Graptolites were a group of marine colonial animals that belong to the group Hemichordata (chordates without a backbone). Most of them floated freely about in the ocean, but some lived attached to the bottom. They lived from the Cambrian to the mid-Carboniferous. As fossils most of the graptolites look like branches of saw blades, with a length from 10 milimetres up to one metre long. They are usually white or silvery against the dark shale. Each "saw tooth" - theke - housed a small animal. The soft parts of the graptolites have never been found, but we assume that the animal could extend a wreath of tentacles from the theke, as shown on the reconstruction. The skeleton shows a close similarity to the presently living feather-gilled animal Rhabdopleura (class: Pterobranchia).
In the free-floating graptolite Rhabdinopora fabelliforme, the branches and thekes grew downwards. It floated around in the Ordovician ocean, and today we find fossils of this graptolite many places in the world.
Only the outer dermal skeleton (the periderm) is known, which is made from an organic substance (collagene-like). The skeleton consisted of two layers; an inner layer of alternating half rings and an outer laminated layer. The planktonic graptolites were spread over a large geographical area. They are therefore good index fossils and we can correlate deposits of similar age all over the world, especially in the Ordovician and the Silurian. Graptolites are common in the Oslo Region List of contents in the graptolite showcase
Publisert 18. mai 2011 16:00