Dinosaur Anatomy Related to Classification: Old and New

Summary Discussion Questions Bibliography

Chapter 1 gave a preliminary definition of a dinosaur as a reptile-or bird-like animal with an upright

Refined Definition of "Dinosaur"

posture that spent most of its life on land. In this chapter we can now expand this definition. A fossil must have the following characters before it can be called a dinosaur:

■ Three or more sacral vertebrae.

■ Shoulder girdle with backward-facing (caudally pointing) glenoid.

■ Asymmetrical manus with less than or equal to three phalanges on digit IV.

■ Acetabulum with open medial wall.

■ Tibia with cnemial crest.

■ Astragalus with a long ascending process fitting into the anterior part of the tibia.

■ Sigmoidally-shaped third metatarsal.

■ Postfrontal absent.

■ Humerus with long deltopectoral crest.

■ Femur with ball-like head on proximal end.

This chapter dealing with dinosaur anatomy should provide a better understanding of how the following can be done:

1 identify anatomical traits essential for clearly distinguishing dinosaur fossils from other closely related forms;

2 classify dinosaurs on the basis of shared traits helping to link evolutionary relatedness of different dinosaur groups; and

3 assemble these groups through a classification based on shared anatomical attributes (cladistics) to form a picture of dinosaur evolution throughout the Mesozoic.

Such a picture is still being sketched to connect dinosaurs to their ancestors and living descendants.

Skeletons are what primarily define dinosaurs, rather than the soft parts of their anatomy or trace fossils. The classification of different dinosaurs into groups based on their anatomical traits depends on correct assessment of their body plans as revealed by their bones, necessitating a thorough knowledge of skeletal anatomy. Such methods are practical because of the geological circumstances that favor preservation of skeletal material. In contrast, dinosaur integuments, that is, derivatives of their skin, including feathers, are rarely found in the fossil record (Chapter 7). Notable exceptions to this generalization are foot impressions preserved as tracks (Chapter 14) and now-less-rare finds of feathered theropods from a Lower

Cretaceous deposit in China (Chapters 9 and 15). These infrequently preserved parts are also anatomical characters of dinosaurs and provide useful supplementary information for independent cross-checks of the interpretations of evolutionary lineages based on skeletal data (Chapter 6). Dinosaur body plans also indicate inferred behavior of dinosaurs based on functional morphology. These then can be compared to behavior indicated by dinosaur trace fossils (Chapter 14) or to the behavior of extant animals that serve as analogues of dinosaurs.

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