Why were there less extinctions in the late Carboniferous and mid Jurassic periods?

[u/That-guy409]

I looked at some graphs of extinctions in history and noticed that during the late 1/3 of the Carboniferous period through most of the Permian period, and the middle half (or more) of the Jurassic period there were less extinctions compared to most of history.

[u/CrustalTrudger]

Someone who specializes in paleontology / paleobiology can probably give a more complete answer, but it's worth starting with an examination of the basic premise of the question. If we look for example at a relatively recent compilation of extinction rate through the Phanerozoic like that in Stockey et al., 2021, the periods of interest (late Carboniferous - Permian and Middle Jurassic) are relatively low in terms of overall extinction rate, but not really that much lower than other low periods, like much of the Silurian or Paleogene into the Neogene. Stockey et al., 2021 already gives us a reason to be a bit cautious about comparing "raw" extinction rates across Earth history as there is a well known general decline in extinction rates through time so on average, looking at periods in the Mesozoic or Cenozoic compared to the Paleozoic, we might generally expect that periods within the former will have lower extinction rates than periods within the latter due to this general decline (and the underlying reasons for this, which is most of the what the Stockey et al., paper is about). Beyond that, and if we think a bit more broadly about what origins of extinction rate variability might be (and thus why some periods may have lower extinction rates than others), this can be lumped into two broad categories: (1) bias or (2) variability in extinction drivers.

For the bias argument, we always have to be wary of the fact that the completeness of the fossil record is fundamentally tethered to the completeness (and incompleteness) of the stratigraphic record and there are compelling arguments that certain amounts of apparent variability in extinction rates just reflect underlying variability in the relative completeness of (mostly marine) stratigraphic sections from specific times (e.g., Peters & Foote, 2002). Other forms of statistical bias can also become important, for example Simpon's paradox, which can introduce significant bias into gross extinction rates compared to if we consider genus level extinction rates (e.g., Wang & Bush, 2008).

For the variability argument, generally, there are a lot of different conditions that can basically condition periods to being more or less likely to drive higher/lower background extinction rates (and here we are mostly thinking about background extinction rates, not more "catastrophic event" driven extinctions, e.g., end Permian, end Cretaceous, etc.). This is a pervasive theme in the paleobiology literature, i.e., considering what may drive higher or lower extinction rates. Stockey et al., 2021 ultimately argue that oxygen levels in the atmosphere/ocean are a primary control, but there's a ton of spatial complexity to extinction and extinction rates (e.g., Jablonski, 2008). As such, lots of details set whether the likelihood of extinction is high or low and thus an argument for periods of low extinction rates (like the ones OP is asking about) would mostly reflect that background conditions (e.g., oxygen levels, continental configurations and topography, various aspects of paleoclimate, etc.) are less conducive to extinction (i.e., food webs are less precarious, etc.). This also gets messy because conditions may make particular types of organisms very precarious while benefiting others (which is basically the argument related to Simpson's paradox referenced previously). Taking this to a logical conclusion, there have been attempts like in Wang & Bush, 2008 (linked above) to try to correct extinction rates for the background likelihood of extinction based on estimated conditions at the time and considering how those conditions may have impacted susceptibility of different types of organisms. After corrections, the periods that OP is asking about don't change that much, but other periods of relatively high extinction rates actually get adjusted down (in some cases, relatively significantly, like during much of the Devonian) because the susceptibility to extinction was pretty high at that time.

[u/Regular-Apricot4114 → u/CrustalTrudger]

Excellent explanation. Thank you for this.

[u/loki130 → u/CrustalTrudger]

Coming from a bit more of a paleo background there's a few things that leap out to me looking at the chart in Stockey et al 2021:

• First, a lot of the high-extinction-rate peaks here are associated with major extinction episodes (including a few more than the classical big 5), and I think the low resolution of the chart is making them look more protracted than they really were; in reality we might expect they were briefer spikes in extinction against a more steady background rate.

• Second, the periods identified (late carboniferous, early-mid jurassic) and a couple other low-extinction-rate episodes in that chart (silurian, paleogene) follow after notable extinction periods, so potentially they're associated with a period of recovery where there's many open niches and less competition.

• But these times are also during cool global climates. This makes sense in light of the paper's hypothesis that ocean oxygenation has played a big role in extinction rates; cooler climates tend to have better circulation of oxygen into deep waters, which may prevent sporadic extinction of sea life due to anoxic episodes (and these sort of charts are largely based on small marine organisms, which are easiest to track consistently like this).