FOSSIL AND RECENT 91 



viii. The caudal skeleton contains a series of strap-shaped uroneurals (see below), 

 and in the Cretaceous ichthyodectids further specializations occur in the shape 

 of the hypurals and the posterior neural and haemal spines (Cavender 1966). 



In features i-v Pachythrissops is more primitive than Allothrissops and Thrissops 

 and therefore may not be separated from them on these features. However, the 

 last three features indicate that Pachythrissops is more advanced than Thrissops 

 and Allothrissops. The absence of a basipterygoid process in Pachythrissops is not 

 definitely established, due to imperfections in preservation, but the several neuro- 

 crania of P. laevis examined show no evidence of such a process. The preoperculum 

 of Pachythrissops differs from Allothrissops and Thrissops in lacking the distinctive 

 projection at the postero- ventral angle and the preopercular sensory canal opens 

 to the surface by two or three very short branches situated at the angle of the bone. 

 Both of these features are common to many lower teleosts and would not invalidate 

 the derivation of Pachythrissops from a form like Allothrissops as implied by Bardack 

 (1965: fig. 3). 



The caudal skeleton, however, makes it almost impossible to ally Pachythrissops 

 with Allothrissops and Thrissops. Pachythrissops laevis shows four elongate 

 uroneurals and a series of about three smaller elements situated posteriorly. Nybelin 

 (1971) described the caudal skeleton of P. propterus and it agrees with P. laevis. 

 In both, there is a full-length neural spine on the second pre-ural centrum and 

 four elongate uroneurals. The first uroneural is short and there is a large gap 

 between the second and third hypurals. Such an arrangement cannot have given 

 rise to that of Allothrissops, etc., and it is highly unlikely that it was derived from the 

 latter. To justify this statement a broader view of the teleost uroneural series is 

 necessary. 



Patterson (1968a) formulated the evolution of the teleost caudal skeleton from the 

 pholidophorid type and described the anatomy of the tail of Leptolepis coryphaenoides 

 from the Upper Lias, which may be taken as representing the most primitive teleost 

 stage. The definition of a teleost proposed by Patterson (1968a) is accepted and 

 would thus include Leptolepis but exclude Pholidophorus. The uroneurals of L. 

 coryphaenoides consist of a graded series of six (sometimes seven, Patterson 1968a : 

 fig. 9c) elements, the anterior member of this series extending lateral to the second 

 or third preural centrum. 



In the upper Jurassic Leptolepis dubia the uroneurals may be divided into two dis- 

 tinct series. The anterior series consists of four strap-shaped elements and these 

 are succeeded by three short uroneurals, the anterior one of which lies lateral to the 

 fourth member of the anterior series. This uroneural arrangement is very charac- 

 teristic and may be traced through to Elops. To attain the Elops grade, a fusion 

 of the first with the second, and the third with the fourth uroneurals has to be 

 postulated. There is both circumstantial and direct evidence for this having hap- 

 pened (Patterson 1968a : 226). The third uroneural of Elops corresponds to the 

 fifth of Leptolepis dubia. The L. dubia type of uroneural disposition is probably 

 basic not only for elopoids, but probably for Clupeomorpha and Protacanthopterygii 

 as well, since the two uroneural series are present in the basal members of these 

 groups. 



