Osteoglossidae, Bonaparte, 1831

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 : 9-11

Osteoglossidae .

Outside of Mormyridae , Osteoglossidae is the most diverse family of Osteoglossomorpha, with four extant genera, classified in two subfamilies ( Osteoglossinae , including Osteoglossum from South America and Scleropages from Southeast Asia and Australia, and Arapaiminae (= Heterotidinae ) including Heterotis from Africa and Arapaima from South America). In all osteoglossids, the large, cycloid scales are reticulate, with a network of furrows across the entire surface of the scales (mormyroids have reticulate furrows on just the posterior field of the scale, with welldeveloped radii on the anterior field; see Hilton, 2003: fig. 39). These furrows define so-called squamules, which have been recovered in the fossil record ( Gayet, Meunier, 1983; Taverne et al., 2007). The crown-group osteoglossids have elongate, posteriorly positioned dorsal and anal fins (these are longer in Osteoglossum than in Scleropages ), a short caudal peduncle, and a large rounded caudal fin.

Osteoglossinae ( Fig. 8 ) have laterally compressed bodies and large, dorsally directed mouths with elongate lower jaws. Two barbels extend from the anterior tip of the lower jaws, and in life these are held horizontally in the water column. Most remarkably are the large pectoral fins that have a long, very robust leading pectoral fin ray. These fin rays support the strong pectoral fins that contribute to the ability of these fishes to float at the surface while hunting prey and leap from the water to capture terrestrial invertebrate and vertebrate prey items above the water line ( Goulding, 1980; Verba et al., 2018). Adults of the two species of Osteoglossum are silver ( O. bicirrhosum ) or greyish-steel ( O. ferreirai ) colored, the yolk-sac larvae of O. bicirrhosum are silver whereas those of O. ferreirai , which is restricted to the Rio Negro, are black with a distinct yellow lateral stripe on the body. The species of Osteoglossum are largely allopatric: O. bicirrhosum is found throughout the Amazon and the Branco river basins, and O. ferreirai is found in the Rio Negro basin, including the Branco, and the Orinoco River, which was likely the result of an introduction ( Escobar et al., 2013). Using a ~1,000 base-pair fragment of the mitochondrial genome, Escobar et al. (2013) calculated a genetic distance of 8.9% between the two species. Scleropages comprises four species, two from Southeast Asia ( S. formosus from Vietnam, Cambodia, Thailand, the Malay Peninsula, Sumatra, and Borneo, and S. inscriptus from Myanmar) and two from Australia ( S. jardinii from the coastal river systems of northern Australia and Papua New Guinea, and S. leichardti from the Fitzroy River basin). Roberts (2012) suggested the subgenus name Delsmania Fowler, 1933 could usefully be applied to the group containing S. formosus and S. inscriptus , with the two Australian species being in the subgenus Scleropages . The four species of Scleropages have dramatic coloration and, at least in the case of S. inscriptus , patterning. Color variants oh S. formosus have been suggested to be distinct species ( Pouyaud et al., 2003), although these are not regarded as valid ( Kottelat, Widjanarti, 2005; Roberts, 2012). A fossil species of Scleropages , † S. sinensis , has been recently described from the Early Eocene Xiwanpu and Yangxi formations of China ( Zhang, Wilson, 2017). Several additional fossil taxa have been interpreted as being close to Osteoglossinae , if not within the subfamily itself, including † Opsithrissops , † Brychaetus , † Foreyichthys , † Heterosteoglossum (see Taverne, 1998; Bonde, 2008; and Forey, Hilton, 2010 for discussion of these and other fossil osteoglossid taxa). Although it bears an elongate lower jaw similar to that of osteoglossids, † Furichthys , from the Early Eocene of Denmark, has been interpreted as stem-group Osteoglossi (= Osteoglossiformes + Mormyriformes; Bonde, 2008).

Arapaiminae comprises two genera of extant fishes ( Arapaima and Heterotis ; Fig. 9 ) and putatively several fossil taxa, including † Joffrichthys from the Paleocene of Canada ( Li, Wilson, 1996b), † Trissopterus, from the Eocene of Italy, and † Sinoglossus from the Eocene of China (although see Murray et al., 2018, who found † Joffrichthys to be a potential stem osteoglossiform). Arapaima , because of the unique configuration of its occipital region ( Hilton et al., 2007), has been identified in the fossil record based on isolated basiocciptal/vertebral elements ( Lundberg, Chernoff, 1992; Gayet, Meunier, 1998). Arapaima has long been considered to be a widespread monotypic genus, with only A. gigas found throughout the Amazon basin. Stewart (2013a,b) argued that four nominal species and a new species should be recognized ( A. arapaima , A. agassizi , A. mapae , A. gigas , and A. leptosoma ). Stewart (2013a) further suggested that A. agassizi had no known specimens and had not been collected for 190 years, and that A. mapae and A. gigas were only known from their holotypes. Several studies have found moderate to low population genetic structure across the range of the genus at various scales ( Araripe et al., 2013; Watson et al., 2013). Most range-wide structure appears to be associated with distance between populations ( Hrbek et al., 2005), and low genetic diversity within smaller portions of its range is suggestive of overfishing (e.g., Araguaia-Tocantins basin; Vitorino et al., 2017). Heterotis niloticus is distributed throughout the Nilo-Sudanese region of Africa from Ethiopia to Senegal, the Chad basin, and Lake Turkana; it has been widely introduced in central and western Africa and is regionally extinct in the upper Egyptian Nile ( Akinyi et al., 2010). In contrast to Arapaima , there is genetic structure of Heterotis , even within relatively small portions of their range (e.g., Hurtado et al., 2013, found significant genetic differentiation between three river basins in Benin). Arapaima and Heterotis are broadly regarded as sister taxa, supported by genetics and several morphological characters (enlarged first infraorbital bone, divided first infrapharyngobranchial, and having the angular, articular, and retroarticular all unfused in the adult [otherwise seen only in † Phareodus]; Hilton, 2003).

As a taxonomic aside, Taverne (1979) named two subfamilies of Osteoglossidae : †Phareodontinae (including † Phareodus, † Brychaetus , † Musperia , and † Phareoides ) and Osteoglossinae (including Osteoglossum and Scleropages ), with Pantodontidae (only Pantodon ) and Arapaimidae as separate families, and the latter containing the subfamilies Heterotinae (= Heterotis + † Paradercetis ) and Arapaiminae (= Arapaima ). Two family group names for the clade including Arapaima and Heterotis have been used in the literature, Arapaimini Bonaparte, 1846 and Heterotidae Cope, 1871. Although the former has priority, the latter is in broad current usage, as Heterotidinae (e.g., Nelson, 1994, 2006; Li, Wilson, 1996a; Hilton, 2003; Nelson et al., 2016). However, the ICZN’s (1999) criteria for reversal of precedence (Article 23.9) are not satisfied, as both family-group names have been used since 1899, and indeed Arapaimidae is also used in current literature (e.g., Taverne, 1998; Bonde, 2008; Akinyi et al., 2010). Therefore, the principle of priority should prevail, and Arapaiminae is the correct name for this family-group taxon ( van der Laan et al., 2014).

The genus-level diversity among the extant Osteoglossidae is dwarfed by the number of fossil genera included in or allied closely to the family ( Forey, Hilton, 2010). Some of these taxa are insertae sedis, but several can be referred to the subfamily †Phareodontinae ( Fig. 10 ), which was erected by Taverne (1979) to encompass † Phareodus (Eocene, North America, Australia, including † Phareoides ), † Brychaetus , (Eocene, Europe, Africa, south Asia, and possibly North America), and † Musperia (Eocene, Sumatra). To this group can be added † Cretophareodus , † Taverneichthys , and † Ridewoodichthys (e.g., see Kumar et al., 2005; Taverne, 2009a,b; Taverne et al., 2009; Forey, Hilton, 2010). Other taxa that are referable to the subfamily, or otherwise near its base (e.g., interpreted to branch off from the stem of the family, or crownward from the phareodontines) include † Brychaetoides , † Phareodusichthys , † Monopteros , † Xosteoglossid , † Magnigena, and several unnamed taxa ( Bonde, 2008; Forey, Hilton, 2010). Two Eocene osteoglossomorphs from Africa, † Singida ( Fig. 11 ) and † Chauliopareion , are frequently considered to be related to Osteoglossidae , primarily either as stem-groups ( Murray, Wilson, 2005; Xu, Chang, 2009; Murray et al., 2018), or as sister-group to Pantodon ( Hilton, 2003) , although other positions have been supported (e.g., stem Osteoglossine, Zhang, 2006; stem Osteoglossi, Bonde, 2008). † Chanopsis (Aptian, Democratic Republic of the Congo) has also been considered to be an osteoglossid ( Bonde, 1996; Taverne, 1998), although Forey, Hilton (2010) questioned this assessment.

A remarkable aspect of the fossil record of Osteoglossidae is that several forms are known from undisputedly marine deposits ( Taverne, 1998; Bonde, 1996, 2008; Forey, Hilton, 2010). This is remarkable because all extant osteoglossomorphs are entirely freshwater forms, and their distribution has been held as a text-book example of vicariance biogeography. However, the occurrence of marine fossil taxa suggests that at least portions of the evolutionary history of Osteoglossomorpha took place in the marine realm ( Taverne, 1998; Bonde, 2008; Forey, Hilton, 2010). Bonde (2008) in fact concluded a marine origin of Osteoglossomorpha, with two or three freshwater invasions, although as many as nine possible invasions into marine habitats was offered as an alternative hypothesis [this latter hypothesis was regrettably miscited as Bonde’s primary conclusion by Forey, Hilton, 2010]). Among the marine forms are † Magnigena (Paleocene, Saudi Arabia), † Brychaetus (Eocene, Europe, Africa, south Asia, and possibly North America), † Heterosteoglossum , † Furichthys , † Xosteoglossid , † Brychaetoides , and an unnamed osteoglossiform (Early Eocene, Denmark), † Monopteros , † Thrissopterus , and † Foreyichthys (Eocene, Monte Bolca, Italy), as well as several unnamed taxa (e.g., a particularly osteoglossid-like partial braincase from the Eocene London Clay; Forey, Hilton, 2010).

Figures

Fig. 8. Osteoglossinae. a. Osteoglossum bicirhossum (UF 189007; 260 mm SL; photo by Z. Randall). b. Scleropages formosus (aquarium specimen; photo by Z. Randall).

Fig. 9. Arapaiminae. a. Arapaima sp. (VIMS 38993; 120 mm SL). b. Heterotis niloticus (CU 95903); Scale bar = 2 cm.

Fig. 10. †Phareodontinae. a. †Phareodus testis, Eocene, Wyoming, USA. (UMA F11332; scale bar = 2 cm). b. †Brychaetus muelleri (BMNH P3898; holotype; scale bar = 5 cm) and c. †Brychaetus muelleri (BMNH 1748; scale bar = 2 cm), Eocene, England.

Fig. 11. †Singida jacksonoides (BMNH P63333, latex peel of holotype). Scale bar = 2 cm.