I don't have time to do a proper post on periodicity - the idea that mass extinctions happen on a semi clockwork fashion periodically. However, the recent paper
that I finished reading and two online friends' posts (here and here) prompted me to write a quick something.
Periodicity came out of a paper done by Raup and Sepkoski back in 1984 based on a database of marine fossils that suggested that there might be a regular pattern to the mass extinctions. They felt they had uncovered a pattern of regular mass extinctions at 26 million years. This, in turn, inspired the idea of Nemesis, a dwarf companion to our sun. There was a hunt for Nemesis, which turned up zilch. Others have suggested that it is actually the sun's orbit around the center of the galaxy and the movement above and below the galactic arms that would cause the periodic signal. The most recent paper that refutes that there could not be a companion star such as Nemesis. It also casts doubt on the galactic orbit having anything to do with the extinctions. They still uncover a 27 million year repeating pattern for the extinctions.
Some of the popular press has stated that the general consensus amongst extinction researchers is that periodicity is real. Unfortunately, that's incorrect. There isn't a general consensus that agrees on this. On the contrary, it looks as though most researchers, other than a few (mostly physics and astronomy types), have rejected the idea. The idea, while perhaps not fringe, is definitely not in the mainstream.
Part of the reason for that is that there are at least two different killers for mass extinctions: impacts (KT Extinction) and vulcanism (Permian-Triassic and Late Triassic Extinctions). There seems to be a third, too: radically cooling (Eocene). Making these rather different extinction mechanisms follow a regular clock seems implausible at best.
My stat is weak enough that I'd rather not take on the paper's analysis directly. However, I can check on the predictions as to where the extinctions ought to be happening and see if there are corresponding generally acknowledged mass extinctions every 27 my. The authors anchor on the KT extinction and state that there ought to be a mass extinction every 27 my for the last 500 million years. Is there?
We'll go back at least to the Permian Extinction. I am rushing this out here.Predicted Extinctions:
11 mya: Miocene, Serravillian
38 mya: Eocene, Bartonian
65 mya: KT Extinction
92 mya: Cretaceous, Turonian
119 mya: Cretaceous, Aptian
146 mya: Jurassic, Tithonian
173 mya: Jurassic, Aalenian
200 mya: Triassic-Jurassic Boundary
227 mya: Triassic, Carnian
254 mya: Permian, Wuchuapingian
281 mya: Permian, Artinskian
308 mya: Carboniferous, Moscovian
335 mya: Carboniferous, Visean
362 mya: Devonian, Famennian
389 mya: Devonian, Givetian
416 mya: Silurian-Devonian Boundary
443 mya: Ordovician-Silurian Boundary
470 mya: Ordovician, Dapingian
497 mya: Cambrian, FurongianActual Extinctions:
14.5 mya: Middle Miocene Disruption (off by 3 my)
33.9 mya: Late Eocene Extinctions (off by 4 my)
65 mya: KT Extinction.
93.5 mya: Cennomanian-Turonian (off by 1.5 mya)
117 mya: Aptian Extinction (off by 2 my)
145 mya: End Jurassic: often considered regional only (off by 1 my)
183 mya: Toarcian Turnover (off by 10 my)
200 mya: Triassic-Jurassic Extinction (spot on)
228 mya: Carnian Extinctions? Questionable, supported, iirc, by Benton (off by 1 my)
251 mya: PT Extinction (off by 3 my)
260 mya: Guadelupian Mass Extinction (off by 6 my)
then we get proverbial crickets. The Carboniferous had little as far as mass extinctions go. In fact, the stretch between the Devonian and the Permian Mass extinctions has been commented on as faunal turnover at a relatively gradual rate due to shifting environments and evolutionary innovation more than anything. However, the Devonian Mass Extinctions have been called one damned thing after another, for a perdiod of 20 to 30 million years extinctions were VERY common. In fact, there are two or three different peaks:
360 mya: Carboniferous-Devonian Boundary/Hangenberg Event (off by 2 my)
375 mya: The Frasnian-Famennian/Kellwasser Event (off by 13 my)
420 mya: Lau event (off by 4 my)
423 mya: Mulde event(off by 7 my)
426 mya: Irevikean event (off by 10 my)
443 mya: Ordovician Mass Extinction (spot on)
Prior to that is highly subject to change. An increasing number of Cambrian taxa are being found in later geological periods. Likewise, there may actually be Ediacarans that make it pretty far into the Paleozoic.What does that say to me?
The whole thing doesn't really hold water for me. Period ending extinctions are the best fit...but...the causes have zilch in common! KT: Impact. TJ: Vulcanism. Ordovician: Glaciation! The patterns
of the extinctions are also different from one another. How an asteroid, a volcano, and a glaciation can have the same cause...well...that stretches credibility a bit.
Anyways, this has taken too long as it is. More, perhaps, later.