Hiodontiformes

Introduction

The Hiodontiformes are represented by two extant species, Hiodon alosoides (goldeye) and Hiodon tergisus (mooneye), which occur in fresh waters, both lakes and streams, of the interior of North America east of the Rocky Mountains. These two species are of minor economic importance, but they are reasonably well known among sport fishers.

The earliest fossil Hiodontiformes thus far known in North America are Eocene species of †Eohiodon (Cavender, 1966; Wilson, 1978; Grande, 1979; Li, Wilson, and Grande, 1997). However, in East Asia the record of the group extends back to the Early Cretaceous †Yanbiania (Li, 1987), a genus sister to the clade containing the Eocene †Eohiodon and the Eocene to Recent Hiodon. In addition, recent studies suggest that †Plesiolycoptera (Zhang and Zhou, 1976) is an even more primitive stem-group member of the Hiodontiformes (Li and Wilson, in press).

Characteristics

All hiodontiform fishes share the following synapomorphies:

1. Hyomandibular bone double-headed.
2. Dermosphenotic bone triradiate.
3. Opercle bone an irregular parallelogram.
4. Dorsal arm of post-temporal bone more than twice as long as ventral arm.
5. Seven-rayed pelvic fin.

Comparison between Hiodon and Eohiodon

The following are some important osteological differences recently identified between Eohiodon and Hiodon:

1. The horizontal arm of the preopercle is proportionally longer in Eohiodon than in Hiodon.
2. Eohiodon has a parallel-sided opercle that is not strongly excavated posteriorly (see Li, Wilson, and Grande, 1997: figure 3-1), while the opercle in Hiodon is strongly excavated both posteriorly and dorsally (see Li and Wilson, 1994, fig. 6).
3. The upper part of the hyomandibula consists of three equal-sized heads (including the opercular process) in Eohiodon (a typical Eohiodon feature), while in Hiodon the dorsal end of the hyomandibular is bridged (a typical Hiodon feature; see Li, Wilson, and Grande, 1997: figure 4).

Recent study also suggests that the lower and upper jaws in Eohiodon woodruffi appear to be intermediate between those of Eohiodon rosei and those of Hiodon. The premaxilla in Eohiodon rosei is similar to that in lycopteriforms and other hiodontiforms, such as Yanbiania (Li, 1987), in having a raised anterior portion. In Hiodon it is anteriorly only as deep as or even slightly shallower than it is posteriorly. The premaxilla in Eohiodon woodruffi has only a slightly raised anterior portion. A distinct feature of the premaxilla in Hiodon, including H. consteniorum (Li and Wilson, 1994), is that it bears a mid-dorsal concavity. The two Eohiodon species do not share this character state with Hiodon.

It should be noticed that in some features of the jaws and infraorbital series, Eohiodon woodruffi is more similar to Hiodon tergisus than it is to Eohiodon rosei and Hiodon alosoides. As in Hiodon tergisus, the posterior end of the maxilla in Eohiodon woodruffi extends back to beyond the vertical midline of the orbit, usually reaching to the level of the posterior one third of the orbit. Correspondingly, the corner of the mouth gape in Eohiodon woodruffi is located at a level near the vertical midline of the orbit. The general outline of the infraorbital series of Eohiodon woodruffi (see Li, Wilson, and Grande, 1997: figure 3-2) is basically the same as that of the extant species Hiodon tergisus, in which the two posterior infraorbitals (io3 and io4-5) are deeper than long, forming a relatively shorter cheek region than in Hiodon alosoides.

Discussion of Phylogenetic Relationships

The Hiodontiformes contain four valid genera: Hiodon (Lesueur, 1818), Eohiodon (Cavender, 1966), Yanbiania (Li, 1987), and Plesiolycoptera (Zhang and Zhou, 1976).

Previous studies suggested that Eohiodon closely resembles Hiodon in osteology (Cavender, 1966; Wilson, 1977, 1978; Grande, 1979; Li and Wilson, 1994), for example in having a caudal skeleton almost identical to that of Hiodon (see Li, Wilson, and Grande, 1997: figure 5). Recently, Li (1994), Li and Wilson (1994, 1996) and Li, Wilson, and Grande (1997) further clarified that Hiodon is sister to the fossil genus Eohiodon. This relationship is supported by at least two synapomorphies uniquely shared by these two genera in the Hiodontiformes sensu stricto Li and Wilson (1996): supraorbital canal ending in frontal, anal fin in male and female sexually dimorphic.

Yanbiania was originally considered to be an early member of the Hiodontidae based on features from the parasphenoid, the basihyal tooth plate, and the caudal skeleton (see Li, 1987). After comparison with North American Eocene Eohiodon and extant Hiodon, a sister-group relationship was recognized between Yanbiania and Eohiodon plus Hiodon based on five derived features:

1. Dermosphenotic bone triradiate.
2. Opercle bone an irregular parallelogram.
3. Dorsal arm of post-temporal bone more than twice as long as ventral arm.
4. Hyomandibular bone double-headed.
5. Pelvic fin with seven rays.

These synapomorphies define the Hiodontiformes (Li and Wilson, 1996a). A complete specimen of Yanbiania recently collected from the type locality confirms the presence of all five synapomorphies. Yanbiania Li 1987 thus demonstrates that the Hiodontiformes had already diverged by Early Cretaceous times.

The early Late Cretaceous genus Plesiolycoptera has an infraorbital series most similar to that in extant Hiodon (see Li, 1987; Li and Wilson, in press). The most important feature supporting a hiodontiform relationship of Plesiolycoptera is the triradiate dermosphenotic, which is unique to the Hiodontiformes among teleosts (Li 1994a, Li and Wilson, 1996a). Another important feature supporting a hiodontiform relationship of Plesiolycoptera is the "L"-shaped post-temporal with a dorsal arm at least twice as long as the ventral arm. This is seen in all hiodontiform species and is also unique to the order (Li and Wilson, 1996a, in press; Li, Wilson, and Grande, 1997). In addition, seven-rayed pelvic fins are found in Plesiolycoptera. This feature characterizes all species included in the Hiodontiformes (Li and Wilson, in press). Further studies of the systematic position of Plesiolycoptera require recovery of better-preserved specimens.

Biogeography

Living species of hiodontiforms are endemic to interior fresh waters of North America east of the Rocky Mountains. However, fossil evidence indicates that the ancient geographic range of this group includes East Asia (from Early Cretaceous to Early Tertiary, see Zhang and Zhou, 1976; Sytchevskaya, 1986; Li, 1987) and far western North America (Early Tertiary, see Cavender, 1966; Wilson, 1978; Grande, 1979; Li and Wilson, 1994; Li, Wilson, and Grande, 1997). Extinction must have been important in the Cenozoic history of the order (Li, Wilson, and Grande, 1997): the present endemic distribution of the Hiodontiformes is thus a remnant of a former widespread northern hemispheric distribution (Wilson and Williams, 1993).

Other Names for Hiodontiformes

• Mooneye, Goldeye

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