246 UPPER CRETACEOUS TELEOSTS 
the origin of a considerable proportion of the recent bathypelagic fauna is accounted 
for when viewed in the light of the competition for food and living space with the 
Acanthopterygii. 
The evolutionary considerations outlined in the present section are summarized 
in Text-figure 102. 
V. CONCLUSIONS 
In the foregoing descriptions and discussion some mention has been made of the 
divergence between on the one hand the Elopomorpha and on the other the salmoni- 
form group and its derivatives. The Elopiformes have been considered in the last 
section to be very close to the halecostomes, with the Elopoidei retaining the greatest 
number of halecostome characters (Nybelin, 1956). Certain specializations, however, 
rule this group out from the main course of teleost evolution (Greenwood, et al., 
1966 : 348). The Salmoniformes do not possess any of the limitations of the 
elopiforms and it has been shown that the major teleost radiations had their origin 
within the Salmoniformes. 
The reality of the divergence between the Elopiformes and the Salmoniformes has 
been indicated but this divergence may in part be due to the absence of fossil 
salmoniforms in deposits earlier than the Albian. The elopiform lineage is distinct 
by the Upper Jurassic and undoubtedly arose somewhat earlier. The salmoniforms, 
first represented in the marine Albian, may have also been present in the Jurassic, 
but in fresh water. If this is the case then the basal members of the two groups 
would possibly have shown a much closer resemblance. The Ichthyotringoidei has 
been proposed as being the most primitive of the Cretaceous salmoniforms so far 
described and certain osteological features, notably the caudal skeleton, are as 
easily comparable with an elopiform as with a salmoniform. Although the two 
lineages might approach one another more closely basally, both probably having been 
derived from the Pholidophoridae, there appears to have been a marked divergence 
in evolutionary potential between the two groups. 
The structure of the jaws has been stressed as being one of the major factors 
influencing the course of later teleost evolution. In the Elopomorpha the jaw is 
non-protrusile and there is no mechanism enabling the premaxilla to be moved 
forwards, while the maxillae form the majority of the upper jaw margin. The 
salmoniform derivatives on the other hand exhibit protrusibility of several types, the 
essential factor being that the jaw is capable of becoming protrusile. Eaton (1935 : 
168) and Alexander (1966 ; 1967a ; I967b : 250) have outlined three main types 
of jaw protrusion, an ostariophysan type, a cyprinodont type and a true acantho- 
pterygian type. Eaton goes on to say that these three types possess an enlarged 
premaxilla with a median dorsal process which suggests a common origin. Con- 
sidering the comments of Allis (1909), Greenwood, et al. (1966), Weitzman (1967) 
and Goody (1968) it is possible that the dorsal processes are homologous and the 
three types share a common ancestry. 
Supporting evidence for the closeness of the basal stocks of the Salmoniformes, 
Myctophiformes, Ctenothrissiformes and Beryciformes is afforded by the structure 
