Why are all adult horse fossils small in certain layers of rock ?

What we know as horses today are evolved from much smaller ancestors.

Explanation:

The older, and smaller, horse fossils are found in the lower layers of rock.

The lumbricus or earthworm has no eyes as it does require to see underground, in addition it has hairs on each of its segments in order to allow grip on the soil it has circular muscle throughout its body to in lateral movement and has no appendages or antennae and a streamlined body, the vent and intestine runs the entire length of the earthworm and can range from 25mm to 300mm