Cambrian Explosion

1. Cambrian Explosion By, Sabrina Perrotta

2. 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

3. 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?

4. 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

5. 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

7. 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”

8. 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”

10. 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…

11. 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..

12. 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

13. 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

14. Rate heterogeneity for mouse and chicken = 5% difference / 300Ma mouse chicken fish fly mosquito plants

15. 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

16. 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

17. 7 genes looked at: ATPase 6, Cytochrome c, Cytochrome oxidase 1, Cytochrome oxidase 2, alpha hemoglobin, beta hemoglobin, NADH 1, and 18S rRNA

18. More of Study 2

20. 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

21. Burgess Shale evidence

22. 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

23. <= arthropods unknown Body Plan Diversity Arthropods Abundance

24. 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

25. 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

27. 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

28. 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)

29. 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?

30. 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?