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Post–K-Pg Radiation of Placentals
From: Ben Creisler
New in Science magazine:
Anne D. Yoder (2013)
Fossils Versus Clocks
Science 339(6120): 656-658
It's a great story, and one that most of us learned in grade school.
Dinosaurs ruled Earth for eons, shaking the ground beneath them as
their colossal forms roamed the dense tropical forests of the
Mesozoic. Mammals were present but were minuscule by comparison,
skulking about in the undergrowth as they foraged for insects. And so
it went until a massive asteroid hit Earth about 66 million years ago,
causing environmental havoc, climate change, and the worldwide
extinction of non-avian dinosaurs [the Cretaceous-Paleogene (K-Pg)
extinction event] (1). Only then did mammals begin to flourish and
diversify into the myriad forms of today. It is a compelling tale, but
one where timing is everything. On page 662 of this issue, O'Leary et
al. (2) offer a fresh perspective on the pattern and timing of
mammalian evolution drawn from a remarkable arsenal of morphological
data from fossil and living mammals (see the figure).
Maureen A. O'Leary, Jonathan I. Bloch, John J. Flynn, Timothy J.
Gaudin, Andres Giallombardo, Norberto P. Giannini, Suzann L. Goldberg,
Brian P. Kraatz, Zhe-Xi Luo, Jin Meng, Xijun Ni, Michael J. Novacek,
Fernando A. Perini, Zachary S. Randall, Guillermo W. Rougier, Eric J.
Sargis, Mary T. Silcox, Nancy B. Simmons, Michelle Spaulding, Paúl M.
Velazco, Marcelo Weksler, John R. Wible, and Andrea L. Cirranello
The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals.
Science 339(6120): 662-667
To discover interordinal relationships of living and fossil placental
mammals and the time of origin of placentals relative to the
Cretaceous-Paleogene (K-Pg) boundary, we scored 4541 phenomic
characters de novo for 86 fossil and living species. Combining these
data with molecular sequences, we obtained a phylogenetic tree that,
when calibrated with fossils, shows that crown clade Placentalia and
placental orders originated after the K-Pg boundary. Many nodes
discovered using molecular data are upheld, but phenomic signals
overturn molecular signals to show Sundatheria (Dermoptera +
Scandentia) as the sister taxon of Primates, a close link between
Proboscidea (elephants) and Sirenia (sea cows), and the monophyly of
echolocating Chiroptera (bats). Our tree suggests that Placentalia
first split into Xenarthra and Epitheria; extinct New World species
are the oldest members of Afrotheria.