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Haemig PD  (2012)  Evolution of Horses. ECOLOGY.INFO 33

Evolution of Horses

Note: This online review is updated and revised continuously, as soon as results of new scientific research become available.  It therefore presents state-of-the-art information on the topic it covers.

Horses have existed for over 50 million years. However, their size, appearance, species diversity and feeding habits have changed greatly during the long course of their evolution. 

In this review, we trace the fossil history of these extraordinary animals, from their beginnings as miniature broncos the size of housecats and small dogs, to their peak radiation in the Miocene of at least a dozen contemporaneous genera of many different sizes, and their subsequent decline to one surviving genus today.

Most of this evolution took place in North America which, for millions of years, was an island continent isolated from the rest of the world by seas, much as Australia is today. Here, for long epochs of time, horses inhabited ancient landscapes, interacted with prehistoric fauna and flora, and numerically dominated ungulate communities. 

We begin this review by quickly summarizing some basic facts about modern-day horse species and their relatives.  Then we start the tale of horse evolution with a look at the miniature broncos of the Eocene.  Unless specifically stated, all examples of horse evolution are from North America.

Living Horses

Today, all living members of the horse family (Equidae) are classified into one genus: Equus.  There are four main groups within the genus (Oakenfull et al. 2000):

Zebras
- Grevy's Zebra (Equus grevyiPhoto 1, Photos 2 and 3
- Plains Zebra (Equus quagga Photo 4, Photo 6
- Mountain Zebra (Equus zebraPhoto 5

Asiatic Wild Asses
- Kulan and Onager (Equus hemionus)
- Tibetan Wild Ass or Kiang (Equus kiang)

African Wild Asses
- African Wild Ass (Equus asinus)

Caballine (True) Horses
- Przewalski's Horse (Equus caballus) (Photo 8, Photo 9)

Origin of domesticated forms:
(1)  The ancestors of domestic horses came from many different populations of wild caballine horses (Vilà et al. 2001; Bendrey 2012).  Przewalski's horse was one of these (see page 2 of this review).
(2)  The wild ancestor of the donkey (Photo 10) is the African wild ass.
(3)  A mule (Photo 11) is the hybrid offspring of a male donkey and a female caballine horse.  It is usually sterile.  A hinny is the hybrid offspring of a female donkey and a male caballine horse.  It is also usually sterile.

Perissodactytla

The horse family is one of three families in the mammalian order Perissodactyla.  The other two families are tapirs (Tapiridae) and rhinoceroses (Rhinocerotidae).  Species of all three families are fast runners, herbivores (browsers and/or grazers) and excellent dispersers of seeds (which pass undigested through their gastrointestinal systems).

The Perissodactyla is an order of mammals in decline.  All of its surviving families were formerly more abundant, diverse and widespread than they are today. 

For example, at a Late Miocene site in Florida dated 9 million years ago, fossils of 21 species of terrestrial ungulates were reported by MacFadden and Hulbert (1990).  Of these, 12 were Perissodactyla (9 horses, 1 tapir, 2 rhinos), 8 Artiodactyla and 1 Proboscidea.  Perissodactyla comprised 74% of all individual ungulates (horses: 58.6%, tapirs 4.2%, rhinos 11.2%).

Today, the Perissodactyla is no longer the dominant ungulate order in the world.  That place has been taken by the order Artiodactyla.   The Artiodactyla include deer, wild pigs, peccaries, bison, wildebeests, mountain sheep, goats, pronghorns, antelopes, giraffes, camels, hippopotamuses and many other taxa.

Eocene Horses

The first horses appeared in the early Eocene of North America, 50 to 56 million years ago.  They were miniature broncos the size of housecats and small dogs, and were diversified into many genera.  These early horses did not have the hoofs of modern horses.  Instead, they had 3 toes on their front feet, and 4 toes on their back feet.

Eocene horses fed mainly on woody and herbaceous vegetation and fruit, rather than on grass.  It is believed that they lived lifestyles similar to those of the duikers of modern-day Africa (Janis 1982; MacFadden 1992).  Duikers (Cephalophus spp., Sylvicapra spp.) are tiny, browsing ungulates of the family Bovidae (Artiodactyla).  They inhabit forests and savannas. 

One of the best-known early Eocene horses was Eohippus angustidens, whose name means "dawn horse."  Fossils of this species were first found during the 19th century in North America.  For many years, dawn horse was believed to be the first horse, but now fossils of earlier horses have been discovered.

Zero Horse

Currently, the earliest known fossil horse is Sifrhippus sandae.  Its scientific name means "zero horse" (Froehlich 2002).   Zero horse was the size of a housecat and is also the smallest known horse (Gingerich 1989, 1991; Froehlich 2002).

Fossils of zero horse have been found in the Clark's Fork Basin of Wyoming and the Bighorn Basin of Montana and Wyoming. They date from a period between 55 and 56 million years ago, which places them near the beginning of the Eocene (Gingerich 1989, 1991; Froehlich 2002; Secord et al. 2012). 

However, zero horse may be much older than these finds because fossils of another, more derived, horse species were found with it at the two sites mentioned in the previous paragraph.  This second horse species, Arenahippus grangeri (sand horse), was larger than zero horse and had more advanced molars (Froehlich 2002).

Oligocene Horses

The Eocene ended approximately 34 million years ago.  After it came the Oligocene, which lasted roughly 10 million years.  During this epoch, all horses continued to be browsers.  However some, such as Mesohippus, added significant amounts of grass to their diets (Solounias and Semprebon 2002). 

The two dominant horse genera of the Oligocene were Mesohippus and Miohippus, which weighed 40 to 55 kilograms.  Both were about 50% larger than most Eocene horses, but still much smaller than the 500 kilograms of many horses today (MacFadden 1992).

Mesohippus appeared in the late Eocene and died out in the mid-Oligocene, 11 million years later (MacFadden 2005).  It was the first horse that had 3 toes instead of 4 toes on its front legs.  Like Eocene horses, it also had 3 toes on its hind legs.  

Miohippus evolved from Mesohippus and the two genera were contemporaneous for about 8 million years (MacFadden 2005). Unlike Mesohippus, Miohippus survived the Oligocene, existing for at least 18 million years, from the late Eocene to the mid-Miocene (MacFadden 2005).  During the next epoch, the Miocene, it radiated into many different horse clades.

Miocene Horses

The Miocene epoch began 24 million years ago and ended about 5 million years ago.  For horses, it was a time of great ecological and evolutionary change.  During this epoch, body sizes, diets and niches diversified, and significant changes in locomotion and anatomy occurred.

During this period, horses also reached the peak of their biodiversity.  For example, the horse subfamily Equinae underwent a major radiation, diversifying from one species, Parahippus leonensis, to 70 species (Maguire and Stigall 2008, 2009).

Diversification of Diets

When the Miocene started, all horses had low-crowned (brachydont) teeth and were mostly browsers, feeding mainly on leaves, twigs and fruits of dicotyledonous plants.  When the Miocene ended, all horses had high-crowned (hypsodont) teeth, and their diets were diverse (Mihlbachler et al. 2011).

For the first time, some horses became grazers, i.e. feeding mainly on grasses and sedges (monocotyledonous plants).  Examples were Protohippus, Calippus, Cormohipparion and Nannippus (MacFadden 2005).  Other horses, such as Parahippus and Merchychippus, ate a mixed diet of both browse and grass (Solounias and Semprebon 2002, MacFadden et al. 1999).  Toward the end of the Miocene, a few hypsodont horses derived from grazers, such as Dinohippus mexicanus and Astrohippus stocki, returned to complete browsing (MacFadden et al. 1999).  However, neither mixed feeding not the secondary adaptation to browsing endured.

The general shift over time from browsing to mixed diets and grazing, was a significant niche change that has endured up until the present (MacFadden 2005).  Modern horses today are mainly grazers.  They add browse to their diet in the winter, but eat mostly monocotyledonous plants (>90%) during the rest of the year (Duncan 1992).

Diversification of Genera

During the Miocene, the climate and vegetation of North America changed.  Grasslands expanded and forests decreased in size.  The advent of extensive grasslands provided a new opportunity for horses to diversify.  Consequently, they colonized this new biome and radiated into many new genera, embracing a grazer diet. 

This adaptive radiation led to an increase in horse genera and the peak of horse diversity in the early Clarendonian Age of the mid-Miocene (11 to 9.5 million years ago).  At that time, there were at least a dozen contemporaneous horse genera living on the central Great Plains of North America (Janis et al. 1998).  The two dominant groups were horses of the tribes Hipparionini and Equini (MacFadden 1992, 2005).  Like other horse groups, both the Hipparions and Equines would later decrease in diversity.  Yet as we shall see, both taxa were contemporaneous in North America up until the Ice Age (Pleistocene).

Horse diversity declined in the late Miocene.  The main reason for this decline was that the guild of browsing horses died out (Janis et al. 2000; 2002).  Why these steeds disappeared is unknown.  However, it appears that ungulate browser diversity decreased on other continents as well during the late Miocene, even among non-horse taxa (Janis et al. 2000).  During this period, there was also an associated increase in the average size of the remaining browsers (Janis et al. 2000).

Body Size Changes

For the first 35 million years of horse evolution, horses were generally small, from 10 to 55 kilograms (MacFadden 1992).  During the Miocene, however, the size of horses diversified:  some horses became larger, others remained similar in size to the early horses, and still others became smaller than their ancestors (MacFadden 1992).  The horses that survive today are those whose ancestors generally increased in size.  The radiation of horses into a diversity of sizes during the Miocene was undoubtedly related to their diversification into different habitats and diets.

Springing Locomotion

As horses spread out of the forests and onto the grasslands during the Miocene, they also became more cursorial.  They evolved a new springing locomotion which made them faster.  This increased speed probably enabled them to better escape predators and exploit distant food patches (McNaughton et al. 1985; Benton 1990; MacFadden 1992).

In hypsodont Miocene horses, the lower legs became elongated, "the interosseous muscle was reduced, and the interosseous tendon was elongated (MacFadden 1992)"  These changes made the horse foot elastic and like a pogo stick (Camp and Smith 1942).  Other morphological changes in muscles and bones, streamlined horses and enabled them to take more efficient, powerful strides (MacFadden 1992).

One-toed Horses

At the beginning of the Miocene, all living horses were tridactyl, i.e. had three toes on each foot.  Later in the Miocene, however, some horses became monodactyl, with one toe on each foot, like modern-day horses.  The horses that became monodactyl all belonged to the tribe Equini and included the genera Pliohippus, Astrohippus and Dinohippus

Early species of these genera were tridactyl, while later species were monodactyl (MacFadden 1992).  A Late Miocene population of one primitive Dinohippus species found at Ashfall Fossil Beds State Historical Park, Nebraska, had a mixed population with some individuals being tridactyl and others being monodactyl (Voorhies 1981).

The modern horse genus Equus, which first appeared in the North American Pliocene, evolved out of this clade of horses and was completely monodactyl.  The Hipparions did not become monodactyl but evolved a similar adaption:  their three toes came to function as one digit (MacFadden 1992).

Pliocene Horses

The Pliocene occurred from 4.5 to 1.8 million years ago.  During these years, which led up to the Ice Age, horse diversity in North America declined from 5 genera to one genus (Equus), and the Caballine horses split off from the Wild Asses and Zebras (Steiner and Ryder 2011)

Two genera died out during the early Pliocene:  Neohipparion and DinohippusNeohipparion was a three-toed horse, while Dinohippus in its final stages was one-toed and the nearest outgroup to the genus Equus. Interestingly, at least some Dinohippus species were principally browsers, even though they possessed high-crowned teeth (MacFadden et al. 1999). However, it is believed that their immediate ancestors were grazers, and that they secondarily acquired the browsing habit (MacFadden et al 1999).

Two other Hipparion horse genera, Ninnippus and Cormohipparion, died out at the end of the Pliocene.  Both were three-toed horses and their occurrence throughout this epoch means that three-toed horses lived in North America right up until the beginning of the Ice Age, a fact usually not generally known.  Nannippus was widespread across the continent, but Cormohipparion was restricted to the southeast (MacFadden 1992).

Nannippus, was called the gazelle-horse.  This genus was composed of small to tiny three-toed grazing horses with slender, elongated limbs (Kurtén and Anderson 1980).  The largest ones reached only the height of Shetland ponies, but "were much lighter in build, with long, graceful legs" (Kurtén 1988).

Equus first appeared in the North American Pliocene and spread across the entire continent (MacFadden 1992, 2005).  The earliest horse of this genus was Equus simplicidens.  It  is believed to be the primitive Equus from which all other Equus evolved.

The endangered Grevy's Zebra (Photo 1, Photos 2 and 3) of modern times is a primitive zebra that preserves many of the characters of Equus simplicidens, such as a long skull with prominent occipital region, slender limbs, small hoofs and large body size (Skinner and Hibbard 1972; Kurtén and Anderson 1980; but see also Forsten and Eisenmann 1995 for minor differences).  Grevy's Zebra inhabits the Horn of Africa, where widespread lawlessness and poaching have greatly reduced its numbers and made it an endangered species.

Finish reading this review on Page 2.

Photograph at top of page by Brenton Nicholls (Australia).

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