Dinosaurs

This climb up the cladogram leaves us wheezing and gasping for air, but at long last situated at Figure4.i2. An archosaur skull with the diagnostic antorbital fenestra.

Antorbital FenestraDimorphodon

Figure 4.13. A candidate for closest relative to Dinosauria: Pterosauria as represented by Dimorphodon, from the Upper Jurassic of Europe.

20 cm

Figure 4.13. A candidate for closest relative to Dinosauria: Pterosauria as represented by Dimorphodon, from the Upper Jurassic of Europe.

20 cm

Dinosauria

Dinosauria

Dinosauromorpha

Ornithodira

Figure 4.14. Cladogram of Ornithodira showing the monophyly of Dinosauria. Derived characters include: at 1, loss of postfrontal, elongate deltopectoral crest on humerus, brevis shelf on ventral surface of postacetabular part of ilium, extensively perforated acetabulum, tibia with transversely expanded subrectangular distal end, and ascending astragalar process on front surface of tibia.

the subject of our book: Dinosauria. Dinosaurs can be diagnosed by a host of shared, derived characters (Figures 4.14 and 4.15). Most strikingly, dinosaurs are united by the fact that, within archosaurs, they possess an erect, or parasagittal stance; that is, a stance in which the plane of the legs is parallel to the vertical plane of the torso (see Figure 4.16). In dinosaurs, an erect stance consists of a suite of anatomical features with important behavioral implications (Box 4.3). The head of the femur (thigh bone) is oriented at approximately 90° to the shaft. The head of the femur itself is barrel-shaped (unlike the familiar ball shape seen in a human femur), so that motion in the thigh is restricted largely to forward and backward, within, as we've seen, a plane parallel to that of the body (see Figure 4.11). The ankle joint is modified to become a single, linear articulation. This type of joint, termed a modified mesotarsal joint, allows movement of the foot only in a plane parallel to that of the body: forward and backward (Figure 4.16). Note that again this situation differs from that in humans, in which the foot is capable of rotating. The upshot of these adaptations of stance is that all dinosaurs are highly specialized for cursorial locomotion (that is, running, as in the "cursor" on a computer screen). Dinosaurs are terrestrial beasts through and through (see Box 4.3).

Beyond their fully erect stance, dinosaurs are diagnosed by a host of derived features (see Figure 4.14). These include loss of a skull roofing bone - the postfrontal - that lies on the top of the head along the front margin of the upper temporal fenestra, an elongate deltopec-toral crest on the humerus, an extensively perforated acetabulum, a tibia with a transversely

Dinosauromorpha

Ornithodira

Figure 4.14. Cladogram of Ornithodira showing the monophyly of Dinosauria. Derived characters include: at 1, loss of postfrontal, elongate deltopectoral crest on humerus, brevis shelf on ventral surface of postacetabular part of ilium, extensively perforated acetabulum, tibia with transversely expanded subrectangular distal end, and ascending astragalar process on front surface of tibia.

Humerus

Humerus

Dinosauria

Figure 4.15. Some of the derived characters uniting Dinosauria. (A) Elongate deltopectoral crest on humerus;

(B) brevis shelf on ventral surface of postacetabular part of ilium;

(C) extensively perforated acetabulum;

(D) tibia with transversely expanded subrectangular distal end, and (E) ascending astragalar process on front surface of tibia.

Figure 4.15. Some of the derived characters uniting Dinosauria. (A) Elongate deltopectoral crest on humerus;

(B) brevis shelf on ventral surface of postacetabular part of ilium;

(C) extensively perforated acetabulum;

(D) tibia with transversely expanded subrectangular distal end, and (E) ascending astragalar process on front surface of tibia.

expanded subrectangular lower end, and an ascending process of the astragalus on the front surface of the tibia (see Figure 4.15).

Was this article helpful?

0 0

Responses

  • Sinit
    What is an antorbital fenestra?
    9 years ago

Post a comment