The Ordovician Period is the second period of the Paleozoic Era and began 485.4 million years ago and ended 443.4 million years ago (when the Silurian Period began). Four continents were present and separated by three main oceans. Laurentia was composed of present-day North America, part of Scotland, and Greenland and was near the equator. Siberia-Kazakhstan was east of Laurentia, slightly north of the equator. The Iapetus Ocean separated these two masses on the south from the continent of Baltica. Avalonia (England, New England, and parts of Canada) was to the west of Baltica. England, Baltica, and Kazakhstan were separated from Gondwana by the Paleotethys Sea. At this ...view middle of the document...
Trilobites and conodonts were abundant during this period. Snails, clams, cephalopods, blastoids, and many creatures of this nature appeared for the first time during this period. Remains of jawless, armored fish make us some of the oldest vertebrate fossils. Reef ecosystems were predominantly controlled by algae and sponges although there were periods of reef collapse due to disturbances. The shallow seas that covered Gondwana became a petri dish for trilobites. The first planktonic graptolites emerged. The Ordovician Period is mostly known for the oldest complete vertebrate fossils of the ostracoderms seen in the picture below.
Colonization of the land was the most revolutionary occurrence of the Period. Microfossils of parts of early land plants and remains of terrestrial arthropods are known from this time. Tentacled mollusks called nautiloids were effective predators. Conodonts were also marine hunters and were finned, eel-like animals with disproportionately large eyes. Sponge reef-dwellers gave way to byozoans which helped build structures similar to coral. Ordovician reefs were home to the first sea lilies. Arthropods (horseshoe crabs) edged slowly into freshwater and shallow lagoons. Evidence shows that the first primitive plants may have begun appearing on the once barren land.
Conodonts Horseshoe Crab
The Ordovician Period came to a close due to freezing conditions that held the planet hostage. The second largest mass extinction of all time took place and half of all marine life was wiped out.
The Silurian Period is the third period of the Paleozoic Era; succeeding the Ordovician Period and preceding the Devonian Period. It began 443.4 million years ago and ended 419.2 million years ago. It is important to note that continental elevations were lower than they are now and the sea levels were significantly higher. Sea levels rose as glaciers that were left behind from the Ice Age, melted. This allowed many organisms to resurface especially since continents were flooded with shallow seas (approximately 330 feet in depth) and coral reefs were created. Coral reefs were most common in seas that occupied a tropical to subtropical climate. Fish flourished and vascular plants took over coastal lowlands while the interiors of continents were overall barren of life. Roderick I. Murchison was a Scottish geologist and in 1835 he came upon rocks in Wales/England and named the rocks after natives named “Silures”. For most of this period, the Panthalassic Ocean covered the regions in the north while the large landmass of Gondwana occupied the southern polar region. In this time period species were distributed globally as opposed to the situation presented in the Ordovician Period.
The majority of North America, Scotland, northwestern Ireland, and the Chukotskiy Peninsula of northeastern Russia all formed the...