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HORSES
    The horses present an excellent set of transitional fossils with over 40 genera and 100 species.  The first horses (equids) evolved from phenacodontid condylarths in the Late Paleocene which had already evolved a specialization of limbs for running and specializations for chewing plant material (molar cusp patterns, jaw musculature).  The teeth of the first horses were still generalized enough that they might have been omnivores rather than full-fledged herbivores. PROPALEOTHERIUM
     Hyracotherium is one of the earliest, most primitive horses which stood up to 50 cm at shoulder (under 2 feet).  The teeth reflected a primitive mammalian condition (3 incisors, 1 canine, 4 premolars, and 3 molars) and they were not specialized for grinding as in modern horses, suggesting that Hyracotherium browsed on soft plant material.    In Hyracotherium, the third finger and toe were already the most prominent digits, the anklebones were modified to allow no lateral movement, and there were 3 toes and 4 fingers (there were also the vestiges of the two lost toes and the lost finger).  Some species of Hyracotherium (H. sandrae, H. tapirinum, H. vasacciense, H. angustidens H. craspedotum) are known from the Eocene of North America while others (such as H. leporinum) are known from the Old World.  Hyracotherium evolved into a number of species such as Propachynolophus (P. gaudryi, P. madani, P. cesserasicus, P. caylux), Pachynolophus (P. duvali, P. livinierensis) Propalaeotherium (which evolved from Propachynolophus; Propaleotherium parvulum, P. messelens, P. isselanum, P. argentonicum, P. hassiacum, P. helveticum), Lophiotherium, and Orohippus. (Storch, 1992; Kitts, 1956; Radinsky, 1966; Radinsky, ; Gingerich, 1981).
hyracotherium hyracotherium
hyracotherium skull
    Hyracotherium lacked the bony bar which encloses the eye orbit from behind (the postorbital bar) found in later horses and they only have a short space (diastema) between their front and rear teeth compared to that of later horses.  One major development in the horses would be the development of specialized teeth (high-crowned teeth) adapted for eating grasses.  The first fossils of grass pollen and the first true savanna environments in earth’s history occur in the Eocene.  Orohippus was very similar to Hyracotherium (for example, it still retained 4 toes on its front feet) and was the ancestor of the later horses (Radinsky, ; MacFadden, ).
orohippus orohippus skeleton
     In the Late Eocene and Oligocene, the climate of North America became drier.  The type of plant life in North America changed and many previous inhabitants of North America (such as a diversity of primitive primates) became extinct.  Grasses evolved in this drier climate and horses became adapted to these new environments.  Epihippus evolved into Mesohippus which evolved into Miohippus. 
mesohippus mesohippus
mesohippus skeleton
Mesohippus and Epihippus still retained a vestige of its 4th finger and a small vestige is sometimes observed on Miohippus as well. 
miohippus archaeohippus
kalobatippus sinohippus

       Miohippus was still a browser of woodland and forest plant material.  Its side toes were still important—in forests, agility (the ability to leap to the side suddenly) can be more important than speed in a single direction.  In the Miocene, Miohippus evolved into a number of horse groups: pygmy horses (such as Archeohippus), browsing horses which migrated to Eurasia and became successful there (Anchitherium, Hypohippus, Megahippus, and Sinohippus), and horses which were now adapted to life on North American plains with specialized teeth for grinding grasses and leg modifications for greater speed.  This last group maintained three toes on each foot and included Parahippus and Merychippus (Forsten, 1982; Thomason, 1986; MacFadden, 1988; Rensberger, 1984).

parahippus merychippus
merychippus merychippus

      In the Late Miocene, Merychippus species evolved into a number of successful species including smaller horses (Protohippus, Calippus), 3 toed browsing hipparion horses (Hipparion, Neohipparion, Stylohipparion, Cormohipparion, and Pseudohipparion; some species migrated to Eurasia and Africa), and one toed horses.  The Hipparion horses in Eurasia were successful and were able to adapt to a wide variety of habitats, ranging from steppe to forest.  Perhaps this success was in part due to the fact that they didn’t have as much competition in Eurasia from the great number of horse species that existed in North America.  Hipparion horses survived for quite some time in Africa, but competition from an ever-increasing number of bovid species (such as antelopes) probably contributed to their demise.  Many species of horse coexisted in geographical areas, suggesting that they lived in different habitats.  The one-toed horses were better adapted to grassland while the side toes of hipparion horses gave them advantages in savannas.  While the side toes of the hipparion horses would only touch the ground when the full weight of the body was placed on the foot, these toes were important when making sudden changes of direction (which is important in savannas where there are still trees to hide predators) (Forsten,  1982; Stirton, 1955; Hulbert, 1982; Webb, 1977; Garces, 1977).

protohippus hipparion
calippus
      Neohipparion produced a number of species including N. floresi, N. arellanoi, N. eurystyle, and N. phosphorum.  A species of Nannipus and a species of Neohipparion came as far east as Florida.  Among Nannipus species, N. lenticularis was ancestral to N. beckensis which was ancestral to N. phlegon.   At most sites, Nannipus is a rare species which is outnumbered by Pliohippus, Astrohippus, and Neohipparion.  There is one site from a shallow-water environment where Nannipus is abundant, suggesting that it specialized to a habitat that other horses rarely entered (Dalquest, 1973; MacFadden, 1984).
nannipus dinohippus
astrohippus
      Neohipparion produced a number of species including N. floresi, N. arellanoi, N. eurystyle, and N. phosphorum.  A species of Nannipus and a species of Neohipparion came as far east as Florida.  Among Nannipus species, N. lenticularis was ancestral to N. beckensis which was ancestral to N. phlegon.   At most sites, Nannipus is a rare species which is outnumbered by Pliohippus, Astrohippus, and Neohipparion.  There is one site from a shallow-water environment where Nannipus is abundant, suggesting that it specialized to a habitat that other horses rarely entered (Dalquest, 1973; MacFadden, 1984).
dinohippus
dinohippus
equus

The genus Equus produced a number of extinct species such as Equus conversidens and E. francisci (a stilt-legged horse).  The first Equus evolved in North America about 3.7  million years ago, were prominent in North American faunas by 3.3 million years ago, and arrive in Europe and Asia between 2.5 and 3 million years ago.  At the onset of the Ice Ages, horses became extinct in North America but one-toed horses thrived in Eurasia and Africa to produce the horses, zebras, donkeys, and asses of today (Lindsey, 1980; Churcher, 1970). A horse species closely related to the African zebra lived in North America (Equus simplicidens) and is referred to as the American zebra (Kurten, 1988).

     Modern one-toed horses still retain remnants of their fibulas (splints which fuse to their tibias) and the bony metapodial splints of the side toes.  There have been rare occurrences of horses born with lateral toes.