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. 



Thus, within the several fossil groups, the origin of fins as 

 parallel developments for stabilization in swimming or even 

 resting on the bottom can be observed. The need for stabi- 

 lization devices was probably related to the development of 

 armor. In each group it is possible that a spine preceded the 

 fin and that a movable fin with cartilaginous basal elements 

 followed. These general observations are a part of Westoll's 

 (and many others') conclusion as to the sequence in arthro- 

 dires. The process of development of the shark fin is not 

 necessarily primitive, and certainly Gegenbaur's archipteryg- 

 ium with its complex skeleton cannot be viewed as primi- 

 tive since it is neither accompanied by a "spine" nor broad- 

 based. 



As regards the origin of paired fins, a part of the above 

 agrees with the fin-fold theory proposed by Balfour, Thacher, 

 and Mivart. This theory holds that the lateral fins are es- 

 sentially comparable to the median ones (their skeleton 

 derived from the stiffening radials). The origin of the girdle 

 involves bracing extensions from the base of the fin and 

 utilization of overlying dermal elements to strengthen these. 

 This type of fold is not a biproduct of an anterior spine. A 

 fin fold of this type is observed in Amphioxus. 



An alternate theory, that of Gegenbaur, which assumed 

 that the girdles were gill arches — the pelvic fins having 

 migrated far back on the body — and that the fin was a 

 development from the rods of the branchial rays, is not sup- 

 ported by any of the facts presented here. The scapulocora- 

 coid of the arthrodire or shark is far from being a gill arch, 

 nor does its relationship to the trunk armor support its being 

 a gill arch. Connection of the pectoral girdle to the head is 

 through dermal elements rather than chondral, and there is 

 no ventral connection between visceral and pectoral skele- 



pelvic bone 



pubic cartila 



iliac cartilage 



Figure 6-65. Ventral views of the pelvic girdles and fin bases of Actinopterygians. A, Acipenser,- 

 B, Polypterus; C, Amia,- D, Lampanyctus. 



186 • THE VERTEBRATE BODY SKELETON 



