Evolution of ear
Washington: An international team of paleontologists has discovered a new species of mammal that lived 123 million years ago in what is now the Liaoning Province in northeastern China.
The newly discovered animal, Maotherium asiaticus, comes from famous fossil-rich beds of the Yixian Formation.
This new remarkably well-preserved fossil, offers an important insight into how the mammalian middle ear evolved.
The discoveries of such exquisite dinosaur-age mammals from China provide developmental biologists and paleontologists with evidence of how developmental mechanisms have impacted the morphological (body-structure) evolution of the earliest mammals and sheds light on how complex structures can arise in evolution because of changes in developmental pathways.
“What is most surprising, and thus scientifically interesting, is this animal’s ear,” said Dr. Zhe-Xi Luo, curator of vertebrate paleontology and associate director of science and research at Carnegie Museum of Natural History.
“Mammals have highly sensitive hearing, far better than the hearing capacity of all other vertebrates, and hearing is fundamental to the mammalian way of life. The mammalian ear evolution is important for understanding the origins of key mammalian adaptations,” he said.
According to the Chinese and American scientists who studied this new mammal, the middle ear bones of Maotherium are partly similar to those of modern mammals.
But, Maotherium’s middle ear has an unusual connection to the lower jaw that is unlike that of adult modern mammals.
This middle ear connection, also known as the ossified Meckel’s cartilage, resembles the embryonic condition of living mammals and the primitive middle ear of pre-mammalian ancestors.
Because Maotherium asiaticus is preserved three-dimensionally, paleontologists were able to reconstruct how the middle ear attached to the jaw.
This can be a new evolutionary feature.
Or, it can be interpreted as having a “secondarily reversal to the ancestral condition,” meaning that the adaptation is the caused by changes in development.
The middle ear morphology in fossil mammal Maotherium of the Cretaceous (145-65 million years ago) is very similar to the mutant morphology in the middle ear of the mice with mutant genes.
The scientific team studying the fossil suggests that the unusual middle ear structure, such as the ossified Meckel’s cartilage, is actually the manifestation of developmental gene mutations in the deep times of Mesozoic mammal evolution.
By studying all features in this exquisitely preserved fossil, researchers believe Maotherium to be more closely related to marsupials and placentals than to monotremes—primitive egg-laying mammals of Australia and New Guinea such as the platypus.
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