Figure 6-41 . Vertebral column and fins of 5usfhenopferon. (After Jorvik) 



massive basal plate with a large anterior spine (Figure 6-39 

 C). The posterior dorsal has simple basals with no apparent 

 radials. Having spines in front of the dorsal fins is typical of 

 many sharks, living and fossil. In Hydrolagus only the ante- 

 rior dorsal has a spine and this is very large (Figure 2-2). 

 In the living cyclostomes (Figure 2-1), median fins are 

 not distinct; the caudal fin fold extends far forward suggest- 

 ing that dorsal and anal fins are, in effect, only specialized 

 parts of the tail fin itself In some fossil agnaths (Pteraspis), 

 a dorsal spine was present in the position of a doral fin 

 (Figures 2-13, 5-28). The tail in the Heterostraci generally 

 had dorsal and ventral rows of large scales. Some of the 

 Osteostraci are assumed to have had two dorsal fins, while 

 others had only a single posterior dorsal. These fins were 

 preceded by a series of large scales or a spine. Although there 

 were no anal fins, the ventral margin of the caudal fin is as- 

 sumed to have been extended out to either side. Anaspids 

 had a row of dorsal scales and an anal fin; Endiolepis is de- 

 scribed as having a distinct dorsal as well as an anal fin. The 

 anal fin of some anaspids was preceded by a spine. 



Caudal fin 



Because the caudal fin is an important contributor to for- 

 ward thrust in swimming, it is well developed in most fishes. 

 The design of this fin is related to the habits of its possessor. 



In actinopterygian fishes, several tail shapes are observed 

 and are described by special terms (Figure 6-42). Sometimes 

 more than just the shape of the fin is involved in the use of 

 these terms, which are built around the Greek word kerkos 

 meaning tail. The nature of the end of the spinal column 

 enters into this descriptive grouping. In the following ac- 

 count it is to be understood that dorsal (epichordal) and 

 ventral (hypochordal) fin folds, above and below the tip of 

 the spinal column, may be involved in the formation of the 

 caudal fin. These folds must be differentiated from the 



posterodorsal and anteroventral lobes of the ventral fin fold 

 which make up the apparent caudal fin (Figure 6-44), The 

 term "lobe" is used variously in referring to the fleshy base 

 of a fin and to a division of that base or of the ray-supported 

 fin. 



In most actinopterygians the tip of the spinal column is 

 bent upward, and this upturned part is associated with a 

 urostyle and hypural plates (Figure 6-43). There is no ap- 

 parent dorsal fin fold; only the ventral fold is developed and 

 this may have two equal lobes; this type is identified as 

 homocercal (Greek homos — common, same, alike). In some 

 teleosts the homocercal form has been retained, but the 

 terminal upturned portion of the spine has been largely or 

 completely done away with (cod, tuna). Another type has 

 the upturned tip of the spinal column elongated and 

 straightened out. The ray-supported fin is a marginal ex- 

 tension above and below this elongated tip. Such a tail, 

 occurring in some deep sea fishes, is identified as the isocer- 

 cal type (Greek isos — equal, similar, in this case referring to 

 the dorsal and ventral fin folds). Reduction of the terminal 

 part of the spine is sometimes accompanied by reduction of 

 the fin itself to a vestige without equal lobes. Such a fin on 

 a rounded caudal lobe or peduncle, in which the axial 

 skeleton is truncated and without hypurals, is identified as 

 the gephyrocercal type (Greek gephyra—Si bridge or inter- 

 mediate). 



In the primitive actinopterygian the tail is strongly heter- 

 ocercal (Greek heteros— other, different), as in Aapenser or 

 Polyodon (Figure 6-44). The dorsal fin fold is lacking or repre- 

 sented by fulcra (spines). The ventral fin fold has a longer 

 posterodorsal than anteroventral lobe. Polyplerus has the tip 

 of the spine slightly upturned, with a small urostyle but with- 

 out distinct hypurals and without an upturned fleshy lobe; 

 this can be described as an abbreviated homocercal tail. 

 Amia and Lepisosteus have retained the upturned fleshy 

 caudal lobe but have developed only the anteroventral lobe 



168 



THE VERTEBRATE BODY SKELETON 



