. Chordate morphology. Morphology (Animals); Chordata. PLEURACANTHUS CLADOSELACHE Figure 6-67. Pectoral girdles and fins of fossil sharks. A, P/euraconthus; B, C/odose/ache. (A after Goodrich, 1930; B ofter Dean, 1909) the point of attachment is high on the centrum. The para- pophysis may then act as the point of attachment of both dorsal and ventral ribs, as it does in the salmon. It might be assumed that originally there were separate points of at- tachment for each of these ribs, the parapophysis serving the dorsal rib and the hemapophysis serving the ventral rib, as in Polypterus. Such a v
. Chordate morphology. Morphology (Animals); Chordata. PLEURACANTHUS CLADOSELACHE Figure 6-67. Pectoral girdles and fins of fossil sharks. A, P/euraconthus; B, C/odose/ache. (A after Goodrich, 1930; B ofter Dean, 1909) the point of attachment is high on the centrum. The para- pophysis may then act as the point of attachment of both dorsal and ventral ribs, as it does in the salmon. It might be assumed that originally there were separate points of at- tachment for each of these ribs, the parapophysis serving the dorsal rib and the hemapophysis serving the ventral rib, as in Polypterus. Such a view finds no support in the fossil re- cord; the parapophysis appears to be the only constant process whether it serves dorsal or ventral rib or both. The hemapophysis has apparently become associated with the ventral ribs in Polypterus. Certainly, the ventral ribs are re- lated to the hemal arches in the tail in fishes. In the teleost the elongated parapophysis appears to be modified into that arch; the dorsal rib has similar relationship in the tetrapod. Another problem is defining parapophysis as opposed to rib. This can only be solved by assuming that the point of articulation is a variable, mechanically determined thing and that the parapophysis of an amphibian is Cjuite differ- ent from that of a mammal. The various groups of vertebrates can be characterized by their ribs. The Chondrichthyes have dorsal ribs or no ribs (Hvdrulagus). Actinopterygians in general have well-devel- oped ventral ribs and less-developed or vestigial dorsal ones. In addition, many teleosts have intermuscular bones distinct from the ribs. The choanates appear to have had both types of ribs; pleural ribs are developed in dipnoans and actinistians but replaced in rhipidistians by dorsal ribs. However, the dorsal ribs of the rhipidistian have acquired a diapophysis con- nection with the neural arch. This suggests that Naef's as- sumption of the compound nature of the tetrapod rib may apply (Figure 6-
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