Front Legs in the Teaching Barn

Bones on the Barn Wall

1Wren Abernathy is spending her eighth-grade spring break helping at Tidewell Farm, which raises pigs, dairy cows, chickens and a few horses. The farm runs a teaching barn for visiting classes. On one wall hang cleaned front-leg skeletons of farm animals that died of old age, each one labeled bone by bone, with a human arm skeleton model beside them for comparison.

2The farm's vet, who teaches the barn tours, showed Wren that the front limbs look very different on the outside. A chicken wing is light and folds against the body. A horse leg is long, with the animal's weight resting on one large hoof. A pig walks mostly on two large middle toes, and two smaller toes sit higher up on each foot. A cow also stands on two main toes, with two small dewclaws behind them.

3Inside, though, the bones follow a similar plan. Each limb starts with one upper bone, the humerus, attached at the shoulder. Below it are two lower bones, the radius and ulna, then a group of small wrist bones and, finally, the bones of the hand or foot. In the horse, the ulna is small and joined to the radius, and two thin splint bones run alongside the long bone above the hoof. The splint bones do not reach the ground.

4A local university lent the barn casts of two fossil front legs from extinct animals in the horse family. The older one, from rock about 55 million years old, belonged to an animal often called the dawn horse, which was about the size of a fox. The younger one, about 35 million years old, came from a larger animal whose middle toe was bigger than its side toes.

5Wren made the table below to compare all six limbs. She counted digits, meaning fingers or toes, but did not count the horse's splint bones or the cow's dewclaws as full digits. She also noted the main job each limb does on the farm, because the same bone plan can be shaped for very different work. When she finished, she asked the vet whether such different animals could share one plan just by chance, and the vet suggested she study the two fossils closely before deciding.