HONR 259C "Fearfully Great Lizards": Topics in Dinosaur Research

Fall Semester 2017
Comparative Anatomy, Homology & Analogy

Diplodocus (left) and Apatosaurus (right; with incorrect skull), at the Dinosaur Hall of the Carnegie Museum of Natural History, 1934

Key Points:
•Osteology is the study of the skeleton.
•The bodies of vertebrates have homologous structures (the same body part, regardless of its particular shape and function) as well as analogous structures (non-homologous parts used for the same function).
•Bones and other features of anatomy are given formal names (normally based on Latin).
•Anatomical directions and landmarks are used to described the position of different anatomical features.
•Major sections of the skeleton include the skull, the axial skeleton (vertebral column and other parts along the main trunk of the body), and the appendicular skeleton (the pectoral girdle and forelimb, and the pelvic girdle and hindlimb.)

Labeled skeletons of Camarasaurus and Tyrannosaurus; image Courtesy of Sauropod Vertebra Picture of the Week

Homologous structures: the same anatomical structure, regardless of function.

Analogous structures: represent different units of anatomy serving the same function.

Comparative anatomy seeks to describe the structure of the bodies of organisms in terms of their homologous structures.

Functions of the skeleton:

Anatomical Directions

Anatomical views: when a specimen is illustrated, the anatomical view represents that surface of the specimen that is shown.

Anatomical landmarks: particular homologous structures on the skeleton (openings, joints, etc.) used for identifying the position of bones or other features of the anatomy.

The skeleton of a dinosaur (or other vertebrate) is divided into a couple of different sections:

(Incidentally, anatomical terms are for the most part based on Latin words. Bones or landmarks with Latin rather than English plurals are noted below)

Complications in Names: The anatomical terminology we use in this course has a long tradition: used by Owen, by 20th Century paleontologists such as Alfred Sherwood Romer and George Gaylord Simpson, and many others. It includes standardized terminology for non-mammalian vertebrates. That said, mammalogists have long used different names for a few of these bones (for instance, the "jugal" is instead called the "zygoma" and the "dorsal" series of vertebrae is subdivided into an anterior "thoracic" section and a posterior "lumbar" section). Furthermore, human anatomy (and veterinarian anatomy) mixes the terminology even more, particularly with regards to anatomical directions. So there may be some difference in names we use here compared to ones you have encountered elsewhere.

Important Bones and Landmarks of the Skull
NOTE: Almost all bones and landmarks of the skull are paired, with one on the right side and one on the left. Although the skulls of vertebrates are composed of many bones, these bones are joined by sutures: depending on the type of suture, the joint can be mobile or immobile.

Teeth are composed of materials (softer dentine and harder enamel) similar to bone. Teeth have a root which fits into the socket of the jaws and a crown covered with enamel which chops, crushes, pulps, tears, slices, and/or grinds food.

Most types of dinosaur teeth do not show occlusion (when one surface meets another). In all types of toothed dinosaur, the teeth are renewed throughout life.

Bones and Landmarks of the Axial Skeleton
Most of the axial skeleton is composed of the vertebral column, itself composed of individual vertebrae (singular, vertebra). Each vertebra contains the following sections:

  • Centrum (pl. centra): the large spool-shaped body
  • Neural arch: an arch of bone on dorsal surface of the centrum
  • Neural canal: the hole through which the spinal chord passes. (Popular conception to the contrary, the spinal cord does not pass through the centra
  • Transverse processes: bony extensions off the lateral sides of the neural arch, for attachment of muscles, tendons, ribs, etc.
  • Neural spine: bony extension off the dorsal surface of the neural arch
  • Various other prongs and crests off the neural arch and centrum, not dealt with in this class

    Skeleton of Giraffatitan, with sections of the vertebral column labeled; image Courtesy of Sauropod Vertebra Picture of the Week

    The vertebral column is divided into four sections in dinosaurs and their relatives:

    Labeled portions of the skeleton of Camarasaurus; image Courtesy of Sauropod Vertebra Picture of the Week

    Labeled skeleton of Triceratops: image courtesy of Sauropod Vertebra Picture of the Week

    Attached to the cervical and dorsal vertebrae are ribs (one on each side). Sacral ribs also exist, but are often fused to the pelvic girdle (see below). Instead of ribs, caudal vertebrae have chevrons, single bones which protect the nerves and blood vessels that run underneath the caudal centra.

    Ventral to the guts of dinosaurs and many other land vertebrates are gastralia (singular gastralium), or "belly ribs". In dinosaurs they are one or two pieces per side.

    Some dinosaurs have dermal ossifications or scutes: bones in the skin of the animal used for armor.

    Bones and Landmarks of the Appendicular Skeleton
    The appendicular skeleton is comprised of the limbs and their girdles (bones that attach the limbs to the axial skeleton.

    The Pectoral Girdle The forelimb is attached to the dorsal part of the axial skeleton by the pectoral girdle. The pectoral girdle is composed of the following bones:

  • Scapula (pl. scapulae): the shoulder blade
  • Coracoid: a bone on the ventral side of the shoulder blade. The shoulder joint of dinosaurs faces mostly posteriorly
  • Clavicle: collar bone. Paired and separate in most dinosaurs, but in meat-eating dinosaurs the clavicles are fused along the midline to form a single bone, the furcula (pl. furculae), or "wishbone".
  • Sternum (pl. sterna): the breastbone. In some dinosaurs it is composed of separate sternal plates; in other it is fused. It is on the ventral surface of the chest

    Left lateral view of the left pectoral girdle and forelimb of Camarasaurus; image Courtesy of Sauropod Vertebra Picture of the Week

    Right lateral view of right pectoral girdle and forelimb of Centrosaurus; image courtesy of The Open Dinosaur Project

    The Forelimb

  • Humerus (pl. humeri: upper arm bone. Meets with the scapula & coracoid at the shoulder, and the radius and ulna at the elbow
  • Ulna (pl. ulnae): (generally) larger and more posterior of the forearm bones. The "funny bone" (techincally the olecranon process) is the backwards-pointing projection of the ulna.
  • Radius (pl. radii): smaller and more anterior of the forearm bones.
  • Manus (pl. manus): the hand. Composed of:

    Anterior view of right manus of Centrosaurus; image courtesy of The Open Dinosaur Project

    The Pelvic Girdle

    Left lateral view of the left pelvic girdle and hindlimb of Camarasaurus; image Courtesy of Sauropod Vertebra Picture of the Week

    The hindlimb is attached to the sacral part of the axial skeleton by the pelvic girdle (aka the pelvis (pl. pelves) or "hips"). The pelvic girdle is composed of three bones on each side:

  • Ilium (pl. ilia): the dorsalmost of the bones, which connects directly to the sacral vertebrae
  • Pubis (pl. pubes>: the lower pelvic bone that always attaches to the ilium anterior to the ischium (see below), although the shaft of the pubis in some dinosaurs points backwards
  • Ischium (pl. ischia): the lower pelvic bone that always attaches posterior to the pubis, and points posteriorly as well
  • Acetabulum (pl. acetabula): the hip socket, where the femur (see below) fits into the pelvis. In most vertebrates there is a sheet of solid bone formed by the pelvic bones on the medial surface of the acetabulum, but dinosaurs are specialized in having a perforate (opened) acetabulum (i.e., only a sheet of cartilage rather than bone on the medial surface).

    Right lateral view of right pelvic girdle and hindlimb of Centrosaurus; image courtesy of The Open Dinosaur Project

    The Hindlimb Note that the structure of the hindlimb is very similar to that of the forelimb.

  • Femur (pl. femora: thigh bone. Fits into the acetabulum by the femoral head, and meets the tibia and fibula (below) at the knee. Often the single largest bone in the body (except for small running dinosaurs, in which the tibia is generally larger).
  • Tibia (pl. tibiae): the main shin bone. Generally thicker than, and medial to, the fibula
  • Fibula (pl. fibulae): smaller and lateral of the shin bones. Note that, popular misconception to the contrary, there is NO such bone as a "fibia"!
  • Pes (pl. pedes): the foot. Composed of:
    Anterior view of right pes of Centrosaurus; image courtesy of The Open Dinosaur Project

    Excellent overview of dinosaur anatomy, and here, and here as well.

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    Last modified: 1 September 2017

    Detail of anatomy of Tyrannosaurus by Scott Hartman, from Scott Hartman's