Mining Mammalian Evolution

Two new studies shed light on the evolution of mammals.

When it comes to understanding the Earth's past, perhaps the most knowledgeable source is, well, rock. Many fossils are essentially stone, formed when hard parts like animal bones and tree bark buried underground are replaced by minerals over time. And to determine when a fossilized organism once walked, swam, or flowered on Planet Earth, scientists often look at the age of the sediment in which it was found. Now, two groups of researchers studying mammalian evolution have struck it rich mining the stories found in rock.

A team of German and Argentine scientists have unearthed the first evidence of early mammals in South America. The tiny fossilized fragment of jawbone lined with complex, crushing molars is thought to be 155 million years old. The finding supports a controversial theory that mammals with complex molars evolved not just in the Northern Hemisphere as traditionally thought, but also in the Southern Hemisphere in a group now extinct except for the platypus and spiny anteater.

Another group of researchers led by Gabriel Bowen of UC Santa Cruz has helped pinpoint the birthplace of many modern groups of mammals. By dating rock layers containing fossils in China and comparing them to those in Europe and North America, the team found strong evidence that hoofed animals and primates originated in Asia about 55 million years ago and subsequently dispersed, possibly in reaction to intense global warming.


Photograph of jaw bone as it was found sitting on a slab of sedimentary rock in the Chubut province of Argentina. This fossil, from the extinct Asfaltomylos patagonicus, represents the first report of a Jurassic mammal to be found in the southern continents.
Photo courtesy of Thomas Martin, Inst. of Geological Sciences, Dept. of Paleontology, Free University of Berlin; Oliver W. M. Rauhut, Edgardo Ortiz-Jaureguizar and Pablo Puerta, Paleontological Museum Egidio Feruglio, Trelew, Argentina.

Site of Patagonian discovery.
Photo courtesy of Thomas Martin.

SEM photos of the dentition of the jaw exhibiting the tribosphenic molar structure.
Photo courtesy of Thomas Martin.