Rodents, Snake Evolution, and Dates

South America was isolated during most of the Cenozoic and evolved a terrestrial vertebrate fauna that included many mammals, including caviomorph rodent. Antoine and colleagues (2011) have now describe South America's oldest known rodents, based on a new diverse caviomorph assemblage from the late Middle Eocene, about 41 million years ago (MYA) of Peru, including five small rodents with three stem caviomorphs. This means rodent dispersal is not linked to the Eocene/Oligocene global cooling and drying episode (about 34 MYA), as previously thought, instead rodents arrived in South America during the much warmer and wetter conditions of the Mid-Eocene Climatic Optimum. Thus, rodents evolved in China about 55 MYA (early Eocene), reached India, Southeast Asia, and Africa by about 46 MYA, and were in South America by 43 MYA. The authors phylogenetic results reaffirm the African origin of South American rodents and support a trans-Atlantic dispersal of these mammals during Middle Eocene times. This discovery further extends the gap of 15 million years between first appearances of rodents and primates in South America. But perhaps of more interest to people, who read this blog, is what impact did it have on snakes? Click on the table to enlarge it.

Rodents are snake food - many species feed on rodents today - and it has been long thought that snakes evolved their macrostomate lineage (snakes with the ability to gape their mouths to swallow excessively large prey) to feed on mammals. Could this prey have been rodents? Rodríguez-Robles et al. (1999) thought rodents were the reason the erycine boas evolved a large gape. Recently Pyron and Burbrink (2011) published a revised list of dates for the appearance of the different lineages of snakes based upon the DNA clock, these dates are shown in the attached table, and the snake clades are shown in their order of appearance. A quick look at the table shows the first mammal eating snakes alive today were the pythons which appeared a mere 40 MYA. Given that pythons probably evolved in Australasia and rodents were not present in Gondwanan it seem probably that pythons evolved there large gape to eat something else - marsupial mammals seem more likely. The earliest snakes with the macro-gape that appear in the list are the acrochordids, completely aquatic snakes, snakes that feed on fish - they were around 84.66 MYA according to this data. Given that boines were in South America 45 MYA, and rodents did not arrive until 43 MYA, it is unlikely they evolved their huge gape to consume the mammals they do today- see video. Therefore, it appears macro-gape snakes may have first evolved their big, elastic mouths to eat big fish. 

Literature
Pierre-Olivier Antoine, Laurent Marivaux, Darin A. Croft, Guillaume Billet, Morgan Ganerød, Carlos Jaramillo, Thomas Martin, Maëva J. Orliac, Julia Tejada, Ali J. Altamirano, Francis Duranthon, Grégory Fanjat, Sonia Rousse, and Rodolfo Salas Gismondi. 2011. Middle Eocene rodents from Peruvian Amazonia reveal the pattern and timing of caviomorph origins and biogeography. Proceedings of the Royal Society B published online before print October 12, 2011, doi:10.1098/rspb.2011.1732. 

Pyron, R. A. and Burbrink, F. T. (2011), EXTINCTION, ECOLOGICAL OPPORTUNITY, AND THE ORIGINS OF GLOBAL SNAKE DIVERSITY. Evolution. doi: 10.1111/j.1558-5646.2011.01437.x 

Rodríguez-Robles, J. A., Bell, C. J. and Greene, H. W. (1999), Gape size and evolution of diet in snakes: feeding ecology of erycine boas. Journal of Zoology, 248: 49–58. doi: 10.1111/j.1469-7998.1999.tb01021.x