Charitomenosuchus leedsi, (Andrews, 1909)

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 : 224-230

CHARITOMENOSUCHUS LEEDSI ( Andrews, 1909)

For measurements, see Tables 7-9 .

Ilium

Overall, the ilium of Charitomenosuchus leedsi ( Figs 48 ; 49 ) stands out from that of most thalattosuchians in displaying a parallelepiped shape (e.g. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Dakosaurus maximus , Lemmysuchus obtusidens , Platysuchus multiscrobiculatus , Macrospondylus bollensis , etc.). Indeed, the different surfaces of Charitomenosuchus leedsi ( Fig. 48 ) are parallel two by two (i.e. dorsal and ventral, anterior and posterior), and their angular relations are different from 90°. In this way, it strongly resembles that of Neosteneosaurus edwardsi NHMUK PV R 3701, NHMUK PV R 3898, and NHMUK PV R 2076. However, the curvature of the anterior margin of Charitomenosuchus leedsi is not present in any specimens of Neosteneosaurus edwardsi .

The ilium of Charitomenosuchus leedsi ( Fig. 48 ) possesses a well-developed postacetabular process like other teleosauroids (i.e. Lemmysuchus obtusidens , Platysuchus multiscrobiculatus , Neosteneosaurus edwardsi , Macrospondylus bollensis , Plagiophthalmosuchus gracilirostris ). Overall, the postacetabular process of Charitomenosuchus leedsi takes the shape of a Lancet arch like that of Lemmysuchus obtusidens : its dorsal (i.e. the iliac crest) and ventral margins are both convex, with the latter showing the greatest intensity. However, at about its mid-length, the concavity of the ventral margin of the postacetabular process switches to become concave. The total anteroposterior length of the iliac crest (i.e. dorsal margin of the postacetabular process) reaches that of the ilium around the peduncles, similar to Lemmysuchus obtusidens . Posteriorly, the postacetabular process culminates in a sharp apex oriented dorsoposteriorly and thus differs from those of Lemmysuchus obtusidens and Platysuchus multiscrobiculatus for which the axis of the postacetabular process is in line with that of the preacetabular process. This dorsal component in the orientation of the postacetabular process of Charitomenosuchus leedsi renders its junction with the preacetabular process more pronounced than in Lemmysuchus obtusidens or Platysuchus multiscrobiculatus , and gives Charitomenosuchus leedsi a distinctive look. The lateral surface of the postacetabular process is relatively flat near its margins, but forms a shallow depression around its base and centre.

The preacetabular process of Charitomenosuchus leedsi ( Fig. 48 ) is proportionally large as its anteroposterior length reaches about half of the total anteroposterior length of the postacetabular process. Comparatively, Plagiophthalmosuchus gracilirostris , Platysuchus multiscrobiculatus and Macrospondylus bollensis also possess an elongated preacetabular process. Overall, the shape of the preacetabular process of Charitomenosuchus leedsi is that of a slender peak whose ventral margin is concave and its dorsal margin is subtlety convex. The preacetabular process of Charitomenosuchus leedsi appears to be pointing anteriorly with a small ventral component, like that of Lemmysuchus obtusidens , Macrospondylus bollensis SMNS 17484a and Neosteneosaurus edwardsi NHMUK PV R 3898. Underneath the preacetabular process, the anterior margin of the ilium strongly bulges not unlike that of Suchodus durobrivensis , which contrasts with those of Lemmysuchus obtusidens or Platysuchus multiscrobiculatus . The junction between the preacetabular process and the anterior margin is achieved through a smooth rounded corner.

Ventrally, the margin of the ilium is strongly undulating with each indentation marking the position of each peduncle, as in other thalattosuchians (e.g. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Lemmysuchus obtusidens , Neosteneosaurus edwardsi , Macrospondylus bollensis , etc.). The junction between the anterior margin of the ilium and the pubic peduncle forms a protruding bulge, as in Thalattosuchus superciliosus NMI F 21731 or Lemmysuchus obtusidens . Laterally, the facet of the pubic peduncle of Charitomenosuchus leedsi is not well defined but appears to be wedge-shaped as in Macrospondylus bollensis and Neosteneosaurus edwardsi , with its dorsoventral height reaching almost 1/3 of its anteroposterior length. The ventral surface of the pubic peduncle possesses a strong indentation along its midline whose intensity is greater than what is observed in Lemmysuchus obtusidens . Ventrally, the pubic peduncle is also wedge-shaped with its medial and anterior margins being relatively flat and the junction between both taking the form of a rounded right-angle corner. Mediolaterally, the pubic peduncle is the thickest at its anterior-most portion, as in all other thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Thalattosuchus superciliosus , Dakosaurus maximus , Lemmysuchus obtusidens , Macrospondylus bollensis , Neosteneosaurus edwardsi ).

The ischial peduncle of Charitomenosuchus leedsi ( Fig. 48 ) also contributes to the ventral margin of the ilium along with the pubic peduncle and the acetabular perforation ( Fig. 49 ). The facet of the ischial peduncle is shaped similar to an isosceles triangle with one of its summit pointing strictly dorsally similar to many other crocodyliforms (e.g. Caiman crocodilus , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus , Congosaurus bequaerti , Hyposaurus natator , Dyrosaurus maghribensis , etc.). Likewise, the facet of the ischial peduncle is also tilted towards the centre of the acetabulum as its posterior corner is set further away from the ilium than its anterior corner. The ischial peduncle greatly protrudes from the ilium posteriorly and in this way forms a bony barrier posteriorly to the acetabulum. Ventrally, the surface of the ischial peduncle is strongly concave and wedge-shaped like the pubic peduncle but with smoother corners. Comparatively, the ischial peduncle is dorsoventrally taller than the pubic peduncle but both display similar lengths anteroposteriorly. Both peduncles are physically separated by the acetabular perforation as in other teleosauroids (e.g. Lemmysuchus obtusidens , Neosteneosaurus edwardsi ). Compared to Lemmysuchus obtusidens , the acetabular perforation of Charitomenosuchus leedsi appears relatively reduced as its anteroposterior extension is more limited. In depth, the acetabular perforation of Charitomenosuchus leedsi is relatively shallow, as in Pelagosaurus typus or Lemmysuchus obtusidens .

The bony acetabulum of Charitomenosuchus leedsi ( Fig. 48 ) accounts for about half of the total lateral surface of the bone, which contrasts with both Lemmysuchus obtusidens and Platysuchus multiscrobiculatus in which the acetabulum covers a wider portion of the lateral surface of the ilium. Due to the shape of the anterior margin of the ilium, the bony acetabulum of Charitomenosuchus leedsi ( Fig. 48 ) is placed directly underneath the preacetabular process as in Pelagosaurus typus or Suchodus durobrivensis , not posteriorly to it as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , Cricosaurus suevicus , and Lemmysuchus obtusidens .

Medially, the ilium of Charitomenosuchus leedsi ( Fig. 48 ) bears two distinct scars marking the position of the sacral rib attachment sites for both sacrals. The sacral rib attachment sites are adjacent and are located around the mid-height of the bone as in Lemmysuchus obtusidens and Neosteneosaurus edwardsi . In Charitomenosuchus leedsi , the posterior attachment site takes the shape of a droplet and is overall greater than the anterior one as it extends posteriorly almost until the tip of the postacetabular process. The area of the posterior attachment site expanding over the postacetabular process presumably received the winglet of the second sacral whereas the more rounded portion next to the anterior attachment site bore the main body of the second sacral rib. The anterior attachment site displays a rather bilobate silhouette with the ventral margin of the scar being more circular than the dorsal one. Each scar is delimited by a thin rim and forms a relatively shallow indentation similar to Suchodus durobrivensis or Lemmysuchus obtusidens . However, Charitomenosuchus leedsi drastically stands out from the latter and other thalattosuchians by displaying a thick elevation directly underneath the dorsal border of each attachment site.

Ischium

The ischium of Charitomenosuchus leedsi ( Fig. 48 ) displays an overall thin shaft, an elongated peduncle bridge and a rather unusual shape for its posterior process. Like other thalattosuchians the anterior peduncle of Charitomenosuchus leedsi is drastically smaller than the posterior peduncle (e.g. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , Cricosaurus suevicus , Geosaurus giganteus , Aeolodon priscus , Lemmysuchus obtusidens , Platysuchus multiscrobiculatus , etc.). In anterior view, the anterior peduncle has an isosceles triangular outline whose greatest axis is oriented mediolaterally. The overall articular surface of the anterior peduncle is also truncated so that the base of the triangle (about 1/3 of its height) faces anteriorly whereas the remaining portion of the peduncle is posteriorly deflected. The base of the triangular articular surface is presumably where the peduncle of the pubis was anchored to the ischium through soft tissues ( Fig. 49 ). The anterior peduncle is borne by the peduncle bridge, which elevates the latter slightly over the posterior peduncle dorsally and also subtlety places it more medially. The peduncle bridge of Charitomenosuchus leedsi is relatively long as it is almost as long anteroposteriorly as the posterior peduncle similar to Macrospondylus bollensis but unlike Lemmysuchus obtusidens . The ventral surface of the peduncle bridge of Charitomenosuchus leedsi is relatively straight whereas its dorsal surface is deeply concave. The latter, combined with the position of the peduncle bridge in relation to the posterior peduncle creates an overall deep acetabular perforation between the peduncles, as in Macrospondylus bollensis and Sericodon jugleri . Unlike metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , Cricosaurus suevicus , etc.), the acetabular perforation of Charitomenosuchus leedsi does not form a sulcus on the medial side of the bone, since it is well defined dorsally to the peduncle bridge. However, the acetabular perforation of Charitomenosuchus leedsi is slightly titled medially compared to Macrospondylus bollensis and Sericodon jugleri .

The posterior peduncle of Charitomenosuchus leedsi corresponds to the mediolaterally widest portion of the ischium. It is almost positioned on the same level dorsally as the anterior peduncle which contrasts with several metriorhynchoids (e.g. Pelagosaurus typus, Thalattosuchus superciliosus , Cricosaurus suevicus , Geosaurus giganteus , etc.). Yet, there are some thalattosuchians that also display similar dorsal extensions for their peduncle but those possess a more reduced acetabular perforation laterally (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Lemmysuchus obtusidens ). Dorsally, the posterior peduncle displays two distinct facets: the articular facet connecting to the ilium, and the facet forming the ventral border of the bony acetabulum. The latter accounts for almost 2/3 of the total dorsal surface of the posterior peduncle and is slightly concave. Its silhouette is that of a rounded triangle with its anterior margin being concave and its lateral one being strongly convex up to the point of being almost hemispherical. The limit between the two sub-surfaces of the posterior peduncle is relatively straight. The medial articular facet of the posterior peduncle is overall triangular in shape. It is oriented medially rather than laterally and its surface is strongly pitted. The posterior peduncle of Charitomenosuchus leedsi appears to be slightly elevated from the shaft as in Macrospondylus bollensis , which is presumably caused by the overall position of the peduncle bridge (which is relatively more ventral in both Charitomenosuchus leedsi and Macrospondylus bollensis ).

Overall, the anterior and posterior margins of the ischium are both concave, with the anterior one displaying a greater intensity and almost appearing hemispherical.The shaft of the ischium of Charitomenosuchus leedsi ( Fig. 48 ), located directly underneath the peduncles, is proportionally long and thin as its total dorsoventral height greatly exceeds (of about 140 %) the anteroposterior length of its constriction. In parallel, the anteroposterior constriction of the shaft (which corresponds to its ventral limit) is not significantly longer than the posterior peduncle, unlike in Macrospondylus bollensis . Ventrally to the shaft, the ischium of Charitomenosuchus leedsi drastically flares out to form the distal blade. Within Thalattosuchia, Cricosaurus suevicus and Lemmysuchus obtusidens also possess the combination of a thin shaft and large distal blade. Posteriorly, the subtle concavity of the posterior margin of the ischium of Charitomenosuchus leedsi is disrupted by a series of undulations before culminating in the posterior process. These undulations in the vicinity of the posterior process appear unique to Charitomenosuchus leedsi (i.e. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , Cricosaurus suevicus , Dakosaurus maximus , Torvoneustes carpenteri , Aeolodon priscus , Lemmysuchus obtusidens , Platysuchus multiscrobiculatus ). The shape of the posterior process of Charitomenosuchus leedsi is also singular: it consists in a relatively straight surface connected to the posterior margin of the ilium and the distal through rounded corners (almost right-angled). Thalattosuchus superciliosus also displays a relatively straight surface for its posterior process, but with a different inclination and size. The posterior process of Charitomenosuchus leedsi is also moderately well-developed as its dorsoventral height reaches about 27% of the total dorsoventral height of the ischium unlike those of Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus LEICT G. 418.1956.13.5, Thalattosuchus superciliosus , or Platysuchus multiscrobiculatus . Comparatively, Aeolodon priscus and Lemmysuchus obtusidens display a proportionally thicker posterior process than Charitomenosuchus leedsi .

The posterior process of the ischium is connected to the anterior process through the ventral margin, which also corresponds to the distal blade. The latter is not straight unlike in several other thalattosuchians (e.g. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, ‘ Metriorhynchus ’ brachyrhynchus LEICT G. 418.1956.13.5, Cricosaurus bambergensis , Dakosaurus maximus , Aeolodon priscus , Lemmysuchus obtusidens , Platysuchus multiscrobiculatus , etc.) but is highly undulating similar to Neosteneosaurus edwardsi . Thalattosuchus superciliosus NHMUK PV R 2054 also displayed a singular distal blade due to an anterior bending. Mediolaterally, the distal blade of the ischium is relatively thick, unlike the anterior and posterior processes. Indeed, on the medial side of the ischium, the distal blade displays a strongly pitted area which corresponds to the ischial suture where both ischia met in vivo ( Fig. 49 ). The anterior process of Charitomenosuchus leedsi ( Fig. 48 ) appears to have been relatively sharp as both the ventral and anterior margins of the ischium are strongly convergent.

Pubis

The pubis Charitomenosuchus leedsi ( Fig. 48 ) shows the combination of a relatively short shaft and a long pubic symphysis, similar to ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Machimosaurus , Lemmysuchus obtusidens , and Neosteneosaurus edwardsi . Overall, the lateral and medial margins of the pubis are concave. However, the concavity of the lateral margin is less harmonious than the lateral one, as it displays undulations along its length similar to Machimosaurus .

The shaft of Charitomenosuchus leedsi accounts for about 1/4 of the total proximodistal length of the pubis. The mediolateral width of the thinnest portion of the shaft (i.e.the constriction) is drastically lesser than that of the pubic peduncle, similar to Pelagosaurus typus, Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Dakosaurus maximus , Geosaurus giganteus , Lemmysuchus obtusidens , Platysuchus multiscrobiculatus , Macrospondylus bollensis . In Charitomenosuchus leedsi , the mediolateral width of the constriction of the shaft corresponds to half of the mediolateral width of the pubic peduncle. From the base of the constriction up until the pubis peduncle, the medial and lateral margins of the shaft are concave but not symmetrical, so that the median of the pubic peduncle is shifted laterally compared to that of the shaft. The pubic peduncle is set on top of the shaft proximally. It displays an oval outline whereas its articular surface is curving, with its centre being concave and its medial corner being set more distally that the lateral corner.

The pubic apron of Charitomenosuchus leedsi is relatively slender up until the apparition of the pubic symphysis, as in Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 3804, Cricosaurus albersdoerferi , Cricosaurus bambergensis , Machimosaurus , Platysuchus multiscrobiculatus . Medially, the margin of the pubis bends at an angle of approximately 153° which drastically expands the size of the pubic apron medially before the pubic symphysis ( Fig. 49 ). The junction between the medial margin of the shaft and the pubic symphysis forms a right angle, as in other thalattosuchians with elongated pubic symphysis regardless of the orientation of the pubic symphysis (e.g.‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , Lemmysuchus obtusidens , Machimosaurus , Macrospondylus bollensis , etc.). The pubic symphysis of Charitomenosuchus leedsi ( Fig. 48 ) is well developed as it accounts for about 31% of the total proximodistal height of the bone. The transition between the pubic symphysis and the distal margin of the pubis is well marked as opposed to Pelagosaurus typus, Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , and Macrospondylus bollensis . In Charitomenosuchus leedsi , the distal margin of the pubis and the pubic symphysis form an angle of approximately 150°, similar to Lemmysuchus obtusidens and Mycterosuchus nasutus . The distal margin of the pubis appears to be overall convex, even though there is a relatively angular bent around its mid-length. Laterally, the distal margin of the pubic apron forms a smooth corner in line with the lateral margin of the pubis (i.e. it does not protrude laterally as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804).

The entire pubic apron of Charitomenosuchus leedsi is slightly twisted so that the medial corner is also pointing ventrally whereas the lateral one also points dorsally. In addition, the posteroventral surface of the pubic apron is slightly concave, emphasizing the curve of the pubic apron ( Fig. 49 ). The pubic apron of other thalattosuchians may show other configurations similarly turning the pubic apron into a shallow spatula (e.g. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Thalattosuchus superciliosus NHMUK PV R 2054 , Lemmysuchus obtusidens , Machimosaurus , etc.).

Overall, the pubis of Charitomenosuchus leedsi ( Fig. 48 ) bears a resemblance to those of Lemmysuchus obtusidens and Neosteneosaurus edwardsi among other teleosauroids (i.e. Machimosaurus , Platysuchus multiscrobiculatus , Macrospondylus bollensis , etc.). The resemblance between Charitomenosuchus leedsi , Lemmysuchus obtusidens and Neosteneosaurus edwardsi is mainly driven by the shape of the pubic apron (including the pubic symphysis) and its relation with the shaft. The shape of the pubic apron is strongly influenced by the size and orientation of the pubic symphysis and both display similar values in Charitomenosuchus leedsi , Lemmysuchus obtusidens , Neosteneosaurus edwardsi , and Mycterosuchus nasutus (the lowest value): the pubic symphysis corresponds to about 28-35% of the total height of the bone, and forms an angle of approximately 28-30° with the median of the shaft. In addition, the distal margin of the pubic apron joining the pubic symphysis and the posterior margin of the pubis is slightly convex in both Charitomenosuchus leedsi and Lemmysuchus obtusidens .

Figures

FIG. 48. — Pelvic girdle elements of Charitomenosuchus leedsi (Andrews, 1909), NHMUK PV R 3806: A, left ilium in medial view; B, left ilium in lateral view; C, left pubis in lateral view; D, left ischium in lateral view; E, left pubis in anterior view; F, right ischium in medial view. Arrow points anteriorly. Target indicates anterior. Scale bar: 1 cm.

FIG. 49. — Pelvic reconstruction of Charitomenosuchus leedsi (Andrews, 1909), NHMUK PV R 3806: A, lateral view; B, anterior view; C, ventral view; D, dorsal view. Arrow points anteriorly. Target indicates anterior. The right ilium, ischium and pubis are mirrored. Scale bar: 5 cm.