Platysuchus multiscrobiculatus, (BERCKHEMER, 1929)

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 : 216-220

PLATYSUCHUS MULTISCROBICULATUS ( BERCKHEMER, 1929)

For measurements, see Tables 7-9 .

Ilium

The ilium of Platysuchus multiscrobiculatus ( Fig. 44 ) displays an anteroposteriorly elongated shape due to the presence of a postacetabular process as in other telesauroids (e.g. Lemmysuchus obtusidens , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , Plagiophthalmosuchus gracilirostris , etc.). The postacetabular process of Platysuchus multiscrobiculatus differs from those of most teleosauroids in being relatively slender: the postacetabular process almost retains the same dorsoventral height throughout its anteroposterior length, giving it a tubular shape. Plagiophthalmosuchus gracilirostris and Macrospondylus bollensis display a relatively slender postacetabular process as well. Posteriorly, the postacetabular process of Platysuchus multiscrobiculatus culminates in a blunt hemispherical peak whose diameter is not significantly smaller than the overall dorsoventral height of the process (i.e. there is no major decrease in the size of the process posteriorly).

The dorsal margin of the postacetabular process is slightly undulating with a small depression around its mid-length. On either side of this recess, the dorsal margin of the postacetabular process is convex. Anteriorly, the postacetabular process gradually moves on to the iliac crest, which corresponds to the dorsal border of the ilium. The latter is convex throughout its length, with the vertex being shifted posteriorly towards the iliac crest-postacetabular process junction. The transition to the preacetabular process is marked by a shallow recess along the dorsal margin of the ilium.

The shape of the preacetabular process grossly mirrors that of the postacetabular process but at a smaller scale: overall, the preacetabular process is relatively elongated anteroposteriorly as well as dorsoventrally thick. Indeed, the dorsoventral height of the preacetabular is retained throughout its anteroposterior length. Also, the peak of the preacetabular process is rather smooth and rounded, but definitely sharper than that of the postacetabular process giving it the appearance of a Lancet arch. Such a combination of shape for the preacetabular and postacetabular processes differs from other thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , Thalattosuchus superciliosus , Cricosaurus suevicus , Geosaurus giganteus , Lemmysuchus obtusidens , Neosteneosaurus edwardsi , Charitomenosuchus leedsi , etc.). The preacetabular process of Platysuchus multiscrobiculatus is in line with the iliac crest and the postacetabular process as in other teleosauroids (e.g. Macrospondylus bollensis , Lemmysuchus obtusidens , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , Plagiophthalmosuchus gracilirostris , etc.). The global orientation of the ilium is uncertain since the position and shape of the sacral rib attachment sites of Platysuchus multiscrobiculatus are unknown. Hence it is unsure if it pointed anteroventrally as in Lemmysuchus obtusidens or more anteriorly as in Charitomenosuchus leedsi and Neosteneosaurus edwardsi .

The junction between the preacetabular process and the anterior margin of the ilium is achieved through a smooth acute angle due to the leaning anterior margin of the ilium, as in Lemmysuchus obtusidens contra Charitomenosuchus leedsi and Neosteneosaurus edwardsi . Indeed, the anterior margin of the ilium of Platysuchus multiscrobiculatus is anteriorly tilted starting from its junction with the preacetabular process. Overall, the anterior margin of the ilium displays a straight outline, and culminates ventrally in an almost hemispherical shape to form the pubic peduncle. In Lemmysuchus obtusidens and Neosteneosaurus edwardsi , this junction also forms a rounded corner, but is less marked.

The posterior margin of the ilium of Platysuchus multiscrobiculatus almost reaches the length of the anterior margin underneath the preacetabular process due to the shortness of the postacetabular process in the dorsoventral direction. Overall, the posterior margin of the ilium appears to be parallel to the anterior margin of the ilium and thus is anteriorly inclined. The junction between the ventral margin of the postacetabular process and the posterior margin of the ilium forms an angle closer to 90° than to an obtuse angle as in Plagiophthalmosuchus gracilirostris contra Lemmysuchus obtusidens , Charitomenosuchus leedsi and Neosteneosaurus edwardsi . The only difference between Platysuchus multiscrobiculatus and the other teleosauroids resides in the global orientation of the margins involved in the aforementioned junction.

The bony acetabulum of Platysuchus multiscrobiculatus ( Fig. 44 ) forms a shallow depression on the lateral surface of the ilium. As in most thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus , Cricosaurus suevicus , Geosaurus giganteus , Lemmysuchus obtusidens , Macrospondylus bollensis , Charitomenosuchus leedsi , etc.), the acetabulum is bordered by the pubic peduncle anteriorly, the ischial peduncle posteriorly, and the supraacetabular crest dorsally. Proportionally, the bony acetabulum occupies a large area of the ilium as in other crocodyliforms (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Tyrannoneustes lythrodectikos , Thalattosuchus superciliosus , Cricosaurus suevicus , Geosaurus giganteus , Lemmysuchus obtusidens , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , Macrospondylus bollensis , Congosaurus bequaerti , Hyposaurus natator , Dyrosaurus maghribensis , Acherontisuchus guajiraensis , etc.). But in Platysuchus multiscrobiculatus this effect is even more emphasized by the shape of the postacetabular process. The supraacetabular crest corresponds to a curved and prominent ridge, located ventrally to the preacetabular and postacetabular processes. Like in Lemmysuchus obtusidens , the supraacetabular crest of Platysuchus multiscrobiculatus extends over almost the totality of the anteroposterior length of the bony acetabulum.

Ischium

The ischium of Platysuchus multiscrobiculatus ( Fig. 44 ) stands out from that of most thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Cricosaurus bambergensis , Aeolodon priscus , Lemmysuchus obtusidens , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , etc.) in displaying an overall T-shape. Indeed, the combination of slender anterior and posterior processes, added to the perpendicularity between the distal blade and the shaft and the overall similar thickness of the anterior and posterior processes, give the ischium of Platysuchus multiscrobiculatus an atypical look. Cricosaurus bambergensis also possesses a perpendicular relation between its distal blade and shaft, and Pelagosaurus typus displays similarly slender anterior and posterior processes almost perpendicular to the shaft.

The anterior and posterior margins of the ischium connecting the proximal peduncles with the distal blade are both strongly concave similar to other teleosauroids (e.g. Lemmysuchus obtusidens , Charitomenosuchus leedsi , Macrospondylus bollensis , Neosteneosaurus edwardsi ). In Platysuchus multiscrobiculatus , the vertex of each margin are relatively facing each other similar to Lemmysuchus obtusidens , Macrospondylus bollensis , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , and also Cricosaurus bambergensis and Cricosaurus araucanensis (but not Cricosaurus suevicus ). The position of the vertex of each concavity also marks the end of the shaft, together forming maximum constriction. In Platysuchus multiscrobiculatus , the maximum concavity of the anterior and posterior margins is set proportionally far from the peduncles, resulting in a long shaft (about 44% of the total dorsoventral height of the bone), adding to the distinctive shape of the bone. Moreover, another particularity of the ischium ( Fig. 44 ) resides in the thickness of the shaft: the anteroposterior length of the shaft reaches about 90% of its dorsoventral length, giving it a stout look.

Distally, the anterior margin of the ischium culminates in a relatively sharp peak: the anterior process of the ischium. The anterior process of Platysuchus multiscrobiculatus does not appear to protrude much anteriorly; indeed, the anteroposterior length of the anterior process is inferior to that of the shaft as opposed to Lemmysuchus obtusidens , for which it is slightly greater. Comparatively, Aeolodon priscus and ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 also possess a relatively short anterior process. The axis of the anterior process of Platysuchus multiscrobiculatus is parallel to the margin of the distal blade, as in Lemmysuchus obtusidens as opposed to Aeolodon priscus . This effect is imputable to the shape of the ventral margin of the anterior process which is aligned with the ventral margin of the distal blade. Hence, there is no rupture along the distal blade of the ischium in Platysuchus multiscrobiculatus , which forms a straight border ventrally. Posteriorly, the distal blade moves on to build the posterior process of the ischium. The latter largely contributes to the T-shape of the ischium due to its narrow configuration. Indeed, the posterior process of the ischium takes a shape similar to that of the postacetabular process of the ilium of Platysuchus multiscrobiculatus : its dorsoventral thickness is almost constant throughout its anteroposterior length. Moreover, the posterior process of the ischium is not significantly taller than the anterior process (only about 1/3 taller dorsoventrally), giving a certain continuity in the appearance of the T-bar. The peak of the posterior process of the ischium of Platysuchus multiscrobiculatus is hemispherical. Comparatively, Macrospondylus bollensis , Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 and Cricosaurus bambergensis show similarly shaped posterior processes.

The posterior peduncle of the ischium of Platysuchus multiscrobiculatus appears to be significantly shorter than the shaft at its shortest anteroposterior portion, as in Macrospondylus bollensis , Charitomenosuchus leedsi , Pelagosaurus typus, Cricosaurus suevicus or Thalattosuchus superciliosus NHMUK PV R 2054 .

Pubis

The pubis of Platysuchus multiscrobiculatus ( Fig. 44 ) is very distinctive among teleosauroids as it shows both a relatively long shaft and flared pubic apron. Comparatively, Machimosaurus also display a long shaft but with a narrow pubic apron, whereas Lemmysuchus obtusidens ( Fig. 59 ) possesses a large pubic apron for a shorter shaft. Charitomenosuchus leedsi and Neosteneosaurus edwardsi also have a moderately long shaft with a wide pubic plate like that of Platysuchus multiscrobiculatus , but the distal shapes are different. In Platysuchus multiscrobiculatus , the thinnest portion of the shaft mediolaterally almost accounts for about half of the mediolateral width of the pubic peduncle, similar to several other thalattosuchians (e.g. Cricosaurus suevicus [ Fig. 19 ], Geosaurus giganteus [ Fig. 30 ], Machimosaurus , Lemmysuchus obtusidens , Charitomenosuchus leedsi , Neosteneosaurus edwardsi ). In comparison, this value is even lesser for Macrospondylus bollensis and Pelagosaurus typus.

The pubic peduncle of Platysuchus multiscrobiculatus is flared proximally. In relation to the median of the shaft, the pubic peduncle is relatively asymmetrical with its medial half being larger than the lateral one. Indeed, the lateral margin of the pubis leading to the peduncle shows a line break in the smooth overall concavity which decreases the size of the peduncle laterally.

The lateral and medial margins of the pubis of Platysuchus multiscrobiculatus are overall concave, with the medial margin of the bone showing several undulations rupturing the harmony of the curve similar to what is observed in Machimosaurus . The distal blade of the pubis drastically flares out from the narrow width of the shaft. The exact shape of the distal blade is uncertain, but the distal blade appears to have been relatively asymmetrical.Indeed, the pubic apron is wider medially than laterally compared to the median of the shaft. In addition, the pubic blade displays a relatively rounded but asymmetrical appearance as it is more expanded medially.

Figures

FIG. 44. — Right and left pubes of Platysuchus multiscrobiculatus (Berckhemer, 1929), SMNS 9930 (holotype): A, right ischium in lateral view and left ischium in medial view; B, right pubis in anterior view; C, right ilium in lateral view. Arrow points anteriorly. Target indicates anterior. Scale bar: 1 cm.

FIG. 59. — Pelvic girdle elements of Lemmysuchus obtusidens (Andrews, 1909), NHMUK PV R 3168 (holotype): A, left ilium in lateral view; B, left ilium in medial view; C, left pubis in lateral view; D, left ischium in lateral view; E, left pubis in anterodorsal view; F, right ischium in lateral view; G, surimposition of both ischia (right and left) of Lemmysuchus obtusidens, NHMUK PV R 3168. Target indicates anterior. Arrow points anteriorly. Scale bar: 1 cm.

FIG. 19. — Pelvic girdle elements of Cricosaurus suevicus (Fraas, 1901), SMNS 9808 (lectotype); the left pelvic girdle elements are relatively complete, with only the femur missing the top of its proximal head (reconstructed in red): A, overview of the pelvic girdle; B, right ilium in lateral view; C, left pubis in anterior view; D, left ilium in lateral view; E, left ischium in lateral view. Target indicates anterior. Arrow points anteriorly. Scale bars: 1 cm.

FIG. 30. — Pelvic girdle elements of Geosaurus giganteus (Von SÖmmerring, 1816), NHMUK PV R 1230: A, right pubis anterior;B, left ilium in lateral view; C, right femur in ventral view; D, right ischium in lateral view. Arrow points anteriorly. Target indicates anterior. Scale bar: 1 cm.