''Megachasma alisonae'' is an extinct species of

megamouth shark The megamouth shark (''Megachasma pelagios'') is a species of deepwater shark. Rarely seen by Human, humans, it measures around long and is the smallest of the three extant taxon, extant filter-feeding sharks alongside the much larger whale sha ...

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 ...

, 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 (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

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 Søvind is a small town, with a population of 1,075 (1 January 2025),Denmark Denmark is a Nordic countries, Nordic country in Northern Europe. It is the metropole and most populous constituent of the Kingdom of Denmark,, . also known as the Danish Realm, a constitutionally unitary state that includes the Autonomous a ...

. 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 in 1973, but only described in 2014, when th ...

''. ''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 more recent 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 ''cret ...

. Due to a lack of fossil 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 Tooth taxa {{paleo-shark-stub