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

Graeme T. LloydDepartment of Palaeontology, Natural History Museum, London, UK

What the deep sea tells us about sampling biasesin the fossil record

Collaborators

Andrew Smith

Jeremy Young

Paul Pearson

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

The fossil record is our only empirical record of the history of life

Generic diversity

N M

aps

Land-based rock and fossil records show strong correlation…

…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?

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?

Geotectonic history

•35,416 species occurrences

•16,197 samples

•205 sites

•4,329 names

•19,349 species occurrences

•3,850 samples

•135 sites

•2,462 names

The database

Planktic foramsCoccoliths

Rock record


Species record


Generic record


Species correlation


Generic correlation


Species detrended


Genera detrended


Subsampling


Modelled versus observed diversity


Model-corrected diversity


Summary


•Exponential rise (opening ocean basin)

Summary




•Coccolith species ~linear rise

•Coccolith genera ~rapid rise followed by slow fall

•Forams: double sawtooth (K-T divides)

Summary








•Yes, strongly

Summary








•Yes, strongly

•How do the two major calcareous groups compare?

•Forams seem to be less biased than coccos

Comparing sampling bias between the land and the deep sea

Testing sampling bias versus common cause

Deep sea

•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?

Land

205 sites, 16,197 samples,36,416 occurrence records

462 sections, 5,563+ samples,22,745 occurrence records

Deep sea Land

The database

Deep sea Land

Rock records

Time (Ma)

Number of cores recovering rock of given age

Number of localities with published nannofossil taxonomic lists

Time (Ma)

Time (Ma) Time (Ma)

Number of species

Raw species diversity

Number of species

Deep sea Land

Species richness

Deep sea Land

Log (Nsites)

Log (species richness) Log (species richness)

Log (Nsites)

Species richness versus rock record (1): raw data

Deep sea Land

Log (species richness) Log (species richness)

Log (Nsites) Log (Nsites)

Species richness versus rock record (2): first differences

Deep sea Land

Orange = empirical patternWhite = diversity at equal subsampling

Time (Ma)

109 samples per bin

Time (Ma)

Species diversity (max) Species diversity (max)

106 samples per bin

Estimating true diversity: 1, subsampling

Time (Ma) Time (Ma)

Species richness Species richness

True richness modelled as invariant (observed richness = sampling)

Deep sea Land

Estimating true diversity: 2, modelling

Yellow = empirical patternBlue-green = model prediction assuming diversity is

invariant and shaped by rock abundance

Deep sea Land

Time (Ma)

Residuals from modelled richness

Time (Ma)

Estimating true diversity: 2, modelling

Time (Ma)

Deep sea Land

Species

Time (Ma)

Species

Time (Ma)

Estimating true diversity: 3, alpha diversity

Mean number of species recorded per site

• 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

Deep sea Land

Summary

Species per genus patterns

• 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

Higher taxa as species proxies

Taxonomic level affects pattern

• 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

Flessa and Jablonski 1985

• 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?

Our database is superior

Species per genus

Number of sites (sampling)

Number of authors (taxonomists)

Long-term correlation (raw)

N sites (rho = 0.95)N authors (rho = 0.93)

Short-term correlation (sampling)

Species per genusN sites

Rho = 0.43

Short-term correlation (taxonomists)

Species per genusN authors

Rho = 0.44

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?

Subsampling (rarefaction by occurrences for sites)

Subsampling (rarefaction by occurrences for papers)

Summary

• How does the species-to-genus ratio change over time?– In a two-step ‘punk eek’ way

Summary


• How does the sampling change over time?

– The same

Summary



– The same

• How does the number of taxonomists change over time?– The same

Summary



– The same


• Do neither, either or both sampling and taxonomists shape the signal?– Both contribute to the pattern

Summary



– 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

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

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

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