488 – 443 MILLION: ORDOVICIAN
- 86% of species lost
- diversity of life increases, fish are the first true vertebrates;
- O2 68 % of modern level;
- mean surface temperature 2 °C above modern level;
- sea level 180m above present day, rising to 220 m and falling sharply to 140 m in end-Ordovician glaciation;
- 100 times as many meteorites struck the Earth per year during the Ordovician compared with today;
- Appalachians first formed, Laurentia and Gondwana hover near equator, wide shallow seas; climate warm but later cools;
- First life on land – primitive liverwort plants and fungi;
- The period ends in ice age and extinction.
|Name||Diameter (km)||Age (megayears)||Dating method||Morphological type||Notes|
|James River, Alberta||4.8||<480||Geological dating||PROBABLE Complex||No surface expression|
|Clearwater East, Quebec||26||~460–470||Rb-Sr melt rocks||CONFIRMED Complex||Chondrite-type|
|Calvin, Michigan||7.24||450 ± 10||Geological dating||CONFIRMED Complex|
|Pilot Lake, North West Territories||6||445 ± 2||K-Ar, 40Ar/39Ar and Rb-Sr||CONFIRMED Complex||Dating based on one sample|
|Bear Swamp, New York||3.5||~444||Geological dating||PROBABLE Simple||No surface expression|
|Glasford, Illinois||~4||443.8-485.4||Geological dating||CONFIRMED Complex||Shattercones, shock metamorphism|
~450 Ma – ORDOVICIAN EXTINCTION
86% of species lost — Graptolite 2-3 cm length
Graptolites, like most Ordovician life, were sea creatures. They were filter-feeding animals and colony builders. Their demise over about a million years was probably caused by a short, severe ice age that lowered sea levels, possibly triggered by the uplift of the Appalachians. The newly exposed silicate rock sucked CO2 out of the atmosphere, chilling the planet.
Jens Ormö, Erik Sturkell, Carl Alwmark & Jay Melosh
ABSTRACT: Approximately 470 million years ago one of the largest cosmic catastrophes occurred in our solar system since the accretion of the planets. A 200-km large asteroid was disrupted by a collision in the Main Asteroid Belt, which spawned fragments into Earth crossing orbits. This had tremendous consequences for the meteorite production and cratering rate during several millions of years following the event. The 7.5-km wide Lockne crater, central Sweden, is known to be a member of this family. We here provide evidence that Lockne and its nearby companion, the 0.7-km diameter, contemporaneous, Målingen crater, formed by the impact of a binary, presumably ‘rubble pile’ asteroid. This newly discovered crater doublet provides a unique reference for impacts by combined, and poorly consolidated projectiles, as well as for the development of binary asteroids.
Paleogeography of Baltica and neighboring cratons at the time of the increased cosmic bombardment following the ~470 Ma asteroid breakup event illustrating the resulting known craters (red dots). Clearwater East is dated to this event (~460–470 Ma). Light blue color represents areas of shallow epicontinental seas, and dark blue areas of deep ocean. This distribution may, however, have varied somewhat due to periodical transgressions and regressions of the sea. The timeline documents the related meteorite falls (black dot and line).