Appendicular Skeleton Legs and Feet

Dinosaur limb bones, especially if they are associated with a manus (anterior foot, or hand) and pes (posterior foot), are wonderful finds, because they can provide information on how dinosaurs moved about their environments. Foot anatomy in

FIGURE 5.5 Pectoral girdle of the Late Cretaceous hadrosaur Edmontosaurus of North America. Denver Museum of Science and Nature, Denver, Colorado.

FIGURE 5.5 Pectoral girdle of the Late Cretaceous hadrosaur Edmontosaurus of North America. Denver Museum of Science and Nature, Denver, Colorado.

particular is valuable for correlating with footprint data, leading to more refined attributions of tracks to their tracemakers (Chapter 14). The bipedal posture of some dinosaurs also means that their hands were freed for other tasks, such as grasping food or potential mates. Specialized functional adaptations suggested by dinosaur limbs have also been the subject of lively discussion, such as why supposed active hunters like Tyrannosaurus or Albertosaurus had such tiny forelimbs with only two fingers in proportion to their large bodies (Chapter 9), the possible function of a "thumb spike" in Iguanodon (Chapter 11), and whether ceratopsians had semi-sprawling versus erect forelimbs (Chapter 13).

Forelimbs (arms) in dinosaurs were attached proximally to the main torso through the pectoral girdle. The pectoral girdle had as its main bones the scapula (shoulder blade) and coracoid, which interacted directly with the clavicle (Fig. 5.5); the latter is present in some dinosaurs such as saurischians (Chapters 9 and 10) and a few ornithischians (Chapter 13). The pectoral girdle interacted with the ribs of the chest region (thoracic ribs) and sternum, with the clavicle (if present) as an intermediary bone. Sterna (plural of sternum) have been reported from some theropods, sauropodomorphs, ornithopods, thyreophorans, ceratopsians, and birds, but they are not always present in the geologic record because some may have been cartilaginous (composed mostly of collagen: Chapter 8) and thus were not preserved. The clavicles fused in some post-Triassic theropods to form a fur-cula, equivalent to a wishbone. The place on the scapula where it articulated with the humerus is the glenoid, which pointed caudally and is yet another trait of dinosaurs (Fig. 5.6). The humerus rotated in whatever range of motion was defined by the glenoid. More importantly, in combination with the length of its forelimbs, it determined whether the dinosaur could have brought food to its mouth with these forelimbs (Chapter 9).

Another characteristic of dinosaurs is a long deltopectoral crest on the humerus. The way to remember this unwieldy term is to use a human body as a guide. The main shoulder and chest muscles are called a deltoid and pectoral, respectively, so any body part related to both of these would have the combined name of deltopectoral. Now think of a raised portion (crest) on the humerus that related to both the deltoid and pectoral muscles of a dinosaur and this image should illustrate the approximate position and purpose of this name for this anatomical landmark. The humerus, just as in humans, formed an elbow joint with the radius and ulna. To work out which one is which, turn your hand so that you are looking at your palm, and then turn it so that you are looking at the back of your hand. The forearm bone that moved to the medial part of your body was the radius, which

FIGURE 5.6 Scapula and its articulation with the humerus and glenoid.

FIGURE 5.6 Scapula and its articulation with the humerus and glenoid.

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  • Grazia Piazza
    How are dinosaur legs joined to the skeleton?
    8 years ago

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