The type of sedimentary rock formed from the skeletons of microscopic living things once in the ocean is limestone.

Here's why:

* Microscopic Organisms: Limestone is primarily composed of calcium carbonate (CaCO3). Many microscopic marine organisms, like foraminifera, coccolithophores, and some types of algae, build their shells and skeletons out of calcium carbonate.

* Accumulation: Over time, these tiny skeletons accumulate on the ocean floor in massive quantities.

* Lithification: The weight of overlying sediment compresses and cements these skeletal fragments, turning them into solid rock, which is limestone.

Limestone is a very common and important rock type, used in construction, agriculture, and even as a source of calcium for various industries.