Chapter 8: Biodiversity and Mass Extinction

Why is it impossible to plot an accurate diagram of species number?

• many species did not fossilise

• low diversity levels in Paleozoic potentially due to ancient rocks being rare + fossils in them metamorphosed/ eroded away → suggestion hat the true pattern of diversification of marine invertebrates had been a rapid rise to modern diversity levels during the Cambrian and Ordovician, and a steady equilibrium level since then

Common features of mass extinctions

1. Many species went extinct, perhaps more than 30% of plants and animals of the time.

2. The extinct organisms spanned a broad range of ecologies, and typically include marine and non‐marine forms, plants and animals, microscopic and large forms.

3. The extinctions were worldwide, covering most continents and ocean basins.

4. The extinctions all happened within a relatively short time, and hence relate to a single cause, or cluster of interlinked causes.

5. The level of extinction stands out as considerably higher than the background extinction level

→ limited evidence of selectivity?

What are the big 5 mass extinctions?

1. end-Ordovician

2. Late Devonian

3. Permian-Triassic

4. End-Triassic

5. Cretaceous-Paleogene

Classifications of mass extinctions

• PTr in a class of its own → 50% of families disappeared→ 80-96% of species

• Intermediate → 20-30% loss of families ; ~50% of species

• “minor” mass extinctions experienced perhaps 10% family loss and 20–30% species loss

Timing of Mass Extinctions: Gradualists vs. Catastrophists

• there is a stepped pattern of decline over a time interval of 0.5–1.5 myr during which 53% of the foraminifera species died out.

• A gradualist would argue that the extinction lasts for more than 0.5 myr, too long to be the result of an instant event.

• A catastrophist would say that the killing lasted for 1–1000 years, and would argue that the stepped pattern is the result of incomplete preservation, incomplete collecting, or reworking of sediment by burrowers

Best evidence of selectivity during mass extinction

• selection against genera with limited geographic ranges

• No evidence for selectivity of bivalve + gastropod genera through mode of life, body size, habitat range but probability of extinction for bivalve genera depended on the number of major biogeographic realms they occupied

• positive survival benefit of a wide geographic range

• genera containing many species survived better than those with only a few

What died out in the End-Ordovician?

Most reef‐building animals, as well as many families of brachiopods, echinoderms, ostracodes, and trilobites died out

Evidence for climatic changes: End Ordovician

• Tropical‐type reefs and their rich faunas lived around the shores of North America and other landmasses that then lay around the equator.

• Southern continents had, however, drifted over the south pole, and a vast phase of glaciation began

• ice spread north in all directions, cooling the southern oceans, locking water into the ice, and lowering sea levels globally

• polar faunas moved toward the tropics, and many warm‐water faunas died out as the whole tropical belt disappeared

When was the Late Devonian ME?

succession of extinction pulses ~ 380 to 360 Ma

What died out in the Late Devonian ME?

• abundant free‐swimming cephalopods were decimated, as were the extraordinary armored fishes of the Devonian.

• Substantial losses occurred also among corals, brachiopods, crinoids, stromatoporoids, ostracodes, and trilobites

• loss of coral‐stromatoporoid reefs led to substantial restructuring of marine seabed communities

Cause of end Devonian ME?

major cooling phase associated with anoxia on the seabed

When did the End -Triassic ME happen?

~201 Ma

What died out in the end Triassic ME?

loss of most ammonoids, many families of brachiopods, bivalves, gastropods, and marine reptiles, as well as by the final demise of the conodonts

Causes of End Triassic ME

• most evidence points to anoxia and global warming following massive flood basalt eruptions located in the middle of the supercontinent Pangea

• suddenness of the event, and the evidence for a series of consequences of flood basalt eruption, including global warming, acid rain, and ocean anoxia, indicate this was a mass extinction similar in causation to the Permian‐Triassic event, but considerably smaller

Extinction event before PTr

end‐Guadalupian mass extinction, about 260 Ma

When was the peak of extinction during PTr

~252 Ma

→ patches peak of eruptions by the Siberian Traps

Sediment before + after PTr

Before extinction

• sediment showed well-oxygenated seabed

After extinction

• black mudstones

• black + associated pyrite indicate anoxia

Siberian Traps

• ~ 3 million km³ of basaltic lava that cover 5 million km² of eastern Russia to a depth of 400–3000 m

• Eruptions over less than 2 myr

• Siberian traps composed of basalt → dark-coloured igneous rock → non-explosive eruptions

• flood basalts form many layers + build up over 1000s of years

Shift in oxygen values at PTr boundary

decrease in the value of the δ18O ratio of about six parts per thousand, corresponding to a global temperature rise of around 16 °C

Killing model in PTr

• increase in Carbon isotope 12

• something else needed→ gas hydrates

• Gas hydrates are generally formed from the remains of marine plankton that sink to the seabed and become buried

• carbon trapped as methane in a frozen ice lattice → earthquakes/ sea water warms → release → runaway greenhouse effect

• Immediate effect of massive volcanic activity: acid rain

• Ocean acidification

PTr survivors

• The few surviving marine invertebrates were those that required minimal food, and who were able to counter the acidified oceans in constructing their calcium carbonate shells

• On land, the survivors included plants that could cope with difficult habitats, and virtually the only reptile was the plant‐eating dicynodont Lystrosaurus

Interpretation of fern spike

• shifts in pollen ratios show a sudden loss of angiosperm taxa and their replacement by ferns, and then a progressive return to normal floras.

• fern spike indicative of a catastrophic ash fall: ferns recover first and colonize the new surface, followed eventually by the angiosperms after soils begin to develop e.g after volcanic eruptions

Evidence for meteorite impact

• unusual clay band at KPg boundary within succession of limestone → unusual levels of iridium

• KPg boundary clays also have shocked quartz

• melt products (KPg spherules) → Sedimentary rocks can be melted only by an unusual process such as a direct hit by an asteroid

Ediacaran Extinction Event

• ~541 Ma

• end of Ediacaran animals

• hard to be sure that all species went extinct at the same time

• some evidence for a nutrient crisis or a major temperature change precipitating the change.

• An older putative mass extinction, at the start of the Ediacaran some 635 Ma, might have been triggered by global cooling, the so‐called “snowball Earth” model

• extinction at the end of the Early Cambrian marked the disappearance of previously widespread archaeocyathan reefs

Late Cambrian Extinction(s)

• potentially a series of 5 extinction events

• 509-485 Ma

• repeated extinctions of trilobites

• following events → animals became more diverse → groups such as articulate brachiopods, corals, fishes, gastropods, and cephalopods diversified dramatically during the great Ordovician radiation

Examples of disaster taxa

• e.g stromatolites, ferns

• After PT→ inarticulate brachiopod Lingula thrived → coped with poorly oxygenated waters

reef gap following PTr

• reefs reassembled in Middle Triassic

• composed of a motley selection of Permian survivors, a few species of bryozoans, stony algae, and sponges. It took another 10 myr before corals began to build true structural reefs

Coal Gap post PTr

• Coals are formed from dead plants, and there were rich coal deposits formed through the Carboniferous and Permian, indicating the presence of lush forests.

• After the acid rain had cleared the land of plant life, no coal formed during the first 20–25 myr of the Triassic

• Late Triassic- forests reappear