The Evolution of the Horse - Coursework Example

The Evolution of the Horse (Co-Evolution with Grasses) A close scrutiny of horses is sufficient to reveal that horses have evidently evolved to be fit for survival. Most people view the horse family as being one of the best examples of the evolutionary process. New fossils continue to be found that have forced some established ideas of horse evolution to change. Despite this, the horse family has largely remained a good example of evolution. The horse family has over the years coevolved with grasses as members of each species adapted to the evolutionary developments of the other species. This paper analyses the evolution of the horse and its co-evolution with grasses. Introduction The evolution of the modern day horse is thought to have begun an estimated over 58 million years ago. In a description of some newly discovered horse fossils published by paleontologist O.C Marsh in 1870. MacFadden (2005b) points out that Marsh described an evolution that took place in a single lineage. In this description, through a series of distinct intermediates, the Eohippus fossil species gradually evolved into what is an almost totally different-looking descendent, the Equus. The American Museum of Natural History further popularized this description by assembling a famous exhibit of these fossil horses. In designing the exhibition, the remains were arranged in such as manner that they would show the gradual evolution form Eohippus (Hyracotherium) to the modern day Equus. Exhibitions such as these were seen to not focus on the evidence of evolution, but on the evolution of the modern day Equus as a model of gradual and straight line evolution. This model was soon to eventually be included in all the biology textbooks. Recent evidence has however clearly shown that the very first horse evolution did not proceed in a straight line as had earlier been supposed. Fossil evidence now shows that the familiar Equus is merely one evolutionary branch of the equine species, and that we only have the illusion of a straight-line evolution because Equus is the only branch of the equine species that survived. New fossil evidences also show that the evolution of the horse was not always smooth and gradual. A number of different traits did not always evolve together and when they did, they evolved at different speeds. Some of these traits also experienced the occasional reversed direction evolution. In addition to this, horse species did not always evolve through a process of gradual transformation of their ancestors (anagenesis). New species sometimes split off from the ancestors through a process of cladogenesis and then proceeded to co-exist with those ancestors for a period of time. Some of the horse species arose suddenly while some arose gradually. Small Eocene Horses The very first Equid was the Hyracotherium, which was a small forest animal of the early Eocene period. The Hyracotherium had a height that ranged between 10-20” at the shoulder. Its appearance was marked by an arched back, short snout, long tail, short neck and short legs that caused it to have doggish looks. It mainly browsed on fairly soft foliage and fruit. Some of its other traits were that it had a small brain with especially small frontal lobes, its legs were rotatable and flexible and all the major bones were present and unfused. It had four toes on each front foot with three toes on its hind legs. It had padded feet that instead of claws had small hooves on each toe. Its primitive mammalian teeth formula included the presence of three grinding molars whose cusps were slightly connected in low crests, 4 clearly distinct premolar teeth, 1 canine as well as three incisors on each side of its jaw. Orohippus (Early Middle Eocene (approx 50 My) The Orohippus during the early-middle Eocene period, there was a gradual and smooth transition that saw the Hyracotherium evolve into the Orohippus. The Orohippus’s traits appeared to be similar to those of the Hyracotherium with the most significant evolutionary trait being in its teeth. flmnh (2014), notes that the Orohippus’s last premolar had changed its shape from that of a premolar to that of a molar an aspect that caused the Orohippus to have one more grinding tooth. The crests of its teeth had also evolved to become more pronounced a characteristic that suggests that the Orohippus’ diet consisted of tougher plant material. The vestiges of the 1st and 2nd toes also vanished. Epihippus (Middle Eocene (aprox 47 My) The Epihippus arose from the Orohippus and had similar traits to it. Tooth evolution was gradually continuing and its last two premolars had changed to molars which gave the Epihippus five grinding well formed low crowned cheek teeth. Late Eocene & Oligocene: The Medium-sized Browsing Horses As they moved from the Eocene to the Oligocene, horses started to change. This change was caused by the changing climate in North America that was progressively becoming drier and the fact that grasses were just beginning to evolve. The previously vast forests were shrinking and this forced the late Eocene horses to respond to these changes by developing tougher teeth. These horses also became leggier and larger so as to have better speed in the open. Mesohippus (Late Eocene approx 40 My) The Mesohippus celer appeared during the late Eocene. At a height of 24” at the shoulder, It was slightly larger than the Epihippus. Its legs were a bit longer, its neck a bit longer, its back less arched and its face and snout distinctly longer. According to flmnh (2014) the Mesophippus had three toes on its front and hind feet as the fourth toe had been reduced to a vestigial nubbin. Its feet were still pad-footed and had a shallow facial fossa. It had similar tooth crests to the Epihippus, which were sharp and well-formed. These were quite suitable for the grinding of vegetation. Its cerebral hemispheres were larger while its last three premolars were now similar to the molars. It only had one simple premolar in front. Miohippus (appox 36 My) Floyd et al. (2007) suggests that the transition to the Miohippus occurred rather suddenly. While it was originally thought that it developed from the Mesohippus, via anagenetic evolution, recent evidence shows that the Miohippus split off from the Mesohippus via cladogenetic evolution. The Miohippus was larger than the Mesohippus and had a slightly longer skull. Its ankle joint had undergone some subtle changes while it’s facial fossa had become more expanded and deeper. Miocene, 18 My: High-Crowned and Spring-Footed During this period, horses transformed from being browsers to grazers so as to take advantage of the large grasslands that had started to appear. Grass is generally more difficult to chew and tends to wear down the teeth more quickly and as such, these grass eaters needed to have tougher ridged teeth. Macfadden (1992) point out that there was a gradual increase in the height of their teeth crowns such that the teeth could now continuously grow as their tops were gradually worn down. The open and large grasslands also necessitated that the horses develop longer legs so as to run faster. This period also saw horses become specialized runner with adaptations such as increased body sizes, and face and leg lengths. The musculature and leg bones became specialized form more efficient forward-and back strides while the leg bones began to gradually fuse together. Instead of the doglike pads, horses now evolved and began to permanently stand on their toes which is an effective speed adaptation. The weight of the animal was now supported by the development of springy ligaments that were seen to run from the big central toe to under the fetlock. Kalobatippus Although not much is known of this genus, its dental formula appears to be somewhere intermediate between the Miohippus and the later Parahippus. Parahippus (Miocene 23 My) The Parahippus was only slightly larger than the Miohippus with similar physical developments and brain capacity. According to Floyd et al. (2007), the Parahippus had began developing a springy ligaments under its foot although it was still three-toed. A number of its crests and cusps were starting to join up and form a series of strong crests formed out of its slightly taller tooth crowns. The Parahippus quickly evolved into the hysodont grazing and fully spring-footed horse that is referred to as the Mercychippus gunteri. Merychippus (Approx 17 My) The Mercychippus was approximately 40” tall with a deeper jaw, a more elongated muzzle and eyes that been moved further back so as to sufficiently accommodate the significantly larger tooth roots. flmnh (2014) points out that the Mercychippus was more intelligent and agile as compared to the earlier horses as a result of its having a notably larger brain with a larger cerebellum and fissured neocortex. It was still 3 toed but had become fully spring-footed. Some of its species had developed full-size side toes while the side toes of others had developed in a manner that saw them only touch the ground during running. The ulna and the radius of its forearm fused to eliminate leg rotation, while the shin and the fibula became greatly reduced. The leg became longer and the central toe developed a convex and large horsey hoof. These traits are seen to all have developed with the combined aim of causing the Mercychippus to be able to run quite rapidly over hard ground. Its teeth still had the parahippus’s distinct grazing tooth crests, but they had now had a thick cement layer in addition to being fully high-crowned. The Mercychippus underwent a process of rapid speciation by the late Miocene that resulted in its giving rise to about 19 new horse species that have been divided into three main groups. This rather explosive burst in horse evolution is commonly referred to as the Mercychippine radiation. The Hipparions which were three-toed grazers, the Protohippines which were a small line of horses that included the Callipus and the Protohippus, and a line of true equines whose side toes was seen to have started to gradually decrease in size. The horse family had managed to reach an apex in its adversity by about 10 My. This diversity saw it become greatly differentiated in regard to its genera and species. The facial fossae of these descendents of the Mercychippine all developed deeper and more elaborate facial fossae. Late Miocene, Pliocene and Pleistocene: One-Toed Horses During this period, the late Mercychippine species are seen to have been large horses having small side toes. These horses eventually gave rise to separate groups of horses that managed to independently lose their side toes. The loss of the side toes is seen to have occurred as a result of the development around the fetlock of side ligaments that are seen to have aided with the stabilization of the central toe during the process of running. These one toed horses are seen to include: Pliohippus (~15My) The Pliohippus arose as a three toed horse. The gradual loss of the side toes is evident throughout the early Pliocene. The Pliohippus are found to be quite similar to the Equus and were for long thought to be the direct descendents of the Equus. However, flmnh (2014), argues that there are two distinct differences between the two, the first of which is that while the Equus’s teeth are very straight, the Pliohippus has strongly curved teeth. Secondly, Wheras the Equus is noted as having no facial fossae, the Pliohippus’s skull has a well developed and deep facial fossae. These similarities and differences indicate that although the Pliohippus is related to the Equus, it did not however give rise to the Equus. Astrohippus (10 My) The Astrohippus is a one-toed horse that is seen to have emerged shortly after the Pliohippus. Floyd et al. (2007) notes that the Astrohippus is thought to have been a descendent of the Pliohippus and had a large facial fossae. Dinohippus (12 My) The Dinohippus is a recently discovered one-toed horse that is thought to have arose about 12 My. The Dinohippus’s ancestors are not exactly known, however it has a number of startling similarities with the Equus and especially so in regards to its teeth, foot morphology and skull. Its facial fossae is noted as having decreased to a significant degree while its teeth are straighter than those of the Mercychippus. During the late Pliocene, the Dinohippus was the most common horse across North America and is thought to have given rise to the Equus. Towards the end of the Pliocene, the Dinohippus underwent a number of gradual changes such as the straightening of the teeth, a decrease in the size of its facial fossae, as well as other changes that saw it smoothly evolve into the Equus. Equus (4My) The Equus is believed to be the genus of all the modern day equines. The first Equus had long necks, were about 13.2 hands tall (which is about the size of a pony), had a rigid spine, deep jaw, flexible muzzle, long nose and fused leg bones that had no rotation. These physical traits caused these first Equus to display the classic horsey body. The Equus also had strong crested straight grazing teeth that were lined with cement. In addition to its being one toed, the Equus had also developed side ligaments in its toes that helped in preventing the hoof from twisting. The genes necessary for the formation of side toes are still retained by the Equus family members. These genes express themselves via the development of vestigial splint bones in toes 4 and 2, these two are around toe 3 which is the large and central toe. Although it is quite rare, modern Equus are sometimes born with small but fully-formed side toes. During the first major glaciations experienced during the late Pliocene period (2.6 Ma), A number of the Equus species are seen to have crossed over to the Old World. Some of these members of the Equus species entered Africa and diversified into the modern day zebras. Others spread across North Africa, Asia and the Middle East and adapted to the desert conditions, these are the asses and onagers. Some Equus species spread into South America while, others spread across the Middle East, Europe and Asia and evolved as the true horse Equus caballus. According to Franzen (2010), comparing the Hyracotherium to the Equus caballus shows that a number of radical changes have occurred in the evolution of the Horse. This is because a comparison of the physical traits of the Equus, and those of the Hyracotherium commonly referred to as the dawn horse and whose fossil evidence shows it was about the size of a large rabbit, proves that the equine evolution has invariably followed a number of extraordinary paths. The gradual change from the Hyracotherium to the Equus is considered to be an example of large-scale and long-term evolution. Modern Equines (Recent) The three toed horses eventually died out and it is thought that this was perhaps due to their being outcompeted by the rather phenomenally successful artiodactyls. As the Ice Ages started, most of North America’s one-toed horses also died out. However, the proliferation of the one-toed Equus is seen to have been quite successful and until about 1 million years ago, there were numerous members of the species across Asia, Africa, South America, Europe and North America. When migrating, these animals moved in enormous herds that must easily have been able to equal the huge wildebeest migrations in Africa and the great North American bison herds. During the late Pleistocene period, there developed a set of widespread and devastating extinctions that resulted in the killing off, of the large mammals in south and North America. All the horse, saber-toothed tigers and mammoths that were in South and North America eventually died out as a result of these extinctions. The extinctions are thought to have been caused by a combination of various combined factors such as overhunting by the humans that had just managed to reach the New World, and a series of extreme climatic changes. As a result of these extinctions, there were no Equids throughout the Americas for the first time in tens of millions of years. The only Equus members that managed to survive to historic times were Equus zebra: This is a little mountain zebra in South Africa that has a characteristic dewlap and gridiron pattern across its rump. Equus caballus: This is the true horse and it once had a number of subspecies. Equus burchelli: The group comprises of Plains zebra of Africa and includes the Chapman’s zebra, Grant’s zebra, the half-striped zebra, Burchell’s zebra and other subspecies. The Plains zebra is what is largely considered by most to be the typical zebra and has rather vertical and wide stripes with a set of thick horizontal stripes on its rump. Equus asinus: This group comprises of the true North African donkeys and asses. The wild asses in Africa are sometimes referred to as Equus africanus. Equus grevyi: This is a big zebra with huge ears and narrow vertical stripes. It is considered to be the most horse-like zebra. Equus hemionus: These are the desert adapted onagers found in parts of the Middle East and Asia and typically include the kiang which was formally referred to as the Equus kiang. The Coevolution of Horses and Grasses One of the cornerstones of Darwin’s theory of evolution is founded on the idea that as the usually few but important resources are gradually utilized, they often tend to become critically short in supply. Plants have been competing with each other for limited resources both intraspecifically and interspecifically for billions of years in a process that has resulted in coevolution. In addition to competing with other plants, plants have also had to contend with the threat of their being consumed by herbivores and have over the years been forced to develop adaptation that help them to deter herbivores from feeding on them. This has resulted in plants and the animals that feed upon them develop an evolutionary relationship that is commonly referred to as coevolution. Coevolution is exemplified by the evolutionary relationship between grasses and horses. During the late Eocene and Oligocene period, changes in climatic patterns resulted in the climate in some parts of the world such as that in North America becoming progressively drier. The vast forests that covered this region started shrinking as grasses stated to evolve in a development that forced the late Eocene period horses to respond to these changes by adapting to the increasingly savanna-like conditions. The expanding savanna land afforded few trees for horses to sufficiently hide themselves and this caused the faster and taller horses to reproduce and survive in a better manner as compared to the slower and shorter horse species. According to Prasad et al. (2005), fossil evidence show that mammals with typical grazing adaptations only occurred during the Oligocene and Miocene which indicates that before these periods, grasses were not present in sufficient quantities to help in forming the major part of a herbivores diet. The postulation that grasses started evolving during the Eocene and Oligocene period is further supported by Stebbins (1981) who points out that prior to the Eocene Epoch; there is no previous evidence to suggest either the occurrence of grasses (Poaceae) or of mammals that had specially adapted teeth that were suitable for feeding on grasses. When horses first evolved at around 60 My. The Hyracotherium which were small Eocene horses mainly browed on the sot leafy bushes. However, as the Earth’s climate gradually cooled down, the expansive woodlands eventually gave way to vast open grasslands with scattered trees. This development forced horses to begin grazing in the open grasslands and altered the evolutionary progress of these two species. According to Williams (2008), as a result of being grazed upon, grasses responded by evolving phytoliths, which are little silica pieces in their leaves. Stromberg (2006) defines these phytoliths as being jagged little grains of sand that gradually wore away the dentine of the horses as the horses continued to feed on the grass. Continued feeding on grass resulted in the teeth of horses being worn down to a point where they could no longer be able to feed and some of these horses started dying off. Sues (2005) argues that although Phytoliths can also serve the function of providing support in plant stems, as is the case with bamboos, it is fairly accepted that phytoliths primarily serve the function of deterring herbivores. However, during the Miocene period, Retallack (2001) postulates that the rather coarse and gritty fodder of grass helped to provide a significant selection pressure that necessitated that ungulates developed hypsodont teeth. This is seen in the case of horses that started developing high-crowned teeth as an evolutionary adaptation to their feeding on the course grasses. The development of higher-crowned teeth resulted in better feeding on the part of the horses as although their teeth wore down rapidly during grazing, they had a higher tooth volume which gradually wore down during the course of their lifetime. This is relative to the earlier horses that had short-crowned teeth that were suitable for feeding on softer plants. In addition to developing high crowns during the Miocene period (18 My) horses also managed to increase the durability of their teeth by infolding their enamels to increase their surface area. This formed more pronounced crests as is seen by the Orohippus which developed more pronounced crests to aid it in the grinding of tough grass material. Starting with the Orohippus during the Middle Eocene Period (approx 50My) the premolars of horses started changing into molars so as to increase the effectiveness of the teeth of horses in grinding the significantly tougher grass plant material. This development is seen to have progressed with the epihippus during the Middle Eocene period (approx 47 My) when the last two premolars of the Ephippus changed to molars a development that gave the Epihippus the advantage of having five well formed low, low crowned grinding cheek teeth. Fossil remains of horses from the Miocene period approx (18 My) shows that the teeth of horses could now continually grow at the top while they were being worn down by its feeding. These horses also developed bigger jaws that could be able to accommodate their bigger teeth as well as larger muscles to aid them in effectively moving these larger jaws so as to grind their food properly. This resulted in the horses developing longer and stronger faces. The heavy grazing on grasses as a result of the increasing grazing of horses and other animals saw them develop other defensive mechanisms. Some varieties of grasses started developing repellants that helped to deter horses and other herbivores from feeding upon them. The repellants work by causing the grass leaves to become unpalatable or less digestible to the herbivores. Some of the grasses evolved a set further and started producing toxins so as to deter horses and other herbivores from feeding upon them. Toxins producing grasses prevented grazing by causing mortality among the herbivores that fed upon them thorough the production of toxins such as cyanide. Conclusion The evolution of the horse did not follow a linear progression but instead, different horse species kept o branching off and evolving along a number of various unrelated routes. At times, several horse species coexisted at the same time having different numbers of toes in addition to their being adapted to different diets. While at times there were relatively long periods where there was only one or very few horse species such as during the Early Eocene period when there was only one horse species; there are other periods that saw an evolutionary burst as a number of species arose such as was the case during the Merychippine radiation. The evolutionary process of the horse is seen to have proceeded and adapted to ecological pressures such as the changing climatic conditions that resulted in a reduction in forest vegetation and subsequently forced the horse to adapt to the consumption of the expanding grasslands. Stebbins (1981) affirms that despite the sometimes large gaps in fossil records, the available information permits a partial degree of synthesis that helps in the revelation of the close connections that happen to exist between the evolution of grasses and the large mammalian herbivores that graze upon them such as horses. References flmnh. 2014. Fossil Horses in Cyberspace: http://www.flmnh.ufl.edu/natsci/vertpaleo/fhc/fhc.htm (Accessed April 2014) Floyd E. A., Mansmann A. R., 2007., Equine podiatry. Philadelphia, Pa: London: Elsevier Saunders. 23 p. Franzen, J. L., 2010, Horsing around, The Rise of Horses: 55 million Years of Evolution: Johns Hopkins University Press. 8 p. MacFadden J. B. 1992, Fossil horses : systematics, paleobiology, and evolution of the family equidae. Cambridge : Cambridge Univ. Press, 1992a. 233 p. MacFadden, 2005b, Fossil Horses: Evidence for evolution: Science, v. 307, p. 1728-1730. Prasad, V., Stroemburg, C. A.E., Almohammadian, H., Sahni, A., 2005, Dinosaur coprolites and the early evolution of grasses and grazers: Science, v. 301, issue 5751, p. 1177 -1180. Retallack, G. J., 2001, Cenozoic expansion of grassland and climatic cooling: Journal of Geology, v. 109, n. 4, p. 407-426. Stebbins, L. 1981. Coevolution of grasses and herbivores: http://www.fws.gov/southwest/es/documents/R2ES/LitCited/LPC_2012/Stebbins_ 1981.pdf (Accessed April 2014) Stromberg, C.A.E., 2006, Evolution of hypsodonty in equids; testing a hypothesis of adaptation: Paleobiology, v. 32, Issue 2, p. 236-258. Sues, H. 2005. Evolution of herbivory in terrestrial vertebrates: perspectives from the fossil record. Cambridge: Cambridge University Press. 224 p. Williams, E. D. 2008. Self-medication in Horses. ProQuest, 2008. 9 p. Read More