416 – 359.2 MILLION: DEVONIAN
- 75% of species lost
- O2 75 % of modern level;
- mean surface temperature 6 °C above modern level;
- sea level 180m above present day gradually falling to 120m;
- Euramerica and Gondwana drift towards each other;
- The Age of Fishes and the Invasion of Land;
- Ocean levels high with wide shallow seas;
- Warm, greenhouse climate;
- Late Devonian extinction – all placodermi (armoured prehistoric fish), and trilobites die off.
|Name||Diameter (km)||Age (megayears)||Dating method||Morphological type||Notes|
|Franktown structure, Ontario||~2||>400||Ordovician sediment cover||PROPOSED Simple||Circular depression|
|Lac La Moinerie, Quebec||8||400 ± 50||Ar40-Ar39 melt rock||CONFIRMED Complex|
|Brent, Ontario||3.8||396 ± 20||K-Ar studies on the coarsely crystalline melt rocks||CONFIRMED Simple||Overflight of Brent Crater|
|Elbow, Saskatchewan||8||395 ± 25||Geological dating||CONFIRMED Complex||Brecciated Devonian strata|
|Nicholson Lake, North West Territories||12.5||389 ± 6.7||Pb/U||CONFIRMED Complex||The large island within the lake is the eroded central peak|
|Flynn Creek, Tennessee||~3.8||382.03 ± 21||Geological dating||CONFIRMED Complex||Shattercones|
|Panther Mountain, New York state||10||~375||Geological dating||PROBABLE Complex||Inverted relief|
~374 Ma – LATE DEVONIAN EXTINCTION
75% of species lost — Trilobite, 5 cm length
Trilobites were the most diverse and abundant of the animals that appeared in the Cambrian explosion 550 million years ago. Their great success was helped by their spiky armour and multifaceted eyes. They survived the first great extinction but were nearly wiped out in the second. The likely culprit was the newly evolved land plants that emerged, covering the planet during the Devonian period. Their deep roots stirred up the earth, releasing nutrients into the ocean. This might have triggered algal blooms which sucked oxygen out of the water, suffocating bottom dwellers like the trilobites.
The Devonian extinction had severe global effects. With a worldwide loss of 60% of existing taxa, every ecosystem was affected. Reef systems were forever changed with the massive deaths of stromatoporoids and tabulate corals. Brachiopods lost their stronghold as the dominant shelled marine invertebrate. Entire classes, such as the agnathan fishes, went extinct. From the loss of microscopic plankton to terrestrial plants, all life on Earth was affected by this major extinction event.Asteroid impacts have become a popular theory examined at extinction events. McLaren (1970) proposed the first bolide-induced extinction for the Late Devonian. To be a single impact, studies have shown that the asteroid would need a diameter greater than ten kilometers. If an asteroid of those proportions impacted earth, it would kill life in the target area, generate earthquakes, tsunamis, wildfires and ballistic molten debris. Tsunamis, especially, would affect shallow marine ecosystems. The blast would heat the atmosphere sufficiently so that nitrogen could combine with oxygen to create nitric oxide and nitric acid. Rain falling in high concentrations could poison upper surface waters and destroy phytoplanktonic life. Calcareous shells would dissolve. Wildfires would produce dioxins and aromatic hydrocarbons, poisoning the environment. Significant addition of carbon dioxide into the atmosphere would create an icehouse effect. Global dust clouds could block sunlight, making photosynthesis impossible. Temperatures could drop below survivable ranges for many organisms.
Evidence for Impact: The Siljan Ring in Sweden, dated to the F-F boundary, has a diameter of 52 kilometers. Many craters have been studied for the Late Devonian extinction event (table), however the dates of many craters are either too wide to be accepted or dates too uncertain based on differing opinions. Larger craters might have been created in the ocean floor, but would now be destroyed by tectonics (McGhee, 1996). Others doubt a single asteroid impact could be responsible for such a severe extinction event.
McGhee GR Jr. 1996. The Late Devonian Mass Extinction. New York: Columbia University Press.
McLaren, D.J. 1970. Time, life and boundaries. Journal of Paleontology, 48,. 801–815.