Transcript Evolution and the Fossil Record The Cambrian and Beyond

Evolution and the Fossil
Record
The Cambrian and Beyond
The nature of the fossil record
How organic remains fossilize
Four categories of fossils
•
1.
2.
3.
4.
defined by method of formation
Compression and impression fossils
Permineralized fossils
Casts and Molds
Unaltered Remains
Compression and Impression
fossils
• Made when organic material is buried in
water or wind-borne sediment before it
decomposes
• The weight of the sediment causes the
structure to leave an impression in the
material it is resting on
• Analogous to footprints in mud or leaves
in wet concrete
• Fig 17.1
Permineralized fossils
• Form when structures are buried in
sediments and dissolved minerals
precipitate in the cells
• Can preserve details of internal structure
• Fig 17.2
Casts and molds
• Molds are unfilled spaces left behind as
organic material decays or dissolves away
• Casts are made when the molds are filled
in with new material which then hardens
into rock
• Provide information about external and
internal surfaces.
• Fig 17.3
Unaltered remains
• mummified remains that are protected
from weathering, animals and
decomposition by bacteria and fungi
• Found in peat bogs, permafrost very dry
desiccating environments (dessert caves).
Preserved in plant resins (amber) Fig 17.4
• Saturated tar sands
Trace Fossils
• Basically these are signs left behind by
living organisms rather than parts of the
organisms themselves
• Includes tracks, burrows, fecal material
• Can be used to get a general idea of the
type of life in some areas
Features of Objects Which
Fossilize
• Durable
• Buried before or shortly after death (usually in
water-saturated sediment)
• Located in areas devoid of oxygen
• Therefore……
Most fossils are of hard materials left in areas
of deposition such as river deltas, flood plains,
marshes, beaches, ocean bottoms and river
beds
• There is an abundant fossil record of organisms
that normally burrow in sediments, such as
bivalves
Strengths and Weaknesses of
the fossil record
•
•
Bias - a potential weakness
3 types of sampling bias
1. GEOGRAPHIC BIAS
2. TAXONOMIC BIAS
3. TEMPORAL BIAS
GEOGRAPHIC BIAS
• Most fossils come from lowland and
marine habitats where the conditions for
fossilization are most prevalent
TAXONOMIC BIAS
• Marine fossils dominate the fossil record
but only 10% of extant species are marine
• 2/3 of extant animal species have no hard
parts which would lend themselves to
being easily fossilized
• Critical parts of plants, like flowers, are
seldom fossilized
TEMPORAL BIAS
• Old rocks are more rare than new rocks
because when tectonic plates subduct
or mountains erode they take their
fossils with them
• Therefore our sampling of ancient life
forms is poor
Biases must be accounted for
• Therefore….
• Paleontologists need to be aware of
limitations in what the fossil record can
tell us
• We need to remember that bias is not,
however, unique to paleontology
• There are many other areas of research
which are biased
DEVELOPMENTAL GENETICS
• can work with only a few model systems
which by no means represent all living
groups
• Examples are roundworms, fruit flies, and
zebra fish for animals
• E. coli and Saccharomyce cerevisieae are
models that are used for molecular and
cell biology
• Ecology focuses on the upland havitats in
North America and Europe.
The Geologic time scale
a look at life through time
Geologic time scale
• Is divided into Eons, Eras, Periods, Epochs,
and Stages
• First formulated as a relative time scale in the
early 1800’s
• Absolute times were added later as more
accurate dating techniques were developed
• The time scale is constantly being refined as
more rocks are sampled and dating
techniques get more sophisticated
Please become familiar with the Phanerozoic Eras periods as
shown below.
Cenozoic Era
(65 mya to today)
Quaternary (1.8 mya to today)
Holocene (11,000 years to today)
Pleistocene (1.8 mya to 11,000 yrs)
Tertiary (65 to 1.8 mya)
Pliocene (5 to 1.8 mya)
Miocene (23 to 5 mya)
Oligocene (38 to 23 mya)
Eocene (54 to 38 mya)
Paleocene (65 to 54 mya)
Phanerozoic Eon
(544 mya to present)
Mesozoic Era
(245 to 65 mya)
Cretaceous (146 to 65 mya)
Jurassic (208 to 146 mya)
Triassic (245 to 208 mya)
Paleozoic Era
(544 to 245 mya)
Permian (286 to 245 mya)
Carboniferous (360 to 286 mya)
Pennsylvanian (325 to 286 mya)
Mississippian (360 to 325 mya)
Devonian (410 to 360 mya)
Silurian (440 to 410 mya)
Ordovician (505 to 440 mya)
Cambrian (544 to 505 mya)
Tommotian (530 to 527 mya)
Entire timeline
The Cambrian “Explosion”
• Called such because almost all of the
currently recognized animal phyla first make
their appearance in the fossil record in the
Cambrian
• The Cambrian spanned “just” 40 million years
• When the fossil record is scrutinized closely, it
turns out that the fastest growth in the
number of major new animal groups took
place during the Tommotian
Important fossil records
• EDIACARAN SHALE
• BURGESS SHALE
• CHENGJIANG BIOTA
EDIACARAN Fauna
• South Australia
• first fossil evidence of multicellular animals
• Pre-Cambrian - 565 mya, late Proterozoic
(Vendian)
• mostly compression and impression
• entirely soft-bodied examples, sponges,
jellyfish etc
• many are trace fossils
BURGESS SHALE
• Slightly younger than Ediacaran shale, 520-515
mya
• British Columbia
• Primarily impression and compression
• Have extraordinary detail
• Wide variety of arthropods (including trilobites),
segmented worms, molluscs, several
chordates, including jawless vertebrates
Not much overlap between the two
except for a few Cnidarians (sea pens)
• Therefore it appears
from these important
fossil records that
there was an
“explosion” of animals
in the Cambrian.
CHENGJIANG BIOTA
•
•
•
•
•
From Yunnan Province in China
veryimportant area
recently made accessible again
very rich in fossils
Found Zygotes and blastocyst that
indicate bilateral symmetry
Was there really a Cambrian
“Explosion” ?
• EVIDENCE FROM MOLECULAR
CLOCKS
• Using molecular clock data from DNA and
protein sequences estimates have been
made on the order of branching in the
animal phylogeny Fig 17.12
Cambrian Explosion
• Estimates show that the
earliest branches
occurred somewhere
between 1200 and 900
mya
• This is hundreds of
millions of years before
they are found in any
fossil record
• This is a highly
controversial area and
implies a long history of
animal evolution for which
we have no fossil record
Evidence From Proterozoic
Rock
• If these projections are correct we should
eventually find fossils of these animals in
the Proterozoic rock
• Some jawless fishes (vertebrates) have
been found in China in the Chengjiang
fauna that are 530 million years old
• This would be indirect evidence that
chordates arose much earlier than this
TAKE HOME MESSAGE?
• The Cambrian explosion is an explosion of
morphological forms but not necessarily of
lineages
• The evolution of these lineages may have been
occurring gradually during the Proterozoic but
existed as small and larva-like organisms which
left no fossils
• However, there is still no explanation for the
dramatic changes in body size in the brief period
of the Cambrian where these fossils are found
What caused the Cambrian
“Explosion”?
• Changes in the ecology of the earth most
likely led to these changes.
ECOLOGICAL CHANGES
• Organisms were filling new niches due to
changes in
• FEEDING BEHAVIORS
– FROM: predominantly either sessile (attached) filter
feeding organisms or those floating high in the water
column living off of plankton
– TO: to a huge variety of feeding mechanisms
• LOCOMOTION.
– FROM:Sessile or free floating organisms
– TO: swimmers, walking, burrowing, both benthic and
pelagic predators, scavengers and on and on
What Factors Led to These
Changes
• Locomotion changes?
– Rising O2 levels 
– allowed larger body size 
– allows evolution of tissues and higher metabolic rates
needed for powered movement
• Shells formation?
– Probably as a result of predator selection pressure
– Have found shells that have holes drilled by predators
– Evidence from the types of holes drilled that predators
were selecting their prey by size.
What other ecological interactions may
have led to selection pressures?
• New types of food such as diversification
in the plankton, may have favored novel
feeding mechanisms
• Anatomy that favors swimming or grasping
(for example) may have been favored as a
way to obtain prey
All of these changes require
• genetic variation to be present
• Would require changes in the genes that
control embryonic development
Macroevolutionary patterns
• An important part of evolutionary research
is looking for broad patterns in the fossil
record
• Can give insight into how macroevolution
may occur
• A common pattern seen in the fossil
record is Adaptive Radiation
Adaptive Radiation
• A single ancestral species diversifies into a
large number of species which occupy a
wide variety of ecological niches
• Where have we seen and talked about
examples of adaptive radiation?
• Darwin’s Finches
• Hawaiian drosophilids
Factors That Trigger Radiation of
Species
• What factors were
responsible for the
radiation in the finches?
• ecological opportunity
– Colonized a habitat that
had few competitors and
wide variety of resourcese
• Leads to morphological
innovations like the beak
types
Ecological Opportunities
• Ecological opportunity
is not created solely
through colonization
events.
• Mass extinction
– Mammals diversified
rapidly after the
dinosaurs became
extinct.
Adaptive radiation
• Morphological innovations lead to
radiation.
• Example: arthropods ( insects, spiders,
crustaceans).
–
–
inhabit a wide variety of niches based on
modification of their jointed limbs
swimming, flying, running, jumping,
grasping, walking
Examples from plants
• as plants moved from
aquatic to terrestrial
habitat in the early
Devonian ( 400mya)
• developed leaves and
vascular tissue
Explosion of flowering plants in
the Cretaceous
• The flower structure allowed a rapid
expansion into new niches
• Pollination strategies
• including co-evolution with insects
• dispersal mechanisms for seeds
Stasis vs. Gradualism
GRADUALISM
• The Darwinian approach
• In this pattern organisms are continually
changing gradually form one from to
another.
• Occurs by a progressive accumulation of
micromutations which leads to the
formation of a new species.
STASIS
• New morphologies appear in the fossil record
and then remain unchanged for millions of
years
• Often, evolutionary innovations appear at the
same time as new species
• This results in morphological evolution that
consists of long periods of no change (stasis)
occasionally punctuated by speciation events
that appear instantly in the geologic record
Gradual changes are rarely
seen in the geologic record
• What do you suppose Darwin would say
when confronted with today’s fossil
record?
• He predicted that gaps in the fossil record
would be filled in over time, with gradual
transitions
WHY STASIS
• Possibly a lack of genetic variation to work
on
– There is strong evidence that lack of genetic
variation is not the cause of stasis
– One living fossil, the horseshoe crab does not
have any less genetic variation than groups
that have evolved significantly
• May be in dynamic stasis. Think of the
finches and how they change with drought
vs. flood years. there is an oscillation back
and forth, fluctuating about a mean, but in
Theory of Punctuated Equilibrium
• Proposed in 1972 by Eldredge and Gould (we
will be reading this paper later.)
• Led to some very heated debates for over 20
years
• Debate revolved around differences in
observing patterns of speciation and change on
a biological time scale of years or decades vs.
a geological time scale of millions of years
• On a biological time scale gradual change and
natural selection are important (and
observable)
EXTINCTION
•
•
•
•
The ultimate fate for all species
Several clear patterns of extinction
Global extinction rates are not constant
Two basic categories of extinctions
– Mass extinctions
– Background extinctions
To be a mass extinction
requires
1. A broad range of organisms being
affected
2. Global extinction
3. Rapid relative to the expected life span
of the taxa that are lost
4. A mass extinction leads to the loss of
over 60% of the species in a period of
a million years
“ The Big Five”
•
•
1.
2.
3.
4.
5.
During the Phanerozoic there have been
5 mass extinctions
Together these account for 4% of all
extinctions
At the terminal Ordovician 440 mya
Late Devonian 365 mya
End-Permian 250 mya
end Triassic 215 mya
Cretaceous-Tertiary (K-T) 65 mya
Background Extinctions
• occurred at constant rates
• make up 96% of all extinctions
• The likelihood of subclades becoming extinct is constant
and independent on how long the taxa have been in
existence
– The probability of a subgroup becoming extinct is
constant over the lifespan of the larger clade
– Rates of extinction are constant within clades but
highly variable across clades
• The extinction rate of marine organisms vary depending
on how far the larvae disperse after the egg is fertilized
– Greater distance leads to greater colonizing ability which might
reduce extinction rate
The K-T Extinction
•
•
Cause is purported to be impact from a
huge meteorite
EVIDENCE
1. iridium sediments
2. unusual elements
1. shocked quartz particles
2. microtektites
Iridium sediments laid down
at the K-T boundary
• Iridium is rare on earth but abundant in
extra-terrestrial objects
• The amount of iridium found from 95
different samples taken in various K-T
boundary sites indicates a likely 10 km
wide meteorite impact
Two unusual elements found
in K-T boundary layers
1. shocked quartz particles
have only been found at the site of
meteorite impact craters
quartz grains that have parallel planes
called lamellae. The deformation is
thought to be due to the shock of impact,
thus “ shocked quartz”
Unusual elements (cont)
2. Microtektites
– tiny glass particles which may be
composed of a variety of source rock
types but all of which originate as
grains melted by the heat of impact
– May be melted in place or ejected by
the impact
– If ejected will take on a teardrop or
dumbbell shape as a result of
solidifying in flight
Locating the crater
• abundant shocked quartz and
microtektites were found in Haiti and
throughout the rest of the Caribbean
• Then in early 1900s evidence from
magnetic and gravitational anomalies
confirmed the existence of a crater in the
Yucatan peninsula. Fig 17.26
• The impact of the meteorite is now
accepted universally but the actual
consequences of the impact are still in
doubt
HOW COULD THE METEORITE KILL?
what are the possible consequences of
such an impact?
1. Vaporization of anhydrite and
seawater cause an influx of SO2 and
water vapor to the atmosphere
– this leads to acid rain
– and scatter of solar radiation which could
cause global cooling
2. Dust-sized particles in the atmosphere
could compound the cooling as well,
by blocking incoming solar radiation
What are the possible consequences of
such an impact (cont)
3. Widespread wildfires are suggested by
soot deposits at many K-T sites
4. The soot could increase smog and
increase cooling
5. Massive earthquakes may have been
triggered
6. Volcanoes
– If volcanoes put ash and sulfur dioxide into
the atmosphere could have caused global
cooling while increase in CO2 may have led
What are the possible consequences of
such an impact (cont)
7. There is much evidence that there was an
enormous tidal wave or tsunami caused by
the impact. Perhaps as high as 4 km
• Sandstone deposits in several areas and
along a 300km strip are interpreted as typical
of what a tsunami might do
• Evidence that an initial splash of
microtektites formed a layer which was then
covered by tsunami-induced deposits and
then a final layer of iridium enriched
particulates that settled down out of the
The decline of many groups of
organisms was not
instantaneous
Many extinctions were probably caused by
interactions between organisms and the
traumatized environment.
•
•
•
•
•
specifically due to disruptions in …
ecological processes
species interactions
biogeochemical cycles
FOR EXAMPLE….
EFFECT ON THE OCEANS
• primary productivity of phytoplankton would
be dramatically reduced
• Local temperature and chemical gradients in
the water would be disrupted
• This would lead to the decimation of marine
and terrestrial biota in the time frame
immediately after the impact
• The decline of many was not immediate and
was drawn out over 500,000 years
Some important characteristics of the
K_T extinction
• 60% TO 80% of all species became
extinct at the end of the Cretaceous
• Losses were not distributed evenly
• Dinosaurs and pterosaurs were wiped
out
• Large bodied mammals disappeared
• Only one order of birds, a shorebird,
survived
• Amphibians, crocodilians, mammals,
Characteristics of K-T extinction(cont)
• Insects virtually unscathed
• Some marine invertebrates were
obliterated
• marine plankton became very scarce
• In North America 35% of land plants were
lost
• Forest communities were replaced by
ferns
Differential survival has not yet
been adequately explained.
• One interesting pattern is emerging
• losses are much more severe in North
America Perhaps because these species
were in the splash zone when heated
material was sent to the north and west
from the impact site
• One outstanding pattern shows that for
bivalves and gastropods, genera with wide
geographic ranges were less likely to get
wiped out than those with narrower ranges
FINI
originally thought to be the impressions of annelid worms
(earthworms), is now interpreted as the feeding traces of
trilobites.
Feed trails
Dwelling traces