MARTIN LAND / SCIENCE PHOTO LIBRARY MARTIN LAND / SCIENCE PHOTO LIBRARY
Colonies of white fossil graptolites, Didymograptus murchisoni, on black shale. Graptolites were colonial animals which lived in abundance in Palaeozoic seas. A colony consists of one or more branches along which are arranged cup- like thecae, each of which originally housed a single animal. The slanting arrangement of the thecae causes the saw-blade appearance of the colony. Members of the genus Didymograptus are called "tuning-fork" graptolites because of their shape. Graptolite means "rock writing" - appropriate because of their resemblance to pencil scribble. They originate from the Ordovician Period, approximately 470 million years ago.
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