Index Fossils
Index Fossils
Lingula anatina is NOT AN INDEX FOSSIL !! ,
This common fossil has existed over a very long geological time and still lives today!
Characteristics
of a good index fossil
Some typical
global index fossils and the ages they indicate..
Early
Pleistocene 0.5 million
Quaternary Period 2.6 million to 10,000
Tertiary
Period 65 to 2.6 million
Eocene
56 to 34 million
Cretaceous
Period 145 to 66 million
Jurassic
Period 201 to 145 million
Triassic
Period 252 to 201 million
Permian
Period 298 to 252 million
Carboniferous Period 359 to 298
Pennsylvanian
Period 323 to 298 million
Mississippian
Period 359 to 323 million
Devonian Period 419 to 359 million
Silurian
Period 444 to 419 million
Ordovician
Period 485 to 444 million
Cambrian
Period 509 to 500 million
Lower Cambrian 529 to 509 million
Older?
Characteristics of a good index fossil
Index fossils (also called type fossils or key fossils) are those that are used to define periods of geologic time.A good index fossil is one with five characteristics:
- it is distinctive
- globally widespread
- abundant
- limited to a particular geologic time and
- is robust and preserves well
Most fossil-bearing rocks formed in the ocean as being buried in sediment is easiest there.
So the major index fossils are marine organisms (global and universal) and only a very few terrestrial organisms are index fossils (limited to young rocks in specific regions and not universal for example: quickly evolving rodent teeth).
Floating eggs and infant stages riding on ocean currents are helpful in giving index fossil worldwide distribution. Good index fossils need to be both --
- abundant and
- vulnerable
Vulnerable to environmental change and extinction.
This combined "boom-and-bust character" is what makes for a good index fossil.
Some index fossils are small or microscopic, part of the floating plankton in the world ocean.
Their tiny bodies rained down all over the ocean, they can be found in all kinds of rocks.
The petroleum industry has made great use of index microfossils, and geologic time is broken down in quite fine detail by various schemes based on calcareous nannofossils, conodonts, diatoms, foraminifera and radiolarians.
Due to plate tectonics, the rocks of the ocean floor are geologically young, as they are constantly subducted and recycled into the Earth's mantle. So marine index fossils older than about 200 million years are normally found in old marine sedimentary strata on land.
Some typical global index fossils and the ages they indicate...
youngest to oldest... Early Pleistocene 0.5
million
Viviparus glacialis
Quaternary Period 1.8 million
Pecten gibbus
Argopecten gibbus
Neptunea tabulata
Tertiary Period
Calyptraphorus velatus
Eocene
Venericardia planicosta
Cretaceous Period 145 to 66 million
Scaphites hippocrepis
Inoceramus labiatus
Jurassic Period
Perisphinctes tiziani
Nerinea trinodosa
Triassic Period
Tropites subbullatus
Monotis subcircularis
Permian Period
Leptodus americanus
Parafusulina bosei
Pennsylvanian Period
Dictyoclostus americanus
Lophophyllidium proliferum
Mississippian Period
Cactocrinus multibrachiatus
Prolecanites gurleyi
Devonian Period 416 to 359 million
Mucrospirifer mucronatus
Palmatolepis unicornis
Silurian Period
Monograptus parultimus
Ordovician Period
Tetragraptus fructicosus
Cambrian Period 509 to 500 million
Paradoxides sp.
Billingselia sp.
Lower Cambrian 529 to 509 million
Archeocyathids
Older - approximately 3,200 to 1,400 million ?
Acritarchs are organic microfossils
Source:http://www.thefossilforum.com/index.php?/gallery/
https://en.wikipedia.org/wiki/Index_fossil
https://en.wikibooks.org/wiki/Historical_Geology/Index_fossils
https://australianmuseum.net.au/the-geological-time-scale