Merychippus is an extinct proto-horse of the family Equidae that was endemic to North America during the Miocene, 15.97–5.33 million years ago. It had three toes on each foot and is the first horse known to have grazed.
Discovery and naming
thumb|left|Restoration of Merychippus insignis
Merychippus was named by Joseph Leidy (1856). Numerous authors <!-- around the turn of the century --> assigned the type species – Merychippus insignis – to Protohippus, but this is ignored. It was assigned to the Equidae by Leidy (1856) and Carroll (1988), and to the Equinae by MacFadden (1998) and Bravo-Cuevas and Ferrusquía-Villafranca (2006). The genus name comes from Ancient Greek μηρυκασθαι (mērukasthai), meaning "to ruminate", and ἵππος (híppos), meaning "horse", but current evidence does not support Merychippus ruminating.
Description
upright|thumb|right|M. sejunctus front and back feet (right) and M. sphenodus lower jaw fragment
Merychippus lived in groups. It was about tall and at the time it was the tallest equine to have existed. Its muzzle was longer, its jaw deeper, and 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 shape of today's horses.
The Miocene was a time of drastic change in environment, with woodlands transforming into grass plains. This led to evolutionary changes in the hooves and teeth of equids. A change in surface from soft, uneven mud to hard grasslands meant there was less need for increased surface area. Equid size also increased, with Merychippus ranging, on average, between 71 and 100.6 kg.
Classification
The genus has been considered non-monophyletic as broadly defined, and including species of unrelated three toed equines. Merychippus gunteri and Merychippus primus are suggested to be early diverging members of Equinae. The type species, “Merychippus” insignis, as well as “M.” californicus, “M.” brevidontus , “M.” goorisi, “M.” republicanus, and “M.” coloradense are suggested to be members of the tribe Hipparionini, while “Merychippus” sejunctus has been suggested to be a member of the tribe Protohippini. Some former species of Merychippus have been assigned to the genera Acritohippus and Parapliohippus in Equini, as well as to Scaphohippus in Protoequini.
References
thumb|Illustration
External links
- Photo of a Merychippus tooth.
- Teeth of Merychippus californicus
- A classification scheme from The Paleobiology Database
- Florida Museum of Natural History
- The Evolution of the Horse
- Biodiversity.org