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Lecture 9

BIOLOGY 1M03 Lecture Notes - Lecture 9: Radiometric Dating, Paleozoic, Cenozoic

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Jon Stone

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BIOLOGY 1M03- Chapter 27: Phylogenies and the History of Life Part 2
Limitations of The Fossil Record
Four limitations to the fossil record are recognized, as biases: habitat, taxonomic,
temporal, and abundance
Habitat bias occurs because organisms that live in areas where sediments actively are
being deposited are more likely to form fossils than are organisms that live in other
Taxonomic bias occurs because some organisms (e.g., those with shells or bones) are
more likely to decay slowly and leave fossil evidence
Temporal bias occurs because more-recent fossils are more common than are ancient
Abundance bias occurs because organisms that are abundant, widespread, and present
on Earth for long time periods leave evidence much more often than do species that are
rare, local, or ephemeral
Paleontologists scientists who study fossils recognize that they are limited to
studying tiny and scattered representatives from the tree of life, yet paleontologists also
know that this is the only way to glimpse what now-extinct life was like
Life’s Tielie
Major events in evolutionary history are marked on the timeline shown in Figure 27.8,
which has been broken into 4 segments
Time intervals were determined on the basis of rock formations or fossilized organisms,
then with radiometric dating
The Precambrian encompasses the Hadean, Archaean, and Proterozoic eons. This period
spans from Earth formation to 542 million years ago (Ma), when most animals appear
In the Precambrian supereon, almost all organisms were unicellular and oxygen levels
were very low
O2levels rose at approximately 2.4 bya and 600 Ma
The interval between 542 Ma and the present is called the Phanerozoic eon and is
divided into three eras: the Paleozoic, the Mesozoic, and the Cenozoic. These eras are
further divided into periods
The Paleozoi old life era covers the interval from 542 to 251 Ma
Many groups including fungi, land animals, and land plants appeared in the Paleozoic
era. This era ends with an obliteration! Almost all multicellular organisms disappear at
the end Permian period
The Mesozoi iddle life eracovers the interval from 251 to 65.5 Ma
Dinosaurs and gymnosperms arose and dominated this era, which ended with the
extinction of the dinosaurs, except birds
The Ceozoi ew life eraincludes the interval from 65.5 Ma to the present
Mammals and angiosperms diversified during this era, which continues to this day.
27.3 Adaptive Radiations
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If rapid speciation in a single lineage is followed by divergence into many different forms
(i.e., a pattern), then a radiation has occurred (i.e.,a process); when evidence for
adaptation is involved, then an adaptive radiation has occurred
Adaptive radiations can be triggered by ecological opportunity and morphological
Soeties, dese groups, ushy rahig diagras, alled star phylogeies or
polytomies can be observed
These star phylogenies represent speciation events that were so rapid that the
branching order cannot be resolved
Ecological Opportunity
Among the most-consistent triggers for adaptive radiations is ecological opportunity,
meaning new resources become available
For example, biologists have documented adaptive radiations into the Anoles lizard
species found on Caribbean islands
On the two islands studied, the same 4 ecological types eventually evolved, because the
islands had similar varieties of habitats. Therefore, similar adaptive radiations took place
independently on the two islands, triggered by the available environment and lacking
Morphological Innovation
Morphological innovation also can trigger adaptive radiations, as was seen in the
Cambrian explosion
Many important diversification events in evolutionary history initiated with the origin of
a key morphological trait that allowed descendants to live in new areas, exploit new
food sources, or move in novel ways
The Cambrian Explosion
The first animals spoges, jellyfish, ad siple wors–appear in the fossil record
approximately 565 Ma, at the end Proterozoic eon
Approximately 50 million years later, animals had diversified into almost all the major
groups living today
This diversification is known as the Cambrian explosion
This period saw what is arguably the most evolutionary change in evolutionary history
The Cambrian explosion is documented by three major assemblages, called the
Doushantuo, Ediacaran, and Burgess Shale fossils
These exceptionally rich deposits before, during, and after the Cambrian explosion make
the fossil record for this event extraordinarily complete
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The Foushantuo Microfossils
The Doushantuo formation in China, which dates to 570 Ma, contains microfossils (tiny
fossil) that researchers have interpreted as sponges and animal embryos in early
developmental stages
These were the first animal types on Earth
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