Artist's reconstruction of Hadrocodium wui.
Credit: Mark A. Klinger, Carnegie Museum of Natural History
CT scan of Hadrocodium brain (pink) through semi-transparent skull. Olfactory bulbs are at front of brain (reddish pink).
Credit: Matt Colbert, University of Texas at Austi
WASHINGTON: Skull scans on two of the oldest known mammal species have shown that that an improved sense of smell may have jump-started brain evolution in the ancestral cousins of present-day mammals.
The findings may help explain why mammals evolved such large and complex brains, which in some cases ballooned 10 times larger than relative body size.
"Now we have a much better idea of the historical sequence of events and of the relative importance of the different sensory systems in the early evolution of mammals," said lead study author Tim Rowe, director of the vertebrate palaeontology laboratory at the University of Texas at Austin.
"It paints a much more vivid picture of what the ancestral mammal was like and how it behaved, and of our own ancestry."
A 3-D view of the brain
The researchers focused on two shrew-like animals - the Morganuocodon and the Hadrocodium - plucked from the Jurassic fossil beds of China for the study published in the current issue of Science.
Using X-ray computed topography, or three-dimensional CT scans, to reconstruct the interior of the skull, researchers were able to see what the brains of the creatures may have been like.
These paper-clip sized, shrew-like critters are thought to be precursors to existing mammals or 'pre-mammals'. The three-dimensional images gave the researchers a magnified, inside view of the brain and nasal cavities of the fossils.
Exploiting a world of information
Rowe and colleagues observed that the nasal cavity and related smell regions were enlarged in the pre-mammal fossils, along with areas of the brain that process olfactory information. Both characteristics indicate an improved sense of smell in pre-mammals.
Comparing the mammal brain endocasts with fossils of other groups, like those of primitive reptiles called cynodonts, revealed that the brains of the Morganucodon and the Hadrocodium were almost 50% larger than the brains of mammal precursors.
Taken together, the results hint that the ability to exploit a world of information dominated by odors and smells made early mammals extraordinarily different from even their closest extinct relatives. The researchers found that the critters also used their fur as a sensor to feel their way around and avoid harm.
