Tuesday, November 25, 2014

Origins — Paleozoic Evolutionary Fauna

Today's lecture will also be conducted by Professor Arne Thorshøj Nielsen.

In this lecture we are going to continue our discussion of fossil records and study the animal life that developed during the Paleozoic era around 540 to 250 million years ago. In a classic study, American paleontologist Jack Sepkoski analyzed the various fossil records of marine animals and divided the distinct faunas into three distinguished groups called the Cambrian, the Paleozoic and the Modern evolutionary faunas.

The oldest fauna by Sepkoski was called the Cambrian evolutionary fauna. It includes the bulk of the fossil species which appeared during the so-called Cambrian Explosion. This was a relatively short evolutionary phase in the beginning of the Cambrian Epoch some 520-540 million years ago during which most major animal groupings appeared, at least in the fossil record. This we will get back to.

The overall diversity increased rapidly during the early part of the Cambrian and then reached a plateau of about a hundred families. The Cambrian fauna declined gradually through the Ordovician and many forms became extinct during the end-Ordovician mass extinction.

The Cambrian fauna was dominated by trilobites and, thus, the Cambrian is also called “the Age of the Trilobites.” Trilobites were arthropods and are thus distant relatives of the modern crustaceans. A typical trilobite would have lived on the sea floor and filtered the mud for food particles.

Also, brachiopods with phosphatic shells, so-called inarticulate brachiopods, were a characteristic constituent of the Cambrian fauna. They lived attached to the sea floor and fed by filtering detritus from the surrounding sea water. Interesting enough, a few representatives still live in the oceans today.

Hyoliths, also, were common in the Cambrian fauna. They are related to the better known bivalves and snails and had a tube formed shell. Many representatives were mud-eaters and they most likely lived on the sea floor.

As mentioned also in previous lectures, it is much discussed what caused the Cambrian Explosion. Why did so many fundamentally different bodies plan to evolve so quickly and in such profusion? The key feature that distinguishes the Cambrian from the Precambrian is the upcoming of a hard skeleton. A hard skeleton suddenly appeared in many different animal groups, which is the reason why they suddenly appeared in the fossil record.

Many researchers believe that there must have been a foregoing evolution in the latest Precambrian which is the reason why so many different animal groups suddenly appeared in the Cambrian when they got potential to become fossilized. It is reasonable to assume that most of the animal phyla actually developed in the latest Precambrian but these ancestral animals were likely small and had no hard, shelly parts, therefore we do not find them as fossils.
The big new thing in the Cambrian was thus the development of a mineralized skeleton.

Another major change is seen within trace fossils: the animals began to burrow more efficiently into the sea floor and graze on the sea floor. The scientists talk about the Cambrian substrate revolution. Many researchers believe that an increase in oxygen level in the atmosphere was the main reason why so many different animals suddenly became capable of forming a hard skeleton. This is supported by investigation of marine environments in the modern world showing that when the oxygen concentration gets below c. 1/10 of normal values the shelled animals disappear but a fauna consisting of non-shelled small animals, for instance worms, still thrive. It is tempting to see this as a model for the Cambrian Explosion.

A hard skeleton provides better muscle attachment, so the animals could increase in size, move and burrow more efficiently and a skeleton also offers protection against predation. It is a popular hypothesis that an arms race played a role in the rapid evolution that took place during the early Cambrian after upcoming of hard skeleton. At the same time the sea level rose very significantly and continents around the world became inundated. This opened up many ecological niches and maybe also supplied extra nutrition to the shallow seas washed out from land. As nearly always, it is highly likely that the Cambrian Explosion reflects the combination of several factors, not just a single one.

The diversity of marine animals increased significantly during the Ordovician. This period of evolution is referred to as the Great Ordovician Biodiversification Event, or, in short, GOBE.

The upcoming fauna is referred to as the Paleozoic Fauna or, more precisely, the Paleozoic Evolutionary Fauna.

Trilobites were still around, but overall the Paleozoic fauna is dominated by brachiopods and other filter feeders, filtering the sea water for food particles. Brachiopods superficially resemble bivalves but are completely different inside. Broadly speaking they live in the same way, though, namely on the sea floor. Brachiopods still exist today; however, bivalves are much more dominant.

Graptolites are another group of filter feeders that was common in the Paleozoic fauna.
Graptolites are colonial animals living afloat in sea water.

Corals and other barrier builders also became common in the Paleozoic fauna – they also filtered the water for food particles. Also predators such as cephalopods developed rapidly. Cephalopods are still common in the modern oceans, such as the likes of squids and octopodes.

The Paleozoic fauna is characterized by burrowing deeper below the sea floor and living higher above it than the Cambrian fauna. The Paleozoic fauna took over slowly and eventually reached a much higher diversity than before. It recovered after the end-Ordovician and the Devonian mass extinctions but was severely diminished at the end of the Permian after which the so-called Modern fauna took over.

In a way the Ordovician biodiversification can be seen as a continuation of the Cambrian Explosion. Nearly no new body forms appeared and the Great Ordovician Biodiversification Event is mainly reflecting a major biodiversity increase at the lower taxonomic level.

Also in this case has there been intense discussion as to what drove this evolution. Again, a continued increase in oxygen level has been suggested and also a continued arms race. Other researchers believe that the surface temperatures in the sea were extremely high in the beginning of the Ordovician Era following which there was a drastic cooling which paved the way for higher evolutionary rates.

A major collision took place in the asteroid belt around 470 million years ago and the Earth was subsequently intensively bombarded with meteorites and large impacts, and some scientist have proposed that this instability could spark rapid evolution. In short, we do not know for sure why this major increase in diversity took place.

The Sepkoski Diagrams show only marine animals. As we will study more about later, land plants started to develop during the Ordovician as well and forests came into existence in the Devonian and further developed during the Carboniferous Period.

Animals invaded land during the Devonian and showed a strong diversification during the Late Paleozoic. Hence plant and animal life diversified much stronger than apparent from the Sepkoski Diagrams showing only marine organisms. Fossils of some of the first terrestrial animals have been found in Greenland. They are of Devonian age.

— Fahim Ferdous Promi

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