. Chordate morphology. Morphology (Animals); Chordata. centrum'' haemapophysrs / parapophysis\ notochord A AMPHIBIAN "URTYPE" vertebral artery^ horizontal septum drapophysis \ ,„-^. B AMNIOTES Figure 6-73. Dorsal and ventral ribs and their relationships to the centrum and neural arch in the primitive amphibian vertebra. A, and the typical amniote vertebra, B. (After Naef, 1929) thoracic cavity which in the mammal functions in respiration; in other amniotes, these rib walls function in support of the body, which frequently lies pressed against the ground. In aquatic forms or small typ
. Chordate morphology. Morphology (Animals); Chordata. centrum'' haemapophysrs / parapophysis\ notochord A AMPHIBIAN "URTYPE" vertebral artery^ horizontal septum drapophysis \ ,„-^. B AMNIOTES Figure 6-73. Dorsal and ventral ribs and their relationships to the centrum and neural arch in the primitive amphibian vertebra. A, and the typical amniote vertebra, B. (After Naef, 1929) thoracic cavity which in the mammal functions in respiration; in other amniotes, these rib walls function in support of the body, which frequently lies pressed against the ground. In aquatic forms or small types such as the living amphibians, the ribs are not important for support but are utilized in the functioning of the axial musculature. Support and protection of the body cavity in the creeping primitive forms was aided by ventral scales and gastralia. The latter are present in the alligator and Sphenodon. Gas- tralia (Figure 6-6) mark the reptile level of differentiation and are presumed to be developed from the rows of bony, ventral scales present in the early amphibians. These ven- tral rows of scales are well marked and fish-like in the small branchiosaurs. Support for the posterior part of the body cavity is given also by the forward-projecting plate of cartilage in Cryplo- branchus (Figure 6-55), the pubis of many reptiles, and the marsupial bones of monotremes. Each of these seems to be an independent response to a common problem. RESUME OF THE BODY SKELETON The vertebrate body skeleton tells essentially the same evolutionary story as the head skeleton, clarifying some areas while being less useful in others. In terms of "verte- brae," it is evident that some sort of column was forming in that there is evidence of reduction of the ventral ribs— Lalimeria has none. With reduction of the ventral rib, dorsal ones appeared, and in Eusthenopleron there are very short dorsal ribs, retaining only the base of the ventral one as their ventral head. In the primitive a
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