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HEXANCHIFORMES [Cow / Six-gill Sharks]
Hexanchus
(Rafinesque, 1810)
Notoryhnchus (Ayres, 1855)

 

The Six-gill Sharks, represented in the Chalk by the Hexanchids Hexanchus and Notorhynchus, possess a relatively small number of highly distinctive clutching / cutting teeth.  These are low and wide, labio-lingually flattened, with a tall bar-like root and a crown comprising an anterior-most main cusp followed by a series of increasingly small angular accessory cusps.  The main cusp is notably enlarged in mature males (compare figures 1A and 1B below).  The lower part of the anterior cutting edge of the main cusp has prominent serrations.  Isolated teeth are scarce in the field.

 

 A  B

1).  Hexanchus ex. gr. microdon -  Labial views of isolated teeth: (A) Lower anterio-lateral of a mature male (note enlarged main cusp) (x8); (B) A 'gracilis'-form lower lateral of a female, or immature male (x8)Images courtesy of Charlie Underwood.

 

 A B

2).  Hexanchus ex. gr. microdon - (A) Labial view of a tooth (x8, Grey Chalk, Glynde, near Lewes, East Sussex, Willett Collection, Booth Museum, BMB 007325, by kind permission of John Cooper); (B) Labial view of an upper anterior tooth (from the front of the palatoquadrate) -  note reduced width of root and reduced number of accessory cusps, and pronounced development of the main cusp (x10, Grey Chalk, Southerham, near Lewes, Randell Collection RR0046).

 

 A  B

3).  Notorhynchus sp.? - Seemingly associated teeth: (A) Lingual view of a tooth; (B) Labial view of a tooth; (x7.5, Grey Chalk, Glynde, near Lewes, East Sussex, Willett Collection, Booth Museum, BMB 007325, by kind permission of John Cooper).

 

 A  B

4).  Modern six-gill sharks for illustration; (A) Bluntnose Six-gill Shark (Hexanchus griseus) (image sourced from ARKive,  Brandon Cole / naturepl.com); (B) Big-eye Sixgill Shark (Hexanchus nakamurai) (image sourced from Bimini Biological Field Station).

 

A B

5).  Notorhynchus aptiensis - (A) Labial and (B) lingual views of a representative isolated Notorhynchus tooth from the Albian Gault Clay of Folkestone (x7.5).  Images (A, B) by kind permission of David Ward.  See also www.gaultammonite.co.uk [Fossils of the Gault Clay]