Through fossil records from Hyracotherium to Mesohippus to Merychippus to Pleshippus to the Equus, one can see the development of dentition, limbs and skull based on the environment that the horses adapted to with time. Environmental changes from the Eocene to the Holocene brought about natural selection as the horses’ initial characteristics could not adapt. In order to survive, horses evolved traits such as stronger limbs, tougher teeth and others that best fit and adapted to their surroundings.
In this lab, we examined these five horse evolution fossils to understand the change in equine morphology in relation to the habitat and the natural selection within the horse population. Hyracotherium, one of the fossils observed, has morphologic characteristics that suggest life in thick, closed canopy forests. The species’ dental characteristic, bunodont, is an indication of a browsers eating habit. In addition, the small skull, small rostrum and a short masseter implies a diet of easily chewable and digestible soft foods, like berries and nuts, mainly found in forests.
Also, their digitgrade limbs are most adaptable to a habitat requiring more maneuverability than speed. In a thick canopy forest, speed would be unnecessary as there would be obstacles of trees, shrubs and wet grounds. Rather, maneuverability would be much more efficient in a forest where the surroundings are bushes, shrubs and trees. This type of limb also becomes easily fatigued and while that is dangerous in an open grassland, this quality is acceptable in a thick canopy forest where running is limited to short periods of time.
Beginning in the Miocene, as the climate became drier and cooler, the increase in grasslands influenced the morphology of horses teeth and jaw. Before the Miocene, horse populations showed signs of bunodont teeth that were meant for living in forests or areas plentiful of soft foods. However, the increase in grasslands evolved the horses’ teeth from bunodont to laphodont and then to selenodont. In order to eat, the horses evolved their dentition and jaws to adapt to the grasslands.
As the only available food source became grass, their masseter needed to be longer so that the muscle would be able to chew and digest the tough grass. In addition, data shows the evolving trend of longer skull lengths and increasing side placements of eyes which can be attributed to the grasslands from the Miocene. Because their food source was low on the ground, horses evolved longer jaws so that they can bend down slightly to eat but keep their heads relatively high up from the ground to use peripheral vision for protection.
As horses began to increase in size, trends show that limbs go from digitigrades to unguligrades; from short weak limbs to longer and stronger limbs. As size increased from the Hyracotherium species to the present Equus species, their limb bones became thicker with higher width/length ratios to adjust to support the added size weight. The number of toes also decreased and the internal struts also strengthened the internal limb bone. As the forests died away, giving way to Miocene’s vast grasslands, horses required legs to give them agility and speed in order to run quickly for long periods of time without fatigue.
To adapt, horses evolved long, columnar limbs that end in a small number of functioning digits but have springy tendons which were highly durable and efficient at transferring muscular energy into forward motion. Despite their speed, the long limbs lacked the muscles aiding rapid locomotion. As the horse limbs became longer, the trade off was the loss of direction and easy maneuverability. Needing to adapt to the new grasslands, horses required speed more than maneuverability to survive.