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Mesozoic Birds--Early Evolution and Completeness of Fossil Record

From: Ben Creisler

New in PLoS ONE:

Neil Brocklehurst, Paul Upchurch, Philip D. Mannion & Jingmai O'Connor (2012)
The Completeness of the Fossil Record of Mesozoic Birds: Implications
for Early Avian Evolution.
PLoS ONE 7(6): e39056.

Many palaeobiological analyses have concluded that modern birds
(Neornithes) radiated no earlier than the Maastrichtian, whereas
molecular clock studies have argued for a much earlier origination.
Here, we assess the quality of the fossil record of Mesozoic avian
species, using a recently proposed character completeness metric which
calculates the percentage of phylogenetic characters that can be
scored for each taxon. Estimates of fossil record quality are plotted
against geological time and compared to estimates of species level
diversity, sea level, and depositional environment. Geographical
controls on the avian fossil record are investigated by comparing the
completeness scores of species in different continental regions and
latitudinal bins. Avian fossil record quality varies greatly with
peaks during the Tithonian-early Berriasian, Aptian, and
Coniacian–Santonian, and troughs during the Albian-Turonian and the
Maastrichtian. The completeness metric correlates more strongly with a
‘sampling corrected’ residual diversity curve of avian species than
with the raw taxic diversity curve, suggesting that the abundance and
diversity of birds might influence the probability of high quality
specimens being preserved. There is no correlation between avian
completeness and sea level, the number of fluviolacustrine localities
or a recently constructed character completeness metric of
sauropodomorph dinosaurs. Comparisons between the completeness of
Mesozoic birds and sauropodomorphs suggest that small delicate
vertebrate skeletons are more easily destroyed by taphonomic
processes, but more easily preserved whole. Lagerstätten deposits
might therefore have a stronger impact on reconstructions of diversity
of smaller organisms relative to more robust forms. The relatively
poor quality of the avian fossil record in the Late Cretaceous
combined with very patchy regional sampling means that it is possible
neornithine lineages were present throughout this interval but have
not yet been sampled or are difficult to identify because of the
fragmentary nature of the specimens.