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Geomagnetic Field Effects on Living Systems

 

6.3.6 Mass Extinctions of Life on Earth

Raup and Sepkoski (1982) identifed fve mass extinctions of life on Earth. Sepkoski’s evolutionary fau­

nas of marine animals are shown by Sepkoski (1984) as “Big Five”:

1. Ordovician–Silurian (O–S) extinction events: ~450–440 Ma at the O–S boundary (Sepkoski,

1984; Baez, 2006). Tis is the second-largest of the fve major extinctions. Two independent stud­

ies simultaneously suggested the cause of the mass extinction was due to global warming, related

to volcanism, and anoxia, and not due, as considered earlier, to cooling and glaciation (Hall, 2020;

Bond and Grasby, 2020).

2. Late Devonian (Late D) extinction: ~375–360 Ma near the Devonian–Carboniferous boundary

(Sepkoski, 1984; Baez, 2006).

3. Permian–Triassic (P–Tr) extinction event: ~252 Ma at the P–Tr boundary (Sepkoski, 1984; Baez,

2006; St. Fleur, 2017). Tis is the Earth’s largest extinction. Te highly successful marine arthro­

pod, the trilobite, became extinct. Te “Great Dying” had enormous evolutionary signifcance

(Erwin, 2006).

4. Triassic–Jurassic (Tr–J) extinction event: ~201.3 Ma at the Tr–J boundary (Sepkoski, 1984; Baez,

2006). Most non-dinosaurian archosaurs, most therapsids, and most of the large amphibians

were eliminated, leaving dinosaurs with little terrestrial competition. Afer that, the Mesozoic

era, characterized by ammonites and dinosaurs, began.

5. Cretaceous–Paleogene (K–Pg) extinction event: ~66 Ma at the K–Pg boundary (Sepkoski, 1984;

Baez, 2006). Tis mass extinction of organisms at the K–Pg boundary was triggered by the col­

lision of a giant meteorite with a diameter of about 10 km on the Yucatan Peninsula in Mexico

(Schulte et al., 2010; Renne et al., 2013). All non-avian dinosaurs became extinct during that time

(Fastovsky and Sheehan, 2005). Mammals and birds, the latter descended from theropod dino­

saurs, emerged as dominant large land animals.

In addition to the fve major mass extinctions, there are numerous minor ones as well. Regarding the

sixth extinction, for example, it has been suggested that the deep environmental transformations that

took place during the Pliocene–Quaternary boundary not only altered patterns of species diversity

and composition but also might have brought a major shif in the functioning of marine ecosystems

(Garreaud et al., 2010). Moreover, the ongoing mass extinction caused by human activity is sometimes

called the sixth extinction (Mason, 2015).

In contrast to the P–T (or P–Tr, Permian–Triassic) boundary issue, not much attention has been paid

to the G–L boundary event; however, the signifcance of the G–L boundary event was re-emphasized

from a diferent aspect relevant to the superocean Panthalassa (Isozaki et al., 2007a). Te timing of the

end-Guadalupian extinction apparently coincides with the onset of the superanoxia in Panthalassa,

i.e., another global scale geologic phenomenon across the P–T boundary (Isozaki, 1997, 2007b). In addi­

tion to the faunal turnover in mid-oceanic plankton (radiolarians) detected in deep-sea chert, shallow

marine sessile benthos (fusulines) also sharply declined in diversity across the G–L boundary in mid-

Panthalassan paleo-atoll complex (Isozaki and Ota, 2001; Ota and Isozaki, 2006). Tese positively sug­

gest the global nature of the G–L boundary extinction and causal environmental change (Isozaki et al.,

2007a).

Regarding the causes of mass extinctions, there are some plausible large-scale events such as astro­

nomical impacts with a giant meteorite at the K–Pg boundary (Schulte et al., 2010; Renne et al., 2013),

and various environmental changes due to volcanic activity during the formation and division of

supercontinents in the P–T boundary, where “plume tectonics” is regarded as promising (Maruyama,

1994).

As shown in Figure 6.8, the pattern of geomagnetic polarity change in the Permian demonstrates

a clear contrast between the early Permian (Cisuralian) to the middle Guadalupian and the late

Guadalupian to Lopingian (Isozaki, 2009, modifed from Gradstein et al., 2004).