Anatomy of Archaeopteryx

Skull. The skull of Archaeopteryx (Figure 10.5a) is tyically archosaurian, with nasal, antor-bital, and eye openings. Some specimens preserve a sclerotic ring, a series of plates that supported the eyeball. The temporal region is poorly known but hints of lower and upper temporal fenestrae are preserved. Archaeopteryx has blade-like, unserrated, recurved teeth.

Arms and hands. The arms are quite long (about 70% of the length of the legs). The hands are about as large as the feet, and each hand bears three, fully moveable, separate fingers. Each finger is tipped with a well-developed, recurved claw. The wrist of Archaeopteryx bears a semi-lunate carpal (Figure 10.5e; see Chapter 9).

Legs and feet. The foot of Archaeopteryx has three toes in front, and a fourth toe lies to the side (or behind; the specimens are flattened). The three in front are more or less symmetrical around digit III, and all the toes all have well-developed claws (Figure 10.5d).

The ankle of Archaeopteryx is a modified mesotarsal joint (see Chapter 4). It preserves a small splint of bone rising up from the center of astragalus, one of two bones in the ankle (see Figure 4.5), to form a tall ascending process. The three foot bones are unfused. The thighs are considerably shorter than the shins, and the fibula is sliver-like as it approaches the ankle.

3. Darwin had just published On the Origin of Species in 1859, proposing that species evolved into other species. Here, a mere two years later, was discovered an apparent "missing link" that mixed "reptilian" and avian features.

Figure 10.4. The beautifully preserved, complete Berlin specimen of Archaeopteryx. (a) Main slab preserving most of specimen; (b) counterslab, preserving opposite side of specimen, primarily impressions. Note the exquisite feather impressions radiating out from the wings and tail.

Long bones. Archaeopteryx has thin-walled long bones with large hollow spaces.

Trunk and tail. The axial skeleton of Archaeopteryx lacks many of the highly evolved features that characterize modern birds. The body is relatively long and shows none of the foreshortening or fusion that one sees in the vertebrae of birds. The sternum is relatively small, with a small keel. A large, strong furcula is present (Figure 10.5f). Also present are gastralia, or belly ribs, which primitively line the belly in many archosaurs (Figure 10.5c).

Archaeopteryx lacks a synsacrum and instead has a primitive, unfused archosaurian pelvis. The pubis is directed downward. The distal end of the pubis (the footplate) is well developed, although the front part is absent.

Archaeopteryx has a long, straight, well-developed tail. Projections from the neural arches (zygapophyses) are elongate, meaning that the tail has little flexibility and has little potential for movement along its length.

Feathers. Archaeopteryx has well-preserved, unambiguous feather impressions. The best-preserved feathers are clearly flight feathers (Figure 10.5b) and are indistinguishable from those of modern birds. Unlike in living birds, however, there are feathers also lining a long, bony tail. These radiate out from the vertebrae, and form an impressive tail plume.

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