''Megachasma'' is a genus of

mackerel sharks The Lamniformes (, from Greek ''lamna'' "fish of prey") are an order of sharks commonly known as mackerel sharks (which may also refer specifically to the family Lamnidae). It includes some of the most familiar species of sharks, such as the grea ...

. It is usually considered to be the sole genus in the 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'' ...

, of which the

basking shark The basking shark (''Cetorhinus maximus'') is the second-largest living shark and fish, after the whale shark. It is one of three Planktivore, plankton-eating shark species, along with the whale shark and megamouth shark. Typically, basking sh ...

is the sole extant member. ''Megachasma'' is known from a single living species, '' Megachasma pelagios''. In addition to the living ''M. pelagios'', 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 ...

'' Megachasma alisonae'' and the

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

–

Miocene The Miocene ( ) is the first epoch (geology), 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 mea ...

'' Megachasma applegatei'' – have been discovered from fossilized tooth remains.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> An early ancestor of ''M. pelagios'' was reported from the

Burdigalian The Burdigalian is, in the geologic timescale, an age (geology), age or stage (stratigraphy), stage in the early Miocene. It spans the time between 20.43 ± 0.05 annum, Ma and 15.97 ± 0.05 Ma (million years ago). Preceded by the Aquitanian (sta ...

of Belgium. However, the Cretaceous-aged ''M. comanchensis'' was reclassified as an odontaspid shark in the genus ''

Pseudomegachasma ''Pseudomegachasma'' ("false megamouth") is an extinct genus of 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 is the only known planktivorous ...

'', and is unrelated to the megamouth shark despite similar teeth morphology. ''Megachasma'' is primarily found in temperate and tropical waters among the Pacific, Atlantic, and Indian oceans. The top sighting spots are 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