The Cambrian Explosion

and Beyond

•Items we will discuss in this section•How Organic Remains Fossilize

•Strengths and Weaknesses of the Fossil Record

•Life Through Time: An Overview

Two things to keep in mind.1. Which part of the organism is

preserved and available for study?

Fossils are very diverse, but there are 5 major categories…..

2. What kinds of habitats produce fossils?

Compression FossilsResult when Organic material is buried in water- or wind-borne sediment before decompositionAs a result of the weight of sand, mud, ash etc. an imprint is left of the structure.This is just like footprints in the mud or leaves on wet concrete.Two-dimensional fossils.Provide information about external surfaces.

Casts and MoldsRemains decay after being buried in sediment

Molds- consist of unfilled spacesCasts-form when new material infiltrates a space, fills it, and hardens into rock.

Preserve information about external and internal surfaces

Permineralized FossilsForm when porous structures are buried in sediments and dissolved minerals precipitate in the pores.

This is just like embedding a tissue in resin before sectioning it

Details of internal structures are preserved

Examples include: fossilized bones and petrified wood

Replacement/ Recrystallization

Form when entire structures are buried in sediments and gradually replaced by other minerals.No details of internal structures are preserved. General information about the 3 dimensional surface is apparent. Sometimes in detail.

Examples include: many shelled species and crinoids.

Unaltered Remains

Preserved in environments that discourage loss from weathering, consumption by animals, and decomposition by bacteria/fungi.Examples : amber, ice, desiccation. 2,000 year old cadavers have been discovered from the iron age. Woolly mammoths with tissues and fur still preservedUnaltered remains represent a small fraction of the fossil record.

Key ingredients:All fossilization processes depend on 3 key features of the specimen

1. Durability – Mostly bones and shells2. Rapid Burial - usually in a water-saturated sediment3. Lack of Oxygen- to discourage decomposition by aerobic

decomposers.

These factors slow decomposition making the specimen more likely to fossilize

This is the reason why most of the fossil record consists of hard structures left in environments such as river deltas, beaches, flood plains etc..

3 types of sampling biasGeographic

o Produced by the tendency for fossils to come from lowland and marine habitats

Taxonomico Marine organisms dominate the fossil record but make up only 10% of extant species o This means that 2/3 of animal phyla living today are underrepresented in the fossil record.

• They lack hard parts that are ideal for fossilizationTemporal

o The Earth’s crust is constantly being recycledo When mountains erode or plates subduct , their fossils go with themo Older rocks are rare while new rocks are common

Studies by Benton and coworkers (2000) o Suggest that older rocks still contain enough fossils to accurately record the order of branching events implied by molecular phylogenies of living groups.

This means that the temporal bias does not prevent us from understanding life’s diversity

The fossil record like any source of Data, has characteristics that limit the types of information that can be retrieved and how broadly the data can be interpreted

Current goals for paleontologists are to recognize the constraints and work within them

• The Ediacaran Fauna– Dated 565-544 mya– None of the fossils found had shells or any other

type of hard parts– Present: sponges, jellyfish, and comb jellies

• These fossilized embryos support the hypothesis that bilaterians evolved prior to the Cambrian Explosion

• The Burgess Shale Fauna– Dated 520-515 mya– Sharply contrast the Precambrian period• Large, complex, and bilateral symmetric forms• Present: arthropods, mollusks, vertebrates, and

echinoderms

• Chordates (pre-vertebrates)– Resemble many of the jawless vertebrates today• Hagfish and lampreys

• Rising oxygen levels in sea water– Due to an increase in photosynthetic algae during

the Proterozoic (Precambrian)– More oxygen makes higher metabolic rates and

larger bodies possible– Larger bodies allow for the evolution of tissues

and higher metabolic rates are required for larger uses of power for increased movement

• Rising levels of atmospheric oxygen– More atmospheric oxygen makes higher metabolic

rates and larger bodies possible– Andrew Knoll and Sean Carroll suggest that a mass

extinction eliminated much of the Ediacaran fauna• This created an opportunity for the smaller organisms to

evolve in response to the change is conditions

• Both hypothesis (oxygen levels and mass extinction) are currently being tested

•Items we will discuss in this sectionAdaptive Radiations

o Ecological Opportunity as a triggero Morphological innovation as a triggero Other Examples: Adaptive Radiations in Land Plants

Stasiso Demonstrating Stasiso Stasis and Speciation in Bryozoanso What is the Relative Frequency of Stasis and Gradualism?o Why Does Stasis Occur?

Occurs when a single or small group of ancestral species rapidly diversifies into a large number of descendants that occupy a wide variety of ecological niches

o I.e. The Galapagos finches and Hawaiian Drosophila Can be seen in a wide array of groups at intervals throughout the history of lifeThere is a prominent pattern

o It is as if the tree of life suddenly sprouts a large number of highly diverse branches

What factors trigger adaptive radiations?Why do only certain lineages diversify broadly and rapidly?

Ecological Opportunity as a TriggerOccurs when a small number of species is suddenly presented with a wide and abundant array of resources, and few competitors

These conditions favor rapid diversification and speciation

Following extinction events rapid diversification occurs

o Extinction of dinosaurs created new opportunities for mammals

Morphological Innovation as a TriggerNot associated with ecological changes

Modifications and elaborations of traits increases success

Occurs when many species occupy the same niche

o Arthropods • Modification of joint limbs to move more efficently and find food

Many new species that appear and then persist for millions of years without apparent change

No burst of speciation

No morphological change

No gradual change over time in response to environmental changes

Niles Eldredge and Stephen Gould1972 broke the Darwin tradition by claiming that stasis is a real pattern in the fossil record and that most morphological changes occur during speciationThis is called the Theory of Punctuated EquilibriumThis has been hotly debated

Which is which?a) Punctuated equilibrium-all

morphological variation occurs at the time of speciation (branching) event

b) Pyletic gradualism-morphology occurs gradually and is unrelated to speciation events. (Darwin’s Theory)

Cambrian Explosion

Debate spurred paleontologist to ask whether stasis is in fact real Does the data support the claim that morphology occurs at

speciation events? Is this seen as the predominant feature of species

histories? Rigorous tests for stasis vs. gradualism are extremely difficult There are certain criteria that must be met for a test to be

acceptable1) The phylogeny of the clade is known, so researchers can

identify which species are ancestral and which descendant2) Ancestral species survive long enough to co-occur with the

new species in the fossil record Each of these are critical however if the second is not fulfilled is impossible

to know if splitting occurred or it was a rapid evolution in the ancestral form without speciation.

Doug Erwin and Robert Anstey (1995) wanted to see how common stasis was. They reviewed a total of 58 studies conducted to test the

theory of punctuated equilibrium spanning a wide variety of taxa and periods

They concluded that “Paleontological evidence overwhelmingly supports a view that….

Speciation is sometimes gradual and sometimes punctuated No one mode characterizes this very complicated process in

the history of life 1/4th of the studies reported gradualism stasis

Of course this led to more questions and theories Is it possible that different types of organisms have distinct

patterns of change through time?

and

Eldredge and Gould’s most prominent claim was “Stasis is Data” In other words lack (……..) is a pattern that needs to be explained

Studies in some species show that no change occurred over millions of years in the fossil record. Why would morphology remain unchanged for so long?

To approach this focus has been directed to living fossils Living fossils are species or clades that show little or no

morphological change over extended periods.

Examples Ginko tree leaves

Current leaves are the same as fossil impressions made 40-mya

Stromatolite-forming bacteria Similar to fossils 1,800 mya

• Horseshoe Crabs- Limulus are identical to fossil species 150 mya• So why have some species remained unchanged while the

radiation of birds, mammals, and flowering plant took place?• Are they changing or are we only seeing the net effect?• Steve Stanley and Xianging Yang (1987)

– Looked at bivalve species that have shown little change over the past 15 million years

– They discovered that the change occurred but that there was little net change within species.

– Many had undergone large fluctuations “zigzag evolution” as they called it.

– Changes tended to fluctuate about a mean value so stasis was perceived as a result

• Though it might appear static, morphology in a lineage may actually fluctuate over time around a long-term average.

• Represent intervals in which 60% of species that were alive went extinct in the span of one million years

While the Big Five are responsible for 4% of all extinctions the other 96% are referred to as Background Extinctions

The K-T Extinction

What killed the dinosaurs?

• The best understood of the Big Five extinction• Evidences include….– Iridium found in the sediments of the Earth’s• Iridium is rare on the earth but highly concentrated in

meteorites

– Also found shocked quartz; either pressurized or melted

– Microtektites spherical or teardrop glass particles associated with impact sites.

• In 1980 conformation of a 180 km crater from an impact on the earth near the Yucatan peninsula of Mexico 65 mya

• It was near a town called Chicxulub

Video

• Vaporization of anhydrite and seawater = influx of enormous amounts of sulfur dioxide and water vapor into the atmosphere– This would form sulfuric acid = acid rain

• Sulfur dioxide scatters solar radiation causing global cooling– Cooling also would have occurred from large

amounts of dust ejected into the atmosphere covering the Earth from solar radiation

• Evidence shows the spread of large fires during the impact period

• Force of impact may have caused massive earthquakes and may have set off volcanoes– Evidence shows the largest magma deposits date

back to the extinction during this period• Impact would have caused an enormous tidal

wave– If asteroid was 10 km wide the wave produced would

have been 4 km high

• Would have effected many marine and terrestrial ecosystems– Estimates claim 60% to 80% became extinct– Early hypotheses stated that the target of

extinction was size selective• Large-bodied organisms suffered most due to their

greater nutrition requirements– Current research shows no correlation between

extinction and body size• Research still goes on!!