Dental evolution of the ‘serrated’ mako shark, Isurus subserratus aka I. escheri (Chondrichthyes, Lamnidae) in the late Neogene of the North Sea Basin

The locality of Langenboom (Noord-Brabant, the Netherlands), also known as Mill, is famous for the massive number of isolated shark teeth recovered between 1995 and 2015 by fossil collectors from dumped upper Miocene and lower Pliocene sands. At this locality, the crenulated teeth of the extinct mako shark Isurus subserratus (Agassiz, 1843), commonly listed in the palaeontological literature with the specific name of escheri, are exceptionally common. Owing to scarcity of these teeth outside the Netherlands, Belgium and Germany, this species is poorly known, and its generic attribution has been for a long time the subject of debate. Based on the morphology and taphonomic condition, we identified four well defined consecutive morphotypes of Isurus subserratus teeth in the abundant Langenboom material. We observe that over time, the size increases, particularly on upper anterior teeth, together with a broadening of the crown. Lower teeth become more massive in appearance, the crenulations on the cutting edges get coarser and more irregular, and lateral cusplets disappear in all tooth positions. These four Langenboom types are also recognised in the sparse existing literature on in situ collected specimens. The gradual dental evolution clearly shows that the species descended from Isurus oxyrinchus Rafinesque, 1810 and is not related to the Carcharodon lineage. Isurus subserratus probably originated in the Serravallian of the North Sea Basin and its occurrence was restricted to the Atlantic coastline of Europe and the United States. By the middle Zanclean, the species seems to have disappeared. The genus Carcharomodus Kriwet, Mewis & Hampe, 2015, recently created to accommodate these teeth, is discussed and discarded.