Geosaurus giganteus, VON SÖMMERRING, 1816

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 : 194-196

GEOSAURUS GIGANTEUS VON SÖMMERRING, 1816

For measurements, see Tables 7-9 .

Ilium

The ilium of Geosaurus giganteus ( Figs 30 ; 31 ) almost takes the shape of an isosceles trapezoid. Indeed, the anterior and posterior margins of the bone appear to display a similar angular relation with the ventral margin of the bone, and the latter seems parallel to the dorsal margin of the ilium. Proportionally, the ilium of Geosaurus giganteus ( Fig. 30 ) is almost as large as the pubis, which is a trait also seen in Suchodus durobrivensis . The preacetabular process of Geosaurus giganteus is sharp but extremely reduced, unlike other metriorhynchoids (e.g.‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , Thalattosuchus superciliosus , Cricosaurus suevicus , Cricosaurus albersdoerferi , Tyrannoneustes lythrodectikos , etc.). Geometrically, the preacetabular process of Geosaurus giganteus is in line with the dorsal margin of the ilium, as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Tyrannoneustes lythrodectikos , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Cricosaurus albersdoerferi . Underneath the preacetabular process, on the lateral surface of the ilium, runs a subtlety protruding arch which constitutes the supraacetabular crest. The supraacetabular crest borders the bony acetabulum dorsally. The latter forms a shallow depression, extending over most of the ilium. Posteriorly, the bony acetabulum is bordered my the ischial peduncle, which is similarly faintly protruding.

The anterior margin of the ilium underneath the preacetabular process shows a subtle concavity over most of its length, which becomes straight or slightly convex where the pubic peduncle starts. The pubic peduncle of does not appear to protrude anteriorly, as in Thalattosuchus superciliosus , Cricosaurus suevicus , Cricosaurus albersdoerferi , and Dakosaurus maximus . On the lateral side of the ilium, the pubic peduncle shows a dorsal extension of about one fourth of the total dorsoventral height of the bone. Also, both the dorsal and ventral margins of the pubic peduncle are undulated. The ischial peduncle is fragmentary but forms the posteroventral corner of the ilium. The posterior margin of the ilium of Geosaurus giganteus appears slightly concave similar to the anterior margin. The junction between the dorsal and posterior margin is achieved through a smooth rounded corner.

Ischium

The ischium of Geosaurus giganteus ( Figs 30 ; 31 ) is fragmentary. The anterior peduncle is rounded and appears reduced like that of other metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , Cricosaurus suevicus , etc.). The peduncle bridge is borne at the extremity of the peduncle bridge, which is formed by two slightly concave margins dorsally and ventrally. Dorsally, at the junction between the peduncle bridge and the posterior peduncle is a notch corresponding to the acetabular perforation. The posterior peduncle of the ischium of Geosaurus giganteus does not extends further dorsally than the anterior peduncle.

The anterior margin of the ischium is strongly concave and takes the shape of a Lancet arch rather than a hemispherical parabola as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Tyrannoneustes lythrodectikos , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Torvoneustes carpenteri , Thalattosuchus superciliosus NHMUK PV R 2054 , and Cricosaurus albersdoerferi . Among teleosauroids, Aeolodon priscus NHMUK PV R 1086 and Charitomenosuchus leedsi also show relatively narrow anterior concavity as in Geosaurus giganteus . The anterior margin of the ischium ends ventrally into a sharp peak constituting the anterior process of the ischium.The latter is formed by a curved (concave) margin dorsally and a more straight one ventrally, and points in the same direction as the anterior peduncle (i.e. anterodorsally).

The shape and orientation of the distal blade of the ischium is however unknown. Anteriorly, the anterior process of the ischium extends slightly further than the anterior peduncle, unlike in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 or Thalattosuchus superciliosus NHMUK PV R 2054 .

Pubis

The pubis of Geosaurus giganteus ( Figs 30 ; 31 ) is highly symmetrical lateromedially, unlike that of other thalattosuchians (i.e.‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Cricosaurus albersdoerferi , Lemmysuchus obtusidens , Neosteneosaurus edwardsi , Macrospondylus bollensis , etc.). Hence, the pubis of Geosaurus giganteus appears more similar to that of extant crocodylians (e.g. Mecistops cataphractus [ Fig. 8 ], Caiman crocodilus [ Fig. 9 ]) due to almost identical lateral and medial margins. This effect is notably imputable to the reduction of the pubic symphysis, but is here emphasized by the decrease in size of the pubic apron mediolaterally. Indeed, in Cricosaurus suevicus and Cricosaurus albersdoerferi , the pubic plate shows a larger pubic apron mediolaterally for a similarly reduced pubic symphysis leading to a mediolaterally unbalanced pubic apron.

The shape of the distal blade, uniting the lateral and medial margin of the pubis is markedly convex and almost hemispherical due to the symmetry of the bone. Its junction with the medial margin is achieved through a short straight surface constituting the pubic symphysis. The latter is practically parallel to the median of the pubic shaft. The latter constitutes almost half of the total height of the pubis. Proximally, the shaft of the pubis flares out to form the pubic peduncle, which is almost twice as large lateromedially as the thinnest portion of the shaft, similar to Cricosaurus suevicus .

Figures

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.

FIG. 31. — Pelvic reconstruction of Geosaurus giganteus (Von SÖmmerring, 1816), NHMUK PV R 1230; iIlium and pubis have been repaired; ischium has been reconstructed based on NHMUK PV R 4763: 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: 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.

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.