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Chapter 14:
Geologic Time
How do we divide geologic time?
Modern time is divided into millennia.
Millennia are divided into centuries.
Centuries are divided into decades.
Decades are divided into years.
Years are divided into months.
Months are divided into weeks.
Weeks are divided into days.
Days are divided into hours, etc.
Geological Time Scale
• Geologic time covers much more time
than the modern calendar.
• Paleontologists have divided Earth’s
history into time units based on the lifeforms that lived during certain periods.
• We start with Eons, which are the
longest divisions of time.
• Eons are then divided into eras.
• Eras are decided by worldwide changes
in the types of fossils present.
• For example, at the end of the
Mesozoic Era many invertebrates
(animals without backbones) became
• Eras are then divided into periods.
• Periods are marked by significant
changes in the fossil record.
• This can include massive expansion of
life or mass extinctions.
• Periods are then divided into epochs.
• Smaller than periods
• Characterized by differences in life
How does evolution relate to the
divisions of the geologic time
Organic Evolution
• The fossil record shows that species have
changed over geologic time.
• Environmental changes can affect an
organism’s survival.
• Those that don’t adapt to changes are less
likely to survive.
• Species: a group of organisms that normally
reproduces only with other members of their
• Offspring of 2 different species normally can
not reproduce (sterile).
Natural Selection
• Charles Darwin proposed the theory of natural
– It states that organisms with characteristics
that are suited to a certain environment have
a better chance of surviving and reproducing
than organisms that do not have these
• He sailed around the world collecting
specimens and data.
– He got some of his best data in the
Galapagos Islands to the west of South
Natural Selection
• He saw that organisms compete with each
other for resources (food and living space),
and must adapt when there is not enough to
go around.
• He saw that organisms in the same species
can show variations that could help or hurt
the organism’s chance of surviving.
Natural Selection Within a Species
Artificial Selection
• Human beings have altered the evolution of
some animals by breeding only animals
with desired traits.
– Cats
– Dogs
– Horses
– Cattle
– Any domesticated animal
• Organisms suited for their environment live
longer and have a better chance of
producing offspring.
• Organisms poorly adapted to an environment
produce few or no offspring.
• A new characteristic becomes common in a
species only if:
– some members already possess that
– the new trait increases the animal’s chance
of survival.
Geologic Time Scale
How do trilobites help explain
• Structure of the hard outer skeleton or
• 3 lobes that run the length of the body
• Body made up of
– Head-cephalon
– Segmented middle-thorax
– Tail-pygidium
Trilobite Timescale
A Variety of Trilobites
AGNOSTIDA - Among the early trilobites, with a basic, clamshelllike appearance.
Suborders Agnostina and Eodiscina.
Representative species pictured here: Ptychagnostus akanthodes
REDLICHIIDA - Including the most primitive trilobites from the lower
Suborders Olenellina and Redlichiina.
Representative species pictured here: Redlichia sp. (Redlichiina)
CORYNEXOCHIDA - An often spiny group united by a shared
hypostomal attachment.
Suborders Corynexochina, Illaenina, and Leiostegiina.
Representative species pictured here: Kootenia sp.
And More . . .
ODONTOPLEURIDA - Very spiny trilobites, a sister group to the
Suborder Odontopleurina; superfamilies Dameselloidea and
Representative species pictured here: Selenopeltis buchii.
LICHIDA - Some of the most ornately sculptured species fall into
this group.
Suborder Lichina; families Lichidae and Lichakephalidae.
Representative species pictured here: Arctinurus boltoni
PHACOPIDA- The well-known Phacops, with its beautiful
compound eyes belongs here.
Suborders Calymenina, Phacopina, and Cheirurina.
Representative species pictured here: Phacops sp.(Phacopina)
And more . . .
PROETIDA - Includes some of the last trilobite species before
the Permian Extinction.
Suborder Proetina, with three Superfamilies.
Representative species pictured here: Proetus granulosus
ASAPHIDA - All share a ventral median suture, and most a
similar development.
Suborder Asaphina, with six Superfamilies comprising ~20% of
all trilobites.
Representative species pictured here: Homotelus sp.
PTYCHOPARIIDA - Bearing the "generic trilobite" body plan,
but many weird variations!
Suborders Ptychopariina and Olenina (Harpina has been
elevated to order Harpetida; see below)
Representative species pictured here: Modocia sp.
And would you believe, more?
HARPETIDA - Bearing the distinctive, broad, often
intricately pitted, cephalic fringe.
In 2002, split out of the Ptychopariida and elevated from
suborder to full order.
Representative species pictured here: Eoharpes sp.
NEKTASPIDA - The so called "soft-shelled trilobites" such
as Naraoia have been classified as an order of trilobites by
some. Click on the image or link to learn more about them,
and to see how they are handled in the 1997 Treatise.
• Trilobites lived in Earth’s oceans for more than
200 million years.
• During Paleozoic Era, some species of trilobites
became extinct and new ones evolved.
• Trilobites showed different characteristics during
the different periods of the Paleozoic Era.
• Trilobite eyes changed over time and tell us about
where they lived in the ocean.
• Trilobite bodies-changed over time as well.
How has Plate Tectonics affected species?
• Earth’s moving plates caused continents to
collide and separate many times.
• Collisions formed mountains and trapped seas
between shifting continents.
• By the end of the Paleozoic Era, the separation
of Pangaea caused wider and deeper seas
between the continents.
• Trilobites environments were changed or
• This is one theory for the extinction of trilobites.