While the dinosaurs ruled the Earth, our small ancestors evolved the crucial characteristics that made them successful after the end of the Cretaceous. Having a big brain and a small litter of offspring are among these, and researchers now have more clues on when those adaptations began.

In an incredibly rare find, researchers have discovered the fossilized remains of a mammal progenitor and her 38 babies. Never before have researchers found the fossilized remains of babies from a mammal precursor. As reported in Nature, the creature, known as Kayentatherium wellesi, is a beagle-size plant-eater that lived 185 million years ago. It has several traits in common with mammals, such as it likely having hair. However, it wasn't quite a mammal just yet.

The babies’ skulls are small replicas of the adult one, which reveals they did not have particularly big brains. Mammalian offspring tend to have big brains compared to their body size. The other piece of evidence is the number of offspring – 38 is definitely not a small litter.

"These babies are from a really important point in the evolutionary tree," lead author Eva Hoffman, a graduate student at the UT Jackson School of Geosciences, said in a statement. "They had a lot of features similar to modern mammals, features that are relevant in understanding mammalian evolution."

Fossils found from just a few million years later show mammal ancestors already sporting big brains and small litters. Rearing many children and developing a big brain are activities that consume a lot of energy. Researchers suggest that our ancestors ended up trading brood power for brain power. These findings are important in our understanding of how we and other mammals evolved into what we are today.

"There are additional deep stories on the evolution of development, and the evolution of mammalian intelligence and behavior and physiology that can be squeezed out of a remarkable fossil like this now that we have the technology to study it," co-author Professor Timothy Rowe explained.

The specimen was collected from a rock formation 18 years ago in Arizona by Rowe, who had no idea it contained dozens of offspring as well. It was another graduate student of his, Sebastian Egberts, who spotted a grain-sized speck of tooth enamel in 2009. Detailed CT scans and subsequence analysis revealed the treasure hidden within.