What is an origins that reproduces among themselves?

Which term describes the number of waves that pass a given point in a given time? crest frequency trough wavelength

Fossils can only be formed when? if mud, silt or tar is in the area to preserve the fossil ierosion does not occur and allows the fossil to form oxygen and microorganisms cannot decay the bones the area is dry and does not contain mud or silt

What was the land like over 500 million years ago? what was under the sea?