Merychippus
Encyclopedia
Merychippus is an extinct proto-horse
of the family Equidae
that was endemic to North America
during the Miocene
from 20.43—10.3 Ma living for approximately .
It had three toes on each foot and is the first horse known to have grazed. Its name means "ruminant
horse", but current evidence does not support Merychippus ruminating.
, but this ignored priority. Its type is Merychippus insignis. It was assigned to Equidae
by Leidy (1856) and Carroll (1988); and to Equinae
by MacFadden (1998) and Bravo-Cuevas and Ferrusquía-Villafranca (2006).
The foot was fully supported by ligament
s, and the middle toe developed into a hoof
, which did not have a pad on the bottom. In some Merychippus species, the side toes were larger, whereas in others, they had become smaller and only touched the ground when running. Its teeth were like those of Parahippus
; the extra crest that was variable in Miohippus
was permanent in Merychippus, and the other teeth were beginning to form a series of tall crests with higher crowns.
By the end of the Miocene era, Merychippus was one of the first speedy grazers. It gave rise to at least nineteen different species of grazers, which can be categorized into three major groups. This burst of diversification is often known as the "Merychippine radiation."
The first was a series of three-toed grazers known as hipparion
s. These were very successful and split into four genera
and at least sixteen species
, including small and large grazers and browsers with large and elaborate facial fossae. The second was a group of smaller horses, known as protohippines, which included Protohippus
and Calippus
. The last was a line of "true equines" in which the side toes were smaller than those of other proto-horses. In later genera, these would be lost altogether as a result of the development of side ligaments that helped stabilize the middle toe during running.
Horse
The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...
of the family Equidae
Equidae
Equidae is the taxonomic family of horses and related animals, including the extant horses, donkeys, and zebras, and many other species known only from fossils. All extant species are in the genus Equus...
that was endemic to North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
during the Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
from 20.43—10.3 Ma living for approximately .
It had three toes on each foot and is the first horse known to have grazed. Its name means "ruminant
Ruminant
A ruminant is a mammal of the order Artiodactyla that digests plant-based food by initially softening it within the animal's first compartment of the stomach, principally through bacterial actions, then regurgitating the semi-digested mass, now known as cud, and chewing it again...
horse", but current evidence does not support Merychippus ruminating.
Taxonomy
Merychippus was named by Leidy (1856). Numerous authors around the turn of the century put the type species in ProtohippusProtohippus
Protohippus is an extinct genus of horse....
, but this ignored priority. Its type is Merychippus insignis. It was assigned to Equidae
Equidae
Equidae is the taxonomic family of horses and related animals, including the extant horses, donkeys, and zebras, and many other species known only from fossils. All extant species are in the genus Equus...
by Leidy (1856) and Carroll (1988); and to Equinae
Equinae
Equinae is a subfamily of the family Equidae, which lived worldwide from the Hemingfordian stage of the Early Miocene to present and in existence for approximately .-Taxonomy:...
by MacFadden (1998) and Bravo-Cuevas and Ferrusquía-Villafranca (2006).
Body mass
Four specimens were examined by M. Mendoza, C. M. Janis, and P. Palmqvist for body mass. The results were:- Specimen 1: 164.7 kg (363.1 lb)
- Specimen 2: 106.4 kg (234.6 lb)
- Specimen 3: 58.9 kg (129.9 lb)
- Specimen 4: 17.2 kg (37.9 lb)
Description
Merychippus lived in herds. It was about 89cm (35in) tall; at the time it was the tallest equine to have existed. The muzzle was longer, the jaw deeper, and the eyes wider apart than any other horse-like animal to date. The brain was also much larger, making it smarter and more agile. Merychippus was the first equine to have the distinctive head of today's horses.The foot was fully supported by ligament
Ligament
In anatomy, the term ligament is used to denote any of three types of structures. Most commonly, it refers to fibrous tissue that connects bones to other bones and is also known as articular ligament, articular larua, fibrous ligament, or true ligament.Ligament can also refer to:* Peritoneal...
s, and the middle toe developed into a hoof
Hoof
A hoof , plural hooves or hoofs , is the tip of a toe of an ungulate mammal, strengthened by a thick horny covering. The hoof consists of a hard or rubbery sole, and a hard wall formed by a thick nail rolled around the tip of the toe. The weight of the animal is normally borne by both the sole...
, which did not have a pad on the bottom. In some Merychippus species, the side toes were larger, whereas in others, they had become smaller and only touched the ground when running. Its teeth were like those of Parahippus
Parahippus
Parahippus is an extinct relative of the modern horse, very similar to Miohippus, but slightly larger, at around tall, at the withers....
; the extra crest that was variable in Miohippus
Miohippus
Miohippus was a genus of prehistoric horse existing longer than most Equidae. Miohippus lived in what is now North America during the Oligocene approximately 32-25 million years ago. While descending genera of this species lived during the Miocene period, the Miohippus was a horse of the Oligocene...
was permanent in Merychippus, and the other teeth were beginning to form a series of tall crests with higher crowns.
Descendants
The first was a series of three-toed grazers known as hipparion
Hipparion
Hipparion is an extinct genus of horse living in North America, Asia, Europe, and Africa during the Miocene through Pleistocene ~23 Mya—781,000 years ago, existing for...
s. These were very successful and split into four genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
and at least sixteen species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, including small and large grazers and browsers with large and elaborate facial fossae. The second was a group of smaller horses, known as protohippines, which included Protohippus
Protohippus
Protohippus is an extinct genus of horse....
and Calippus
Calippus
Calippus may refer to:* Calippus of Syracuse, a student of Plato and Syracusean tyrant.* Callippus, a Greek astronomer and mathematician.* Calippus , a small lunar crater.* Calippus, an extinct relation of the modern horse....
. The last was a line of "true equines" in which the side toes were smaller than those of other proto-horses. In later genera, these would be lost altogether as a result of the development of side ligaments that helped stabilize the middle toe during running.