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94-507: Whippomorpha or Cetancodonta is a group of artiodactyls that contains all living cetaceans ( whales , dolphins , and porpoises ) and the hippopotamids . All whippomorphs are descendants of the last common ancestor of Hippopotamus amphibius and Tursiops truncatus . This makes it a crown group . Whippomorpha is a suborder within the order Artiodactyla (even-toed ungulates). The placement of Whippomorpha within Artiodactyla

188-615: A virus that helped pave the transition from egg-laying to live-birth . The word placenta comes from the Latin word for a type of cake , from Greek πλακόεντα/πλακοῦντα plakóenta/plakoúnta , accusative of πλακόεις/πλακούς plakóeis/plakoús , "flat, slab-like", with reference to its round, flat appearance in humans. The classical plural is placentae , but the form placentas is more common in modern English. The placenta has evolved independently multiple times, probably starting in fish , where it originated multiple times, including

282-406: 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

376-412: A day grazing. They normally feed only on land, though occasional consumption of aquatic vegetation has been observed. 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 ). All whippomorphs are placental mammals, meaning that embryos are fed by the placenta , which draws nutrients from

470-517: A few bumps. In contrast, camels and ruminants have bumps that are crescent-shaped cusps ( selenodont ). Placenta Mammalian placentas probably first evolved about 150 million to 200 million years ago. The protein syncytin , found in the outer barrier of the placenta (the syncytiotrophoblast ) between mother and fetus, has a certain RNA signature in its genome that has led to the hypothesis that it originated from an ancient retrovirus : essentially

564-399: A form in which it comprises several distinct parts connected by blood vessels. The parts, called lobes, may number two, three, four, or more. Such placentas are described as bilobed/bilobular/bipartite, trilobed/trilobular/tripartite, and so on. If there is a clearly discernible main lobe and auxiliary lobe, the latter is called a succenturiate placenta . Sometimes the blood vessels connecting

658-504: A more land-suitable version of a whale's flippers. 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. There is strong resemblance between the dentition of primitive cetaceans and primitive ungulates, which seemingly cements the position of Cetacea within Artiodactyla. In addition, both cetaceans and artiodactyls possess two distinct components in their ears,

752-494: 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). Some attempts have been made to rename

846-465: A recognizable species in popular culture. 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. Hippopotamuses face a number of threats from humans. Common hippopotamuses are classed as vulnerable , and are subject to habitat destruction as

940-529: A relative of the baby. Nepalese think of the placenta as a friend of the baby; the orang Asli and Malay populations in Malay Peninsula regard it as the baby's older sibling. Native Hawaiians believe that the placenta is a part of the baby, and traditionally plant it with a tree that can then grow alongside the child. Various cultures in Indonesia , such as Javanese and Malay, believe that

1034-992: A result of agriculture , water management , climate change and development of housing and urban areas. 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 , poaching , and logging . Cetaceans have also had an extensive history with humans. The primary threats to 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. Some evidence also exists that human-generated sound may account for increases in

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1128-659: A shared evolutionary history between cetaceans and hippopotamuses. Furthermore, some genetic sequences have been found in both whales and hippopotamuses that are not present in the genomes of other mammals. This would indicate that these groups share ancestry. Whippomorpha's placement within Artiodactyla can be represented in the following cladogram : Tylopoda (camels) [REDACTED]   Suina (pigs) [REDACTED]   Ruminantia (ruminants) [REDACTED]     Hippopotamidae (hippopotamuses) [REDACTED]   Cetacea (whales) [REDACTED] It

1222-576: A similar aquatic lifestyle. Hippopotamuses appeared in the late Miocene and occupied Africa and Asia—they never got to the Americas. The camels ( Tylopoda ) were, during large parts of the Cenozoic , limited to North America; early forms like Cainotheriidae occupied Europe. Among the North American camels were groups like the stocky, short-legged Merycoidodontidae . They first appeared in

1316-477: A simple stomach that digests food. Thus, they were grouped together as non-ruminants (Porcine). All other even-toed ungulates have molars with a selenodont construction (crescent-shaped cusps) and have the ability to ruminate , which requires regurgitating food and re-chewing it. Differences in stomach construction indicated that rumination evolved independently between tylopods and ruminants ; therefore, tylopods were excluded from Ruminantia . The taxonomy that

1410-474: A slim build, lanky legs, and a long tail. Their hind legs were much longer than their front legs. The early to middle Eocene saw the emergence of the ancestors of most of today's mammals. Two formerly widespread, but now extinct, families of even-toed ungulates were Entelodontidae and Anthracotheriidae . Entelodonts existed from the middle Eocene to the early Miocene in Eurasia and North America. They had

1504-473: A stocky body with short legs and a massive head, which was characterized by two humps on the lower jaw bone. Anthracotheres had a large, porcine ( pig -like) build, with short legs and an elongated muzzle . This group appeared in the middle Eocene up until the Pliocene , and spread throughout Eurasia, Africa, and North America. Anthracotheres are thought to be the ancestors of hippos, and, likewise, probably led

1598-639: A time, allowing them to swim and surface during rest periods. Hippopotamuses surface to breathe every three to five minutes, a process that is partially subconscious, allowing them to do it whilst sleeping. 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. 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. A hippo normally spends up to five hours

1692-451: A transitional organism from freshwater to seawater , as the isotopic analysis of the bones and teeth of Ambulocetus indicate that it inhabited estuaries . The second branch of Whippomorpha is thought to have developed into the family Anthracotheriidae , 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

1786-424: A very flexible body, contributing to their speed by increasing their stride length. Many even-toed ungulates have a relatively large head. The skull is elongated and rather narrow; the frontal bone is enlarged near the back and displaces the parietal bone , which forms only part of the side of the cranium (especially in ruminants). Four families of even-toed ungulates have cranial appendages. These Pecora (with

1880-419: Is a matter of some contention, as hippopotamuses were previously considered to be more closely related to Suidae (pigs) and Tayassuidae (peccaries). 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. Modern whippomorphs all share

1974-461: Is critical for embryo survival. The placenta also provides a reservoir of blood for the fetus, delivering blood to it in case of hypotension and vice versa, comparable to a capacitor . Numerous pathologies can affect the placenta. The placenta often plays an important role in various cultures , with many societies conducting rituals regarding its disposal. In the Western world , the placenta

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2068-416: Is explained by the fact that hippos simply developed their semi-aquatic adaptations at a much later time than their cetacean cousins. 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

2162-411: Is increased. The increased diameter and straighter flow path both act to increase maternal blood flow to the placenta. There is relatively high pressure as the maternal blood fills intervillous space through these spiral arteries which bathe the fetal villi in blood, allowing an exchange of gases to take place. In humans and other hemochorial placentals, the maternal blood comes into direct contact with

2256-614: Is limited to the Arctic Ocean . By comparison, modern hippopotamuses are confined entirely to the African continent. Despite once being widespread across Europe , Africa and Asia , hippos are now considered vulnerable, and are limited to the lakes, rivers and wetlands of southern Africa. 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

2350-475: Is more adapted to a squeezing mastication , which is characteristic of omnivores . Camels and ruminants have fewer teeth; there is often a yawning diastema , a designated gap in the teeth where the molars are aligned for crushing plant matter. The incisors are often reduced in ruminants, and are completely absent in the upper jaw . The canines are enlarged and tusk-like in the Suina, and are used for digging in

2444-457: Is most often incinerated . Some cultures bury the placenta for various reasons. The Māori of New Zealand traditionally bury the placenta from a newborn child to emphasize the relationship between humans and the earth. Likewise, the Navajo bury the placenta and umbilical cord at a specially chosen site, particularly if the baby dies during birth. In Cambodia and Costa Rica , burial of

2538-708: Is therefore focused on anthracotheres (family Anthracotheriidae); one dating from the Eocene to Miocene was declared to be "hippo-like" upon discovery in the 19th century. A study from 2005 showed that the anthracotheres and hippopotamuses had very similar skulls , but differed in the adaptations of their teeth. It was nevertheless believed that cetaceans and anthracothereres descended from a common ancestor, and that hippopotamuses developed from anthracotheres. A study published in 2015 confirmed this, but also revealed that hippopotamuses were derived from older anthracotherians. The newly introduced genus Epirigenys from Eastern Africa

2632-525: Is thus the sister group of hippos. Linnaeus postulated a close relationship between camels and ruminants as early as the mid-1700s. Henri de Blainville recognized the similar anatomy of the limbs of pigs and hippos, and British zoologist Richard Owen coined the term "even-toed ungulates" and the scientific name "Artiodactyla" in 1848. Internal morphology (mainly the stomach and the molars) were used for classification. Suines (including pigs ) and hippopotamuses have molars with well-developed roots and

2726-413: Is traditionally thought to be sterile , but recent research suggests that a resident, non-pathogenic , and diverse population of microorganisms may be present in healthy tissue. However, whether these microbes exist or are clinically important is highly controversial and is the subject of active research. The placenta intermediates the transfer of nutrients between mother and fetus. The perfusion of

2820-593: 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. Whippomorpha diverged from other cetartiodactyls approximately 59 Myr , whilst whales diverged from hippos approximately 55 Myr. The first branch contained ancestors of Cetacea; semi-aquatic protowhales such as Pakicetus in

2914-404: Is vulnerable to persistent hypoxia or intermittent hypoxia and reoxygenation, which can lead to generation of excessive free radicals . This may contribute to pre-eclampsia and other pregnancy complications . It is proposed that melatonin plays a role as an antioxidant in the placenta. This begins at day 17–22. Placental expulsion begins as a physiological separation from the wall of

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3008-694: The Old World , exist today only in the Americas . South America was settled by even-toed ungulates only in the Pliocene, after the land bridge at the Isthmus of Panama formed some three million years ago. With only the peccaries, lamoids (or llamas ), and various species of capreoline deer , South America has comparatively fewer artiodactyl families than other continents, except Australia, which has no native species. The classification of artiodactyls

3102-428: The cancer testis antigen PAGE4 and expressed in cytotrophoblasts , CSH1 and KISS1 expressed in syncytiotrophoblasts , and PAPPA2 and PRG2 expressed in extravillous trophoblasts. The placenta begins to develop upon implantation of the blastocyst into the maternal endometrium , very early on in pregnancy at about week 4. The outer layer of the late blastocyst, is formed of trophoblasts , cells that form

3196-507: The hind legs have a reduction in the number of toes to three. The central axis of the leg is between the third and fourth toe. The first toe is missing in modern artiodactyls, and can only be found in now-extinct genera. The second and fifth toes are adapted differently between species: When camels have only two toes present, the claws are transformed into nails (while both are made of keratin , claws are curved and pointed while nails are flat and dull). These claws consist of three parts:

3290-426: 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 . Both hippos and whales have an unusually large and strangely shaped larynx , which enables

3384-412: The ruminants as the sister group of cetaceans. Subsequent studies established the close relationship between hippopotamuses and cetaceans; these studies were based on casein genes , SINEs , fibrinogen sequences, cytochrome and rRNA sequences, IRBP (and vWF ) gene sequences, adrenergic receptors , and apolipoproteins . In 2001, the fossil limbs of a Pakicetus (amphibioid cetacean

3478-500: 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 . Hippopotamoidea was formerly included to Suiformes with Suidae (pigs) and Tayassuidae (peccaries). Most of

3572-416: The barrier function of the placenta. The placenta grows throughout pregnancy . Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy week 14 (DM). In preparation for implantation of the blastocyst, the endometrium undergoes decidualization . Spiral arteries in the decidua are remodeled so that they become less convoluted and their diameter

3666-546: The booming calls of whales underwater and the unique noises produced by hippos to communicate while submerged. Whippomorphs have always had complex cultural and social relationships with humans. Hippopotamuses have a reputation for extreme aggression towards humans. Hippos are incredibly territorial and protective of their young, and are the deadliest mammal in Africa, killing between two and three thousand people each year. Despite this, hippos remain popular zoo animals and

3760-747: 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. Artiodactyl Cetartiodactyla Montgelard et al. 1997 Artiodactyls are placental mammals belonging to the order Artiodactyla ( / ˌ ɑːr t i oʊ ˈ d æ k t ɪ l ə / AR -tee-oh- DAK -tih-lə , from Ancient Greek ἄρτιος , ártios  'even' and δάκτυλος , dáktylos  'finger, toe'). Typically, they are ungulates which bear weight equally on two (an even number) of their five toes (the third and fourth, often in

3854-412: The earliest known hippopotamus fossils were around 15 Myr. 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. Thus the large difference in time between the discovery of cetacean and hippopotamid fossils

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3948-462: The early Eocene (about 53 million years ago). Since these findings almost simultaneously appeared in Europe , Asia , and North America , it is very difficult to accurately determine the origin of artiodactyls. The fossils are classified as belonging to the family Diacodexeidae ; their best-known and best-preserved member is Diacodexis . These were small animals, some as small as a hare , with

4042-609: The early Cenozoic ( Paleocene and Eocene ), which had hooves instead of claws on their feet. Their molars were adapted to a carnivorous diet, resembling the teeth in modern toothed whales , and, unlike other mammals, had a uniform construction. The suspected relations can be shown as follows:   Artiodactyla [REDACTED]   Mesonychia † [REDACTED]   Cetacea [REDACTED] Molecular findings and morphological indications suggest that artiodactyls, as traditionally defined, are paraphyletic with respect to cetaceans. Cetaceans are deeply nested within

4136-400: 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. 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

4230-521: 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. Later studies obtained findings that indicated almost 11,000 orthologous genes between cetaceans and hippopotamuses, in addition to numerous positive indicators of

4324-419: The exception of the musk deer ), have one of four types of cranial appendages: true horns, antlers , ossicones , or pronghorns . True horns have a bone core that is covered in a permanent sheath of keratin, and are found only in the bovids . Antlers are bony structures that are shed and replaced each year; they are found in deer (members of the family Cervidae ). They grow from a permanent outgrowth of

4418-522: The fetal chorion , though no fluid is exchanged. As the pressure decreases between pulses , the deoxygenated blood flows back through the endometrial veins. Maternal blood flow begins between days 5–12, and is approximately 600–700 ml/min at term. Deoxygenated fetal blood passes through umbilical arteries to the placenta. At the junction of umbilical cord and placenta, the umbilical arteries branch radially to form chorionic arteries . Chorionic arteries, in turn, branch into cotyledon arteries . In

4512-633: 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 orders is that many artiodactyls (except for Suina ) digest plant cellulose in one or more stomach chambers rather than in their intestine (as perissodactyls do). Molecular biology, along with new fossil discoveries, has found that cetaceans ( whales , dolphins , and porpoises ) fall within this taxonomic branch, being most closely related to hippopotamuses . Some modern taxonomists thus apply

4606-960: The former; the two groups together form a monophyletic taxon, for which the name Cetartiodactyla is sometimes used. Modern nomenclature divides Artiodactyla (or Cetartiodactyla) in four subordinate taxa: camelids (Tylopoda), pigs and peccaries (Suina), ruminants (Ruminantia), and hippos plus cetaceans (Whippomorpha). The presumed lineages within Artiodactyla can be represented in the following cladogram :   Tylopoda (camels) [REDACTED]   Suina (pigs) [REDACTED]   Tragulidae (mouse deer) [REDACTED]   Pecora (horn bearers) [REDACTED]   Hippopotamidae (hippopotamuses) [REDACTED]   Cetacea (whales) [REDACTED] The four summarized Artiodactyla taxa are divided into ten extant families: Although deer, musk deer, and pronghorns have traditionally been summarized as cervids (Cervioidea), molecular studies provide different—and inconsistent—results, so

4700-674: The frontal bone called the pedicle and can be branched, as in the white-tailed deer ( Odocoileus virginianus ), or palmate , as in the moose ( Alces alces ). Ossicones are permanent bone structures that fuse to the frontal or parietal bones during an animal's life and are found only in the Giraffidae . Pronghorns , while similar to horns in that they have keratinous sheaths covering permanent bone cores, are deciduous. All these cranial appendages can serve for posturing, battling for mating privilege, and for defense. In almost all cases, they are sexually dimorphic, and are often found only on

4794-459: The genus Poeciliopsis . Placentation has also evolved in some reptiles . The mammalian placenta evolved more than 100 million years ago and was a critical factor in the explosive diversification of placental mammals. Although all mammalian placentas have the same functions, there are important differences in structure and function in different groups of mammals. For example, human, bovine, equine and canine placentas are very different at both

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4888-438: The girl skill in digging clams, and expose boys' placentas to ravens to encourage future prophetic visions. In Turkey , the proper disposal of the placenta and umbilical cord is believed to promote devoutness in the child later in life. In Transylvania and Japan , interaction with a disposed placenta is thought to influence the parents' future fertility. Several cultures believe the placenta to be or have been alive, often

4982-406: The gross and the microscopic levels. Placentas of these species also differ in their ability to provide maternal immunoglobulins to the fetus. Placental mammals, including humans, have a chorioallantoic placenta that forms from the chorion and allantois . In humans, the placenta averages 22 cm (9 inch) in length and 2–2.5 cm (0.8–1 inch) in thickness, with the center being

5076-401: The ground and for defense. In ruminants, the males' upper canines are enlarged and used as a weapon in certain species (mouse deer, musk deer, water deer ); species with frontal weapons are usually missing the upper canines. The lower canines of ruminants resemble the incisors, so that these animals have eight uniform teeth in the frontal part of the lower jaw . The molars of porcine have only

5170-474: The group Archaeoceti , which developed into the exclusively aquatic ancestors of modern cetaceans. 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

5264-568: The intervillous spaces of the placenta with maternal blood allows the transfer of nutrients and oxygen from the mother to the fetus and the transfer of waste products and carbon dioxide back from the fetus to the maternal blood. Nutrient transfer to the fetus can occur via both active and passive transport . Placental nutrient metabolism was found to play a key role in limiting the transfer of some nutrients. Adverse pregnancy situations, such as those involving maternal diabetes or obesity , can increase or decrease levels of nutrient transporters in

5358-417: The jaw structure of fossils of select species, particularly Anthracotherium , seem to indicate that it was an ancestral form of modern hippopotamuses. 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

5452-601: The late Eocene and developed a great diversity of species in North America. Only in the late Miocene or early Pliocene did they migrate from North America into Eurasia. The North American varieties became extinct around 10,000 years ago. Suina (including pigs ) have been around since the Eocene. In the late Eocene or the Oligocene , two families stayed in Eurasia and Africa; the peccaries , which became extinct in

5546-407: The limbs are predominantly localized, which ensures that artiodactyls often have very slender legs. A clavicle is never present, and the scapula is very agile and swings back and forth for added mobility when running. The special construction of the legs causes the legs to be unable to rotate, which allows for greater stability when running at high speeds. In addition, many smaller artiodactyls have

5640-587: The lobes get in the way of fetal presentation during labor , which is called vasa previa . About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal mature placenta. Some 350 of these genes are more specifically expressed in the placenta and fewer than 100 genes are highly placenta specific. The corresponding specific proteins are mainly expressed in trophoblasts and have functions related to pregnancy . Examples of proteins with elevated expression in placenta compared to other organs and tissues are PEG10 and

5734-634: The males are consistently larger and heavier than the females. In deer, only the males boast antlers, and the horns of bovines are usually small or not present in females. Male Indian antelopes have a much darker coat than females. Almost all even-toed ungulates have fur, with the exception being the nearly hairless hippopotamus. Fur varies in length and coloration depending on the habitat. Species in cooler regions can shed their coat. Camouflaged coats come in colors of yellow, gray, brown, or black tones. Even-toed ungulates bear their name because they have an even number of toes (two or four)—in some peccaries,

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5828-463: The males. One exception is the species Rangifer tarandus , known as reindeer in Europe or caribou in North America, where both sexes can grow antlers yearly, though the females' antlers are typically smaller and not always present. There are two trends in terms of teeth within Artiodactyla. The Suina and hippopotamuses have a relatively large number of teeth (with some pigs having 44); their dentition

5922-410: The maternal-fetal barrier. Deterioration in placental functioning, referred to as placental insufficiency , may be related to mother-to-child transmission of some infectious diseases. A very small number of viruses including rubella virus , Zika virus and cytomegalovirus (CMV) can travel across the placental barrier, generally taking advantage of conditions at certain gestational periods as

6016-432: The mother's body. They are k-selected organisms, producing a limited number of offspring, but with a high rate of survival. 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

6110-421: The mouse deer, often reaches a body length of only 45 centimeters (18 in) and a weight of 1.5 kilograms (3.3 lb). The largest member, the hippopotamus, can grow up to 5 meters (16 ft) in length and weigh 4.5 metric tons (5 short tons), and the giraffe can grow to be 5.5 meters (18 ft) tall and 4.7 meters (15 ft) in body length. All even-toed ungulates display some form of sexual dimorphism :

6204-948: The name Cetartiodactyla ( / s ɪ ˌ t ɑːr t i oʊ ˈ d æ k t ɪ l ə / ) 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. The roughly 270 land-based even-toed ungulate species include pigs , peccaries , hippopotamuses , antelopes , deer , giraffes , camels , llamas , alpacas , sheep , goats and cattle . Many are herbivores, but suids are omnivorous, whereas cetaceans are entirely carnivorous. Artiodactyls are also known by many extinct groups such as anoplotheres , cainotheriids , merycoidodonts , entelodonts , anthracotheres , basilosaurids , and palaeomerycids . Many artiodactyls are of great dietary, economic, and cultural importance to humans. The oldest fossils of even-toed ungulates date back to

6298-428: The newborn with a carbon copy of the mother's long-term humoral immunity to see the infant through the crucial first months of extrauterine life. IgM antibodies, because of their larger size, cannot cross the placenta, one reason why infections acquired during pregnancy can be particularly hazardous for the fetus. The placenta and fetus may be regarded as a foreign body inside the mother and must be protected from

6392-412: The normal immune response of the mother that would cause it to be rejected . The placenta and fetus are thus treated as sites of immune privilege , with immune tolerance . For this purpose, the placenta uses several mechanisms : However, the placental barrier is not the sole means of evading the immune system, as foreign fetal cells also persist in the maternal circulation, on the other side of

6486-465: The outer layer of the placenta. This outer layer is divided into two further layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer. The syncytiotrophoblast is a multinucleated continuous cell layer that covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlying cytotrophoblasts, a process that continues throughout placental development. The syncytiotrophoblast contributes to

6580-428: The placenta develops. CMV and Zika travel from the maternal bloodstream via placental cells to the fetal bloodstream. Beginning as early as 13 weeks of gestation, and increasing linearly, with the largest transfer occurring in the third trimester, IgG antibodies can pass through the human placenta, providing protection to the fetus in utero . This passive immunity lingers for several months after birth, providing

6674-408: The placenta has a spirit and needs to be buried outside the family house. Some Malays would bury the baby's placenta with a pencil (if it is a boy) or a needle and thread (if it is a girl). In some cultures, the placenta is eaten, a practice known as placentophagy . In some eastern cultures, such as China , the dried placenta ( ziheche 紫 河 车 , literally "purple river car") is thought to be

6768-474: The placenta is believed to protect and ensure the health of the baby and the mother. If a mother dies in childbirth, the Aymara of Bolivia bury the placenta in a secret place so that the mother's spirit will not return to claim her baby's life. The placenta is believed by some communities to have power over the lives of the baby or its parents. The Kwakiutl of British Columbia bury girls' placentas to give

6862-435: The placenta potentially resulting in overgrowth or restricted growth of the fetus. Waste products excreted from the fetus such as urea , uric acid , and creatinine are transferred to the maternal blood by diffusion across the placenta. The placenta functions as a selective barrier between maternal and fetal cells, preventing maternal blood, proteins and microbes (including bacteria and most viruses ) from crossing

6956-486: The placental barrier. The trophoblast is the outer layer of cells of the blastocyst (see day 9 in Figure, above, showing the initial stages of human embryogenesis). Placental trophoblast cells have a unique genome-wide DNA methylation pattern determined by de novo methyltransferases during embryogenesis . This methylation pattern is principally required to regulate placental development and function, which in turn

7050-442: The plate (top and sides), the sole (bottom), and the bale (rear). In general, the claws of the forelegs are wider and blunter than those of the hind legs, and they are farther apart. Aside from camels, all even-toed ungulates put just the tip of the foremost phalanx on the ground. In even-toed ungulates, the bones of the stylopodium (upper arm or thigh bone) and zygopodiums (tibia and fibula) are usually elongated. The muscles of

7144-427: 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

7238-413: The question of phylogenetic systematics of infraorder Pecora (the horned ruminants) for the time being, cannot be answered. Artiodactyls are generally quadrupeds . Two major body types are known: suinids and hippopotamuses are characterized by a stocky body, short legs, and a large head; camels and ruminants, though, have a more slender build and lanky legs. Size varies considerably; the smallest member,

7332-535: The rate of cetacean strandings . Whales were inspirations for many mythical creatures, including the Leviathan , which was 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

7426-430: The risk of postpartum bleeding may be reduced in women offered active management of the third stage of labour, however there may be adverse effects and more research is necessary. The habit is to cut the cord immediately after birth, but it may be no medical reason to do this; on the contrary, not cutting the cord could sometimes help the baby in its adaptation to extrauterine life , for preterm infants. The placenta

7520-472: The sequence with that of other living beings—the more similar they are, the more closely they are related. Comparison of even-toed ungulate and cetaceans genetic material has shown that the closest living relatives of whales and hippopotamuses is the paraphyletic group Artiodactyla. Dan Graur and Desmond Higgins were among the first to come to this conclusion, and included a paper published in 1994. However, they did not recognize hippopotamuses and classified

7614-668: The size of a wolf) and Ichthyolestes (an early whale the size of a fox) were found in Pakistan. They were both archaeocetes ("ancient whales") from about 48 million years ago (in the Eocene). These findings showed that archaeocetes were more terrestrial than previously thought, and that the special construction of the talus (ankle bone) with a double-rolled joint surface, previously thought to be unique to even-toed ungulates, were also in early cetaceans. The mesonychians , another type of ungulate, did not show this special construction of

7708-472: The suborder Cetancodonta , due to the misleading utilization of the suffix -morpha for a crown group, as well as the risk of confusion with the clade Hippomorpha (which consists of equid perissodactyls ); however Whippomorpha maintains precedence. 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

7802-404: The surface in order to breathe for the first time. Hippopotamus calves suckle on land. 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. Whippomorpha is a suborder located within

7896-399: The surface of the placenta and further divide to form a network covered by a thin layer of cells. This results in the formation of villous tree structures. On the maternal side, these villous tree structures are grouped into lobules called cotyledons . In humans, the placenta usually has a disc shape, but size varies vastly between different mammalian species. The placenta occasionally takes

7990-521: The talus, and thus was concluded to not have the same ancestors as cetaceans. The oldest cetaceans date back to the early Eocene (53 million years ago), whereas the oldest known hippopotamus dates back only to the Miocene (15 million years ago). The hippopotamids are descended from the anthracotheres, a family of semiaquatic and terrestrial artiodactyls that appeared in the late Eocene, and are thought to have resembled small- or narrow-headed hippos. Research

8084-449: The thickest, and the edges being the thinnest. It typically weighs approximately 500 grams (just over 1 lb). It has a dark reddish-blue or crimson color. It connects to the fetus by an umbilical cord of approximately 55–60 cm (22–24 inch) in length, which contains two umbilical arteries and one umbilical vein . The umbilical cord inserts into the chorionic plate (has an eccentric attachment). Vessels branch out over

8178-479: The traditional order Artiodactyla and infraorder Cetacea are sometimes subsumed into the more inclusive Cetartiodactyla taxon. An alternative approach is to include both land-dwelling even-toed ungulates and ocean-dwelling cetaceans in a revised Artiodactyla taxon. In the 1990s, biological systematics used not only morphology and fossils to classify organisms, but also molecular biology . Molecular biology involves sequencing an organism's DNA and RNA and comparing

8272-435: The uterus. The period from just after the child is born until just after the placenta is expelled is called the "third stage of labor". Placental expulsion can be managed actively, for example by giving oxytocin via intramuscular injection followed by cord traction to assist in delivering the placenta. Alternatively, it can be managed expectantly, allowing the placenta to be expelled without medical assistance. Blood loss and

8366-502: The villi, these vessels eventually branch to form an extensive arterio-capillary-venous system, bringing the fetal blood extremely close to the maternal blood; but no intermingling of fetal and maternal blood occurs ("placental barrier"). Endothelin and prostanoids cause vasoconstriction in placental arteries, while nitric oxide causes vasodilation . On the other hand, there is no neural vascular regulation, and catecholamines have only little effect. The fetoplacental circulation

8460-421: The water surface, where such organisms are more vulnerable to predation . 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 ) and a lack of sweat glands. Webbing is also present between the toes of hippopotamuses;

8554-426: Was hotly debated because ocean-dwelling cetaceans evolved from land-dwelling even-toed ungulates. Some semiaquatic even-toed ungulates ( hippopotamuses ) are more closely related to ocean-dwelling cetaceans than to other even-toed ungulates. Phylogenetic classification only recognizes monophyletic taxa; that is, groups that descend from a common ancestor and include all of its descendants. To address this problem,

8648-412: 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. 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

8742-483: Was significant as the involucrum was a morphology thought previously to be exclusive to cetaceans, a synapomorphy . This feature irrefutably linked cetaceans to raoellids. It is thought that early whales such as Nalacetus and Pakicetus were restricted to freshwater environments, as modern hippopotamuses are. 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

8836-621: Was widely accepted by the end of the 20th century was:   Suidae [REDACTED]   Hippopotamidae [REDACTED]   Tylopoda [REDACTED]   Tragulidae [REDACTED]   Pecora [REDACTED] Modern cetaceans are highly adapted sea creatures which, morphologically, have little in common with land mammals; they are similar to other marine mammals , such as seals and sea cows , due to convergent evolution . However, they evolved from originally terrestrial mammals. The most likely ancestors were long thought to be mesonychians—large, carnivorous animals from

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