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- near the start of the Ordovician, however the first bona fide examples date to the Middle Ordovician. The diversification of the Discosorida, in terms of genera, peaked at the beginning in the Middle Ordovician (modern Darriwilian stage) followed by a decline in the Upper Ordovician (modern Sandbian and Katian stages) only to peak again in the Middle Silurian. Afterwards their diversity declined drastically and remained low until their end in the late Devonian. Some were endogastrically curved, with the lower, siphuncle side concave, others

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