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Vertebrates
The vertebrate part of the animal kingdom (Vertebrata) contains fish, amphibians, reptiles, birds, mammals and graptolites.
Vertebrate fossils are far less common than invertebrate fossils, partly because vertebrates are far outnumbered by invertebrates, and partly because the vertebrae fall apart when a vertebrate dies, so that good, whole specimens are extremely rare. Also, most vertebrate remains are difficult to identify without knowledge.
Graptolites
Graptolites are some of the most unusual and also some of the most important vertebrates of the Lower Palaeozoic times. As fossils, they are found either as white films or as solid shapes made
of pyrite. Because people thought they looked like ancient writing, they were given the name graptolites, meaning stone writing.
Graptolites are made of either one, two, four, or many, branches, known as stipes. Along each stipe (a kind of backbone) are many small cups. In each cup was a food-gathering organ.
The earliest graptolites, which tend to be colonial and consist of numerous branches, are found in rocks of Cambrian age. They are thought to have been floating colonies, held up by some form of air bag. The less branched forms appeared in the Ordovician Period, and rapid evolution made them especially useful as index fossils for the Lower Palaeozoic Era. Because they were free floating, they were able to move about through all of the seas, and so they are very useful for comparing Lower Palaeozoic rocks around the world.
(Below) The parts of a graptolite.
Air sac for buoyancy or attachment
Support or nema
Series of tube-like thecae
Branch or stipe
(Below) Dictyonema, a primitive dendroid graptolite from the Cambrian Period.
Recognising graptolites
Graptolites are mainly found as thin films or rods made of pyrite. They are normally found by breaking open sheets of shale. Because they are impressions and not usually solid objects, and because they are often small, they can easily be missed. Most graptolites are spotted by imagining their shape to be long, thin, saw-like blades.
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