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The Ordovician Period
430 million years ago
430 to
500 million years ago
Mass extinction: trilobites, crinoids and brachiopods greatly reduced in numbers
Gastropods, crinoids, trilobites, brachiopods, graptolites increase in numbers again. First corals and vertebrates
Brachiopods became a more important form
of life; echinoderms were also more important, especially crinoids. In the Ordovician Period they were still small. Gastropods became larger and more common, as did molluscs, although their number was small. Crinoids continued to thrive.
Corals made their first appearance in the Ordovician Period, first as tabulate forms, then, by mid-Ordovician, as rugose forms as well. Graptolites had become abundant, and the fact that they were free floating and so found everywhere makes them useful index fossils. The first vertebrates also lived in the Ordovician Period. They were the ancestors of fishes and all other vertebrates.
At the end of the Ordovician Period there was a second global catastrophe and mass extinction when about a quarter of all families died. The trilobites fared worse than most other families. Large numbers of brachiopods and echinoids also suffered.
The Silurian Period
(Above) Ordovician trilobite: Ogygenus.
(Below) Ordovician graptolite: Didymograptus.
395 million to 430 million years ago
Corals and crinoids abundant. First land plants, growth in arthropods, fish with jaws
The Silurian Period is short, and no new major
groups of organisms evolved. Corals became
more numerous and large reefs formed. Trilobites
and graptolites still survived, but in restricted numbers. On the other hand, scorpion-like
arthropods grew to be enormous, and some
example fossils show they reached 3 metres in
length, the largest arthropods ever to be on earth. The first fish with jaws developed but were rare. The first plants appeared on land. At this time, however, the land plants had no leaves.
(Above) Silurian graptolite: Monograptus.
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