The diagram below shows the layers in a rock having a brachiopod: a rock column labeled rock is shown. the column has three layers. the bottommost layer of the rock has two identical organisms marked as, brachiopod, in the key and one shaded circle, the middle layer has a two squares, the top layer has one star shape. in the key the shaded circle is marked as x, the square is marked as y and the star is marked as z which statement about the rock fossils is true? organism x existed for a short period of time. organism y became extinct before organism x. organism x and organism y existed at the same time. organism z became extinct much before organism x.