The Silurian Wenlock Reef
If somebody had stood on the beach on the northwest coast of the microcontinent of Avalonia looking out towards the Iapetus Ocean, here's what they might have seen. The warm sea shelters a teeming community of living things. Corals and sponges build a reef framework visible through the clear, shallow blue water. Other creatures cling on to the reef, or hunt among its nooks and crannies for food or shelter.
We know all of this from looking at the fossils found in the Wenlock limestone, and the rock layers (strata) of similar age that surround it.
The Wenlock Reef formed a band of rock which stretches parallel to the ancient coastline. The sponges and corals grew in water that was deep enough for them to be safe from the destructive power of most waves breaking on the shoreline, but shallow enough to allow sufficient light to filter through enabling green algae to produce food.
This map of seawater depth and the Silurian coastline was drawn by a scientist who investigated the animal communities recorded by the Wenlock fossils and looked for patterns in their distribution. He noticed that brachiopods seemed to show a particular pattern of distribution.
Brachiopods with thinner shells lived in deeper water where there was less food but also fewer predators.
Only free-swimming creatures spent any time in the deepest, colder and darker water, off the shelf where all the reef creatures lived, so only their fossils are found in the rocks deposited here.