Mammal Skeletons

The skeleton of each vertebrate species is shaped both by its evolutionary ancestry and by ongoing adaptations to its way of life. In this lab you'll have an opportunity to closely examine a number of bone specimens and learn their important features, and to get an introduction to comparative functional anatomy.

Objectives

By the time you complete this lab, you should be able to identify some specific bones and other features of some vertebrate skeletons and compare and contrast these features in terms of functional anatomy. Be prepared to answer detailed compare and contrast questions on skeletons and skulls. For the introduction page (the page you're on now), you should be able to do the following by the time you complete the lab:

• Recognize all the structures listed in bold type on this page, and compare and contrast them among the various skeletons in lab.
• Recognize these types of joints and explain how they differ: suture, symphysis, synovial joint.
• Recognize the regions of the vertebral column (cervical, thoracic, lumbar, sacral, caudal) and explain how they differ.
• Compare and contrast these bones among the different the skeletons listed on this page:
  • Pectoral girdle and forelimb: scapula, clavicle, humerus, ulna, radius.
  • Pelvic girdle and hind limb: femur, tibia, fibula

After you do this page, go on to the other pages and work on the objectives you see there. To be sure you've got it all, go to the skeleton lab review page after you've completed the whole lab.

Specimens

• Cat skeleton
• Monkey skeleton
• Human skeleton
• Mole skeleton
• Loose vertebrae

Skeleton concepts

The main purpose of this lab isn't to get you to memorize the names of all the bones, it's to get you thinking about comparative functional anatomy. However, if you're going to talk about skeletal anatomy, you need to know the names of a few bones and other skeletal features.

Joints

A joint is any connection between two bones. Some joints are flexible, and some aren't. Identify the following types of joints on a human skeleton. Inflexible joints come in two main types:

• Suture: Two bones are tightly connected, with no flexibility and no cartilage between them. Example: bones of the cranium (main part of skull, not including the jaw).
• Symphysis: Two bones less tightly connected, with very little flexibility and often some cartilage joining them together. Example: connection between the left and right sides of the lower jaw (mandibles). Also the left and right pubic bones.

Synovial joints (also called diarthroses) are more flexible. A synovial joint typically has two main parts: the condyle, which is a rounded part that sticks out, and the fossa, which is a concave part that loosely holds the condyle. Synovial joints come in a range of styles, but many fit into one of these categories:

• Ball-and-socket joint: The ball is a condyle, and the socket part is a fossa. Examples: shoulder and hip joints.
• Hinge joint: Moves only in a single plane, unlike the more flexible ball-and- socket. Example: knee.
• Pivot joint: Allows rotation around the axis of the bone. Example: vertebrae of the neck, moving when you turn your head. Also the joint between the radius and the humerus in your elbow; when you rotate your hand from facing up to facing down, the radius pivots around its long axis. Meanwhile, the joint between the humerus and the ulna acts like a hinge joint.

There are also other styles of joints, and many real joints don't fit neatly into one of these categories. Also, a joint isn't usually a direct connection between two bones; there are other tissues involved, such as cartilage to allow smoother movement and ligaments to help stabilize the joint.

Muscle attachments

Muscles must attach to bone to allow for locomotion. The attachment is usually indirect, with a tendon connecting the muscle to the bone. In many cases, tendons or muscles are attached to processes, which are simply parts of the bone that stick out to allow for attachments. If it's a large muscle, the bone must have a large process for its attachment. Each muscle normally has two attachments on two different bones. The origin is the proximal attachment (closer to the body's center), and it doesn't move when the joint is flexed. The insertion is the more distal attachment (farther out on a limb), and it moves when the joint is flexed. As you look at skulls and skeletons in the rest of the lab, you can study animal function by comparing the relative sizes of the processes that hold specific muscles.

Directional terms

These terms are used to describe where anatomical features are located.

• Anterior: toward the head.
• Posterior: toward the tail.
• Dorsal: toward the back.
• Ventral: toward the belly.
• Proximal: closer to the backbone.
• Distal: further from the backbone.

Axial and appendicular skeleton

Vertebrate skeletons are divided into the axial skeleton (the body's main axis, including the vertebral column and the skull) and the appendicular skeleton (the limbs and their supporting bones; "appendicular" refers to the fact that this part of the skeleton supports the appendages). 

Tetrapods

"Tetrapod" means 4-legged, or, more precisely, 4-limbed. Dogs are tetrapods; so are humans and birds. Fish aren't tetrapods, because they lack arms or legs. However, snakes are considered tetrapods because their ancestors had four legs and were part of the tetrapod lineage. The word tetrapod is both a description of the body and a designation of a clade; all the tetrapods form a monophyletic group.

Human skeleton

The human skeleton is a good place to start with learning the names of the bones of mammals. Fortunately, most of the bones have more or less the same names in other species of animals. Here is a diagram of a human skeleton, from (Wikipedia).

Axial skeleton

Vertebral column (backbone): Each vertebra has a vertebral body and a vertebral foramen, which is the space that encloses the spinal cord dorsal to the vertebral body. The bony arch that creates the vertebral foramen is called the neural arch. The dorsal side of each vertebra also has a spinous process, which provides a place for muscle attachments.

Between the vertebrae there are intervertebral discs, which form slightly flexible symphyses between the vertebrae. The interior of each disc contains a soft nucleus which is derived from the notochord. For more information, see the Wikipedia page on intervertebral discs.

While the backbone of the fish is divided into two regions (trunk and tail), the vertebral columns of mammals are differentiated into several regions:

• Cervical vertebrae: The neck region, between the skull and the thoracic vertebrae. There are seven cervical vertebrae in all mammals, whether large or small. You can recognize individual cervical vertebrae because they have an opening on each side (called the transverse foramen) which provides a space for both blood vessels and nerves. Only cervical vertebrae have these openings, regardless of mammal species.
• Thoracic vertebrae: The chest region. Thoracic vertebrae have flattened areas (facets) on each side to articulate with ribs.
• Lumbar vertebrae: Between the thoracic region and the sacral region, the lumbar vertebrae don't have ribs attached.
• Sacral vertebrae: The sacral vertebrae are fused together into the sacrum, which in turn is fused with the hip bones to make up the pelvic girdle.
• Caudal vertebrae: The tail. In humans, the 3-5 caudal vertebrae are very small; together, they're called the coccyx.

If you look at some loose (disarticulated) vertebrae, you should be able to recognize with region of the vertebral column they belong to, even if you don't know what species they came from. See Vertebra on Wikipedia for more information.

Skull: The skull is also part of the axial skeleton. See the Skulls page for more on skull characteristics.

Appendicular skeleton: pectoral girdle and arms

The pectoral girdle consists of the bones that support the arms. For humans, the pectoral girdle consists of two bones on each side:

• Scapula: The shoulder blade. Provides attachments for some back muscles and helps form the socket of the shoulder.
• Clavicle: The collarbone. Helps form the socket of the shoulder and provides attachments for stabilizing muscles in front.

Appendicular skeleton: pelvic girdle and legs

The pelvic girdle consists of the hip bones, which connect the legs to the vertebral column. The hip bones (ilium, ischium, and pubis) are tightly fused together so they functionally become a single bone, which is also called the innominate bone. The innominate is in turn tightly connected to the sacrum, which is a set of vertebrae that are fused and flattened to connect to the pelvic girdle.

In the legs and feet, you should recognize these main bones:

• Femur
• Tibia
• Fibula
• Tarsals
• Metatarsals

Is this skeleton male or female? See radiologypics to find out.

Monkey skeleton

As you might expect, the bones of the monkey skeleton are very similar to those of a human. Imagine that this skeleton was standing upright -- how would you recognize that it's a monkey and not a human?

You should be able to recognize the bones labeled on this picture.

Cat skeleton

Cat skeleton image by Przemek Maksim, Wikimedia Commons.

Note the shape of the ribcage. In cats, it's deeper (dorsal to ventral) than it is wide (left to right). In humans, it's the other way around. Why? How does this compare to other animals?

The clavicles (collarbones) are very small, and they don't articulate with any other bone (in fact, in prepared cat skeletons, the collarbones must be held on with wire). How would this size reduction affect the stability and mobility of the shoulder joint?

The cat is a good example of digitigrade locomotion, described below.

Numbered bones:

The bones listed in bold are the ones you might be asked about on the lab exam.

Dog skeleton

We don't have a dog skeleton in the lab, but here's a nice old (public domain) illustration:

Take a close look and think about how this skeleton uses more or less the same set of bones as a human, but optimized for four-legged running. Some things to note:

• The ribcage is deep dorsoventrally (from top to bottom) and narrow laterally (side to side). Compare this to a human ribcage. Why would this be the case? Which mammals have narrow ribcages, and which have wide ones?
• The scapulas (shoulder blades) are positioned on the sides, not on the back as on a human. When a dog runs, the scapulas can swing forward to back, effectively increasing the length of the stride.
• There are no clavicles (collarbones). Cats have small clavicles, while dogs have none at all. Why do you think this would be the case? Why would humans be different? What about a macaque (the monkey skeleton in lab)?
• The neck vertebrae have large lateral processes. What do these do? Look at these in conjunction with the skull of a coyote or fox.

Styles of posture & locomotion

Most mammals have more or less the same bones in their limbs, with some important variations related to their styles of locomotion. Tetrapod locomotion can be be divided into three styles, depending on how much of the foot is in contact with the ground on each stride.

Mole skeleton

Moles are not built for speed; they’re bult for strength. Moles are small mammals that live underground and eat earthworms and other invertebrates. They are adapted to a fossorial (digging) lifestyle. Moles’ closest relatives are shrews, which are small insect-eating mammals that do not dig.

Don’t confuse moles with gophers, another common digging animal. Gophers are rodents, and they eat mostly plants. If small mammals are digging up your garden and eating your plants, those are probably gophers.

The mole's skeleton shows some interesting adaptations for digging.

• Extended ulna. Find the mole's ulna, one of the forearm bones. Your own ulna forms the point of your elbow and extends along your forearm to your little finger. On the proximal end (closest to the body) of the mole's ulna, there is a large process extending beyond the elbow. This extended ulna looks and acts like a lever, allowing the mole to forcefully "swim" its way through the soil.
• Short humerus with large flat areas for muscle insertions.
• Keeled sternum. The sternum has a ridge, or keel, that sticks out from the mole's chest. This provides extra surface area for the attachment of large pectoral (chest) muscles used in digging. Compare the slightly keeled sternum of the mole to the massively keeled skeletons of birds, which rely primarily on chest musculature for flying.
• The mole’s extra "thumbs". How many fingers does the mole have? Look closely. Are you sure those are all fingers? To understand the mole’s hand anatomy, take a look at the images in the reference articles listed below. What appears to be a large, flat, blade-like thumb is actually an extended sesamoid bone (technically called the os falciforme), not a thumb. In humans and many other mammals, the sesamoid is a small bone in the hand, but in the mole it becomes extended to act like an extra thumb that helps in digging.

The mole skeleton we have in lab is Talpa europea, the european mole. If you see a wild mole in our area, it's probably a broad-footed mole, Scapanus latimanus. If you happen to find a dead mole, take a look at its hands. You might not see an extra thumb visible, but that extra os falciforme bone is hidden inside.

Mole  skeleton references

The Mole Loses Its Mysterious Second Thumb Science

Circumventing the polydactyly ‘constraint’: the mole's ‘thumb’: Christian Mitfutsch et al., 2011. Biology Letters.

The mole's humerus from the BBC show Secrets of Bones.

Terminology review

On a lab exam or quiz, I might ask you to recognize any of the following bones or other features. You should be able to recognize them on the skeletons of a human, monkey, or cat.

1. Compare and contrast the clavicles of the cat and monkey skeletons. Why are they different? What about the clavicles of a human or a dog?
2. What features of the mole skeleton show that this animal is adapted for digging? Why does the mole seem to have six fingers on its hands?
3. What are the regions of the mammalian vertebral column? Why are the vertebrae different in the different regions, and how can you recognize which region a particular vertebra comes from?
4. What do the terms plantigrade, digitigrade, and unguligrade mean? Why are these characteristics important? Give an example each.
5. Define symphysis, suture, and synovial joints. Give an example of each from the human skeleton.

References & further reading

Mammals

Bone I.D. A website intended to help people identify bones.

Veterinary Gross Anatomy: Ungulate Dissection from Veterinary Anatomy at  the University of Minnesota College of Veterinary Medicine. Detailed pictures and descriptions of horse anatomy.

Anatomical Adaptation for Cursorial Locomotion from Veterinary Anatomy at  the University of Minnesota College of Veterinary Medicine. Overview of anatomical specializations for running, comparing herbivores and carnivores.

Animal movement

Bodies in Motion (paywall) from National Geographic. This page includes outstanding motion graphics showing the skeletons of various kinds of vertebrates in motion. Unfortunately, it's only visible to National Geographic subscribers. I don't usually include subscriber-only links, but this one is excellent; you might be able to view it through the library.

General Reference Books

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.