skulls of a mammal that lived alongside the dinosaurs may be offering
scientists a glimpse into the evolution of the middle ear.
The separation of the three
tiny middle ear bones — known popularly as the hammer, anvil and stirrup — from
the jaw is a defining characteristic of mammals. The evolutionary shift of those tiny bones, which started out as joints in ancient reptilian
jaws and ultimately split from the jaw completely, gave mammals greater
sensitivity to sound, particularly at higher frequencies (SN: 3/20/07). But finding well-preserved skulls from ancient
mammals that can help reveal the timing of this separation is a challenge.
Now, scientists have six
specimens — four nearly complete skeletons and two fragmented specimens — of a newly
described, shrew-sized critter dubbed Origolestes
lii that lived about 123 million years ago. O. lii was part of the Jehol Biota, an ecosystem of ancient
wetlands-dwellers that thrived between 133 million and 120 million years ago in
what’s now northeastern China.
The skulls on the nearly
complete skeletons were so well-preserved that they were able to be examined in
3-D, say paleontologist Fangyuan Mao of the Chinese Academy of Sciences in
Beijing and colleagues. That analysis suggests that O. lii’s middle ear bones were fully separated from its jaw, the team reports online December 5 in Science.