Mammal Skulls

Objectives

This page is intended to help you understand the comparative functional anatomy of mammal skulls. After studying this page and the accompanying specimens in lab, you should be able to:

• Name the major anatomical features of mammal skulls and describe their functions.
• Identify various mammal skulls in terms of which mammalian order they represent, and describe the specific features that allow you to make this identification.

Specimens

Various skulls of mammals, representing these orders:

• Carnivora: Carnivores such as coyotes, foxes, cats, otters, and sea lions.
• Cetartiodactyla: Two-toed ungulates such as deer, pigs, and cows; this group was traditionally classified as the order Artiodactyla. More recently, DNA studies have shown that this order also includes cetaceans (whales and dolphins, which were formerly classified as the order Cetacea). In lab, we have examples of both traditional artiodactyls such as deer and cetaceans such as dolphins. Both are classified as Cetartiodactyla, even though their skulls look very different.
• Lagomorpha: Rabbits, hares, and pikas.
• Perissodactyla: horses and zebras.
• Primates: Monkeys, humans.
• Rodentia: mice, etc.

Skull Terminology: Coyote

One of your tasks for today's lab is to learn how to identify skulls from various orders of mammals. Before you can do that, you need to learn to look at skulls and to describe what you see. For that, you'l need some terminology. There are two complementary sets of terms for describing skulls: the names of major anatomical features and the names of the individual bones that make up the skull. On this page, you will see the main anatomical features labeled, because those features will help you understand how skulls work and help you identify them. In this lab, you won't need to remember the names of all the bones that make up the skull, but there will be a few specific bones described later that might be on a test.

Use the coyote skull as an example to learn the anatomy, then find the same features on the skulls of other species.

 

First, you should remember some basic terms for describing bone shapes:

• Process: any bony structure that sticks out, generally forming a place for a muscle to attach.
• Fossa: an indentation in a bone. The fossa is the socket part of a ball-and-socket type joint.
• Condyle: the convex part of the joint; the ball part in a ball-and-socket joint.

Now examine these specific skull features:

• Orbit: the eye socket. In a carnivore such as this coyote, the orbits face forward, providing good binocular vision. In a species that's more likely to be prey than predator (such as a rodent or a deer), the orbits face to the sides, providing a wide field of view but less effective binocular vision.
• Sagittal crest: a ridge along the top of the head, formed where the parietal bones from each side meet in the middle. The sagittal crest forms an attachment point for extra-large temporalis muscles. The temporalis is one of the main muscles responsible for biting and chewing; it extends from the sagittal crest down to the coronoid process. An animal with a powerful bite is likely to have large temporalis muscles, a sagittal crest, and large coronoid processes. Many mammals do not have a sagittal crest; the muscles simply attach to the sides of the skull.
• Coronoid process: a flat structure sticking up from the mandible (lower jaw). The temporalis muscle attaches to the medial (inner) side of the coronoid process.
• Auditory bulla (also called typmanic bulla): a hard, compact structure that contains the inner ear structures.
• Occipital condyles: part of the joint between the skull and the first cervical (neck) vertebra.
• Mandible: the lower jaw bone.
• Mandibular condyle: on the mandible, the convex part of the hinge joint for the jaw.
• Mandibular fossa: on the skull, the concave part of the hinge joint for the jaw. The mandibular fossa is in the temporal bone, and the joint is called the temporomandibular joint.
• Angular process: an attachment point for the masseter muscle, which (along with the temporalis) helps in biting and chewing.
• Nuchal crest: a crest that extends laterally across the back of the skull, forming an attachment point for strong neck muscles. (The nuchal crest is also called the lambdoidal crest, but I won't use that term in this class.)
• Zygomatic arch: also called the cheekbone. Formed by the zygomatic bone and part of the temporal bone, the zygomatic arch provides some protection for the eye and also provides an attachment point for the masseter muscle used for chewing. The other main jaw muscle, the temporalis, runs along the side of the skull from the temporal bone and sagittal crest to attach to the coronoid process of the jaw, and it passes inside the zygomatic arch. If the temporalis muscle is large, the zygomatic arch must be wide to accommodate its size.
• Foramina (singular: foramen): An opening in the skull, providing a place for nerves or blood vessels to pass through the bone. The largest is the foramen magnum, through which the spinal cord passes. There are numerous  other small holes in the skull — too many to remember in one day, and they won't be on the test!

More about the temporomandibular joint (TMJ): This joint functions as a hinge for the jaw. In carnivores, it's typically a tight hinge. However, in other orders, the hinge is loose. A loose TMJ allows for sideways motion of the jaw which is important for animals such as cattle and sheep, which grind sideways when chewing their food. The TMJ is loose if the mandibular condyle is open and shallow, and if the mandibular condyle moves freely within the fossa. The temporomandibular joint is found only in mammals, not in other vertebrates; see the reptile skulls page for more on this.

Coyote skull: temporalis & masseter muscles

The temporalis and masseter are large muscles for mastication (chewing and biting), and they vary in relative size and shape among the different orders of mammals. (Illustrations below by Alice Wang.)

In a carnivore such as this coyote, the temporalis is larger than the masseter; it originates from the sagittal crest and inserts into the large coronoid process. This arrangement allows for a powerful bite with the jaws wide open, but isn't suited to sideways grinding with the molars. Note that the temporalis must pass inside the space of the zygomatic arch. An animal with a powerful bite will probably have a large zygomatic arch, both because it needs to provide enough space for the temporalis and because it must be stout enough to support the masseter.

In orders of mammals that grind their food with sideways jaw movements (such as Rodentia and Cetartiodactyla), the temporalis is small and the masseter greatly enlarged. The masseter is attached directly to the zygomatic arch, which must be thick and strong in mammals with powerful masseters.

Coyote: Order Carnivora

Now that you have examined the coyote skull in detail, let me summarize the features that identify it as a member of the order Carnivora. You should be able to identify these features on the coyote or on skulls of other carnivores:

• Large sagittal crest: A distinctive feature when it's present (as in the coyote), but not all carnivores have it.
• Orbits face forward: providing good binocular vision, this arrangement is common for predators but not for their prey.
• Mixed dentition with large canine teeth, crushing molars, and distinct carnassial teeth.
• Tight temporomandibular joint, allowing for a powerful bite but not sideways grinding motion of the jaw.
• Nuchal crest is wide and well-defined. This provides a space for the attachment of neck muscles that are used for moving the head sideways while tearing apart prey.

Some of these features will also be present in members of other mammalian orders.

Black-tailed deer: Order Cetartiodactyla

This skull provides a good example of a cetartiodactyl, with adaptations to a herbivorous lifestyle and to avoiding predators.

This skull differs from the coyote in several significant ways:

• Orbits are on the side of the skull, providing excellent peripheral vision but limited binocular vision.
• Cheek teeth are continuously growing, an adaptation to the continual wear of a diet of vegetation. Also note the grooves in these teeth, produced as the lower jaw grinds from side to side. All the cheek teeth look the same, unlike the specialized teeth of carnivores.
• No upper incisors; the lower incisors meet a bony plate on the upper jaw. (This is true for deer and cattle, but not for pigs).
• Large gap (diastema) between the incisors and the cheek teeth.
• No canine teeth. Most, but not all, artiodactyls lack canine teeth (but see the pig skull for an exception).
  
• Temporomandibular joint is loose, allowing for sideways grinding of food.
• Large angle on mandible for attachment of the large masseter muscles that are used for sideways grinding of food. Unlike the Carnivora skulls, there is no hook-shaped angular process, just a large angle of the jaw.
• Antlers. The antlers are not a permanent feature of the skull; they form each year and fall off at the end of the breeding season. The antlers themselves are not made of bone, but this skull has bony supporting structures that provide a place for the antlers to form.
• No sagittal crest.
• Narrow nuchal crest. Provides an attachment point for a nuchal ligament and for muscles that hold the head up, but does not allow for the large muscles that allow for tearing sideways movements in Carnivora.

The deer displays many classic features of the order Artiodactyla, but some other artiodactyls may lack some of these features.

Pig (Order Cetartiodactyla)

The pig is an unusual cetartiodactyl, adapted to a wide-ranging omnivorous diet.

Which features tell you that this is an artiodactyl? Which ones might make you think it's something else?

Rear view

From the rear, you can see some additional features:

• Tall, narrow nuchal crest. Similar to that of a deer, but completely different from the wide nuchal crests of carnivores such as coyotes.
• Long paroccipital processes. These structures provide attachments for some chewing muscles called digastric muscles.
• Foramen magnum. Every skull has a foramen magnum, the hole for the spinal cord. On either side of the foramen magnum are the occipital condyles, which articulate with the first vertebra (which is called the atlas).

Like other artiodactyls, pigs have fairly a loose temporomandibular joint, allowing them to grind their food with sideways movements of the jaw. Take a look at this video from XROMM (X-Ray Construction of Moving Morphology) at Brown University for a good look at these lateral jaw movements. Pigs are more omnivorous than most cetartiodactyls, and their teeth are more varied. Their cheek teeth are covered with a solid layer of enamel, unlike the layers of enamel and dentine that you can see in the teeeth of the deer.

Porcupine (Order Rodentia)

The order Rodentia includes larger species such as beavers and porcupines, along with smaller representatives such as mice and rats. The porcupine provides a good example of typical rodent skull characteristics.

Important features to note include:

• Large, continuously growing upper and lower incisors (two upper and two lower). The incisors only have enamel on their front faces; the back parts of the teeth are made of softer dentin. The upper and lower incisors don't meet directly, so they tend to sharpen each other as they continuously grow.
• No canine teeth; there is a gap (diastema) between the incisors and the molars.
• Large, continuously growing molars with alternating layers of enamel and dentin on the biting surfaces.
• Loose temporomandibular joint.
• Orbits face outward.
• No (or very small) sagittal crest. Note that the beaver skull, which looks similar to the porcupine (but bigger), does have a small sagittal crest.
  

Rodent skulls are distinctive, but it's possible to mistake a rabbit skull (order Lagomorpha) for a rodent.

Rabbit: order Lagomorpha

Rabbit skulls resemble those of rodents, but there are some distinct differences.

• Small holes (fenestrae) in many of the skull bones. This feature alone makes lagomorphs distinctly different from rodents. It has been suggested that these fenestrae are important for cooling the animal's blood, but it's not clear to me why this would be uniquely important for the lagomorphs. These fenestrae aren't found in other orders.
• Four upper incisors. The front pair of incisors resembles those of a rodent, but there is a second, smaller pair hidden behind them. Rodents don't have these.

For comparison, here is an X-ray of a rabbit skull:

 

Note the length of the teeth, which grow continuously. The chewing faces of the teeth aren't completely covered with enamel. As the teeth wear down, they expose layers of hard enamel and softer dentin. As the dentin wears faster than the enamel, the teeth continually show new sharp edges for chewing. The molars are similar to those of rodents or artiodactyls.

Reference:

Eastern Cottontail Rabbit on Digimorph. This page shows a detailed 3-D X-ray scan of a rabbit skull; if you click on roll, pitch, or yaw, you can rotate the skull in space.

Chimpanzee: order Primates

The same order as humans.

Sample test questions

On the lab exam you might be presented with any of these skulls, along with questions such as:

1. What order does this specimen belong to? (You should know the scientific names of the orders.)
2. What specific features does skull A have that tell you it's not in the same order as skull B?
3. If you can't identify the order of a skull, what features would tell you whether the animal was a plant eater or a meat eater?
4. What is this skull feature? (See "Skull terminology" above for possible answers.)
5. What is the temporomandibular joint? How does this joint differ among mammal skulls from the Carnivora compared to the Cetartiodactyla, and why is the difference significant? What groups of vertebrates have a temporomandibular joint, and which don't? (See the reptile skulls page for more information on this.)

References and further reading

California Academy of Sciences skull exhibit.

Digital Morphology. Amazing 3-dimensional views of skulls and other specimens.

Rodent Jaws on Animal Diversity Web (University of Michigan): Rodent jaws are adapted for chewing and grinding food, and they typically have very large masseter muscles. Different groups of rodents show different styles of chewing muscle structure; you can view it in excruciating detail on this site.

Hystricognathy vs Sciurognathy in the Rodent Jaw: A New Morphometric Assessment of Hystricognathy Applied to the Living Fossil Laonastes (Diatomyidae). If you are deeply fascinated by rodent jaws, you might want to look at this research article from the scientific journal PLOS One. It's far beyond Bio 6A, but it provides an example of how comparative functional anatomy can be used in a modern research context.

Skull on Wikipedia. A good overview.

Guide to Identification of Pinniped Skulls of the Farallones National Marine Sanctuary. A brief pdf handout with detailed photos of the skulls of California seals and sea lions.

Books

Elbroch, Mark, 2006. Animal Skulls, A Guide to North American Species. Stackpole Books. This is an outstanding book for anyone with an interest in skulls.

Kardong, Kenneth, 2012. Vertebrates: Comparative Anatomy, Function, Evolution (6th Edition). This  textbook is an excellent resource for understanding vertebrates.