Evolution Of The Horse

For some of my equine science posts I have decided to look through my lecture notes and past assignments and edit and add to them. The first topic I saw that was interesting was evolution of the horse. This was from my equine anatomy module. 

There are two main methods to study evolution; these are fossil records and genetic markers. The horse family has very good fossil records.

Horses are in the order Perissodactyla; this includes tapirs and rhinos as well as horses and zebras etc. The very early members of Perissodactyl were very small- around the size of a terrier. Horses are in the suborder Hippomorpha. There are three families in this; horses are in the family Equidea. 350 different equid species have been found. Modern equine show wide genetic diversity when breeding with each other but as they don’t produce fertile young they are separate species. For example, a horse bred with a donkey produces a mule but this cannot be bred as most mules will be infertile.

The horse has evolved in a number of ways: -

• They have increased in size
• Reduce number of digits
• Increased length of skull
• Increased depth of molars
• Change in premolars towards molars
• Change in eye structure

There have probably been other changes that can’t be seen on fossil records such as changes in behavior.

54-38 million years ago, all horses appear to have 4 toes and premolars. Orohippus is an examples of this.

38-26 million years ago the horse walked on 3 toes and the premolars were disappearing. Mesohippus and Miohippus are examples and are getting bigger. Most were still browse feeders in forests but some were starting to live on plains. Merychippus was another variety of the same animal and also had 3 toes on all four feet but was living a bit later on.

26-7 million years ago Parahippus had started to show signs of adapting to plains. Parahippus possessed 3 digits but the lateral ones were shorter and would only touch the floor on rough terrain. They were starting to get elongation of teeth.

By 7 million years ago the lateral digits had disappeared. Pliohippus was one of the first kind of horses that walked on one digit as today's Equus does. This had curved teeth however, so is thought unlikely to be an ancestor of the modern horse.

The modern day Equus is thought to have evolved from Dinohippus. Fossil records have found that Dinohippus rapidly spread in the old world and evolved into zebras an asses which as in the Equus category.

Dinohippus.

In Eurasia, horse fossils have been found from around 6,000 years ago. It is thought that these were domesticated horses as knowledge of capturing, taming and rearing horses spread quickly with people.

(The images in this post do not belong to me.)

Love Laura

xxxx