Recovery of the carcharhinid from the “SUE” Locality is surprising given the freshwater environment from which the fossils derive. This specimen shows clear evidence of reworking (rounded edges and a shiny patina), suggesting that it was not originally deposited at the same time as the Tyrannosaurus rex fossil. That said, there are three modern species of carcharhinid shark that either regularly invade euryhaline habitats or are restricted to freshwater ecosystems: Carcharhinus leucus (Bull shark) (Müllerand Henle, 1839) and two species of Glyphis (including the Ganges shark) (Compagno and Cook, 1995; Martin, 2005). The combination of evidence for taphonomic reworking with the known occurrence of some modern relatives invading freshwater habitats makes it difficult to infer life habits of this shark.
The second novel discovery is Galagadon nordquistae n. gen. n. sp. As mentioned above, there are currently three species of orectolobiform chondrichthyan known from the Hell Creek Formation: Protoginglymostoma estesi (Herman, 1977) (Cappetta, 2006), Restesia americana (Cook et al., 2014), and Chiloscyllium sp. (Cook et al., 2014). Galagadon n. gen. is the only Hell Creek Formation orectolobiform currently known that possesses abundant folds and plications on the labial tooth surface. With the inclusion of Galagadon n. gen., a fourth species can now be included within the freshwater ecosystems. Increasing the species-richness of aquatic ecosystems is not surprising when one considers the complexity and heterogeneity of habitats available to orectolobiformes during the Late Cretaceous. For over 30 million years prior to the deposition of the Hell Creek Formation, the WIS inundated the central portion of North America, generating marine and estuarine ecosystems for orectolobiforms to occupy. Despite all extant orectolobiforms living strictly in marine ecosystems (Martin, 2005), tolerance to varying salinity levels within this group is documented by occurrences of orectolobiform fossils discovered across salinity gradients from the marine into upland river systems (e.g., Kirkland et al., 2013), and the “SUE” locality is considered a freshwater environment based on both its distance to the paleoshoreline and the occurrence of amphibian and baenid turtle remains. Terrestrial landscape heterogeneity due to topographic and climatic microcosms during the Late Cretaceous in Laramidia is posited to have caused an increase in vertebrate biodiversity (Gates et al., 2010a, 2012), which is similar to biozonation identified by Nicholls and Russell (1990) for the seaway itself. As such, it seems reasonable that a number of small-bodied sharks could live together in such diverse habitats, or perhaps they are occurring in short, successive stratigraphic intervals. Many extant carpet-and bamboo shark species also exhibit overlapping or peripatric distributions (Corrigan and Beheregaray, 2009; Compagno et al., 2005).
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We provide the first analytically supported phylogenetic positions for several Mesozoic orectolobiform taxa by incorporating both modern and ancient taxa through combined evidence data matrices, allowing for preliminary exploration of the evolution and biogeography of the clade. This study adds to others (e.g., Adent and Capetta, 2001; Peart et al., 2015) that combine extant and fossil species to explore the history of shark evolution.
