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 : 178-184

‘ METRIORHYNCHUS’ BRACHYRHYNCHUS ( EUDES-DESLONGCHAMPS, 1867)

NHMUK PV R 4763

For measurements, see Tables 7-9.

Ilium

The ilium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 ( Fig. 22 ) stands out from that of other metriorhynchoids in displaying an overall isosceles triangular shape, with its anterior and posterior margins almost equal in length with the ventral margin. Like for other derived metriorhynchoids, the ilium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is strongly reduced and lacks a postacetabular process. However, the preacetabular process of the ilium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is well developed as it reaches about 59 % of the dorsoventral height and about 48 % of the anteroposterior length of the ilium at the peduncles ( Fig. 22 ). Furthermore, the overall thin and elongated shape of the preacetabular process of NHMUK PV R 4763 is not found in other metriorhynchoids. The preactebular process of the ilium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is directly in line with the dorsal margin of the ilium similar to most metriorhynchoids except some Thalattosuchus superciliosus specimens (i.e. SMNS 10116 and NMI F21731). In ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, the dorsal margin of the ilium is extremely short (as opposed to that of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804) and almost appears non-existent between the size of the posterior margin of the ilium and that of the preacetabular process. Geosaurus giganteus also possesses a short dorsal margin but the later is more pronounced due to the smaller inclination with the posterior margin. Indeed, in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 the junction between the dorsal and posterior margins of the ilium is obtained through an obtuse angle (about 140°), as in most metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Tyrannoneustes lythrodectikos , Thalattosuchus superciliosus NHMUK PV R 2054 but not SMNS 10116 and NMI F21731, Cricosaurus araucanensis , etc.). The posterior margin of the ilium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is only subtlety concave, as opposed to ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Tyrannoneustes lythrodectikos , Thalattosuchus superciliosus , and Suchodus durobrivensis .

The ventral margin of the ilium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is strongly undulating and shows three successive recess areas: two marking the centre of each peduncle, and one for the acetabular perforation. The latter marks the transition between the peduncles ventrally but does not separate them on the lateral side of the ilium as in other derived metriorhynchoids. The acetabular perforation of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is more pronounced than in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Tyrannoneustes lythrodectikos , Suchodus durobrivensis , and Cricosaurus species. However, it is less marked than in Thalattosuchus superciliosus or Dakosaurus maximus . The ischial peduncle of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is tall as it almost doubles the height of the pubic peduncle. Furthemore, the ischial peduncle strongly protrudes laterally and its articular facet is strongly concave with an important anterior orientation, which differs from other metriorhynchoids. The pubic peduncle of the ilium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is less concave than the posterior peduncle and its anterior edge follows the curve of the anterior margin, as in Thalattosuchus superciliosus NHMUK PV R 2054 and Suchodus durobrivensis . The outline of the pubic peduncle on the lateral surface of the ilium is wavy, similar to other metriorhynchoids. However, in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 the pubic peduncle forms a pointed summit where the supraacetabular crest starts, unlike in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Tyrannoneustes lythrodectikos , Cricosaurus araucanensis , Thalattosuchus superciliosus where they meet in a recess. The anterior margin of ilium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is gently convex throughout, as in Thalattosuchus superciliosus NHMUK PV R 2054 , Suchodus durobrivensis , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, or Tyrannoneustes lythrodectikos . The supraacetabular crest of the ilium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is slightly arched and rigidly follows the shape of the anterior margin, unlike in most metriorhynchoids where the supraacetabular crest differs from the anterior margin dorsally to curl up and/or form a wide rugged area (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Thalattosuchus superciliosus , Cricosaurus araucanensis , etc.). The supraacetabular crest of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is also scarred along its surface indicating the existence of a cartilage cap in vivo. The bony acetabulum of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is bordered by the ischial peduncle posteriorly and the supraacetabular crest anteriorly and dorsally. It forms a relatively narrow (anteroposterioly) and mediolaterally deep recess compared to ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804. The bony acetabulum of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 displays a rugged surface as in Thalattosuchus superciliosus , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Tyrannoneustes lythrodectikos , and Suchodus durobrivensis (where it is at least preserved). The whole acetabulum hollow of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 stretches up to 69% (22.6 mm) of the dorsal height and 45% (16.53 mm) of the anteroposterior length of the ilium

On the medial side of the ilium, the sacral rib attachment sites for both sacrals are borne towards the dorsal portion of the bone as in other derived metriorhynchoids. Each scar is composed of a round base and a slender but more deeply rugged dorsal portion extending from the round base to the dorsal margin of the ilium. The circular base is raised rather than imprinted, similar to ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Tyrannoneustes lythrodectikos , Suchodus durobrivensis but unlike in Thalattosuchus superciliosus , Cricosaurus araucanensis , or Dakosaurus maximus . The overall shape of the sacral rib attachment sites of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 strongly differ from those of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Tyrannoneustes lythrodectikos , and Suchodus durobrivensis which are bilobate and less medially protruding.

Ischium

The ischium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 stands out from most metriorhynchoids in displaying a slender and tubular posterior process, culminating in a reactively thick and rounded apex. In comparison, Pelagosaurus typus (SMNS 17758) also shows a tubular posterior process but the latter displays a thinner apex. The apex of the posterior process of the ischium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 forms a squared end with rounded angles ( Figs 22 ; 23 ), which is also seen in some teleosauroids (e.g. Proexochokefalos cf. bouchardi and Charitomenosuchus leedsi , and Aeolodon priscus MNHN.F.CNJ 78 to a lesser extent). The posterior process corresponds to the thinnest portion of the ischium mediolaterally. The distal blade of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 appears relatively straight like most metriorhynchoids, which contrasts with Thalattosuchus superciliosus NHMUK PV R 2054 . Anteriorly, the distal blade forms a sharp apex – the anterior process – whose dorsal margin is slightly curved (concave) like other metriorhynchoids. The anterior process of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is directly in line with the distal blade, as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Dakosaurus maximus , and Torvoneustes carpenteri . On the medial side, the distal blade bears a pitted or rugged texture over most of its length indicating the presence of a cartilage cap in vivo but also the area where both ischia were connected. Anteriorly, the pitted texture stops around the base of the anterior process and becomes smoother as in other metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Thalattosuchus superciliosus , Dakosaurus maximus , etc.). Posteriorly, the ischial suture stops at the base of the posterior process. Also, the surface of the ischial suture is not perpendicular to the lateral surface of the distal blade but rather protrudes to form an angle of approximately 45°with the latter, indicating that both ischia were presumably set at about 90° ( Fig. 25D ).

Proximally, the ischium bears two protuberances: the anterior and posterior peduncles ( Figs 22 ; 25 ). The posterior peduncle of the ischium is the biggest of the two and its medial surface connects directly to the ilium. Comparatively, the lateral margin of the posterior peduncle is slightly longer anteroposteriorly than the medial one, and was part an attachment site for the hip cartilage (cartilago acetabularis [ Cong et al. 1998]) which covered all portions involved the in acetabulum in vivo ( Tsai & Holliday 2015). The proximal surface of the posterior peduncle is concave to form the ventral portion of the acetabulum, and is relatively close to the shaft of the ischium which differs from the protruding posterior peduncle of Dyrosauridae (e.g. Acherontisuchus guajiraensis ). Oppositely, the anterior peduncle of the ischium is greatly protruding from the shaft of the ischium and from the posterior peduncle, thus creating an open space between the two peduncles called the acetabular perforation ( Romer 1956). The anterior peduncle of the ischium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 (about 26-28% of the length of the posterior peduncle) is greatly reduced in size compared to Dyrosauridae (e.g Hyposaurus natator NJSM 23368, Dyrosaurus maghribensis OCP DEK-GE 252 and 255) or Crocodylia (e.g. Palaeosuchus palpebrosus RVC-JRH-PP1 [ Fig. 7 ]; Mecistops cataphractus [ Fig. 8 ]; Caiman crocodilus [ Fig. 9 ]). The anterior peduncle of metriorhynchoids is also reduced compared to that of teleosauroids as its mediolateral width is about the same size as its anteroposterior length (contra Lemmysuchus obtusidens , Macrospondylus bollensis , Charitomenosuchus leedsi , etc.).

The peduncle bridge of the ischium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, which bears the anterior peduncle, stems from the lateral surface of the ischium and is thus decentred compared to the posterior peduncle. The base of the peduncle bridge is also directly connected to the base of the posterior peduncle, which is the reason why the acetabular perforation appears shallow when looking at the bone laterally (like in Cricosaurus araucanensis , Cricosaurus suevicus , Dakosaurus maximus , Torvoneustes carpenteri , or several teleosauroids like Aeolodon priscus , Lemmysuchus obtusidens , Neosteneosaurus edwardsi, Proexochokefalos cf. bouchardi, Teleosaurus sp. ). However, this imprint is scattered when looking at the medial side of the ischium as is other thalattosuchians. In addition, the peduncle bridge of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is strongly arched towards the medial side of the bone as to bring the anterior peduncle directly underneath the pubic peduncle of the ilium, similar to Cricosaurus araucanensis , and Cricosaurus suevicus among metriorhynchoids. The dorsal surface of the peduncle bridge, forming the ventral margin of the acetabular perforation, is strongly concave and medially tilted. Hence, the acetabular perforation of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 actually forms a relatively deep hollow which is tilted towards the sagittal plane (medially-bound). The combination of the curvature of the peduncle bridge along with its dorsal concavity creates a relatively large opening for the acetabular perforation, which counterbalances the fact that the position of the peduncle bridge laterally obstruct the canal. In ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 and other thalattosuchians (e.g. Thalattosuchus superciliosus , Dakosaurus maximus , Lemmysuchus obtusidens , etc.), most of the acetabular perforation is actually constituted by the ischium (as it is greatly reduced on the ilium). Comparatively, the acetabular perforation of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is downsized in relation to extant crocodylians (e.g. Mecistops cataphractus ) and dyrosaurids (e.g. Acherontisuchus guajiraensis ), and was likely covered by a membrane. Hence, the dorsal or medial portion of the peduncle bridge of the ischium was presumably the attachment site for one of the intrinsic ligaments, the anterior portion of the Ligamentum capitis femoris.

The anterior and posterior peduncles of the ischium emerge from the shaft of the ischium, which constitutes the shortest portion of the bone anteroposteriorly. This constriction flares out distally to form the distal blade: its posterior and anterior surfaces are both concave, with the posterior surface displaying a greater radius of curvature. In NHMUK PV R 3804 (holotype of ‘ Metriorhynchus ’ cultridens), NHMUK PV R 4763, LEICT G.418.1956.13.5 and LEICT G.418.1956.13.6 all display differing radius of curvature for the anterior concavity, leading to differences in the angle between the posterior peduncle to the distal blade, and in the width of the neck constriction.

There is a difference between the ischium of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 and ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 (holotype of ‘ Metriorhynchus ’ cultridens; see Figs 22 , 24 and Young et al. [2010, 2013] for taxonomic affiliation), where the former displays a slender ischial neck compared to the latter. Indeed, when both are resized ( Fig. 5 ) the NHMUK PV R 3804 (holotype of ‘ Metriorhynchus ’ cultridens) appears shorter and thicker: its neck is longer anteroposteriorly and its anterior concavity is characterized by a shorter radius of curvature. Moreover, the two specimens are also distinguishable based on the inclination of the proximal surface of the posterior peduncle with the distal margin of the distal blade: NHMUK PV R 3804 (holotype of ‘ Metriorhynchus ’ cultridens) displays a greater angle with the distal blade than NHMUK PV R 4763 does, while both exhibit the same height between the distal blade and the posterior-most portion of the posterior peduncle. The ischium of LEICT G.418.1956.13.6 ( Fig. 23 ) appears more similar to NHMUK PV R 3804 (holotype of ‘ Metriorhynchus ’ cultridens), but unfortunately comparisons cannot be taken further as the only shared bone of the pelvic girdle between the three specimens is the ischium. The distinct differences of ‘ Metriorhynchus ’ brachyrhynchus (NHMUK PV R 4763 and NHMUK PV R 3804 holotype of ‘ Metriorhynchus ’ cultridens, Fig. 24 ), and LEICT G.418.1956.13.6 ( Fig. 23 ), could presumably be attributed to intraspecific variation as both have been attributed to the same species ( Young et al. 2010; 2011b; 2012; 2020a, b). Sexual dimorphism could hypothetically explain such differences, as a change in size and shape of the ischium (specifically the length between its posterior peduncle and its distal blade) would directly affect the dimensions of the abdominal cavity. This view supports the conclusions of Herrera et al. (2017) with metriorhynchids being viviparous.

Pubis

The pubis of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 ( Fig. 22 ) strongly differs from that of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 in displaying a stronger shaft constriction along with more pronounced proximal and distal flaring. Indeed, the medial and lateral margins of the pubis of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 are both symmetrically markedly concave as opposed to the less incurvated margins of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804. In addition, the mediolateral constriction of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 reaches less than 1/3 of the total shaft length whereas the constriction of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 corresponds almost to half of the shaft length.

The proximal peduncle of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 is mediolaterally large as it accounts for twice the mediolateral thickness of the shaft constriction. Compared to ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, the peduncle of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763is not dorsoventrally twisted in relation to the shaft so that its greatest axis lies parallel to that of the shaft. Distally, the pubic apron of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 apperas to markedly flare out but it is missing its extremity. The overall shape of the shaft and peduncle of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 superficially resembles that of Suchodus durobrivensis among Thalattosuchia.

Figures

FIG. 22. — Pelvic girdle elements of ‘Metriorhynchus’ brachyrhynchus (Eudes-Deslongchamps, 1867), NHMUK PV R 4763: A, left pubis in anterior view (obtained from 3D model); B, left ilium in medial view; C, left ischium in medial view; D, left ilium in lateral view; E, superimposed outlines of the ischia of NHMUK PV R 4763 and NHMUK PV R 3804. Target indicates anterior. Arrow points anteriorly. Pictures of ‘Metriorhynchus’ brachyrhynchus, NHMUK PV R 4763, copyright from The Trustees of the Natural History Museum. Scale bar: 1 cm.

FIG. 23. — Ischium of ‘Metriorhynchus’ brachyrhynchus(Eudes-Deslongchamps,1867) (skull identified by Dr Mark Young on 19th September 2012,as stipulated in the records):A, LEICT G.418.1956.13.5,right ischium in medial view;B, LEICT G.418.1956.13.6,left ischium in lateral view.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. 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. 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. 5. — A metriorhynchoid schematic pelvic girdle in lateral view, based on ‘Metriorhynchus’ brachyrhynchus (Eudes-Deslongchamps, 1867) (NHMUK PV R 3804, holotype of ‘Metriorhynchus’ cultridens & NHMUK PV R 4763): A, pelvic girdle with associated vocabulary; B, C, reconstruction of the soft tissues, based on the work of Tsai & Holliday (2015). Scale bars: 1 cm.