Geoscience Reference
In-Depth Information
classification they shared with squids and cuttlefish. They lived
between late Devonian and Cretaceous times, and reached their great-
est diversity and success in the Mesozoic. Friedrich Quenstedt
(1809-1889) was a professor of palaeontology at the University
of T
¨
bingen and he interested a young student, Albert Oppel
(1831-1865), in these fascinating animals. Oppel was not to live long,
but in his short life he overshadowed Quenstedt because he succeeded
in demonstrating that the Jurassic of southern Germany contained
many ammonites which, crucially, appeared to have a short range. He
mapped out the distribution of fossils in particular beds and recog-
nised that narrow horizons could be characterised by overlapping
ranges of ammonite species. These Concurrent Range Biozones are
now known as 'Oppel Zones' and the fossils that characterise parti-
cular biozones are known as index fossils. Ammonites provide a zonal
precision of as little as 200,000 years. In other cases the range of a
single species can provide a Total Range Biozone, while evolutionary
sequences of fossils can be used to define a Consecutive Range
Biozone where one particular species rapidly evolved into another
species which in turn was succeeded by another. Other fossil groups
are also useful in zonation, and trilobites and graptolites have been
used as index fossils for the Cambrian and Ordovician respectively.
The most useful zone fossils are those that had a global distribution
and a rapid evolutionary turnover, and that are commonly found.
Soon after Oppel'swork, zonations formany areas began to appear
in the geological literature, and palaeontologists expended a lot of effort
in determining the ranges of fossils. They wanted to know when each
type first appeared and when they disappeared, to ascertain whether a
particular species had biostratigraphical potential. In 1909 and 1910
Amadeus William Grabau (1870-1946) and Hervey Woodburn Shimer
(1872-1965) published their landmark two-volume North American
Index Fossils that provided an illustrated condensation of all the sys-
tematic works published for that continent and illustrated the most
important fossils that stratigraphers would wish to identify during the
course of their attempts to correlate and date sequences of fossiliferous
