NO. 5 TELESCOPING OF THE CETACEAN SKULL 3 



GENERAL CONDITIONS IN ZEUGLODONTS 



Concerning the zeuglodonts there is little to say. All of the pub- 

 Hshed figures as well as the rather scanty material which I have 

 been able to examine agree in indicating that the bones of the skull 

 retain their original relationships (pi. 5, fig. i). Highly specialized 

 on this primitive ground plan, and in a direction toward elongation 

 and narrowing of the post-rostral portion, the zeuglodont skull, in its 

 general structure, appears to be removed from rather than antecedent 

 to the line of development which led to the telescoped, broadened con- 

 dition of the post-rostral region seen in the skulls of all modern 

 cetacea. It seems not improbable that the zeuglodonts were for the 

 most part animals with relatively long bodies and small heads as 

 compared with the living whales and porpoises ^ and that the culmi- 

 nating point in their characteristic line of development is indicated 

 by Basilosaurus, the genus in which these tendencies appear to have 

 been carried to the greatest extreme. The superficial resemblance 

 which the zeuglodonts bear to reptiles as a result of these peculiarities 

 has olPten been noticed ; and it should be observed in the present con- 

 nection, that, like the zeuglodonts, the extinct marine reptiles seem 

 to have been without any tendency toward cranial telescoping. It 

 may not be impossible that in both instances the relatively small 

 head was not subjected to the mechanical forces needed to call forth 

 the peculiar reaction which has been the dominant factor in the 

 development of the skull in modern cetaceans (see pp. 38-39) . A fur- 

 ther reason for regarding the known zeuglodonts as probably not di- 

 rectly ancestral to any of the recent whales is the circumstance that in 

 spite of the extreme degree and peculiar character of the general spe- 

 cialization attained by some members of the zeuglodont group the 

 dentition of these animals appears to have been, even in such an aber- 

 rant type as Basilosaurus, uniformly undergoing a simple and not very 

 unusual process of reduction in the normal mammalian manner, a 

 tendency which would not lead by any known process to the remark- 

 able and unique condition of polyodonty through which the modern 

 cetacea have either once passed or are now in.^ While it appears to me 



^ The proportion of head length to total length in Basilosaurus is about as 

 I to 12. It is not sufficiently known in other members of the group. In living 

 whales it is usually somewhere near i to 6 (less in Kogia and more — even as 

 high as I to 2^ — in the balsenids). 



* The two principal explanations of the origin of this polyodonty — intercala- 

 tion of milk teeth among the teeth of the permanent dentition, and the 

 splitting up of serrate permanent teeth into numerous simple elements — are 

 purely hypothetical, resting on no processes actually observed. See Winge, 

 Smithsonian Misc. Coll., Vol. y2, No. 8, pp. 50-56, July 30, 1921. 



