Thursday, December 20, 2012

Squamates & the Cretaceous Extinction

The carnivorous lizard Palaeosaniwa stalks a pair of hatchling Edmontosaurus as
 the snake Cerberophis and the lizard Obamadon look on. Obamadon gracilis is
a small polyglyphanodontian named after President Obama. These squamates 
disappeared with the dinosaurs at the Cretaceous extinction.

More than 9,000 living species of snakes and lizards exploit an extraordinary range of ecological niches and habitats. The history of this radiation extends deep into the Mesozoic. After the appearance of crown squamates in the Jurassic, lizards and snakes underwent a Cretaceous radiation, and by the late Cretaceous most major groups had appeared, including iguanians, geckos, skinks, anguids, and platynotans, as well as many lineages of  snakes. The Cretaceous–Paleogene (K-Pg) extinction that ended the Mesozoic and the dinosaurs is considered to have had little effect on squamate evolution with the exception of the marine mosasaurs. And, all major squamate lineages are thought to have survived the end of the Cretaceous. Now a new study by Longrich et al. (2012) suggests otherwise.

A revision of fossil squamates from the Maastrichtian and Paleocene of North America shows that lizards and snakes suffered a devastating mass extinction coinciding with the Chicxulub asteroid impact. Species-level extinction was 83%, and the K-Pg event resulted in the elimination of many groups of lizard groups and a dramatic decrease in morphological disparity. Survival was associated with small body size and perhaps large geographic range. The recovery was longed with diversity not approach Cretaceous levels for 10 million years after the extinction, and it resulted in a dramatic change in faunal composition. 

Nicholas R. Longrich, Bhart-Anjan S. Bhullar, and Jacques A. Gauthier. 2012. Mass extinction of lizards and snakes at the Cretaceous–Paleogene boundary PNAS 2012 ; published ahead of print December 10, 2012, doi:10.1073/pnas.1211526110

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