Metriorhynchus brachyrhynchus, (EUDES-DESLONGCHAMPS, 1867)

Scavezzoni, Isaure, Fischer, Valentin, Johnson, Michela M. & Jouve, Stéphane, 2024, Form and function of the pelvic girdle of Thalattosuchia and Dyrosauridae (Crocodyliformes), Geodiversitas 46 (6), pp. 135-326 : 184-188

‘ METRIORHYNCHUS’ BRACHYRHYNCHUS ( EUDES-DESLONGCHAMPS, 1867)

NHMUK PV R 3804

For measurements, see Tables 7-9 .

Ilium

The ilium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 ( Fig. 24 ) bear the typical metriorhynchoid characteristics, which are an overall small size in relation to the other pelvic bones, and the absence of a postacetabular process. The preacetabular process of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 is short but lateromedially thick and points anterodorsally (forms an angle of approximately 36° with the ventral margin of the ischial peduncle) rather than strictly anteriorly like extant crocodylians (e.g. Crocodylus niloticus NMW 31137) and dyrosaurids (e.g. Congosaurus bequaerti MRAC 1806 ). This difference in inclination of the preacetabular process can be attributed to the different way the ilium is borne by the sacrals: the ilium of thalattosuchians is rather tilted in relation to the coronal plane (e.g. Herrera et al. 2017) compared to extant crocodylians (e.g. Palaeosuchus palpebrosus [ Fig. 7 ], Mecistops cataphractus [ Fig. 8 ], Caiman crocodilus [ Fig. 9 ]) or dyrosaurids ( Congosaurus bequaerti , Hyposaurus natator , Acherontisuchus guajiraensis ). The preacetabular process of NMH PV R 3804 strongly differs from that of NHMUK PV R 4763, and resembles those of Suchodus durobrivensis , Thalattosuchus superciliosus and more specifically Tyrannoneustes lythrodectikos .

The anterior margin of the ilium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 underneath the preacetabular process is convex, and meets distally with the pubic peduncle of the ilium. The dorsal margin is globally reduced as the anterior and posterior margins almost directly connect with the large base of the preacetabular process ( Fig. 24 ). The junction between the dorsal and posterior margins is achieved through an obtuse and rounded corner, similar to those of Suchodus durobrivensis , Tyrannoneustes lythrodectikos and Thalattosuchus superciliosus NHMUK PV R 2054 . The posterior margin of the ilium is concave and ends distally to form the posterior corner of the ischial peduncle of the ischium. The ventral margin of the ilium is slightly undulated: it bears the pubic peduncle of the ilium anteriorly, and the ischial peduncle of the ischium posteriorly. The undulation of the ventral margin of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 is more subtle than that of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 or Thalattosuchus superciliosus , and closely resembles that of Tyrannoneustes lythrodectikos and Suchodus durobrivensis . The ischial peduncle represents the region where the ilium is the thickest mediolaterally. Comparatively, the ischial peduncle is subtlety longer dorsoventrally than the pubic peduncle, so that both peduncles appear to have the same height. Like in other metriorhynchoids, the ischial peduncle takes the shape of an isosceles triangle whose orientation is both lateral and anterior. The outline of the pubic peduncle diplays three successive triangular mounds, with the supraacetabular crest originating from the hollow between the two first summits. On the lateral side of the ilium, the pubic peduncle gradually transitions to the ischial peduncle through a recess but without rupturing the articular surface (as in other metriorhynchoids). There is a subtle opening along the ventral margin of the ilium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 corresponding to the acetabular perforation. The acetabular perforation almost indiscernible in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, as in Suchodus durobrivensis , Tyrannoneustes lythrodectikos , and Cricosaurus species. The acetabular perforation of the ilium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 does not separate the peduncles of the ilium but is rather indicating the transition between both. Indeed, the acetabular perforation on the ilium is slightly shifted anteriorly, thus ending up within the posterior-most portion of the pubic peduncle. Furthermore, the peduncles of the ilium meet each other on the lateral surface of the ilium, well above the ventral margin of the bone, meaning that both peduncles where brought together presumably at the expense of the acetabular perforation. This highly contrasts with dyrosaurids ( Congosaurus bequaerti , Hyposaurus natator , Acherontisuchus guajiraensis ) and extant crocodylians (e.g. Palaeosuchus palpebrosus [ Fig. 7 ], Mecistops cataphractus [ Fig. 8 ], Caiman crocodilus [ Fig. 9 ]) where both peduncles are entirely separated by a large acetabular perforation. In ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and other metriorhynchoids (e.g. Suchodus durobrivensis , Thalattosuchus superciliosus , Dakosaurus maximus , etc.), the ilium is greatly reduced while still conserving the bony acetabulum and the connections to the other hip bones (i.e. the peduncles), and this reduction presumably happened at the expense of the acetabular perforation and the postacetabular process. The bony acetabulum is limited dorsally by the supraacetabular crest, and runs all the way down to the peduncles ventrally. The supraacetabular crest is a parabolic-shaped ridge that is prominent anteriorly and fades posteriorly. The supraacetabular crest is not centered on the ilium, and its posterior border is slightly closer to the posterior margin of the bone than its anterior border is to the anterior margin of the ilium unlike in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763. The area comprising the anterodorsal surface of the acetabulum up to the anterior margin of the ilium, across the supraacetabular crest, is rugged thus reflecting the anchoring of cartilage. Like in other metriorhynchoids, this coarse area extends over the anterior portion of the acetabulum as the rest of the surface is smoother (e.g. Tyrannoneustes lythrodectikos , Suchodus durobrivensis , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , etc.). The anterior margin of the ilium and supraacetabular crests are, however, strongly pitted and presumably hosted a structure similar to the acetabular labrum of extant crocodylians. In vivo, the bony acetabulum was probably covered in cartilage which likely extended over the ischium and pubis, forming a puboischial synchondrosis like that of extant crocodylians ( Claessens & Vickaryous 2012; Tsai & Holliday 2015).

The attachment sites for the two sacral ribs are visible on the medial side of the ilium. They are recognizable by their overall bilobate shape, with the biggest lobe positioned ventrally as in Tyrannoneustes lythrodectikos , Suchodus durobrivensis , and ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763. The anterior attachment site is slightly larger than the posterior one, and both show a relatively smooth ventral lobe along with a rougher dorsal lobe indicating the existence of cartilage joining the ribs and the ilium in vivo. The attachments sites do not form depressions on the ilium, but are rather in relief or raised, unlike in Thalattosuchus superciliosus , Cricosaurus araucanensis or Dakosaurus maximus . Furthermore, the sacral rib attachment sites are adjacent, and both situated relatively high on the ilium near the preacetabular process, which is similar to other derived metriorhynchoids (e.g. Suchodus durobrivensis , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Cricosaurus araucanensis , Thalattosuchus superciliosus , Tyrannoneustes lythrodectikos , etc.). Yet, all these traits differ from dyrosaurids (e.g. Congosaurus bequaerti , Hyposaurus natator , Acherontisuchus guajiraensis ) which show distinct elliptic attachments sites for each sacral process. In addition, in dyrosaurids the attachments sites for each sacral are separated by the acetabular perforation of the ilium, and are borne near the ventral margin of the bone. The ventral deflection of the pelvic girdle of metriorhynchoids, which presumably happened concurrently with its reduction, can be held responsible for the dorsal position of the sacral rib attachment sites near the preacetabular process. In comparison, dyrosaurids possess short and upright sacral processes which are anchored along the anterior and posterior margins of the ilium, leading to pelvic girdles positioned higher dorsally than in metriorhynchoids.

The shape of the ilium – both overall and in detail – of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 is strongly similar to that of Tyrannoneustes lythrodectikos so that both putatively belong to the same taxon. At the present stage ‘ Metriorhynchus ’ brachyrhynchus is a polyphyletic wastebasket taxon ( Waskow et al. 2018) as it is filled with markedly differing specimens on both cranium and postcranium levels (see ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 for a striking example). In parallel, the cranium (skull and mandible) of Tyrannoneustes lythrodectikos has been introduced with some characters that appear sometimes approximate and dubious (see Foffa & Young 2014), especially pertaining to ‘ Metriorhynchus ’ brachyrhynchus ( Waskow et al. 2018) .

Ischium

The ischium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 ( Fig. 24 ) possesses a short and thick shaft similar to Tyrannoneustes lythrodectikos and Dakosaurus maximus among metriorhynchoids. Indeed, the shaft of the ischium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 is 1/3 longer anteroposteriorly as it is tall proximodistally. Still, the minimum thickness of the shaft (defining the extension of the shaft) is not drastically greater than the largest anteroposterior length of the posterior peduncle like Tyrannoneustes lythrodectikos , and Dakosaurus maximus to a lesser extent. As in other metriorhynchoids, the acetabular perforation is strongly reduced on the ischium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, and corresponds to a shallow notch between both peduncle which then transitions to a shallow groove on the medial margin of the bone (e.g. Thalattosuchus superciliosus , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Tyrannoneustes lythrodectikos , etc.). The anterior margin of the ischium underneath the peduncles is strongly concave even semicircular (greater than in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763). The anterior margin culminates ventrally to form the anterior process of the distal blade, which is sharp as in several metriorhynchoids (i.e. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 and LEICT G.418.1956.13.5, Geosaurus giganteus , Dakosaurus maximus , and Torvoneustes carpenteri ). The posterior margin of the ischium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 appears slightly concave as opposed to its anterior margin.

Pubis

The pubis of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 ( Fig. 24 ) displays a well-developed pubic symphysis like other thalattosuchians. The proximal peduncle of the bone is oval in proximal view, but its anterior margin shows a deep extension towards the shaft (vs Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , Macrospondylus bollensis , Charitomenosuchus leedsi , Neosteneosaurus edwardsi ). Also, the greatest axis of the pubic peduncle is not in line with that of the section of the shaft but is tilted at about 30° ( Fig. 25 ). Overall, the pubis of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 takes the shape of a palette knife with a rod-like shaft and a distal blade bearing a straight surface medially (i.e. the large pubic symphysis; Fig. 28 ) and a rounded one laterally. Thereby, it differs from that of extant crocodylians which show an almost isosceles triangular distal blade. Indeed, in Caiman crocodilus ( Fig. 9 ) or Mecistops cataphractus ( Fig. 8 ), the lateral and medial margins are almost symmetrical thus displaying a reduced pubic symphysis. The dyrosaurids Hyposaurus natator and Dyrosaurus maghribensis (OCP DEK-GE 252 and OCP DEK-GE 255) further differ from ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 as they also possess a rather symmetrical isosceles triangular distal blade, but this is not the case for Cerrejonisuchus improcerus which displays a strongly asymmetrical pubis.

Comparatively, the pubes of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and Cerrejonisuchus improcerus may appear similar with a short and concave medial margin of the shaft leading to a straight portion of the distal blade, and a more elongated lateral margin of the shaft ending with a rounded portion of the distal blade (the posterior protuberance). In both cases, the medial margin of the shaft and the distal blade meet at an angle slightly greater than 90°. Yet, the main difference resides in the junction between the lateral margins of the shaft and the distal blade: while those are aligned in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, in Cerrejonisuchus improcerus they form an angle lesser than 180°. The overall flat lateral margin of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 along with its larger distal blade, creates a smooth and gradual transition between the shaft and the distal blade. On the contrary, in extant crocodylians (e.g. Mecistops cataphractus [ Fig. 8 ]) and in other thalattosuchians the shaft of the pubis flares out both laterally and medially to form the distal blade (e.g. Pelagosaurus typus, Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Geosaurus giganteus , Lemmysuchus obtusidens , Macrospondylus bollensis , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , Mycterosuchus nasutus ).

In ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, the pubic blade displays a posterior protuberance similar to what is observed in dyrosaurids (i.e. Hyposaurus natator , and Dyrosaurus maghribensis OCP DEK-GE 252 & OCP DEK-GE 255), but unlike most thalattosuchians (e.g. Pelagosaurus typus, Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Geosaurus giganteus , Lemmysuchus obtusidens , Macrospondylus bollensis , Charitomenosuchus leedsi , Neosteneosaurus edwardsi ). Comparatively, Mycterosuchus nasutus also possesses a posterior protuberance but of greater intensity than in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804.

Figures

FIG. 24. — Pelvic girdle elements of ‘Metriorhynchus’ brachyrhynchus (Eudes-Deslongchamps,1867), NHMUK PV R 3804 (holotype of ‘Metriorhynchus’ cultridens Andrews, 1913): A, right ilium in lateral view; B, right ilium in medial view; C, right pubis in anteroposterior;D, left ischium in lateral view. Target indicates anterior. Arrow points anteriorly. Scale bars: 1 cm.

FIG. 7. — Snapshot of the pelvic girdle of the alligatoroid Palaeosuchus palpebrosus (Cuvier, 1807), RVC-JRH-PP1: A, in anterior view; B, in lateral view. Note the orientation of the pubes. Cross indicates posterior view. Arrow points anteriorly. CT scan courtesy of Professor John Hutchinson (see https://skfb.ly/6ByyV). Scale bars: 1 mm.

FIG. 8. — Right pelvic girdle of Mecistops cataphractus Cuvier, 1825, RBINS 18374: A, ilium in lateral view; B, ischium in lateral view; C, pubis in ventral view; D, ilium in medial view. Cross indicates posterior. Arrow points anteriorly. Scale bar: 1 cm.

FIG. 9. — Left pelvic girdle of Caiman crocodilus (Linnaeus, 1758), NHMW 30900: A, ilium in medial view; B, ilium in lateral view; C, ischium in medial view; D, ischium in lateral view; E, pubis in posterior view. Target indicates anterior. Arrow points anteriorly. Scale bar: 1 cm.

FIG. 25. — Reconstruction of the pelvic girdle of ‘Metriorhynchus’ brachyrhynchus (Eudes-Deslongchamps, 1867), NHMUK PV R 4763 and NHMUK PV R 3804 (holotype of ‘Metriorhynchus’ cultridens): A, NHMUK PV R 3804, right lateral view; B, NHMUK PV R 4763, right lateral view; C, NHMUK PV R 3804, anterior view; D, NHMUK PV R 4763, anterior view; E, NHMUK PV R 3804, ventral view; F, NHMUK PV R 4763, ventral view. Target indicates anterior. Arrow points anteriorly. The right ischium and left ilium and pubis of NHMUK PV R 3804 are mirrored. The sacral ribs of NHMUK PV R 4763 have been modified to contact the ilium. Scale bars: 5 cm.

FIG. 28. — Pelvic girdle elements of Suchodus durobrivensis Lydekker, 1890, NHMUK PV R 2618: A, right ilium in lateral view; B, right ilium in medial view; C, right pubis in anterodorsal view; D, right pubis in lateral view; E, left pubis in posteroventral view. Target indicates anterior. Cross indicates posterior. Arrow points anteriorly. Scale bar: 1 cm.