5.1 Ancient biodiversity: Origin of the Cambrian Evolutionary Fauna - Arne Thorshøj Nielsen

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En provenance du cours de University of Copenhagen

Origins - Formation of the Universe, Solar System, Earth and Life

344 notes

University of Copenhagen

Origins - Formation of the Universe, Solar System, Earth and Life

344 notes

The Origins course tracks the origin of all things – from the Big Bang to the origin of the Solar System and the Earth. The course follows the evolution of life on our planet through deep geological time to present life forms.

À partir de la leçon

Origin of the marine Cambrian and Palaeozoic Evolutionary Faunas / Diversity in deep time / Origin of predation and the Mesozoic Arms race

In module 5, Arne Thorshøj Nielsen and Jan Audun Rasmussen will show you how the higher life forms, particularly the marine animals, evolved in the oceans, after the sudden appearance of a hard skeleton in many different animal groups during the Cambrian Explosion 540 My ago. You will be introduced to the changing major evolutionary faunas through time, and also see how clever strategies to kill or avoid being killed, major extinction events and many other factors controlled the evolution that eventually resulted in the modern marine faunas.

5.1 Ancient biodiversity: Origin of the Cambrian Evolutionary Fauna - Arne Thorshøj Nielsen9:15

5.2 Ancient biodiversity: Origin of the Palaeozoic Evolutionary Fauna - Arne Thorshøj Nielsen 10:03

5.3 Ancient biodiversity: Origin of the Modern Evolutionary Fauna - Part 1 - Jan Audun Rasmussen9:30

5.4 Ancient biodiversity: Origin of the Modern Evolutionary Fauna: Part 2 - Jan Audun Rasmussen7:00

5.5 Ancient biodiversity: Origin of predation and the Mesozoic Arms Race - Jan Audun Rasmussen11:54

Rencontrer les enseignants

Henning Haack
Associate Professor
Natural History Museum
James Connelly
Professor
Natural History Museum of Denmark
Vivi Vajda
Professor
Department of Geology, Lund University
Jon Fjeldså
Professor, Curator
Thomas Gilbert
Professor
Michael Houmark-Nielsen
Associate Professor
Natural History Museum of Denmark
Bent Erik Kramer Lindow
Curator
Natural History Museum of Denmark
Gilles Guy Roger Cuny
Associate Professor
Natural History Museum of Denmark
Ole Seberg
Associate Professor
Natural History Museum of Denmark
Gitte Petersen
Associate Professor
Natural History Museum of Denmark
Tais Wittchen Dahl
Assistant Professor of Geobiology
Arne Thorshøj Nielsen
Associate Professor
Natural History Museum of Copenhagen
Martin Vinther Sørensen
Associate Professor, Curator
Natural History Museum of Denmark
Svend Stouge
Associate Professor
Natural History Museum of Denmark
Danny Eibye Jacobsen
Associate Professor, Curator
Natural History Museum of Denmark
Jan Audun Rasmussen
Associate Professor
Natural History Museum of Denmark
Emily Catherine Pope
Assistant professor
Natural History Museum of Denmark

Hi. My name is Arne Thorshøj Nielsen and I am a palaeontologist working on Cambrian

and Ordovician fossils at the Natural History Museum in Copenhagen. Welcome to this lecture

where I will tell about ancient biodiversity and the

evolution of animal life with special focus on the marine life. As an introduction

I will also tell you about fossils.

You have in previous lectures

heard that life evolved during the Precambrian, maybe from as early as 3.8 billion yrs ago,

but animal life as such did not evolve before the very end of the Precambrian.

The Cambrian, starting at 541 million yrs ago, is characterized by the upcoming of hard shells

in many different animal groups and this sparked a very rapid evolution of animals.

This event, developing during some 20 mill yrs, is referred to as the Cambrian explosion. The present lecture

deals with the major trends in evolution of animal life from the beginning of the Cambrian to the end of the Permian.

That is from about 540 million years ago when animals with a hard skeleton appeared

to the major mass extintion, at the end of the Permian, approx. 250 million years ago

where higher life was close to being wiped out on Earth.

The modern world is teeming with life almost everywhere. It is unknown precisely how many

animal species exist today, but some researchers estimate that we are dealing with approx. 1.5

million species and many, many more small undescribed forms. A little more than 200.000

animal species are presently described from the modern seas and oceans. The species richness

of animals in an environment is referred to as the biodiversity.

With maybe about 1.5 million animals living on the planet today, it is obvious than one

should expect the presence of many more fossil species in the geological record since there

has been animal life on this planet for about 600 million years. It is reasonable to assume

that several tens of millions of different species have existed through geological time,

but the fact is that only a quarter of a million fossil species have been described so far.

Although many fossil species undoubtedly remain to be found and named, we will never come

anywhere near describing the true number of species that have existed in the past.

How come? Well, the problem is that most dead animals never become fossils, they are simply

recycled. The soft tissue decays and the hard parts are usually broken down or dissolved.

It is therefore more the exception than the rule that a dead animal becomes a fossil.

That said, animals with hard parts are obviously more likely to become fossilized than soft-bodied

animals and the potential for being preserved as a fossil increases if the dead animal is

rapidly buried by sediment. The processes that affect an animal – or

a plant for that matter - between death and burial are referred to as taphonomy. The taphonomic

processes introduce what could be called a filter between the living community and the

fossil community. Only very, very rarely will the fossil community have precisely the same

species composition as the living community and in most cases the fossil community will

contain only a fraction of the animal species that actually lived in the environment.

Under very special circumstances a larger proportion of the fauna may be preserved as

fossils, including forms without hard skeleton. Such rare fossil occurrences are referred

to as Lagerstätten. Lagerstätten provide a unique window into ancient biodiversity,

demonstrating how many different animals lived in the environment. In the Cambrian 3 such

localities are particularly famous namely the Chengjiang fauna in China, the

Sirius Passet fauna in Greenland and the Burgess Shale in Canada. These fossil sites provide

a unique glimpse of the Cambrian biodiversity. At the same time they also demonstrate how

few species that are actually preserved under normal circumstances.

Based on the fossils found in the Burgess Shale calculations have been made, showing

that less than 15% of these animals would have been preserved as fossils under normal

circumstances. This should be kept in mind evaluating the species richness of fossiliferous

strata: Usually less than 15% of the species living in an environment may be anticipated

preserved - that is, preferably those with hard parts. Analyzing the biodiversity of

the past is therefore not an easy task.

In spite of all the difficulties working with the incomplete fossil record pioneering work

on ancient biodiversity was made by the American palaeontologist Jack Sepkoski. 54a 00:06:33,780 --> 00:06:37,300 He compiled known fossil taxa from the marine realm and

recognized 3 so-called evolutionary faunas, named the Cambrian fauna, the Palaeozoic fauna

and the Modern fauna. His initial work has subsequently been improved by other scientists

but his basic conclusions are still accepted by most workers. His diagrams show the diversity

of marine taxa at family level. It should be noted that a family may consist of only one

species or many species or genera and this is not shown. His diagram also shows the major mass

extinctions that occurred several times in the history of life.

Especially five such events were dramatic, namely at the end Ordovician,

in the late Devonian, end Permian, end Triassic and end Cretaceous. These are called the big five.

Overall the sepkoski diagram shows that the marine animals have become more and more diverse

through time. It is, however, discussed among biologists and palaeontologists how evolution works.

Darwin predicted a slow, gradual evolution, with slow accumulation of beneficial

mutations, others have pointed out that this fits poorly with the fossil record, mostly

showing rapid changes and long periods where no or only few changes take place. Fossil examples

of gradual changes are actually rare. This school of palaeontologists has suggested that evolution

takes place in steps, so-called punctualistic evolution. They further suggest that evolution

mostly takes place in small populations where mutations quickly can spread because of the

limited number of individuals. It is, in any case, obvious that mass extinctions recurrently

wiped out a significant proportion of the fauna, allowing for development of news forms

afterwards. A well-known example is the radiation of mammals after extinction of the dinosaurs at

the end of the Cretaceous some 66 mill yrs ago.

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