Skulls: Reptiles vs. Mammals

Gila monster

The Gila monster is a lizard, native to the American Southwest. It is among the few venomous lizards, and has an unusually "armored" skull. In other respects, it is similar to most lizards, and the skull specimen we have in lab is a good representative for showing some differences between reptile and mammal skulls.

Jaw

In lizards and other reptiles, the lower jaw (mandible) is formed from several bones joined together. This multi-part mandible is the original vertebrate style, as found in fish and amphibians. Mammals, in contrast, have a single bone forming each side of the mandible. This evolutionary change affects the hinge joint of the jaw and, indirectly, the middle ear bones.

In the skull of this gila monster lizard, the mandible has a clearly defined dentary (the bone that holds the teeth), while several other bones are fused together to form the rest of the mandible. Also, the jaw joint seems to have “extra” bones compared to a mammal. Two of the main bones that create the jaw joint are the quadrate and the articular (which is part of the "other bones of the mandible" in this photo). The articular is the region of the mandible that directly articulates with the quadrate. Compare this to the jaw of a mammal.

Mammal jaws, compared

In mammals, the hinge joint of the jaw is the temporomandibular joint, where the mandibular condyle fits into the mandibular fossa (which is part of the temporal bone). See the skulls page for more detail. The quadrate and articular bones are not part of the jaw hinge. So where did these bones go in mammals?

The answer is that the quadrate and articular bones, once they were no longer part of the jaw hinge, evolved dramatically. They became reduced in size and eventually turned into the incus and malleus, two of the middle ear bones of mammals.

The temporomandibular joint is only found in mammals. In reptiles and other vertebrates, the mandible does not directly articulate with the temporal bone.

Note on the mammalian temporal bone: The temporal bone of mammals is formed by the fusion of several bones, including the squamosal. For the purposes of this lab, we can say that the part of the temporal bone we’re looking at is the same as the squamosal bone in reptiles or amphibians. I won't ask you the names of these bones on the lab exam.

Secondary palate

In the Gila monster, there is no bony secondary palate. See the discussion of the alligator skull below for an explanation of this.

Occipital condyle

The occipital condyle is the structure on the back of the skull that allows it to articulate with the first neck vertebra. Most reptiles have only one occipital condyle, while mammals have two.

References for lizards

Lizard skulls from Udo Savalli at ASU.

Alligator

We have an alligator skull and a whole skeleton of a very small alligator. Compare the alligator skull to the Gila monster. Although it's very different in size and shape, in most respects it is structurally similar. However, there is one very important difference:

The secondary palate

The term "palate" refers to the roof of the mouth. The primary palate is also the bottom of the neurocranium, or braincase. Thus, the primary palate is like a wall between the brain and the inside of the mouth. In frogs and Gila monsters, that's all there is in terms of the palate.

Look at the underside of the alligator skull. The nostrils open into a nasal air passage that can carry the air to the back of the throat, where the air could enter the trachea. The nasal air passage is completely separated from the mouth. The structure separating the nasal air passage from the mouth is called the secondary palate. This structure is highly advantagous; it means that an alligator can float with its nostrils above water and its mouth open underwater, and it can still breathe. Or, alternatively, it can eat and breathe at the same time. You no doubt do this yourself; that's because mammals also have a complete secondary palate.

When an alligator breathes through its nose, the air is drawn into the external nares (nostrils) and passes through the space between the primary palate and the secondary palate before being drawn through the internal nares at the back of the throat and then into the trachea.The internal nares are the small openings near the posterior end of the palate. The larger openings, called palatine foramina, are covered by soft tissue in living alligators.

Also, note that the alligator, like other reptiles, has a single occipital condyle forming the joint with the first cervical vertebra. Mammals have two.

The secondary palate, in addition to providing a passage for air, makes the skull much stronger in terms of the forces generated by a powerful bite. Here is a bear skull for comparison:

It's easy to see how these skulls are able to withstand extreme forces.

Alligators are unusual among reptiles in having a complete secondary palate. In general, fish, amphibians and most reptiles lack a solid bony secondary palate, while mammals all have it. For comparison, look at the skulls of a frog, salamander, gila monster, and bird.

References for alligators

Alligator skeleton from Udo Savalli at ASU.

Snake

We have a snake skull and whole snake skeleton in lab.

This snake's jaw hinge is similar to that of other reptiles; the quadrate bone plays a key role. However, the snake's quadrate can hinge in two places, allowing the snake to open its mouth wide to swallow very large prey.

Snakes don't chew their food. Most other reptiles (including lizards) don't, either. In fact, we mammals are unusual in our dedication to mastication (chewing). Mastication requires a fairly precise fit between the upper and lower jaws, which mammals can achieve thanks to our tight temporomandibular joint. Look at the snake skull and imagine what might happen if it decided to chew. The loose jaw hinge that allows the snake to swallow large prey would also allow the mandible to move freely, making chewing inefficient and letting the upper and lower teeth contact each other unpredictably. Also, this loose jaw construction does not support the musculature needed for powerful chewing and biting.

References for snakes

Snake skulls from Udo Savalli at ASU.

Review

Concepts & terminology

This page describes some important evolutionary events in the evolution of vertebrate skulls. It's necessary to use some specialized vocabulary in order to describe the skull features involved, but the vocabulary itself isn't the main point; the words are simply tools to help you understand the skulls. Here are some terms I might ask you about on the lab exam.

• Jaw hinge joint
• Mandible (I won't test you on specific bones such as the dentary or articular)
• Mastication
• Occipital condyle
• Quadrate bone and its role in the jaw hinge
• Primary palate
• Secondary palate
• Middle ear bones: incus and malleus

Review questions

1. Amphibians and reptiles (including birds) use the quadrate and articular bones as part of the hinge joint of the jaw. In mammals, these bones are not part of the jaw hinge. What happened to these bones in mammals? What is the functional significance of this change?
2. What is the functional significance of the secondary palate? Which vertebrates have a fully developed secondary palate?
3. Suppose you found a skull and you had no idea what sort of animal it came from. What features would help you determine whether it was a reptile or a mammal?

References & further reading

Evolution of mammalian auditory ossicles on Wikipedia.

Evolution of the mammalian middle ear from the UC Museum of Paleontology.

Gila monster on Wikipedia

Kardong, Kenneth, 2012. Vertebrates: Comparative Anatomy, Function, Evolution (6th Edition). This big, expensive textbook is an excellent resource. It is the main source I used in making these pages.

Your Inner Fish. This is an outstanding book by Neil Shubin as well as a PBS documentary based on the book. I highly recommend both. For a video on the evolution of middle ear bones, go to the video page and click on "We hear with the bones that reptiles eat with."

Lend an ear to a classic tale of mammalian evolution. Hoffman, 2021. Nature. "Newly discovered fossil evidence has led to a re-evaluation of one of the fundamental transitions in mammalian evolution: the transformation of bones of the lower jaw into those of the middle ear." An easy-to-read news article about recent research.

Alligator skulls in 3-D

There are some excellent 3-dimensional reconstructions of alligator skulls online.

Alligator mississippiensis on DigiMorph. Take a look at the 3D volume rendered movies and the slice movies. These provide an excellent look at the secondary palate, once you figure out what you're looking at.

3D Alligator on Casey Holliday's site at the University of Missouri.

Visible Interactive Alligator at the Witmer Lab site, Ohio University. See also The 3D Alligator: A new anatomical resource for education & research.