92 



nsppu2 



ELOPIFORM FISHES 



ep1-3 



un 7 



Fig. 45. Pachythrissops laevis Woodward. Caudal skeleton in left lateral view, 

 on B.M.N. H. 40333 (holotype) and P.29392. 



Based 



Allothrissops and Thrissops, however, have retained the Leptolepis coryphaenoides 

 type. Instead of the fifth uroneural remaining short, this and succeeding uroneurals 

 extend basally to lie lateral to the preural and ural centra. Concomitant with this 

 development the anterior uroneurals tend to extend forward and in the Cretaceous 

 ichthyodectids, which also show at least five elongate uroneurals, the anterior 

 members of the series extend as far forwards as the fourth preural centrum. The 

 development of this unique uroneural arrangement is probably correlated with 

 greater rigidity in the tail. The body form of the ichthyodectids suggests fast 

 swimming fishes. The caudal fin is very deeply forked with the caudal fin-rays 

 crossing the hypurals at a steep angle and those of the upper lobe lying in line with 

 the uroneurals. In the Cretaceous ichthyodectids, and to a lesser degree in Allo- 

 thrissops and Thrissops, various degrees of hypurostegy are shown. Hypurostegy 

 is by no means confined to the ichthyodectids but is found in several groups of higher 

 teleosts, e.g. Scombridae, yet the combination of this feature with the uroneural 

 development is unique. 



The caudal skeleton of Pachythrissops is virtually identical with that of Leptolepis 

 dubia in showing two distinct series of uroneurals, the anterior group consisting of 

 four uroneurals and the posterior of three. To derive an Allothrissops- and Thrissops- 

 like caudal skeleton from the Pachythrissops type a simplification of the uroneural 



