A fossil unearthed in northeastern China has middle ear anatomy somewhere between more primitive and modern mammals, confirming for the first time transitional steps in the evolution of the important structure.

	Carnegie Museum of Natural History Above: The newly discovered Chinese Cretaceous nocturnal mammal Yanoconodon was about the size of a chipmunk. It is estimated to have weighed less than one ounce and was about 5 inches long. Below: An artist's rendering of the reconstructed skeleton. Click photos for larger image.

Scientists suspected the malleus or hammer, incus or anvil, and stapes or stirrup, split off from a location at the hinge of the lower jaw to become separate structures.

The new find, 125 million-year-old Yanoconodon allini, has middle ear bones that are partly separated from the jaw, but remain connected by a bridge of ossified cartilage.

A couple of other fossils contain pieces of that bridge, called Meckel's cartilage, but not the whole thing. Scientists could not be certain the bony oddity wasn't distorted or the result of a preservative quirk.

"This time it is different because we found in this particular fossil the connecting part [from] the middle ear to that particular bone," explained Zhe-Xi Luo, curator of vertebrate paleontology at the Carnegie Museum of Natural History and lead author of the paper, which was published today in Nature.

Humans and other modern mammals have a Meckel's cartilage during embryonic life. It is reabsorbed during fetal development, allowing the ear to separate from the jaw, he added.

With its unusually high number of vertebrae and short limbs, the 5-inch-long Yanoconodon looked somewhat like a lizard or salamander. But it was more closely related to marsupials, such as the opossum.

	Carnegie Museum of Natural History A map showing where the Yanoconodon was discovered. Click photo for larger image.

It was the curiously long spine that caught Dr. Luo's eye when collaborators from Nanjing University sent him, in 2003, pictures of the fossil they had just found. Yanoconodon had 26 thoracic and lumbar vertebrae, while most extinct and living mammals have about 20.

"I was absolutely stunned when I saw it on the e-mail," Dr. Luo said. "We got a general sense that the body form of this new mammal is very atypical or very exceptional for your normal mammalian look."

He and a Carnegie museum preparator traveled to China to begin work on the specimen, focusing on the vertebrae. Its spinal anatomy turned out to be a real-life version of an experiment conducted by a Utah research group, which found mutating certain developmental genes in lab mice and rats altered the shape of their vertebrae.

With Yanoconodon, "we essentially have this phenomenon documented in the fossil record," Dr. Luo explained. "Gene mutation may have played a significant role in getting extra vertebral segments and also in forming this rather funny structure of the vertebral column that made this animal much more lizard-like than any other mammals."

The Carnegie and Nanjing team then began work on the specimen's teeth, and in that process discovered that the transitional middle ear structure had been preserved.

Placement of ear bones has implications for lifestyle, as well as hearing. When they were part of the jaw, sound was predominantly conducted through bone. Many early mammals were diggers and burrowers living a subterranean existence, "so this makes sense," Dr. Luo said.

With the development of the tympanic membrane and the migration of the middle ear bones away from the jaw, air conduction of sound improved. Even now, most mammals are nocturnal, not diurnal as humans are, he said.

"This hearing adaptation, this extra-sensitivity of the ear, is fundamental to our capacity to be active in the night," Dr. Luo noted. In turn, that might have permitted early mammals to find more food and to avoid potential predators, such as dinosaurs.

He added the Yanoconodon fossil again demonstrates that intricate, complex structures can develop through the process of evolution.

As Dr. Luo put it: "We have now a clear case documenting why a very elaborate and very delicate and very sophisticated ear structure came about and how it came about."