A NEW SPECIES OF ISCHYODUS (CHONDRICHTHYES: HOLOCEPHALI: CALLORHYNCHIDAE) FROM UPPER MAASTRICHTIAN SHALLOW MARINE FACIES OF THE FOX HILLS AND HELL CREEK FORMATIONS, WILLISTON BASIN, NORTH DAKOTA, USA

Abstract:  A new species of chimaeroid, Ischyodus rayhaasi sp. nov., is described based primarily upon the number and configuration of tritors on palatine and mandibular tooth plates. This new species is named in honour of Mr Raymond Haas. Fossils of I. rayhaasi have been recovered from the Upper Maastrichtian Fox Hills Formation and the Breien Member and an unnamed member of the Hell Creek Formation at sites in south‐central North Dakota and north‐central South Dakota, USA. Ischyodus rayhaasi inhabited shallow marine waters in the central part of the Western Interior Seaway during the latest Cretaceous. Apparently it was also present in similar habitats at that time in the Volga region of Russia. Ischyodus rayhaasi is the youngest Cretaceous species of Ischyodus known to exist before the Cretaceous/Tertiary extinction, and the species apparently did not survive that event. It was replaced by Ischyodus dolloi, which is found in the Paleocene Cannonball Formation of the Williston Basin region of North Dakota and is widely distributed elsewhere.

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