Cambrian explosion
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Cambrian Explosion. By, Sabrina Perrotta. What is the Cambrian Explosion?. Occurred around 500 to 540 Ma Time when fauna seemed to “Explode” and radiate into many branches Increase in abundance, diversity, and complexity of fauna. Cambrian Explosion Controversy.

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Cambrian Explosion

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Cambrian explosion

Cambrian Explosion

By,

Sabrina Perrotta


What is the cambrian explosion

What is the Cambrian Explosion?

  • Occurred around 500 to 540 Ma

  • Time when fauna seemed to “Explode” and radiate into many branches

  • Increase in abundance, diversity, and complexity of fauna


Cambrian explosion controversy

Cambrian Explosion Controversy

  • When did the explosion exactly occur?

  • Was it a really an “explosion” of diversity or was there a gradual increase that just wasn’t preserved?

  • What was the cause of the explosion?

  • Can the fossil record be read as a timeline, or are there events not seen in the fossil record?

  • Is the fossil record complete?

  • Does rapid diversification track phylogeny?


Fact vs inference

Facts

Lack of late Vendian fossils

Rarity of Precambrian infaunal traces

Species originate from some starting point

Inference

Complete absence of ancestors of modern groups at same time

Absence of coelomatic animals that made them

As groups originate they appear with out delay in the fossil record

Fact vs. Inference


Arthropod evidence

Arthropod Evidence

  • Make up 40% of the Burgess Shale

  • Calcification of cuticle important for fossilization

  • Earliest record of trilobites was found to be geographically differentiated


Arthropod evidence continued

Arthropod evidence continued..

  • Earliest trilobites found in fossil record date to the lower Cambrian

  • Found morphologically the same, structurally different

  • Wide geographic ranges

  • Evidence suggests arthropods must have originated before the “explosion”


Trace fossil evidence

Trace Fossil Evidence

  • Track the behavioral patterns of fauna

  • The do NOT represent body fossil diversity, the DO represent diversity in behavior

  • Show increase in biodiversity by increase in activity after the “explosion”


More on trace fossils

Pre- Explosion

Shallow water environments

Low oxygen concentration in deep oceans

Inadequate supply of organic detritus

Post-Explosion

Deeper water environments

Deeper burrows due to increased competition

Increase in abundance, diversity, and complexity

More on trace fossils…


More on trace fossils1

Shows increase in trace fossils after “explosion”, marking a change in hunting strategies, ability to penetrate substrates, new methods of locomotion

Deep water environments “flooded with trace fossils” due to increase in water depth and increase in biodiversity

Rise in trace fossils coincides with the timing of explosion

More on trace fossils..


Molecular clock evidence

Molecular Clock evidence

  • Study 1 done by Peterson et al 2003

  • Earliest bilaterian fossil=555 Ma, this study used the fact that the vertebrates split from dipterans 900 MA to calibrate molecular clock

  • Purpose of study=to prove there is a pronounced rate difference between vertebrates and dipterans


Study 1 continued

Study 1 continued..

  • Used 7 different amino acid sequences from 23 in group taxa (equaling 11 different invertebrate calibration points)

  • Total RNA from 17 taxa were prepared from live animals

  • Fragments are PCR amplified, cloned, and sequenced


Cambrian explosion

Rate heterogeneity for mouse and chicken = 5% difference / 300Ma

mouse

chicken

fish

fly

mosquito

plants


More of study 1

More of Study 1

  • Results:

    The last common bilaterian arose between 573 and 656 Ma

    Find that molecular rates in vertebrates slow down with increasing time because of gene duplication events increasing the number of interactors for each protein, slowing down the rate of molecular evolution

    Support of the explosion reflecting the diversification of bilaterian phyla


Study 2 of molecular clock evidence wray et al 1996

Study 2 of molecular clock evidence (Wray et al. 1996)

  • Estimated divergence times between metazoan phyla based on tendency for nucleotide and amino acid sequences to diverge

  • Rates of divergence vary with time and taxa, sequences from many taxa can show a mean rate of divergence


Cambrian explosion

7 genes looked at:

ATPase 6, Cytochrome c, Cytochrome oxidase 1, Cytochrome oxidase 2,

alpha hemoglobin,

beta hemoglobin, NADH 1,

and 18S rRNA


More of study 2

More of Study 2


Results of study 2

Results of Study 2

  • Strong correlation between genetic distance and divergence time ( p<.0001)

  • The rate of heterogeneity found proves mineralized skeletons evolved independently in several phyla at different times

  • Metazoans must have begun to diverge before the “explosion”, and there was an extended period of divergence throughout the Proterozoic


Burgess shale evidence

Burgess Shale evidence


Burgess shale continued

Burgess Shale continued..

  • Preservation due to rapid burial from turbidity clouds of sediment that form in an anoxic environment, which becomes conducive for fossil preservation

  • Fauna makes up around 120 genera and 12 major groups


Cambrian explosion

<= arthropods

unknown

Body Plan Diversity

Arthropods

Abundance


Burgess shale continued1

Burgess Shale continued..

  • Shows an increase in trace fossils, but different animals can produce the same trace fossil

  • Most fauna present are benthic, but the few pelagic fauna found suggest a shallow water origin

  • Use the burgess shale to show the diversity of fauna to occur post- explosion


Cambrian substrate revolution

Cambrian Substrate Revolution

  • Before the Cambrian explosion, seafloor sediment had well developed microbial mats with poor vertical orientated bioturbation = suggesting fairly stable, low water content

  • After the Cambrian explosion, seafloor microbial mats become scarce, and there is an increase in vertically orientated bioturbations


Cambrian substrate revolution1

Cambrian substrate revolution

  • Change in substrate due to:

    Development of predation foster evolution of mineralized skeletons

    Seafloor sediment change , with decrease in microbial mats and an increase in homogeneity of oceans, cause an increase in disturbance to benthic organisms causing an increase in bioturbation


Cambrian substrate revolution continued

Cambrian Substrate Revolution continued.

  • Caused..

    Extinctions (of microbial mats)

    Adaptations (animals learn to adapt to new conditions by developing anchoring techniques)

    Environmental Restrictions (certain fauna can only live in shallow water environments)


Controversy

Controversy

Do you believe there truly was an

“explosion” of rapid diversification?

Do you believe there was a gradual

Increase in diversification that occurred

slowly throughout time?


Discussion questions

Discussion Questions ???????

  • What do you think was the trigger for the Cambrian Explosion?

  • When do you think the explosion happened?

  • Do you believe there was an “explosion” of diversity?

  • Do you feel the fossil record is a reliable indicator of the first appearance of phyla?


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