evolution & the fossil record
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7/28/2019 Evolution & the Fossil Record
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The fossil record:
• provides direct evidence of
evolution
•shows that lineages change
time
• gives information about
e process o evo u on(modes)
• gives information on the
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rate of evolution
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• Sedimentary Rock – formed by deposition and solidification of
sediments - the only fossil-bearing rocks
• gneous oc - coo e mo en roc orme y ex rus on rom
volcanoes and by upwelling of magma at the edges of crustal
plates
• Metamorphic Rock - formed by alteration of sedimentary or
igneous rock under high pressures and temperatures
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Plate tectonics
•• Convection cells in athenosphere bring magma to the surface in
certain areas – mid-oceanic ridges – which causes sea floor
sprea ng
• Plates move at 5-10 cm per year
• Plates impinging on other plates can cause mountain building
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Measuring geological time
• ra o sotopes ecay exponent a y at measura e rates
• Nt = N0*ert
• Gives rate of decay or ½ life (t ½)
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e.g 40K decays to 40Ar
t ½ = 1.3 * 109 years
Sum of decay product and remaining undecayed atoms gives
= 40 40 = 40 0 , t
In ractice: use t to find r r = ln 1/2 / t
then use r, Nt and N0 to solve for t, the age of the rock.
Or: Or: N 1/ 2
0
n
1ln
t N
t
⎜ ⎟⎝ ⎠=
⎛ ⎞ ( )
01/ 2ln *
ln 2
t
t N
t
⎜ ⎟⎝ ⎠=
2⎝ ⎠
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In the case of K/Ar datin an estimate of the ori inal amount of 40K in the rock can be made by adding the amount present now to
the amount of 40Ar now divided by 0.112. The division by 0.11240.
decays to 40Ar. (The other decay product is 40Ca.)
1/ 240
1ln 1 * *
0.112
Ar t
K t
+⎜ ⎟⎝ ⎠
=
Potential problem : there may have been some of the decay
pro uc n e roc o eg n w – u s s ess e y or decay products like Ar because Ar is a gas and leaves heated
rock
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Sedimentary rocks and the fossil record
• r nc p e o uperpos t on - upper se mentary ayers represent
more recently deposited sediments
The principle of superpositioncan be violated where rocks
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geologic upheavals
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• Stratigraphic column - is not continuous over the surface of the
g o e - se ments were epos te n r e ep so es n erent
regions of the world - provides local “snapshots” of geologic time
• Fossilization is a rare event. Relative to the number of species
that have ever existed, few are likely to have been fossilized • Species with hard body parts are best preserved – shells, plates,
-
represented in the fossil record
• Erosion, weathering, metamorphic processes have made theoss recor more ncomp e e - o er me per o s are ess we
represented than newer time periods
• Some organisms were common in specific time periods, and
fossilized well - “index fossils” – accurate ages of rocks rich inan index fossil provide a time when that species was common. So
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estimate of the age of the sediment and other associated fossils
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Paleozoic era (540 mya) - divided into 6 periods
– ,arose rapidly during this time, first vertebrates (agnathans)
• Ordovician – 500 mya – diversification of many animal phyla
- end marked by mass extinction
• Silurian – 440 mya – many agnathans, first jawed fishes
, ,
vascular plants
• Devonian – 410 mya – “Age of fishes” - great diversification of fish types, including sharks, bony fishes, first amphibians, first
ferns, first seed plants, mass extinction at end of period
• – -
plants, etc. (plant remains from this period produced many coal
deposits), first winged insects, and first reptiles
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• erm an – mya - ur er vers ca on – rs mamma - e
reptiles – mass extinction of most marine life at end of period
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Mesozoic era – (250 mya) - divided into 3 periods
• r ass c – rst cont nenta separat on – rea up o Pangea –
diversification of both marine and terrestrial forms – including
first dinosaurs and mammals
• Jurassic – 200 mya - “Age of dinosaurs” – diversification of many reptile groups, first birds, mammals diversify but most
species are small, gymnosperms become dominant plant life
• Cretaceous – 145 mya - complete continental separation,cont nue vers cat on o nosaurs, r s, an mamma s,
increased diversity of flowering plants, - end marked by a
massive extinction – (called “Cretaceous-Tertiary boundary”)
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Cenozoic era – 65 m a – with two eriods• Tertiary – 65 mya - continents approached modern positions
mammals diversified and filled niches previously filled by
reptiles, diverse flowering plants and pollinating insects,
diversification of teleost (spiny finned) fishes
• uaternary – 2 mya – repeate g ac at ons, ext nct on o many
large mammals, evolution of modern humans, agriculture
The Tertiary Period is divided into 5 Epochs: Paleocene, Eocene,
Oligocene, Miocene, and Pliocene.
The Quaternary Period is divided into 2 Epochs: Pleistocene and Holocene. We are currently in the Holocene Epoch.
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The fossil record is necessarily incomplete but there are many
things that can still be concluded about evolution
Gradual evolution has been demonstrated in man rou s where
geologic strata have been deposited regularly - eg. Stickleback skeletal features show gradual evolution in multiple
c arac er s cs a eren ra es an mes
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Gradual evolution is well documented in marine organisms
because sediments are regularly deposited along shorelines,
river mouths, and deep basins.
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well documented in
horseshoe crabs.
Horseshoe crabs areoften called “living
oss s ecause t e
lineage dates to the
Cambrian.
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Macroevolution - the origin of higher taxa over long periods of
geologic time is demonstrated in the fossil record
• The evolution of tetrapods from fishes
• The evolution of birds as a lineage of dinosaurs• The evolution of mammals from re tiles
• The evolution of cetaceans from terrestrial mammals
• -
n sp e o e ncomp e eness o e oss recor , e pa ern oappearance of lineages in the fossil record largely matches the
phylogenetic sequences estimated independently.
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Rhipidistian fishes appeared in the early Devonian (408 mya), had
a comp ex o nte s u w t many ones, teet on severa onesin the jaws, lateral line canals on the head, internal and external
nostrils, lobed fins with bony supports, and respired with both
gills and lungs
Eusthenopteron
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The first amphibians appeared in the late Devonian (380 mya).
The had a com lex skull similar to rhi idistians, with teeth onthe same bones. They had internal and external nostrils and
lateral line canals on the head. They respired with both lungs and
, .
appendages, stronger pectoral and pelvic girdle and increased sizeof bones in limbs
Ichthyostega
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Tiktallik is one of several intermediate forms
known between Rhipidistians and
Amphibians. It had gills, lungs, lobed fins,
and a neck.
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The link between dinosaurs and birds is the most famous link -
“ ”- -late Jurassic - when dinosaurs were common.
It had many reptilian features -
a long tail with many
vertebrae, teeth in its jaws,
.
closely resembled a theropod
dinosaur.
Its body was
well feathered
and it appeared to have flight
feathers on its
forelimbs.
hoatzin
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The fossil record of theropods suggests that feathers evolved before
, .“preadaptation” - a feature that evolves for one purpose but is later
used for another purpose.
Synapomorphies:
hollow long bones
crescent shaped wrist bone
expanded breastbone
small feathersvaned feathers
sickle-shaped claw on foot
opposable hind toe
s ort takeeled breastbone and no
teeth
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The evolution of modern mammals from a reptilian ancestor is
- Carboniferous and extending through the early Jurassic the
progression of intermediate forms is clear.
Synapsid reptiles were an earlygroup of reptiles characterized by a
temporal fenestra on each side of the
skull. This character is retained in
The transition from synapsid reptiles
to mammals involved several
changes in different characteristics atdifferent times.
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Teeth became more specialized in
form and function.
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The size of the brain increased.The legs moved to a position
under the body.
The jaws became simpler:
eventually became the entire
lower jawthe jaw joint simplified from a
articulation with the quadrate to
squamosal
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cusps
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Former jaw bones became the two of the three bones of the
mammalian inner ear.
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The evolution of each of these characteristics is
documented in
,
semi-aquatic, and fullyaquatic forms.
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Loss of rear limbs,
stiffening of forelimbs,
increased flexibility of
the vertebral column are
all apparent inintermediate forms.
Ichthyosaurs - descended
from a terrestrial reptile -
many cetacean
characteristics long
e ore e ce aceanlineage began - a case of
convergent and parallel
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evolution in multiple
characteristics.
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The fossil record records many instances of “evolutionary trends”.
– characteristics of intermediate forms (“missing link”) but also have
features that the intermediate ancestor shouldn’t have had. Thus
ey pro a y represen s e ranc es a re a ne c arac er s cs o
an ancestor that was a link to other forms but had also evolved newcharacteristics since diverging from the common ancestor - overall
several trends are still clear.
Evolutionary trends in horses
• • Teeth : evolution of complex ridges of enamel (lophs) with change
in diet from leaves (browsers) to grasses (grazers)
• aws: e onga on - w ncrease space e ween nc sors an
molars and shift in position of molars toward the front
• Leg length and body size: increased – associated with change in
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habitat from forests to plains
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Although gradual evolution is documented in the fossil record,
many neages ave st nct gaps. e gaps ave een nterpreteas due to the incomplete nature of the fossil record. The gaps have
also been interpreted as being due to a phenomenon called
“punctuated equilibria.”
,
where species change little for long periods of time (evolutionary
stasis or equilibrium) and then appear to change very rapidly to anew orm punc ua on .
Their model proposes that small populations of a species evolve to
a new form (allopatrically) without leaving any fossils and then thenew form migrates to the range of the parent species where it
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.
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The same data can often
be inter reted b radual
and punctuated models.
Eldridge and Gould’s
mo e st ncorporates a
series of small, but veryra id evolutionar
changes.
Toda Eldrid e has associated punctuations with periods of
rapid ecological change, such as
extinctions.
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Rates of evolution vary greatly in fossil record - generally high
rates o evo ut on are seen w en a new neage rst comes ntoexistence - there is often considerable evolutionary
“experimentation” following origin of a new type of organism
Rates of evolution appear to be slower in earlier periods. Thismay be a bias due to fewer intermediate representatives in
older strata and less clear lines of descent
Rates can vary greatly - measurements of rates in livingorgan sms can e muc g er an a seen n e oss
record - rate estimates from fossils are likely to be
underestimates due to an inability to measure rates of evolution
over short time spans using stratigraphic layers
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