Anna AudlerLS1, Section 2JID: 304295761Horse EvolutionThe evolution of the horse can be traced to the first members of the horse linage, the Hyracotherium. The Hyracotherium lived in an environment that had thick, semitropical forests and fern swamps and fed off of nuts, berries, and other shrubs and trees (Life Science Core, 2008-14). The Hyracotherium was able to survive these conditions due to its small morphological characteristics. Overall, as revealed by skull size and length of the central metatarsal, Hyracotherium was small; therefore, it would have been able to hide itself in the forest. Additionally, it had three toes. These structures would have increased the animal’s balance and maneuverability, allowing it to navigate the thick forest with ease. Hyracotherium’s teeth and jaw characteristics also suggests that it lived in a closed canopy environment. They had Butodont molars with multiple cuspids, suggesting that it fed on a variety of softer and harder foods like berries and nuts that would be found in such an environment. Eye morphology also speaks to a thick forest environment. Hyracotherium’s eyes face forward. In dense foliage, it would be more important to see what is ahead, rather than to the sides. Individuals with these favorable characteristics would have survived via the principles of natural selection. As the environment slowly changed from thick forests to open grasslands, Equine morphologies changed with it. This transition lead to the arrival of Mesohippus and Merychippus. Both had larger skulls and central metatarsals than Hyracotherium, and still shared with their ancestor in having three toes. However, in both of the newer species, the middle digit became hooved, and Merychippus only had the hooved toe touching the ground. This echoes the more open environment and that they no longer needed the same amount ofmaneuverability that Hyracotherium did in the denser forests. Mesohippus did still have more Bunodont-like teeth, while Merychippus’ teech appear to be flatter and more like Lophodont molars. As the environment changed to contain more grasses, species had to become grazers. Grasses harder to break down, so Merychippus needed flatter teeth to chew and grind their food better. Also, teeth also needed to become longer, as chewing grass wears down the teeth. Essentially, the longer the teeth, the more times an animal can eat, and as a result, the longer the animal can live. When the environment transitioned into the grasslands we see today, Early Equus and Equus evolved. They are both bigger than the species before it, and have larger skulls and limbs to match. The two metatarsals started to fuse to the central metatarsal, providing extra support tothe limb, leaving one larger hoofed toe. Equus’ loss of two toes is a tradeoff of maneuverability and agility for strength and speed. This allowed it to be able to to outrun their predators. Eye position also moved further back on the skull, allowing individuals to be able to eat while still being able to see above the grass for any predators. .The evolution from Hyracotherium to Equus produced an organism with fewer toes, largerand flatter teeth, and a larger face and body. It is important to note, however, that some of these characters displayed allometry. While the central metatarsal grew longer and wider, those two measurements changed the same relative amount in all species. The same trend is seen when the skull length and central metatarsal length, and the length of the masseter and the central metatarsal are compared. Each species’ changes based on a similar