Giraffe ( Giraffa camelopardalis )
|Owen , 1848
The cloven-hoofed animals , also Paarzehige ungulates or Paarzeher (Artiodactyla, formerly Paraxonia), are in the classical system one order of mammals (Mammalia). They are mainly herbivorous animals, which, in contrast to the odd-toed ungulates, are usually characterized by an even number of toes (two or four). This group includes some of the economically most important groups of mammals such as cattle , pigs , camels , goats and sheep , but also other well-known animals such as giraffes , hippos , deer and antelopes . Molecular biology studies have shown that the even ungulates are likely to be paraphyletic about whales . This means that some groups (especially the hippos) are more closely related to the whales than to the other members of this order. Modern phylogenetic systematics accordingly group cloven-toed ungulates and whales to form the common taxon of Cetartiodactyla . The artifacts therefore form a form taxon , i.e. a group that does not represent a closed community of descent, but is characterized by common features.
Artifacts can be divided into four sub-categories:
- The pigs (Suina) consist of the real or old world pigs and the umbilical pigs or peccaries.
- The hippos belong to their own suborder, the Ancodonta (including the whales as the closest related group, also called Cetancodonta).
- The camels - including the New World species - are the only recent representatives of the subordination of the callous soles (Tylopoda).
- The ruminants (Ruminantia) consist, among other things, of giraffes , deer and horned bearers , which include, for example, cattle , goat-like animals and animals known as gazelles and antelopes .
The artifacts have some common features, especially in the construction of the limbs, which are not found in the ( recent ) whales that are highly adapted to aquatic life. Common derived features ( synapomorphies ) of the ungulates were a special structure of the ankle bone with two joint rollers ( trochlea tali proximalis and distalis ) and the elongated last lower anterior tooth . However, the discovery of this special talus in fossil whales in 2001 gave new food to discussions about the systematics (see below ).
General build and coat
The cloven-hoofed animals are medium to large animals to fours ( quadruped move). There are two types of shape, which clearly differ in their physique. Pig-like animals and hippos, for example, are characterized by a stocky trunk, short legs and a large head. Callus soles and ruminants, on the other hand, have a rather slim physique and long, thin legs. The size of the ungulates varies considerably. The smallest representatives are the deer piglets , which often only reach 45 centimeters head length and a weight of 1.5 kilograms. The largest representatives are - with a head body length of up to 5 meters and weight 4.5 tons - the hippopotamus or - with a height of up to 5.5 meters and a head body length of 4.7 meters - the giraffe . With regard to size, almost all species have a sexual dimorphism : the males are consistently larger and heavier than the females. There are also gender differences in forehead weapons: in deer, only the males usually have antlers and the horns of the hornbeams are usually significantly smaller or completely absent in the females. There can also be sexual dimorphisms with regard to other aspects such as teeth or coat color. For example, the deer goat antelope , in which the fur of the males is dark brown, while that of the females is ocher.
With the exception of the almost hairless hippos, all representatives of this order are covered with thick fur , the length and color of which vary depending on the habitat. Species in cooler regions can change their fur . Inconspicuous, camouflaging fur colors such as yellow, gray, brown or black tones predominate.
Limbs and musculoskeletal system
External structure of the limbs
Artifacts get their name because they usually have an even number of toes (two or four) - only in some umbilical pigs has the number of toes on the hind legs been reduced to three. The central axis of a leg lies between the third and fourth toe in an even-toed ungulate. These two middle toes are best developed. The originally present first toe is always missing in today's even-toed ungulates, it is only documented in fossil representatives. The second and fifth toes are designed differently: in the hippopotamus , they are directed forward and fully functional. In the other pair of ungulates, they are arranged backwards (and are referred to as "anal toe" or "dewclaw") or completely reduced. In the case of pigs and deer piglets, the anus toes are still used on soft, muddy ground and increase the surface area. In most cases, however, they no longer touch the ground. In some groups, such as camels and giraffes , regression has progressed so far that the second and fifth toes are no longer even present as rudiments .
In hippos , all four toes point forward.
In pigs and many other cloven-hoofed animals, the second and fifth toes point backwards as dewclaws.
In camels only two toes are present, the claws are transformed into nails.
In spite of the term "even-toed ungulate", the term hoof for the toe end organ is strictly speaking reserved for horses ; in the case of the art of cloven-hoofed animals, one speaks correctly of claws . These claws are made of horny substance and originally consist of three parts: the plate (above and on the sides), the sole (below) and the ball of the foot (at the back), which can, however, be fused to different extents. In general, the claws of the forelegs are wider and more blunt than those of the hind legs, and they gape more sharply apart. With the exception of the camels, all artifacts are tip-toe , which means that they only touch the tip of the foremost toe link on the ground. In camels, the horns on the feet are reduced to nails , the toes rest on an elastic pad made of connective tissue that forms a wide sole surface. Hence the name of the subordination of the calluses (Tylopoda), whose only recent representatives are the camels.
There is a tendency for the metapodia ( metacarpal and metatarsal bones ) to grow together in the ungulates . In pigs and hippos, they are still separate and only coupled by tight connective tissue. In camels and ruminants , the 3rd and 4th metapodium are fused to form a unit, the main metatarsal bone, the origin of which from two bones is often still visible through longitudinal grooves on the front and back ( sulcus longitudinalis dorsalis and palmaris or plantaris ).
The bones of the stylopodium ( upper arm or thigh bone ) and zygopodium ( ulna and radius or tibia and fibula ) are usually elongated. The muscles of the limbs are mainly localized close to the trunk, which means that even ungulates often have very slender legs. There is never a collarbone , the shoulder blade is very flexible, its swinging back and forth provides additional mobility when running fast.
The special construction of the legs ensures a stiff position of the lower limbs. Rotational movements of the legs are hardly possible any more, but the immobility causes greater stability when moving faster. In addition, many smaller cloven-toed ungulates have a very flexible torso, the flexibility of which increases the stride length when escaping. In addition to the selection pressure to achieve high speeds when fleeing, the specialized construction of the limbs also increases energy savings when moving slowly, for example when eating.
Head and teeth
Many ungulates have relatively large heads. The front (pre-orbital) part of the skull is often elongated and narrow, the nasal bones tapering forward in one or two points. The frontal bone is enlarged towards the back (caudad) and displaces the parietal bone , which in some ruminants (e.g. in many horned animals ) no longer has any part in the upper (dorsal) part of the roof of the skull.
The lips are flexible and strongly muscled. In some groups such as the pig-like species, but also in some ruminants such as the saiga and the dikdik , a trunk has been formed by lengthening the upper lip .
Four families of cloven-hoofed animals, the giraffe-like , deer , pronghorn carrier and Hornträger have forehead weapons. The group of forehead weapon bearers (Pecora) includes with the musk deer and water turning but also representatives without forehead weapons . The forehead weapons (usually referred to as horn or antler) are mostly outgrowths of the frontal bone that are built differently. The giraffe-like have bone cones that are covered with hairy skin. Deer are characterized by antlers that grow from cone-shaped bones ("rose bushes"). It consists of bone substance and is shed and newly formed every year after the mating season. In contrast, the horns of the hornbeams consist of horn substance on a bone cone and are usually retained for a lifetime. The skin that covers the bone cone secretes horny cells that eventually form a hard horny sheath. The oldest horn layers are shifted further and further towards the horn tip. With the exception of the four-horned antelope (and some domestic animal breeds, e.g. Jacob's sheep ), all horned animals have two horns. Finally, the horns of the forked horns are formed similarly to those of the horned ones, in contrast to this, the horn sheaths are shed annually.
The forehead weapons can be used for display, the fight for mating privilege and defense. In almost all cases they are sexually dimorphic , that is, larger in males than in females. In some species such as almost all deer, some forest goats and the okapi , the females generally lack them.
The dentition of the ungulates is variable, but two trends can be identified. The pigs and hippos have a relatively large number of teeth (some real pigs even have the original number of teeth of the higher mammals (44)). The teeth are more adapted to a squeezing chewing process, which corresponds to the omnivorous diet of these animals. In camels and ruminants, the number of teeth is reduced, there is often a gap in the teeth known as a diastema , and the molars are geared towards grinding the plant food.
The incisors are often reduced, in ruminants they are completely absent on the upper jaw, instead the lower incisors press against a dental plate . The canine teeth are different: in the pig-like species they are enlarged and developed like a tusk, they are used to dig in the ground and for defense. In ruminants, the upper canine tooth in males of species without forehead weapons (deer piglets, musk deer , water deer ) is enlarged and is used as a weapon in the fight for mating privilege. In species with forehead weapons, however, the upper canine is usually missing. The lower canines of the ruminants resemble the incisors, so that these animals have eight uniform teeth in the front part of the lower jaw.
The molars of the pig-like species are low-crowned and have a few cusps. In contrast, those of the camels and ruminants have a high crown and the humps are formed into crescent-shaped melting strips (selenodont).
The sense of smell , which, as in most mammals, is very well developed, is primarily used to perceive the environment . The sense of hearing is also well developed, supported by the flexible auricles found in many species . Unlike many other mammals also is sight pronounced at least in ruminants and camels. Above all, movement vision is developed, immobile objects tend not to be perceived. Similar to many other animals, which have to be constantly alert to predators, the eyes are attached to the side of the head, which allows an almost complete all-round view and the earliest possible detection of threats.
Soft tissue anatomy
In order to adapt to the difficult to digest plant food, the ungulates have developed some peculiarities of the digestive tract , which are particularly pronounced in the ruminants. In the area of the mouth there are often additional salivary glands and the oral mucosa is often very keratinized in order to avoid injuries from hard plant parts and to enable the roughly chewed food to be transported more easily.
In the structure of the stomach , the ungulates probably show the highest specializations among all mammals, whereby several stomach sections have developed independently of each other. It is most easily built in real pigs, which still have a simple, sack-shaped stomach. In umbilical pigs and hippos, several blind sacks were developed in which the food is broken down by microorganisms. The stomach of the camels is three-parted and is divided into two fore-stomachs, which, however, in contrast to those of the ruminants , are equipped with glands , and the actual abomasum . Camels can ruminate, but are not included in the ruminant taxon .
Finally, the stomach of ruminants is divided into three or four sections: the rumen ( rumen ), the reticulum ( reticulum ), the leaf stomach ( omasum ) and the abomasum ( abomasum ). The stag piglets, classified as particularly primitive within the ruminants, lack the leaf stomach, otherwise all species of this subordination show the same structure and the same digestive system. The forestomach are free of glands. Here, the roughly chewed food is broken down by microorganisms and returned in small portions to the oral cavity, where it is chewed on again before it enters the actual stomach and is digested further. (For details see the article ruminants ). This digestion method offers two advantages: on the one hand, the difficult to digest plant food can be broken down and used in the best possible way. On the other hand, the duration of the actual food intake - especially with the body posture with the head close to the ground, which is unfavorable for the perception of the environment - is shortened, which is advantageous with regard to the threat posed by predators; ruminating can then take place in sheltered places.
The intestines of cloven hoofed animals is like many herbivorous mammals generally very long, the small intestine is heavily looped. In contrast to the odd-toed ungulates , where fermentation only takes place in the intestine, the caecum and colon are simpler and less voluminous.
Urinary and sexual apparatus
The structure of the urinary and sexual apparatus of the ungulates also shows some abnormalities. The penis is bent in an S-shape when at rest and rests in a skin pocket on the abdomen. It is fibroelastic, which means that the erectile tissue is only slightly developed and the erection mainly causes this curvature to be stretched and thus lengthened, but hardly thickened. This structure of the penis is found in a similar way in the whales and is an indication of the close relationship. The testicles are located in the scrotum and thus outside the abdominal cavity . The ovaries of many females go through a descent ( Descensus ovarii ) - comparable to the testicular descent of many male mammals - and are located near the pelvic entrance at the level of the fourth lumbar vertebra . The uterus has two horns ( uterus bicornis ).
Some ungulates have developed peculiarities in the circulatory system. The heart of real pigs has a pair of cartilage deposits between atria and ventricles. In some ruminants, two heart bones ( ossa cordis ) are formed that stabilize the aortic opening .
The number of mammary glands is variable and, like all mammals, roughly correlates with the litter size. Originally there were probably two rows of teats from the armpit to the groin area. This original arrangement can still be found in some real pigs, which also have the largest litter size of all articulated ungulates. In most cases, however, there has been a reduction in the number of teats, and the remaining cloven-hoofed animals only have one or two pairs of teats. In some species these form an udder in the groin region.
Secretory glands in the skin are present in almost all species and can be located in a wide variety of places, for example in front of the eyes, behind the horns, on the neck or back, on the feet or in the anal region.
distribution and habitat
Cloven-hoofed animals, with the exception of the Australian - oceanic space that Antarctica widespread and many remote islands in the world. The species focus today is in Africa and Asia . In America the group is relatively poor in species, especially in South America , where only umbilical pigs, New World camels ( llamas and vicunas ) and deer occur. Here, other groups such as the extinct South American ungulates and some rodents (for example capybaras , pampas hares or agoutis ) have occupied similar ecological niches . Humans have distributed various artifacts as domestic or hunting animals worldwide, so that these animals can be found almost everywhere today where there are humans.
Even-toed ungulates inhabit almost all habitats, from tropical rainforests and steppes to desert areas and high mountain regions . The greatest biodiversity, however, is found in open habitats such as grasslands and open forests. These animals are pronounced ground dwellers, only a few species lead a semi-aquatic (partially in the water) way of life, such as the hippopotamus. Some species have colonized the high mountains and are excellent climbers.
Way of life
Social behavior and activity times
The social behavior of the ungulates is variable. In general, however, there is a tendency to join together in larger groups, but there are also animals that live solitary or permanently in pairs (such as the deer piglets). In species that live in groups, a hierarchy often develops, both among males and females. However, a number of species also live in harem groups , which means that a single male gathers a few females and the common offspring around him and does not tolerate any rivals. In other species, the females and young animals form their own groups for most of the year, while the males live solitary or in bachelorette groups and only visit the female groups during the mating season. With many even-toed ungulates, during the mating season there is bitter fighting for the mating privilege between the males, which is fought with the forehead arms, the tusk-like canine teeth or in some other way.
Many ungulates are territorial and mark their territory, for example with glandular secretions or urine. In addition to species that are true to their location all year round, there are also animals that undertake seasonal migrations in search of better feeding places.
No general statements can be made about the activity times. There are day, crepuscular and nocturnal representatives as well as species in which the day-night scheme can vary depending on the season or habitat.
Most of the ungulates are herbivores , the range of food for which can vary depending on the species and habitat. Often grass, herbs or leaves, but also other parts of plants such as tubers or fruits are consumed. Real pigs, umbilical pigs and, to a lesser extent, deer piglets are omnivores that supplement their diet with insects, worms and sometimes small vertebrates. Most species depend on a daily intake of water, but some dry-living species can survive for weeks without drinking - the best-known example is the camels.
Reproduction and Life Expectancy
In general, there is a tendency for the ungulate to long gestation periods , small litter size and high degree of development of the newborns. As with many other mammals, species in temperate or polar regions have a fixed mating season, while species in tropical areas can often reproduce year-round. According to the way of life, a polygynous mating behavior predominates , so a male often mates with several females. The mating usually takes place through the mammalian typical "Aufreiten" only by the camels it is performed lying down.
The gestation period varies between 4 and 5 months for pigs, deer piglets and musk deer , 6 to 10 months for hippopotamuses, deer and horned animals , 10 to 13 months for camels and 14 to 15 months for giraffes . Usually one or two pups are born, but with some pigs it can be up to ten.
The newborns of all species flee the nest and are born with their eyes open and, with the exception of the generally hairless hippos, hairy. The striped or spotted fur of the young animals, which is used as camouflage and is lost as they grow up, is typical of some ungulates (such as pigs or deer). The young of some species spend their first weeks with their mother in a sheltered place, others can run soon after birth and follow the herd within a few hours or days.
Life expectancy is usually 20 to 30 years, as with many mammals, smaller species often have a shorter life span than large species. Animals like hippos, cattle and camels, which can reach 40 to 50 years, are the oldest .
Predators and parasites
Depending on their size and habitat, the ungulates have different natural enemies, but it is often predators such as cats , dogs or bears that hunt these animals. Other predators are crocodiles , large birds of prey and, in the case of small species and juveniles, giant snakes .
The animals that parasitize cloven-hoofed animals include tapeworms and roundworms , botflies , fleas , animal lice and flukes , but these only have a weakening effect on the animals when they are heavily infested.
Human and arthropod
Since the earliest times, ungulates have been hunted by humans for a variety of reasons: to eat their meat, to make their fur into clothing, and to use their forehead weapons, bones and teeth as weapons or tools. Later, humans not only limited themselves to hunting, but also tried to keep some species close to them and to breed them. The domestication of animals began v later than the eighth millennium. BC, when wild goats , wild sheep and wild cattle , a little later also the wild boar , were domesticated as domestic goats , domestic sheep , domestic cattle and domestic pigs . At first, livestock served primarily as a supplier of food, later animals were also used for work, for example since around 3000 BC. The dromedary and the lama . The process of domestication was complex, genetic studies indicate that in many domestic animals in different regions this step took place several times independently of one another.
Today even ungulates are kept for a variety of reasons. These are primarily the enjoyment of their meat , the production of milk and the processing of their skin or fur into leather and other clothing or the shearing to obtain the wool . Some species are also used as working, draft, riding or pack animals, such as domestic cattle , water buffalo , yak or various camels.
With regard to domestication, two basic types can be distinguished. On the one hand, there are animals that have been bred in various breeds , which are distributed worldwide and which in some cases differ considerably from the wild form, such as domestic cattle , domestic pigs , domestic goats and domestic sheep . Other domestic animals have largely remained in their area of origin and have not changed much compared to the wild form, such as the reindeer , some cattle (such as the water buffalo , the banteng , the gaur or the yak ) and camels (such as the dromedary , the trample , the llama or the alpaca ).
Some wild ungulates are not only hunted for food, but also for hunting reasons. Such practices, which are carried out not out of necessity, but to win trophies , are sometimes heavily criticized and have pushed some species, such as the Alpine ibex or the Arabian oryx , to the brink of extinction.
The degree of endangerment of the individual ungulate species is different. Some species have been able to expand their habitat as cultural followers (such as the wild boar ) or have been brought by humans as parked animals or runaway pets to regions in which they were not previously native. Some even-toed ungulates have also benefited from the fact that their predators (primarily predators ) as food competitors of the cattle breeders have been decimated by them, in some cases considerably.
In return, many ungulates have declined significantly and some ungulates have even been exterminated. The reasons for this lie in hunting and, more recently, in the increasing destruction of their habitat. Several species of gazelle (the Algerian gazelle ), several Malagasy hippopotamus species , the blue goat and the Schomburgk deer are extinct . Even the wild cattle , the root form of the house cow , has disappeared in the 17th century. Two species, the Arabian oryx and the Saudi gazelle , are listed by the IUCN as extinct in the wild , which means that only stocks in breeding programs or zoos exist. 14 species are " critically endangered ", including the Mendes antelope , the Kouprey , the wild form of the trample , the David deer , the Przewalski gazelle , the saiga , the Vietnamese wood cattle and the dwarf wild boar . Another 24 species are listed as endangered and 36 species are listed as vulnerable .
Systematics and tribal history
The systematics of the ungulates is hotly debated. The reason for this is that, on the one hand, they are a morphologically (by their physique) clearly defined order , but on the other hand the whales have evolved from them and some groups (especially the hippos) are more closely related to them than to the other cloven-hoofed animals. This makes the ungulates in phylogenetic (defined by the evolution of the tribe) systematics - which have become more authoritative in recent times - to a paraphyletic taxon , i.e. a group that descends from a common ancestral form but does not include all descendants of this ancestor. Since the phylogenetic classification by way only monophyletic taxa recognizes, that is, groups derived from a common ancestor, and include all descendants of this ancestor must the cloven with whales as a Cetartiodactyla designated taxon are summarized. Here, the traditional systematics will first be presented and then the phylogenetic view of the ungulates will be explained.
The traditional system
The traditional system of the ungulates
Carl von Linné already postulated a close relationship between camels and ruminants. Henri de Blainville recognized the similar structure of the limbs of these animals to those of pigs and hippos and the English zoologist Richard Owen coined the term "even-toed ungulates" ("pair-toed ungulates") and the scientific name Artiodactyla in 1848.
Since then, the composition of this group has been clear and has hardly ever been questioned. The structure of the stomach and the molars served for the internal systematics . For example, pigs, peccaries and hippos have low-crowned molars and a simple stomach, and they digest directly without chewing the chew. That is why they were grouped together as non-ruminants (Nonruminantia) or pig-like species in the broader sense (Suina or Neobunodontia). All other artifacts have high-crowned molars with a selenodontic structure (crescent-shaped melting strips) and are able to ruminate, which is why they form the group of Selenodontia. Differences in the structure of the stomach suggested that the ability to ruminate twice developed independently of one another; therefore the camels are not counted among the actual ruminants (Ruminantia), but compared to them as callous soles (Tylopoda). Within the ruminants, the primeval, forehead -weapon-free stag piglets stand in opposition to all other groups that are grouped together as forehead-weapon bearers (Pecora).
From a purely morphological point of view, the following presumed ancestry relationships emerged, which were largely recognized until the end of the 20th century:
The traditional position of the whales
The recent whales are highly adapted sea creatures, which outwardly have little in common with other mammals - similarities with other marine mammals such as seals and manatees are based solely on convergence . But it stands to reason that they must have evolved from land-dwelling mammals. For a long time, the mesonychia was considered the most likely candidate for the ancestors of the whales . Some of these were gigantic, carnivorous animals from the early Cenozoic ( Paleocene and Eocene ) that had hooves instead of claws on their feet. Their limbs did not have the pair-hoofed structure of the ankle bone , which was not known from fossil whales until recently. Their molars were adapted to an animal diet and are similar to the teeth of today's toothed whales, which are designed for fish-eating food and, in contrast to other mammals, have a uniform (homodontic) structure.
The mesonychia were believed to be close relatives of the ungulates, so it was well recognized that the ungulates and whales are the closest living relatives of each other. This close relationship could also be confirmed by morphological similarities, for example in the structure of the penis or the arrangement of the bronchi . The presumed parentage can be reproduced as follows:
The phylogenetic system
In the 1990s, a start was made on developing the biological system, not only from the point of view of body structure and fossil evidence, but also by means of molecular biological studies. The attempt is made to obtain genetic information by sequencing the DNA and RNA and to compare it with the data of other living beings in order to determine the degree of relationship based on the degree of similarity. This method was and is used in numerous living things and has significantly changed the systematics of many taxa. These methods were also carried out on the cloven-hoofed and whales, with the surprising result that the closest relatives of the whales, the hippos and the cloven-hoofed, are a paraphyletic group.
Among the first to come to this conclusion were Dan Graur and Desmond Higgins with a study published in 1994. However, they did not take the hippos into account and thought the ruminants were the sister group of the whales. Subsequent investigations then came to the conclusion that the hippos are the closest living relatives of the whales; this was based on, among other things, casein genes , SINEs , fibrinogen sequences, cytochrome and rRNA sequences, IRBP and vWF gene sequences, adrenoreceptors and apolipoproteins approved. In one of these studies, Claudine Montgelard, Francois M. Catzeflis and Emmanuel Douzery first proposed the name Cetartiodactyla in 1997, which is composed of the scientific names of whales (Cetacea) and artiodactyla.
In 2001 two extraordinary fossil finds caused a sensation. In Pakistan , parts of the limb skeleton of the approximately wolf-sized Pakicetus and the fox-sized Ichthyolest , two primeval whales from the Eocene around 48 million years ago, which are grouped together as Pakicetidae , were found. These findings not only showed that the early whales were land-bound to a greater extent than previously assumed, but also clearly showed the special construction of the ankle bone with a double-rolled articular surface. This feature was previously considered the exclusive feature of the ungulates and since it has now also been discovered in early whales, the close relationship between the two groups could also be morphologically proven. In the later whales, the hind limbs were so extensively reduced that no conclusions about possible ancestry can be drawn from the structure of the hind legs of these animals. The mesonychia do not show this special structure of the ankle bone, so a descent of whales from them was excluded.
The special construction of the ankle bone shows a close relationship between cloven-hoofed animals and whales, but cannot answer the question of whether the cloven-hoofed animals are paraphyletic. Morphological investigations were therefore carried out to support the molecular biological findings of the close relationship between hippos and whales. Similarities could be found in the arrangement of the cusps of the molars , in the structure of the metatarsals and in the skull , which support a sister group relationship between these two taxa. Whether the most noticeable commonality, the loss of the fur and the sebum glands , is a common characteristic or an independently developed adaptation to the aquatic way of life is controversial.
The problem is that the oldest whale species lived in the early Eocene (around 53 million years ago), while the oldest known hippopotamus only lived in the Miocene (around 15 million years ago). Since the common ancestor of whales and hippos must have lived before the first whales, there is a 40 million year gap in the fossil history of hippos. In view of the comparatively good fossil find rate of the ungulates it seems unlikely that there are no remains of any ancestor of the hippopotamus. Some studies explained the late emergence of the hippos with the fact that they developed from relatives of the umbilical pigs , but this seems unlikely due to the molecular findings. The focus of research was therefore directed towards the Anthracotheriidae , a group of cloven-hoofed animals that was widespread from the Eocene to the Miocene and was described as "hippopotamus" when it was discovered in the 19th century. A study from 2005 showed that the genealogically younger Anthracotheriidae in particular have a skull structure very similar to that of the hippopotamus, but with a different tooth design. As a possible scenario, however, it was assumed that the whales and the Anthracotheriidae descended from a common ancestor and the hippos evolved from the Anthracotheriidae. A study published in 2015 was able to confirm this, but it also revealed that the hippos did not evolve significantly from ancestral history, as was assumed at the time, but can be derived from more original representatives of the anthracotheria. The newly introduced genus Epirigenys from eastern Africa therefore represents the sister group of hippos.
The overwhelming molecular findings and also some morphological indications suggest that the ungulates are paraphyletic with respect to the whales and form a taxon cetartiodactyla with them, whereby the monophyly of the cetartiodactyla is well supported by molecular and anatomical indications. Modern systematics divide the cetartiodactyla into five subordinate taxa, each of which is also very likely to be monophyletic: the calluses (Tylopoda), the pig-like (Suina), the ruminants (Ruminantia), the hippos (Ancodonta) and the whales (Cetacea).
The hippos are considered to be the sister group of the whales - including fossil taxa, a clade of hippos and Anthracotheriidae should represent the closest related group of these marine mammals. The ruminants are also likely to be more closely related to whales and hippos than to the other cloven-hoofed animals - this has so far only been investigated in molecular biology, but not morphologically, and is therefore controversial. Camels are the oldest recent sideline of the ungulates, which confirms the assumption of the convergent development of the ability to ruminate in camels and ruminants. The suspected lineages within the Cetartiodactyla can be shown in the following cladogram:
The four taxa of the Cetartiodactyla, which are grouped as artiodactyla, are divided into ten recent families:
- The calluses (Tylopoda) comprise only one family, the camels (Camelidae). It is a species-poor group of animals that are well adapted to extreme habitats - the old world camels in deserts and the new world camels in high mountain regions.
- The pig-like (Suina) are composed of two families:
- The ruminants (Ruminantia) consist of six families:
- The stag piglets (Tragulidae) are the smallest artifacts and the most primitive ruminants, they inhabit forests in Africa and Asia.
- The giraffe-like (Giraffidae) are composed of two externally different genera, the giraffes and the okapi .
- The musk deer (Moschidae) are a group of deer-like, but antlerless animals living in East Asia.
- The Gabelhorn carrier (Antilocapridae) comprise only one way out in North America living pronghorn .
- The deer (Cervidae) are made up of around 45 species, which are characterized by antlers that usually only the males wear. Some common species in Europe are among the deer , Dam - and red deer , the elk and reindeer .
- The hornbeams (Bovidae) are the most species-rich and diverse group of ungulates. These include the cattle , goat-like , gazelle-like and several groups known as antelopes .
- The hippos (Hippopotamidae) comprise two species, the (real) hippopotamus and the pygmy hippopotamus . For the reasons mentioned above, they are kept in their own subordination, ancodonta.
- For the internal classification of the whales see Whales: Systematics .
The greatest systematic problem within the subordinate taxa concerns the ruminants. It is generally accepted that the deer piglets are the sister group of the remaining five families, which are grouped together as forehead weapon bearers (Pecora). The system is confusing within the forehead weapon carriers. Although deer, musk deer and fork-horned bearers have traditionally been grouped together as deer-like (cervioidea), different molecular studies provide different - and inconsistent - results, so that the question of a phylogenetic system of forehead weapon bearers cannot currently be answered.
In December 2007, Hans Thewissen , Professor in the Department of Anatomy at Northeastern Ohio Universities Colleges of Medicine and Pharmacy, presented an alternative family tree hypothesis. According to his research, the closest relatives of the early whales were an extinct cloven-ungulate group called the Raoellidae, and both taxa together make up the sister group of the rest of the cloven-ungulates, including the hippos:
His findings come from the investigation of a new skeleton from the Kashmir region in Pakistan . It was a representative of the genus Indohyus , which is assigned to the Raoellidae. Mainly due to a bony ring on the temporal bone (bulla), called involucre, which was known previously only of whales, as well as other features of the Vorbackenzähne (premolars) and the bone structure of the close relationship was established.
The Cetartiodactyla are classified within the higher mammals in the superordinate order of Laurasiatheria , a group of mammals named after their presumed place of origin, the continent Laurasia . It is still unclear to which large groups within the Laurasiatheria the Cetartiodactyla are more closely related, but what is certain is that the previously proposed group of " ungulates " (ungulata) is not a natural group. Various molecular studies reveal at least different possible lineages: According to one hypothesis, the cetartiodactyla form a common taxon Fereuungulata together with the odd-toed ungulates and the ferae ( pangolins and predators ) - although it remains unclear whether the unpaired ungulates are more closely related to the cetartiodactyla or the ferae. Another, more recent hypothesis, on the other hand, combines the odd ungulate, ferae and bats to form a taxon Pegasoferae and sees the Cetartiodactyla as their sister group.
The oldest fossils of the ungulates come from the early Eocene (around 55 million years ago). Since these finds appeared almost simultaneously in Europe, Asia and North America, it is very difficult to determine the place of origin of the artifacts more precisely. These early forms are summarized in the group of the Dichobunoidea - their best known and best preserved representative is Diacodexis from the family of the Diacodexeidae . They are small (initially only about the size of a rabbit) with a slim build, long, thin legs and a long tail. The hind legs were significantly longer than the front legs, the typical toe structure was not yet fully developed and the teeth were low-crowned and simply built. Herbertlutzius also comes from the early phase of the development of the ungulates . The size of today's hedgehog is the smallest known representative of this mammal order and which belongs to the Dichobunidae closely related to the Diacodexeidae . Already in the early to middle Eocene there was radiation and the emergence of the ancestors of most of today's suborders.
The earliest whales are recorded from the early Eocene (around 53 million years ago) and are believed to have evolved on the Indian subcontinent . From then on, due to the sea-dwelling way of life, they followed a completely separate line of development (see evolution of the whales ).
The Entelodontidae and the Anthracotheriidae were two once widespread but now extinct groups of the ungulate . The Entelodontidae existed from the Middle Eocene to the Early Miocene in Eurasia and North America. They had a stocky body with short legs and a massive head, which was characterized by two bones on the lower jaw. The Anthracotheriidae had a large, pig-like build, the legs were short and the snout was remarkably elongated. This group appeared in the mid Eocene and spread across Eurasia, Africa, and North America. It lived up to the Miocene or Pliocene - from Asia there is, however, an uncertain find from the early Pleistocene . Presumably the Anthracotheriidae were the ancestors of the hippos , perhaps they led a similar aquatic way of life and were displaced by them. The hippos themselves appeared in the late Miocene and are documented from Africa and Asia - they never reached America.
The calluses (Tylopoda) were restricted to North America during much of the Cenozoic era , but early forms such as the Cainotheriidae are also documented from Europe. The North American callous soles included groups such as the stocky, short-legged Merycoidodontidae , the Protoceratidae equipped with forehead weapons and the actual camels (Camelidae). These first appeared in the late Eocene and developed a great biodiversity in North America. Only in the late Miocene or early Pliocene did they immigrate to Eurasia and in the late Pliocene to South America. They still live in these continents today, in North America they became extinct around 10,000 years ago for reasons that are not exactly known.
Representatives of the pig species (Suina) have been known since the Eocene. In the late Eocene or Oligocene the two families that still exist today were formed, the Old World pigs , which were always restricted to Eurasia and Africa, and the umbilical pigs , which died out in the Old World and are now only found in America.
The ruminants (Ruminantia) have also been known since the Eocene with numerous early forms that are probably similar to today's stag piglets . These early forms, which are summarized as tragulina, lived up to the Miocene in Africa, Eurasia and North America, then they died out except for the recent stag piglets. In the Oligocene or Miocene, the remaining representatives who are still alive today developed, most of which were restricted to the Old World. The hornbeams (Bovidae), which settled in Africa from the early Miocene onwards , experienced a great deal of radiation , where they largely displaced the previously dominant odd-toed ungulates and hyraxes and occupied the ecological niche of the large herbivores. Within the giraffe-like (Giraffidae) arose next to today's species also the cattle giraffes , which developed deer-like forehead weapons. The deer (Cervidae) also developed numerous species, but remained rather restricted to Eurasia. Horned bearers and deer came to North America relatively late (Late Miocene or Early Pliocene), but on this continent the group of forked horns had spread with numerous species, of which today only one species, the pronghorn , has survived.
South America was only colonized by the arthropod in the Pliocene, around three million years ago, when the land connection at the Isthmus of Panama closed. With umbilical pigs , New World camels and deer , the South American cloven-hoofed fauna has remained species-poor compared to the other continents. When the large mammals became extinct at the end of the Pleistocene , the even-toed ungulates were less affected than other groups of mammals, and species extinction on a larger scale only occurred among the North American camels. One reason for this is that the larger representatives of the even-toed ungulates were found in Africa and, in contrast to the other continents, hardly any large mammals went extinct there at the time.
Most of the information in this article has been taken from the sources given under literature; the following sources are also cited:
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