Extinction

Extinction marks the end of a species.

Species have a finite existence: they originate in a speciation event, and ultimately at some point, the last members of a species die. This is extinction. It is a process that happens all the time, but at times in the Phanerozoic (i.e. the last 542 million years) we see pulses of extinction. The biggest five pulses, we call mass extinctions.

Introduction

We're going to cover:

History of thought behind extinction and the extinction of species – Section 1.
Mass extinctions: definitions and patterns – Section 2.
Mass extinctions: causes – Section 3.
The big five mass extinctions – Section 4.
The sixth mass extinction – Section 5.

Extinction is a fundamental and unavoidable process – but all extinctions will impact on the ecosystem in which a species was found. As such, they matter... And this is an important topic. Since the industrial revolution, humanity has largely benefited from processes that have also resulted in the systematic destruction or wholescale change of nature on a global scale. I believe there is a strong moral argument to be made, that you, me, and everyone else, should inherently care about biodiversity and the state of the planet. But even if we don't take that as read, and we are only interested in humanity – as we will learn over the course of this material – extinctions can destabilise many systems upon which we as a species, our cultures, and our economies depend. It is vital we understand them.

1 – What is extinction?

Have we always known that species go extinct? If not, how did we come to this idea? What causes extinctions, and are all groups equally at risk? In our first video we look at all these questions.

Summary

The hypothesis that extinction existed developed in the early 19^th century. Georges Cuvier was a key player in this area.
(Georges Cuvier's full name is actually Jean Léopold Nicolas Frédéric, Baron Cuvier. What a name!)
Extinction happens all the time, and there are background extinction rates.
We can define extinction as the death of the last member of a species, and functional extinction as a point beyond which a population cannot recover.
Thre are intrinsic factors that impact extinction risk, and extrinsic factors that often drive extinctions.

2 – Mass extinctions

We've already encountered the idea that extinction can happen in pulses. Let's explore that in a bit more detail, and find out what makes a mass extinction.

Summary

Mass extinctions are hard to define. Each is different to the other. But generally, we can say they have in common the following:

A large proportion (> 30%) of species go extinct at the same time.
Extinction occurs across a range of environments, and is global.
This happens over a relatively short time, and is presumably related to a single cause.
Extinction levels should be appreciably higher than the background extinction rate.

It's sometimes useful to think about mass extinctions in terms of proximal kill mechanisms (i.e. those things that kill individuals) and ultimate ones (those that cause the proximal ones).
Correlation ≠ causation.

Something to think about

Why do you think so many of our geological periods coincide with extinctions?

3.1 – Patterns and causes

We have touched on some proximal kill mechanisms, now let's look at the ultimate kill mechanisms.

Summary

There is increasing evidence that large igneous province eruptions (LIPs) might drive a number of mass extinctions.
But not all LIPs appear to be linked to extinctions, so we need to work on understanding the mechanisms.
Meteorites have also been implicated in some extinctions.
What is left after a mass extinction – i.e. disaster taxa – tend to be generalists.
Recovery can take between 5 and 30 million years.

3.2 – Major taxa

In the next video we look at the big five mass extinctions of the Phanerozoic. On the way through I mention a load of groups of fossils and when they go extinct.

Consider this section of the website a crash course in those groups, for those of you that have not met them before. For those groups you know about, please do feel free to scroll on down!

On the phylogeny website I introduce the idea that there is a single tree of life, and that the trees that we build are hypotheses aimed at approximating this. Here is one such hypothesis for the relationships of some major animal groups:

I've also marked on a bunch of clades. (Remember what these are?) Clades are delineated by coloured boxes. So, let's meet some major fossil groups, learn a few basics, and put them on the tree of life.

Brachiopods

Meet a member of the phylum Brachiopoda:

Brachiopod Peregrinella peregrina, Late Cretaceous, France. ~8 cm long.

These animals originate in the Cambrian, were hugely successful filter feeders in the oceans of the Palaeozoic Era, and still exist, but are nowhere near as successful today. On our tree of life, we can find these in the clade Lophotrochozoa – a group united by developmental biology, and a feeding structure called the lophophore.

Bryozoa

Next up we have the Bryozoa, another phylum of lophotrochozoans, closely related to the brachiopods!

A fossil bryozoan (Polypora elliptica), from the Carboniferous Thrifty Formation of Eastland County, Texas. Max ~4.5cm.

The fossils of these creatures look like nets - they are actually a number of animals living in a colony, and filter feeding. They appeared in the fossil record in the early Ordovician, and are still around today.

Echinoderms

Echinoderms are the only animal group to have pentaradial (five-way) symmetry. They are the sister group to the hemichordates, and in turn, to the chordates on our tree of life - in the deuterostome clade. Here is a fine example:

Blastoid Pentremites robustus, Lower Carboniferous, found in Indiana. Specimen ~4.5 cm long.

The Echinodermata includes the stalked crinoids/sea lilies, sea urchins, starfish, and brittle stars. Their fossil record is super duper interesting as there is quite a lot of extinct diversity (they have been around from the Cambrian, and are alive today, but only five lineages, out of many more, survive), and they went through a phase in the Cambrian where some had no symmetry at all! The above is a member of an extinct group called the Blastoids.

Ostracods

Ostracods are small crustacean arthropods (so in the clade ecdysozoa). Here is a rock chock full of them (they look a bit like kidney beans as they have a bivalved carapace around their body):

A ~15 cm Ordovician limestone packed full of fossil debris, including ostracods fromLafayette County, Wisconsin.

The ostracods appear towards the end of the Ordovician, and they are pretty much ubiquitous as small fossils from that point onwards. They are used to date rocks, and to understand the environments they were deposited in. They are unique in having individual sperm which, once uncoiled, are actually longer than their entire body.

Trilobites

Trilobites are the only major arthropod group to have gone extinct. Boo. They were around from the Cambrian to the end of the Permian, and during this time – especially earlier in this range, they were the most speciose animals, as far as we can tell from the fossil record.

Specimens of Homotelus bromidensis an Ordovician trilobite. These were found in the Bromide Formation of Carter County, Oklahoma. They are housed by the Paleontological Research Institution, Ithaca, New York (specimen number PRI 45505). Longest dimension of this piece of rock is ~14 cm.

They're gorgeous, aren't they?

Ammonoid

Ammonoids were really successful cephalopods (the group that also includes squid, octopodes, and cuttlefish). They are thus molluscs (and lophotrochozoans too!). Unlike the other groups I mentioned, these molluscs had external shells, which are often preserved in the fossil record:

This is an ammonite cephalopod - Cleoniceras besairiei. It is Cretaceous in age. Maximum diameter ~12 cm.

They were around from the Devonian to the end Cretaceous, and were free swimming creatures. They are really useful for dating rocks… and studying evolution.

Eurypterids

Otherwise known as sea scorpions, these were aquatic chelicerates (i.e. a member of the arthropod group that includes the arachnids and horseshoe crabs [which are not, usefully, true crabs]). Aren't they cool?

A Silurian sea scorpion (Eurypterus remipes) from Herkimer County, New York. Fossil ~12.5 cm in length.

These creatures were around from the Ordovician to the end Permian, and were predators in the Palaeozoic oceans, especially those of the Silurian.

Corals

There are two groups of extinct corals, that were around during the Palaeozoic – Rugosa and Tabulata. Corals alive today are a member of a group (Scleractinia) that appeared during the Triassic. Corals are cnidarians in our tree – so metazoans, but not quite bilaterally symmetrical ones. They lack, for example, a through-gut. Here are the extinct groups:

Rugose corals

Here is an example of the order Rugosa, around from the Ordovician to the end Permian. Bear in mind this group can be either solitary (as you see here) or form colonies.

A fossil rugose coral (Heliophyllum halli). This is Middle Devonian in age, sourced from the Moscow Formation of Erie County, New York. Specimen ~11cm long.

Tabulate corals

This is a member of the other major fossil order - the Tabulata. This group was also around from the Ordovician to the end Permian. Tabulate coral were only colonial.

A tabulate coral (Emmonsia emmonsii) – also Devonian in age, but from the Onondaga Limestone of Genesee County, New York. Longest dimension of specimen ~8.5 cm.

Bivalves

More molluscs! These ones are the bivalves - think muscles, clams and oysters:

This is a fossil bivalve – Mercenaria mercenaria. It is quaternary in age, and was found in St. Mary's County, Maryland. Specimen ~10 cm wide.

Bivalves have, uh, two valves, with a body in between. They appear in the early Cambrian, and are around today: they became prominent filter feeders in the Mesozoic oceans. They are common as fossils.

Gastropods

A final mollusc group for your delectation. Slugs and snails are gastropods, and there are a lot of marine members of this group that have spiral shells with good preservation potential – so are common in the fossil record.

This is a fossil gastropod called Naticarius plicatella. Note that inside its aperture, you can see a thing called the operculum (trap door) which is not normally preserved. This particular critter is Pliocene in age (it's from the Pinecrest Beds of Sarasota County, Florida, FYI). Specimen ~3.5 cm long.

Conodonts

3D models for these microfossils are hard to come by, I'm afraid, sorry. They represent the feeding apparatus of an extinct vertebrate group. They were around from the Cambrian to the end Triassic. If you want to learn more about them, this is a good source. These are somewhere around the chordates in our tree.

Dinosaurs

Given that I couldn't find a 3D model of the above, here is a 3D model of a dinosaur. These creatures did pretty well for along time, surviving from the Triassic until today. Well, the non-avian (i.e. non-bird) dinosaurs died out, but the clade as a whole is still very much with us.

This is actually a model of a cast of a Tyrannosaurus rex skull. But is still cool.

Dinosaurs aren't massively common as fossils compared to, say, marine invertebrates, but many people spend time studying them, their evolution, and their ecology. Tyrannosaurus rex is a large theropod - a member of the group that gave rise to birds. If you want to learn more about dinosaurs, EES currently offers a third year dinosaur palaeobiology module. In case you didn't guess it, these are chordates.

4 – The big five

Now we've met some of the major players in the history of life on earth, we can learn... all about how many of these cool groups died out (!?). There are, as we have learned, five major mass extinctions in the Phanerozoic. Let's look at them in a bit more detail!

Summary

We've now met the big five.

End Ordovician – ~445 Ma, substantial turn over in marine fauna, perhaps due to continental arrangement and associated climate change.
Late Devonian – pulses lasting from 380 to 360 Ma, which hit cephalopods and armoured fish, corals, brachiopods, and echinoderms. Plus trilobites. Caused by climate change?
End Permian – the biggest (251 Ma). Almost everything died. Major groups to go include trilobites, eurypterids, tabulate and rugose corals, and more. Probably caused by the eruption of the Siberian traps.
End Triassic – led to the loss of many brachiopods, bivalves, gastropods, and marine reptiles (plus the conodonts) at 200 Ma. Potential cause is the central Atlantic magmatic province associated with opening of Atlantic.
End Cretaceous – we say goodbye to the ammonoids, and the non-avian dinosaurs. Resulted from a pesky extra-terrestrial impactor that hit in what is today Mexico.

5 – The Sixth

So that is the past. Where are we now in terms of extinction?

Summary

Current extinction rates are between 100 and 1000 times background extinction rate.
Despite large error bars, this supports the idea that humans are causing – and you and I are in the middle of – the sixth mass extinction.
Drivers include habitat alteration caused by our activities, and anthropogenic climate change.
The insects are a good example of the potential impact this will have.

Bonus stuff!

That was all a bit depressing, wasn't it? But it's important.

As ever though, there is some bonus material for you – some extra things you may like to read if you find this interesting.

Want to learn more about #6?

There is a lot of discussion out there on the current mass extinction. This paper is a useful overview of where we are (well, were in 2011), where there is uncertainty, and the context for the current extinction. If you want further quantification, here is another good paper, published in Science, in 2015 that tries to quantify the extinction levels to assess the current state of play. This study looks at species populations, rather than extinctions, to try and assess the state of biodiversity today.

Does everyone agree?

This is science, and so – as with any human endeavour – there are lots of competing factors and views. Here is one, in an article in the Atlantic. Here is a paper looking at the current state of biodiversity in a different way - and arguing that species extinction is not our most pressing issue.

What do you think? Is it useful to talk about the sixth mass extinction, or are we not there yet? Is the entire discussion semantic?

What about my questions, above?

If the more philosophical side of things is of interest to you, this paper discusses some of these. Be warned, that it covers some heavy topics, and indeed, may reach conclusions with which you will disagree. There is also a distinct element of ethics associated with these questions and potential answers.

Something cheerful!

I got really bummed out when writing this lecture. This video of dogs wearing booties helped take my mind off the grim realities of 21^st century life. It may do for you too.