Metriorhynchus brevirostris

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 : 531-533

publication ID	https://doi.org/ 10.1093/zoolinnean/zlaa092
publication LSID	lsid:zoobank.org:pub:B4E27A94-49A1-4B90-9990-CC87A65B4E3A
DOI	https://doi.org/10.5281/zenodo.7017053
persistent identifier	https://treatment.plazi.org/id/5E272C7F-FFB1-FFE1-1967-FD777080F8A9
treatment provided by	Felipe
scientific name	Metriorhynchus brevirostris
status

Treatment

METRIORHYNCHUS BREVIROSTRIS ( HOLL, 1829)

( FIGS 1–3 View Figure 1 View Figure 2 View Figure 3 )

v 1820 Upper jaw of the fossil crocodile from Havre in the Museum of the Academy of Geneva – De la Beche, lithograph.

v 1824 Tête à museau plus court [partim] – Cuvier, pp. 152– 153, pl. 10, figs 5–7.

v* 1829 Steneosaurus brevirostris sp. nov. – Holl, p. 88.

v* 1831 Gavialis Jurinii sp. nov. – Gray, p. 57. [sic] [partim]

v* 1832 Metriorhynchus geoffroyii gen. et sp. nov. – von Meyer, p. 106. [partim]

v 1836 Anterior extremity of the upper jaw of Steneosaurus – Buckland, p. 36, pl. 25’ fig. 3.

v 1837 Metriorhynchus Geoffroyii (von Meyer) – Bronn, p. 520, pl. 26, figs 7b,d, 8a,b. [sic] [partim]

v 1845 Steneosaurus rostro-minor [sic] Geoffroy Saint-Hilaire – Pictet, p. 46, pl. 1, fig. 2 [partim]

v 1853 Steneosaurus rostro-minor [sic] Geoffroy Saint-Hilaire – Pictet, p. 492–493, pl. 25, fig. 9 [partim]

v 1973 Metriorhynchus geoffroyi (von Meyer) – Steel., p. 45, fig. 18 (10). [sic]

v 1987 Metriorhynchus geoffroyi (von Meyer) – Adams- Tresman, p. 193. [sic]

v 2010 Metriorhynchus geoffroyii (von Meyer) – Young et al., p. 803.

Holotype: MHNG V-2232, partial cranial rostrum.

Casts of holotype: MGCL 9868 , PIMUZ A/III 82 and OUMNH unnumbered.

Type locality and horizon: Le Havre , Département de Seine-Maritime, Haute-Normandie, France. Kimmeridgian, Upper Jurassic.

Etymology: From Latin brevis, short, and rostrum, snout. The combination with the genus results in ‘the

short snouted moderate snout’ (or ‘moderate short snout’), which is perhaps unfortunate.

Diagnosis: Metriorhynchid crocodylomorph with the following unique combination of characters: premaxilla posterodorsal processes terminate level to the M3 alveoli; the three 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; premaxilla is transversely broad level to the narial fossa posterior margin; constriction at the premaxilla–maxilla contact; having at least 13 maxillary alveoli anterior to the palatine anterior processes; maxillary alveoli M1–M13 are irregularly shaped ovals, with the post-M3 alveoli becoming more circular in shape; nasal anterior processes terminate level to the M8 alveoli; lacks a fully ossified internarial bar (thus an undivided external naris); the anterior-margin of the narial fossa is posterior to the P1 alveoli and the posterior margin terminates slightly posterior to the premaxillary tooth-row.

Description

The holotype of Metriorhynchus brevirostris (MHNG V-2232; Figs 1–3 View Figure 1 View Figure 2 View Figure 3 ) is the cranial rostrum, preserving the premaxilla, maxilla and the anterior process of the nasals. In palatal view only, the premaxilla and maxilla are preserved, and there are no complete tooth crowns. Overall, the snout is long and narrow, with a slightly concave upper margin ( Figs 1 View Figure 1 , 2 View Figure 2 ). In dorsal view, widest part of the premaxillae is slightly wider than the anterior portion of the maxillae.

Premaxilla and external nares: In dorsal view, the premaxilla is an anteroposteriorly elongated rhomboid ( Fig. 1 View Figure 1 ). The premaxillary posterodorsal processes are not particularly elongated for a metriorhynchid, terminating approximately level to the third (M3) maxillary alveoli. The suture between the premaxilla and maxilla is not entirely clear, but it appears to be almost straight (rather than curved or strongly interdigitating), with the posterodorsal processes converging posteriorly. The premaxilla only contacts the maxilla along its posterior margins. In lateral view, the external surfaces are slightly convex ( Fig. 2 View Figure 2 ). The ornamentation on the external surfaces is hard to discern due to encrustations.

The external naris aperture is largely circular, undivided and situated within the larger narial fossa ( Figs 1–3 View Figure 1 View Figure 2 View Figure 3 ). The shape of the narial fossa is unclear, due to the poor preservation of the anterior margin, but it looks to have been oval-shaped. There is no evidence of an ossified narial bar, but the posterior-end of the bar is preserved, slightly overhanging the external naris. The anterior-margin of the narial fossa is posterior to the P1 alveoli, while the posterior-margin of the narial fossa is slightly posterior to the premaxillary tooth-row.

In palatal view, three widely spaced alveoli can be seen ( Fig. 1B View Figure 1 ). The first alveoli (P1) are the smallest and are circular in shape. The P2 alveoli are larger and more oval in shape, while the P3 alveoli are the largest in the premaxilla and are oval in shape. The premaxilla–maxilla contact is an anteriorly directed ‘V’-shape created by the maxilla palatal anterior process overlapping the premaxilla. The maxillary palatal process terminates level to the anterior margin of the P3 alveoli. It appears as though the incisive foramen is positioned medial to the P2 alveoli but, given the preservation of the rostrum, this is not entirely clear. The palatal processes of the premaxilla form the anterior margin of the M1 alveolar margin.

Maxilla: Approximately the anterior half of the maxilla is preserved ( Fig. 1 View Figure 1 ). The external surfaces of the maxillae are slightly convex. The ornamentation is difficult to distinguish from the encrustations, but along the alveolar margins on the right side, there appears to be natural external surface texture ( Fig. 2B View Figure 2 ). It is composed of well-developed anteroposteriorly aligned ridges, suggesting that the rostrum may have been ornamented in life. In Tyrannoneustes lythrodectikos ( Foffa &Young, 2014) and Dakosaurus maximus ( Young et al., 2012) there are clear shifts in ornamentation patterns across the maxilla; given the preservation of the holotype we cannot determine if that is true for Metriorhynchus brevirostris . In lateral view, the preservation of the specimen precludes us from determining of there were reception pits on the lateral margins of the maxillae ( Fig. 2 View Figure 2 ).

In palatal view, 12 alveoli are preserved on the right maxilla (anterior section of the 13 th alveolus is visible), while there are 13 complete alveoli on the left maxilla (with the anterior section of the 14 th alveolus visible) ( Fig. 1 View Figure 1 ). All alveoli are circular to subcircular in shape. The interalveolar spaces are variable in size. The M1 and M2 alveoli are closely packed, from M2 to M7 the interalveolar spaces are wider, between M7 and M8 there is the largest preserved gap, M8–M10 has gaps similar in size to those of M2–M7; finally, from M10 posteriorly the interalveolar spaces become small. Paired palatal grooves are present on either side of the maxilla midline. The grooves appear to terminate level to the M5 alveoli. In Ty. lythrodectikos these grooves terminate level to the M4 alveoli ( Foffa & Young, 2014), and in the ‘English rostrum’ from the ‘E-clade’ these grooves continue anteriorly along maxilla terminating level to the M2 alveoli ( Young et al., 2020a).

Nasals: Little of the nasals are preserved,only the anterior processes. The anterior processes form the characteristic converging triangular shape seen in thalattosuchians (e.g. see Fraas, 1902; Andrews, 1913) ( Fig. 1 View Figure 1 ). The nasals are paired, with the anterior processes terminating

level to the M8 alveoli. Due to the encrustations the ornamentation is hard to discern, but the external surface has anteroposteriorly aligned grooves.