HONR 259C "Fearfully Great Lizards": Topics in Dinosaur Research
Spring Semester 2007
Comparative Anatomy, Homology, and Osteology
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.
Guides to postcranial anatomy and
- Anterior (towards the tip of the snout)/Posterior (towards the tip of the
- Dorsal (up and out through the spine)/Ventral (down and out through
- Medial (towards the middle)/Lateral (towards the sides)
- Proximal (towards the trunk)/Distal (away from the trunk)
- Proximal and distal are normally used only for the limbs, and occasionally for the tail
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
- The skull, composed of:
- The cranium (braincase, face, and upper jaw)
- The mandible (lower jaw)
- The postcranium (everything posterior to the cranium), composed of:
- The axial skeleton (spine, ribs, and related features of the neck, trunk, and tail)
- The appendicular skeleton (forelimb, hindlimb, and their girdles)
(Incidentally, anatomical terms are for the most part based on Latin words. Bones or landmarks
with Latin rather than English plurals are noted below)
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.
- Orbit: eyesocket
- Naris (pl. nares): nostril socket
- Antorbital fenestra: a large opening in the facial bones of dinosaurs and their relatives,
anterior to the orbit and posterior to the naris (fenestra, pl. fenestrae:
an large opening in the skeleton, from the Latin word for "window")
- Teeth. In dinosaurs and most other land-dwelling vertebrates, the teeth are found
in three main bones: two on each side of the upper jaw, and on on each side of the lower jaw
- Premaxilla (pl. premaxillae): anterior of the tooth-bearing bones of
- Maxilla (pl. maxillae): posterior of the two tooth-bearing bones of
- Lacrimal: bone separating the antorbital fenestra and orbit, contains the tear
- Postorbital: bone posterior to the orbit
- Jugal: the "cheek bone", ventral to the orbit
- Temporal fenestrae: openings in the back part of the skull for attachment and
expansion of the jaw muscles. In dinosaurs and their relatives, there are two temporal
fenestrae on each half of the skull (left and right):
- Infratemporal fenestra: also called the lateral temporal fenstra, opening
on the side of the skull
- Supratemporal fenestra: opening on the top of the skull
- Nasal: bone along the top of the snout dorsal to the naris and maxilla
- Braincase: a collection of bones which surrounds the brain cavity
- Foramen magnum: Latin for "great opening", the hole in the back of the braincase
where the spinal cord emerges from the brainstem
- Occipital condyle: a condyle (rounded knob joint) composed of several different
bones just ventral to the foramen magnum; the connection between the cranium and the backbone
- Dentary: the tooth-bearing bone of the mandible; in mammals the whole of the mandible
is composed of just the dentary, but in dinosaurs and most other vertebrates there are various
- Mandibular fenestra: in dinosaur and their relatives, an opening on the lateral
surface of the mandible surrounded by the dentary and the postdentary bones
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 surfae meets another).
In all types of toothed dinosaur, the teeth are renewed throughout life.
Bones and Landmarks of the Axial SkeletonCentrum (pl. centra): the large spool-shaped body
Most of the axial skeleton is composed of the vertebral column, itself composed of
individual vertebrae (singular, vertebra). Each vertebra contains the following
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
The vertebral column is divided into four sections in dinosaurs and their relatives:
- Cervical: the neck
- Dorsal: the back
- Sacral: the hips
- (Sometimes the sacral vertebrae are fused into a single unit, called a sacrum)
- Caudal: the tail
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
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 brestbone. In some dinosaurs it is composed
of seperate sternal plates; in other it is fused. It is on the ventral surface of
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
Radius (pl. radii): smaller and more anterior of the forearm bones.
Manus (pl. manus): the hand. Composed of:
- Carpals: various small bones of the wrist. The wrist as a whole is called
the carpus (pl. carpi)
- Metacarpals: the long bones of the palm of the hand. These are numbered I-V, with
I being the medialmost (the one to which the thumb is attached) and V being the lateralmost
(the one to which the pinky is attached). All the metacarpals as a unit are called the
metacarpus (pl. metacarpi)
- Digits: fingers. Digits are numbered I-V as above, with I being the thumb, and
V being the pinky. Digits are composed of individual finger bones or phalanges
(singular phalanx). The distalmost, claw- or hoof-bearing phalanx is called the
The Pelvic Girdle
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).
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
Pes (pl. pedes): the foot. Composed of:
- Tarsals: various small bones of the ankle. The wrist as a whole is called
the tarsus (pl. tarsi. Two tarsals of importance in dinosaurs are the two
proximal tarsals, the astragalus (pl. astragali) and calcaneum (pl.
calcanea), which fit onto the distal ends of the tibia and fibula
Incidentally, dinosaurs and their closest relatives lack a heel (which is formed in
other land vertebrates by a backwards projection of the calcaneum)
- Metatarsals: the long bones of the body of the foot. These are numbered I-V, with
I being the medialmost (the one to which the big toe is attached) and V being the lateralmost
(the one to which the little toe is attached). All the metacarpals as a unit are called the
metatarsus (pl. metatarsi). Unlike humans and bears, but like cats and dogs
and horses (and birds...), dinosaurs held their metatarsi upright, so that their ankles
did not normally touch the ground
- Digits: toes. Digits are numbered I-V as above, with I being the big toe, and
V being the little toe. Digits are composed of individual finger bones or phalanges
(singular phalanx). The distalmost, claw- or hoof-bearing phalanx is called the
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Last modified: 12 January 2007