Outcrop area is not necessarily a good proxy for predicting
(terrestrial) diversity, see e.g. Dunhill (2012), Dunhill et al.
(2014), Walker et al. (2017), and getting outcrop area/exposure
area on a global scale is a non-trivial task (even less so the
density of fossils per formation). So far, we are still waiting
for a good proxy that reflects sampling/research effort, etc.
The authors tried to account for various sampling biases with the
data that was available at the time. If you try to implement the
modelling approach they took you will see that it is a very neat
framework (especially its flexibility is quite nice, as pointed
out by Mitchell et al., 2019), but
it required hell a lot of work to carry out. They went way beyond
of what similar studies did in the past, which also tried to
correct for sampling bias (e.g., Benson & Mannion, 2012;
Benson & Upchurch, 2013; Tennant et al., 2016).
Obviously, there will always be room for improvements. Chiarenza
et al. (2019) is a very neat approach to the topic, and definitely
needs further consideration. I would argue that (at least the way
it is implemented at the moment) the results of Chiarenza et al.
(2019) can also be interpreted in a different way and do not
necessarily show what they are claiming they show (see also the
reviewers' comments in the supplement). Nonetheless, it is
definitely an exciting contribution to the issue.
I am not so sure what you mean with "actually known fossils". The
results are obviously based on what is known at the moment - but
that is trivial and also holds true for the opposite
interpretation of the fossil record. So, I think I misunderstand
you here: What do you mean by this?
The shape of the curves is determined by the mathematical
properties of the involved models. For a model with constant
speciation/cladogenetic rates (I will stick to speciation since
that is the commonly used term) and extinction rates (with spec.
rates > ext. rates) you expect a linear increase in the number
of speciation events in log space through time. If speciation rate
decreases through time and is ultimately surpassed by extinction
rate, you expect a quadratic relationship (also holds true for the
opposite case, where speciation rate increases through time +
extinction rate remains constant).
You then fit your models to the data and assess which model
performs best. That will tell you whether non-avian dinosaurs as a
whole were in decline or not (in terms of speciational
capability). You can add various covariates to account for other
effects involved (e.g., sampling bias, extrinsic controls on
speciation dynamics, etc.), but it is not necessary for the model
per se.
Note also, that for some ornithischian subclades, no decline is
recovered.
I did not want to start a discussion (not enough time, I am
afraid), but merely point out, that the following statement does
not necessarily reflect current consensus:
"The idea that few new species _did_
evolve in the last few million years of the Cretaceous has long
been abandoned: it was based simply on the fact that the Campanian
record of North America is better than the Maastrichtian record of
North America."
Benson, R. B. J. & Mannion, P. D.
Multi-variate models are essential for understanding vertebrate
diversification in deep time
Biology Letters, 2012, 8, 127-130
Benson, R. B.
& Upchurch, P.
Diversity trends in the establishment of terrestrial
vertebrate ecosystems: interactions between spatial and
temporal sampling biases
Geology, 2013, 41, 43-46
Chiarenza, A. A.; Mannion, P. D.; Lunt, D.
J.; Farnsworth, A.; Jones, L. A.; Kelland, S.-J. &
Allison, P. A.
Ecological niche modelling does not support
climatically-driven dinosaur diversity decline before the
Cretaceous/Paleogene mass extinction
Nature Communications, 2019, 10,
1091
Dunhill, A. M.
Problems with using rock outcrop area as a paleontological
sampling proxy: rock outcrop and exposure area compared with
coastal proximity, topography, land use, and lithology
Paleobiology, 2012, 38, 126-143
Dunhill, A. M.;
Hannisdal, B. & Benton, M. J.
Disentangling rock record bias and common-cause from
redundancy in the British fossil record
Nature Communications, 2014, 5, 4818
Mitchell, J. S.; Etienne, R. S. &
Rabosky, D. L.
Inferring diversification rate variation from phylogenies with
fossils
Systematic Biology, 2019, 68, 1-18
Tennant, J. P.;
Mannion, P. D. & Upchurch, P.
Sea level regulated tetrapod diversity dynamics through the
Jurassic/Cretaceous interval
Nature Communications, 2016, 7, 12737
Walker, F. M.;
Dunhill, A. M.; Woods, M. A.; Newell, A. J. & Benton, M.
J.
Assessing sampling of the fossil record in a geographically
and stratigraphically constrained dataset: the Chalk Group of
Hampshire, southern UK
Journal of the Geological Society, 2017,
174, 509-521
On 05/12/2019 18:10, David Marjanovic
wrote:
Extrinsic Factors.
Because the fossil
record has long been known to be incomplete (50, 51), it
is possible that the observed slowdown and downturn are
byproducts of undersampling. This assumption would imply
that there is a systematic downward bias in the
phylogeny toward recent times, which would be counter to
the usual expectation for poor sampling (50, 51).
Here, to test the effect of such biases, we fitted
additional models with appropriate covariates, including
stage-level formation counts (because formation count is
widely reported to be associated with sampling bias) (9, 10, 12, 35, 44, 52, 53),
taxon-specific formation counts (the number of
formations in which a taxon is found), taxon-specific
collection count (the number of fossil collections in
which a taxon is represented), cladewise valid taxa
counts (the known underrepresentation in the phylogeny)
(54),
fossil quality scores (state of preservation) (55), and
body size (smaller taxa are less likely to be preserved)
(56).
As an indirect
measure of the influence of geography on speciation
dynamics, such as segregation by geographic barriers (30), we
used Mesozoic eustatic sea-level reconstructions (34) as
an additional covariate in our models (mean sea-level
value along each terminal branch). We also tested the
ecological limit on clade diversification or the
possible effects of niche saturation by adding a measure
of intraclade diversity taken as the number of
contemporary branches (including internal branches) for
each taxon (the number of tips in time-sliced trees) (48). All
data files are available in Datasets S1âS13.
As far as I can tell, all this assumes that if the
quality of preservation is the same in two formations,
then they will produce the same number of specimens if all
else is equal; not all else is equal, and the corrections
for this are listed. What is not listed, as far as I
understand, are the area on which each formation is
exposed now (as opposed to the area over which it was
deposited; that's what the sea level correction is for),
and the density of fossils in each formation (apparently
higher in the Dinosaur Park than in the Hell Creek,
AFAIK).
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On top of all that, the emphasis on actually known
fossils (spelled out at the start of the "Materials and
Methods" section) means that all conclusions are heavily
biased toward North America. If there was a decline there
but an increase in Africa, we're simply not going to find
out anytime soon.
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Finally, the curves in the figures are awfully smooth
for spanning so much time. Why would they be? How would
that work?
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(...Also, may I request that people stop saying
"speciation" when they mean cladogenesis? There's only one
of 150 species concepts under which those are reliably the
same thing, and it's never used.)
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