1.2 Getting a Grip: Dinosaurs and Mass Extinction
The Bigger Picture of general extinction rates over Deep Time.
First off, it is essential to know that most of the extinction that has happened over the billion years or so of multicellular life have not been taking place as mass events, as David Raup (1991, 80-85; 1994, 6760) has indicated. Smaller scale extinction events can occur during an overall radiation of new forms, Quantal & Marshall (2009), while adaptive radiations can spawn cryptic “extinction” pulses when all that has happened is the diversification rate has shifted slightly, Crisp & Cook (2009). Many individual groups show localized extinction events in exactly this less dramatic way, such as the several Triassic collapses covered in Steven Stanley (2009) of ammonoids (nautilus-like cephalopods that were major marine predators during the Mesozoic) and conodonts (chordate cousins hanging on from the Cambrian that are discussed more in Chapter 2 of Downard (2004).
Moreover, it isn’t even the case that the background extinction rate has remained constant. Fastovsky & Weishampel (1996, 388-390) noted there appears to be a general decline in the rate at the family level, and if you look more specifically at cases like the invertebrate genera extinctions (Figure 1 below) you see those are likewise substantially higher back in the Cambrian and into the early Ordovician than later on. The review by Valentine (2004, 453-458) made similar observations. These suggest larger dynamic patterns (dare I say, evolutionary ones) are at work, where the earliest models of a newly appeared lineage are more prone to failing the tests of life, while the forms that persist to successfully leave descendant lineages are those that have been honed into a more reliable configuration, less subject to perturbation unless something really drastic happens.
It is that deviation from the norm during mass extinctions that grab people’s attention, though. The Ordovician, Devonian, Permian, Triassic & Cretaceous periods all ended in die-offs so intense that significant changes afterward in what was alive justified putting new labels on them (especially so for the Permian extinction, ending the Paleozoic Age and ushering in the Mesozoic, in turned capped by the Cretaceous event). But it is a sobering reminder of how vast the carpet of Deep Time has been to realize that these five big episodes of mass extinction, momentous though they certainly were for things living at the time, together involve only 5% of all extinctions.
In other words, 95% of life has died out the old fashioned way, not in mass extinction events.
Oceanic plankton are stars in this area of comparative stability: while they go extinct given enough time just like everything else that has ever lived, they tend to do so fairly gradually as climate changes filter out the less successful. For example, the radiolarians over the last 17 million years reported by Kamikuri et al. (2009), with an uptick in their extinction rates 15-11 Ma coinciding with dropping sea temperatures during the formation of the Antarctic ice sheet.
Rich et al. (1996, 103) highlight one special yardstick: “Many a paleontologist has lived a long and useful life without seeing fossil flagellates, ciliates, or even radiolarians. But no one who deals with so-called invertebrates can afford to overlook the Foraminifera, whose name is commonly shortened to ‘forams.’ Not only are they the most abundant and best-preserved fossil protists; they also are the most useful of index fossils. No one knows how many oil wells they have helped locate or how many formations they have helped to identify and date.” Summarizing work dating back into the 1980s, E. Thomas (2003, 319-320) noted how “Rapid extinction of many deep-sea benthic foraminiferal species at the same time is very unusual in earth history, and most faunal changes of deep-sea faunas occur gradually, over hundreds of thousands to a few million years.”[FIGURE 001]
Figure 1. Adapted from Prehistoric Life (2009, 33). Mass extinction episodes are highlighted by an icon () and darker bordered boxes, which may be compared to the more graded rate data in Figure 2 below. The ups and downs of the cycles show interesting variety: note the overall declining rate before the abrupt Ordovician peak, contrasting with a steadily rising rate for the Devonian (cumulatively represent a lot of life checking out before the end of the period), in turn contrasting with the steeper spikes during the later three events.
The hardy foraminifera experienced a major decimation exactly once in their entire history, and it wasn’t associated with a mass event, making it through the K-T extinction just fine, for example, Alegret et al. (2012). Their crisis took place around 56 Mya during what was first called the Early Eocene Climate Optimum (EECO), then the Middle Eocene Climatic Optimum (MECO), and finally the Paleocene-Eocene Thermal Maximum (PETM). The Eocene was already a very warm period (reaching temperatures equaling the PETM five million years later) before a long-term chill over the last 50 million years punctuated by ice ages. Surveys by Kunzig (2011) and Kump (2011) stressed its implications for present global warming due to the aberrant conditions prevailing during the PETM spike: CO2 levels in the Earth’s atmosphere reached four times present levels, the equivalent of igniting the planet’s current fossil fuel reserves in one go, Kunzig (2011, 94-96).
There was increase in volcanism during the PETM, such as in the Caribbean basin during this period, Bralower et al. (1997), steadily baking rocks and releasing CO2 and methane, as well as methane hydrate outgassing from oceanic deposits (methane being an even stronger greenhouse gas than carbon dioxide, incidentally, though with lesser total impact because of its lower atmospheric concentration), D. J. Thomas et al. (2002), Dickens et al. (2003), Zachos et al. (2005), and P. Pearson (2010) on Bijl et al. (2010), and Kerr (2011b), as well as injections of carbon from melting permafrost, DeConto et al. (2012). Though some calcified ostracods (tiny marine arthropods) were temporarily disrupted during the PETM, Steineck & Thomas (1996), the foraminifera were especially devastated as the methane belch undercut the carbonates needed to build their shells, E. Thomas (2003).
Foraminifera tend to stratify by depth, illustrated by Fortey (2009, 61), a clue to how a dominant form could crash if the temperature, chemistry or nutrient balance alters beyond their tolerance range. Maslin & Thomas (2003) survey some of the dynamics of methane hydrates in sequestering and releasing carbon, while part of the reason for the general foraminifera success story is their diversified ability to respire nitrates along with oxygen, Piña-Ochoa et al. (2010).
Far removed from their benthic habitat, humans have nonetheless relied on forams for some time: Fortey (2009, 68, 228) and Stow (2010, 191-192) noted Egypt’s pyramids are built of limestone packed with one species, the 40-50 Mya coin-shaped Nummulites gizehensis, while Hofstadter (2009, 72) connected the abrasive properties of their tiny “test” shells to lens grinding for Galileo’s early telescopes.
Parenthetically, the PETM would get a big knock later in the Eocene from a most peculiar cause: the aquatic Azolla fern that proliferated in the Arctic Ocean, which because of plate movements had a poor circulation system and so when the carbon-rich ferns died they sank into the anoxic bottom, disrupting the carbon cycling system and dragging the climate downward as CO2 levels plunged, Brinkhuis et al. (2006). The opening of the Tasmanian Gateway at this time contributed to the cooling trend, Bijl et al. (2013), as did the collision of India with Asia, where the rise of the Himalayas chilled the Tibetan plateau along with spawning a new monsoon system that shifted moisture circulation globally, Zhisheng et al. (2001), Gupta & Thomas (2003), Gupta et al. (2004), and Irving (2008) re Kent & Muttoni (2008). Tudge & Young (2009, 40-42, 52-56) offer a tidy overview of this in their argument on Eocene primate evolution, and Hodges (2006) illustrates the causal dynamics of orogeny on climate. Adding to the mix (and perhaps not coincidentally) there was also a geomagnetic reversal during the PETM, Y. Lee & Kodama (2009).