Paleozoic Era: Ordovician Period 

485–444 Ma

The Ordovician: a great diversification

The Cambrian Explosion wasn’t the only major evolutionary radiation in the Paleozoic Era, nor was it the biggest—that title goes to the aptly named Great Ordovician Biodiversification Event (or GOBE for short). This event saw a fourfold expansion of marine animal genera, including the diversification of brachiopods, cephalopods, crinoids, graptolites, conodonts, and some of the earliest fish known, along with building diversity of trilobites and other arthropods. Most of the Earth's land masses were collected together in the supercontinent Gondwana, which was moving through the southern hemisphere towards the south pole. Surrounded by extensive seas of the Panthalassic, Iapetus, and Paleo-Tethys Oceans, the shallow platform around Gondwana was teeming with this new animal diversity. Amongst the animals shown here, the large trilobite Isotelus scurries across the seafloor while large predators lurk behind, including the giant orthoconic nautilus, Endoceras—a huge cephalopod related to today’s octopus but wearing a straight, conical shell. The nautiloids weren’t the only giants here, though, as Pentecopterus also draws near. This large eurypterid, or sea scorpion, is known from nearby Iowa, where it was discovered, and could reach a length of 2 meters. Swimming into the neighboring Silurian Period are also several forms of very early fishes, including the bony, helmeted Xiushanosteus and cartilaginous Shenacanthus most recently reported from an exceptional fossil deposit in the earliest Silurian of South China, which provides excellent support to divergence estimates of jawed fishes originating here in the late Ordovician. In the far background, in fact on land, the broad ranging mountain is to signify the Taconic orogeny, the mountain-building event that ceased around 440 million years ago and was one of the collisional events responsible for building the Appalachian Mountains. In addition, fossil spores comparable to those of younger land plants have been found in Ordovician rocks, providing our strongest evidence to-date that plants had started to invade terrestrial environments during this interval. The end of the period was ushered with the Earth's first big biosphere crash, known as the Late Ordovician Mass Extinction (or LOME). The earliest of the "big five" mass extinction events, the LOME was also one of the biggest, ranking 2nd on the list, with approximately 85% of all known species going extinct. While studies are still debating the cause of this extinction, the positioning of Gondwana over the south pole, allowing for extensive glaciation and a cooling climate, may have played a role.