Their distinctive scales are used as index fossils for the relative dating of rock layers. |
They are very abundant in some Paleozoic formations and include useful index fossils. |
Carboniferous and Permian strata often contain useful index fossils belonging to this group. |
These rocks are interbedded in Emsian marine sedimentary successions that are well documented by index fossils. |
Unfortunately, geochronologists assume that index fossils represent an absolute pattern. |
Thus, the Silurian-Devonian boundary is anchored to the first occurrence of specific index fossils. |