Dakosaurus (Von Quenstedt, 1856)

Young, Mark Thomas & Andrade, Marco Brandalise De, 2009, What is Geosaurus? Redescription of Geosaurus giganteus (Thalattosuchia: Metriorhynchidae) from the Upper Jurassic of Bayern, Germany, Zoological Journal of the Linnean Society 157 (3), pp. 551-585 : 570

IMPLICATIONS FOR DAKOSAURUS

With the exclusion of the longirostrine forms with nonserrated teeth from the genus Geosaurus , the taxa currently included within Dakosaurus needs to be reviewed. The genus was erected by Von Quenstedt (1856), and currently comprises large-sized brevirostrine metriorhynchids with distinctively robust skull morphology. Based upon the optimization of dental characters, the genus Aggiosaurus can now be conclusively referred to Dakosaurus ( Buffetaut, 1982b, raised the possibility of a synonymy). The only specimen of Aggiosaurus , Aggiosaurus nicaeensis Ambayrac, 1913 , is a poorly preserved upper jaw, within a slab of limestone, from the upper Oxfordian of south-east France ( Ambayrac, 1913a, b; Buffetaut, 1982b). However, based upon the great size and robustness of the dentition (with some crowns being up to 12 cm in length), and their low number, the only genus of metriorhynchid it could possibly be is Dakosaurus (see Appendix 2 for the character apomorphy lists). As such, Aggiosaurus is here considered a junior synonym of Dakosaurus .

In the phylogeny obtained, Dakosaurus and Geosaurus form a monophyletic clade within Geosaurinae , in which all species could be assigned to Geosaurus . This is possible, as the type species of Dakosaurus was originally described as G. maximus . Despite these considerations, we believe that the species assigned within Dakosaurus should not be transferred to Geosaurus because: (1) their morphology is distinctive, especially the morphology of the teeth serrations, thereby allowing for the prompt characterization of both genera; (2) the serrations seen in both genera are likely to be nonhomologous, but are certainly of distinct morphology; (3) the prompt characterization allows for the easy identification of isolated teeth into distinct genera; (4) Geosaurus and Dakosaurus , as proposed here, are neither paraphyletic nor polyphyletic; (5) the transfer of Dakosaurus taxa to Geosaurus is unnecessary, would imply several taxonomic changes, and would introduce further confusion into the current taxonomy ( Fig. 8 ).

However, the maintenance of Dakosaurus demands an additional but single modification, affecting D. lapparenti from the late Valanginian of south-east France. This species is based upon quite fragmentary specimens, from which the rostrum, mandible, and most of the postcrania are missing. It also possesses nonfaceted serrated teeth. However, the teeth are blade-like, and the dentition of D. lapparenti is keeled (a raised ridge) (E. Roberts, pers. comm., 2007). Such keels have not been reported in other species of Dakosaurus . In all other cases, Dakosaurus ( D. manselii , D. maximus , and D. andiniensis ) possess denticulate carinae with a distinct morphology from the one observed in Geosaurus . With effect, our phylogenetic results place this species as the sister group of G. giganteus + G. grandis , and not to other Dakosaurus . Therefore, this species is transferred to Geosaurus , and should be referred to as G. lapparenti .

The age for D. lapparenti is usually given as Hautervian (e.g. Hua & Buffetaut, 1997), following Debelmas & Strannoloubsky (1957). However, Debelmas (1958: 43) rapidly corrected this mistake when referring to a Dakosaurus pelvis, assigning these remains to the Valanginian. In fact, comparison between the ammonites from the locality named by the authors and recent ammonite zonation for the Early Cretaceous of Tethys ( Wippich, 2003) clearly supports an upper Valanginian age (peregrinus ammonite zone) for D. lapparenti . As such, there are no metriorhynchid specimens known from the Hautervian.

Figures

Figure 8. Metriorhynchid taxonomy in historical perspective: main changes in taxon names from their original description (left column) to this paper (right column), passing through definitions by Fraas (1902) and Steel (1973). The cladogram on the right is a reduced version of the strict consensus tree in Figure 7. The grey areas indicate monophyletic genera in this taxonomic arrangement.