Form and function of the pelvic girdle of Thalattosuchia and Dyrosauridae (Crocodyliformes) Author Scavezzoni, Isaure Universite de Liège, Evolution and Diversity Dynamics Lab, All. du Six Août 14, 4000 Liège (Belgique) isaure. scavezzoni @ gmail. com v. fischer @ uliege. be isaure.scavezzoni@gmail.com Author Fischer, Valentin Universite de Liège, Evolution and Diversity Dynamics Lab, All. du Six Août 14, 4000 Liège (Belgique) isaure. scavezzoni @ gmail. com v. fischer @ uliege. be v.fischer@uliege.be Author Johnson, Michela M. Department of Palaeontology, Staatliches Museum für Naturkunde Stuttgart, Museum am LÖwentor, Rosenstein 1, 70191 Stuttgart (Germany) michela. johnson @ smns-bw. de michela.johnson@smns-bw.de Author Jouve, Stéphane Sorbonne Universite, BUPMC - Pôle Collections, Tour Zamansky, 15 étage, bureau 1513, 4 place Jussieu, 75252 Paris Cedex 05 (France) stephane. jouve @ sorbonne-universite. fr stephane.jouve@sorbonne-universite.fr text Geodiversitas 2024 2024-05-02 46 6 135 326 https://sciencepress.mnhn.fr/sites/default/files/articles/pdf/g2024v46a6.pdf journal article 10.5252/geodiversitas2024v46a6 1638-9395 11106598 urn:lsid:zoobank.org:pub:6ACF6A79-9149-4781-808D-478668673EB6 AEOLODON PRISCUS ( VON SÖMMERRING , 1814 ) For measurements, see Tables 7-9 . The specimenNHMUK PV R 1086 is a juvenile, whereas the specimen MNHN .F.CNJ78 corresponds to a more mature individual. Differences in shape are presumably related to ontogeny. Ischium The ischium of Aeolodon priscus NHMUK PV R 1086 ( Fig.37 ) displays a relatively short and thick shaft, whereas it appears more slender and elongated in MNHN .F.CNJ78 ( Fig. 38 ). In comparison, Macrospondylus bollensis also displays a thick shaft. The anterior and posterior margins of the ischium of Aeolodon priscus NHMUK PV R 1086 , notably constituting the shaft, are both concave. However, the posterior margin of the ischium switches to convex at about its mid-length as it forms the posterior process of the ischium. The posterior process of Aeolodon priscus NHMUK PV R 1086 is relatively thick as its dorsoventral height accounts for roughly half of the total height of the bone. The posterior process of NHMUK PV R 1086 is dorsoventrally thicker than that of MNHN .F.CNJ78 due to differences in the concavity of their posterior margin. In comparison, Sericodon jugleri displays a relatively thick posterior process, whose extremity is slightly dorsoventrally thicker than Aeolodon priscus MNHN.F.CNJ 78, but not as broad as in NHMUK PV R 1086. Another factor adding to the relative thickness (i.e. almost 1/2) of the posterior process of Aeolodon priscus NHMUK PV R 1086 is the angular relation between the ventral margin of the ischium and the shaft. The ventral margin is almost perpendicular to the median of the shaft, whereas this angular relation is slightly greater for MNHN .F.CNJ78 thus increasing its total dorsoventral height and giving it a more slender appearance. In MNHN .F.CNJ78, the posterior margin of the ischium displays a more subtle concavity but with a greater amplitude. Hence, the posterior margin of the ischium of Aeolodon priscus MNHN.F.CNJ 78 appears almost entirely concave, and culminates posteriorly to form a dorsoventrally thinner posterior process compared to Aeolodon priscus NHMUK PV R 1086 . FIG . 36. — Pelvic girdle elements and overview of Plagiophthalmosuchus gracilirostris ( Westphal, 1961 ) , NHMUK PV OR 14792 (holotype): A , right ilium in dorsal view; B , right ilium in lateral view; C , overview of Plagiophthalmosuchus gracilirostris . Arrow points anteriorly. Scale bars: A, B, 1 cm; C, 5 cm. FIG . 37. — Pelvic girdle elements of Aeolodon priscus (Von SÖmmerring, 1815), NHMUK PV R 1086 (holotype): A , right ischium in medial view; B , left ischium in lateral view; C , overview of Aeolodon priscus . Scale bars: 1 cm. In Aeolodon priscus , the ventral margin of the ischium is straight over most of its surface. Unlike inMNHN.F.CNJ78, the linearity of the margin of Aeolodon priscus NHMUK PV R 1086 is ruptured by a bent of about 160°located where the anterior process starts (i.e. the portion of the bone anterior and ventral to the thinnest portion of the shaft), similar to the metriorhynchoid Thalattosuchus superciliosus . The anterior process of Aeolodon priscus is a sharp bony process constituted by a ventral straight margin and a slightly curved (concave) dorsal one, as in most teleosauroids (like in Teleosaurus sp. [Geoffroy Saint Hilaire 1825], Macrospondylus bollensis , and Proexochokefalos cf. bouchardi ). It points anterodorsally as in other thalattosuchians (i.e. Thalattosuchus superciliosus , Cricosaurus albersdoerferi , Lemmysuchus obtusidens , etc.). The anterior process of Aeolodon priscus ( Fig. 37 ) does not appear to protrude anteriorly further than the anterior peduncle, as in some metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 and Thalattosuchus superciliosus NHMUK PV R 2054 ). The anterior margin of the ischium of Aeolodon priscus ( Figs 37 ; 38 ) is concave underneath the peduncles. The degree of concavity of the anterior margin in Aeolodon priscus NHMUK PV R 1086 is comparable to that of Geosaurus giganteus and thus also takes the shape of a Lancet arch. Dorsally, the anterior margin culminates to form the anterior peduncle but this portion is partially obstructed. The dorsal portion of the anterior peduncle and the peduncle bridge of Aeolodon priscus NHMUK PV R 1086 are visible on the right ischium, and the outline of the anterior peduncle appears to be circular in medial view, as in other thalattosuchians (i.e. Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus albersdoerferi , Lemmysuchus obtusidens , Macrospondylus bollensis , Proexochokefalos cf. bouchardi , Teleosaurus sp. , etc.). The dorsal border of the peduncle bridge appears relatively straight or faintly concave. Globally, the anterior peduncle appears short, especially in relation to the posterior peduncle as in Sericodon jugleri but unlike in Cricosaurus suevicus . A small notch marks the transition between the anterior and posterior peduncles; it corresponds to the acetabular perforation. The latter is smaller than in Sericodon jugleri and Macrospondylus bollensis , but appears to have the same mediolateral orientation which is uncommon in teleosauroids. The posterior peduncle sticks out from the rest of the shaft as it protrudes in all directions. Still, the shaft of the ischium of Aeolodon priscus is not significantly longer anteroposteriorly than the posterior peduncle unlike in Lemmysuchus obtusidens , and thus resembles those of Neosteneosaurus edwardsi , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Torvoneustes carpenteri , Tyrannoneustes lythrodectikos , Thalattosuchus superciliosus NHMUK PV R 2054 , or even Dakosaurus maximus . The posterior peduncle is located more ventrally than the anterior peduncle, which overhangs the ischium dorsally as in other thalattosuchians. On the medial side of the ischia of Aeolodon priscus NHMUK PV R 1086 , there is a crescent-shaped area underneath the anterior peduncle extending as far ventrally as the base of the shaft (i.e. anteroposterior narrowing) and as far posteriorly as the beginning of the posterior peduncle. The latter displays a drastic slimming down mediolaterally from the surrounding areas. Such a hollowed area on the medial side of the ischium is commonly found in thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus albersdoerferi , Torvoneustes carpenteri , Neosteneosaurus edwardsi , Lemmysuchus obtusidens , etc.). On the medial side of the bone, the distal blade of Aeolodon priscus NHMUK PV R 1086 ( Fig. 37 ) is deeply scarred with perpendicular grooves as in other thalattosuchians. This region presumably bore cartilaginous structures in vivo assuring, most notably, the connection between both ischia.