The Evolution of Middle Ear
Bones in Mammals
Robert Davis
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What are the origins of mammalian middle ear
bones?
How have they evolved over time to become
such a distinguishing structure in mammals?
Did the middle ear bones evolve from a common
ancestor of all mammals, or have there been
separate divergence points for each mammalian
sub-group?
Background information: Terms to
know
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Homoplasy-similarity among structures that
arose through independent evolution instead
of descent from a common ancestor; due to
convergent evolution, parallelism, or reversal
Homology-similarity between species that
results from inheritance of traits from a
common ancestor
Cynodonts-”dog teeth,” extinct mammal like
reptiles that gave rise to modern mammals; in
order Therapsida
Therapsids-Order containing pre-mammal
mammaliaforms
Terms cont…
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Synapsida- Class containing order Therapsida,
mammaliaforms and reptiles
 Newer evolutionary trees consider mammals to
be in clade within the class synapsida, while older
classifications still have mammals in a separate
Class Mammalia
Mammaliaforms-transitional animals classified as
pre-mammals, somewhere between reptiles and
mammals
Therian- group of mammals containing placentals
and marsupials.
Dentary- mandible
Eutriconodont- early mammals, now extinct; order
within Class Mammalia
Mammalian Middle Ear Bones
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Tympanic
Malleus
Incus
Stapes
Incus
Malleus
Stapes
Tympanic
Picture of middle ear bones from the
Smithsonian.
Where did the middle ear bones
originate?
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The middle ear bones in mammals are
derived from bones in the dentary bone
(mandible) of Therapsids
In the Therapsid dentary, there were
three bones used in the jaw hinge: the
angular, articular and quadrate
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The tympanic, malleus, and incus are
homologous to the angular, articular, and
quadrate.
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Tympanic = Angular
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Malleus = Articular
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Incus = Quadrate
Transitional Stages of Evolution
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In early Therapsids, the quadrate, angular
and articular were directly connected to
the dentary bone in the jaw hinge.
These bones were essential for jaw
movement.
These bones were also utilized for crude
hearing capabilities.
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The next stage of evolution came with the
separation of these bones and the dentary.
After the bones separated from the jaw, they
formed the postdentary rod.
It is believed that these the mammalian middle
ear bones evolved at least twice.
It is believed that monotreme and therian
ancestors evolved very similar middle ear bones,
but in two separate paths.
Monotremes
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In monotreme ancestors, there is a
mandibular trough, that is believed to
have accommodated the postdentary rod.
This rod is believed to have been
connected to the trough by ligaments.
This rod was used for hearing and the
quadrate, angular, and articular were no
longer needed in the jaw hinge.
Therians
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In therians, the mandibular trough no longer
held the quadrate, angular, and articular.
These bones were held to the jaw by ossified
Meckel’s cartilage.
This bridge of cartilage and bone extended away
from the dentary and approached the area that
would make up the modern ear cavity.
Figure 1. From
article
“Homoplasy in
the Mammalian
Ear” taken from
Journal of
Science.
Separation of
the middle ear
bones from the
post dentary
(mandible) in
monotremes,
and therians.
Morganucodon—cynodont; one of the earliest
relatives of all mammals
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Morganucodon- cynodont mammaliaform
related to mammals
Morganucodon fossils have maintained the
Meckel’s cartilage bridge that linked the
ear bones to the dentary.
Evidence from Yanoconodon
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Yanoconodon is a fossil eutriconodont
found in the Yan mountains in China.
Yanoconodon fossil retains the middle
ear bones and ossified Meckel’s
cartilage.
Excellent example of transitional form of
ear bone from Therapsids to Mammals.
Figure 2. Ear bones from different
mammaliaforms, and points of origin
from dentary. a. Mammaliaform
Morganucodon b. Eutriconodont
Yanoconodon c. the ectotympanic
(blue), malleus (green) and incus
(brown) of modern Ornithorhynchus d.
Eutriconodont Repenomamus e.
Ossified Meckel's cartilage of
Repenomamus f. Ossified Meckel's
cartilage of Yanoconodon g. Middle
ear of Yanoconodon h. Middle ear
bones of adult Ornithorhynchus i.
Embryonic Ornithorhynchus
Further Research
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Compare the structures of middle ear
bones in existing mammals to see if there
are any significant differences among
living mammals.
Trace the evolution of other structures of
the ear to see if they evolved with the
middle ear bones or separately.
Video explaining evolution of
mammalian middle ear bones
http://www.youtube.com/watch?v=TZyYHG
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