Introduction to Dawn Horse

Dawn horse is also known as Eohippus as their biological name which is alternatively also known as Hyracotherium. This name was given by a famous paleontologist of America named Othniel Charles Marsh based on their structural similarities with the Hyracotherium genus. This genus was named so by an early British paleontologist named Richard Owen. Eohippus meaning any genus of horse that is from the lower Eocene and are primitive and small-sized that the size of the horses found today. Their special characteristics include 4 toed forefeet and 3 toed hind feet.

Alternative Eohippus meaning dawn horse that is small-sized primitive mammals that belong to the horse family but are now extinct. Eohippus is extinct that is a part of a diversified clad of large mammals that have hooves like cattle, camel, giraffe, pigs, and many more. They primarily fall under the taxonomy of horses that includes extinct species like horses, zebras, and donkeys whose presence was identified by the study of various fossils. Its fossil study indicates that it originated and had its habitat in North America as well as Europe in an early geological time span called Yprisian which is the oldest and the lowest stratigraphic age of the Eocene and its time span is between 56 to 47.8 Ma. The skeleton portrayal of Dawn’s horse is given below.


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Among all the other odd-toed hoofed hind leg mammals, Hyracotherium (Eohippus meaning) is the most identical and relatable ancestry of this genus called the Perissodactyla. This refers to the group of mammals that has one or three (odd-numbered) hoofed toes on each of the hind feet like a pig, ass, zebras, horses, or rhinoceros. 


Features of Eohippus

Hyracotherium is believed to be existing in an early geological time span called Yprisian which is the oldest and the lowest stratigraphic age of the Eocene and its time span is between 56 to 47.8 Ma.and they were approximately the size of a fox but the sizes varied a little depending on the species of dawn horse.  They were usually of size 30 to 60cms which is about 1-2 feet tall with a verified skull dimension depending on the species. Few of them had a very small skull and neck and thus possessed relatively shorter faces but the other species had a long elongated skull with a long neck like that of a horse mouth. They also had a springy arched back which is common for all the species of Hyracotherium. Its head was short and its eyes were situated just at the center of its skull. It had a round back with a striped coat on it for camouflaging itself. It was a four-legged mammal with four-toed hoofs on each one of the two forelegs and three hooped toes (odd-numbered) on each of the two hind legs.

It is about 12 inches tall of its shoulder and the weight of Hyracotheruim is estimated to vary from the weight of a big cat that is 2 to 5kgs to up to a weight of a pony that is 15 to 25 kgs. It possessed a large sharp canine probably to use it to fight for the harems of the females, whereas, the other set of teeth were crowned. This indicates it was primarily herbivorous and fed mostly on plants, grasses, and shoots. It has been seen that its hind legs were longer than the length of its forelegs and thus it used them for running in order to escape from the predator. Their body was well above the ground level and they had a lean constructed limb with hoofed toes that supported them while they were willing to stand vertically supported by a surface underneath. 

Although they have four toes each on two forelegs and three toes each on hind legs, only three toes were functional primarily with Hyracotherium. Each toe had a small hoop at the end of it. The bones situated at the lower limbs that are called Radius and Ulna are not fused with each other like that in the horses that are existing in today’s world. They are not joined with one another and had a gap that made them two distinct bones altogether. This indicates that they were capable of having some rotational movement and it was easy for them to move around the bumpy and rough terrains of the forests. They are said to be the ancestors of another extinct species called strange Chalicotheres and enormous Titanotheres. Technically Eohippus were the immediate successors of Hyracotherium.


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Evolution of Eohippus

Eohippus species were first found in an archeological time span of early Eocene that is about 52 million years ago. Before the fossils were discovered the evaluation of Eohippus to Equus was believed to be quite simple in the initial period of time. But then with the increase in the number of the equid fossils the actual progression or evaluation of Equus from that of its primary ancestor  Eohippus is observed to be highly complex and is in the form of multibranched progression. Thus the straightforward and simple progression map of the evolution of the horse was replaced by a more elaborative model of progression with numerous branches spreading in many directions out of which evaluation of modern horse from dawn horse is just a single stream of the multi-branched evaluation map. 

In 1951, one of the famous paleontologist named George Gaylord Simpson was the first one to discover in his study that evaluation of modern horse is not the ultimate progression of the entire lineage of genus equids but is only a part of all the genus of horses that have been able to survive through the process of evolution. The early species that have sequentially evolved into modern horses from  Eohippus was first discovered on the basis of the discovery of the fossils in North America in 1879 by a paleontologist named Othniel Charles Marsh. Since then the theory of progression in a sequence from dawn horse or Eohippus to the modern horse has been considered as one of the prime examples in studying a complete complex multi-branched evolutionary process and the concept was famous by the name of Thomas Huxley. 

The sequences of the transition fossils were collected and restored by the American Museum of Natural History and are exhibited to broadcast the linear lineage of simplistic evaluation of the horse species and are widely included as the prime study of the complete progression process of genus equids through the centuries. It has been drawn from the study of the fossils of Eohippus that they had 44 teeth that were low crowned, that is, three incisors, four premolars, three molars, and one canine on each side of their jaws. This is a typical arrangement of teeth generally found in herbivorous browsing mammals. Its molars are seen as bumpy, uneven, and dull which can only be used for grinding of foliage. The apex of the molars was slightly connected to the crests. 

This shows that they fed on soft foliage and grasses or fruits while scampering around bushes as that of a modern barking deer or rib-faced deer. It usually had a small brain with a smaller frontal lobe. As their limbs were long, especially the hind legs which indicate that it was the beginning for adaptation to run and to run with a greater speed to escape the predator. All the bones which are otherwise fused in the modern horse were observed to be loose and unfused and were treated as two separate bones that allowed the rotational movement of the legs and it was easy for them to run on bumpy and rocky surfaces with ease. Its hock joints, as well as wrists, were much lower to the ground level and they had forelimbs that had developed five toes. Out of those five toes, one of the big thumb toes was off the ground and the rest four were functional and had proto-hooves. The rare hind limb also had five tows out of which only three toes had the proto-hooves while the first and fifth vestigial toes are much off the ground in each of the two hind limbs. 

As the evaluation progressed these toe hooves disappeared among all the modern horse species. Their feet were padded as that of a dog or cat family member but the claws were replaced with hooves. The Eohippus went through some significant changes over a span of 20 million years. The most significant changes that were noticed with the fossils available was the change in the shape of the teeth in order to change with the change in the diet over the years. The Eohippus Equidae shifted from eating only foliage and fruits primarily to grazing and browsing feeds. During the Eocene timeline, the Eohippus species were branched out into various other categories of Equidae that are of the new category and characteristically different from Eohippus. Years later, there were thousands of fossils of these Equidae species that were buried in the Eocene layer and are located primarily in the North American strata along the Wind river basin located in Wyoming.  

Similar kinds of fossils were also seen in Europe such as propalaeotherium but this is not considered as the ancestor of modern horses. Some of the pictures of the evaluation of Equidae species are illustrated below for a clear understanding.


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Discovery of Eohippus

A scientist named Othniel C. Marsh in 1876 studied a skeleton that was similar to that of the Equidae species and named that skeleton Eohippus Validus, which means a horse-like hippo that is commonly known as dawn horse in today’s time. In 1932, another British scientist, Richard Owen had described a similar fossil that was formally pointed out in the paper published by Clive Forster much later of the discovery. E. Validus was then included in the category of genus Hyracotherium which was then of the top priority among all the genus of horses known by then and thus Eohippus became the second most popular genus and a synonym for Hyracotherium. But recently it was found that Hyracotherium was a part of the paraphyletic group which means it includes the group of species that has its last common ancestor with all its descendants except a few that belong to the monophyletic subgroup, and therefore, only H. Leporinum is a part of this genus. As E. Validus was found to be highly identical to the ancient genus Hyracotherium and thus the binomial nomenclature of E. Validus was done as Eohippus angustidens.    


Characteristics

There has always been a big debate whether Eohippus or Hyracotherium should be considered as the first ancestor of the modern horse and should be classified as the “first horse”. Though it is quite impossible to find out the ansistarial form of any extinct species by comparing the fossils that are about 50 million years in age. Today Hyracotherium is classified as the family member of palaeothere by many paleontologists around the globe and thus it is distinct as perissodactyl who is famously known as odd hooped toe mammals that are giant plant eaters and are ancestors of the horse family. These are also classified as Brontotherium and are biologically known as Brontotheres. Though many of the paleontologists believe that Eohippus also belongs to the family of perissodactyl but it is mostly distinguished as a member of Equidae, though it has been a subject of debate to date. 

Thus few of the distinctive characteristics of a dawn horse is that they had odd toe limbs, that is, they had four hooped toes in each of the two forelimbs and three hooped toes in each of the hind limbs and as the limbs were lean and long they had acquired the ability to run faster. They also had crowned teeth that counted 44 in total that helped them to eat soft plants, grasses, and shoots. They had spots like deer that helped them camouflage themselves from the predators and kept them alive.


Habitat and Food 

According to the study of their fossils they mostly lived in dense forests and also in rocky and bumpy areas which is clear from the construction of their limbs and their bone structure it. Scientists have also found the remains of the monkeys around their fossils that indicated that they were browsers and did not make one single place as their habitats. Looking at their crowned teeth it is obvious that they fed on low bushes and plants that had soft leaves and shoots.

FAQs (Frequently Asked Questions)

1. Why are Eohippus Called Dawn Horses?

Ans. earlier to the discovery of a scientist named Marsh another British scientist have studied a similar fossil that was formerly named as genus Hyracotherium which was then of the top priority among all the genus of horses known by then and thus Eohippus became the second most popular genus and a synonym for Hyracotherium. As Eohippus in Latin means hippo or horse-like thus it was commonly known as Dawn horse.

2. When Was Eohippus Alive?

Ans. According to the age determined of the fossils that were found in the Eocene layer and are located primarily in the North American strata, Eohippus was determined to be alive about 55 million years ago.