Mycterosuchus nasutus, (Andrews, 1909), (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 : 212-214

MYCTEROSUCHUS NASUTUS ( ANDREWS, 1909)

For measurements, see Tables 7-9 .

Ischium

The most distinguishing feature of the ischium of Mycterosuchus nasutus ( Fig. 41 ) is the combination of a thin and elongated shaft with a long and narrow posterior process. Indeed, the ischium of Mycterosuchus nasutus ( Fig. 41 ) displays strongly concave anterior and posterior margins, resulting in a slender shaft as in Teleosaurus sp. (NHMUK PV R 238 and OUMNH. J1638) and Charitomenosuchus leedsi (NHMUK PV R 3806). Comparatively, the ischium of Proexochokefalos cf. bouchardi ( MJSN SCR010-374) also displays a slender shaft and an overall slender ischium. The slender shaft of Mycterosuchus nasutus strongly contrasts with the thicker shaft of the other aeolodontines Aeolodon priscus and, presumably, Sericodon jugleri .

Dorsally and ventrally to the maximum constriction of the shaft, the ischium of Mycterosuchus nasutus drastically flares out which gives it a very distinctive aspect like Charitomenosuchus leedsi and Teleosaurus sp. (NHMUK PV R 238 and OUMNH. J1638). However, the posterior margin of the ischium of Mycterosuchus nasutus stands out in rapidly displaying a change in concavity directly dorsally to the constriction. As a result, the posterior margin of the ischium underneath the posterior peduncle forms a marked bulged, rather than being straight as in Teleosaurus sp. (NHMUK PV R 238 and OUMNH. J1638) or concave throughout as in Charitomenosuchus leedsi . The edge of the posterior peduncle of Mycterosuchus nasutus forms a thin ridge on both its lateral and medial sides, which appears similar to Indosinosuchus potamosiamensis ( Martin et al. 2019a) , Platysuchus multiscrobiculatus but contrasts with Sericodon jugleri , Teleosaurus sp. , Macrospondylus bollensis, Proexochokefalos cf. bouchardi, Neosteneosaurus edwardsi , Lemmysuchus obtusidens and presumably Machimosaurus . Only the base of the peduncle bridge of Mycterosuchus nasutus is preserved, which shows a partially obstructed acetabular perforation laterally as in most teleosauroids (except Sericodon jugleri , Charitomenosuchus leedsi and Macrospondylus bollensis ).

Underneath the shaft, the posterior margin of the ischium of Mycterosuchus nasutus is relatively straight until it reaches the apex of the posterior process, as in most teleosauroids (e.g. Teleosaurus sp. (NHMUK PV R 238 and OUMNH. J1638), Macrospondylus bollensis, Proexochokefalos cf. bouchardi, Neosteneosaurus edwardsi , and Lemmysuchus obtusidens ). It contrasts with the irregular posterior margin of Charitomenosuchus leedsi , and also with the other aeolodontines Aeolodon priscus and Sericodon jugleri which have a straight but shortened posterior margin. Like in most thalattosuchians, the ventral margin of the ischium of Mycterosuchus nasutus – corresponding to the ischial blade – is relatively straight throughout its length, although it shows a subtle bulge around its mid length. Due to the shape and relative inclination of the posterior and ventral margin of the ischium, the posterior process of Mycterosuchus nasutus displays a proportionality long and narrow outline compared to other aeolodontines Aeolodon priscus and Sericodon jugleri . Still, the posterior process of Mycterosuchus nasutus is less extended than in Teleosaurus sp. (NHMUK PV R 238 and OUMNH.J1638) and Macrospondylus bollensis among teleosauroids, and it is also dorsoventrally thicker than in Platysuchus multiscrobiculatus . Posteriorly, the posterior process of Mycterosuchus nasutus extends to form a slightly rounded apex with the greatest axis of the concavity being parallel to the ventral margin of the ischium, giving it a dorsoventrally symmetrical aspect as in Pelagosaurus typus, Teleosaurus sp. (NHMUK PV R 238 and OUMNH.J1638), Platysuchus multiscrobiculatus, Proexochokefalos cf. bouchardi, and Lemmysuchus obtusidens . The slightly convex apex of Mycterosuchus nasutus contrasts with the angular shapes of Aeolodon priscus , Sericodon jugleri , Charitomenosuchus leedsi and Neosteneosaurus edwardsi , and the marked semi-circular apex of Pelagosaurus typus, Platysuchus multiscrobiculatus and Macrospondylus bollensis . In this way, the degree of convexity of the apex of Mycterosuchus nasutus is similar to those of Proexochokefalos cf. bouchardi and Lemmysuchus obtusidens , but at a smaller size.

In Mycterosuchus nasutus , the ischial suture is mainly oriented ventrally as in the other aeolodontines ( Aeolodon priscus and Sericodon jugleri ), and also as in Pelagosaurus typus, Proexochokefalos cf. bouchardi, Neosteneosaurus edwardsi , and hence is not clearly visible on the medial side of the ischial blade.

Pubis

The pubis of Mycterosuchus nasutus ( Figs 41 ; 42 ) is distinguished from that of other thalattosuchians in possessing a prominent posterior bulge or protuberance at the intersection between the distal blade and the lateral margin of the pubis (e.g. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Cricosaurus albersdoerferi , Cricosaurus bambergensis , Geosaurus giganteus , Lemmysuchus obtusidens , Machimosaurus , Macrospondylus bollensis , Neosteneosaurus edwardsi ). The taxa ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 also possesses a posterior protuberance but it is more subtle than in Mycterosuchus nasutus . Among dyrosaurids, only Cerrejonisuchus improcerus possesses a strongly protruding posterior protuberance.

The lateral and medial margins of the pubis of Mycterosuchus nasutus forming the entirety of the pubic apron are not symmetrical. Indeed, the medial margin of Mycterosuchus nasutus is concave throughout whereas its lateral margin is concave proximally but starts undulating distally, similar to what is observed in Suchodus durobrivensis and also Machimosaurus to a lesser extent. The lateral margin of Mycterosuchus nasutus starts to bulge (and thus forms an undulation) at the same time as the pubic apron expands distally. Hence, the bulge of the lateral margin marks the beginning of the distal flare out of the pubic plate similar to Suchodus durobrivensis but unlike Machimosaurus .

Overall, Mycterosuchus nasutus appears to possess a well-developed pubic symphysis, but its size relative to the total length of the pubis cannot be calculated. However, the intuition of a relatively long pubic symphysis for Mycterosuchus nasutus comes from the fact that the total length of the pubic symphysis (reconstructed as on Fig. 41 ) almost equals the remaining mediolateral length of the distal blade of the pubis. The junction between the medial margin of the pubis and the pubic symphysis of Mycterosuchus nasutus is unknown, but was presumably forming a right-angled corner as in other thalattosuchians with a well-developed pubic symphysis (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Lemmysuchus obtusidens , Machimosaurus , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , etc.). In Mycterosuchus nasutus , the pubic symphysis is set at an angle of approximately 27° with the median of the shaft (and upper portion of the pubic apron), which is close to the value of Charitomenosuchus leedsi but slightly lesser than the 30°of Lemmysuchus obtusidens and Neosteneosaurus edwardsi . Despite this, the inclination of the pubic symphysis of Mycterosuchus nasutus falls within the same overall range as Lemmysuchus obtusidens , Charitomenosuchus leedsi and Neosteneosaurus edwardsi .

The pubic symphysis of Mycterosuchus nasutus laterally transitions to the distal blade of the pubis through an angle of approximately 150°, which is greater than in Neosteneosaurus edwardsi , but is similar to Lemmysuchus obtusidens and especially Charitomenosuchus leedsi . The exact shape of the distal blade of Mycterosuchus nasutus is unknown but appears to have been convex based on the preserved portions. as in other thalattosuchians (e.g. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Geosaurus giganteus , Lemmysuchus obtusidens , Machimosaurus , Macrospondylus bollensis , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , etc.). The thickness of the distal blade of the pubis of Mycterosuchus nasutus in the anteroposterior direction is greater than that of the pubic symphysis ( Fig. 42 ), similar to ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Lemmysuchus obtusidens , and Neosteneosaurus edwardsi . Like in other thalattosuchians, the pubic apron of Mycterosuchus nasutus ( Fig. 42 ) is concave anterodorsally (e.g. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 3804, Lemmysuchus obtusidens , Macrospondylus bollensis , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , etc.).

Figures

FIG. 41. — Pelvic girdle elements of Mycterosuchus nasutus (Andrews, 1909): A, B, NHMUK PV R 2617, holotype; C, D, CAMSM J.1420 (referred); A, left pubis in anterior view; B, left pubis in posterior view; C, left ischium in medial view; D, left ischium in lateral view; E, left pubis in posterior view; F, left pubis in anterior view. Target indicates anterior. Cross indicates posterior. Pictures of CAMSM J.1420 courtesy of Dr. Michela Johnson. Scale bars: 1 cm.

FIG. 42. — Reconstruction of the pelvic girdle of Mycterosuchus nasutus (Andrews, 1909), NHMUK PV R 2617: A, right lateral view; B, anterior view; C, ventral view; D, dorsal view. Target indicates anterior. Arrow points anteriorly. The right pubis is mirrored. The ilium, ischium and first sacral are reconstructed from Sericodon jugleri Von Meyer, 1845, MJSN SCR010-312, and only serve as a qualitative representation to better highlight the remains of Mycterosuchus nasutus NHMUK PV R 2617. Scale bar: 5 cm.