Merychippus is an extinct genus of ancestors of the current horse. In fact, it constitutes the third link in the evolution of the horse, between the Mesohippus and the Pliohippus. This genus of animals lived in the Miocene epoch, which belonged to the Neogene period of the Cenozoic Era.
It was described in 1856 by the renowned American paleontologist Joseph Leidy, who also established the type species of this genus, Merychippus insignis. The representatives of this genus inhabited ecosystems with prairies and savannas in which there were abundant bushes that served as food. These animals moved through these meadows in groups, constituting very well established herds.
Graphic representation of Merychippus and comparison with the height of the average human being. Source: Nobu Tamura (http://spinops.blogspot.ca/)
characteristics
Morphology
In regards to its appearance, the Merychippus was very similar to today's horses. They had an average height of between 90 cm and 120 cm. This genus is recognized as the first that began to develop the characteristic shape of horses, with its elongated snout.
Likewise, they had quite voluminous and large eyes. Their teeth were also large and had a series of protrusions called ridges. They also had wide crowns. This allowed them to successfully feed on slightly more resistant plants.
These animals, as is well known, were quadrupeds. Its limbs had three fingers, the middle being the most developed, with a hoof. In some species it is believed that the lateral fingers were also highly developed.
Reconstruction of the bone structure of the Merychippus. Source: H. Zell
In addition to this, scientists have suggested that the cranial capacity of the Merychippus was superior to that of its predecessors, so they apparently had a larger brain that made them more agile and intelligent.
Reproduction
Members of the genus Merychippus were dioecious, which means that there were both female and male individuals. When it comes to reproduction, scientists and specialists can only guess, since the elements they have are fossil records that are sometimes not even complete.
However, taking into account the taxonomic classification and the location of these animals within the Chordata phylum and the Mammalia class, it is possible to establish how their reproduction would have been.
Fertilization
Primitive horses, members of the genus Merychippus, reproduced sexually. This implied that there must be the fusion or union of gametes or sex cells. In this case, the gametes that were united were the egg and the sperm, in order to generate a new individual.
Thanks to the similarity that these animals had with current horses, it is feasible to affirm that fertilization was internal, that is, the male deposited the sperm inside the female using a copulatory organ.
As with many of today's large mammals, scientists agree that each female produced only one egg for each ovulation. In such a way that in each fertilization only a single individual would be formed or a maximum of two, in the event of a multiple pregnancy.
Pregnancy and birth
Because these animals were located within the group of mammals, it is stated that their embryonic development should be similar to that of current mammals. In this sense, once fertilization occurred, a single cell was formed, known as a zygote.
Later it began to undergo a series of transformations until three layers of undifferentiated cells appeared, known as ectoderm, mesoderm and endoderm. Each of these layers gave rise to the tissues and organs that made up the complete individual.
The fetus developed inside the female's body, so they could be considered viviparous. During gestation, the fetus received all the nutrients from the mother's body directly through a structure known as the placenta, as is the case with all mammals.
The time that the pregnancy lasted is not yet clear. However, as it bears some similarity to current horses, it can be said that it could last approximately 11 months.
After this time, the female went into labor, in which she gave birth to a foal that still had to remain under the mother's care for some time.
Finally, the foal could reach maturity a few years after its birth. On average, about three to four years later it was ready to breed.
Nutrition
As with current horses and as with their ancestors, the horses of the genus Merychippus were herbivorous animals. This means that they fed on plants.
Because the habitats in which they developed were grasslands and large expanses of plains, they fed mainly on small shrubs that had succulent and highly nutritious leaves. The characteristics of his teeth, especially his incisors, allowed him a more efficient chewing of the grass and therefore a better processing of food sources.
Digestion
Taking into account the resemblance that these specimens must have with current horses and the fact that they were classified within the Mammalia class, it is correct to say that their digestive system was very similar to that of current herbivorous mammals, specifically horses.
In view of this, the transit of food through the digestive tract of the animal can be inferred. First, in the oral cavity, food was cut and ground by specialized teeth for this purpose. Here they were also subjected to the action of the various digestive enzymes typical of saliva, which began to process them, preparing them for subsequent absorption.
Subsequently, the food bolus passed into the esophagus, from where it was directed to the stomach. There, thanks to the action of gastric juices, the nutrients were fragmented to facilitate the subsequent absorption process.
In the intestine, it was where the absorption of the nutrients took place, that is, the passage of these into the bloodstream. However, it was possible that the organism of these animals was not able to digest and absorb all the components of plants. This is why, surely, in your digestive tract there would be microorganisms, more specifically bacteria that contribute to the degradation of these components.
Finally, the components that were not assimilated, passed to the rectum and were expelled through the anus in the form of feces.
References
- Bravo, V. and Ferrusquia, I. (2006). Merychippus (Mammalia, Perissodactyla) from the middle Miocene off state of Oaxaca, southeastern Mexico. Geobios 39 (6).
- Evolution of the horse. Taken from: britannica.com
- Hooker, JJ (1994). "The beginning of the equoid radiation." Zoological Journal of the Linnean Society 112 (1–2): 29-63
- Horse evolution over 55 million years. Taken from: chem.tufts.edu
- L. Carroll. 1988. Vertebrate Paleontology and Evolution. WH Freeman and Company, New York