Hiodontidae, Valenciennes, 1847

Hiodontidae View in CoL View at ENA .

Hiodontidae ( Fig. 3 View Fig ), which is regarded as the living sister group of all other extant Osteoglossomorpha ( Taverne, 1979; Li, Wilson, 1996a; Hilton, 2003; Zhang, 2006; Wilson Murray, 2008), with one or two genera ( Hiodon and † Eohiodon ); the fossil taxa † Yanbiania and † Jiaohichthys from the Early Cretaceous of China and † Plesiolycoptera from the Mid Cretaceous of China are stem group Hiodontiformes. Hiodon comprises two extant species ( H. alosoides and H. tergisus ), both found exclusively in the freshwater rivers and lakes throughout much of North America east of the Rocky Mountains. These fishes have a generalized, laterally compressed body, with large eyes, a forked caudal fin, and a silvery body with cycloid scales ( Hilton et al., 2014). The parasphenoid and basihyal toothplate are armed with large, caniniform teeth that serve the so-called “parasphenoid-tongue bite apparatus” ( Hilton, 2001). The osteology of Hiodon has been described by Taverne (1977) and Hilton (2002), with specific aspects of its skeleton described by others ( e.g., caudal skeleton, Schultze, Arratia, 1988) due in part to its overall plesiomorphic morphology, which has led to its use as a representative basal teleost in broad based systematic analyses (see discussion and references by Hilton, 2002).

The three species of † Eohiodon from the Early Eocene of western North America have been regarded as close relatives of the extant genus Hiodon ( Li et al., 1997a; Hilton, Grande, 2008; Fig. 3b View Fig ). Indeed, because of the absence of any synapomorphies distinguishing the species of † Eohiodon from those of Hiodon , Hilton, Grande (2008) regarded it as a synonym of Hiodon . The two extant species of Hiodon possess a post-pelvic bone, and this is considered a synapomorphy of the extant taxa ( Hilton, 2003), although the condition in most fossil taxa, including † H. consteniorum and the species of † Eohiodon , is unknown ( Hilton, 2003). Murray et al. (2010: fig. 10) illustrated a fragmentary bone that they interpreted as a postpelvic bone in † Schuleichthys brachypteryx, a species from the Early Cretaceous of China that was left as incertae sedis at the base of Osteoglossomorpha. These authors suggested that the presence of a postpelvic bone in † Schuleichthys was a character of a broader group and therefore resurrected the genus † Eohiodon (see also Murray et al., 2018). However, we find the published photograph documenting the postpelvic bone in † Schuleichthys to be unconvincing, and maintain that until this element is clearly seen in taxa outside of the extant taxa, it should be considered to be a synapomorphy of these two extant taxa. Regardless, there has yet to be any synapomorphies identified that group the taxa previously included in the genus † Eohiodon ( i.e., all diagnostic characters cited for the genus, such as low vertebral and fin ray counts, are plesiomorphic, being similar to stem group Hiodontiformes and † Lycopteridae ). We therefore support the interpretation that those taxa previously included in † Eohiodon should be regarded as stem group Hiodon ( Hilton, Grande, 2008).