194 ELOPIFORM FISHES 



Bardack (1965) suggests that the plethodonts were derived from elopoids but the 

 reasons given do not stand critical examination. The similarity in dentition is a 

 convergent development (p. 207), the similarity in neurocranial architecture is merely 

 due to primitive characters (flat roof, medially united parietals and roofed post- 

 temporal fossae), and the common possession of an ossified interorbital septum is 

 again an example of convergence to meet a similar functional need. 



The osteoglossomorphs are anatomically diverse, but the evolutionary trends which 

 have influenced osteoglossomorph anatomy are quite distinct from those manifest 

 in the elopiform complex. The osteoglossomorphs show : 



a. The development (in the vast majority) of a predatory method of feeding 

 which is imposed upon the basic teleostean parasphenoid-basibranchial/basi- 

 hyal bite. This trend has resulted in a relatively immobile palate and upper 

 jaw and the retention of both a basipterygoid process and a well-ossified 

 ethmoid region (for a fuller discussion of osteoglossomorph jaw adaptations 

 see Greenwood et al. 1966). 



b. Development of an otophysic connection in some ; but this is never developed 

 in either the megalopid or the clupeomorph fashion (Greenwood 1963). 



c. Reduction and consolidation of the circumorbital elements (Nelson 1969b). 



d. Close association between the upper jaw and the infraorbitals. 



e. Development of a unique type of caudal skeleton (Greenwood 1966 ; Cavender 

 1966 ; Nelson 1969b) which can only be derived from the type found in Liassic 

 leptolepids or pholidophorids. 



The diversity of the Osteoglossomorpha makes overall comparison with the 

 elopiforms difficult, but in none of the osteoglossomorph groups are there significant 

 features with counterparts in the elopiforms (for Ichthyodectidae see Bardack 1965 ; 

 for Tselfatioidei see Patterson 1967c ; for Osteoglossiformes and Mormyriformes see 

 Greenwood 1966 and Greenwood et al. 1966). 



Palaeontological evidence suggests that the osteoglossomorph complex is an 

 ancient one ; the Hiodontoidei are first known from the Upper Jurassic (Greenwood 

 1970b), the highly specialized Tselfatioidei from the Albian, and the Ichthyodectidae 

 are recorded from the Tithonian (possibly as early as the Oxfordian, Bardack 



I965)- 



Ichthyodectids such as Allothrissops from the Kimmeridgian, and Thrissops from 

 the Oxfordian appear to be the least specialized osteoglossomorph fishes. Even in 

 these forms, however, there are features which militate against their having been 

 derived from, or having given rise to, elopiform fishes. Allothrissops, for instance, 

 is more specialized than elopiforms in showing a reduced supratemporal, a reduction 

 in the size of the antorbital and a restricted parasphenoid dentition. It also ap- 

 parently shows no distinct dermethmoid element (Patterson 1967a : fig. 2). The 

 supraorbital-infraorbital canal connection of Allothrissops is developed in a different 

 way from that seen in elopiforms. In Allothrissops the connection is indirect, via 

 the otic sensory canal, whereas in elopiforms it is direct. 



Specialized features of primitive elopiforms which rule out their being considered 

 ancestral to Allothrissops and related teleosts include : loss of the basipterygoid 

 process, reduction in snout ossification, reduction of branching of the preopercular 



