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Dinosaur body mass evolution study

From: Ben Creisler

New in open-access PLoS Biology:

Roger B. J. Benson, Nicolás E. Campione, Matthew T. Carrano, Philip D.
Mannion, Corwin Sullivan, Paul Upchurch, David C. Evans (2014)
Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of
Sustained Ecological Innovation on the Avian Stem Lineage.
PLoS Biol 12(5): e1001853.

Large-scale adaptive radiations might explain the runaway success of a
minority of extant vertebrate clades. This hypothesis predicts, among
other things, rapid rates of morphological evolution during the early
history of major groups, as lineages invade disparate ecological
niches. However, few studies of adaptive radiation have included deep
time data, so the links between extant diversity and major extinct
radiations are unclear. The intensively studied Mesozoic dinosaur
record provides a model system for such investigation, representing an
ecologically diverse group that dominated terrestrial ecosystems for
170 million years. Furthermore, with 10,000 species, extant dinosaurs
(birds) are the most speciose living tetrapod clade. We assembled
composite trees of 614–622 Mesozoic dinosaurs/birds, and a
comprehensive body mass dataset using the scaling relationship of limb
bone robustness. Maximum-likelihood modelling and the node height test
reveal rapid evolutionary rates and a predominance of rapid shifts
among size classes in early (Triassic) dinosaurs. This indicates an
early burst niche-filling pattern and contrasts with previous studies
that favoured gradualistic rates. Subsequently, rates declined in most
lineages, which rarely exploited new ecological niches. However,
feathered maniraptoran dinosaurs (including Mesozoic birds) sustained
rapid evolution from at least the Middle Jurassic, suggesting that
these taxa evaded the effects of niche saturation. This indicates that
a long evolutionary history of continuing ecological innovation paved
the way for a second great radiation of dinosaurs, in birds. We
therefore demonstrate links between the predominantly extinct deep
time adaptive radiation of non-avian dinosaurs and the phenomenal
diversification of birds, via continuing rapid rates of evolution
along the phylogenetic stem lineage. This raises the possibility that
the uneven distribution of biodiversity results not just from
large-scale extrapolation of the process of adaptive radiation in a
few extant clades, but also from the maintenance of evolvability on
vast time scales across the history of life, in key lineages.

Author Summary

Animals display huge morphological and ecological diversity. One
possible explanation of how this diversity evolved is the "niche
filling" model of adaptive radiation—under which evolutionary rates
are highest early in the evolution of a group, as lineages diversify
to fill disparate ecological niches. We studied patterns of body size
evolution in dinosaurs and birds to test this model, and to explore
the links between modern day diversity and major extinct radiations.
We found rapid evolutionary rates in early dinosaur evolution,
beginning more than 200 million years ago, as dinosaur body sizes
diversified rapidly to fill new ecological niches, including
herbivory. High rates were maintained only on the evolutionary line
leading to birds, which continued to produce new ecological diversity
not seen in other dinosaurs. Small body size might have been key to
maintaining evolutionary potential (evolvability) in birds, which
broke the lower body size limit of about 1 kg seen in other dinosaurs.
Our results suggest that the maintenance of evolvability in only some
lineages explains the unbalanced distribution of morphological and
ecological diversity seen among groups of animals, both extinct and
extant. Important living groups such as birds might therefore result
from sustained, rapid evolutionary rates over timescales of hundreds
of millions of years.