AQUATIC MAMMALS 



Caudal conditions in living and known fossil Cetacea seem to be of 

 three sorts ; one represented by the zeuglodont Basilosaurus, the second by 

 Zeuglodon os'tr'ts, and the third by all odontocetes and mysticetes. Basi- 

 losaurus and its ilk had an anguilliform type of body, the tail being ex- 

 cessively long. Posterior to the thorax the zygapophyses did not articu- 

 late and the transverse and spinous processes were short, indicating that 

 the spinal musculature was not developed to a degree where it could 

 handle flukes of the modern cetacean sort. These vertebral details show 

 that there was great mobility of the tail to allow for serpentine motions 

 and it seems certain that the propulsive mechanism must have been in 

 the nature of some sort of continuous fin fold running in a fore and aft 

 direction, although this may not have been of entirely uniform width. 

 Presumably there was a symmetrically placed pair of these extending for 

 most of the length of the tail. As the caudal expansion of living ceta- 

 ceans is horizontal it is certainly reasonable to assume that this was also 



Figure 32. Six stages in the ontogenetic development of the cetacean flukes, 

 after Ryder. 



the case in zeuglodonts, and that it progressed by undulations in the 

 vertical plane after the manner of the traditional sea serpent. 



The second sort of cetacean caudal condition is represented by Zeuglo- 

 don osnis. In this the tail length was comparable to modern porpoises, 

 and although the high spinous processes of the thorax and anterior lum- 

 bar region indicate that there was powerful musculature in these areas 

 for control of the tail, the spines of the posterior lumbar region and 

 tail were remarkably weak, showing that here there were no heavy mus- 

 cles. It is therefore doubtful if this animal had abruptly expanded 

 flukes ; and the tail was too short to have anguilliform keels upon either 

 side. It therefore appears not unlikely that it was provided with a cau- 

 dal equipment more nearly resembling the manati than a living por- 

 poise. Incidentally there were likely at one time or another zeuglodonts 

 which had a caudal propulsive apparatus fairly intermediate in character 

 between that of Basilosaurus and Zeuglodon osiris. 



[198] 



