Metriorhynchus brevirostris (Holl, 1829)

Young, Mark T., Brignon, Arnaud, Sachs, Sven, Hornung, Jahn J., Foffa, Davide, Kitson, James J. N., Johnson, Michela M. & Steel, Lorna, 2021, Cutting the Gordian knot: a historical and taxonomic revision of the Jurassic crocodylomorph Metriorhynchus, Zoological Journal of the Linnean Society 192 (2), pp. 510-553 : 543

COMPARISONS BETWEEN METRIORHYNCHUS BREVIROSTRIS AND OTHER METRIORHYNCHIDS

While the holotype of Metriorhynchus brevirostris (MHNG V-2232) lacks autapomorphic characters, it is readily distinguishable from better-known metriorhynchid species. It has a unique combination of metriorhynchid symplesiomorphies and a slightly posteriorly retracted narial fossa (see: Young et al., 2020a). Furthermore, it lacks the autapomorphies seen in the rostra of the Late Jurassic genera Dakosaurus , Plesiosuchus , Torvoneustes , the ‘E-clade’ taxa, Cricosaurus and Rhacheosaurus (see: Young et al. 2012, 2020a; Abel et al., 2020). These include the rostral shortening and resultant reduction in tooth count seen in Geosaurini, the amblygnathous rostrum and lateral plates seen in Dakosaurus , the robust triangular rostrum and elongated palatines of Plesiosuchus , and the mediolaterally expanded maxillary alveoli seen in the ‘E-clade’ (see: Wilkinson et al., 2008; Young et al., 2012, 2020a; Abel et al., 2020). Furthermore, Me. brevirostris lacks the ossified internarial bar and more noticeable narial fossa retraction seen in the rhacheosaurin genera Cricosaurus and Rhacheosaurus (see: Young et al., 2020a).

Metriorhynchus brevirostris can be distinguished from all known Middle Jurassic metriorhynchids in that the narial fossa anterior margin is slightly retracted, being level to the P2 alveoli rather than the P1 alveoli (see: Young et al., 2020a). Compared to the Callovian geosaurines, the presence of at least 13 maxillary alveoli anterior to the palatines is higher than the dental formulae of ‘ Metriorhynchus ’ brachyrhynchus and Tyrannoneustes lythrodectikos ( Table 6 ; Fig. 7 ). Moreover, the nasal terminates level to the M8 alveoli in Me. brevirostris , whereas the nasal terminates level to the M 6 in ‘ Me. ’ brachyrhynchus (NHMUK PV R 3804) and Ty. lythrodectikos (PETMG R176) (see Fig. 7 ). The fact that the premaxilla posterodorsal processes terminate level to the M3 alveoli in Me. brevirostris also differentiates it from Ty. lythrodectikos , where these processes terminate level to the M5 alveoli (PETMG R176, see Fig. 7 ; Foffa & Young, 2014). Metriorhynchus brevirostris can also be distinguished from the Callovian metriorhynchine Maledictosuchus riclaensis as it lacks the welldeveloped reception pits on the lateral margins of the anterior maxilla seen in Mal. riclaensis ( Parrilla-Bel et al., 2013) . Metriorhynchus brevirostris is also distinct from Gracilineustes leedsi as it lacks: the exceptionally high tooth count, closely packed alveoli and the poorly developed-to-almost absent maxillary ornamentation of Gr. leedsi ( Andrews, 1913) .

There are notable differences between Metriorhynchus brevirostris and Th. superciliosus . Although the rostrum of Me. brevirostris has encrustations, the maxillary ornamentation is noticeably not as pronounced as it is in Th. superciliosus (compare Figs 1A – 6 and 7C ; Andrews, 1913). In dorsal view, there is a constriction at the premaxilla–maxilla contact in Me. brevirostris , followed by a noticeable broadening of the premaxilla at approximately the level of the narial fossa posterior margin ( Fig. 1A ); Th. superciliosus lacks both of features, with the posterior premaxilla and anterior maxilla having a largely constant width (PETMG R17 and MNHN.F RJN 256; see Figs 6 , 7C ). In Me. brevirostris the premaxillary alveoli successively increase in size, with the P1 alveoli being circular in shape and the P3 alveoli being the most oval of the premaxillary tooth-row ( Fig. 1B ); whereas in Th. superciliosus the premaxillary alveoli are largely comparable in size, with the P1 and P3 alveoli being oval in shape, and the P2 being the most circular (PETMG R17 and MNHN.F RJN 256; see Figs 6 , 7C ). The anterior maxillary alveoli of Th. superciliosus are fairly tightly packed, with the interalveolar spaces reduced to thin laminae (especially for the M2–M3 alveoli; PETMG R17 and MNHN.F RJN 256); whereas in Me. brevirostris the interalveolar spaces are much broader, even between the M1–M2 and M2–M3 alveoli ( Fig. 1 ). In Me. brevirostris , the premaxilla–maxilla suture is almost straight in dorsal view ( Fig. 1A ), whereas in Th. superciliosus it has a more ‘jagged’ interdigitating profile (PETMG R17 and MNHN.F RJN 256; see Figs 6 , 7C ). The nasal proceeds slightly further anteriorly in Me. brevirostris than in Th. superciliosus , with seven maxillary alveoli anterior to the nasals in Me. brevirostris ( Fig. 1 ); whereas in Th. superciliosus there can be eight (MNHN.F RJN 256, NHMUK PV R 2051), nine (GLAHM V930, NHMUK PV R 2030, PETMG R17) or 10–11 (GLAHM V962).

Figures

Figure 1. Metriorhynchus brevirostris holotype MHNG V-2232. A, dorsal view; B, palatal view. See text for anatomical abbreviations.

Figure 6. Thalattosuchus superciliosus lectotype MNHN.F RJN 256. A, dorsal view; B, palatal view; C, left lateral view. Scale bar = 10 cm.

Figure 7. Comparative plate of metriorhynchid anterior cranial rostra. A, B, Metriorhynchus brevirostris, holotype MHNG V-2232 in dorsal and palatal views, respectively; C, D, Thalattosuchus superciliosus, referred specimen PETMG R17 in dorsal and palatal views, respectively; E, F, ‘Metriorhynchus’ brachyrhynchus, referred specimen NHMUK PV R 3804 (holotype of ‘Metriorhynchus’ cultridens) in dorsal and palatal views, respectively; G, H, ‘Metriorhynchus’ palpebrosus, holotype OUMNH J.29823 in dorsal and palatal views, respectively; I, J, Tyrannoneustes lythrodectikos, referred specimen PETMG R176 in dorsal and palatal views, respectively; K, L, Plesiosuchus manselii, holotype NHMUK PV OR 40103 in dorsal and palatal views, respectively. See text for anatomical abbreviations. Scale bar = 5 cm.