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More recently, however, fossils of other fish-like creatures, the 530-million-year-old Early Cambrian fossil dubbed Haikouella and then the 515-million-year-old middle Cambrian animal Pikaia have been promoted as the world's earliest chordate (Heeren 2000).
The very first jawed fish appeared in the Late Ordovician epoch and now-extinct worm-shaped marine animals called graptolites thrived in the oceans.
Glaciation locks up water from the ocean, and the interglacials free it, causing sea levels repeatedly to drop and rise.
The vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches.
Mollusks, which had also appeared during the Cambrian, became common and varied, especially bivalves, gastropods, and nautiloid cephalopods.
It was long thought that the first true chordates appeared in the Ordovician period in the form of fossils of the fish-like Ostracoderms found in strata traced to the Middle Ordovician (Gregory 1935).
The world's largest trilobite, Isotelus rex, was found in 1998, by Canadian scientists in Ordovician rocks on the shores of Hudson Bay.
Panthalassic Ocean covered much of the northern hemisphere, and other minor oceans included Proto-Tethys, Paleo-Tethys, Khanty Ocean (which was closed off by the Late Ordovician), Iapetus Ocean, and the new Rheic Ocean.
Marine fungi were abundant in the Ordovician seas, apparently decomposing animal carcasses, and other wastes.
The Ordovician period came to a close in a series of extinction events that, taken together, comprise the second largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct.
In what was to become North America and Europe, the Ordovician period was a time of shallow continental seas rich in life.
Trilobites and brachiopods in particular were numerous and diverse.