Saturday, 28 July 2012

A New Species Of Ancient Rodent Which Hints At The Oldest Grassland Yet

Just over a week ago, I was on holiday in Ecuador. I spent four days trekking through the Andes Mountains, climbing two peaks and admiring the spectacular scenery produced by the destruction of the Pangaean super continent over 200 million years ago. Today the region consists of lush, agricultural land in the north, changing into scrub land and then rocky desert plateaus in the south. However, due to the fact that the rocks are volcanic, there are very few fossils to build up an image of what the region was like millions of years ago.

The 32.5 million year old jaw of Eoviscaccia frassnettii
Yet a team of palaeontologists composed of scientists from the American Museum of Natural History, University of California, Santa Barbara and Case Western Reserve University have found the remains of two new species of rodents from 32.5 million year old rocks in the Tinguiririca River Valley in the Chilean Andes. One of the two (named Eoviscaccia frassinettii), after examination, is now believed to be the oldest chinchilla (a mouse-like creature) on Earth.

The second, named Andemys termasi which translates as 'the mouse of the Andes,' is the creature which hints at the ecological past of the region. The tiny rodent is the second oldest on the continent, the first being 41 million year old close relative from Peru. What is interesting is that the ancient relative had small crowns on its back teeth, indicating that it fed on soft food such as fruits. The younger Andemys had much larger crowns.

The 32.5 million year old jaw of Andemys termasi
This indicates that it fed on tougher foods. The specific pattern on the teeth showed that the staple of Andemys's diet was grass. What the change, coupled with the time difference, indicates is that the entire region experienced a massive ecological change from something (possibly forest or jungle based on fossils older than 32 million years) to open grass plains. 'The Tinguiririca chinchilla replicates a dental pattern appearing in many other South American herbivores such as Notoungulates—hooved animals that are now extinct—at that time.'

'This pattern is called hypsodonty,' said lead author Ornella Bertrand, who conducted the research through the Museum's Annette Kade Graduate Student Fellowship Program. A general interpretation of this pattern is the spread of grassland environments. What makes this case special is that it is the oldest yet, providing evidence for what might be the oldest grassland on Earth. However, it is possible that there are older examples. Such plants only appeared after the K-T boundary extinction, but 33 million years is a long time for a group as successful as the grasses to evolve and go global.

'The island continent of South America represented a land of evolutionary opportunity for the ancestors of chinchillas and other caviomorph rodents,' said co-author Darin Croft of Case Western Reserve University in Cleveland. 'These remarkable rodents came to fill an amazing variety of ecological niches and today are among the most characteristic Neotropical mammals.' This would all change 30 million years later when North and South America collided.