Elopiformes, Notacanthifomies and Anguilliformes: Relationships 



D. G. Smith 



NOTACANTHIFORMES 



THE Notacanthiformes is composed of two clearly defined 

 families, the Halosauridae and Notacanthidae. Overall, the 

 Halosauridae is the more primitive family. McDowell (1973) 

 divided it into two subfamilies: the Halosaurinae, containing 

 only Halosaurus, and the Halosauropsinae, containing Halo- 

 sauropsis and Aldrovandia. The notacanthids show a number 

 of specializations not found in the halosaurs, involving mainly 

 the mouth and dorsal fin. The Notacanthidae contains either 

 two or three genera, depending on the placement of Lipogenys. 

 McDowell recognized only Nolacanlhus and Polyacanthonolus 

 in the Notacanthidae while assigning Lipogenys to a separate 

 family. He considered the Lipogenyidae and Notacanthidae to 

 form a suborder of the Notacanthiformes, the Notacanthoidei, 

 which stood opposed to the Halosauroidei. Greenwood (1977), 

 however, felt that Lipogenys was closely related cladistically to 

 Polyacanthonolus and that those two genera formed the sister 

 group of Notacanlhus. A classification of the Notacanthiformes 

 based on Greenwood's interpretation would be as in Fig. 44. 



Notacanthiform larvae cannot yet be identified confidently 

 below the ordinal level and hence can tell us little about rela- 

 tionships within the order. Smith (1970) gave reasons to suspect 

 that the Tiluropsis form (short head, vertically elongate eye) 

 belongs to the Halosauridae. Circumstantial evidence suggests 

 that the Tiluriis form (short head, normal eye) is the larva of 

 the Notacanthidae. Tilunis is the only notacanthiform larva 

 found in the Mediterranean. Although adult notacanthids of 

 both Notacanlhus and Polyacanthonolus occur in the Mediter- 

 ranean, halosaurs apparently do not (McDowell, 1973). The 

 identity of the third basic type of notacanthiform larva, known 

 as Leptocephalus giganteus (long head, normal eye), cannot even 

 be guessed at this point. 



Anguilliformes 



The Anguilliformes, the true eels, is the largest and most 

 specialized of the elopomorph orders. A definitive classification 

 of the Anguilliformes does not yet exist. The scheme that follows 

 can be considered an outline that will be filled in and modified 

 as studies continue. 



The eels can be divided into two groups: those in which the 

 frontal bones are fused, and those in which they remain as 

 separate right and left elements. This observation dates back to 

 Regan (1912), but its phylogenetic significance has not always 

 been agreed upon. Regan himself said nothing about it one way 

 or another; he simply used it as a key character. A case can be 

 made for the view that the fusion of the frontals was a single 

 event that occurred quite early in the evolutionary history of 

 eels and therefore reflects a real phylogenetic division. On the 

 whole the fused-frontal group contains more primitive members 

 than the divided-frontal group, although the fused condition is 

 itself a derived character state. Except for Anguilla. all the di- 

 vided-frontal eels are markedly specialized, including pelagic 

 and fossorial representatives. Yet in none of these lines has a 

 fusion of the frontals been among the modifications. Of the more 



specialized members of the fused-frontal group, all but the Ser- 

 rivomeridae can be clearly traced back to more primitive mem- 

 bers, all with perfectly fused frontals. It is more parsimonious 

 to assume that fusion took place once at a point early in an- 

 guilliform evolution than to assume that it occurred several 

 times early but not at all later on. 



The number of families in the fused-frontal group is still 

 somewhat uncertain. Ten are provisionally recognized here. The 

 Synaphobranchidae, Simenchelyidae, and Dysommatidae are 

 closely related and could easily be considered subfamilies of the 

 Synaphobranchidae (Robins and Robins, 1976). They combine 

 some very primitive characters with some peculiar specializa- 

 tions and do not seem to be intimately related to any of the 

 other families. The Nettastomatidae shares several advanced 

 characters with certain congrids and could be considered a de- 

 rivative of that group. The interrelationships of the remaining 

 families are not clear; the resemblances involve mainly primi- 

 tive characters. The Ophichthidae is a large and morphologically 

 diverse family containing both generalized and highly modified 

 forms. It is united by certain specialized characters such as a 

 ventrally displaced posterior nostril, a reduced caudal fin, and 

 numerous branchiostegal rays that overlap on the ventral mid- 

 line. The Congridae (including Macrocephenchelyidae) is also 

 a large family, but without the extreme variety of external mor- 

 phology found in the Ophichthidae. Its specializations are more 

 subtle and consist mainly of trends in several characters. The 

 Colocongridae and Muraenesocidae have at various times been 

 included in the Congridae, but again the resemblances are main- 

 ly in primitive characters. Neither family fits the pattern of 

 character modification found in the Congridae, and both show 

 at least one primitive character that is absent in nearly all con- 

 grids: separate hypohyals. The Muraenesocidae is here restricted 

 to Muraenesox itself and its close relatives Congresox, Cyno- 

 ponticus and Sauromuraenesox. Of the other genera previously 

 referred to this family, Hoplimnis has been removed to the 

 Nettastomatidae (Smith, 1979; Smith and Castle, 1982), and 

 Xenomystax (including Paraxenomyslax) probably belongs in 

 the Congridae. The Derichthyidae and Serrivomeridae are mid- 

 water eels, the former relatively little modified, the latter highly 

 modified. The Serrivomeridae was formerly associated with the 

 Nemichthyidae, but this seems unlikely. The completely fused 

 frontals and massive palatopterygoid arcade of serrivomerids 

 difier strikingly from the partially fused frontals and reduced 

 pterygoid found in nemichthyids. 



There are eleven families of eels with divided frontals: the 

 Anguillidae, Moringuidae, Heterenchelyidae, Myrocongridae, 

 Xenocongridae, Muraenidae, Nemichthyidae, Cyematidae, Sac- 

 copharyngidae, Eurypharyngidae, and Monognathidae (the 

 monognathids actually have fused frontals, but they are clearly 

 related to the saccopharyngids and eurypharyngids and the fu- 

 sion seems secondary). Although they are more clearly defined 

 than the fused-frontal families, their interrelationships are still 

 uncertain. Except for the Anguillidae, they are all distinctly 

 specialized, either for burrowing (Moringuidae, Heterenchelyi- 

 dae), for midwater life (Nemichthyidae, Cyematidae, Sacco- 



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