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E N L A R G E I M A G
Double take. The middle ear of theplatypus may have evolved
separatelyfromthat of placental mammals andmarsupials.
The middle ear that endows mammals with their sense of hearing
isone of the masterpieces of evolution. And it now seems that this
is an
instrument so nice, nature invented it twice.
The three delicate bones of the mammalian middle ear--the
malleus,
incus, and stapes--work in concert to amplify and internally
direct
sound waves so that they can be converted into nerve impulses.
But
before they became specialized for hearing, the bones of the
middle
ear were part of the jaw of synapsids, reptile-like ancestors
of
mammals. The fossil record shows that these bones migrated from
the
jaw to the middle ear over the course of evolution. Because all
modern
mammals have the same middle ear, and because itsjawbone
origins
are so peculiar, scientists have long assumedthat themiddle
ear
arose just once in the course of mammalian evolution.
But the discovery of a well-preserved fossil jawbone on the
southern coast of Australia has cast doubt on this
classic story. The bone was found in 2003 by a team led by
husband and wife paleontologists Thomas Rich and
Patricia Vickers-Rich, at Museum Victoria and Monash University,
respectively, in Melbourne, Australia. The pair
immediately recognized that it belonged to Teinolophos, a
distant ancestor of the platypus and other egg-laying
mammals, but not of marsupials and placental mammals.
After consulting with James Hopson, an evolutionary biologist at
the University of Chicago, they realized that the
jawbone still carried the tell-tale imprint of the three "middle
ear" bones, which had not yet migrated. Therefore,
the middle ear must have evolved at least twice: once in the
lineage that gave rise to the platypus, and again in
the lineage that gave rise to marsupials and placentals like us.
The researchers report their findings in today's
issue ofScience.
This evolutionary twist is a surprise to most, says Thomas
Martin, a paleontologist at the SenckenbergResearch Institute in
Frankfurt, Germany. And he says it gives vindication to
embryologists who, when
comparing the way the middle ear forms between egg-layers like
the platypus and other mammals, "noticed
certain minor differences."
Related sites
Hopson's homepage
Vickers-Rich's homepage
Rich's homepage
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