Introduction

Neogene alepisauroid fishes frequently emerge from the Northern Fossa Magna region in Nagano Prefecture, central Japan (e.g. Uyeno, 1967; Suzuki, 2008). A fossil palatine of an alepisauroid was found as a fragment from an outcrop of the middle Miocene Yokoo Formation exposed along the Kangawa River in the Motohara area of Sanada-machi, Ueda City, Nagano Prefecture, central Japan (Figure 1). The alepisauroid was collected from the upper part of the formation, and identified as Omosudis sp. belonging to the family Alepisauridae. Until now, alepisauroid fish fossils have been known from only a few specimens from the middle Miocene in Japan: Polymerichthys nagurai Uyeno, which was described as a new alepisauroid and placed within its own family by Uyeno (1967), from the middle Miocene Tubozawa Formation of the Hokusetsu Subgroup in Aichi Prefecture; Alepisauroidei? from the middle Miocene Bessho Formation in Nagano Prefecture by Uyeno (1979); and Alepisaurus sp., which is obviously close to Alepisaurus ferox Lowe, from the middle Miocene Iseyama Formation in Nagano Prefecture by Suzuki (2005, 2008).

The origin of the order Aulopiformes clade is estimated at 140 Ma in the Early Cretaceous, with a possible range into the Late Jurassic (Davis and Fielitz, 2010). The estimated divergence dates for the family Alepisauridae and the genus Omosudis Günther are in the Early Cretaceous. Isolated tooth fossils suggested to belong to an unidentified alepisauroid taxon have been described from the Early Cretaceous Barremian deposits of the Oliete subbasin of Alcaine in northeastern Spain (e.g. Harry, 1953; Patterson, 1993; Kriwet, 2003). The earliest definite fossil representative of the Recent alepisauroid fish Alepisaurus paronai d'Erasmo was discovered in the Miocene of Rosignano, Tertiary Piedmont Basin of Italy (Patterson, 1993).

Figure 1.

Map showing the tossil locality of USKF-101 (marked by ★). Based on the topographical map “Sanada” scale 1:25,000 published by the Geospatial Information Authority of Japan.

Figure 2.

The fossil palatine of Omosudis sp., USKF-101. Scale bar indicates 10 mm. Narrow arrows indicate the wide pulp cavities of the two teeth. Thick arrows indicate the basally elongated postapical barbs.

Alepisauroid fishes (Alepisauroidei sensu Davis, 2010) are bathypelagic to mesopelagic predatory fishes which have diagnostic palatine and dentary teeth. As is well known, fossil alepisauriform tooth morphologies allow for specific identification (Kriwet, 2003). If the fossil teeth are not isolated but arranged in the palatine or dentary, identification appears to be even more probable, because it is possible to confirm that the teeth are anteriorly inclined, posteriorly inclined or not inclined. In fact, the palatine tooth characteristics indicate a meaningful taxonomic identification at the generic level. Judging from the diagnostic characterisics of the well preserved palatine teeth, this fragment of the palatine can be classified as a member of the family Alepisauridae, and as compared with that of extant Omosudis lowii Günther, the fossil is considered to belong to Omosudis.

The specimen described here is noteworthy because, to date, it is the first reliable fossil record of Omosudis from the middle Miocene in Japan and appears to mark the earliest occurrence of this Recent genus in the Northwest Pacific region. This paper aims to provide palatine tooth morphologies of the fossil material assigned to Omosudis, and also aims to propose a better identification of fossil alepisauroids at the generic level by analyzing the palatine tooth morphology.

Figure 3.

Drawing of the fossil palatine of Omosudis sp., USKF-101. Scale bar indicates 10 mm.

Regional geology and the alepisauroid locality

The alepisauroid fossil specimen treated in this paper was collected by the present author in a recent year from an outcrop of the Yokoo Formation which is well exposed along the Kangawa River in the Motohara area of Sanadamachi, Ueda City, Nagano Prefecture, central Japan (Figure 1). Middle Miocene deep-water sediments, which yield abundant fish remains such as isolated bones, teeth and scales, are distributed around the fossil localities. The sedimentary rocks distributed in Ueda City range in age from early to middle Miocene and are divided into four formations: the Oomineyama, Tarouyama, Yokoo and Iseyama formations in ascending order (Yamagishi, 1964). The Yokoo Formation, which is the objective of the present study, is situated in the eastern margin of the Utsukushigahara Belt of the Northern Fossa Magna Region and is an equivalent stratigraphic unit to the upper part of the Uchimura Formation. The Uchimura Formation is the standard referential stratigraphy of the Northern Fossa Magna region in Nagano Prefecture (Kosaka et al., 1992).

Omosudis sp. was collected from the upper part of the Yokoo Formation. The formation primarily consists of alternating beds of muddy sandstone and mudstone (Suzuki, 2007, 2012). On the basis of radiolarians and other associated fossils such as Mizuhobaris izumoensis, the alepisauroids-bearing strata are considered to be of middle Miocene age (Blow, 1969; Noda et al., 1986; Kubota and Kosaka, 1990; Tanabe et al., 2016).

Material and method

The fossil teeth are not isolated but are still arranged in the palatine and show the directions of tooth inclination. Comparative materials are specimens of Omosudis lowii Günther, 1887 from collections of the Fish Division and Center for Molecular Biodiversity Research of the National Museum of Nature and Science, Tsukuba, Japan. Two relevant specimens are: NSMT-P40156, from Suriname, western Atlantic coast of South America; and NSMT-P 93514, from east of the Mariana Basin, in the northwestern Pacific. Morphological comparisons are made with O. lowii which were described and figured by Günther (1887), Ege (1958), Rofen (1966), Rosen (1973) and Nielsen and Jespersen (1986).

Figure 4.

Schematic drawing of a fang-type palatine tooth, showing descriptive terms used in this paper. Terms mostly from Kriwet (2003).

The Yokoo specimen described here, a fragment of a palatine with two palatine teeth (P1, P2), is registered with the prefix USKF and housed in Uedasouzoukan, Ueda City, Nagano Prefecture, Japan (USKF-101, Figures 2, 3). In order to discuss the morphology, it is necessary to define a few descriptive terms (Figures 4, 5). The terminology used in this study follows that adopted by Kriwet (2003). Morphological terms in the systematic description used in this paper are shown in Figure 4. The classification is referred to in Ege (1958), Rofen (1966), Rosen (1973), Nelson (2006) and Davis (2010). Figure 5 compares the important palatine tooth characteristics of extant Omosudis and similar alepisauroid genera.

Systematic description

Figure 5.

Brief comparison of diagnostic palatine tooth characteristics of Omosudis and similar alepisauroid genera. — indicates that the characteristic does not exist.

Discussion

Although the Yokoo specimen is only partly preserved, the palatine teeth bear some diagnostic characteristics offering a close resemblance to those of extant Omosudis. The grooves on the palatine surface are also observable in similar fossil specimens such as Alepisaurus sp. belonging to the family Alepisauridae, which was collected from the middle Miocene Iseyama Formation (Suzuki, 2008, p. 44, fig. 2a, b). Similar grooves are also present in the current genus Alepisaurus Lowe. The genus Omosudis is the most closely related taxon of the genus Alepisaurus (e.g. Gregory and Conard, 1936; Gosline et al., 1966; Smith and Ethel, 1973; Johnson, 1982; Baldwin and Johnson, 1996). Omosudis is distinct from similar alepisauroid genera such as Alepisaurus, Anotopterus Zugmayer and extinct Polymerichthys Uyeno in the general appearance of the palatine tooth (Figure 5). In the Yokoo specimen, as in O. lowii, the appearance is fang-like, but it is saberlike in Polymerichthys, knife-like in Alepisaurus and dagger-like in Anotopterus (Carnevale, 2007; Nazarkin, 2016). In the Yokoo specimen, as in O. lowii, a postapical barb is present, but it is absent in Anotopterus, Alepisaurus and Polymerichthys. In the Yokoo specimen, O. lowii and Alepisaurus, palatine teeth are posteriorly inclined, but they are anteriorly inclined in Anotopterus and Polymerichthys. The palatine tooth characteristics of the fossil specimen described here are strongly similar to extant O. lowii samples (original descriptions and figures are Günther, 1887, p. 201, pl. 52, fig. c-c'; Ege, 1958, p. 13, fig. 2; Rofen, 1966, p. 468, fig. 165A, D, p. 469, fig. 166; Rosen, 1973, p. 437, fig. 61; Nielsen and Jespersen, 1986, p. 62, fig. 1A, B, p. 63, fig. 2A, B). Omosudis lowii is characterized by five diagnostic characteristics: enormously large, fang-like outline due to a basally elongated postapical barb (limited to P1 and P2), wide pulp-cavity surrounded by a thin dentine layer, nearly straight to arcuate and smooth anterior cutting edge, rounded and smooth posterior cutting edge and apico-basal striations (e.g. Lowe, 1833; Maul, 1946; Marshall, 1955; Gibbs and Wilimovsky, 1966; Rofen, 1966; Rosen, 1973; Johnson, 1974; Goody, 1976; Kriwet, 2003). It is particularly worth mentioning that the posterior cutting edge is rounded but cannot be recognized in this fossil specimen.

Figure 6.

Schematic drawings of fang-type short postapical barb teeth. A, palatine or dentary tooth of Muraenesox cinereus (Muraenesocidae) (redrawn from Carpenter, 1999); B, palatine or dentary tooth of Leptostomias robustus (Stomiidae) (redrawn from Nakabo, 2002); C, palatine or dentary tooth of Evermannella balbo (Evermannellidae) (redrawn from Johnson, 1982); D, palatine or dentary tooth of Trichiurus lepturus (Trichiuridae) (redrawn from Long, 1991).

The Yokoo specimen is assignable to the genus Omosudis belonging to the family Alepisauridae by having the following characteristics: enormously large, posteriorly inclined teeth each with sharply pointed apex, apico-basal striations, a nearly straight to arcuate anterior cutting edge, a wide pulp cavity surrounded by a thin dentine layer and a fang-like outline due to a basally elongated postapical barb. In any case, important characteristics correspond to the original descriptions, figures and comparative materials of Omosudis lowii. The essential diagnostic characteristic for assignment of the fossil to Omosudis is P1 and P2 that are fang-like in shape due to a basally elongated postapical barb. Other fish that also have fang-like teeth such as the genera Muraenesox McClelland (Muraenesocidae), Leptostomias Gilbert (Stomiidae), Evermannella Fowler (Evermannellidae) and Trichiurus Linnaeus (Trichiuridae) differ in that their palatine or dentary teeth include a short postapical barb that only develops near the tip of the apex (e.g. Rosen, 1973; Johnson, 1982; Long, 1991; Carpenter, 1999; Nakabo, 2002; Kriwet, 2003; Nelson, 2006) (Figure 6).

The morphological resemblance between the fossil Omosudis and extant Omosudis lowii indicates their close relationship. Importantly, the palatine teeth described here show the reliable evidence of similarity. That is, the presence of a basally elongated postapical barb, thought to have functioned as a specialized feeding mechanism, is a diagnostic characteristic of Omosudis. The presence of a postapical barb has often been assumed as one of several diagnostic characteristics of some extant alepisauroids, but it seems justified to assign the Yokoo specimen to Omosudis.

In some extant alepisauroids, especially Alepisaurus, Anotoptems and Omosudis, reclining replacement teeth exist within a single row (Marshall, 1955; Gibbs and Wilimovsky, 1966; Rofen, 1966; Rosen, 1973; Johnson, 1974; Kriwet, 2003). However, the replacement teeth cannot be recognized in the Yokoo specimen because this is the anteriormost portion of a palatine.

Concluding remarks

It is evident from the discussion above that the presence of articulated dental remains, even if they are fragmentary specimens, provide better information to identify fossil alepisauroid remains than isolated teeth. It should be emphasized that the diagnostic characteristics of fossil teeth being arranged in a single row can be used for assigning fossil alepisauriform teleosts, even though the isolated fossil tooth morphology can allow for specific identification at the generic level. This paper provides verifiable palatine tooth morphologies of the fossil material assigned to Omosudis. The method in this paper allows utilization of incomplete specimens. It remains unclear whether the fossil Omosudis sp. is the same species as Omosudis lowii. Species level identification currently remains difficult because of exceedingly rare occurrences and insufficient preservation. However, this is the first reliable fossil record from the Middle Miocene in Japan and appears to mark the earliest occurrence of this recent genus in the Northwest Pacific region.