''Megachasma'' is a genus of sharks. It is usually considered to be the sole genus in the distinct family Megachasmidae, though suggestion has been made that it may belong in the family

Cetorhinidae Cetorhinidae is a family of filter feeding mackerel sharks, whose members are commonly known as basking sharks. It includes the extant basking shark, ''Cetorhinus'', as well as two extinct genera, ''Caucasochasma'' and ''Keasius ''Keasius'' is ...

, of which the

basking shark The basking shark (''Cetorhinus maximus'') is the second-largest living shark and fish, after the whale shark, and one of three plankton-eating shark species, along with the whale shark and megamouth shark. Adults typically reach in length. ...

is currently the sole extant member. ''Megachasma'' is known from a single living species, ''

Megachasma pelagios The megamouth shark (''Megachasma pelagios'') is a species of deepwater shark. It is rarely seen by humans and is the smallest of the three extant filter-feeding sharks alongside the relatively larger whale shark and basking shark. Since its di ...

''. In addition to the living ''M. pelagios'', however, two extinct megamouth species – the

Priabonian The Priabonian is, in the ICS's geologic timescale, the latest age or the upper stage of the Eocene Epoch or Series. It spans the time between . The Priabonian is preceded by the Bartonian and is followed by the Rupelian, the lowest stage of t ...

'' M. alisonae''Shimada, Kenshu, and David J. Ward. "The oldest fossil record of the megamouth shark from the late Eocene of Denmark, and comments on the enigmatic megachasmid origin.

/ref> and the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

–

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

'' M. applegatei'' – have also recently been proposed on the basis of fossilized tooth remains. An early ancestor of the recent species ''

'' was reported from the early Miocene (Burdigalian) of Belgium. However, the Cretaceous-aged ''M. comanchensis'' has been recently reclassified as an odontaspid shark in the genus ''

Pseudomegachasma ''Pseudomegachasma'' ("false Megamouth shark, megamouth") is an extinct genus of Filter feeder, filter-feeding shark that was closely related to the modern sand tiger shark. It is known from Cretaceous strata in Russia and the United States, and ...

'', and is in fact unrelated to the megamouth shark despite similar teeth morphology. They are a very rare genus of shark that are mainly found in temperate and tropical waters among the Pacific, Atlantic and Indian oceans. The top siting spots were Taiwan, Japan, and the Philippines. Watanabe, YY, Papastamatiou, YP. Distribution, body size and biology of the megamouth shark Megachasma pelagios. J Fish Biol. 2019; 95: 992– 998. https://doi-org.edu/10.1111/jfb.14007

References

Shark genera Megachasmidae Fish genera with one living species {{Shark-stub