Cricosaurus suevicus, (FRAAS, 1901)

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 : 174-176

CRICOSAURUS SUEVICUS ( FRAAS, 1901)

For measurements, see Tables 7-9 .

Ilium

The ilium of Cricosaurus suevicus ( Fig. 19 ) displays an overall triangular outline, similar to other metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , Thalattosuchus superciliosus , Cricosaurus albersdoerferi , Dakosaurus maximus , Geosaurus giganteus ). The orientation of the preacetabular process of Cricosaurus suevicus is almost in line with the dorsal margin of the ilium as in Cricosaurus albersdoerferi , but also ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , Thalattosuchus superciliosus (NHMUK PV R 2054 and NMI F21731), or Geosaurus giganteus . The preacetabular process is welldeveloped as it reaches about the anteroposterior length of the pubic peduncle. Comparatively, among metriorhynchoids, the preacetabular process of Suchodus durobrivensis , Thalattosuchus superciliosus , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Tyrannoneustes lythrodectikos , Cricosaurus albersdoerferi and Geosaurus giganteus are proportionally shorter than that of Cricosaurus suevicus , and only ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 exceeds its length. The preacetabular process of Cricosaurus suevicus is also stout with its consequent dorsoventral thickness. This trait is also recovered in Cricosaurus albersdoerferi , and to a lesser extent in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Tyrannoneustes lythrodectikos and Thalattosuchus superciliosus NHMUK PV R 2054 . The mediolateral thickness of the preacetabular process appears to reach its climax towards its base; there the lateral surface of the ilium gradually sinks towards the centre of mass of the bone (where the acetabulum lies). This elevated area on the lateral side of the ilium underneath the preacetabular process corresponds to the supraacetabular crest. Like in other metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , Thalattosuchus superciliosus , Dakosaurus maximus , etc.), the supraacetabular crest of Cricosaurus suevicus forms a thick prominent dome bordering the acetabulum dorsally.

The anterior margin of the ilium underneath the preacetabular process is relatively straight (notably due to a smooth transition between the anterior margin and the pubic peduncle), as in Cricosaurus albersdoerferi , Suchodus durobrivensis , Dakosaurus maximus , and Geosaurus giganteus . The junction between the anterior margin of the ilium and the pubic peduncle is smooth (like in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , Tyrannoneustes lythrodectikos or Geosaurus giganteus ) and does not form a bump nor a depression unlike in Thalattosuchus superciliosus .

The ventral margin of the ilium of Cricosaurus suevicus is undulating and hence marks the position of each peduncle. Similar to other thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , Thalattosuchus superciliosus , Dakosaurus maximus , Geosaurus giganteus , Lemmysuchus obtusidens ), the dorsal border of the acetabular perforation is found underneath the second posterior half of the pubic peduncle, where the ventral margin of the ilium is concave. The articular surfaces of the peduncles form a continuum, which fills almost half of the height acetabulum dorsoventrally. The posterior-most portion of the ischial peduncle protrudes laterally in order to form the posterior border of the bony acetabulum and act as a physical obstacle. The ventral margin of the ischial peduncle is located more dorsally than that of the pubic peduncle, similar to other thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , Thalattosuchus superciliosus , Dakosaurus maximus , Geosaurus giganteus , Tyrannoneustes lythrodectikos , Lemmysuchus obtusidens , Macrospondylus bollensis , etc.). In Cricosaurus suevicus and Cricosaurus albersdoerferi , there is a smooth transition between the dorsal and posterior margins of the ilium, so that the entire surface extending from the tip of the preacetabular process up until the ischial peduncle appears convex.

Ischium

The ischium of Cricosaurus suevicus ( Fig. 19 ) is similar to other thalattosuchians in possessing well-developed anterior and posterior processes (e.g. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , Cricosaurus bambergensis , Dakosaurus maximus , Macrospondylus bollensis , Lemmysuchus obtusidens , Aeolodon priscus , Neosteneosaurus edwardsi , Teleosaurus sp. , Proexochokefalos cf. bouchardi, etc.).

In Cricosaurus suevicus , the anterior peduncle is smaller than the posterior peduncle but not as drastically as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , Torvoneustes carpenteri . Indeed, the dorsoventral height of the anterior peduncle reaches about half of the anteroposterior length of the posterior peduncle. Like in other metriorhynchoids, the anterior peduncle of Cricosaurus suevicus is ovoid in shape, and is located at the extremity of a relatively long peduncle bridge (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , Geosaurus giganteus , Torvoneustes carpenteri ). Contrary to ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , Torvoneustes carpenteri , and Cricosaurus araucanensis , the peduncle bridge of Cricosaurus suevicus is dorsoventrally as thick as the anterior peduncle, and its thickness remains constant throughout its length as in Geosaurus giganteus . This effect is imputable to the dorsal margin of the peduncle bridge which is concave and medially facing so that its lateral rim is situated more dorsally than the medial one, as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Cricosaurus araucanensis , and Dakosaurus maximus . Also, the peduncle bridge of Cricosaurus suevicus is not centred as its base is closer to the lateral side of the ischium. In addition, the peduncle bridge as a whole is curved towards the medial side of the ischium to contact the ilium medially.

The tilted dorsal surface of the peduncle bridge, coupled with its laterally position and medial curvature forms an opening for the acetabular perforation.

Similar to Dakosaurus maximus , Cricosaurus araucanensis , Torvoneustes carpenteri and Cricosaurus albersdoerferi , the posterior peduncle of Cricosaurus suevicus does not excessively protrude from the shaft of the ischium since its articular facet is located almost on the same level as the acetabular perforation, and as its dorsoventral length does not extend further posteriorly than that of the shaft (i.e. the dorsal margin of the ischium is relatively straight and perpendicular to the articular facet of the posterior peduncle). The articular facet of the posterior peduncle forming the ventral border of the bony acetabulum is slightly concave, however the other side connecting to the ilium is not recovered. The indentation for the acetabular perforation is shallow on the lateral side of the ischium.

The shaft of the ischium is located directly ventrally to the peduncles and is identifiable as the portion of the bone extending from the base of the peduncles up until the anteroposterior constriction of the bone (see Material and methods). The anterior and posterior margins of the ischium underneath the peduncles, which form the shaft, are both markedly concave as in Cricosaurus bambergensis and Dakosaurus maximus . Still, the shaft of Cricosaurus suevicus is proportionally thick since the anteroposterior length of this section exceeds that of the posterior peduncle (here by about 1/2), which is a trait found in other thalattosuchians regardless of the degree of concavity of the anterior and posterior margins (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Torvoneustes carpenteri , Thalattosuchus superciliosus , Dakosaurus maximus , Geosaurus giganteus , Lemmysuchus obtusidens , Macrospondylus bollensis , etc.).

In Cricosaurus suevicus the anterior process of the ischium is not recovered. In parallel, the shape of the posterior process of the left ischium of Cricosaurus suevicus is very characteristic: its dorsal margin is highly sinusoidal starting from its junction with the neck of the ischium, and its extremity is therefore thick and rounded. From what is preserved of the right ischium, it appears that the posterior process apex was elliptic as in Lemmysuchus obtusidens and Torvoneustes carpenteri (and presumably Dakosaurus maximus ), with a relatively flat distal blade ventrally.

Pubis

The pubis of Cricosaurus suevicus ( Fig. 19 ) is closer to that of extant crocodylians (e.g. Caiman crocodilus [ Fig. 9 ] or Mecistops cataphractus [ Fig. 8 ]) than to that of most thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus , Dakosaurus maximus , Lemmysuchus obtusidens , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , etc.) in possessing a relatively reduced pubic symphysis and subsequent extended distal blade. This feature is found in other highly pelagic forms among Thalattosuchia, namely rhacheosaurines (e.g. Cricosaurus albersdoerferi , Cricosaurus bambergensis ) and some geosaurines (e.g. Geosaurus giganteus ). However, the species Cricosaurus araucanensis shows a larger pubic symphysis and thus differs from other Cricosaurus species. Overall, the pubis of Cricosaurus suevicus is large, especially in relation to the ilium as in Cricosaurus albersdoerferi and Geosaurus giganteus where the mediolateral breadth of the pubic plate exceed the anteroposterior length of the ilium.

The lateral and medial margin of the pubis of Cricosaurus suevicus are both strongly concave, but with differing intensity and position of their vertex.As a consequence, the shaft displays a rather undulating appearance with asymmetrical margins. Indeed, the medial portion of the shaft appears almost straight as it is subtlety concave, whereas the lateral margin is intensely titled towards the thinnest portion of the bone and the vertex of the lateral concavity. The asymmetrical appearance of the shaft is also recovered in the pubic peduncle whose centre is shifted laterally. In addition, the pubic peduncle is twice as large mediolaterally as the thinnest portion of the shaft is, as in Geosaurus giganteus , Pelagosaurus typus or Macrospondylus bollensis . Suchodus durobrivensis also displays a pubic peduncle larger than its shaft, but the difference is more subtle.

The substantial degree of concavity of the lateral and medial margins of the pubis encompass the important mediolateral enlargement of the pubic apron. The distal margin of the pubis of Cricosaurus suevicus – i.e. the distal blade – is convex with its vertex shifted medially, giving it a highly asymmetrical look. Thalattosuchian pubes are usually convex between the lateral margin and the pubic symphysis e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus , Dakosaurus maximus , Geosaurus giganteus , Lemmysuchus obtusidens , Macrospondylus bollensis , etc.), but the relative shortness of the distal blade in some taxa presumably reduces the possibility of asymmetry. The junction between the distal blade and the medial margin of the pubis is achieved through a smooth round corner corresponding to the pubic symphysis.

Figures

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. 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.