Ordicician Period (488 - 443 Mya) (1 janv. 48800000 av. J.-C. – 1 janv. 44300000 av. J.-C.)

Description:

During this period, the area north of the tropics was almost entirely ocean, and most of the world's land was collected into the southern supercontinent Gondwana.
Ordovician strata are characterized by numerous and diverse trilobites and conodonts (phosphatic fossils with a tooth-like appearance) found in sequences of shale, limestone, dolostone, and sandstone. In addition, blastoids, bryozoans, corals, crinoids, as well as many kinds of brachiopods, snails, clams, and cephalopods appeared for the first time in the geologic record in tropical Ordovician environments. Remains of ostracoderms (jawless, armored fish) from Ordovician rocks comprise some of the oldest vertebrate fossils.