Animals > Chordates > Mammal Skulls
See Bio 6A lab manual, Chapter 16.
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.
This page shows some examples of skulls labeled with the terminology that is typically used in describing and identifying them.
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.
- Sagital 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.
- 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): There are numerous small holes in the skull, providing places for either nerves or blood vessels to pass. There are far too many to remember, 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.
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.
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.
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.
To learn more:
- 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.
This page updated November 29, 2012