Gymnarchidae, Bleeker, 1859

Hilton, Eric J. & Lavoué, Sébastien, 2018, A review of the systematic biology of fossil and living bony-tongue fishes, Osteoglossomorpha (Actinopterygii: Teleostei), Neotropical Ichthyology 16 (3), pp. 1-35 : 7

Gymnarchidae .

A single species, Gymnarchus niloticus , is included in this family ( Fig. 7 ), and is found distributed throughout tropical Africa from Senegal to Ethiopia in the Ghazal and Jebel systems, White Nile, and Nile River to Lake Nasser ( Sudan) in northeast Africa, and in the Gambia, Senegal, Niger, Volta, Ouémé and Chad rivers of western Africa ( Azeroual et al., 2010). It has an elongate, cylindrical body with a broadly rounded head and a dorsal fin that runs most of the length of its body; anal, caudal, and pelvic fins are lacking. It reaches 1.67 m in length and 18.5 kg ( Bigorne, 1990). Its osteology has been described by Taverne (1972), and aspects of its skeleton are illustrated by Benveniste (1994). Fossil remains identified as Gymnarchus are known from several localities throughout central and northern Africa (e.g., Pliocene deposits of Chad, Otero et al., 2009), including the Late Eocene Birket Qarun Formation in Egypt ( Murray et al., 2010), which is the oldest record of the family.

Gymnarchidae is broadly considered to be the sister group of Mormyridae ( Taverne, 1979, 1998; Bonde, 2008; classified as a subfamily of Mormyridae in some classifications, e.g., Greenwood, 1971; Lauder, Liem, 1983), although in most analyses and classifications it is not explicitly coded for, being subsumed into the supraspecific terminal group Mormyroidea (e.g., Li, Wilson, 1996a; Wilson, Murray 2008) or left uncoded (e.g., Hilton, 2003; Zhang, 2006). Although she did not designate it as a monotypic family, Benveniste (1994) recovered Gymnarchus as the sister-group of Petrocephalus + Mormyrinae. In this study, several unambiguous autapomorphies were identified distinguishing Gymnarchus from other mormyroids (absence of a supraoccipital crest, absence of basibranchial toothplates, absence of the first pharyngobranchial, absence of supraneurals, absence of the caudal fin, a dorsal fin with more than 100 fin rays, absence of an anal fin, a small posttemporal bone formed primarily by the ventral limb, the condition of having the m. posterior intermandibularis absent and the m. interhyoideus present); Gymnarchus also has an edentulous parasphenoid, although this is homoplastically found within Osteoglossomorpha in Heterotis and some mormyrids ( Benveniste, 1994).

Figures

Fig. 7. Gymnarchidae. Gymnarchus niloticus (VIMS 22064). Scale bar = 2 cm.