Discussion

To date, the remains of only two individuals of P. saturator have been identified from the Maastrichtian type section. The holotype of P. saturator was recovered in 1998 (Dortangs et al., Reference Dortangs, Schulp, Mulder, Jagt, Peeters and de Graaf2002), while the only other skeletal material that we are aware of, a posterior mandibular unit, lay hidden in collections made in the latter part of the 19th century, having been labelled Mosasaurus hoffmanni, until its ‘rediscovery’ well over a decade later (Schulp et al., Reference Schulp, Polcyn, Mateus and Jacobs2013a). Prognathodon sectorius, earlier referred to as L(e)iodon sectorius (see Schulp et al., Reference Schulp, Polcyn, Mateus, Jacobs, Morais and Everhart2008), is almost equally rare (e.g. Kuypers et al., Reference Kuypers, Jagt, Peeters, de Graaf, Dortangs, Deckers, Eysermans, Janssen and Arpot1998). So far, of this taxon only partial jaws and isolated teeth crowns are available. Thus, this new material not only provides a welcome addition to a scant record, it has also prompted us to reflect on the paucity of Prognathodon, and more specifically Prognathodon saturator, in comparison to relatively common mosasaur taxa in area such as Mosasaurus and Plioplatecarpus, and even Carinodens, although the latter is mainly represented by isolated (shed) tooth crowns. Much progress has been made recently in identifying niche exploitation and resource partitioning in the type Maastrichtian mosasaur fauna (Schulp, Reference Schulp2006; Schulp et al., Reference Schulp, Vonhof, Van der Lubbe, Janssen, Van Baal, Mulder, Jagt and Schulp2013b) and, more generally, in aspects of the geographic and stratigraphic distribution of tetrapods (Mulder, Reference Mulder2003) and vertebrate taphonomy in the Maastrichtian type area (e.g. Janssen et al., Reference Janssen, Van Baal, Schulp, Jagt, Jagt-Yazykova and Schins2011). The relatively depleted carbon stable isotopic values recorded for tooth enamel (Schulp et al., Reference Schulp, Vonhof, Van der Lubbe, Janssen, Van Baal, Mulder, Jagt and Schulp2013b) suggest that both species of Prognathodon would have foraged in more offshore and/or deeper-water settings, thus differentiating them from the less depleted taxa Carinodens and Plioplatecarpus. This alone, however, does not provide an adequate explanation of the rarity of Prognathodon as compared with Mosasaurus, which exhibits an isotope signature ranging from roughly similar to even more depleted values. Even though tooth morphology suggests that a difference in preferred prey existed between M. hoffmanni and P. saturator, the question remains whether the shallow marine type Maastrichtian ecosystem would have supported two similarly large apex predators at the same time. A degree of habitat partitioning, with P. saturator further offshore in an open marine setting, appears likely. Tooth morphology of and bite force estimates for Prognathodon saturator both suggest that large marine turtles could have formed part of its diet (Schulp, Reference Schulp2006). Although the holotype of P. saturator does not preserve any stomach contents, turtle fragments from the gut of a smaller-sized specimen of Prognathodon from the late Campanian of Alberta, Canada, lends support to such a scenario (Konishi et al., Reference Konishi, Brinkman, Massare and Caldwell2011). The large marine turtle Allopleuron hofmanni has previously been suggested to have exploited the sea grass meadows of the type Maastrichtian (Mulder, Reference Mulder2003). Seen in this light, the relatively common occurrence of Allopleuron vs the rarity of Prognathodon saturator would be puzzling even if a population size difference in this assumed predator–prey ratio of more than an order of magnitude is being considered. An interesting hypothesis could be that smaller juvenile or subadult specimens were a more convenient prey for P. saturator. In view of the fact that some extant marine turtles are known to undergo a shift in diet and foraging area during ontogeny (e.g. Reich et al., Reference Reich, Bjorndal and Bolten2007), the observed absence of juvenile Allopleuron in the type Maastrichtian (Janssen et al., Reference Janssen, Van Baal, Schulp, Jagt, Jagt-Yazykova and Schins2011) would offer a possible explanation for the paucity of P. saturator in the area. If, indeed, both P. saturator and juvenile Allopleuron spent most of their time in more offshore waters, this would indeed account for their near-absence in the shallow, near-shore setting of the type Maastrichtian. However, testing this hypothesis requires the unlikely discovery of (multiple) P. saturator skeletons with preserved gut contents featuring the recognisable remains of juvenile Allopleuron.

Along a different line of reasoning, we are not aware of any record of (isolated, i.e. shed) tooth crowns of P. saturator from the Maastrichtian type area. This observation does lend support to the idea that P. saturator would have preferred a habitat further offshore, avoiding the near-shore shallows of the Maastricht area. In this case only floating carcasses and the occasional disoriented individual would have ended up in the type Maastrichtian strata.