… In Which the Author Jaws About Ears

So here’s the thing…

We all know that the jaw joint is basically a hinge. The lower jaw pivots at this point to open and close the mouth. For most animals, the lower jaw is a solid piece that contains teeth that works against the solid upper jaw that also has teeth.

Every animal that has a jaw has the same joint, between the articular bone in the lower jaw and the quadrate bone in the upper jaw. Fish are like this. Frogs, lizards, snakes, crocodiles, birds, turtles…. Everything.

Except Mammals. Naturally.

The jaw joint in mammals is between the dentary bone making up the bulk if not all of the lower jaw, and the squamosal bone of the skull.

But – and here’s the kicker – the quadrate and articular bones still exist in mammals, and still make an important hinge joint… in the middle ear. This, friends, is what distinguishes ‘mammals’ from every other vertebrate on the planet. It’s not hair, or milk, or placentas. It’s the structure of the jaw, and of the middle ear.

But Penny, you say. How could that be?

Well hold on, because this is cool.

Let’s start with an organism that already has a jaw, like an early amphibian. Eoherpeton will do.

Eoherpeton, a reptilomorph amphibian. Make note of the bones of the upper and lower jaws, especially the dentary, angular, surangular and quadrojugal. The quadrate (upper jaw) and articular (lower jaw) bones that form the joint are hidden behind the quadratojugal and surangular.

In early amphibians and primitive ‘reptiles’ the jaw joint is formed by the quadrate and articular bones, that are tucked in and hidden by the quadratojugal and surangular bones.

Another bone that you can’t see in the above drawing, but is important later is the columella, or stapes. This bone, though large and clunky in early land vertebrates, gets light and small to form the stapes (or stirrup), one of the three tiny bones in the mammalian middle ear and the only bone of the middle ear in every other land vertebrate. The stapes is in contact with the quadrate bone, thus making the lower jaw an important component of the hearing apparatus in most vertebrates.

The quadrate-articular jaw joint and a single bone in the middle ear (the stapes) is the model for all birds, dinosaurs, amphibians, and anything called ‘reptile’. Something changed though, in the lineage leading to mammals, that made this joint lose its jaw function and join the stapes in the middle ear.

So how did this happen?

The ‘whys’ are unclear, but we do have a good sense of ‘what’ took place. Here are some trends:

  • Enlargement of the dentary bone at the expense of all the other bones of the lower jaw.
  • Development of the coronoid process of the dentary, which provides surface area for the attachment of the jaw muscles.
  • Development of the reflected lamina of the angular, which later works with the surangular bone to support the tympanic membrane for hearing
The skull of Thrinaxodon, a cynodont synapsid that still has the quadrate-articular jaw joint. Note the enlargement of the dentary, the coronoid process, and the reflected lamina of the angular.

As the bones of the lower jaw except for the dentary are reduced, and they become more and more involved with hearing, though still performing the functions of the jaw joint. The surangular begins to contact the squamosal bone, and a facet or hollowed out groove forms to allow a rocking motion. This is the beginning of the Glenoid Fossa that forms the squamosal part of the modern mammalian jaw. Later, with further reduction of the surangular bone, the dentary contacts the squamosal bone forming the mammalian dentary-squamosal jaw joint.

Probainognathus, a cynodont synapsid that simultaneously has both the quadrate-articular jaw joint and the squamosal- dentary jaw joint.

With the establishment of the dentary-squamosal jaw joint, the quadrate-articular joint, with the tympanic membrane supported by the reflected lamina of the angular and the surangular bone function only for hearing.

View of the inside of the jaw of Morganucodon, an early mammal that shows only the dentary-squamosal jaw joint.

Once the bones of the ear separated from the jaw, they evolved into the structures we’re more familiar with today. The Hammer-Anvil-Stirrup (Malleus-Incus-Stapes) are actually Articular-Quadrate-Stapes, and were once intimately associated with our jaws. Which is totally why you can hear yourself chew. Or why when you have an ear infection you jaw hurts. Or how a rotten tooth can cost you your hearing.

The mammal jaw and ear are deeply entwined with each other in evolutionary history. And it’s the structure of the ear that sets mammals apart from all other land vertebrates.

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