Dakosaurus maximus, (PLIENINGER, 1846)

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 : 196-202

DAKOSAURUS MAXIMUS ( PLIENINGER, 1846)

For measurements, seeTables 7-9. The specimen SMNS 8203 is fragmentary and hence includes potential reconstructions.

Ilium

The ilium of Dakosaurus maximus ( Figs 32 ; 33 ) seems to display the typical triangular silhouette of metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , Thalattosuchus superciliosus , Cricosaurus suevicus , Cricosaurus albersdoerferi , etc.), conveying the absence of a postacetabular process.

The anterior margin of the ilium underneath the preacetabular process is straight as in Cricosaurus suevicus , Cricosaurus albersdoerferi , Suchodus durobrivensis , and Geosaurus giganteus . The bony acetabulum of Dakosaurus maximus forms a relatively marked hollow, extending up until about half the height of the ilium dorsoventrally as in other metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus , Cricosaurus suevicus , etc.). Moreover, the acetabulum of Dakosaurus maximus appears to be bordered anteriorly by a laterally prominent pubic peduncle-supraacetabular complex acting here as a physical barrier. Such a structure is also observed ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Tyrannoneustes lythrodectikos , Suchodus durobrivensis , and maybe Cricosaurus suevicus , but does not seem present in Thalattosuchus superciliosus . Comparatively, this pubic peduncle– supraacetabular complex is still less protruding laterally than the ischial peduncle is, which is a common relation found in all other crocodyliforms. The acetabular perforation of Dakosaurus maximus forms a shallow notch on the ventral margin of the ilium, and also marks the position of the ischial peduncle.

The sacral rib attachment sites on the medial side of the ilium are set in a way indicating that the position of the ilium in vivo was anteriorly tilted (i.e. the ventral margin of the ilium was not placed horizontally) similar to several thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , Tyrannoneustes lythrodectikos , Thalattosuchus superciliosus , Lemmysuchus obtusidens , Charitomenosuchus leedsi , etc.), but unlike extant crocodylians (e.g. Caiman crocodilus [ Fig. 9 ] or Mecistops cataphractus [ Fig. 8 ]) and dyrosaurids (e.g. Congosaurus bequaerti , Hyposaurus natator , Dyrosaurus maghribensis , Acherontisuchus guajiraensis ). The exact shape of the sacral rib attachment sites is uncertain, but they seem to have been distinct bilobate structures (at least along their ventral margins). The sacral rib attachment sites form deep imprints on the ilium of Dakosaurus maximus , similar to Thalattosuchus superciliosus but unlike the raised ones of Suchodus durobrivensis , Tyrannoneustes lythrodectikos , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763.

Ischium

The ischium of Dakosaurus maximus ( Figs 32 ; 33 ) stands out from that of most thalattosuchians in displaying the combination of a short and thick shaft and a short anterior process along with a dorsoventrally thick posterior process. Torvoneustes carpenteri also displays a short anterior process and thick posterior process, but its shaft is slightly more elongated.

The posterior peduncle of Dakosaurus maximus is large as its anteroposterior width almost reaches that of the shaft at its thinnest portion or constriction. Unfortunately, the posterior peduncle is partially trapped with sediments so the exact shape of its articular surfaces is unclear. In parallel, the posterior peduncle of Dakosaurus maximus does not significantly protrude dorsally from the shaft as in most thalattosuchians (e.g. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Geosaurus giganteus , Torvoneustes carpenteri , Tyrannoneustes lythrodectikos , Aeolodon priscus , Lemmysuchus obtusidens , Neosteneosaurus edwardsi, Proexochokefalos cf. bouchardi). This effect is due to the relative position of the base of the peduncle bridge, which is located near the base of the posterior peduncle in Dakosaurus maximus . Indeed, the peduncle bridge stems from the proximal edge of the ischium. Another consequence of this situation is the impression of an almost non-existent or reduced acetabular perforation on the lateral side of the bone, unlike in Macrospondylus bollensis and Charitomenosuchus leedsi which display a deeper acetabular perforation laterally. Presumably, the acetabular perforation of Dakosaurus maximus formed a titled burrow on the medial side of the bone like all other thalattosuchians displaying the same configuration (especially methriorhynchoids). The acetabular perforation is borne by the dorsal surface of the peduncle bridge, which is unfortunately ruptured shortly after the start of its anterior slimming.

The shaft as a whole is thick (anteroposteriorly longer than dorsoventrally high) similar to ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and Tyrannoneustes lythrodectikos , but unlike the more slender ones of Cricosaurus suevicus or Cricosaurus albersdoerferi . Torvoneustes carpenteri and Thalattosuchus superciliosus NHMUK PV R 2054 possess a slightly slender shaft than that of Dakosaurus maximus but not too markedly. The posterior and anterior margins of the ischium, constituting notably the shaft, are both concave with the anterior margin displaying the greatest intensity. However, the posterior margin of the shaft appears to become straight or slightly convex shortly after transitioning from the shaft to the posterior process. Hence, the ischium gradually widens ventrally to form the distal blade, which possesses a sharp anterior process and a relatively large one posteriorly. The distal blade of Dakosaurus maximus is incomplete but appears to have been relatively straight throughout. The margin of the distal blade forms an angle of approximately 58° with the median of the shaft like that of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, but which differs from the more erect ones of Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus bambergensis and Torvoneustes carpenteri .

The anterior process of the ischium of Dakosaurus maximus is in line with the rest of the ventral margin of the distal blade which contrasts with Thalattosuchus superciliosus NHMUK PV R 2054 and Pelagosaurus typus among metriorhynchoids. Compared to the posterior process, the anterior process of Dakosaurus maximus appears strongly reduced ( Fig. 32 ) as in Torvoneustes carpenteri , Cricosaurus araucanensis ; both the anteroposterior length and dorsoventral height (at its base) of the anterior process are markedly inferior to those of the posterior process (even partially incomplete). Whereas the anterior process is always anteroposteriorly shorter than the posterior process, the difference in dorsoventral height is usually less marked in most metriorhynchoids (e.g. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus NHMUK PV R 2054 , etc.). It is possible that this dissimilarity in the shape of the ischium reflects a difference in the muscles arrangement.

Similar to Thalattosuchus superciliosus NHMUK PV R 2054 and ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, the anterior process of Dakosaurus maximus is mediolaterally thicker than the rest of the distal blade and the junction of both ischia forms a platform.

The posterior process of Dakosaurus maximus is large and greatly exceeds the size of the anterior process, as in Cricosaurus araucanensis , Torvoneustes carpenteri and Lemmysuchus obtusidens . Indeed, the dorsoventral height of the base of the posterior process can fit more than twice that of the anterior process. The exact shape of the apex of the posterior process of Dakosaurus maximus is not preserved but presumably formed a blunt and rounded extremity similar to Cricosaurus araucanensis , Torvoneustes carpenteri and Lemmysuchus obtusidens . The shape of the posterior process is hypothetically reconstructed from the overall slope of the posterior margin of the ischium, which is more gentle than in Thalattosuchus superciliosus NHMUK PV R 2054 or ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763.

Pubis

The pubis of Dakosaurus maximus ( Fig. 32 ) is fragmentary and is missing most of its pubic plate. The extension of the pubic symphysis is therefore unknown and two distinct reconstructions are proposed on Fig. 32 . Comparatively, the closest taxa to Dakosaurus maximus – Suchodus durobrivensis and Geosaurus giganteus ( Young et al. 2020a) – possess extremely distinct pubic shapes. The peduncle of the pubis of Dakosaurus maximus is also poorly preserved, but appears to be larger than the thinnest portion of the shaft but not as markedly as in Cricosaurus suevicus or Geosaurus giganteus . Hence, the lateral and medial margins of the pubis of Dakosaurus maximus are both concave, with the lateral margin showing a slightly greater intensity of curvature. The portion extending from the base of the peduncle until the thinnest portion of the bone corresponds to the shaft. The shaft of the pubis of Dakosaurus maximus is strongly reduced in length and appears amongst the shortest of Thalattosuchia (e.g. Suchodus durobrivensis & Geosaurus giganteus ).

Figures

FIG. 32. — Left and right ilia of Dakosaurus maximus (Plieninger, 1846) Von Quenstedt, 1856, SMNS 8203: A, left ilium in lateral view; B, left ilium in medial view; C, left ischium in lateral view; D, left pubis in lateral view; E, left pubis in anterior view. Target indicates anterior. Arrow points anteriorly. Scale bar: 1 cm.

FIG. 33. — Pelvic reconstruction of Dakosaurus maximus (Plieninger, 1846), SMNS 8203: A, lateral view; B, anterior view; C, ventral view; D, dorsal view. Arrow points anteriorly. Target indicates anterior. The right pubis is mirrored. Scale bar: 5 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.