This is the first of a 3 part series that will teach you all about mass extinctions, travelling through time to share the details of the mass extinctions that have occurred in the past, all the way up to the one that is currently happening.

Extinctions are a normal part of life in fact, species of organisms are always going extinct. The background rate of extinction defines the ‘normal’ rate that extinctions occur, without human interference. This rate is has been recently estimated to be about 0.1 extinctions per million species per year (E/MSY). In addition to this, new species are always emerging in a type of punctuated equilibrium model, leading to a constantly changing amount of biodiversity on Earth through time. 

A mass extinction is defined as an event in which a least 75% of all species on Earth disappear. This must occur on a global scale, with corresponding evidence in both the geologic and biological records. For past extinctions, determining what 75% of species at that time was and what the coinciding geological markers are, are difficult and nearly impossible tasks.

There is no real way of determining with certainty how many species existed in the past and when they went extinct. The main source of evidence of past biodiversity is in the fossil record which has some major issues. A type of ‘bias’ exists in the fossil record, meaning that very specific geographic and geologic conditions need to be present for an organism to be fossilized.

Essentially, the organism needs to be in the right place at the wrong time, in other words, all the conditions for fossilization are there, but so is the thing that kills them. Ideally, that place is an anoxic environment that will slow or stall decomposition. It must also be saturated with carbonates, otherwise the soils will use the organism’s bones as a source of carbonate, dissolving them rather than preserving them. Chance is also a key element for fossilization: for fossils to be preserved for many years, it must occur in a place where sediments are deposited, lithified (compressed and built into the rock), then uplifted and preserved rather than eroded.

These very specific conditions were obviously not present everywhere, thus, only certain species over the past millions of years were fossilized. Marine animals, and some land animals and plants are the main components of the fossil record, leaving out a huge number of species of insects and bacteria that often don’t have opportunities to be fossilized. In addition to this, for these fossils to be discovered in modern times, they must be fairly abundant meaning that we are most likely only uncovering the most common species of the past. 

In terms of the geologic record, it is where scientists can infer the conditions of the past depending on the thickness of layers of certain elements in the sedimentary rocks. For example, a thick iridium layer could infer a meteorite crash on Earth (however further evidence is required to confirm this) because iridium is uncommon on Earth, but very common in extra-terrestrial materials. Another example is the use of oxygen isotope ratios in sediments and ice cores as indicators of temperature (ie. markers for glaciations). Colder waters contain higher O¹⁸/O¹⁶ ratio because H₂O¹⁶ is favored over H₂O¹⁸ for evaporation since it is lighter. Therefore if conditions on Earth were cold, the H₂O¹⁶ would be evaporated and trapped in ice and snow on land, leaving greater O¹⁸ in marine sediment records.

The main issue with the geologic record is that rocks don’t last forever. Earth’s crust is constantly being constantly renewed (sea floors spreading) and destroyed (subduction), meaning that most of the really old rocks are all gone. The oldest rocks on Earth are found in places where subduction does not occur, therefore they are fairly sporadic, making it difficult to discern past conditions.

Even today, the definitive number of species on Earth cannot be determined, but calculating extinction rates is a lot simpler, since we do not have to rely on old and sporadic fossils and sediments.

The periods when extinctions occurred are determined when, over a geologically “short” period of time, a significantly lower variety of fossils are discovered indicating a sharp drop in biodiversity during that time. If this drop coincides with addition markers such as an iridium spike (and many others) then this could indicate a mass extinction event.

The geologic markers and other astounding evidence found all over the world for what is believed to be the cause of the Permian-Triassic mass extinction will be discussed in next week’s post.