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Note that the time scale above shows the Carboniferous period as represented by the Pennsylvanian and Mississippian separately. This is not always done.
Geological time: interactive site: http://learn.genetics.utah.edu/content/variation/time/
Geological time: http://www.pbs.org/wgbh/evolution/change/deeptime/index.html
The three eras of the geological time scale
The earth’s history can be traced over MANY millions of years, so scientists have developed a geological time scale to help visualize these periods. This vast amount of time is divided into eons, eras and periods for easier reference. You MUST know the names of the three ERAS:
So, for example, the most recent eon is divided into 3 eras, called the
Paleozoic (meaning ancient life)
Mesozoic (meaning middle life)
Caenozoic (meaning recent life)
You don’t have to memorize the periods, only the eras . It is important that you become familiar with these names, so that you can use such information in a test or exam. The end of each era is marked by a series of catastrophic extinctions, which wiped out many of the previously successful species. Examine the diagram below, showing eras & periods.
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(Ma = million years)
Examine the pie chart and the table, which has the main events of each period
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PALEOZOIC |
Cambrian period Explosion of multi-cellular life, many trilobites in seas, modern groups develop |
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Ordovician and Silurian periods Invertebrates with shells, first fish, first plants with vascular tissue |
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Devonian period Arthropods on land, first trees, many primitive fish, first amphibians and insects |
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Carboniferous period Coal formed, Gondwana is under ice sheets, 1st reptiles develop, many diverse insects |
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Permian period Glossopteris trees in Gondwana, many marine Molluscs, mammal-like reptiles |
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MESOZOIC |
Triassic period First dinosaurs, first small mammals develop |
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Jurassic period Dinosaurs develop many forms, 1st birds develop, conifers form, ammonites in seas |
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Cretaceous period Flowering plants and insects evolve, more dinosaurs develop, placental mammals |
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CAENOZOIC |
Many different forms of mammals and birds develop, the earth cools down after widespread heating, modern animals develop, hominids develop |
The following are the major events in each era of the time scale:
- During the Pre-Cambrian time, life in general consisted of bacteria, simple algae and simple unicellular organisms. The best examples of such early life are the stromatolites, large mounds of cyanobacteria in sediment along the continental shelf that oxygenated the early atmosphere and allowed other aerobic life forms to exist.
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Paleozoic
: Started with an “explosion” of multicellular life, called the
Cambrian Explosion. Marine
trilobites
were common. The first invertebrates and fish are found, later the first
amphibians and insects. Diverse land plants develop and coal swamps form in
certain areas. The first reptiles form. This era ended with the
massive
Permian Extinction, wiping out many successful species
Figure 4: Several trilobite fossils 
- Mesozoic : The Mesozoic era starts with the Triassic period, which saw the rise of the dinosaurs as the world’s dominant organisms. The dominant plants were Gymnosperms (cone bearers, like cycads and pine trees). Later, the first mammals develop and flowering plants or Angiosperms are formed. Birds develop, as well as the first placental mammals. This era ends with the Cretaceous Extinction (the so-called KT-boundary), that wiped out the dinosaurs 65 million years ago.
Dinosaur
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- Coenozoic : Mammals developed further, including primates. Development of birds and flowering plants is evident. Global cooling occurs (i.e. the most recent ice ages, which caused a drop in sea levels. This allowed development of land bridges between North America and Asia, also between India and Sri Lanka and between Australia and the islands to its north. These land bridges assisted greatly in the migration of species to new land masses).
Hominid evolution started during the Coenozoic. This will be discussed more fully in Grade 12.
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Two types of flowering plants (Angiospermae)
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Critical evaluation
The geological column summarises a commonly accepted evolutionary theory about life’s history. As with most scientific theories, not all scientists agree about the theory. Criticisms include the following:
- The sequence (order) of fossils given in the geological column is not found anywhere on earth. Instead, there are many places where ‘older’ layers are found on top of ‘younger’ layers.
- There are many gaps in every sequence. To explain this, palaeontologists suggest that about 2/3 of all the kinds of organisms that ever lived could not have left any fossils. (Palaeontologists are scientists who study fossils and try to use then to reconstruct the past.) It is reasonable to expect that many soft-bodied creatures would not have been fossilised. However, it is strange that there are gaps in the fossil records even for creatures with hard shells and strong bones.
- The dating methods used to date rocks might not give valid dates. This is discussed in the section about fossils and dating.
THE “MISSING LINK” BETWEEN DINOSAURS AND BIRDS
It’s been believed for many years that modern birds developed from reptilian ancestors, as dinosaur fossils often showed AVIAN or ‘bird-like’ features. There was thus much excitement when the first fossil of a “missing link” between birds and dinosaurs was found, the so-called Archaeopteryx. Around 10 such fossils have been found to date. Read the information below:
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Left is a specimen of Archaeopteryx , and right is an artist’s impression of the skeleton in an upright position. Note the very fine feather impressions, including the flight feathers of the wings. Despite its obvious avian nature, Archaeopteryx has a hand virtually identical to other dinosaurs such as Velociraptor .
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Exercise 1
Use the pictures below and on the previous page to
compare the skeletons
of a dinosaur,
Archaeopteryx
and a modern bird. In your answer, give
differences and similarities
between
Archaeopteryx
and dinosaurs, and between
Archaeopteryx
and birds.
Solution
COMPARING THE SKELETONS OF DINOSAURS, Archaeopteryx & BIRDS:
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Archaeopteryx vs Dinosaur: SIMILARITIES: |
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| 1 Jaws have teeth | |
| 2 Hand / arm has claws | |
| 3 Long bony tail present | |
| 4 Presence of gastralia or dermal ribs (not attached to spine) | |
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Archaeopteryx vs Dinosaur: DIFFERENCES: |
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| 1 Long forelimbs, like wings | Short forelimbs |
| 2 Feathers present | No feathers |
| 3 Hand has three claws | Hand has five claws |
| 4 Furcula / wish bone present | No furcula present |
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Archaeopteryx vs Modern bird SIMILARITIES: |
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| 1 Feathers are present | |
| 2 Forelimbs are long and wing-like | |
| 3 Furcula / wish bone present (fused clavicles) | |
| 4 Bones of the lower forelimb are separate | |
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Archaeopteryx vs Modern bird DIFFERENCES: |
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| 1 Teeth in jaws | No teeth in the beak |
| 2 Claws on forelimbs | Forelimbs without claws |
| 3 Long bony tail | Short tail bones / pygostral present |
| 4 No breast bone | Breast bone with a keel |
Activity: Critical Analysis of Archaeopteryx
Some scientists consider Archaeopteryx to be a transitional fossil between dinosaurs and birds. Other scientists disagree. Below are two arguments given by scientists to support their interpretation that Archaeopteryx is not a transitional fossil between dinosaurs and birds. For each of these arguments, state whether the scientists are questioning the DATA, or the CONCLUSIONS about the Archaeopteryx fossil.
Exercise 2
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Watkins, Hoyle, Wickramasinghe, Watkins, Rabilizirov and Spetner (1985) published an article where they claimed that the feather impressions of Archaeopteryx were forged. They provide evidence suggesting that chicken feathers were pressed into a cement mix, and these were applied over dinosaur fossils:
Watkins, R.S., Fred Hoyle, N.C. Wickramasinghe, J. Watkins, R. Rabilizirov, and
L.M. Spetner (1985), “
Archaeopteryx: A Photographic Study,” British Journal of Photography, 132:264-266, March 8.
These scientists are questioning the DATA / CONCLUSIONS made by other scientists about Archaeopteryx.
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Some scientists argue that even if the feathers on Archaeopteryx were not forged, there is no evidence that the fossil is transitional between reptiles and birds, since the fossil is compatible with the fossil of a bird.
These scientists are questioning the DATA / CONCLUSIONS made by other scientists about Archaeopteryx.
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Some scientists argue that birds have been found in younger rocks than the rocks
in which Archaeopteryx was found, and so Archaeopteryx cannot be the ancestor to birds.
These scientists are questioning the DATA / CONCLUSIONS made by other scientists about Archaeopteryx.
Solution
- Data
- Conclusions
- Conclusions
The “missing link” between fish and amphibians:
In 1938, an East London fisherman caught a deep-sea fish he had never seen, so it was taken to the East London museum, where it was identified as a coelacanth by Professor JLB Smith and his assistant, Marjorie Courtenay-Latimer. This caused an international uproar, because the fish was previously known only from fossils and had been believed to be extinct. People were amazed that fossil coelacanth found in rock strata 70 –350 million years old could look exactly like the large blue fish before their eyes! This started a search for more specimens and 14 years later, another one was caught off the Comoro Islands in the Indian Ocean, north of Madagascar and more have been found since, including in deep crevices at St. Lucia in northern KZN.
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Scientists agree that the coelacanth is probably the oldest living vertebrate and they are thought to be closely related to the freshwater fish that is seen as the ancestor of all land animals. The coelacanth’s internal organs are in many ways similar to those of amphibians, which are the earliest life forms to have colonized land. Unlike amphibians, its spine is supported by a permanent notochord, a cartilage and oil tube – it never develops a bony vertebral column like other fish do. The body is lobe-finned, not ray-finned like modern fish, although it is not known to “walk” on the ocean floor. Many scientists had thought that the coelacanth’s fins looked like “legs” (it was referred to as ‘old four legs’) and that it was in the process of developing limbs for use on land, but here was a specimen, millions of years later, that still had the same fin structure. Its nostrils are also not blind-ending as in fish; they open into the mouth and can be used for breathing.
Assignment:
The modern coelacanth-finding throws doubt on an assumption palaeontologists use when reasoning about what the fossil record says about extinctions.
- What is the assumption the modern coelacanth-finding throws doubt on?
- How does the modern coelacanth-finding throw doubt on this assumption ?
