Written in Stone – Mass Extinctions
Extinctions are defined as the death of the last individual in a species or group whereas a Mass Extinction is defined as the loss of a significant proportion of the animal kingdom.
The loss of one species can result in the loss of another dependant species (e.g. in the food chain).
Extinctions are usually the ultimate fate of all species due to changes in the environment.
Extinctions can be in waves or pulses, stepwise or gradual. Even after catastrophic extinctions there are usually some individuals left which have naturally evolved sufficiently to survive the change in the environment.
The taxonomy of Animalia is:
· Kingdom (One)
· Phylum
· Class
· Order
· Family
· Genus
· Species (Many)
The fossil record shows evidence that Mass Extinctions occurred in the following periods:
· Ordovician
· Devonian
· Permian
· Triassic
· Cretaceous
The causes include:
· Asteroid impact (Iridium evidence; Shocked Quartz; Craters)
· Volcanism (Flood Basalts; CO2; SO2 etc.)
· Sea Level Change (Loss of Habitat)
· Climate Change (Cooling and Heating)
It should be noted that these causes are interrelated. For example an Asteroid impact can cause climate change and climate change can cause sea level change.
Mass extinctions can be identified through the fossilised remains of many ancient life forms. These include:
Graptolites; Trilobites; Ostracods; Gastropods; Bivalves; Nautiloids; Ammonites; Brachiopods; Echinodea; Bryozoans; Conodonts; Plankton; Corals; Foraminifera.
Each of these species has a different response to changing environments and can therefore be used to deduce the causes of the extinction.
From the fossil record it is likely that this extinction was caused by climate cooling leading to the formation of massive ice sheets and a corresponding drop in sea level. There was also a loss of oxygen in the sea and rivers (Anoxia). The primary cause for this cooling is not understood.
The main cause is believed to be a large asteroid impact and associated Tsunamis. The oxygen levels in sea and fresh water also fell. There was also both warming and cooling of the atmosphere.
This extinction was massive and it could be said that it was when all life nearly died. About 95% of all species died out. For example coral reefs were absent for 7 to 8 MY.
The causes were just about anything including: asteroid impact; climate cooling; climate warming; sea level drop; volcanic eruption (flood basalt + CO2 +SO2). During this time it is likely that there was a ‘Volcanic Winter’ bringing darkness and acid rain.
During this extinction all Conodonts disappear from the geological record and Cephalopods were reduced to one family. The causes were asteroid impact, volcanism, sea level change and anoxia.
This is probably the most well known extinction commonly known as the C/T extinction which did for the Dinosaurs. They had however been in decline over 2 to 3 MY before this. Snakes and crocodiles survived along with ichthyosaurs. Ammonites had been in decline but became extinct at C/T.
The main cause that has been popularised is that the cause was a giant asteroid impact. The Iridium layer in the geological record supports this. The discovery of the giant crater at Chixulub has been linked to this impact but more recent dating gives a 100,000 year discrepancy.
It is now believed that the asteroid impact was only one factor in the extinction. Massive volcanism (Deccan Traps in India) producing large amounts of CO2 (warming) and atmospheric dust (cooling) was also a large factor.
Since the last ice age the main effect on the animal kingdom has been humans. This has been the result of mankind’s destruction of the habitat and hunting.
Examples are the Dodo which was made extinct by sailors using it as food and the Tasman Tiger (actually a wolf!).
Many other species have been brought close to extinction by us and this is an ongoing process. The list includes:
· Bonobos – Poaching, logging and disease
· Bumble Bees – Intense farming practice
· Cod – Over fishing
· Coral Reefs – Sea temperature rise and acidification of the oceans
· Dolphins – 70% of species threatened
· Tuna – Over fishing
· Whales – Over fishing 50% of species under threat
· Bats – Loss of habitat
· British Ladybird – Import of Harlequin
· Trees – Logging; Clearance for farming
Kate has studied the idea that mass extinctions are periodic in nature. This idea stems from the apparent regularity of extinctions. However there are gaps in this pattern and she has concluded in her PhD thesis that there is no such regular pattern exists.
Looking in detail at the fossil record there are a number of effects that cloud the issue:
· Multiple naming of the same species discovered by different people in different sites
· Sampling Biases
o Large outcrops have more fossils
o Some sites are more collected than others
· Depositional Hiatuses
o Gaps in rock strata
o Missing strata and fossils
· Reworking of Fossils
o Movement from one stratum to another
· Preservational Biases
o Close to land only 10% of fossils are preserved
Added to this environmental changes drive the evolutional process. Hence can one say that a species has really become extinct when it has evolved to combat the environmental change?
Kate increasing believes that, under the current definition, mass extinctions never really happen!
Dick Harris
October 2014