I had not heard of the axis wobbling theory. Interesting, thanks Greg.
Extinction theory is a HUGE topic, people have written volumes on these events. But in short, there have been five previous major extinction events, excluding the current one underway. These were:
Late Ordovician extinction, 439mya
Late Devonian extinction, 362mya
End Permian extinction, 248mya
Late Triassic extinction, 205mya
End Cretaceous extinction, 65mya
There were other mass extinctions, but these are the ‘Top 5’, and of these the Permian was the biggest, with, as you say Tony, some estimates putting as high as 95% of all species becoming extinct. Though the Cretaceous extinction is, of course, the most famous due to the loss of the dinosaurs.
Ordovician Extinction, culminating 439mya. (22% marine families)
Cephalopods: Some nautiloid families, including the first nautiloid order the Ellesmeroceratina
Others: Some brachiopods, graptolites.
Possible cause: In the late Ordovician times the sea level dropped due to ice age and massive glaciation. Ice sheets spreading out from the poles would have lowered the sea level. Marine life on the continental shelves would have suffered vast habitat reduction. At the very end of the period, the opposite happened, the sea level suddenly rose polluting the continental shelves with oxygen-deficient deep water.
Devonian Extinction, culminating 362mya (21% marine families)
Cephalopods: Nautiloid families and the nautiloid order Discosorida. The obscure and short-lived Anarcestida and Clymeniida (two of the earliest ammonoid groups)
Others: Some forms of coral, 75% brachiopods, graptolites.
Possible cause: Poorly understood. Series of events, probably a drastic global cooling. Some evidence of meterorite impact; possible pollution of continental shelves by deep water.
Permian Extinction culminating 248mya (57% marine families, 95% total life)
Cephalopods: The nautiloid order Bactritida, the ammonoid goniatites and the ammonoid order Prolecanitida
Others: Trilobites. Devastating for terrestrial life.
Possible cause: Gradual event over 10 million years or so. Possible meteorite impact. All land masses locked together into Pangea causing overheating and vast climatic change, coupled with intense volcanic activity in Siberia causing dramatic rise in CO2 levels. Consequent alteration in oceanic currents. Drop in sea level at end of Permian due to glaciation with effects as above.
Triassic: Extinction culminating 205mya (20% marine families)
Cephalopods: Final orthoconic nautiloids, the ammonoid ceratites and the one of the earliest coleoid groups, the Aulacoceratida
Others: conodonts, many brachiopods, many forms of crinoid.
Possible cause: Climatic change causing increased rainfall in an otherwise arid period. This could have led to a change in surface temperature, and pH and salinity levels in the sea.
Cretaceous Extinction event 65mya (15% marine families, poss. 75% all families)
Cephalopods: the last few remaining families of ammonites and belemnites.
Others: Dinosaurs (dwindling anyway), sea reptiles, final pterosaurs, some land plants.
Possible cause: Meteorite impact, crater has been discovered in the Yucutan Peninsula, with layers of iridium, an element abundant in meteorites, present in clays the world over. Intense volcanic activity in the Deccan Traps in India, either caused by effects of the meteorite or India moving northwards sitting on top of a hot mantle plume. Blanketing of sky due to either/both events would have reduced plant photosynthesis causing a knock-on effect on terrestrial life and on the marine life via phytoplankton.
I totally agree, there may not be a single cause for these extinctions, often major extinctions seem to be a chain of interconnecting events. It goes without saying that although extinction events are terminally bad things for the species involved, they allow the survivors to diversify and fill vacant niches. For those survivors they are a wonderful opportunity.
Hope that is of some interest!
