Who’s been writing on the rocks?
Although there’s a disappointing amount of graffiti found on the world’s rocks, this is not the result of any human writing. This photo shows a specimen of a type of fossil known as a “graptolite”.
The resemblance of this fossil to writing has not gone without notice; the name graptolite has in it the Greek root “graptos” which translates to written.
Graptolites were colonial animals, similar in that sense to modern day corals, with multiple individual organisms inhabiting a single structure. Early graptolites are found in the Cambrian period and were typically attached to the seafloor; later in geologic time other species developed as floating colonies.
The rocks they are found in are most commonly shales; fine-grained sediments deposited in quiet water. When a colony would die off, the remnants would fall or sink to the bottom of the ocean where the sediment would gradually entomb them. When the mud was lithified into rock, graptolites like these left their remnants as coatings and patterns of organic carbon on the rock surface.
As graptolites have wide distribution around the world, existed for hundreds of millions of years, and evolved into different forms with time, they are useful as “index fossils” – fossils that can be used to correlate single geologic layers across large distances on the planet’s surface. Find an identical graptolite in two rocks on opposite sides of the planet and you’ve probably found two units that were deposited at the same time.
This photo is 1 cm in height.
Image credit: Black Country Museum (Creative Commons) https://www.flickr.com/photos/blackcountrymuseums/5074221181/ http://geologymatters.org.uk/collections/getrecord/WAGMU_Geology_Temp_36/ http://www.encyclopedia.com/doc/1O13-graptolite.html http://en.wikipedia.org/wiki/Special:BookSources/0-375-40119-9
