Fossils

In this presentation you will: Investigate the extent to which fossils support the theory of

evolution Examine the development of the horse through the fossil record Explore the different methods of fossilization

ClassAct SRS enabled.

Fossils

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In this presentation you will explore the process of fossilization, investigate the fossil history of the horse and consider to what extent fossil evidence supports the theory of evolution.

Fossils are any form of preserved remains thought to come from a living organism. They include imprints and impressions, hard skeletal structures and entire organisms.

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Fossils as Evidence for Evolution?

Fossil evidence is not enough to prove that evolution happened. However, it does support a theory that suggests that living things have become more complex through time.

Older rocks contain very few fossils and these are very simple in nature. Younger rocks contain a lot more fossils and these have more complex structures.

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Fossils Suggest Increasing Complexity

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Many fossils that appear in older rocks do not appear in younger rocks. In evolutionary theory, this is thought to be when species appeared and when they became extinct.

Evidence suggests that both geographic regions and climatic conditions have varied through time. Since organisms adapt to changing environments, these changes may have triggered new species to evolve and other species to become extinct.

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Explaining the Gaps in the Fossil Record

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The various explanations for the gaps in the fossil record include:

• Only a fraction of fossils have been discovered

• Organisms do not fossilize easily

• New species can appear too quickly to leave intermediate fossils records

• Dead organisms decompose quickly or are eaten

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Which of the following offers the most likely scientific explanation for the extinction of a species?

Question

A) It adapted to its environment too quickly

B) It just disappeared

C) It failed to adapt to its environment

D) It literally turned into another species

Extinct

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Which of the following could be used to explain a gap in a fossil record?

Question

A) Organisms sometimes become extinct

B) Organisms do not always adapt to their environments

C) Fossilization sometimes takes place too quickly

D) The conditions for fossilization are not always present

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The Fossil History of the Horse

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The fossil record of the horse is one of the few fossil records that is almost complete.

The fossils come from the North American sedimentary deposits, that is, rock formed by layers of material that accumulated and hardened over time, formed over the last 54 million years.

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The oldest odd toed, hoofed mammals belonged to a genus (a group made up of one or more species) called Hyracotherium, found throughout North America and Europe during the early Eocene (about 54 million years ago).

Hyracotherium

By the beginning of the Oligocene (about 38 million years ago), it was extinct everywhere except North America.

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The Hyracotherium was small, lightly built and adapted for running. The limbs were short and the feet were elongated so that the digits were almost vertical. There were four digits in the forelimbs and three digits in the hindlimb.

Hyracotherium Features

The incisors were small and the molars large, but with few bumps. They were able to chew soft vegetation.

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34

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Forelimb digits

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The probable course of development from Hyracotherium to Equus involved at least 12 genera (groups of several species) and several hundred species.

Hyracotherium to Equus

The major changes seen in the development of the horse over this time were related to locomotion and feeding. It is thought that these changes represent adaptations to changing environmental conditions.

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Hyracotherium to Equus

Eocene. 56,000,000 Years ago

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Heightabout 40 cm

Forelimb

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Hyracotherium to Equus

Oligocene. 38,000,000 Years ago

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3

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Heightup to 2 feet

Forelimb

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Hyracotherium to Equus

Miocene. 26,000,000 Years ago

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3

4

Heightup to 3.2 feet

Forelimb

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Hyracotherium to Equus

Pliocene. 7,000,000 Years ago

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3

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Heightabout 3.2 feet

Forelimb

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Hyracotherium to Equus

Pleistocene. 1,000,000 Years ago

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Heightup to 5.2 feet

Forelimb

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The fossil record of the horse is very important. This is because it shows progressive change based on homologous structures.

Homologous Structures

Homologous structures are structures that are found in different organisms that are believed to have a common evolutionary origin, for example, the forelimb of the horse.

Hyracotherium

Mesohippus

Merychippus

Pliohippus

Equus

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Over the last 54 million years, each species of ‘horse’represents a stage of development that was successful for several hundred years, as demonstrated by fossil records, before becoming extinct.

Stages of Development

When one species became extinct, another closely related species often took over.

Extinct

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Adapting to Changing Climates

Other fossil records suggest changing climatic conditions, indicating that each species was adapted to the conditions around at that time.

The transitions in the development of the horse were not regularly spaced in time, nor is the fossil record complete.

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The Modern Horse

All modern horses are thought to be descended from the genus Pliohippus. The genus Equus arose in North America during the Pleistocene age and migrated into Eurasia (now Europe) and Africa where it gave rise to zebras and asses as well as the modern horse.

The horse became extinct in North America several thousand years ago, at a time which co-incided with the arrival of humans. It was then re-introduced by the Spaniards about 500 years ago.

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3

It what two major areas has the horse developed over the last 54 million years?

Question

A) Locomotion and reproduction

B) Locomotion and feeding

C) Reproduction and feeding

D) Feeding and excretion

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Homologous structures are structures found in different species that are believed to have different evolutionary origins.

Question

Hyracotherium

Mesohippus

Merychippus

Pliohippus

Equus

Answer True or False.

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The Fossilization Process

Fossils are the preserved remains or traces of organisms that were once alive. Fossilization of a whole plant or animal is very rare.

Often, just hard parts become fossilized, like the wood and seeds of plants, and the bones and teeth of animals.

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Preservation of Unaltered Hard Parts

The shells of invertebrates and single-celled organisms, or vertebrate bones and teeth may be preserved unaltered. This is because they are made of calcium, phosphorus or silica salts which can be resistant to decay.

Examples include:

• Bones and teeth of vertebrates

• Shells of clams and snails

• Plant hard woods

• Insect skeletons

• Some types of sponges

• Echinoderms (star fish)

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Preservation of Unaltered Hard Parts

The hard parts of many fossil organisms have beenchemically altered by the addition, removal, orrearrangement of chemical constituents.

Petrifaction (The filling of pores e.g. in wood, shell, or bone by minerals.)

Replacement (The molecule-by-molecule substitution of another mineral e.g. in coral fossils.)

Examples of this include:

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Imprints of Hard Parts in Sediment

Impressions or molds are the imprints of an organism (or part of an organism) in the sediment. A shell buried in sandstone may be leached or dissolved by groundwater, leaving a mold of the shell in the surrounding sandstone. A mold can be made of the inside or the outside of the specimen.

A cast may be produced if a mold is filled with sediment or mineral matter. A cast is a replica of the original.

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Preservation of Unaltered Soft Parts

In rare circumstances, the soft parts of an animal may be preserved. The two most common methods of soft part preservation are freezing and desiccation (drying or mummification).

Soft parts of organisms such as insects or small frogs may be preserved if the organism becomes trapped in pine resin (later altering to amber). Larger animals may become trapped in oily, tar-like asphalt.

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Trace Fossils

Trace fossils are markings in the sediment made by the activities of organisms. They result from the movement of organisms across the sediment surface, or the tunnelling of organisms into the sediment, or the ingestion and excretion of sedimentary materials.

In many cases, tracks of animals are the only record of an organism’s existence. For example, in many places, dinosaur tracks are much more abundant than dinosaur bones.

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Only the hard parts of organisms can be fossilized.

Question

Answer True or False.

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A mold is a replica of the original organism.

Question

Answer True or False.

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Summary

After completing this presentation you should be able to:

Identify how fossils support the theory of evolution

Describe the development of the horse through its fossil record

Describe the different processes of fossilization