|
What is a fossil ?
Fossils are the remains of old shells inlaid in rocks, and also bones and teeth of animals from the past, insects trapped in amber, petrified logs, dinosaurs footprints left in the mud... among other things. It is saud that fossils are any trace left by plants or animals that lived during prehistoric times.
How do they form ?
Most fossils were formed when an organism remained burried under sediments, usually under water, just after its death. Under these conditions, fleshy parts decompose and disappear rapidly; whereas hard parts, like bones, does not degrade. Sometimes, these hard structures stay almost intact, but most of the time their pores and open spaces impregnate with minerals brought by water, and all the structure becomes rocky. For petrified wood, the original material can be replaced by minerals, silicon in particular, and some characteristics like the annual growing rings are kept as well as some cellular structures.
In another type of fossilization, the original structure is completely dissolved and only an empty space, or mould, remains in the rock. This mold has the exact shape as the original material. Sometimes, other materials goes inside this mold and produce a copy of the original.
What is a stratigraphic fossil ?
A stratigraphic fossil is a fossil from a geologic era which is delimited in time. In field stratigraphy, it allows the datation of one or many layers (stratum) where the fossil is present.
To be qualified as a stratigraphic fossil, a specy has to:
- have had wide geographic extension (which allows to correlate some distant places around earth) ;
- have existed in a short period in geologic timescale (in order to get a good stratigraphic "resolution" by delimiting a short time period and thus, thin ground layers) ;
- have been profuse (a necessary condition to find enough fossilized specimens).
In other words, an animal specy (generally) or vegetal (rarely) is a good stratigraphic fossil if it is an ubiquist specy (present in many environments), of rapid evolution and small size to be well preserved and thus easily recognizable. Fossils not having these characteristics are said "panchronic" and are not used in biostratigraphy.
Reminder on "Earthcaches": There is no container or logbook on the given coordinates. Just visit the site and answer the questions by e-mail. There is no need to wait for my reply, feel free to log immediately after. If the answers are incorrect, I will let you know. Happy caching!
To validate your visit:
1-. Find the block from the picture. Which stratigraphic fossil can you see ?
2-. Explain in a few words why this fossil is stratigraphic and do you think it comes from Nord-Pas-de-Calais or not ?
3-. What size is the biggest one ?
You can contact me in: English, French, Catalan or Spanish.
|