Evolution and the Fossil Record

The Cambrian and Beyond

The nature of the fossil record

How organic remains fossilize

Four categories of fossils

• defined by method of formation

1. Compression and impression fossils

2. Permineralized fossils

3. Casts and Molds

4. 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 bias1. 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

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)

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)

Please become familiar with the Phanerozoic Eras periods as shown below.

Phanerozoic Eon (544 mya to present)

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 unchangedunchanged 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 the fossil record we perceive it as stasis

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 decadesyears or decades vs. a geological time scale of millions of yearsmillions of years

• On a biological time scale gradual change and natural selection are important (and observable)

• On a geological scale “instantaneous” seeming changes could actually be taking millions of years

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”

• During the Phanerozoic there have been 5 mass extinctions

• Together these account for 4% of all extinctions

1. At the terminal Ordovician 440 mya

2. Late Devonian 365 mya

3. End-Permian 250 mya

4. end Triassic 215 mya

5. 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 particleshave 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 to global

warming

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 atmosphere on top of that

The decline of many groups of organisms was not

instantaneous

• specifically due to disruptions in …• ecological processes • species interactions • biogeochemical cycles• FOR EXAMPLE….

Many extinctions were probably caused by interactions between organisms and the traumatized environment.

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, and

turtles were little affected

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