evolution & the fossil record

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



• 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



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



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



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.



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



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.



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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.



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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evolution & the fossil record

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