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4.2.2 Geological timescale
Daniel Williamson
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1 Geological Timescale
Figure 1
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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:
Geological time:
http://learn.genetics.utah.edu/content/variation/time/
http://www.pbs.org/wgbh/evolution/change/deeptime/index.html
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1.1 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.
Figure 2
1 http://learn.genetics.utah.edu/content/variation/time/
2 http://www.pbs.org/wgbh/evolution/change/deeptime/index.html
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(Ma = million years)
Examine the pie chart and the table, which has the main events of each period
Cambrian period
Explosion of multi-cellular life, many trilobites in seas, modern groups develop
Ordovician and Silurian periods
Invertebrates with shells, rst sh, rst plants with vascular tissue
Devonian period
Arthropods on land, rst trees, many primitive sh, rst amphibians and insects
Carboniferous period
Coal formed, Gondwana is under ice sheets, 1st reptiles develop, many diverse insects
Permian period
Glossopteris
trees in Gondwana, many marine Molluscs, mammal-like reptiles
MESOZOIC
Triassic period
First dinosaurs, rst small mammals develop
Jurassic period
Dinosaurs develop many forms, 1st birds develop, conifers form, ammonites in seas
Cretaceous period
Flowering plants and insects evolve, more dinosaurs develop, placental mammals
CAENOZOIC
Many dierent forms of mammals and birds develop, the earth cools down after widespread heating,
modern animals develop, hominids develop
Cambrian period
Explosion of multi-cellular life, many trilobites in seas, modern groups develop
Ordovician and Silurian periods
Invertebrates with shells, rst sh, rst plants with vascular tissue
Devonian period
Arthropods on land, rst trees, many primitive sh, rst amphibians and insects
Carboniferous period
Coal formed, Gondwana is under ice sheets, 1st reptiles develop, many diverse insects
Permian period
Glossopteris
trees in Gondwana, many marine Molluscs, mammal-like reptiles
MESOZOIC
Triassic period
First dinosaurs, rst small mammals develop
Jurassic period
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Dinosaurs develop many forms, 1st birds develop, conifers form, ammonites in seas
Cretaceous period
Flowering plants and insects evolve, more dinosaurs develop, placental mammals
CAENOZOIC
Many dierent forms of mammals and birds develop, the earth cools down after widespread heating,
modern animals develop, hominids develop
Cambrian period
Explosion of multi-cellular life, many trilobites in seas, modern groups develop
Ordovician and Silurian periods
Invertebrates with shells, rst sh, rst plants with vascular tissue
Devonian period
Arthropods on land, rst trees, many primitive sh, rst amphibians and insects
Carboniferous period
Coal formed, Gondwana is under ice sheets, 1st reptiles develop, many diverse insects
Permian period
Glossopteris
trees in Gondwana, many marine Molluscs, mammal-like reptiles
MESOZOIC
Triassic period
First dinosaurs, rst small mammals develop
Jurassic period
Dinosaurs develop many forms, 1st birds develop, conifers form, ammonites in seas
Cretaceous period
Flowering plants and insects evolve, more dinosaurs develop, placental mammals
CAENOZOIC
Many dierent forms of mammals and birds develop, the earth cools down after widespread heating,
modern animals develop, hominids develop
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PALEOZOIC
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Cambrian period Explosion of multi-cellular life, many trilobites in seas, modern groups develop
Ordovician and Silurian periods Invertebrates with shells, rst sh, rst plants with vascular tis
Devonian period Arthropods on land, rst trees, many primitive sh, rst amphibians and insects
Carboniferous period Coal formed, Gondwana is under ice sheets, 1st reptiles develop, many dive
Permian period
MESOZOIC
Glossopteris
trees in Gondwana, many marine Molluscs, mammal-like reptiles
Triassic period First dinosaurs, rst small mammals develop
Jurassic period Dinosaurs develop many forms, 1st birds develop, conifers form, ammonites in seas
Cretaceous period Flowering plants and insects evolve, more dinosaurs develop, placental mamma
CAENOZOIC
Many dierent forms of mammals and birds develop, the earth cools down after widespread heating, m
Table 1
1.2 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.
Figure 3:
•
Paleozoic
Stromatolites
: Started with an explosion of multicellular life, called the Cambrian Explosion. Marine
trilobites were common. The rst invertebrates and sh are found, later the rst amphibians and
insects. Diverse land plants develop and coal swamps form in certain areas. The rst reptiles form.
This era ended with the massive
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Permian Extinction, wiping out many successful species
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Figure 4:
•
Mesozoic
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Several trilobite fossils
cycads and pine trees).
formed.
dinosaurs
Gymnosperms (cone bearers, like
: The Mesozoic era starts with the Triassic period, which saw the rise of the
as the world's dominant organisms. The dominant plants were
Later, the rst mammals develop and owering plants or Angiosperms are
Birds develop, as well as the rst 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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Figure 5:
•
cycad
Coenozoic :
plants
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Mammals developed further, including primates. Development of birds and owering
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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Figure 6
Figure 7
Two types of owering plants (Angiospermae)
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Australopithecus
Figure 8:
Models of
Figure 9:
Baby mammoth
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, an early hominid
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1.2.1 Critical evaluation
The geological column summarises a commonly accepted evolutionary theory about life's history. As with
most scientic 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.
1.2.2 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 rst fossil of a
missing link between birds and dinosaurs was found, the so-called Archaeopteryx.
have been found to date. Read the information below:
Figure 10
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Around 10 such fossils
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Figure 11
Left is a specimen of
Archaeopteryx
, and right is an artist's impression of the skeleton in an upright
position. Note the very ne feather impressions, including the ight feathers of the wings. Despite its obvious
avian nature,
Archaeopteryx
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has a hand virtually identical to other dinosaurs such as Velociraptor
.
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Figure 12:
Artist's impression of the animal
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Figure 13:
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A mounted archeopteryx model. Note the teeth.
Exercise 1
Use the pictures below and on the previous page to
Archaeopteryx
(Solution on p. 19.)
compare the skeletons of a dinosaur,
and a modern bird. In your answer, give dierences and similarities between Ar-
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chaeopteryx and dinosaurs, and between Archaeopteryx and birds.
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1.2.2.1 Activity: Critical Analysis of
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Archaeopteryx
Some scientists consider Archaeopteryx to be a transitional fossil between dinosaurs and birds.
scientists disagree.
Other
Below are two arguments given by scientists to support their interpretation that Ar-
chaeopteryx 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
(Solution on p. 22.)
a. 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.
b. 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.
c. 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
Ar-
chaeopteryx.
1.2.3 The missing link between sh and amphibians:
In 1938, an East London sherman caught a deep-sea sh he had never seen, so it was taken to the East
London museum, where it was identied as a coelacanth by Professor JLB Smith and his assistant, Marjorie
Courtenay-Latimer. This caused an international uproar, because the sh 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 sh before their eyes! This started a search
for more specimens and 14 years later, another one was caught o 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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Figure 14:
Coelacanth display
Figure 15:
Madge Courtenay-Latimer and the coelacanth
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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 sh 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 sh do. The body is lobe-nned, not ray-nned
like modern sh, although it is not known to walk on the ocean oor. Many scientists had thought that
the coelacanth's ns 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
n structure. Its nostrils are also not blind-ending as in sh; they open into the mouth and can be used for
breathing.
Assignment:
The modern coelacanth-nding throws doubt on an assumption palaeontologists use when reasoning about
what the fossil record says about extinctions.
a.
b.
What is the assumption the modern coelacanth-nding throws doubt on?
How does the modern coelacanth-nding throw doubt on this assumption ?
An extinction event can be recognised by fossils of that type not being found in higher (i.e. younger) rock.
Although no coelacanth fossils are found in rock younger than that assumed to be 100 million years old,
coelacanths are not extinct: they are alive today (extant).
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Solutions to Exercises in this Module
Solution to Exercise (p. 14)
COMPARING THE SKELETONS OF DINOSAURS,
SIMILARITIES:
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)
Archaeopteryx vs Dinosaur:
DIFFERENCES:
1 Long forelimbs, like wings
2 Feathers present
Short forelimbs
No feathers
3 Hand has three claws
Hand has ve claws
4 Furcula / wish bone present No furcula present
Archaeopteryx vs Modern bird
SIMILARITIES:
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
Archaeopteryx vs Modern bird
DIFFERENCES:
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
SIMILARITIES:
1 Jaws have teeth
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Archaeopteryx
& BIRDS:
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2 Hand / arm has claws
3 Long bony tail present
4 Presence of gastralia or dermal ribs (not attached to spine)
Archaeopteryx vs Dinosaur:
DIFFERENCES:
1 Long forelimbs, like wings
2 Feathers present
Short forelimbs
No feathers
3 Hand has three claws
Hand has ve claws
4 Furcula / wish bone present No furcula present
Archaeopteryx vs Modern bird
SIMILARITIES:
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
Archaeopteryx vs Modern bird
DIFFERENCES:
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
SIMILARITIES:
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)
Archaeopteryx vs Dinosaur:
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DIFFERENCES:
1 Long forelimbs, like wings
2 Feathers present
Short forelimbs
No feathers
3 Hand has three claws
Hand has ve claws
4 Furcula / wish bone present No furcula present
Archaeopteryx vs Modern bird
SIMILARITIES:
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
Archaeopteryx vs Modern bird
DIFFERENCES:
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
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Archaeopteryx vs Dinosaur: SIMILARITIES:
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)
Archaeopteryx vs Dinosaur: DIFFERENCES:
1 Long forelimbs, like wings
Short forelimbs
2 Feathers present
No feathers
3 Hand has three claws
Hand has ve claws
4 Furcula / wish bone present
No furcula present
Archaeopteryx vs Modern bird SIMILARITIES:
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
Archaeopteryx vs Modern bird DIFFERENCES:
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
Table 2
Solution to Exercise (p. 16)
a. Data
b. Conclusions
c. Conclusions
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