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Near-stasis in the diversification of Mesozoic tetrapods (free pdf)



A new open-access paper that might be of interest:


Benson RBJ, Butler RJ, Alroy J, Mannion PD, Carrano MT, Lloyd GT 2016
Near-stasis in the long-term diversification of Mesozoic tetrapods.
PLoS Biol 14(1): e1002359
http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1002359

Abstract

How did evolution generate the extraordinary diversity of vertebrates
on land? Zero species are known prior to ~380 million years ago, and
more than 30,000 are present today. An expansionist model suggests
this was achieved by large and unbounded increases, leading to
substantially greater diversity in the present than at any time in the
geological past. This model contrasts starkly with empirical support
for constrained diversification in marine animals, suggesting
different macroevolutionary processes on land and in the sea. We
quantify patterns of vertebrate standing diversity on land during the
Mesozoic–early Paleogene interval, applying sample-standardization to
a global fossil dataset containing 27,260 occurrences of 4,898
non-marine tetrapod species. Our results show a highly stable pattern
of Mesozoic tetrapod diversity at regional and local levels,
underpinned by a weakly positive, but near-zero, long-term net
diversification rate over 190 million years. Species diversity of
non-flying terrestrial tetrapods less than doubled over this interval,
despite the origins of exceptionally diverse extant groups within
mammals, squamates, amphibians, and dinosaurs. Therefore, although
speciose groups of modern tetrapods have Mesozoic origins, rates of
Mesozoic diversification inferred from the fossil record are slow
compared to those inferred from molecular phylogenies. If high
speciation rates did occur in the Mesozoic, then they seem to have
been balanced by extinctions among older clades. An apparent 4-fold
expansion of species richness after the Cretaceous/Paleogene (K/Pg)
boundary deserves further examination in light of potential taxonomic
biases, but is consistent with the hypothesis that global
environmental disturbances such as mass extinction events can rapidly
adjust limits to diversity by restructuring ecosystems, and suggests
that the gradualistic evolutionary diversification of tetrapods was
punctuated by brief but dramatic episodes of radiation.

Author Summary

Vertebrates invaded the land more than 360 million years ago. Since
then, they diversified to more than 30,000 tetrapod species today,
including birds, mammals, squamates, and amphibians. The fossil record
provides our best window onto diversification across such long spans
of time, but is unevenly sampled. Previous studies counted observed
families of fossil tetrapods and supported an expansionist model,
entailing large and unbounded diversity increases through time. We
applied methods that correct for differences in sampling through time
and space to a comprehensive species-level database of Mesozoic to
early Cenozoic fossil tetrapods. We find strong evidence that tetrapod
diversity increased during the Mesozoic, but that the long-term net
rate of diversification was low; species richness only doubled or
tripled over 190 million years. This is enigmatic because today’s high
biodiversity could not have been realised at such a slow rate.
Diversification rates must have been much higher during other
intervals, or rapid diversification might have been concentrated
during brief episodes such as the earliest Cenozoic. Patterns of
diversification on geological timescales and their relationships to
hypothesised drivers such as ecological opportunity and environmental
volatility must receive renewed scrutiny if we are to understand how
land vertebrates and other animals attained the high biodiversity seen
today.

-- 
David Černý