The Ordovician period is placed between 488.3±1.7 to 443.7±1.5 million years ago.1 It is preceded by the Cambrian period and is followed by the Silurian Period as seen in Figure 1.1 Initially, only these two periods existed, until 1979 when Charles Lapworth defined the Ordovician period as a period of its own based on fossil fauna that was different from both the Cambrian and the Silurian Period.1 The dates given for the start and end of the Ordovician period were found using the radiometric dating method on different fossils, which is thought to be extremely accurate.1
The Ordovician period was started by an extinction called the Cambrian-Ordovician extinction which lasted about 44.6 million years, and ended with a mass extinction event known as the Ordovician-Silurian extinction event that wiped out approximately 60% of all the marine genera.1
Organisms found in the Ordovician period were exclusively aquatic organisms. A small exception to this may have been extremely primitive non-vascular plants near the waterline of coasts. Insects, amphibians, and reptiles did not exist. Even the fish that existed near the end of the Ordovician period were extremely primitive, and would be unrecognizable today.3
Ordovician Environment
Plate Tectonics
The current placement of our continents is vastly different from what the continents looked like in the Ordovician period. Continental drift has had an enormous impact on plate tectonics and the way the earth looks today structurally.3 During the Ordovician period, most of the land on earth was in the form of one supercontinent, Gondwana, which consisted of what is now Southern Europe, Africa, South America, Antarctica, and Australia.4 Throughout the Ordovician period, Gondwana was moving towards the South Pole, which contributed to global climate change as discussed below.5
In addition to moving longitudinally, there was also some latitudinal movement of the plates as well, which contributed to new crust formation, leading to volcanic and earthquake regions.3 An example of this latitudinal plate motion is the northward drift of the Baltoscandian Plate (northern Europe).4
Climate
The beginning of the Ordovician period had a climate that was extremely hot. This is likely due to high levels of carbon dioxide, which would then produce a greenhouse effect, warming the earth.1
An estimate of marine water temperatures is 45°C 1, which is approximately 10°C warmer than the absolute warmest marine waters found on earth today- the Persian Gulf.6
This warm temperature is thought to have restricted the marine life from further diversifying.1 Throughout the period, the climate became cooler, and ocean temperatures became closer to present-day equator ocean temperatures1 which are approximately 30°C.7
The plate tectontics of the time also had a large effect on different aspects of the climate. As Gondwana moved south, glaciers began to form in what is the present day Sahara region of Africa.5 This climate change is a proposed reason for the mass extinction at the end of the Ordovician. The lateral continental plate motions caused the ocean currents to change and the opening of the Atlantic ocean.4
Sea Levels
The sea levels varied throughout the Ordovician period, but were predominantly high overall, being the highest in the entire paleozoic era.8 Most of the continent Gondwana was underwater, with only the newly forming Appalachian mountains and the surrounding area being above sea level.3 Due to the flooding of Gondwana, regional drowning also occurred which caused carbonate sedimentation to stop. 4
In the early Ordovician period, the sea level rose steadily, before it came to a halt in the mid Ordovician period. In the end Ordovician it rose once again before it fell right before the extinction.8 The highest sea levels in the period could be found at the beginning of the end Orodovician period.8
Ordovician Life
Following the Cambrian period, the Ordovician period brought a new level of complexity in terms of ecological systems and organisms, that can still be seen in the present-day to a certain extent.9 The fauna that emerged from the Ordovician became a template for all organisms and fauna to follow.9
Although the Cambrian period is most famous for the explosive rise in organismal diversity, the Ordovician period also featured an explosion in diversity, equally as important and incredible.9 The marine faunal genera increased by four times which resulted in approximately 12% of all Phanerozoic marine fauna.9
A key distinction between the Ordovician period and the Cambrian period is the vast diversity of the carbonate shell-secreting organisms in the Ordovician compared to almost no diversity in the Cambrian period.9 It was originally thought that the first vertebrates appeared in the Ordovician period, in the form of a fish.9 However, it has since been determined that likely the first vertebrate arose in the early Cambrian period.9
Early Ordovician Life
At the beginning of the Ordovician period, many new types of organisms joined the trilobites 9, which are now extinct marine arthropods.10 These organisms included various types of mollusks and echinoderms which are still very prominent in today’s ecosystems.9 Also included in these organisms were the first sea stars, which are still very well established in Earth’s oceans.9 Continuing from the Cambrian, green algae were the most common plant organism in the early Ordovician.9
Mid Ordovician Life
In the middle of the Ordovician period, the trilobites that were earlier joined by other organisms were replaced by a more mixed ecosystem of organisms.9 The first rugose corals appeared, and brachiopods, bryozoans, mollusks, cornulitids, tentacultitids, and echinoderms all thrived.9 There was a large increase in both the diversity and intensity of bioeroding organisms9, which erode hard ocean substrates.11 By the end of the mid-Ordovician period, trilobites were no longer predominant, and on their way to extinction.
Late Ordovician Life
In the late Ordovician period, prior to the extinction, the first jawed fish appeared, which was a major step and probably the closest organism in appearance to what fish of present day look like.9 Also very apparent by the late Ordovician period was the odd developments of what was left of the trilobites.9 Various trilobites developed very odd spines and nodules which are hypothesized to have been defense mechanisms against predators such as primitive shark like organisms.9
Trilobites also had developed snouts in the shape of shovels for plowing through the sea bottom, and long eyestalks which are also believed to have been in response to the need to detect predators.9 However, some trilobites’ eyes disappeared altogether, suggesting that there were advantages to loss of sight, which perhaps involved an increase in efficacy of other predator detecting senses.9
In terms of plant life, fossil spores from land plants have been found in the uppermost layers of the Ordovician sediments.9 Due to this discovery it is likely that the first land fungi involved arbuscular mycorrhiza fungi which form a symbiotic relationship with other plants providing nutrients.9 Further evidence of this is fossilized fungal hyphae and spores found in Wisconsin with a date of about 460 million years ago which places the fossils directly in the Ordovician period.9
End-Ordovician Extinctions
Fatalities
In the midst of a biodiversity boom, the Ordovician period ended with the mass extinction of almost all life.12 Approximately 443 million years ago there was an extinction resulting in the fatality of approximately 85% of all sea life, which was the majority of life at the time, given that most of the continent Gondwana was underwater.13
There were two major death periods, with about 1 million years in between them.12 The extinction targeted over 50% of trilobite families and 66% of all brachiopod and bryozoan families.13 Other fauna specifically affected were bivalves, echinoderms, and corals.13 The extinction event ended when melting glaciers caused the sea level to stabilize once more, and thus stop the constant destruction of coastline habitat.12
Potential Causes
There are three different main theories as to the cause of the extinction. The main theory, with the most evidence backing it, is that the movement of the continent Gondwana into the south polar region caused global cooling, glaciation, and thus a fall in sea level.12 A fall in sea level would significantly impact life on Earth because it would eliminate habitats along the continental shelves, where many organisms thrived. Evidence of glaciation can be found in deposits in the Sahara desert.12
The glaciation event was preceded by a drop in carbon dioxide, which in turn affect any shallow water and life inhabiting it.12 In addition to this blow, the glaciation locked up water from the world ocean, and the interglacials freed it, causing sea levels repeatedly to drop and rise.12 When the water level dropped, it eliminated many ecological niches, which when returned were lacking in entire families.12
The consistent change in the water level caused a shift in the location of bottom water formation from low to high latitudes.12 This created an oxygenated environment in bottom water where some organisms began to develop a niche and thrive.12 This would have aided in the extinction, as these organisms would have survived, however, anoxic conditions soon would return and wipe these newfound niches out as well.12 Any species which survived were those which could effectively deal with change and many are still around in the present day.12
A less supported theory for the extinction, suggested by a small group of scientists is that there could have been a gamma-ray burst originating from hypernova within 6000 light-years of Earth.12 A ten-second gamma-ray burst would have stripped Earth’s atmosphere of half its ozone, exposing all of Earth’s organisms to a high level of ultraviolet radiation.12 Although this theory is consistent with some of the events surrounding the extinction, all evidence is fairly ambiguous, making the theory far less accepted than the glaciation theory.12
The third theory, which is also less widely supported by scientists is that the pattern of volcanic weathering in the time period suggests a brief ice age.12 Plate tectonics of the time were quite active, and thus there was an abundance of volcanic activity, which was balanced by heavy weathering of the uplifting Appalachian mountains, which collected carbon dioxide.12 When the volcanic activity stopped, the balance was upset and the weathering continued, causing an extremely fast decrease in carbon dioxide.12
1. Ordovician Period: http://en.wikipedia.org/wiki/Ordovician
2. Geological Time Scale: http://www.britannica.com/EBchecked/topic/452280/Permian-Period/69920/Permian-life
3. Orovician Environment: http://members.wolfram.com/jeffb/Fossils/drift.shtml
4. Ordovician: Tectonics and Paleoclimate: http://www.ucmp.berkeley.edu/ordovician/ordotect.html
5. Ordovician Climate and Plate Tectonics: http://paleobiology.si.edu/geotime/main/htmlversion/ordovician4.html
6. Temperature of Ocean Water: http://www.windows2universe.org/earth/Water/temp.html
7. Pacific Ocean:http://en.wikipedia.org/wiki/Pacific_Ocean
8. Paleoclimatology:http://www.sciencedirect.com/science/article/pii/S0031018210004785
9. Ordovician Life: http://en.wikipedia.org/wiki/Ordovician#Life
10. Trilobite: http://en.wikipedia.org/wiki/Trilobite
11. Bioerosion: http://en.wikipedia.org/wiki/Bioerosion
12. Ordovician – Silurian Extinction: http://en.wikipedia.org/wiki/Ordovician–Silurian_extinction_event
13. BBC Ordovician Extinction: http://www.bbc.co.uk/nature/extinction_events/Ordovician–Silurian_extinction_event
