Suchodus durobrivensis, LYDEKKER, 1890

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 : 192-194

SUCHODUS DUROBRIVENSIS LYDEKKER, 1890

For measurements, see Tables 7-9 .

Ilium

The ilium of Suchodus durobrivensis can easily be identified as metriorhynchoid as it is reduced in size compared to the pubis ( Figs 28 ; 29 ), and as it does not display a postacetabular process. Similar to ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, the preacetabular process is not parallel to the ventral margin of the ilium and points anterodorsally. The dorsal margin of the ilium forming a short iliac crest, directly posterior to the preacetabular process, is more developed in Suchodus durobrivensis than in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, as it is almost as long as the preacetabular process. The anterior margin of the ilium is convex while the posterior margin is concave. The ventral margin is undulating, with the concave portion corresponding to the ischial peduncle, and the overall convex part belonging to the pubic peduncle. Like in other metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Geosaurus giganteus , Tyrannoneustes lythrodectikos ), the acetabular perforation of Suchodus durobrivensis is reduced to a slight indentation along the ventral margin of the ilium. Hence, it does not separate the peduncles as in teleosauroids (e.g. Charitomenosuchus leedsi , Neosteneosaurus edwardsi , Lemmysuchus obtusidens , etc.). Similar to Thalattosuchus superciliosus and Dakosaurus maximus , the acetabular perforation of Suchodus durobrivensis is positioned underneath the anterior portion of the ischial peduncle, whereas it was placed within the extension of the pubic peduncle in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763. In ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and Tyrannoneustes lythrodectikos , the acetabular perforation appears to be in between the peduncles. As opposed to ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , and Dakosaurus maximus , the acetabular perforation of Suchodus durobrivensis is strongly reduced. Only Tyrannoneustes lythrodectikos GLAHM V1145 and ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 display an acetabular perforation so shallow it appears almost indistinguishable. The acetabulum, and the pubic and ischial peduncles take up almost all of the available space on the ilium of Suchodus durobrivensis similar to other metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , Dakosaurus maximus , Tyrannoneustes lythrodectikos , etc.). Thereby, the reduction of the ilium happened at the expense of the acetabular perforation and postacetabular process in order to preserve the parts involved in the hip articulation as in other derived metriorhynchoids.

The ischial peduncle is dorsoventrally higher than the pubic peduncle (about 1.8 times taller), and also protrudes laterally, leaving a sharp edge posteriorly (like in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763). The ischial peduncle takes the shape of triangle pointing dorsally as in other crocodyliformes, whereas the pubic peduncle is more rectangular as in Geosaurus giganteus . The pubic peduncle of Suchodus durobrivensis seems slightly domed dorsally but does not display distinct peaks as in other metriorhynchoids (e.g. Thalattosuchus superciliosus , Tyrannoneustes lythrodectikos , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763).

The majority of the lateral surface of the ilium is covered by an elliptic concave area, the bony acetabulum, which appears slightly offset posteriorly. Most of the acetabulum is scarred, and this rugged area extends over the whole anterior portion of the ilium, overpassing the supraacetabular crest as well as in Tyrannoneustes lythrodectikos , Thalattosuchus superciliosus , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763. This region was presumably the anchoring site of a structure similar to the acetabular labrum of extant crocodylians. The acetabulum comprises two distinct concave subareas: the anterior strongly rugged and shallow portion, and the posterior deeper part. In vivo the whole acetabulum was presumably covered in cartilage, with the actual articular capsule probably centered over the posterior concavity as the anterior portion would potentially form, or at least partake in, the anterior wall of the capsule (i.e. the acetabular labrum). The supraacetabular crest demarcates the dorsal border of the acetabulum. In Suchodus durobrivensis , it appears like a sheared parabolic ridge leaning posteriorly, with the anterior rim prominent and the posterior one smoothed. The ventral portion of the supraacetabular crest stems from the anterior-most portion of the ilium, unike in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and Tyrannoneustes lythrodectikos . The dorsal portion of the supraacetabular crest is offset in relation to the anterior margin of the ilium, as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Thalattosuchus superciliosus , Tyrannoneustes lythrodectikos .

On the medial side, the ilium is convex at the site where the sacral processes attach to the bone as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Tyrannoneustes lythrodectikos . Similar to other derived metriorhynchoids, the attachments sites are located dorsally near the preacetabular process. This further differs from dyrosaurids (e.g. Congosaurus bequaerti , Hyposaurus natator , Dyrosaurus maghribensis , or Acherontisuchus guajiraensis ) and extant crocodylians (e.g. Palaeosuchus palpebrosus [ Fig. 7 ], Mecistops cataphractus , or Caiman crocodilus [ Fig. 9 ]) where the sacral rib attachment sites are fully separated and positioned along the ventral margin of the ilium. The two attachment sites of Suchodus durobrivensis are bilobed and adjacent, and their global outline is slightly in relief or raised (like in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and Tyrannoneustes lythrodectikos ). They are not clearly discernible from one another, except for the bilobate ventral margin which hints at the exact position of the split. Compared with ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, or Tyrannoneustes lythrodectikos , the sacral rib attachment sites of Suchodus durobrivensis are less pitted, but occupy a larger area of the ilium as they account for 65.8% of the anteroposterior length of the bone.

Pubis

The pubis of Suchodus durobrivensis ( Figs 28 ; 29 ) displays a rod-like shaft culminating in a truncated distal blade. The lateral and medial margins of the shaft are concave, with the medial margin possessing the greatest degree of curvature and the shortest length. The distal extremity of the pubic plate shows a straight margin medially (i.e. the pubic symphysis) and a convex one ventrolaterally, with a gradual transition between the two. Medially, the concave margin of the shaft and the straight one of the pubic symphysis meet at an angle of approximately 90°, and form a pointed peak. Laterally, the shaft gradually transforms into the pubic plate with an undulation of small intensity. The rugged area marking the existence of cartilage in vivo is present on the whole distal extremity of the pubic plate, and extends over a small portion of the lateral margin. The pubic peduncle is oval in proximal view, with its posterior margin extending slightly towards the shaft in posterior view. The shaft represents the thickest portion of the bone anteroposteriorly.

There are several differences on the shaft and distal blade which make the distinction between Suchodus durobrivensis and other metriorhynchoids. Overall, the pubis of Suchodus durobrivensis is asymmetrical mediolaterally with an extended pubic symphysis but narrow pubic plate mediolaterally. In this way, the pubis of Suchodus durobrivensis differs from that of most thalattosuchians, extant crocodylians (e.g. Caiman crocodilus [ Fig. 9 ], Mecistops cataphractus [ Fig. 8 ]) and dyrosaurids (e.g. Hyposaurus natator , Dyrosaurus maghribensis ). Like other thalattosuchians, the pubis of Suchodus durobrivensis displays an oval peduncle proximally. In Suchodus durobrivensis , the shape of the medial and lateral margins are strongly dissimilar: the medial margin of the pubis is strongly concave whereas the degree of concavity is lesser for the lateral margin. Among metriorhynchoids, a similar difference is observed in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and Geosaurus giganteus . The pubis of Suchodus durobrivensis presents a strong constriction of the shaft relatively far from the peduncle of the pubis (at about 35% of the total length of the pubis), as in Cricosaurus suevicus , Cricosaurus bambergensis , Geosaurus giganteus . Among metriorhynchoids, Pelagosaurus typus, Cricosaurus araucanensis , and Dakosaurus maximus also present a strong constriction but it is located proportionally more closely to the peduncle.

In Suchodus durobrivensis ( Fig. 29 ) the transition between the lateral margin of the shaft and the distal blade is both smooth and gradual, as it does not show a posterior protuberance as in‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804. Consequently, the arched distal margin of the pubic plate is shorter and has a smaller degree of curvature in Suchodus durobrivensis , and also appears more elliptic (as opposed to other mertriohynchoids). The shortness of the distal blade namely accounts for the narrow appearance of the pubic plate of Suchodus durobrivensis . In addition, the relative size of the pubic symphysis (about 32% of the total pubic length) and its angular relation with the distal blade (about 140°) accentuate the narrow appearance of the bone. Similar to other metriorhynchoids with a developed pubic symphysis, the junction between the medial margin and the pubic symphysis of Suchodus durobrivensis forms a pointed right-angled corner (e.g. Thalattosuchus superciliosus , Cricosaurus araucanensis , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804).

Figures

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.

FIG. 29. — Pelvic reconstruction of Suchodus durobrivensis Lydekker, 1890, NHMUK PV R 2618: A, lateral view; B, anterior view; C, ventral view; D, dorsal view. Arrow points anteriorly. Target indicates anterior. The left ilium and pubis are mirrored. Scale bar: 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. 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. 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.