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. The tympanic, malleus, and incus are homologous to the angular, articular, and quadrate Tympanic = Angular Malleus = Articular Incus = Quadrate 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. 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. 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. 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. The evolution of the mammalian middle ear and jaw joint were pivotal steps in the evolution of mammals. It is also a great example of how classical comparative anatomy, paleontology and developmental biology have come together to piece together how this remarkable transformation of jaw joint to ear ossicles was able to come about. The homologies of the malleus, incus and stapes to the articular, quadrate and columella, and tympanic ring and gonial to the angular and prearticular suggested by comparative anatomy 175 years ago have been recently confirmed by molecular and developmental biology. The recent discovery of new mammali form fossils has allowed careful documentation of the shift from primary to secondary jaw articulation, creating an opportunity to follow the transformation of the post-dentary skeletal elements. This fossil data has been complemented by the study of marsupial development, providing insight into the changing role of the malleus and incus, and the relationship of the primary and secondary jaw joints. We now have a number of unanswered questions. What are the signalling molecules involved in the interactions between the condylar process and the glenoid fossa that create the novel mammalian jaw joint? What are the mechanisms that lead to transformation of Meckel’s cartilage into a ligament allowing isolation of the ear from the jaw? What controls the distribution of secondary cartilages? What controls the timing of differentiation an.

1. 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. The tympanic, malleus, and incus are homologous to the angular,
articular, and quadrate
Tympanic = Angular
Malleus = Articular
Incus = Quadrate
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. 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. 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. 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.
The evolution of the mammalian middle ear and jaw joint were pivotal steps in the evolution of
mammals. It is also a great example of how classical comparative anatomy, paleontology and
developmental biology have come together to piece together how this remarkable transformation
of jaw joint to ear ossicles was able to come about. The homologies of the malleus, incus and
stapes to the articular, quadrate and columella, and tympanic ring and gonial to the angular and
prearticular suggested by comparative anatomy 175 years ago have been recently confirmed by
molecular and developmental biology. The recent discovery of new mammali form fossils has
allowed careful documentation of the shift from primary to secondary jaw articulation, creating
an opportunity to follow the transformation of the post-dentary skeletal elements. This fossil data
has been complemented by the study of marsupial development, providing insight into the
changing role of the malleus and incus, and the relationship of the primary and secondary jaw
joints. We now have a number of unanswered questions. What are the signalling molecules
involved in the interactions between the condylar process and the glenoid fossa that create the
novel mammalian jaw joint? What are the mechanisms that lead to transformation of Meckel’s
cartilage into a ligament allowing isolation of the ear from the jaw? What controls the

2. distribution of secondary cartilages? What controls the timing of differentiation and cessation of
growth of the ossicles relative to the jaw? With the new tools available to us we hope to be able
to address some of these questions and provide insights into the mechanisms that lie behind
evolution.
Solution
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. The tympanic, malleus, and incus are homologous to the angular,
articular, and quadrate
Tympanic = Angular
Malleus = Articular
Incus = Quadrate
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. 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. 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. 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.
The evolution of the mammalian middle ear and jaw joint were pivotal steps in the evolution of
mammals. It is also a great example of how classical comparative anatomy, paleontology and
developmental biology have come together to piece together how this remarkable transformation
of jaw joint to ear ossicles was able to come about. The homologies of the malleus, incus and
stapes to the articular, quadrate and columella, and tympanic ring and gonial to the angular and
prearticular suggested by comparative anatomy 175 years ago have been recently confirmed by
molecular and developmental biology. The recent discovery of new mammali form fossils has
allowed careful documentation of the shift from primary to secondary jaw articulation, creating
an opportunity to follow the transformation of the post-dentary skeletal elements. This fossil data

3. has been complemented by the study of marsupial development, providing insight into the
changing role of the malleus and incus, and the relationship of the primary and secondary jaw
joints. We now have a number of unanswered questions. What are the signalling molecules
involved in the interactions between the condylar process and the glenoid fossa that create the
novel mammalian jaw joint? What are the mechanisms that lead to transformation of Meckel’s
cartilage into a ligament allowing isolation of the ear from the jaw? What controls the
distribution of secondary cartilages? What controls the timing of differentiation and cessation of
growth of the ossicles relative to the jaw? With the new tools available to us we hope to be able
to address some of these questions and provide insights into the mechanisms that lie behind
evolution.