. Chordate morphology. Morphology (Animals); Chordata. Figure 6-62. Pelvic girdles and fins of choanate fishes as seen in ventral view; A, Eusfhenopferon; B, Lafimerio; C, Profopferus. scapula" extending medially from its dorsal end. Chlainydo- selachus also has this element. In many sharks such a piece is lacking; the scapulocoracoid merely tapers to a point. The scapulocoracoids are joined ventrally through a separate piece of cartilage which has been compared with the ster- num. The presence of this median piece cannot be detected in the adult. The base of the fin is generally built ar
. Chordate morphology. Morphology (Animals); Chordata. Figure 6-62. Pelvic girdles and fins of choanate fishes as seen in ventral view; A, Eusfhenopferon; B, Lafimerio; C, Profopferus. scapula" extending medially from its dorsal end. Chlainydo- selachus also has this element. In many sharks such a piece is lacking; the scapulocoracoid merely tapers to a point. The scapulocoracoids are joined ventrally through a separate piece of cartilage which has been compared with the ster- num. The presence of this median piece cannot be detected in the adult. The base of the fin is generally built around three ele- ments: a propterygium, a mesopterygium, and a metapter- ygium. From these numerous radials extend out. The varia- tions in shark fins are multitudinous but generally there are three basal units as in Squaliis. Occasionally only two are present, the propterygium being absent (Heterodonlus). In fossil sharks this "uniformity" is lost; Cladosetachus and Cladodus appear to have only the metapterygium (Figure 6-67). A most peculiar fossil "shark" is Pleuracanthus (Figure 6-67), which has a suprascapular element and a biserial archipterygial type of fin. A similar but much shorter and rounder fin is observed in Chondrenchelys. The presence of both broad-based and concentrated fins (the archipterygium) in sharks and shark-like fishes sug- gests that there was considerable experimentation with a newly developed structure and that in the sharks, as repre- sented by the living fauna, only a single basic type has come down to the present. The pectoral appendage of Hydrolagus (Figure 6-66) is comparable to that of the shark. The scapulocoracoids are more broadly joined ventrally and the base of the fin lacks an evident mesopterygium. The radials are longer and separated by gaps like those of an actinopterygian. This type of appendage and girdle fit well into the fossil array sug- gesting that the chimaerid pattern is independent of that of the present da
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