That SoAm mid-Jurassic Motherlode!

In the shadow of Cerro Cóndor, a 600-metre-high limestone bluff in Patagonia, two young palaeontologists gaze over waves of mountain ridges running west towards the Andes. Diego Pol and Ignacio Escapa, from the Egidio Feruglio Palaeontological Museum in Trelew, Argentina, have spent years trekking the winding gravel trails here in the Chubut River valley, meeting only wandering guanacos, rheas and sheep. Already, their team has hand-dug half a dozen quarries in nearby canyons that have yielded globally important fossils.

But many prizes remain among the uncharted sediments of the Middle Jurassic, a geological epoch spanning 160 million to 180 million years ago, when dinosaurs, plants and early mammals were all undergoing key evolutionary changes. This time period holds crucial clues to the explosion of evolutionary diversity in both dinosaurs and mammals. The oldest known dinosaur remains, for instance, are around 230 million years old; the oldest known fossil mammals have been dated at 193 million years ago1. Both groups diversified to an enormous extent during the Middle Jurassic2, yet relatively few sediments of that age have been studied. That makes Chubut province in southern Argentina a rare opportunity. "This has the potential to be a global landmark for the Middle Jurassic," says Pol. "For the Southern Hemisphere, it already is."

The Argentine finds may open a little-understood palaeontological window, just as China's rich fossil beds have illuminated the early history of mammals, dinosaurs, reptiles and birds. Chubut is "an amazing region because you get fairly complete skeletal material, which allows you to answer many evolutionary questions", says Peter Makovicky, a palaeontologist at the Field Museum in Chicago who has explored much of Argentina.

"The discoveries from the Middle Jurassic of Argentina are no ordinary field finds," adds Zhe-Xi Luo, a curator at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, who has published on the earliest mammals from China. "They are of such a significant nature that the whole early mammalian evolutionary paradigm must be changed."

It seems the deposits are from the Toarcian to the Oxfordian. For some reason I believe that there is a definite lack of good sequences in that time period. Or is that a NorAm problem? Or am I just plain mistaken? If I am not, this is a major find just for the fact that we will be able to get some idea of the fauna of the time. Indeed, the article goes on to describe some spectacular finds they have already come across with respect to mammalian-synapsid evolution.

Now if only we could find a terrestrial fossiliferous layer from the tropics from during the PT transition... *sighs*