|Monograptid graptolites from Silurian (c.430 million years old) slates at Coalpit Bay, Donaghadee, Co. Down.|
Although often looking like little more than pencil lines on slate, these long extinct animals are more closely related to the vertebrates (animals with backbones) than many other more familiar types of soft-bodied animals. A graptolite actually represents a colony of interconnected animals, or zooids, with each zooid occupying an individual theca. These thecae form the 'teeth' along the 'saw-blade', or stipe, of fossil graptolites. The evolution of graptolites (other than the dendroids, which barely changed) shows a general trend towards increasing simplicity, starting with many branched and often curved forms early in their history and ending with unbranched straight forms throughout their last 50 million years until their extinction.
Most graptolites were planktonic, floating near the surface of the sea from early Ordovician (510 million years ago) times until their extinction in the early Devonian (390 million years ago). However, one group of graptolites, known as Dendroids on account of their richly-branched form, lived on the sea floor and survived for longer. The dendroids first appeared in the late Cambrian (515 million years ago) and survived until the early Carboniferous (330 million years ago).
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