Thalattosuchus superciliosus, (Blainville, 1853), (Blainville), 1852

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 : 163-168

THALATTOSUCHUS SUPERCILIOSUS (BLAINVILLE IN EUDES-DESLONGCHAMPS & BLAINVILLE, 1852)

For measurements, see Tables 7-9 .

Ilium

The ilium of Thalattosuchus superciliosus ( Figs 13 ; 14 ) resembles those of other metriorhynchids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Tyrannoneustes lythrodectikos , Cricosaurus suevicus , etc.) due to the absence of a postacetabular process. In this way, it differs from that of teleosauroids (e.g. Lemmysuchus obtusidens , Platysuchus multiscrobiculatus , Mycterosuchus nasutus ( Andrews 1909) Andrews, 1913 , Neosteneosaurus edwardsi , etc.).

The ilium of Thalattosuchus superciliosus ( Figs 13 ; 14 ) displays an overall triangular appearance. There appears to be at least two morphotypes of Thalattosuchus superciliosus : one notably portrayed by the specimens NHMUK PV R 2054, GLAHM V1146 and presumably GLAHM V1005 (the ‘de Blainville’ morphotype), and the other exemplified by SMNS 10116, NMI F21731, and NHMUK PV R 1530 (the ‘Leeds’ morphotype). This list is not exhaustive. Overall, the ilium of the ‘de Blainville’ morphotype has relatively smoother posterior and dorsal margins with a proportionally longer preacetabular process, whose apex is thicker and blunt. On the contrary, the ilium of the ‘Leeds’ morphotype has a more broader-stocky appearance with irregular dorsal and posterior margins due to enlarged sacral rib attachment sites, and its preacetabular process is thin and sharp. In addition, the lateral facet of the pubic peduncle of the ‘Leeds’ morphotype is relatively more extended dorsally than in ‘de Blainville’ morphotype.

The preacetabular process of Thalattosuchus superciliosus NHMUK PV R 2054 is well-developed with its anteroposterior length almost reaching that of the pubic peduncle, unlike those of Thalattosuchus superciliosus SMNS 10116, Thalattosuchus superciliosus NMI F 21731, Suchodus durobrivensis , and also Geosaurus giganteus . However, the preacetabular process of all specimens of Thalattosuchus superciliosus are proportionally shorter anteroposteriorly than that of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763. Still, the preacetabular process of Thalattosuchus superciliosus NHMUK PV R 2054 is relatively thicker dorsoventrally than those of Thalattosuchus superciliosus SMNS 10116, Thalattosuchus superciliosus NMI F 21731, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 and Suchodus durobrivensis ( Fig. 28 ). The preacetabular process of Thalattosuchus superciliosus NHMUK PV R 2054 displays almost entirely parallel dorsal and ventral margins, further differing from Thalattosuchus superciliosus SMNS 10116, Thalattosuchus superciliosus NMI F 21731, and also Suchodus durobrivensis which display convergent margins at the apex. Consequently, the apex of the preacetabular process of Thalattosuchus superciliosus NHMUK PV R 2054 appears blunt and slightly rounded. At the junction between the preacetabular process and the anterior margin of the bone, the ventral border of the preacetabular hook begins to curve, participating in the posterior widening of the process in all specimens of Thalattosuchus superciliosus . The base of the preacetabular process represents one of the mediolaterally thickest points of the bone for all Thalattosuchus superciliosus specimens ( Fig. 14 ).

The anterior margin of the ilium of NHMUK PV R 2054 and SMNS 10116 underneath the preacetabular process is slightly convex throughout its length, whereas that of NMI F21731 displays a hollow directly above the junction between the anterior margin and the pubic peduncle which appear slightly prominent ( Figs 13 ; 14 ). Indeed, the pubic peduncle of NMI F21731 appears to form a more protruding bulge than those ofNHMUK PV R 2054 and SMNS 10116 which are so weakly bulging that it is almost unnoticeable. In this way, NMI F21731 resembles the dyrosaurid ilia (i.e. Congosaurus bequaerti or Hyposaurus natator ). The pubic peduncle of Thalattosuchus superciliosus ( Figs 13 ; 14 ) is anteroposteriorly shorter than the ischial peduncle, unlike Suchodus durobrivensis and ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 (for which both are relatively equal). The ventral surface of NHMUK PV R 2054 appears to be slightly concave as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, whereas SMNS 10116 and NMI F21731 display a relatively flat to subtlety convex surface as in Suchodus durobrivensis , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, and Tyrannoneustes lythrodectikos . The ventral margin of the pubic peduncle of Thalattosuchus superciliosus is separated from that of the ischial peduncle by a small indentation – the acetabular perforation – whereas the articular facets of both peduncles form a continuum laterally. In NMI F21731, the dorsal border of the pubic peduncle is also proportionally more stretched out dorsally compared to the other Thalattosuchus superciliosus specimens (i.e. NHMUK PV R 2054 and SMNS 10116) and other metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Geosaurus giganteus , Tyrannoneustes lythrodectikos , Cricosaurus araucanensis , etc.). The outline of the lateral facet is not consistent across the different Thalattosuchus superciliosus specimens, with NHMUK PV R 2054 displaying a single triangular peak (shifted towards the acetabular perforation) whereas SMNS 10116 and NMI F21731 possess two distinct peaks of differing size and shape ( Figs 13 ; 14 ).

The ischial peduncle of Thalattosuchus superciliosus ( Figs 13 ; 14 ) is overall similar to that of other metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Dakosaurus maximus , etc.): its outline is triangular (dorsally oriented apex) and its posterior half protrudes laterally to form the posterior border of the bony acetabulum. The ventral surface of the ischial peduncle is slightly concave. It is also marked with deep scars indicating the iliac suture with the ischium.

The bony acetabulum is bordered by two protruding processes: the ischial peduncle posteriorly and the supraacetabular crest dorsally. The latter forms an arched ridge ventral to the preacetabular process. The surface of the supraacetabular crest is deeply scarred in NHMUK PV R 2054 and NMI F21731 ( Figs 13 ; 14 ). In NMI F21731, the supraacetabular crest strongly impacts the shape of the anterior margin of the ilium, whereas in NHMUK PV R 2054 and SMNS 10116 it only forms a subtle bulge. In all specimens, the supraacetabular crest extends posteriorly as far as the dorsal peak of the ischial peduncle.

The posterior margin of the ilium is markedly concave in all Thalattosuchus superciliosus specimens ( Figs 13 ; 14 ). This feature is found in some other metriorhynchoids, such as Suchodus durobrivensis , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Tyrannoneustes lythrodectikos and Geosaurus giganteus . In comparison, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 possesses a relatively straight posterior margin. The junction between the posterior margin and the dorsal margin is achieved through a posteriorly emerging corner in Thalattosuchus superciliosus , similar to Suchodus durobrivensis . In NHMUK PV R 2054 and SMNS 10116 ( Figs 13 ; 14 ), the dorsal margin of the ilium up until the base of the preacetabular process is relatively straight, whereas that of NMI F21731 is markedly concave. Still, the shape of the dorsal margin of SMNS 10116 is disturbed at about its mid-length by a portion of the posterior sacral rib attachment site (on the medial side of the bone) which sticks out dorsally (and hence can be seen in lateral view). In NHMUK PV R 2054 and NMI F21731, the sacral rib attachment sites for the sacrals do not exceed dorsally over the margins of the bone and thus can only be seen medially. There is a small bump marking the intersection between the dorsal margin of the ilium and the base of the preacetabular process in the NHMUK PV R 2054, whereas this junction is more angular in NMI F21731 and SMNS 10116 ( Figs 13 ; 14 ). The intersection between the dorsal margin of the ilium and the base of the preacetabular process actually corresponds to the dorsal-most portion of the anterior attachment site on the medial side of the bone. The latter more or less influences the shape of the junction, depending on the lateromedial inclination of the dorsal surface of the preacetabular process (see differences between SMNS 10116 and NMI F21731 on Fig. 14 ).

The attachment sites of the sacral ribs are visible on the medial side of the ilium. They are easily identifiable as shallow impressions and are individually surrounded by a ridge.In this way, they differ from the sacral rib attachment sites of Suchodus durobrivensis , Tyrannoneustes lythrodectikos , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763. NMI F21731 has more pronounced indentations, and it is the only one possessing tall ridges directly ventral to the sacral rib attachment sites. The position of both imprints are identical in all Thalattosuchus superciliosus specimens ( Figs 13 ; 14 ), and are located near the dorsal border of the ilium similar to other metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , Tyrannoneustes lythrodectikos , Cricosaurus araucanensis , etc.). However the outline of each imprint is variable: in NMI F21731 both imprints are bilobate, whereas in SMNS 10116 and NHMUK PV R 2054 the sacral rib attachment sites are more bowed. Both attachment sites are relatively the same size, with some subtle variations across the specimens: in NHMUK PV R 2054 the anterior one is bigger, in SMNS 10116 both seem equal, and in NMI F21731 the posterior one is greater.

Ischium

The ischium of Thalattosuchus superciliosus ( Fig. 13 ) displays the typical thalattosuchian triangular shape (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Cricosaurus suevicus , Dakosaurus maximus , Lemmysuchus obtusidens , Neosteneosaurus edwardsi ) due to a well-developed distal blade. The general outline of the ischium shows a strongly concave anterior margin culminating ventrally into a pointed process, an almost flat ventral margin joined to another almost flat posterior margin through a bevelled corner. In GLAHM V1005, the posterior margin of the ischium is not flat but slightly bulged. The ischium of the ‘de Blainville’ morphotype displays a relatively slender shaft and proportionally longer peduncle bridge compared to the ‘Leeds’ morphotype. In the ‘Leeds’ morphotype, the anterior peduncle also seems dorsoventrally thicker.

As in other thalattosuchians, the anterior peduncle is slightly taller dorsally than the posterior one, and is also located further away from the shaft. Indeed, the junction between the anterior peduncle and the shaft of the ischium is achieved through an elongated bony structure called the peduncle bridge. Proportionally, the peduncle bridge of Thalattosuchus superciliosus displays a considerable length (longer than in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Cricosaurus suevicus , or Lemmysuchus obtusidens , resulting in a more anteriorly projected anterior peduncle. The dorsal surface of the peduncle bridge is slightly concave throughout as it forms the ventral border of the acetabular perforation. Mediolaterally, the peduncle bridge is thinning down towards its base whereas it flares out dorsoventrally in the same direction. In Thalattosuchus superciliosus , the acetabular perforation does not appear to form a deep indentation on the medial surface of the ischium which results in both the lateral and medial surfaces of the peduncle bridge appearing similar. This feature contrasts with the more pronounced acetabular perforation of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 Cricosaurus araucanensis , Dakosaurus maximus , Torvoneustes carpenteri , Lemmysuchus obtusidens , Macrospondylus bollensis , Charitomenosuchus leedsi , and Neosteneosaurus edwardsi .

Similar to other metriorhynchoids, the anterior peduncle is reduced in size and extension and is ovoid in shape (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Cricosaurus suevicus , Torvoneustes carpenteri , etc.) contra teleosauroids (e.g. Lemmysuchus obtusidens , Macrospondylus bollensis , Neosteneosaurus edwardsi , etc.). The anterior peduncle is pitted over almost its entire surface conveying the presence of an extensive cartilage cap in vivo (presumably connecting with the ilium and pubis as in extant crocodylians; Claessens & Vickaryous [2012]; Tsai & Holliday [2015]). The anterior peduncle of Thalattosuchus superciliosus does not appear to have contacted the ilium ( Fig. 15 ), but was situated directly ventrally to the posterior bump of the pubic peduncle of the ilium, thereby circling the acetabular perforation.

The posterior peduncle of Thalattosuchus superciliosus NHMUK PV R 2054 ( Fig. 13 ) is anteroposteriorly longer and mediolaterally wider than the anterior peduncle, as in other thalattosuchians. Dorsally its articular surface is separated in two distinct areas: the facet medially oriented which is highly scarred as it was fused with the ilium, and the facet laterally oriented which is smooth and concave. The latter is also the largest and formed the ventral border of the bony acetabulum. The junction between the two areas forms a thick ridge. In dorsal view, the overall outline of the posterior peduncle resembles an ellipse but with an anteriorly flat portion, which borders the acetabular perforation posteriorly.The posterior peduncle is low-built and does not significantly stick out from the shaft of the ischium (unlike the anterior one).

Directly underneath the peduncle is the shaft (or neck) of the ischium ( Fig. 13 ). This portion forms the junction between the distal blade and the peduncle. The neck of the ischium is markedly larger anteroposteriorly than the posterior peduncle is as in Lemmysuchus obtusidens , Macrospondylus bollensis , Neosteneosaurus edwardsi , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Tyrannoneustes lythrodectikos , and Torvoneustes carpenteri . Comparatively, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 and Dakosaurus maximus do not display such a difference between the length of the posterior peduncle and that of the shaft. The shaft gradually flares out ventrally to form the distal blade. The latter is well-developed as it constitutes most of the surface of the ischium. Still, like Dakosaurus maximus and Torvoneustes carpenteri , the distal blade of Thalattosuchus superciliosus is overall shorter than in Pelagosaurus typus and ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763.

Anteriorly, the distal blade culminates in a sharp process whereas it forms a thicker extremity posteriorly, similar to other thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Lemmysuchus obtusidens , Macrospondylus bollensis , Neosteneosaurus edwardsi ). The anterior process of the distal blade shows a dorsal component in its orientation due to a bend along the ventral margin of the ischium. This bend is located at about 1/3 of the length of the ventral margin anteriorly, and constitutes an angle of approximately 157°. The presence of a marked angle interrupting the otherwise flat surface of the ventral margin of the distal blade appears to be unique to Thalattosuchus superciliosus ( Fig. 13 ). On the medial side of the bone, the ventral margin of the distal blade is scarred with deep ridges and sulci perpendicular to the border of the bone. The area is also bevelled medially, starting from the ventral corner of the distal process up until the peak of the anterior process. The entire bevelled and scarred surface corresponds to the suture where both ilia met in vivo. Posteriorly, the distal blade forms a posteroventrally facing bevelled corner, rather than a relatively squared one as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 or rounded one as in Lemmysuchus obtusidens . It also differs from the more pointed one of Neosteneosaurus edwardsi . From the top corner of the posterior process until the shaft, the posterior surface of the distal blade is relatively straight and constitutes the thinnest portion of the bone mediolaterally.

Pubis

There is no major differences observed between the two morphotypes based solely on the pubis. The pubis of Thalattosuchus superciliosus ( Figs 13 ; 14 ) presents a proportionally long shaft along with a relatively quadrangular pubic plate. The lateral and medial margins of the bone present a similar degree of concavity, with their apex situated almost at the mid-height of the bone. The concavity of the margins results in a mediolaterally wide peduncle and pubic plate.The proximal widening of the peduncle is a common trait among Thalattosuchia (e.g. Suchodus durobrivensis , Cricosaurus suevicus , Dakosaurus maximus , Geosaurus giganteus , or Lemmysuchus obtusidens ), even if some taxa lack it (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804). The peduncle of Thalattosuchus superciliosus shows a teardrop profile (whose great axis is parallel to that of the shaft), as well as an undulating articular surface whose lateral extremity is more elevated proximally than the medial one. Posteriorly, the margin of the articular surface of the peduncle drops slightly more than its counter part on the anterior surface of the bone.

The overall symmetry of the pubic plate contrasts with those of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Cricosaurus suevicus , Lemmysuchus obtusidens , but resembles those of Geosaurus giganteus , Hyposaurus natator and Dyrosaurus maghribensis Jouve, Iarochène, Bouya & Amaghzaz, 2006 . However, the shape is relatively different, as the angle between the ventral border of the pubic blade and the pubic symphysis is greater in Thalattosuchus superciliosus (about 120°, Figs 13 ; 14 ) than in dyrosaurids (about 90°), giving a quadrangular appearance to Thalattosuchus superciliosus . Lemmysuchus obtusidens also shows a great angle (of about 140°) between its pubic symphysis and ventral margin, but its pubis does not appear symmetrical due to a less convex ventral margin and its subsequent more distal intersection with the posterior margin.

Unlike ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and Suchodus durobrivensis , the length of the pubic symphysis of Thalattosuchus superciliosus is short in relation to the size of the pubic plate and the pubis as a whole. However, in Thalattosuchus superciliosus , the ventral margin of the pubic blade is longer than in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and Suchodus durobrivensis , resulting in a less straight posterior margin and a proportionally mediolaterally wider pubic plate. Comparatively, rhacheosaurines and geosaurines possess the mediolaterally widest pubic blades and the shortest pubic diaphyses.

Figures

FIG. 13. — Pelvic girdle elements of Thalattosuchus superciliosus (Blainville, 1853), NHMUK PV R 2054: A, left ilium in lateral view; B, left ischium in lateral view; C, left pubis in lateral view; D, right pubis in anterior view; E, left pubis in anterior view; F, left ilium in medial view. Target indicates anterior. Arrow points anteriorly. Scale bars: 1 cm.

FIG. 14. — Right ilia and pubis of Thalattosuchus superciliosus (Blainville, 1853), SMNS 10116 (A-D) and NMI F21731 (E-G): A, E, right ilium in lateral view; B, F, right ilium in anterior view;C, G, right ilium in medial view; D, right pubis in anterior view.Target indicates anterior.Arrow points anteriorly. Scale bars: 1 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.

FIG. 15. — Pelvic girdle of Thalattosuchus superciliosus (Blainville, 1853), NHMUK PV R 2054: A, lateral view; B, anterior view; C, ventral view; D, dorsal view. Target indicates anterior. Arrow points anteriorly. The right ilium, ischium and pubis are mirrored. Scale bar: 5 cm.