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- Today I Found Out
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Want to learn more? Here are some keywords to get your googling started:
Cambrian Period – Geologists cut the past up into segments. The Cambrian is a geologic "period" that lasted from 570-505 million years ago.
Ordovician Period – The geologic period that followed the Cambrian and lasted from 505 million years ago to about 440 million years ago.
Cambrian Explosion of Life – The evolutionary event (during the Cambrian period) in which the first marine animals diversified into all kinds of different forms. All of the basic body plans – like segmented/non-segmented, asymmetric/symmetric, radial/bilateral – emerged during this time.
Great Ordovocian Biodiversification Event (GOBE) – Another big evolutionary event that happened about 40 million years after the Cambrian explosion. Often described as a "filling out" of the taxonomic tree, whose main branches sprouted during the Cambrian.
Late-Ordovician Extinction – The major mass extinction event at the end of the Ordovician period. Considered by most paleontologists to be one of the 5 biggest mass extinctions since animals first evolved on Earth.
Hirnantian Extinction – Another term for the Ordovician Extinction. "Hirnantian" refers to the layers of rock (the "stratigraphic sequence") that were laid down during a short span of time at the end of the Ordovician. (Geologists break up the stratigraphic record into small chunks called "stages." The Hirnantian is one stage.)
Glacial Striations – The parallel scratches in bedrock made as a glacier moves over its surface (the scratches are actually made, not by the ice itself, but by rocks stuck to the bottom of the glacier)
Glacial Moraine – The big pile of soil and rocks that glaciers push in front of them as they move. (Note: technically, this is called a "terminal moraine." There are also "medial" and "lateral" moraines that run along the length of glaciers).
Glacial Till – The debris dropped by a glacier/ice sheet when it melts. Usually a big pile of all sizes of rocks, from sand and mud to boulders.
Animal of the Ordovician – All the animals still lived in the oceans at this point in Earth's history. Some of the major groups include coral (tabulate and rugose), brachiopods, trilobites, graptolites, and conodonts.
Credits (and Twitter handles):
Script Writer: Emily Elert (@eelert)
Script Editor: Kate Yoshida (@KateYoshida)
Video Illustrator: Ever Salazar (@eversalazar)
Video Director: Emily Elert (@eelert)
With Contributions From: Henry Reich (@minutephysics), Alex Reich (@alexhreich), David GoldenBerg, Rachel Becker and Peter Reich
Music by: Nathaniel Schroeder: http://www.soundcloud.com/drschroeder
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Image Credits: Ordovician Glacial Striations – Flickr user Sebastian Luening
Dowdeswell, J. A., Ottesen, D., Rise, L., & Craig, J. (2007). Identification and preservation of landforms diagnostic of past ice-sheet activity on continental shelves from three-dimensional seismic evidence. Geol Geology, 35(4), 359.
Ghienne, J. et al (2014). A Cenozoic-style scenario for the end-Ordovician glaciation. Nature Communications Nat Comms, 5, 4485.
Harper, D. A., Hammarlund, E. U., & Rasmussen, C. M. (2014). End Ordovician extinctions: A coincidence of causes. Gondwana Research, 25(4), 1294-1307.
Heron, D. L., & Craig, J. (2008). First-order reconstructions of a Late Ordovician Saharan ice sheet. Journal of the Geological Society, 165(1), 19-29.
Heron, D. P., & Howard, J. (2010). Evidence for Late Ordovician glaciation of Al Kufrah Basin, Libya. Journal of African Earth Sciences, 58(2), 354-364.
Krug, A. Z., & Patzkowsky, M. E. (2004). Rapid recovery from the Late Ordovician mass extinction. Proceedings of the National Academy of Sciences, 101(51), 17605-17610.