''Megachasma alisonae'' is an extinct species of

megamouth shark 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 taxon, extant filter-feeding sharks alongside the relatively larger whale shark and basking shark. ...

that lived during the Eocene (

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 ...

, ca. 36 mya). It is the oldest fossil record of a megamouth shark. The type fossil was recovered from the Søvind Marl Formation in Denmark and consists of a single tooth. Based on comparison with the teeth of the recent megamouth species (''Megachasma pelagios''), the length of the animal has been estimated at .

Discovery and naming

Megachasma alisonae Carcharocles auriculatus

The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

and only known specimen of ''M. alisonae'' is NHMUK PV P73711, an isolated tooth. It is nearly complete and either belonged to the right side of the upper jaw or the left side of the lower jaw. It was discovered on a sea cliff in the Pyt Member of the Søvind Marl Formation in

Denmark ) , song = ( en, "King Christian stood by the lofty mast") , song_type = National and royal anthem , image_map = EU-Denmark.svg , map_caption = , subdivision_type = Sovereign state , subdivision_name = Danish Realm, Kingdom of Denmark ...

. The tooth was discovered in a bulk sediment sample from the upper of the member that was collected in 1988. The species was described by Kenshu Shimada and David J. Ward in 2016. The specific name honors Alison Ward, who aided David J. Ward with the work that resulted in the type specimen's discovery.

Classification

''M. alisonae'' belongs to the same genus as the modern

and the extinct species ''

Megachasma applegatei ''Megachasma applegatei'' is an extinct species of megamouth shark from the Oligocene to early Miocene (28-23 Mya) of the Western United States. The type fossil was discovered in the San Joaquin Valley The San Joaquin Valley ( ; es, Valle ...

''. ''Megachasma'' itself belongs to the family Megachasmidae. Molecular studies have estimated that this family arose in the

Upper Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...

. Due to a lack of fosil material, it has been hypothesized that the dental changes in this family occurred slowly enough to make their teeth harder to differentiate from other families of sharks early in their evolution. The tooth morphology of ''M. alisonae'' has a greater resemblance to that of Odontaspididae than seen in the other species of ''Megachasma''. This supports the hypothesis that Megachasmidae is closely related to a group of sharks with odontaspidid-like teeth, perhaps the odontaspids themselves. The tooth morphology of ''M. alisonae'' also provides information needed to identify older megachasmid fossils.

References

Megachasmidae Prehistoric sharks Fossil taxa described in 2016 {{paleo-shark-stub