The world without dinosaurs
In the arid expanse of Chubut Province, Patagonia, a sequence of rock layers holds the history of a planetary reset. These layers, known as the Lefipán Formation, straddle one of the most important moments in Earth's history: the Cretaceous-Paleogene (K-Pg) extinction event, 66 million years ago. This is the cataclysm, caused by an asteroid impact, that eliminated all non-avian dinosaurs. The formation, which can be up to 380 meters thick, was once a deltaic to shallow marine environment connected to a nascent Atlantic Ocean. Its sediments precisely record the world just before the impact and the new world that emerged in its wake.
Fossil evidence from the Lefipán Formation shows that before the extinction, the ecosystem had many plesiosaurs like Aristonectes parvidens swimming in the sea. On land, the small, shrew-like mammals that lived under the feet of dinosaurs were present but not dominant. One of the last mammals of the dinosaur age found here is Cocatherium lefipanum, known only from a single molar just 2.9 millimeters long. It represents one of the oldest known therian mammals (the group that includes marsupials and placentals) in the Southern Hemisphere. The extinction event was devastating for Patagonia, with studies of fossil leaves indicating a plant extinction rate as high as 92%. But for the surviving mammals, the disappearance of the dinosaurs was an unprecedented opportunity.
The rise of mammals
The fossils found in the Paleocene layers of the Lefipán Formation, just meters above the K-Pg boundary, show a dramatic change. With the dominant dinosaurs gone, mammals began to diversify with incredible speed. Within the first million years after the extinction, mammals radiated into the vacant ecological roles. They grew larger, evolving from creatures weighing less than a pound to some that were over 100 pounds. This rapid increase in body size was a direct response to the new availability of food and territory.
This Patagonian site is one of the few places in the Southern Hemisphere that documents this transition so clearly. The recovery here was also remarkably fast. Evidence from insect damage on fossil leaves suggests that Patagonian ecosystems recovered in about 4 million years, more than twice as fast as ecosystems in North America. This early mammal explosion was a story of brawn over brains. Fossil analyses show that while mammal bodies grew significantly larger almost immediately after the extinction, their brain-to-body size ratio decreased. It was more important to be big than to be smart in this new, emptier world. Larger, more complex brains would not evolve for another 10 million years, when competition between mammal species became more intense.