Introduction

The megamouth shark, Megachasma pelagios (Lamniformes, Megachasmidae), is a large elasmobranch, which was first discovered off Hawaii in 1978 (Taylor et al., 1983). On the basis of its large body size (up to ca. 5.5 m) and unique morphology, the discovery of this species is considered to be one of the ichthyological highlights of the 20th century (Berra, 1997; Nakaya, 2010).

Together with the basking shark, the whale shark, and mobulid rays, the megamouth shark is the fourth planktivorous elasmobranch lineage (Taylor et al., 1983). The fossil records of planktivorous elasmobranchs are important for clarifying the processes whereby elasmobranchs have filled the ecological niche for giant plankton feeders (Friedman et al., 2010). However, the fossil record of these three planktivorous sharks is limited, partly because of their small tooth size (generally <5 mm in height). Recent summaries of the megamouth shark fossil record suggest that this genus is reported from only approximately 10 localities, including North America, South America, and Europe (De Schutter, 2009; Cappetta, 2012; Shimada et al., 2014b). To the best of our knowledge, the Asian record of the megamouth shark was documented only in an unpublished doctoral dissertation by Kuga (1985). The description was based on a single piece of material housed in a private collection, and the specimen is not accessible at present.

The present study reports a new discovery of a megamouth shark fossil from a Neogene or lower Quaternary deposit on Okinawa Island, Japan. The specimen is noteworthy because, to date, it is the only verifiable fossil record of a megamouth shark from Asia.

Locality and geological setting

The specimen was collected on a beach along the west coast of the southern region of Okinawa Island (Figure 1), where the lithology consists of the Shimajiri Group, the Chinen Formation, and the Ryukyu Group (Iryu et al., 2006). The specimen was ex situ, but it almost certainly came from the Shimajiri Group or the Chinen Formation, which is broadly exposed in the area. The rocks of the two stratigraphic units in the area mainly consist of muddy very fine-grained sandstone, thereby suggesting that it was deposited in an outer shelf or deeper environment. The Shimajiri Group is known to include a large number of elasmobranch remains, some of which (Megaselachus megalodon, Parotodus henedini, and Cosmopolitodus hastalis) were described previously (Uyeno and Oshiro, 1982). The geologic ages of the Shimajiri Group and the Chinen Formation are estimated to be Miocene to lower Pleistocene on the basis of the planktonic foraminiferal and calcareous nannofossil assemblages (Tanaka and Ujiié, 1984; Imai et al., 2013).

Systematic paleontology

Figure 1.

Map of Okinawa Island showing the location of the fossil megachasmid specimen.

Figure 2.

Tooth of fossil megachasmid shark (NMNS-PV 22659) from Okinawa, Japan. Lingual (A), labial (B), apical (C), basal (D), mesial (E), and distal (F) views. Scale = 5 mm.

Discussion

The morphology of the specimen suggests that it belongs to the genus Megachasma. The specimen can be distinguished from other lamniform genera by its small tooth size, strongly lingually flexed central cusp, and a massive root with a strong lingual protrusion (Cappetta, 2012).

There is much debate on the classification of megamouth shark fossils. De Schutter (2009) tentatively divided the Cenozoic megamouth sharks into two morphological groups: (1) “Megachasma sp.,” characterized by the general presence of lateral cusplets and long, well separated root lobes, including early Miocene specimens from California and early Miocene?—early Pliocene? specimens from Belgium; and (2) “M. cf. pelagios,” characterized by a general absence of lateral cusplets and poorly separated short root lobes, including late Mioceneearly Pliocene specimens from Chile, eastern USA (Florida and North Carolina), and Greece.

Shimada et al. (2014b) subsequently erected a new species, Megachasma applegatei, based on numerous isolated teeth, including 67 type specimens, collected from Oligocene—early Miocene deposits in western USA (California and Oregon). The authors accepted De Schutter's two morphological groups (i.e., “Megachasma sp.” and “M. cf. pelagios”) and suggested that “Megachasma sp.” should be assigned to M. applegatei. However, the taxonomic status of “M. cf. pelagios” of De Schutter (2009) was not stated by Shimada et al. (2014b), in part because De Schutter's (2009) specimens were based on a private collection with tenuous chronostratigraphy. In the present study, we follow the division into these two morphological groups and identify NMNS-PV 22659 as belonging to “M. cf. M. pelagios” because it lacks lateral cusplets and displays a weakly bilobate root.

It remains unclear whether the fossil “M. cf. pelagios” is the same species as the extant M. pelagios. The morphological resemblance between the fossil “M. cf. pelagios” and M. pelagios indicates their close relationship (De Schutter, 2009). However, it should be noted that the crown slenderness of NMNS-PV 22659 (crown height/crown width = 1.02) fell outside the range of the extant M. pelagios (total range, 1.1–2.4; interquartile range, 1.5–2.0; Yabumoto et al., 1997; Shimada et al., 2014b). Future discussion on the taxonomic status of “M. cf. pelagios” should be based on larger sample sizes, but this is beyond the scope of the present study.

Fossil megamouth sharks also include one Mesozoic species, Megachasma comanchensis (Shimada, 2007). This species was erected for four isolated teeth obtained from a mid-Cretaceous (Cenomanian) deposit in Colorado, USA (Shimada, 2007). Later, Shimada et al. (2014a) suggested that this species may not belong to the family Megachasmidae but to the family ‘Odontaspidae’, so its taxonomic position is still unsettled. This species is distinguishable from NMNS-PV 22659 in having a well developed nutrient groove and wide and flattened basal attachment surfaces of the tooth root (Shimada, 2007).

Kuga (1985) described a single isolated tooth found from the Tomioka Formation (Pliocene) of Fukushima Prefecture, Japan, and identified this fossil as Megachasma cf. pelagios. However, the description was based on a specimen in a private collection, and the specimen is not accessible at present. The non-Asian occurrences of Cenozoic megamouth shark fossils were summarized by Cappetta (2012) and Shimada et al. (2014b) as follows: Chattian of California, USA (Phillips et al., 1976); late Chattian-Aquitanian of California and Oregon, USA (Shimada et al., 2014b); Aquitanian of Mexico (González Barba and Thies, 2000); Miocene of Oregon (Taylor et al., 1983); upper Miocene of Chile (Cappetta, 2012); Tortonian of Greece (Keupp and Bellas, 2002); Zanclean of North Carolina, USA (Purdy et al., 2001); lower Miocene?-lower Pliocene? of Belgium (De Schutter, 2009); and Neogene of Florida, USA (De Schutter, 2009). The extant megamouth shark is known to have a global distribution in tropical to temperate seas (Compagno, 2002; Nakaya, 2010). Similar to the extant species, the worldwide occurrences of the fossil megamouth shark may reflect its circumglobal distribution.

In addition to the megamouth shark, the large planktivorous basking shark (Cetorhinus) lived around Japan during the Cenozoic. A tooth of the basking shark was reported from Miocene deposits (Karasawa, 1989), and many gill rakers were discovered in several localities of Oligocene—Pleistocene rocks in Japan (Kikuchi and Takaoka, 1979; Uyeno et al., 1983, 1984; Nakagawa and Yasuno, 1985; Tomita and Oji, 2010). On the basis of current knowledge, only three shark and one batoid lineages are recognized as having acquired a planktivorous lifestyle during elasmobranch life history. The present discovery of a fossil megamouth shark from Japan confirms that at least two of these four planktivorous elasmobranch lineages were represented in the western North Pacific during the Neogene or the early Pleistocene.