THE TAIL 



Although it is not denied that the vertically expanded tail of an 

 aquatic mammal might eventually develop into perfect vertical flukes 

 without ever showing any definite asymmetry, it is deemed more likely 

 that there will be marked asymmetry exhibited during some stage of the 

 process. If this be so the form of the tail might be either epibatic (the 

 upper lobe longer and larger than the lower) as in most sharks, or 

 hypobatic (the lower lobe longer than the upper) as in some of the 

 extinct marine reptiles. The latter condition would probably be the 

 more likely, for I believe an epibatic condition of the tail in any air- 

 breathing vertebrate is unknown. 



Before abandoning the subject of tails that have expanded in the 

 vertical plane it may be well briefly to discuss conditions in some of 

 the extinct aquatic reptiles. Always in the flukes of the Cetacea and 

 Sirenia the caudal vertebrae extend straight toward the rear and pass 

 through the center of the tail to the vicinity of its medial notch. Thus 

 the caudal tendons are enabled to operate practically from the posterior 

 border of the tail and the force exerted upon each of the two lobes is 

 symmetrical. Always in reptiles having a bilobed tail, however, the ver- 

 tebrae followed the lower lobe to the tip, the caudal axis bending 

 sharply ventrad at the tail base (peduncle). This clearly indicates a 

 fundamental difference that from the very beginning has underlain the 

 evolution of these two sorts of tails. Fraas has offered a reconstruction 

 of what he considered to have been four of the stages in the attainment 

 of this reptilian development. The precise shape of each tail is largely 

 speculative, of course, and it is questionable whether in Mixosaurus the 

 upper lobe should not be placed farther back (fig. 33), because the 

 greater height of the spines near the tail base may really indicate that 

 an augmented muscle mass existed at this point ; but the principle seems 

 sound, for only this sort of gradual, asymmetrical development of the 

 tail could account for the situation of the caudal vertebrae within the 

 lower lobe. 



Obviously in the case of sharks the development of the epibatic tail has 

 been the most favorable for bottom feeding habits, so that the shorter 

 lower lobe would not drag upon the bottom. Equally obvious is the ad- 

 vantage of a markedly asymmetrical hypobatic tail for swimming near 

 the surface, so that no high upper tail lobe will project above the water. 

 As deeper swimming was habitually indulged in a higher upper lobe 

 could develop, this ultimately attaining the size of the lower lobe. 



Von Huene (1922) mentioned that in the latipinnate ichthyosaurs 

 (as Mixosaurus) the tail seems to have been a very poor propeller and 



[191] 



