Theropod Skeletons





Stereochemistry Terminology
FIGURE 5.1 Orientation terminology as applied to anatomical features in vertebrates, using the skeletons of the Early Cretaceous theropod Deinonychus antirrhopus (left) and a modern human Homo sapiens (right).

variations in a species and the nature of fossilization, paleontologists usually only have pieces of what used to be entire bones. Thus, dinosaur paleontologists need to be extremely skilled anatomists who are used to working with fragmentary material in their reconstructions and restorations of ancient vertebrates. This tradition extends back to the eighteenth and nineteenth centuries, with paleontologists such as Georges Cuvier and Richard Owen (Chapter 3).

Before progressing with dinosaur anatomy, learning front, rear, sides, and other references on a dinosaur skeleton is necessary. Learning this orientation requires familiarity with some directional terms that anatomists use to describe surfaces on an animal's body or its parts. The head region of an animal is its anterior, whereas its rear end is the posterior. The same terms can be applied to any bone or place on a dinosaur skeleton that is located more toward a forward- or backward-facing region, hence the anterior part of the tibia, the posterior part of the fibula. These terms are also applied as the prefixes "ant" or "post", such as antorbital fenestra or postorbital fenestra. Another prefix used for bones that occur in front of another bone is "pre" (e.g., premaxilla, predentary). These prefixes should not be confused with the adjectives dorsal and ventral; dorsal is on the back and ventral is on the front of a standing person, but they denote top and bottom when applied to a crawling baby. For example, chordates are animals characterized by a dorsal nerve cord, which means that the nerve cord is located near the animal's spinal area as opposed to its ventral belly region.

The medial (middle) part of the body can be defined on the basis of proximity to the midline, which demonstrates the bilateral symmetry (left-right sameness) of a typical vertebrate body, observable from either the dorsal or ventral surface of an animal. The medial part of the body can also define its axis, of which its associated bones are the axial skeleton (skull, spine, hips, and tail). Any body parts extending to the sides of the body and away from the midline, such as appendages, are lateral and these bones comprise the appendicular skeleton. Dorsal, ventral, medial, and lateral can be applied as directional modifiers to any given skeletal part or area. For example, a hat held by its chinstrap is ventral to the jaw and lateral to the cheek, whereas the hat itself is dorsal to all other skull bones and covers the medial surface of the skull.

Appendicular parts also have their own modifiers based on how close they or their parts are to the medial part of the body. The proximal parts of a limb are in close proximity to the medial part of the body, but the distal parts of that same limb are more distant. For example, the longest leg bone in the human body, the femur, fits into the hip region and so is more proximal than foot bones (metatarsals, tarsals, and phalanges, in increasingly distal order) on the same leg.

Probably the easiest way to divide a dinosaur skeleton is to use the terms cranial and postcranial. Cranial refers to all of the bones and other features associated with the skull, which is at the anterior part of the dinosaur, whereas any bones or features toward the posterior of the skull are considered postcranial. This seemingly disparate division is justified by the complexity of some dinosaur skulls, which can contain more than 30 different bones and as many as 200 individual teeth (Chapter 9); the description of these parts details their distinctive characters. Postcranial parts can also be considered as oriented toward the caudal (tail) region of the dinosaur.

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