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https://sites.google.com/site/oddtoedungulates1/home/natural-selection

https://www.history.com/news/horse-domestication-happened-across-eurasia-study-shows

https://www.amnh.org/exhibitions/horse/the-evolution-of-horses/on-your-toes/

http://chem.tufts.edu/science/evolution/HorseEvolution.htm

https://www.livescience.com/55207-tapir-facts.html

http://carnivoraforum.com/topic/9774523/1/

http://eqsci.weebly.com/natural-evolution.html

https://www.britannica.com/animal/perissodactyl/Form-and-function#ref51658

http://cenozoiclife.blogspot.com/2016/04/perissodactyla-odd-toed-ungulates.html

http://netnebraska.org/basic-page/television/wild-horses-evolution

http://www.equinespot.com/types-of-horses.html

https://www.britannica.com/topic-browse/Animals/Mammals/Hoofed-Mammals-Odd-toed

https://www.thoughtco.com/odd-toed-hoofed-mammals-130482

http://www.ultimateungulate.com/perissodactyla.html

http://animals.jrank.org/pages/3238/Odd-Toed-Ungulates-Perissodactyla-PHYSICAL-CHARACTERISTICS.html

https://www.thoughtco.com/odd-toed-hoofed-mammals-130482

https://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/perissodactyla-odd-toed-ungulates

https://www.sciencedaily.com/releases/2011/09/110901142100.htm

https://www.britannica.com/animal/Thoroughbred
https://biology.tutorvista.com/organism/kingdom-animalia.html

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Another of Darwin's theories is the occurrence of natural selection, which means that the fittest traits for an organism will continue to be passed down through generations because they help the species to survive the longest and continue to reproduce. This also means that the weaker traits will gradually be reduced in the gene pool of the species because those organisms with those traits will die or be killed off before they can pass down those traits. The type of natural selection that has occurred within the odd toed mammals is directional, which means an extreme trait is the most fit for the organisms within their environment. A specific trait that shows this in the odd toed mammals is their ability to run. Because of their skeletal structure they have developed the ability to run at speeds up to 30 miles per hour. This includes all 3 species: horse, rhino and tapir. This characteristic allows them to escape from danger such as a predator attack.

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Descent with modification is Darwin's theory that, over time, as organisms, evolve they change.  These changes are usually physical and happen because of various factors such as environmental changes, overpopulation or predators.  An example of this happened specifically through horse ancestry beginning with the Mesohippus to the Equus or modern day horse.  As the horse evolved, various changes occurred such as a wider skull, reduction in the number of toes, and an increase in overall size. Such changes occurred over time in order to allow the horse species to continue to survive and adapt to its environment. The modern day characteristics that the modern day horses now allow them to gain great speed especially in the open fields and low grasslands they live on. It has also been found as well that in today's horses, there are still small remnants of vestigial structures of other toes above the horses feet, which goes to further prove how over time the horse evolved from ancestors who have more toes which needed in times when they lived in more forest lands which called for the extra support.

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Artificial selection is when humans breed together animals to create new offspring based on certain desirable traits.  This has played a large role in horse evolution as humans have domesticated and bred horses for specific purposes such as farming, transportation, hunting and sport. Humans had begun to domesticate and breed wild horses 4,000 - 6,000 years ago. An example of this is the thorough breed horse. The thorough bred horse originated in England for the purpose of racing and jumping. It had been thought that they had been made between a cross between Arab and Barb horses who had come to England back in the 3rd century. Because of the artificial selection, it led to a new type of breed of horses that were fit for their specific purpose of performance. Thoroughbreds have gained characteristics from both horses such as speed and spirit from the Arab horse while also obtaining stamina and round hips from the Barb horses. The change breeding has had on horses has even gone as far as to classifying domesticated horses as separate species than of wild horses. Breeding has also led to over 350 different breeds of horses to exist today. Just as when any other organism evolves, if the breeding results in new animals with fit traits for their specific purpose as well as for survival and reproducing, then a new breed is made and evolution once again takes place.

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Geographic isolation played a large role in the separation and diversification of the group. Beginning when Eocene period began, it is believed the oldest common ancestor had evolved in India at the time when it was still and Island, and by the time it had moved and formed into the other continents, it allowed for the migration of the ancestors to move about other parts pf the world. It is also believed the earliest horse ancestors horses started in what is now North America. They crossed over the land bridge that connected North America to Northern Asia and from there they spread through Asia, Europe and Africa.  The rhinos started to appear in Eurasia and, along with horses, migrated to Africa during the Miocene period.  This gave rise to the modern day rhino and zebra found in Africa today.  The horses and tapir migrated from North America to South America in the Pliocene period.  During this time, the continents had still been shifting and moving. (continental drift).  This resulted in the breaking of land bridges and further separating the continents.  This geographically isolated the already migrated mammals across different continents, forcing them to evolve differently based on their different environments. This is also why Tapirs only live on the Western Hemisphere, while the Rhinos only live on the Eastern Hemisphere. On top of that, wild horses in North America had ended up dying out between 13,000 to 11,000 years ago, and once again no horses were able to come and migrate back into the continent by then. The reason horses live there today is because sellers had introduced them back into the land and as some let some of the horses free they began to revert back their old behavioral patterns making wild horses again such as the modern day mustangs.

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The type of speciation that occurred in the evolution of odd toed mammals was gradual.  The first odd toed mammals appeared in the early part of the Eocene period, about 55 to 40 million years ago.  Over time, the group evolved and there were many different species having many different characteristics that eventually led to the odd toed mammals we see today.  For example, horses started in the beginning of the Eocene period and it wasn't until the beginning of the Oligocene period (22 million years later) that its next major evolutionary changes started to appear.  We do not see another major change until the middle of the Miocene period (about 20 million years later). As the evolution continues on, we can also notice how the changes are made for the most part are not very different from the last, except for the increase in overall size and decrease in the amount of toes.   The evolution continues this way until we reach the modern day horse, which has been in existence for the past 11,500 years.

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Chromosomal analysis is able to show how related species are to one another by identifying similar genetic traits.  Even though all 3 of these species have different numbers of chromosomes (Tapir: 80, Rhino: 82-84, Horse: 64), they still have very similar structures.  For example, chromosomes 21, 18 and 11 for the Tapir, Rhino and Horse respectively are similar. Likewise for chromosomes 6, 3 and 3 respectively.

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Tapir

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Rhino

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The type of evolution that occurred in odd toed mammals was divergent evolution.  This means that, as the evolution in the group progressed, the variations in species became more separate and diverse.  While all starting from a common ancestry, we see the early forms of horse, tapir and rhino beginning to branch out with different characteristics.  One reason for this was the migration of sub-populations of the original group migrating to different parts of the world and having to adapt to different environments, habitats and threats.  By the early to middle Eocene period, the earliest signs of diversification had already begun to form and though subtle, were shown through little variances in teeth and skulls. This increase in diversity and the early forms of multiple different species appeared: the horse, rhino, tapir, and several others, some of whom are now extinct.  The phylogentic tree is also able to show how species branch off from one another over time. In this tree for example, it shows how all the animals started it off with a common ancestor, to the begging of them breaking off from one another. The horse broke off earlier than from the rhinos and tapirs, in the early Eocene period, while fossils had shown how the rhinos and tapirs were still almost exactly alike until the end of the period where they were diversified enough to be classified as separate species.

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Horse

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Embryology is able to show many similarities between organisms in the early stages of life.
The two comparisons shown here are between a zebra and a domesticated horse (closely related to as far a their genus Equus but are still different species).  One characteristic that the embryos share is indication of an initial snout formation.  Another similar trait is the starting formation of a tail bone.  Both also show a longer neck and legs, with the front legs forming first, which also is also a very important characteristic for the embryos to have since all new born calves must be able to walk as soon as they are born.  These similarities help to prove the close relationship between these organisms.

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The skeletal structure of these 3 modern species have these similarities. They all have a backbone that rests above their ribs and legs. It is primarily controlled by their back legs. They also have no collar bone. These features support their ability to run. They also have a large rib cage in order to contain and protect their internal organs. All 3 have an elongated face in order to help with grazing and finding food. They also have longer and bent hind legs for proper support. They have tail bones to assist with balance. Although they have different numbers of toes, they all have a longer middle toe that is used as the primary support for their weight and balance.

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Grevy’s Zebra
Animalia
Chordata
Mammalia
Perrisodactayla
Equidae
Equus
grevyi

White Rhino
Animalia
Chordata
Mamalia
Perrisodactayla
Rhinocerotidae
Ceratotherium
simum

Mountain Tapir
Animalia
Chordata
Mamalia
Perrisodactayla
Tapirdae
Tapirus
pinchaque

Taxonomy is how every species of organism are classified as. As the classification goes further and further into the name, it gets more and more specific, making each group smaller and smaller. Organisms are put into these groups based on certain characteristics they have and are with all who have the same as the particular requirements for that level. As shown with the organisms above, they are very closely related as they have the majority of their classifications in common. Animalia means they are multi-cellular heterotrophs, Chordata means they are vertebrates, mammalia means they are warm blooded and give birth to live young, and perrisodactayla, which the group that is most specific to them, means they have an odd amount of hoofed toes.

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Horse

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Tapir

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Rhino

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Foot comparison - Tapir, Rhino and Horse

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protapirus

33.9 - 23 mya (Oligocene)

It lived in North America and Europe. It is most closely related to modern day tapirs. This fossil shows the same elongated facial structure and the same location of teeth and the same large eye socket as the modern tapir.

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Pliohippus

5.3-2.6 mya (Piocene)

Primarily lived in what is now North and South America. It is the species that came before the modern day horse. Therefore, it has very similar traits - long legs, long neck, elongated facial structure and long back that is able to rest on the legs. The only thing that was different in this species was that it was slightly shorter than the modern horse (4 feet tall at the shoulder). It was the first horse species to have a single toe, similar to the horses of today.

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Ceratotherium Neumaryi

Late Miocene 23-5 mya

It lived in the Mediterranean area and was very similar to modern day rhinos. It had the same wide jaw, horn on the top of its head and the same amount (24) and shape of teeth. Like modern day rhinos, this mammal had no front teeth.

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Eohippos (Dawn Horse)

Early Part of the Eocene Epoch (55.8 mya - 33.9 mya)

The Dawn Horse was one of the earliest ancestors of the of the order as well as the oldest known horse. It lived in the Northern Hemisphere and it was very small compared to the modern day horse at 2 feet long. It had legs similar to a horse - with stronger back legs than in the front. It also had an elongated facial structure. Lastly, it had the same curved back bone.

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Woolly Rhino

Middle Pliocene to end of Ice Age (Pleistocene) (5.3 mya - 11,700 years ago

The Woolly Rhino had lived in Northern Europe and Asia, and had even been found to be depicted in wall drawing by humans. They traveled alone or in small family groups just as the modern-day Rhino. In the fossil below, compared to a Rhino of today, it is very similar in structure as it also has a wide jaw and low crowned teeth along with molars and premolars that contributes greatly in helping with browsing when looking for food.

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1. Location found & habitat specific

1. Majority live in South Africa, Namibia, Zimbabwe, and Kenya, live in grassy savannas and woodlands in grassy clearings

2. Physical characteristics

1. Second largest land mammal after elephant, males = 3.6 tons, females = 1.7 tons, known for their square upper lip, longer skull for a rhino, less sharp and defined forehead, more pronounced shoulder hump, have almost no hair and two horns and front average at 60cm

3. Behavioral characteristics

4. Tend to avoid heat, mostly active in early morning, late afternoon and evening, at highest heat find shade, and rinse themselves in shallow mud pools as well as rinse themselves off of parasites, usually group in about 14 with mainly females and their calves, adult males defend their territories with scraped dung piles, males fight for females to breed with, often times injuring one another, most sociable rhinos yet still very territorial as many other rhinos are, peaks of breeding are between October and December though happens all year long, almost only eat short grass.

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1. Location found & habitat

2. Columbia, Ecuador, and Northern Peru, mountain forests or high treeless plateaus that are described as wet and tropical mountain forests constant covered by clouds, found between elevations of 2,000 and 4,000 meters

3. Physical characteristics

4. Smallest and furriest of the tapirs, long dark brown to black hair, lips lined in white, adults have two hair free patches on their lump, weigh between 150-250 kilo, young have watermelon patterned coat, do have even toed front hooves (4) but still have odd toed hooves on hind legs (3), large stocky body, shorter legs, round ears, flexible nose

5. Behavioral characteristics

6. Comfortable in water and can swim well, well adapted to traveling in harsh conditions of forested hills and other harsh weather, main activity happens in first hours of dusk and sunrise, more active on cooler than warmer days, important for dispersing seeds for South American forests, feed on leaves, twigs, and fruits, have very good sense of smell and hearing

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There are many physical traits that the animals within the Perissodactyla order share:

1. Feet - Their most notable features is their toes because they have an odd amount, as stated by their common name Odd Toed Mammals. Horses only have one per foot, Tapirs have an even amount of four on the front, but still have an odd amount of three in the back, and rhinoceroses have three. Their feet also include hooves.

2. Collar Bones - Another feature they have is an absent collar bone. The absence of this structure in them allows for a greater advantage in running.

3.  Backbone - balanced largely on the forelegs and pushed by the hind legs. This structure also allows for great running skills and also can withstand great amounts of weight from the organism.

4. Digestive System - They have very simple digestive systems that includes a small stomach,  a long intestine, and huge colon. They also do not have a gall bladder.

5. Other Important Characteristics - elongated nose, large nostrils, tails (with hair)

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1. Location found & habitat

1. They live in Eastern and Southern Africa on the grasslands and savannas - aka short grasslands and desert scrublands, vegetation is more abundant

2. Physical characteristics

1. Medium sized, long heads and necks, slender legs, long and upright ears that can move, upright mane, long tail with hair at the tip, single-hoofed, black skin under coat, about 770-990 pounds, thickest stripes of all zebras which are used to confuse predators' vision

3. Behavioral characteristics

1. Live in family groups (harems) led by stallion, communicate with one another through facial expressions as well as bark, bray, snort, or huff, when in danger stallion alerts others, usually runaway from predators and also use power kick as defense that can cause serious damage to predator, mainly eat grass as well as bark, fruit, and leaves.

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1. Strictly Herbivores -  they eat from a wide range of plants such as leaves, grassy vegetation, fruits, etc. depending the specific habitat they live in.

2. Obtaining Food - they typically browse and forage for their food all around their habitat.

3. Herds - they usually organizes themselves into bans/herds. Typciallly they stay in the same herds they were born into, but will somtimes also be chased off and into other feeding grounds and new herds.

4. Offspring - The males of the herd will fight in order to mate with certain females. After, the female picks the winner. The Gestation period for females is long, and lasts for an average of about a year to a year and a half. They usually give birth to only one calf at a time, and when it is born it is able to walk and stand almost immediately

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They live on the continents of North America, South America, Europe, Asia, and Africa.

they are not limited to any other type of vegetation or climates.

Grass is one of the most major parts of all of their diets, so it is required that their habitat includes it.

They also need to be a near a permanent water supply for frequent bathing and drinking.

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Also known as odd toed mammals, this order of mammals includes  three major families of animals commonly known as horses, rhinoceroses, and tapirs.