Concurrent Range Zone

My previous series of blogs dealt with different species of Turritella in order to demonstrate how paleontologists "tell time" using a temporal (through time) succession of species of the same genus. This new blog shows another technique that is commonly used to "tell time."  It is called the concurrent-range zone = overlap in time of two different fossils (species or genera), or stated in another way, overlap of the first (FAD) and last (LAD) occurrence of two or more fossils.  FAD = first appearance datum; LAD = last appearance datum. Simply stated, if you have, in a layer of sedimentary rock, the co-occurrence of two fossils that overlap in time, then the age of the layer is the age of the overlap interval.

                                fossil 2

M
D            LAD
S	/////////////	age of layer = S
O	FAD
C

               fossil 1

A detailed example of a concurrent-range zone using fossil bivalves is shown below (the concept is that if these two genera occur in the same layer of rock, the geologic age of the rock is late Pliocene):

Swiftopecten parmeleei (Dall, 1898)
juvenile, height 33 mm

Swiftopecten parmeleei (Dall, 1898)
early adult, height 71 mm

Cyclocardia occidentalis Conrad, 1855
height 13 mm