Question

The splint bone of present day horse is a vestige of
A. Fourth toe
B. First toe
C. Second toe
D. Second and fourth toes in limbs

Answer 1

Hint: Vestigiality is the retention of some or all the ancestral functions in a given species during the process of evolution. Its analysis is done by comparing homologous features in related species.

Complete Answer:
The evolution of the horse, a mammal of the family Equidae, occurred over a 50 million years ago i.e. the Eocene Epoch.
- Evolution of Horses: Eohippus stood up to about 42.7 to 50.8 cm high, unusually small in comparison with the modern horse, and had an arched back and raised hindquarters. The legs ended in padded feet with four functional hooves on each of the forefeet and three on each of the hind feet, single-hoofed feet of modern equines. The brain was far smaller and less complex than that of today’s horse. The teeth were adapted to a browser’s diet.

- Orohippus, a genus from the middle Eocene, and Eohippus, a genus from the late Eocene, resembled Eohippus in size and in the structure of the limbs.
- By the late Oligocene, Mesohippus had evolved into a somewhat larger form known as Miohippus. In Mesohippus, the fourth toe on the forefoot had been reduced to a vestige, so that both the forefeet and hind feet had three functional toes and a footpad.
- During the Miocene and Pliocene epoch were horses like Parahippus, Merychippus, etc. Most of these horses, including Hipparion, Neohipparion, and Nannippus, had three-toed feet of their ancestors. One exception was the Pliohippus which led to the one-toed horse, the direct predecessor of Equus.
- Equus is the genus to which all modern equines, including horses, asses, and zebras, belong.
- The ancestors of the horse came to walk only on the end of the third toe and both side (second and fourth) "toes". Skeletal remains show wear on the back of both sides of metacarpal and metatarsal bones, commonly called the "splint bones". They are the remnants of the second and the fourth toes. Modern horses retain the splint bones and are often believed to be useless attachments, but they play an important role in supporting the carpal joints (front knees) and even the tarsal joints (hocks).

Hence, the correct option is D, “second and fourth toes in limbs”.

Note: The line leading from Eohippus to the modern horse exhibits the following evolutionary patterns: increase in size, reduction in the number of hooves, loss of the footpads, lengthening of the legs, fusion of the independent bones in the lower legs, elongation of the muzzle, increase in the size and complexity of the brain, and development of crested, high-crowned teeth suited to grazing.