Hexanchus microdon (Agassiz, 1835)

Adolfssen, Jan S. & Ward, David J., 2015, Neoselachians from the Danian (early Paleocene) of Denmark, Acta Palaeontologica Polonica 60 (2), pp. 313-338 : 316-318

Hexanchus microdon (Agassiz, 1835)

Fig. 2H, I.

1835–44 Notidanus microdon sp. nov.; Agassiz 1835: 3, pl. 27: 1; 1843: 222; 1844: pl. 36: 1, 2.

1952 Notidanus ( Hexanchus ?) microdon Agassiz ; Arambourg 1952: 40, pl. 1: 1–55.

1977 Notidanus ( Hexanchus ?) microdon Agassiz ; Herman 1977: 98,

pl. 3: 7.

2002 Hexanchus microdon (Agassiz) ; Antunes and Cappetta 2002: 108, pl. 4: 3, 4.

Material.—Three almost complete upper jaw teeth ( MGUH 29825, MGUH 29826, GMV 2012-58) and two fragmentary teeth from the lower jaw collected from the Ce in Stevns gual (C 2) views. D–G. Sphenodus lundgreni ( Davis, 1890) . D. MGUH1406 , holotype (Br2) anterior lateral tooth in labial (D 1), lingual (D 2), and distal (D 3) views, with magnification of folds (D 4, D 5, respectively). E . MGUH 29822 (Br2) anterior tooth in labial (E 1), lingual (E 2), and distal (E 3) views. F . MGUH 29823 (Br2) lateral tooth in labial (F 1), lingual (F 2), and mesial (F 3) views. G . MGUH 29824 (Br2) rare posterior tooth in labial (G 1), lingual (G 2), and mesial (G 3) views. H, I. Hexanchus microdon (Agassiz, 1843) . H . MGUH 29825 (Ce) fragment from a lower jaw tooth. I . MGUH 29826 (Ce) fragment from an upper jaw tooth in lingual view. J. Gladioserratus sp ., MGUH 29827 (Br2) in lingual view. K, L. Heptranchias howelli Reed, 1946 . K . OESM-10046-14 (Br2) lower jaw tooth in labial (K 1) and lingual (K 2) views. L . OESM-10046-15 (Br2) upper jaw tooth in labial view. M. Notidanodon brotzeni Siverson, 1995 , MGUH 1399 (Br2) crown of tooth .

Kridtbrud. Incomplete teeth of Hexanchus are common in the Faxe quarry (Br2).

Description.—The dentition in Hexanchus is both monognathic disjunct, dignathic and gynandric heterodont. Teeth from the upper jaw are strongly labio-lingually compressed with a prominent main cusp that is slightly inclined towards the commissure, with a flat labial face and a convex lingual face. The mesial cutting edge of the main cusp is only partly preserved, but fine serration is visible. A single low distal cusplet is present and strongly inclined towards the commissure. The distal end of the tooth is missing, so it is unclear whether there was a second distal cusp or not. The root is high and wide and labio-lingually compressed.

Teeth from the lower jaw are strongly labio-lingually compressed with a distally inclined main cusp. The main cusp is serrated on the lower of its half mesial cutting edge and is either straight or slightly sigmoid mesially on the upper half. The distal cutting edge is convex. The distal cusplets are distally inclined with convex mesial and distal cutting edges. The maximal number of distal cusplets observed was five, but it was in a broken specimen with clear signs of having had more.

Remarks.—The teeth are not well preserved but do not differ significantly from those referred to H. microdon from the Late Cretaceous chalks and from the Fiskeler. Of the two Recent species, H. griseus ( Bonnaterre, 1788) and H. nakamurai Teng, 1962 ), the dentition of H. microdon closely resembles those of H. nakamurai in having lower teeth with lower crowns making them relatively wider than those of H. griseus Herman, Hovestadt-Euler, Hovestadt, and Stehmann, 1994 ). The name H. microdon is problematic as it is applied to almost any H. nakamurai- like Hexanchus from the Late Cretaceous and early Paleogene. In particular, in Morocco it is conventionally used for all Hexanchus specimens in the Maastrichtian, Paleocene, and early Eocene, whereas in NW Europe it is restricted to the Cretaceous and H. agassizi is used for Paleocene and early Eocene specimens. A revision is required but it is beyond the scope of this paper. Teeth of H. agassizi , as the name is currently used, differ from those of H. microdon by their smaller size. H. microdon differs from H. gracilis ( Davis, 1887) mainly in having more distal cusplets, up to ten in H. microdon versus up to six in H. gracilis ( Cappetta 1980) . All these differences could be due to a combination of ontogenetic and sexual dimorphism.

Stratigraphic and geographic range.—Maastrichtian in northern Europe to the Ypresian in Morocco ( Cappetta 1980, 1987).