Equidae

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Horses
Przewalski horses

Przewalski horses

Systematics
Class : Mammals (mammalia)
Subclass : Higher mammals (Eutheria)
Superordinate : Laurasiatheria
without rank: Scrotifera
Order : Unpaired ungulate (Perissodactyla)
Family : Horses
Scientific name
Equidae
Gray , 1821

The Equidae are the representatives of the family of horses (also equines ) within the odd-toed ungulates (Perissodactyla). This family was originally very extensive, but only today includes the genus horses ( Equus ) with the wild horse (including the house horse ) and different donkey and zebra species . The tribal history of the horse family began in the early Eocene around 56 million years ago and is one of the most extensive and best preserved of a group of mammals . Due to their preference for hard grass and their associated way of life in open landscapes, today's horses are currently the most specialized representatives of the odd-toed ungulate order.

features

Asiatic donkey ( Equus hemionus )
Grevy's zebra ( Equus grevyi ) with foal
Tibetan wild ass ( Equus kiang )

Today's horses include medium-sized to large mammals whose head-torso length varies from 200 to 300 cm, while the shoulder height reaches about 110 to 150 cm. The weight is between 175 and 450 kg. Characteristic features are the strong body, the long limbs and the likewise long neck with the large head and the high-crowned ( hypsodontal ) molars. Also noticeable are the neck mane and the tail, which looks like a tail due to the long hair and is sometimes more than 110 cm long, but whose bony structure is usually only half the length. The most characteristic feature of today's horses, however, are the hands and feet each consisting of only one toe, which is why the recent representatives are also referred to as "equine". Extinct members of the horse group could also get bigger, some cultivated forms of today's domestic horse breeds sometimes considerably exceed the dimensions of the recent wild forms.

In terms of tribal history, members of the Equidae had a clearly different appearance. Above all the earliest members such as Sifrhippus , but also the equine Hyracotherium or Propalaeotherium , all come from the early to middle Eocene , were very small and sometimes weighed less than 10 kg. In their habitus they resembled the other early odd-toed ungulates with their slender limbs and the clearly convex curved spine . In addition, they had four toes on their front feet and three toes on their back feet, an archaic feature that is only found in the tapirs of today's odd-toed ungulates . The clearly convex shaped spine is still interpreted as an adaptation to a way of life in a more closed, forest-like biotope , where these animals lived as shy bush hats. A distinction between the genera of the early horse representatives and other basal perissodactyls, such as the early tapir-like Hyrachyus , which appeared at the same time , is largely only possible on the basis of the teeth.

The development to today's morphology with the characteristic straight running back and only one functional toe per limb took place gradually. The increase in leg length and body weight was accompanied by a stiffening of the rear section of the spine (lumbar vertebrae). This resulted in the typical “stiff” gallop of today's horses, which deviates significantly from the “flexible” sprint of other fast-moving mammals such as the cheetah . The number of toes was reduced from the first four-toed ( tetradactyl ) horses to three- toed ( tridactyl ) and single-toed ( monodactyl ) horses . The first single-toed horses ("equine") can be traced back to the Miocene . The reduction in the number of toes also led to a change in gait. Were original horse still having a front portion of the hands and feet on ( digitigrade ) so touched later forms only with the last phalange the ground ( toe-goers ). This also applies to today's horses ( Equus ). The development of monodactyly is limited to the most recent development phase of the horses from the tribe of Equini . The real peculiarity of the horse's foot is therefore not found in the reduction in the number of toes and the formation of only one functional toe per limb. Rather, the most important change is the transition from a rather flat foot of the toe walker to the steep foot of the toe walker, which evolved much earlier and can also be demonstrated in tridactylic horses. One of the reasons for the monodactyly of today's horses and their immediate close relationship may therefore be a change in the nutritional strategy towards more rambling hikes in open landscapes. The reduced number of toes reduces the frictional energy between foot and substrate, which is particularly advantageous at higher speeds such as trot and gallop.

The transformation of the horse's body also includes other changes in the musculoskeletal system. At the distal ( distal ) ends of the joint metapodials a nearly circular joint surface, which comprises about at today's horse 270 ° and in the center a formed in equines sagittal has bone back. This led to much better and far more expansive longitudinal movements and prevented the toe bones attached there from shearing sideways. Horses of the early Miocene such as Mesohippus had less well developed joint ends, which only covered 150 °, and a poorly developed middle back. Furthermore, the reconstruction of the knee joint is part of it, which became more and more asymmetrical in the course of development - the length of the inner joint roller at the lower end of the thigh bone increased disproportionately compared to the outer one. The process for the formation of an asymmetrical knee joint began at least 12 million years ago and represents an adaptation of the equidae on the one hand to the sometimes enormous increase in body size, and on the other hand to open landscapes and a fast type of locomotion. However, the formation of the ankle bone ( astragalus or talus ) is limited. the maneuverability in rather closed landscapes strongly, which was possibly already developed in very early equidae.

Other changes mainly took place in the skull. In addition to the general increase in size - including the brain capsule - this also largely affects the dental apparatus and the frontal area of ​​the face. The earliest horse representatives from the Eocene , such as Sifrhippus , possessed the complete dentition of modern mammals with three incisors , one canine , four premolars and three molars per jaw branch. While the number of teeth in equidae changed only slightly - today's horses usually have only reduced the canine and the anterior premolars - there was, however, a shift in the posterior row of teeth with the premolars and molars in relation to the position of the orbit and of the symphysis without increasing the length of the row of teeth relatively sharply. If the eye socket was still above the first or second molar in early horse representatives , in today's horses it is above or slightly behind the last molar. This forward displacement of the rear dentition was a result of the change in the horse's diet from soft to hard plant food, which encouraged the development of high-crowned teeth, which took place especially during the Miocene . The attachment surfaces for the masseter muscles also increased due to changes in the anterior facial area. It is noteworthy, however, that in today's domestic horses, depending on the size of the individual breeds, these evolutionary changes can only be partially understood, which may be a result of the different final phases of ontogenetic development within these breeds.

A specialty of horses are the already mentioned high-crowned molars, of which three to four premolars and three molars are formed per jaw arch . In today's horses, however, the foremost premolar is often missing. The high-crowned teeth wear out due to the heavy mechanical stress when chewing the hard grass. In principle, only the uppermost part of the tooth protrudes from the alveolus , the rest remains in the tooth socket and is gradually pushed out. The root is short and open and only closes when the tooth is almost chewed off. There is also a noticeable increase in the size of the root. It is assumed that the part of the tooth crown remaining in the tooth socket acts as a “replacement root” and weakens the shear forces that act when chewing. Only when the tooth crown is almost completely worn then requires a larger tooth root.

Today's horses have only a slight sexual dimorphism . Mares differ from stallions by their slightly smaller body size and the absence or significantly smaller formation of the canine tooth. In the earliest horses, gender differences are difficult to determine. From the Miocene onwards, however, clear differences in the canines of the canine teeth of Miohippus or Hipparion can be made out, which were twice as long in males as in females. Some researchers believe that the enlargement of the canines at the stallions with the formation of Rossigkeitsgesichtes accompanied, one the Flehmen similar facial expression with raised lips, ears and head stretched and thus clearly visible canine in pairing willing stallions. According to this, this behavior should go back up to 20 million years.

distribution

The current distribution of horses as a wild form extends from Eurasia to Africa and includes largely open habitats from boreal steppe landscapes to tropical savanna areas . They occur in the lowlands as well as in mountainous areas and highlands up to over 4000 m above sea level. Earlier horse representatives were much more widespread. The family originated in North America . Especially with the adaptation to open landscapes, an unprecedented expansion of the equidae arose, which comprised around a dozen genera from the late Miocene five million years ago. From North America they spread to their present habitats. They reached Eurasia in the lower Miocene around 20 million years ago. Africa was first entered in the Upper Miocene at least 5 million years ago, while South America was settled in the Upper Pliocene after the formation of the Isthmus of Panama about three million years ago. Only Oceania and Antarctica do not belong to the natural range of the Equidae. The relatives of the horses populated quite different biotopes . Early equidae were more likely to be found in closed or semi-open forest landscapes; it was only later that they developed into the highly specialized open land inhabitants known today.

Way of life

Social behavior and reproduction

Domestic siren stallion ( Equus asinus ) tries to cover a mare

In principle, two different models of social behavior can be distinguished. Some of the recent horses, such as the Przewalski horse , the house horse , the mountain zebra and the plains zebra , live in stable herds or group associations, which are usually led by a dominant stallion. These “ harems ” consist of three to six members, sometimes more, but usually less than ten individuals, in hierarchical ranks. The pairing takes place mainly within the group. Younger males sometimes form "bachelor groups". In both group associations the ranking is regulated by dominance. The extent of the individual action areas is often very large and can clearly exceed 10 km² (up to over 100 km²), the boundaries and much traveled paths are marked with excrement and urine by the dominant male animal . Such herd associations largely exist in dry areas with changing seasons and a fluctuating food supply. The Grevy's zebra , African and Asian donkeys , on the other hand, live as territorial loners and defend their territory from other species. Herd formations sometimes take place, but these are very unstable. As a rule, there are only groups of mares and foals. Such a social structure occurs in areas with little fluctuations in the food supply and without strong seasonalization of the climate. It is assumed that this way of life with mother-young animal associations and solitary stallions is the more primitive one that was also present in the earliest horses. Stable herd associations may not have developed until the Miocene, with the advent of open landscapes.

The gestation period is between 12 and 13 months, after which a young animal is usually born and suckled for a maximum of two years. This as a k-strategy called form of reproduction (long gestation period, singular young, long service phase) was already in phylogenetically watching old horse representatives. In a fossil of a mare of the extinct genus Propalaeotherium from the Eckfelder Maar ( Rhineland-Palatinate ) and in at least eight female individuals of the likewise fossil genus Eurohippus from the Messel pit near Darmstadt ( Hesse ) - both sites belong to the Middle Eocene - Evidence that the animals were pregnant at the time of death, with only one fetus formed at a time. This means that this type of reproduction seems to have been developed very early in the equidae and their relatives. In addition, she points out that the very early horse representatives already lived in close social associations, since this form of reproduction is now mainly known from herbivorous mammals that live in close social associations. This could also be demonstrated in other early horses, such as the Pseudhipparion , on the basis of finds from the Ash Hollow Formation in Nebraska . Numerous representatives of this horse genus were killed here in a catastrophic volcanic eruption in the late Miocene.

Today's horses live between 20 and 25 years of age, and in captivity they are known to be up to 40 years old. Since the life expectancy of mammals depends heavily on body size, a correspondingly shorter maximum age must be assumed for the earliest horses, some of which weigh less than 10 kg. Experts assume about three to four years for these horse representatives. With the increase in height, the age gradually lengthened. However, it is unclear how the life expectancy of subsequently dwarfed fossil horses such as Pseudhipparion is to be assessed . The predators of today's horses are mainly the big cats , but also wolves and hyenas .

nutrition

Plains zebra ( Equus quagga ) eating
Wild horse ( Equus ferus )

All of today's horse representatives specialize in hard, silicic acid-containing plant foods and therefore prefer grasses as their main source of nutrition . Their predominant habitat therefore includes little wooded, open terrain, which also makes them one of the most specialized, recent unpaired ungulate group . Because the hard grass quickly wears out the teeth due to its high silica content in the form of embedded opal phytolites, the horses developed high-crowned ( hypsodontal ) teeth with enamel folds on the chewing surface and a high proportion of dental cement . This special structure protects against excessive abrasion of the teeth. In addition, typical horizontal grinding patterns occur on the teeth. The traces of abrasion on the molars also showed that some horse species of the Equus genus , especially in the Pleistocene , were possibly also adapted to a mixed vegetable diet, which included soft plant material in addition to hard grass food.

The earliest horse representatives from the Eocene were, however, more adapted to food containing fruit , as indicated by their low-crowned ( brachyodont ) molars and the high proportion of tooth enamel as well as the characteristic bumps on the chewing surfaces ( bunodont ). Their preferred habitat comprised more closed or semi-open forest landscapes. At the latest in the Oligocene , teeth emerged from this with crowns that were still low, but clearly raised and transversely positioned enamel ridges ( lophodont ), which indicate an increased change to leafy vegetarian food. The first horse representatives with high-crowned molars were recorded in the early Miocene at least 18 million years ago and belong largely to the line of the more modern Equinae . However, some of the older equidae from the subfamily of the Anchitheriinae also developed approaches to hypsodontic teeth. The process of the formation of high-crowned teeth went hand in hand with climatic changes during this time to drier and cooler conditions and the simultaneous expansion of open savannah and steppe landscapes . It is associated with the increase in dental cement and the significant folding of tooth enamel . However, this development does not include all early horse representatives, some stayed with the soft, others specialized in mixed vegetable food.

Like all unpaired ungulates, horses are rectum fermenters that digest a large part of the food they eat in the appendix and colon with the participation of numerous microorganisms . The intestinal tract is extremely large and can hold up to 210 liters in today's horses. Finds from the Messel Pit from the middle Eocene also show that such a digestive system is by all means a very old development. In the early equine Hallensia traces ( bacteriography ) of part of the internal organs caused by bacteria could be detected, whereby a very large appendix could be detected. This confirms the assumption that the earliest unpaired ungulates practiced a similar form of food processing.

Systematics

External system

The horse family (Equidae) belongs to the order of the odd ungulate . The sister taxon of the Equidae forms the extinct family of the Palaeotheriidae , which are primarily recorded from Europe and became extinct in the Oligocene . The Palaeotheria include the genera Palaeotherium , Olagiolophus and Leptolophus . In research it is controversial whether Propalaeotherium and Hyracotherium belong to the Palaeotheria or to the equidae, in the latter they are partly assigned to the subfamily Hyracotheriinae . In contrast to the early representatives of the horses, the palaeotheria have higher tooth crowns on the molars, a larger nasal cavity, longer vertebrae and, compared to the metatarsal bones, longer metacarpal bones . They are also stratigraphically younger than the earliest horses. Together, however, the Equidae and Palaeotheriidae form the superfamily Equioidea (equine species) and the suborder Hippomorpha (equine relatives) within the odd ungulate . The latter form the counterpart to the Ceratomorpha with today's rhinos and tapirs . The separation of the horse line from the Ceratomorpha took place after molecular genetic studies 56 million years ago.

Internal system

Abbreviated internal systematics of the family Equidae (mainly North American representatives)
according to Mihlbachler et al. 2011
  Equidae  

 early horses ( Orohippus , Epihippus , Mesohippus , Miohippus )


  Anchitheriinae  

Clade  1 ( Megahippus , Hypohippus , Kalobahippus )


   

Clade  2 ( Archaeohippus , Desmahippus , Parahippus )


  Equinae  
  Hipparionini  

Clade  1 ( Acritohippus , Pseudhipparion , Neohippus , Merychippus )


   

Clade  2 ( Hipparion , Cormohipparion , Nannihippus )



  Equini 

Clade  1 ( Protohippus , Calippus , Scaphohippus )


   

Clade  2 ( Plioparahippus , Pliohippus , Astrohippus , Onohippidion , Dinohippus , Equus ) 







Template: Klade / Maintenance / Style

The systematic division of horses is widely discussed. The origin of the Equidae family is in North America. Certain lines of development in the Old World, such as Anchitherium or Hipparion, are often only viewed as "side branches". The current subdivision of the family Equidae was made in 1989 by Robert L. Evander , who significantly differentiated two subfamilies with Anchitheriinae and Equinae . A basal (phylogenetically old) position of the subfamily Hyracotheriinae within the Equidae, as often warned by Central European researchers, was not taken into account. The Anchitheriinae are characterized by three toes on the limbs and highly specialized low-crowned molars. In addition, they represent the first group of horses with significant increases in size. The Equinae in turn are divided into the tribes Hipparionini and Equini. A characteristic feature of both tribes is the increase in dental cement in the molars due to the specialization in grass forage. Both the Hipparionini and the early Equini have three toes each, only the latest horses have singular toes. Originally a third tribe was introduced by Evander with Protohippini, José L. Prado and María T. Alberdi moved this in 1996 to the level of a subtribe within the Equini and compared it with the modern Pliohippina and today's horses. Richard C. Hulbert Jr. proposed a similar structure in 1988 .

As things stand today and taking into account more recent results, the group of Equidae includes the following taxa:

  • Equidae Gray , 1821
  • Tribus Hipparionini (= Hippotheriini) Quinn , 1955
  • Tribus Equini Gray , 1821
  • Plioparahippus
  • Pliohippus Marsh , 1874
  • Astrohippus Stirton , 1940
  • Hippidion (= Parahipparion , Hyperhippidium ) Owen , 1869
  • Onohippidium Moreno , 1891
  • Boreohippidion Avilla, Bernardes & Mothé , 2015
  • Dinohippus Quinn , 1955
  • Haringtonhippus Heintzman, Zazula, MacPhee, Scott, Cahill, McHorse, Kapp, Stiller, Wooller, Orlando, Southon, Froese & Shapiro , 2017
  • Equus (= Sussemionus , Plesippus , Allohippus , Asinus , Hippotigris , Dolichohippus , Onager , Hemionus , Kraterohippus , Kolpohippus , Sterrohippus , Neohippus ) Linnaeus , 1758

From a genetic point of view, the intrafamily variability of equidae is relatively high. More recent genetic studies of recent and extinct species and genera from the Holocene and Pleistocene have shown several more closely or more closely related groups. Especially today's donkeys and zebras, which are traditionally regarded as a stenonine group of the genus Equus , have very variable relationships. From a genetic point of view, the genus Equus is closely related to Hippidion and Haringtonhippus ; from a comparative anatomical point of view, Dinohippus is also closely related .

Tribal history

Evolutive trends

Size comparison of a full-grown primeval horse, Museum für Naturkunde (Berlin)
Evolution of horses, compiled from skeletons from the State Museum for Natural History Karlsruhe

The tribal history of horses is one of the best-documented fossil records among mammals. It is a prime example of gradual evolutionary change and is characterized by the development from small, multi-toed, leaf-eating forest dwellers to long-legged, single-toed grass-eaters. However, this development was not a linear result, but often led to the emergence of side lines and evolutionary dead ends and is to be regarded as a result of the very extensive, but ultimately only partial fossil record.

The origin and a large part of the evolution of horses took place in North America , but time and again individual representatives migrated to other continental areas and developed their own lines there. In addition to the general development in size, general evolutionary trends include adaptations to open landscapes with the associated changes in the morphology of the musculoskeletal system and the formation of high-crowned teeth in the course of adaptation to hard grass. Above all, the development of high-crowned teeth also caused further changes in the facial skull to develop more massive masticatory muscles. These developments took place independently several times in the individual horse lines.

Eocene

Reconstructed skeleton of Hyracotherium as a representative of the early equine species

One of the earliest and best-known equines is Hyracotherium (once synonymous with Eohippus , but this is now recognized again as an independent genus of the earliest horses) from the Eocene - a small forest animal with a shoulder height of only about 20 centimeters, which with its small, four-humped molars leaves and fruits chewed. As already mentioned, the systematic position of Hyracotherium is controversial.

The oldest clear member of the horse family is Sifrhippus , who lived in North America at the beginning of the Eocene almost 56 million years ago. It was still very similar to the Hyracotherium , to which part of the Sifrhippus species were originally attributed. This primordial horse is likely to be closely related to basal unpaired ungulates such as Hallensia or similar forms, some of which are also still close to the Palaeotheriids. It lived in open forests and lived mainly on soft vegetable foods. Among other things, Pliolophus emerged from it, but only a few skeletal elements are known. Another descendant was Orohippus , who differed from Pliolophus in the construction of teeth and toes and probably preferred a somewhat harder vegetable diet . The genus lived in North America about 50 million years ago. His descendant Epihippus , who lived around 47 million years ago, was also restricted to North America.

Almost at the same time as the oldest North American horses, Erihippus appeared in Asia as well. The shape was documented in 2018 based on several remains of jaws from the Lingcha formation in the Chinese province of Hunan .

Oligocene

Mesohippus

From early hyracotherine horses like Orohippus and Epihippus the subfamily of the Anchitherinae developed in the Oligocene . The genus Mesohippus emerged as one of its first representatives in the late Eocene, around 40 million years ago, in North America . This horse had a shoulder height of around 60 centimeters and an average weight of 42 kg. The back was no longer as curved as in its ancestors, and the legs , neck and skull were longer, especially the intermaxillary bone lengthened, so that the distance between the front and rear teeth ( diastema ) became wider. Mesohippus had three functional toes in the back and front, but also a rudimentary fourth toe in front . Like its forerunners, the animal still walked on paws. At the beginning of the Oligocene, around 35 million years ago, the genus Miohippus developed from Mesohippus . At around 54 kg, this horse was larger than Mesohippus and had a slightly longer skull.

Miocene

Skeletal reconstruction of the Hipparion

In the Miocene , the climate changed fundamentally. In North America in particular, it became drier, forests shrank, and open grasslands emerged. This created a significant trend in horse evolution . The horses gradually adapted to the new conditions of a steppe landscape . They became larger in order to be able to move more quickly in the open landscape and, as they gradually changed their diet from leaves to grass, they developed enlarged tooth crown surfaces; later representatives also increased the tooth crowns. However, the teeth of the Anchitherinae were not as high-crowned as the later horse representatives.

According to the Oligocene forms ( Mesohippus , Miohippus ), later genera of the Anchitherinae lived in North America in the Miocene, which experienced an initial significant increase in body size, with Kalobahippus already reaching around 400 kg. The genus Anchitherium , which gave the subfamily its name , also reached Eurasia via the Bering Bridge . It was the first horse species to repopulate this continent after the early equine species disappeared in the Eocene. Anchitherium was widespread from China to Spain, but its line died out without any offspring . Other North American genera of the Anchitheria such as Parahippus , which appeared about 23 million years ago, developed to the ancestors of today's horses . The animal still had three toes, but had longer teeth than its ancestors. From him Merychippus developed 18 million years ago , which is already counted to the same subfamily ( Equinae ) as today's horses. This horse was about 1 meter tall and the skull already resembled that of a modern horse. Also Merychippus had three toes, but the animal was already completely on tiptoe. The teeth had high furrows with a thick layer of enamel .

From Merychippus -like animals on the one hand the side branch of the Hipparionini developed , to which Hipparion and Hippotherium are counted, on the other hand, the Equini, to which today's species belong. While Merychippus was restricted to North America, Hipparion immigrated to Eurasia about 12 million years ago over the Beringland Bridge and replaced anchitherium there . Hipparion was the first horse to reach the African continent.

The lateral toes slowly disappeared within the equini, which were developing further in North America. One of its oldest representatives was Pliohippus , who lived in North America around 15 million years ago. It already looked very similar to modern horses and was already monoed.

Plio-Pleistocene

Representation of the Hippidion

Dinohippus from the Upper Miocene and Pliocene of North America is considered to be the ancestor of the genera Equus , Haringtonhippus, and Hippidion . The three genera emerged in the course of the Pliocene, beginning around 5 million years ago. In contrast to the later horse species, Dinohippus still had the skull pits so typical of Miocene horses, the function of which is unknown. Equus was the only representative of horses in the strict sense ( tribe Equini) to come from North America to Eurasia about 2.5 to 3 million years ago. These were members of the more original Equus form, the so-called stenonine horses, named after the extinct Old Pleistocene species Equus stenonis . These horses spread very quickly across Eurasia and had reached South Africa no later than 2 million years ago. With the appearance of this advanced form, the older hipparions also disappeared in the Old World, having previously become extinct in North America. Today's zebras and donkeys developed from the stenonine forms. About 1.5 million years ago or a little later, the first caballine horses of the genus Equus (also known as real horses, named after Equus caballus , today's domestic horse ) came from North America to Eurasia and formed the ancestral form of today's wild and domestic horses. In America the horses of the genus Equus as well as Hippidion and Haringtonhippus died out in the course of the Quaternary extinction wave at the latest 10,000 years ago.

literature

  • Bruce J. MacFadden: Fossil Horses. Systematic, Paleobiology, and Evolution of the family Equidae. Cambridge University Press, 1992
  • Jens Lorenz Franzen: The primeval horses of the dawn. Origin and evolution of horses. Elsevier, Munich 2007, ISBN 978-3-8274-1680-3 .

Individual evidence

  1. ^ A b c Peter Grubb: Equus burchelli. Mammalian Species 157, 1981, pp. 1-9
  2. ↑ a b c d G. S. Churcher: Equus grevyi. Mammalian Species 453, 1993, pp. 1-9
  3. ↑ a b Deb Bennett and Robert S. Hoffmann: Equus caballus, Linnaeus, 1758. Mammalian Species 628, 1999, pp. 1-14
  4. ^ Arthur Walton and John Hammond: The Maternal Effects on Growth and Conformation in Shire Horse-Shetland Pony Crosses. Proceedings of the Royal Society of London, Series B Biological Sciences 125, 1938, pp. 311-335
  5. ^ A b Ross Secord, Jonathan I. Bloch, Stephen GB Chester, Doug M. Boyer, Aaron R. Wood, Scott L. Wing, Mary J. Kraus, Francesca A. McInerney and John Krigbaum: Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum. Science 335 (Feb. 24), 2012, pp. 959-962
  6. ↑ Meinolf Hellmund: Skeleton reconstruction of Propalaeotherium hassiacum (Equidae, Perissodactyla, Mammalia) based on the finds from the Eocene Geiseltal (Saxony-Anhalt, Germany). Hallesches Jahrbuch für Geoswissenschaften, Series B, Supplement 12, 2000, pp. 1–55
  7. ^ Katrina Elizabeth Jones: New insights on equid locomotor evolution from the lumbar region of fossil horses. Proceedings of the Royal Society B283, 2016, S. 20152947 doi: 10.1098 / rspb.2015.2947
  8. ^ Christine M. Janis and Raymond I Bernor: The Evolution of Equid Monodactyly: A Review Including a New Hypothesis. Frontiers in Ecology and Evolution 7, 2019, p. 119, doi: 10.3389 / fevo.2019.00119
  9. ↑ a b c d e f g h i Bruce J. MacFadden: Fossil Horses. Systematic, Paleobiology, and Evolution of the family Equidae. Cambridge University Press, 1992
  10. ↑ John W. Hermanson and Bruce J. MacFadden: Evolutionary and functional morphology of the knee in fossil and extant horses (Equidae). Journal of Vertebrate Paleontology 16 (2), 1996, pp. 349-357
  11. ↑ a b Christine M. Janis, Boris Shoshitaishvili, Robert Kambic and Borja Figueirido: On their knees: Distal Femur asymmetry in Ungulates and its relationship to body size and locomotion. Journal of Vertebrate Paleontology 32 (2), 2012, pp. 433-445
  12. ^ A b Aaron R. Wood, Ryan M. Bebej, Carly L. Manz, Dana L. Begun and Philip D. Gingerich: Postcranial Functional Morphology of Hyracotherium (Equidae, Perissodactyla) and Locomotion in the Earliest Horses. Journal of Mammal Evolution 18, 2011, pp. 1-32
  13. ↑ a b c d David J. Froehlich: Quo vadis eohippus? The systematics and taxonomy of the early Eocene equids (Perissodactyla). Zoological Journal of the Linnean Society, 134, 2002, pp. 141-256
  14. ^ A b Leonard Radinsky: Ontogeny and Phylogeny of horse skull evolution. International Journal of Organic Evolution, 38 (1), 1984, pp. 1-15
  15. ↑ a b Caroline AE Strömberg: Evolution of hypsodonty in equids: testing a hypothesis of adaptation. Paleobiology 32 (2), 2006, pp. 236-258
  16. ↑ Nikos Solounias, Melinda Danowitz, Irvind Buttar and Zachary Couppee: Hypsodont crowns as additional roots: A new explanation for hypsodonty. Frontiers in Ecology and Evolution 7, 2019, p. 135, doi: 10.3389 / fevo.2019.00135
  17. ↑ Eberhard Trumler: The "Rossigkeitsgesicht" and similar expressive behavior in solipeds. Zeitschrift für Tierpsychologie 16, 1959, pp. 478-488
  18. ↑ a b c Ludovic Orlando, Jessica L. Metcalf, Maria T. Alberdi, Miguel Telles-Antunes, Dominique Bonjean, Marcel Otte, Fabiana Martin, Véra Eisenmann, Marjan Mashkour, Flavia Morello, Jose L. Prado, Rodolfo Salas-Gismondi, Bruce J. Shockey, Patrick J. Wrinn, Sergei K. Vasil'ev, Nikolai D. Ovodov, Michael I. Cherry Blair Hopwood, Dean Male, Jeremy J. Austin, Catherine Hänni and Alan Cooper: Revising the recent evolutionary history of equids using ancient DNA. PNAS 106, 2009, pp. 21754-21759
  19. ↑ Manuel Salesa, Israel M. Sánchez and Jorge Morales: Presence of the Asian horse Sinohippus in the Miocene of Europe. Acta Palaeontologica Polonica 49 (2), 2004, pp. 189-196
  20. ↑ Tamara A. Franz-Odendaal, Thomas M. Kaiser and Raymond L. Bernor: Systematics and dietary evaluation of a fossil equid from South Africa. South African Journal of Science 99 (September / October), 2003, pp. 453-459
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  26. ^ A b c d Matthew C. Mihlbachler, Florent Rivals, Nikos Solounias and Gina M. Semperbon: Dietary Change and Evolution of Horses in North America. Science 331, 2011, pp. 1178-1181
  27. ↑ Bruce J. MacFadden: Fossil Horses - Evidence for Evolution. Science 305, 2005, pp. 1728-1730
  28. ↑ Jens Lorenz Franzen: Origin and systematic position of the Palaeotheriidae- In: Donald R. Prothero and RM Schoch (eds.): The evolution of the Perissodactyls. New-York 1989, pp. 102-108
  29. ^ A b Donald R. Prothero: Classification of the Perissodactyla. In: Donald R. Prothero and RM Schoch (Eds.): The evolution of the Perissodactyls. New-York 1989, pp. 530-537
  30. ↑ a b Jens Lorenz Franzen: The Equoidea of ​​the European Middle Eocene (Geiseltalium). Hallesches Jahrbuch für Geoswissenschaften B17, 1995, pp. 31–45
  31. ↑ Christelle Tougard, Thomas Delefosse, Catherine Hänni and Claudine Montgelard: Phylogenetic Relationships of the Five Extant Rhinoceros Species (Rhinocerotidae, Perissodactyla) Based on Mitochondrial Cytochrome b and 12S rRNA Genes. Molecular Phylogenetics and Evolution 19, 2001, pp. 34-44
  32. ^ A b Robert L. Evander: Phylogeny of family Equidae. In: Donald R. Prothero and RM Schoch (Eds.): The evolution of the Perissodactyls. New-York 1989, pp. 109-126
  33. ^ José L. Prado and María T. Alberdi: A cladistic analysis of the horses of the tribe Equini. Journal of Palaeontology 39 (3), 1996, pp. 663-680
  34. ^ A b José Luis Prado and María Teresa Alberdi: Fossil Horses of South America: Phylogeny, Systemics and Ecology. Springer, 2017, pp. 1–149
  35. ↑ Richard C. Hulbert Jr .: Cormohipparion and Hipparion (Mammalia, Perissodactyla, Equidae) from the Late Neogene of Florida. Bulletin of the Florida State Museum, Biological Sciences 33 (5), 1988, pp. 229-338
  36. ↑ Véra Eisenmann and Vasiliev Sergej: Unexpected finding of a new Equus species (Mammalia, Perissodactyla) belonging to a supposedly extinct subgenus in late Pleistocene deposits of Khakassia (southwestern Siberia). Geodiversitas 33 (3), 2011, pp. 519-530
  37. ↑ Leonardo Dos Santos Avilla, Camila Bernardes and Dimila Mothé: A New Genus for Onohippidium galushai MacFadden and Skinner, 1979 (Mammalia, Equidae), from the Late Hemphillian of North America. Journal of Vertebrate Paleontology 35 (3), 2015, p. E925909
  38. ↑ Raymond L. Bernor, Miranda J. Armor-Chelu, Henry Gilbert, Thomas M. Kaiser and Ellen Schulz: Equidae. In: Lars Werdelin and William Joseph Sanders (eds.): Cenozoic Mammals of Africa. University of California Press, Berkeley, Los Angeles, London, 2010, pp. 685-721
  39. ↑ a b c Peter D. Heintzman, Grant D. Zazula, Ross DE MacPhee, Eric Scott, James A. Cahill, Brianna K. McHorse, Joshua D. Kapp, Mathias Stiller, Matthew J. Wooller, Ludovic Orlando, John Southon, Duane G. Froese and Beth Shapiro: A new genus of horse from Pleistocene North America. eLife 6, 2017, p. e29944 doi: 10.7554 / eLife.29944
  40. ↑ Raymond L. Bernor, Wang Shiqi, Liu Yan, Chen Yu and Sun Boyang: Shanxihippus dermatorhinus comb. nov. with comparisons to Old World hipparions with specialized nasal apparatus. Rivista Italiana di Paleontologia e Stratigrafia 124 (2), 2018, pp. 361–386
  41. ↑ a b Bin Bai, Yuan-Qing Wang and Jin Meng: The divergence and dispersal of early perissodactyls as evidenced by early Eocene equids from Asia. In: Communications Biology 1, 2018, p. 115 doi: 10.1038 / s42003-018-0116-5
  42. ↑ Ann Forstén: Mitochondrial DNA time-table and the evolution of Equus: comparison of molecular and paleontological evidence. Annales Zoologici Fennici 28, 1992, pp. 301-309
  43. ↑ Stephen Jay Gould: Illusion Progress. Frankfurt am Main, 1996 (here pp. 81–99)
  44. ^ Donald R. Prothero and Neil Shubin, The evolution of Oligocene horses. In: Donald R. Prothero and RM Schoch (Eds.): The evolution of the Perissodactyls. New-York 1989, pp. 142-175
  45. ↑ Jaco Weinstock, Eske Willerslev, Andrei Sher, Wenfei Tong, Simon YW Ho, Dan Rubenstein, John Storer, James Burns, Larry Martin, Claudio Bravi, Alfredo Prieto, Duane Froese, Eric Scott, Lai Xulong and Alan Cooper: Evolution, Systematics , and Phylogeography of Pleistocene Horses in the New World: A Molecular Perspective. PLoS Biology 3 (8), 2005, p. E241 doi: 10.1371 / journal.pbio.0030241
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  47. ^ A. Azzaroli: Ascent and decline of monodactyl equids: a case for praehistoric overkill. Annales Zoologici Fennici 28, 1992, pp. 151-163

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