Paleontologists Discover New Mammal from Mesozoic Era

March 14, 2007
Paleontologists Discover New Mammal from Mesozoic Era
This artist's representation shows Yanoconodon, a 125-million-year-old mammal fossil found in China. Credit: Nicolle Rager Fuller, National Science Foundation

An international team of American and Chinese paleontologists has discovered a new species of mammal that lived 125 million years ago during the Mesozoic Era, in what is now the Hebei Province in China.

The new mammal, documented in the March 15 issue of the journal Nature, provides first-hand evidence of early evolution of the mammalian middle ear--one of the most important features for all modern mammals. The discovery was funded by the National Science Foundation (NSF).

"This early mammalian ear from China is a rosetta-stone type of discovery which reinforces the idea that development of complex body parts can be explained by evolution, using exquisitely preserved fossils," said H. Richard Lane, program director in NSF's Division of Earth Sciences, which co-funded the discovery with NSF's Division of Environmental Biology and its Assembling the Tree of Life (AToL) program.

Named Yanoconodon allini after the Yan Mountains in Hebei, the fossil was unearthed in the fossil-rich beds of the Yixian Formation and is the first Mesozoic mammal recovered from Hebei. The fossil site is about 300 kilometers outside of Beijing.

The researchers discovered that the skull of Yanoconodon revealed a middle ear structure that is an intermediate step between those of modern mammals and those of near relatives of mammals, also known as mammaliaforms.

"This new fossil offers a rare insight in the evolutionary origin of the mammalian ear structure," said Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History (CMNH) in Pittsburgh, Pa. "Evolution of the ear is important for understanding the origins of key mammalian adaptations."

Mammals have highly sensitive hearing, far better than the hearing capacity of all other vertebrates, scientists have found. Consequently, paleontologists and evolutionary biologists have been searching for more than a century for clues to the evolutionary origins of mammal ear structure.

Mammalian hearing adaptation is made possible by a sophisticated middle ear of three tiny bones, known as the hammer (malleus), the anvil (incus) and the stirrup (stapes), plus a bony ring for the eardrum (tympanic membrane).

The mammal middle ear bones evolved from the bones of the jaw hinge in their reptilian relatives. However, paleontologists long have attempted to understand the evolutionary pathway via which these precursor jaw bones became separated from the jaw and moved into the middle ear of modern mammals.

"Now we have a definitive piece of evidence, in a beautifully preserved fossil split on two rock slabs," said Luo. "Yanoconodon clearly shows an intermediate condition in the evolutionary process of how modern mammals acquired their middle ear structure."

Yanoconodon is about 5 inches (or 15 cm) long and estimated to weigh about 30 grams. Its teeth are notable for the three cusps in a straight line on molars (thus known as a triconodont) for feeding on insects and worms. It has a long body, short and sprawling limbs and claws that were ideal for either digging or living on the ground.

In addition to its unique ear structure, Yanoconodon also has a surprisingly high number of 26 thoracic ("chest") and lumbar ("waist") vertebrae, unlike most living and extinct terrestrial mammals that commonly have 19 or 20 thoracic and lumbar vertebrae. The extra vertebrae give Yanoconodon a more elongated body form, in contrast to its relatively shorter and very primitive limb and foot structures. The new mammal also has lumbar ribs, a rare feature among modern mammals.

"The discoveries of exquisitely preserved Mesozoic mammals from China have built the evidence such that biologists and paleontologists are able to make sense of how developmental mechanisms have impacted the morphological evolution of the earliest mammals," said Luo.

The article is authored by Luo and his collaborators, Peiji Chen and Gang Li of Nanjing Institute of Geology and Palaeontology, China, and graduate student Meng Chen of Nanjing University.

Source: NSF

Explore further: Lopsided ear function can lead to lopsided brain development

Related Stories

Lopsided ear function can lead to lopsided brain development

March 13, 2018

Left-right differences in ear function have been found to lead to asymmetric brain development that affects the preferred direction of turning movement in mice. In a multi-national study publishing 13 March in the open access ...

Sound localization: Where did that noise come from?

March 12, 2018

The ability to estimate distances to sound sources accurately can be crucial for survival. A new study carried out under the direction of Professor Lutz Wiegrebe at the Department of Biology at Ludwig-Maximilians-Universitaet ...

Climate change may be hazardous to your health

March 13, 2018

Doctors around the country are already seeing evidence that climate change is affecting the health of their patients. In Florida, people are asking for more medication; as heat waves aggravate their medical conditions, such ...

Fish use deafness gene to sense water motion

December 21, 2017

Fish sense water motion the same way humans sense sound, according to new research out of Case Western Reserve University School of Medicine. Researchers discovered a gene also found in humans helps zebrafish convert water ...

Recommended for you

Modern humans interbred with Denisovans twice in history

March 15, 2018

Modern humans co-existed and interbred not only with Neanderthals, but also with another species of archaic humans, the mysterious Denisovans. While developing a new genome-analysis method for comparing whole genomes between ...

Are palaeontologists naming too many species?

March 14, 2018

A comprehensive new study looking at variations in Ichthyosaurus, a common British Jurassic ichthyosaur (sea-going reptile) also known as 'Sea Dragons', has provided important information into recognizing new fossil species.


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.