Hexanchus gracilis: A shark from the Lower Chalk of Hunstanton

S M Bowerman (UK)

Sharks of the genus Hexanchus belong to the family Hexanchidae and are more commonly known as six gilled sharks or cow sharks. They are well known for the difference in shape between teeth of the upper and lower jaw. They are first recorded in the Jurassic. Smart (2001) states that Hexanchidae are represented in the English late Cretaceous by two species: Hexanchus microdon and H. gracilis, both species being known only from rare isolated teeth. I collected a single specimen of H. gracilis, which is the subject of this short article.

It was found at Hunstanton, in Norfolk in the UK (TF 673414), from the Upper Cretaceous Lower Chalk, Middle Cenomanian, Varians Chalk, Schloenbachia varians zone.

It is an incomplete specimen from the lower jaw. The specimen shows only two cusps of a possible seven. These appear to be accessory cusps. The posterior aspect of the specimen is not preserved therefore the principle cusp is not seen. The cusps show no signs of serration and the specimen remains in chalk matrix with the lingual aspect uppermost. The root is rectangular and slightly convex. Tooth width is 5mm, and its height is 3mm.

Further reading

Fossils of the Chalk: Palaeontological Association Guide No 2 (2nd edition), edited by Andrew B Smith and David J Batten, The Palaeontological Association, London (2002), 374 pages (Paperback), ISBN: 0901702781

References

Smart, P. J. 2001. An undescribed late Cretaceous Notorynchus tooth (Chondrichthyes, Vertebrata) from the English Chalk. Proceedings of the Geologists’s Association, 112, 59-62 “Hexanchid sharks are first recorded in the Jurassic”.

Additional material by E. R. Matheau-Raven Cretaceous Sea Red Rock & Chalk Fossils Of Hunstanton & The Norfolk Coast http://www.fossiliferous.co.uk.