158 (3): 661–682. doi:10.1111/j.1096-3642.2009.00558.x. ISSN   1096-3642.
  35. Lewison, Rebecca; Pluháček, Jan (2016-06-16). "IUCN Red List of Threatened Species: Hippopotamus amphibius". IUCN Red List of Threatened Species. Retrieved 2020-11-20.
  36. Ransom, Chris; Robinson, Philip; Collen, Ben (2015-02-23). "IUCN Red List of Threatened Species: Choeropsis liberiensis". IUCN Red List of Threatened Species. Retrieved 2020-11-20.
  37. Government of Canada, Fisheries and Oceans Canada (2018-08-07). "Researching human impacts on marine mammals". www.dfo-mpo.gc.ca. Retrieved 2020-11-20.
  38. Hildebrand, John A. (2012). "Marine mammals and anthropogenic sound". Proceedings of the Seventh ACM International Conference on Underwater Networks and Systems - WUWNet '12. New York, New York, USA: ACM Press. p. 1. doi:10.1145/2398936.2398949. ISBN   978-1-4503-1773-3. S2CID   30035540.