Question

Evolution of horse from Hyracotherium involved
A. Lengthening of limbs and increase in size
B. Progressive loss of digits and lengthening of retained digits causing shifting of soft footed to paddle footed
C. Increased complexity of molars and polarization of premolars
D. All the above

Answer 1

Hint: The species Hyracotherium lived in marshes areas. But, today horses live on lands and grasses. Due to this change, their body evolved to be stronger and longer in order to adapt.

Step by step answer:The evolution of horses is dependent on environmental conditions. The initial species went through a long sequence of evolution to be the present-day horses. Hyracotherium or Eohippus inhabited in marshy forests with soft vegetation and protection. The number of forests decreased due to unfavorable climatic conditions. As a result, Hyracotherium shifted to land with hard grass. So, the molar teeth of these organisms became longer, stronger, and saw-like in order to chew grass. Neck and snout became longer to watch out for the enemies. Legs modified to enhance their capabilities for running fast. Hyracotherium species were spread throughout Asia and Europe. They had a smaller body, head, and neck. Their forelimbs had four and hind limbs had only three nailed digits. Incomplete cement and no serrations upon molar teeth were also observed in the fossils of these species. Mesohippus evolved from Hyracotherium about 9 million years ago. The size of the species was about modern sheep. All of their limbs had three nailed digits similar to hyracotherium. Molar teeth evolved into having some serrations. Merychippus evolved from Mesohippus 60 million years ago. Merychippus has the size of a mule. The significant change was absorbed in molar teeth. They grew larger and were cemented and serrated. Pilohippus evolved from Merychippus about 5 million years ago. Their limbs had only a single hoofed digit. The rest of the limbs were reduced to small splint bones hidden beneath the skin, molars remained long and strong. The horses today, evolved from Pilohippus have great height; single hoofed digits and the rest of them do not exist anymore. They have snouts, limbs, molars, and a longer neck.
 So, the correct option is option D

Note: The fossils of Hyracotherium were discovered by Richard Owen was a paleontologist in the year 1841. He renamed the species to Hyracotherium from Eohippus. It was the size of a fox. These fossils helped in deriving the evolvement and the process very closely.