Long-sought fossil mammal with transitional middle ear found in China

April 14th, 2011 - 6:28 pm ICT by ANI  

London, Apr 14 (ANI): Palaeontologists have announced the discovery of Liaoconodon hui, a complete fossil mammal from the Mesozoic found in China that includes the long-sought transitional middle ear.

The specimen was found by palaeontologists from the American Museum of Natural History and the Chinese Academy of Sciences.It shows the bones associated with hearing in mammals, the malleus, incus, and ectotympanic, decoupled from the lower jaw, as had been predicted, but were held in place by an ossified cartilage that rested in a groove on the lower jaw.

The new research also suggests that the middle ear evolved at least twice in mammals, for monotremes and for the marsupial-placental group.

“People have been looking for this specimen for over 150 years since noticing a puzzling groove on the lower jaw of some early mammals,” Jin Meng, curator in the Division of Palaeontology at the Museum and first author of the paper, said.

“Now we have cartilage with ear bones attached, the first clear paleontological evidence showing relationships between the lower jaw and middle ear,” Meng revealed.Mammals, the group of animals that includes egg-laying monotremes like the platypus, marsupials like the opossum, and placentals like mice and whales, are loosely united by a suite of characteristics, including the middle ear ossicles.

The mammalian middle ear, or the area just inside the eardrum, is ringed in shape and includes three bones, two of which are found in the joint of the lower jaw of living reptiles.

This means that during the evolutionary shift from the group that includes lizards, crocodilians, and dinosaurs to mammals, the quadrate and articular plus prearticular bones separated from the posterior lower jaw and became associated with hearing as the incus and malleus.

The transition from reptiles to mammals has long been an open question, although studies of developing embryos have linked reptilian bones of the lower jaw joint to mammalian middle ear bones.

The new fossil, Liaoconodon hui, fills the gap in knowledge between the basal, early mammaliaforms like Morganucodon, where the middle ear bones are part of the mandible and the definitive middle ear of living and fossil mammals.

Liaoconodon hui is a medium-sized mammal for the Mesozioc (35.7 cm long from nose to tip of tail, or about 14 inches) and dates from 125 to 122 million years.

It is named in part for the bountiful fossil beds in Liaoning, China, where it was found.

The species name, hui, honours palaeontologist Yaoming Hu who graduated from the American Museum of Natural History-supported doctoral program and recently passed away.

The fossil is particularly complete, and its skull was prepared from both dorsal and ventral sides, allowing Meng and colleagues to see that the incus and malleus have detached from the lower jaw to form part of the middle ear.

These bones remain linked to the jaw by the ossified Meckel’s cartilage that rests in the groove on the lower jaw. The team hypothesizes that in this early mammal, the eardrum was stabilized with the ossified cartilage as a supporting structure. “Before we did not know the detailed morphology of how the bones of the middle ear detached, or the purpose of the ossified cartilage,” Meng said.

“Liaoconodon hui changes previous interpretations because we now know the detailed morphology of the transitional mammal and can propose that the ossified cartilage is a stabilizer.

“I’ve always dreamed of a fossil with a good ear ossicle. Now, we have had this once in a lifetime discovery,” Meng added.

The findings have been published in Nature. (ANI)

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