The early Jurassic might be famous as the point in prehistory that dinosaurs began to grow into giants, but something else was growing larger at that time too: the brains of early mammal-like animals. That could be because smell and touch were vital for their survival during the age of the dinosaurs.
In the 1980s, palaeontologist Timothy Rowe visited the fossil collection at Harvard University with an earnest request: could he please crack open a rare, 190-million-year-old skull of a tiny mammal to determine the shape of its brain? The eyes of Harvard’s curators widened behind their spectacles; their lips pursed; their brows wrinkled.
“The standard response was, ‘No way! Sit on your hands and be patient, and sooner or later we’ll have non-destructive techniques to answer those questions,'” Rowe recalls. “It was so frustrating because I really wanted to know what the brain was like, but these fossils were treated like a Rembrandt or a Vermeer.”
Three decades later, Rowe’s waiting is over. His team at the University of Texas at Austin recently used high-resolution X-ray computed tomography (CT) to create 3D maps of the skulls of two ancient mammals, faithfully reproducing the shapes of the brains they once contained. The digital moulds suggest that their brains evolved to meet the need for acute senses of smell and touch.
There will be mammals
Rowe examined fossil Jurassic skulls from China, remains of the pygmy shrew-like creatures Morganucodon oehleri and Hadrocodium wui â€“ animals so old that they may in fact be forerunners to true mammals.
Mammals have rather large brains for their body size compared with other animals. The difference is the neocortex: a six-layered hunk of brain tissue that is much larger and more complex in mammals. As expected, the digital brains of the early mammals had big neocortices â€“ but something else contributed to their overall size.
Staring back at Rowe from the computer screen were two prominent bumps on the front of the brains: the olfactory bulbs, where smell is processed (see image, top right). The bulbs were much larger than Rowe expected, suggesting that smell was extremely important for early mammals, just as it is for certain mammals today, such as bears and bloodhounds.
Tucked in the wrinkly folds of the digital brains the researchers found evidence of strong motor coordination skills and a keen sense of touch. Rowe speculates that the animals depended on specialised hair follicles attached to nerves to learn about their environments, as some mammals now use whiskers. We know that even these early mammals were covered in hair because a fossil of one of them, Castorocauda lutrasimilis, remarkably preserves evidence of a thick pelt that covered its body.
“The story of becoming a mammal is the story of developing the most sensitive and high-resolution olfactory system,” says Rowe, “and secondary to that is touch and motor skills.”
“It’s a beautifully done paper,” says Lori Marino, who studies the evolution of mammalian brains at Emory University in Atlanta, Georgia. “It’s important work because up to now we haven’t had a whole lot of information about what parts of the brain have expanded in different groups. It tells us something about our preconceptions of how the mammalian brain evolved: it wasn’t just the neocortex that expanded. It gives a new respectability to smell.”
The evident importance of smell and touch to these tiny proto-mammals hints at their lifestyle. The 190-millionâ€“year-old animals probably navigated dark burrows and skittered through leaf litter hunting insects â€“ activities greatly helped by sensitive smell and touch.
“Having a great sense of smell is also consistent with the idea these mammals may have been nocturnal,” explains Rowe. Despite recent evidence of nocturnal behaviour in dinosaurs, it’s generally thought that most of these animals were active at day and asleep at night. “That’s when the mammals came out. With a great sense of smell, it doesn’t matter if it’s dark,” he says. “Smell might be what made it possible for early mammals to come out and find food and mates. In the early Jurassic, that was what drove their evolution.”