192 ELOPIFORM FISHES 



movement of the premaxilla and maxilla is such that a delicate sensory canal would 

 be stretched and broken. The isolated ethmoid commissure remains as a superficial 

 line of pores within the skin of many ' lower ' teleosts (Nybelin 1967a) . 



The Elopiformes are traditionally linked with the Clupeiformes (Denticipitoidei 

 and Clupeoidei) (Regan 1909 ; Berg 1940 ; Bertin & Arambourg 1958). As sug- 

 gested by Greenwood et al. (1966) there is little evidence to support this proposal. 

 If a relationship exists it is probably to be found below the teleost level. 



Clupeiform anatomy has been influenced by the following trends : 



a. Development of an otoplwsic connection involving an intimate intracranial 

 association between the swimbladder and the inner ear. 



b. Development of a recessus lateralis, and elaboration of the sensory canal 

 system on the cranial bones. This is often accompanied by the loss of a 

 well-defined lateral line canal on the trunk. 



c. Development of a temporal foramen between the frontal and parietal which 

 houses a region of sensory tissue, the ' sinus temporalis ' (see Patterson 1970a 

 for a discussion of this sensory tissue). 



d. Deepening of the suspensorium with an increasing mobility of the dermal jaw 

 elements, particularly the supramaxillae. (The engraulids and chirocentrids are 

 exceptional in this respect ; Denticeps has no supramaxillae, Greenwood 1968) . 



e. Development of microphagous feeding habits (except Chirocentrus) which 

 principally involves reduction or loss of teeth on the dermal jaws, gill arches 

 and palate and the development of epibranchial organs (Nelson 1967). 



f. Development of a unique type of caudal skeleton, characterized by a free 

 first hypural, fusion of the second hypural with the first ural centrum and 

 fusion of the first uroneural with the first preural centrum (Gosline i960, 

 1961 ; Greenwood et al. 1966 ; Ca vender 1966). Denticeps is somewhat more 

 primitive (Greenwood 1968). 



g. Lateral compression of the body accompanied by the development of scutes 

 in the ventral mid-line (a few also show dorsal scutes, e.g. Diplomystus, 

 Hyperlophus and Potamalosa) . 



These clupeiform trends are quite distinct from any seen in the elopiforms. In 

 those elopiforms with an otophysic connection (Megalopidae) it is entirely extra- 

 cranial and involves hypertrophy of the intercalar, an element which in clupeomorphs 

 is reduced to a tiny cap of bone. Lateral compression of the body, also seen in the 

 megalopids, is never associated with the development of scutes. 



The Upper Cretaceous species of Diplomystus are in many respects the most primi- 

 tive clupeiforms known (Patterson 1967a). A derivation of Diplomystus from a 

 basal elopiform, such as the Upper Jurassic Anaethalion vidali, is unlikely in view 

 of the fact that Diplomystus has retained a basipterygoid process and a heavily 

 ossified mesethmoid (for the primitive and advanced condition of the ethmoid 

 elements see Patterson 1970b). Conversely, elopiforms could not have been derived 

 from a Diplomystus-like clupeoid since the latter is already specialized towards the 

 clupeoid condition in showing pre-epiotic fossae, a specialized path of the supra- 

 temporal commissure and certain specializations of the tail (Patterson 1967a : fig. 8 ; 

 Greenwood 1968 : fig. 32 ; Cavender 1966 : fig. 4). 



