Pleural Of Phalanx

FIGURE 5.7 Phalangeal formula applied to a human hand as an example.

FIGURE 5.7 Phalangeal formula applied to a human hand as an example.

Phalanx Formula Theropod

is just below (posterior to) your thumb. Distal to the radius and ulna were, in order, the carpals (wrist bones), metacarpals, phalanges (plural of phalanx), and unguals (claws or hooves); the latter two compose the digits (or fingers). The phalanges are divided from the metacarpals in a human by the location of the knuckle joints and the same is true of dinosaus.

The main difference between a dinosaur manus and a human hand is seen in the asymmetry of the dinosaur manus, which can be discerned through a phalangeal formula (Fig. 5.7). To demonstrate this formula, turn your hand so that you are looking at the back (dorsal surface) of it. Label your fingers from the thumb (closest to the midline of your body) to the smallest finger from I to V, then count the number of separate bones (each phalanx) in each of these labeled fingers. Test the results by comparing them with the following data:

The data should confirm the observation of this phalangeal formula in humans: 2-3-3-3-3. Dinosaurs have, as another trait distinguishing them from their ancestors, a manus that has such asymmetry with less than or equal to three phalanges on digit IV and less than or equal to two phalanges on digit V. In some dinosaurs, digits IV and V became modified or reduced enough in their manus that they eventually became vestigial or disappeared, which gave some dinosaurs (particularly theropods) three-fingered hands. A few theropods, such as Tyrannosaurus, even evolved two-fingered hands (Chapter 9). A similar circumstance happened with digits I and V in the pes of some dinosaurs, which resulted in such dinosaurs leaving four- and three-toed tracks (Chapter 7). However, other dinosaurs retained all five digits on either their manus or pes (Chapters 10 and 12), indicated in some dinosaurs' tracks as well as their skeletons (Chapter 14).

In the posterior portion of a dinosaur, hind limbs were associated with the sacral vertebrae (collectively called the sacrum) by the pelvic girdle. As mentioned earlier, the sacrum was connected with the hips (dorsal and medial to the ilium), and the ball-like, proximal end of the femur fitted into the acetabulum (Fig. 5.8A). Distal to the femur were the tibia and fibula, where the tibia, more medial than the fibula, formed the knee joint with the femur. The tibia is key to two dinosaurian traits: it has a cnemial crest and its distal, anterior surface fits with the ascending process

Dinosaurs Process
FIGURE 5.8 Characters for dinosaurs involving the appendicular skeleton. (A) Sacrum, proximal end of the femur, and the fit of the latter into the acetabulum. (B) Tibia, showing two traits of dinosaurs: cnemial crest and astragalus.

of an anklebone called the astragalus (Fig. 5.8B). The astragalus and another ankle-bone, the calcaneum, collectively formed a dinosaur's tarsals. These terms relate directly to the evolution of dinosaurs from reptilian ancestors with different arrangements of their tarsals (Chapter 6).

Distal from the tarsals and relating to a dinosaur's feet were the metatarsals, phalanges, and unguals. Although humans normally walk with their metatarsals in contact with the ground, in most cases, dinosaurs had metatarsals above the ground (Chapter 7). Such a condition, called digitigrade, is also observable in dogs, which have a posteriorly pointing joint between the metatarsals and ankle bones (Fig. 5.9A). A digitigrade stance can be approached in humans by standing on the balls of the feet or wearing high-heeled shoes (Fig. 5.9B) but strictly speaking is achieved only by on-point ballet dancers who stand on the tips of their toes. This stance contrasts with relaxing and standing with most of the body weight on the metatarsals (heels), which is plantigrade. By far, most dinosaurs were digitigrade, placing their weight on their phalanges (Fig. 5.9C), although some dinosaurs walked plantigrade under some conditions (Chapter 14).

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