. Chordate morphology. Morphology (Animals); Chordata. several lines of modification. One line acquired an anterior spine, and, through continued basal concentration, the spine alone remained and increased in size. The jointed limb of the antiarch is a modified spine. WestoU (Figure 6-71) has documented a rather different sequence beginning with rather heavily armored types with long spines and no fins, to types with reduced spines and small fins, to types lacking the spine but with a large fin. The fin of the antiarch is a derivative of the primitive spined conditon (Figure 6-72). Westoll bas
. Chordate morphology. Morphology (Animals); Chordata. several lines of modification. One line acquired an anterior spine, and, through continued basal concentration, the spine alone remained and increased in size. The jointed limb of the antiarch is a modified spine. WestoU (Figure 6-71) has documented a rather different sequence beginning with rather heavily armored types with long spines and no fins, to types with reduced spines and small fins, to types lacking the spine but with a large fin. The fin of the antiarch is a derivative of the primitive spined conditon (Figure 6-72). Westoll bases his opinion on the se- quence of appearance of forms in the fossil record; how- ever, it is doubtful that the record is so good as to prove this order. There is some doubt that heavily armored types are primitive; they are undoubtedly more easily preserved. Stensio's sequence is more in agreement with the fin-fold theory and the embryological development of fins as ob- served in sharks. However, what is true for the sharks may not apply to arthrodires, since it has already been observed that these two groups are quite distinct. Pelvic fins are presumed to be among the several pairs observed in acanthodians and are known in at least one arthrodire, Coccosteus decipiens. In this form there is a strong iliac process with an expanded ventral margin from which extends a broad-based fin. Fossil agnath fishes reveal something as to the origin of fins. In the earliest osteostracans, slight ventrolateral folds are present (Figure 5-31), then spines appear projecting out fi-om this fold, and gradually cephalic horns are formed (Figure 5-32). Behind these large cephalic horns a flap-like fin developed in some of the later types. Heterostracans developed slight pectoral spines but no lateral fins (Figures 5-28, 5-29). The anaspids (Figure 5-28) and living cyclo- stomes are finless. The former may have scales forming a fin fold ventrolaterally and this fold may have an anterior spine. Th
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