What the deep sea tells us about sampling biases in the fossil record

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What the deep sea tells us about sampling biases in the fossil record

  1. 1. What the deep sea tells us about sampling biases in the fossil record Graeme T. Lloyd Department of Palaeontology, Natural History Museum, London, UK
  2. 2. CollaboratorsAndrew Jeremy Paul Smith Young Pearson
  3. 3. Talk Outline• Introduction• Deep sea record of Coccolithophores and planktic forams – Deep sea rock and fossil records – Correlations and modelling – Sampling-corrected richness• Deep sea vs. land-based record of Coccolithophores – Deep sea vs. land rock and fossil records – Correlations and modelling – Sampling-corrected richness: common signal?• Deep sea coccolithophore species-per-genus patterns – An unusual result! – Potential explanation(s) – Separating signals• Conclusion
  4. 4. The fossil record is our only empirical record of the history of life
  5. 5. Land-based rock and fossil records show strong correlation… N Maps Generic diversity
  6. 6. …but what about the deep sea?•Most microfossil groups are highly cosmopolitan…•…and massively abundant (1000s specimens per gram)•Many remarkably continuous sections (>10 million years)•Phylogenies often incorporate ancestors•Well studied (DSDP/ODP/IODP)•The best fossil record we have?
  7. 7. Comparing coccolithophore and planktic foraminifera deep sea rock and fossil records•Questions:•How does the deep sea rock record change over time?•How does the deep sea fossil record change over time?•Are the deep sea rock and fossil records correlated?•How do the two major calcareous groups compare?
  8. 8. The database Coccoliths Planktic forams•35,416 species occurrences •19,349 species occurrences•16,197 samples •3,850 samples•205 sites •135 sites•4,329 names •2,462 names Geotectonic history
  9. 9. Rock recordCoccoliths Planktic forams
  10. 10. Species recordCoccoliths Planktic forams
  11. 11. Generic recordCoccoliths Planktic forams
  12. 12. Species correlationCoccoliths Planktic forams
  13. 13. Generic correlationCoccoliths Planktic forams
  14. 14. Species detrendedCoccoliths Planktic forams
  15. 15. Genera detrendedCoccoliths Planktic forams
  16. 16. SubsamplingCoccoliths Planktic forams
  17. 17. Modelled versus observed diversityCoccoliths Planktic forams
  18. 18. Model-corrected diversityCoccoliths Planktic forams
  19. 19. Summary•How does the deep sea rock record change over time? •Exponential rise (opening ocean basin)
  20. 20. Summary•How does the deep sea rock record change over time? •Exponential rise (opening ocean basin)•How does the deep sea fossil record change over time? •Coccolith species ~linear rise •Coccolith genera ~rapid rise followed by slow fall •Forams: double sawtooth (K-T divides)
  21. 21. Summary•How does the deep sea rock record change over time? •Exponential rise (opening ocean basin)•How does the deep sea fossil record change over time? •Coccolith species ~linear rise •Coccolith genera ~rapid rise followed by slow fall •Forams: double sawtooth (K-T divides)•Are the deep sea rock and fossil records correlated? •Yes, strongly
  22. 22. Summary•How does the deep sea rock record change over time? •Exponential rise (opening ocean basin)•How does the deep sea fossil record change over time? •Coccolith species ~linear rise •Coccolith genera ~rapid rise followed by slow fall •Forams: double sawtooth (K-T divides)•Are the deep sea rock and fossil records correlated? •Yes, strongly•How do the two major calcareous groups compare? •Forams seem to be less biased than coccos
  23. 23. Comparing sampling bias between the land and the deep sea
  24. 24. Testing sampling bias versus common causeDeep sea Land •Correlations between sampling and diversity are common •Two main explanations: sampling-bias and common cause •For coccolithophores we have two records; ideal to test •Sampling-bias predicts diversity will track sampling •Common cause predicts shared diversity •What do the two rock records look like? •What do the two fossil records look like? •Are the rock and fossil records correlated? •Is there evidence for a common palaeobiodiversity?
  25. 25. The databaseDeep sea Land 205 sites, 16,197 samples, 462 sections, 5,563+ samples, 36,416 occurrence records 22,745 occurrence records
  26. 26. Rock recordsDeep sea Land Number of cores recovering Number of localities with rock of given age published nannofossil taxonomic lists Time (Ma) Time (Ma)
  27. 27. Species richnessDeep sea Land Raw species diversity Number of species Number of species Time (Ma) Time (Ma)
  28. 28. Species richness versus rock record (1): raw dataDeep sea LandLog (Nsites) Log (Nsites) Log (species richness) Log (species richness)
  29. 29. Species richness versus rock record (2): first differencesDeep sea LandLog (Nsites) Log (Nsites) Log (species richness) Log (species richness)
  30. 30. Estimating true diversity: 1, subsamplingDeep sea Land 109 samples per bin 106 samples per bin Species diversity (max) Species diversity (max) Time (Ma) Time (Ma) Orange = empirical pattern White = diversity at equal subsampling
  31. 31. Estimating true diversity: 2, modellingDeep sea LandTrue richness modelled as invariant (observed richness = sampling) Species richness Species richness Time (Ma) Time (Ma)Yellow = empirical patternBlue-green = model prediction assuming diversity is invariant and shaped by rock abundance
  32. 32. Estimating true diversity: 2, modellingDeep sea LandResiduals from modelled richness Time (Ma) Time (Ma)
  33. 33. Estimating true diversity: 3, alpha diversityDeep sea LandMean number of species recorded per site Species Species Time (Ma) Time (Ma) Time (Ma)
  34. 34. SummaryDeep sea Land • The recorded history of coccolithophorid diversity over last 150 Ma changes dramatically according to whether data is drawn from land-based records or deep-sea records • Coccolithophorid diversity correlates strongly to the shape of the rock record it is recovered from • Subsampling, modeling and estimates of mean alpha diversity all point to a third, much more uniform diversity irrespective of which record is used
  35. 35. Species per genus patterns
  36. 36. Higher taxa as species proxies• Used since the earliest diversity curves…• …and continue to be (e.g. Alroy et al. 2008)• Originally pragmatic (less data required)• Then argued that species are inadequate• But, adequacy of higher taxa to represent species-level patterns is essentially untested
  37. 37. Taxonomic level affects pattern
  38. 38. Flessa and Jablonski 1985 • Only explicit test of species-to-higher taxon ratio • Compared families to number of named species in Zoo. Record (Raup 1976) • Pattern of change differs • Families become more speciose
  39. 39. Our database is superior• Species are standardised (synonyms)• Species are assigned to genera• Species are often widespread• Species are long-ranging• Species are comparatively stable taxonomically• Questions:• How does the species-to-genus ratio change over time?• How does the sampling change over time?• How does the number of taxonomists change over time?• Do neither, either or both sampling and taxonomists shape the signal?
  40. 40. Species per genus
  41. 41. Number of sites (sampling)
  42. 42. Number of authors (taxonomists)
  43. 43. Long-term correlation (raw)N sites (rho = 0.95)N authors (rho = 0.93)
  44. 44. Short-term correlation (sampling)Species per genus Rho = 0.43N sites
  45. 45. Short-term correlation (taxonomists)Species per genus Rho = 0.44N authors
  46. 46. Correlations• Both number of sites and number of authors significantly correlate with species-per-genus• Fit 3 models: – spg ~ N sites – spg ~ N taxonomists – spg ~ N sites + N taxonomists• Which is the best explanatory model? – Akaike weights = N Sites (marginally more than a combined model) – Variance partitioning = a combined model• So is it sites or combined?
  47. 47. Subsampling (rarefaction by occurrences for sites)
  48. 48. Subsampling (rarefaction by occurrences for papers)
  49. 49. Summary• How does the species-to-genus ratio change over time? – In a two-step ‘punk eek’ way
  50. 50. Summary• How does the species-to-genus ratio change over time? – In a two-step ‘punk eek’ way• How does the sampling change over time? – The same
  51. 51. Summary• How does the species-to-genus ratio change over time? – In a two-step ‘punk eek’ way• How does the sampling change over time? – The same• How does the number of taxonomists change over time? – The same
  52. 52. Summary• How does the species-to-genus ratio change over time? – In a two-step ‘punk eek’ way• How does the sampling change over time? – The same• How does the number of taxonomists change over time? – The same• Do neither, either or both sampling and taxonomists shape the signal? – Both contribute to the pattern
  53. 53. Summary• How does the species-to-genus ratio change over time? – In a two-step ‘punk eek’ way• How does the sampling change over time? – The same• How does the number of taxonomists change over time? – The same• Do neither, either or both sampling and taxonomists shape the signal? – Both contribute to the pattern Genera are not an accurate proxy for species
  54. 54. Conclusion: what does the deep sea tells us about sampling biases in the fossil record?• The deep sea record shows the same correlation with sampling as land-based studies• This argues in favour of the sampling-bias interpretation and not the common cause• The deep sea record is more biased than the land-based• The deep sea coccolithophore record is more biased than the deep sea planktic foram record• Once sampling has been accounted for there is convergence on a single palaeobiodiversity estimate (at least for Coccolithophores)• Taxonomic structure (species-per-genus) for deep sea Coccolithophores is biased by both sampling and the number of taxonomists

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