The Evolution of Middle Ear Bones in Mammals Robert Davis

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 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… 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 Tympanic Malleus Incus Stapes

Picture of middle ear bones from the Smithsonian. Malleus Incus Stapes Tympanic Picture of middle ear bones from the Smithsonian.

Where did the middle ear bones originate? 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

Transitional Stages of Evolution 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.

Monotremes 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 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

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 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 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=TZyYHGObgk8