Catching evolution in the act

Many scientists have long suspected that the tiny bones in the middle ears of all modern-day mammals evolved from bones in ancient reptilian jaws, but direct evidence was lacking. Now, paleontologists have unearthed fossils that appear to show the transition.

Mammals have three delicate bones that transfer sound in their middle ears. Reptiles don’t have such a configuration but do have three bones that make up the rear edges of their lower jaws, says Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh. Scientists have assumed that some transitional creatures had an intermediate configuration of bones. Indeed, some early mammals had a distinct groove precisely where the three bones appear in reptiles (SN: 2/12/05, p. 100: Available to subscribers at Groovy Bones: Mammalian ear structure evolved more than once). However, no one had found transitional fossils that preserved the small bones, which probably were surrounded by soft tissue.

Luo and his colleagues describe in the March 15 Nature the fossils of Yanoconodon allini, an early mammal that lived about 125 million years ago in what is now northeastern China. The remains preserve a fragment of cartilage that holds two of the three bones in the expected position. The new fossils corroborate previous researchers’ hypotheses, and they reveal crucial aspects of the evolving ear structure, Luo’s team contends.