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 CRICOSAURUS ARAUCANENSIS ( GASPARINI & DELLAPÉ , 1976 ) For measurements, see Tables 7-9 . Ilium The ilium of Cricosaurus araucanensis ( Figs 16-18 ) stands out from that of other metriorhynchoids in displaying an almost isosceles triangular outline but with one side (corresponding to the dorsal border of the bone) disrupted by the shape of the sacral rib attachment sites. The latter form another distinctive trait of Cricosaurus araucanensis (and presumably other Cricosaurus Wagner, 1858 species : see Cricosaurus albersdoerferi and Cricosaurus bambergensis ) as they strongly protrude from the medial surface of the bone, hence impacting its outline in lateral view. FIG . 16. — Left ilium of Cricosaurus araucanensis ( Gasparini & Dellapé, 1976 ) , MLP 72-IV-7-1 (holotype): A , lateral view; B , medial view; C , dorsal view; D , anterior view. Arrow points anteriorly.Target indicates anterior. 3D models of Cricosaurus araucanensis ( MLP 72-IV-7-1), courtesy of Dr Yanina Herrera.Scale bar: 1 cm. The preacetabular process of Cricosaurus araucanensis is anteroposteriorly long and dorsoventrally thin and accounts for about 1/3 of the total height of the bone dorsoventrally similar to Cricosaurus suevicus . It is possible that the tip of the preacetabular process of Cricosaurus albersdoerferi and Cricosaurus bambergensis is broken, which could explain their relatively smaller size. The preacetabular process of Cricosaurus araucanensis is in line with the iliac crest as in Cricosaurus suevicus and Cricosaurus albersdoerferi . However, in Cricosaurus araucanensis the dorsal margin of the bone and the iliac crest do not coincide as it is the case in most metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Tyrannoneustes lythrodectikos , Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , Geosaurus giganteus , etc.). Indeed, the iliac crest of Cricosaurus araucanensis corresponds to a hollow rather than a ridge due to the protruding attachment sites ( Fig. 16 ). This posterior hollow forms a relatively straight line and culminates ventrally to form the posterior corner of the ischial peduncle. The ischial peduncle laterally protrudes from the ilium to constitute the posterior border of the bony acetabulum. The lateral facet of the ischial peduncle of Cricosaurus araucanensis displays the typical triangular shape found in other metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , Tyrannoneustes lythrodectikos ). The ventral surface of the ischial peduncle is wedge-shaped and slightly concave, similar to other thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus , Lemmysuchus obtusidens , Charitomenosuchus leedsi , Neosteneosaurus edwardsi ). Anteriorly, the ventral surface of the ischial peduncle transitions to the ventral surface of the pubic peduncle through a small indentation: the acetabular perforation. On the lateral surface of the ilium, the lateral facet of the ischial peduncle meets with that of the pubic peduncle without interruption. Comparatively, the lateral facet of the pubic peduncle is dorsoventrally shorter than that of the ischial peduncle similar to most thalattosuchians (with the exceptions of Pelagosaurus typus [ Fig. 10 ] and Neosteneosaurus edwardsi ). Ventrally, the articular surface of the pubic peduncle of Cricosaurus araucanensis is also wedge-shaped with its concavity laterally facing. The medial side of the ilium bears the sacral rib attachment sites for the sacral ribs. Those are located along the dorsal margin of the ilium and strongly protrude dorsally, hence markedly impacting the outline of the bone in lateral view ( Figs 16-18 ). Similar to other thalattosuchians, the sacral rib attachment sites of Cricosaurus araucanensis are adjacent and share a margin mesially (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , Thalattosuchus superciliosus , Dakosaurus maximus , Lemmysuchus obtusidens , Charitomenosuchus leedsi , Neosteneosaurus edwardsi ). The sacral rib attachment sites of Cricosaurus araucanensis are in relief with their surrounding margin forming a bulge. Hence, the sacral rib attachment sites of Cricosaurus araucanensis appear to protrude from the surface of the ilium, in the way of a wax seal on an envelope. The outline of the sacral rib attachment sites of Cricosaurus araucanensis are bilobate with the biggest lobe positioned ventrally, as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Tyrannoneustes lythrodectikos , Suchodus durobrivensis , and Thalattosuchus superciliosus NMI F 21731. Ischium The ischium of Cricosaurus araucanensis ( Fig. 17 ) displays a relatively thin shaft like other Cricosaurus species and is dorsoventrally longer than anteroposteriorly wide (e.g. Cricosaurus suevicus and Cricosaurus bambergensis ). Moreover, the shaft of Cricosaurus araucanensis appears relatively small compared to the overall size of the bone due to the position of the maximal constriction of the shaft closer to the proximal peduncle than the mid-height of the bone, similar to Dakosaurus maximus and Torvoneustes carpenteri among metriorhynchoids. This effect is partly due to the large size of the posterior process of the ischium of Cricosaurus araucanensis which increases the overall size of the bone. Indeed, the dorsoventral height of the posterior process accounts for about half of the total proximodistal height of the bone as in Dakosaurus maximus and Torvoneustes carpenteri . Some other thalattosuchians display an enlarged posterior process but are found mainly among Teleosauroidea (e.g. Aeolodon priscus , Sericodon jugleri , Lemmysuchus obtusidens ). Both the anterior and posterior margins of the ischium are concave, but with differing curvatures. The exact shape of the posterior process of the ischium of Cricosaurus araucanensis is unknown due to a partially preserved posterior margin, but appears to have been more similar to Torvoneustes carpenteri than to other Cricosaurus species due to its large size (e.g. Cricosaurus suevicus and Cricosaurus bambergensis ). Indeed, the posterior process of Cricosaurus araucanensis is as long anteroposteriorly as it is high dorsoventrally and thus contrasts with the more slender process of most metriorhynchoids (e.g. Pelagosaurus typus , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus bambergensis ). Due to the relative breadth of the posterior process of the ischium of Cricosaurus araucanensis , it is plausible that the apex of the process was more rounded than sharp as in Torvoneustes carpenteri and Lemmysuchus obtusidens . The ventral margin of the ischium of Cricosaurus araucanensis appears relatively straight, and extends anteriorly to form the anterior process. The anterior margin of the ischium is strongly concave, which contrasts with the posterior margin. The apex of the anterior process is not preserved but the hypothetical extensions of both the anterior and ventral margins result in a sharp and thin junction. Hence, the posterior and anterior process of the ischium of Cricosaurus araucanensis are strongly asymmetrical with the anterior process appearing overall reduced, not unlike Dakosaurus maximus and Torvoneustes carpenteri . The anterior peduncle of the ischium of Cricosaurus araucanensis is located more dorsally (or proximally) than the posterior peduncle, similar to Pelagosaurus typus , Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Cricosaurus albersdoerferi , Torvoneustes carpenteri , and Geosaurus giganteus . Some teleosauroids also possess a laterally reduced acetabular perforation but their anterior peduncle moderately protrudes dorsally: Aeolodon priscus , Proexochokefalos cf. bouchardi , Teleosaurus sp. Geoffroy Saint-Hilaire, 1825 , Lemmysuchus obtusidens , and Neosteneosaurus edwardsi . The anterior peduncle of Cricosaurus araucanensis displays an overall circular outline as in Thalattosuchus superciliosus NHMUK PV R 2054 but to a lesser extent. Indeed, the articular surface of the anterior peduncle of Cricosaurus araucanensis is greater than in Thalattosuchus superciliosus and other metriorhynchoids, resulting in the anterior peduncle being dorsoventrally thicker than the mid-section of the peduncle bridge. In addition, the anterior peduncle of Cricosaurus araucanensis is not entirely in line with the peduncle bridge so it does not point in the exact same direction; the anterior peduncle has a dorsal component in its orientation. FIG . 17. — Left ilium of Cricosaurus araucanensis ( Gasparini & Dellapé, 1976 ) , MLP 72-IV-7-1 (holotype) and right ischium and left pubis of MLP 72-II-27-6 (referred): A , left ilium of MLP 72-IV-7-1 in lateral view; B , left ischium of MLP 72-IV-7-1 in lateral view; C , left ischium of MLP 72-II-27-6 in medial view; D , left pubis of MLP 72-II-27-6 in anterior view. Arrow points anteriorly. Target indicates anterior. Pictures of Cricosaurus araucanensis ( MLP 72-IV-7-1 and MLP 72-II-27-6), courtesy of Dr Yanina Herrera. Scale bars: 1 cm. The peduncle bridge of the ischium of Cricosaurus araucanensis is relatively short and thick with its dorsal margin concave and its ventral margin convex as in other thalattosuchians. The peduncle bridge is connected to the shaft at the base of the posterior peduncle and thus obstructs the acetabular perforation laterally. For this reason, the posterior peduncle does not appear to protrude from the shaft at all which is similar to Cricosaurus suevicus and Cricosaurus albersdoerferi among metriorhynchoids. The base of the peduncle bridge is not centred on the bone but is shifted laterally, creating space medially. In addition, the peduncle bridge is curved medially to connect with the pubic peduncle of the ilium, creating additional space for the acetabular perforation. The posterior peduncle of Cricosaurus araucanensis is anteroposteriorly and mediolaterally larger than the anterior peduncle as it connects to the ilium dorsally. The posterior peduncle is composed of two distinct articular facets dorsally: the medial one which connects to the ilium, and the lateral one which borders the acetabulum ventrally. The medial facet is wedge-shaped and is oriented mediodorsally. The surface of the lateral facet is slightly concave, displays a relatively quadrangular shape and is larger than the medial facet (about 2/3 of the total surface). The acetabular perforation of the ischium of Cricosaurus araucanensis is greater than its counterpart on the ilium. As the peduncle bridge stems from the base of the posterior peduncle, the acetabular perforation appears nearly non-existent laterally. The medial curvature of the peduncle bridge leaves room for the acetabular perforation, which also creates a faint burrow on the medial side of the ischium at the base of the peduncle bridge. A similar configuration is found in other thalattosuchians displaying a reduced acetabular perforation laterally (e.g. ‘ Metriorhynchus ’ brachyrhynchus 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Torvoneustes carpenteri , Aeolodon priscus , Lemmysuchus obtusidens , Teleosaurus sp. , Proexochokefalos cf. bouchardi , Neosteneosaurus edwardsi , etc.). However, the hollow left by the acetabular perforation on the medial side of the bone appears less pronounced in Cricosaurus araucanensis , presumably due to the length and dorsal extension of the peduncle bridge which creates a large opening between the bones ( Fig. 18 ). Pubis The pubis of Cricosaurus araucanensis ( Fig. 17 ) possess a relatively long and thin shaft as in other Cricosaurus (i.e. Cricosaurus suevicus and Cricosaurus bambergensis ). Indeed, the maximum mediolateral constriction of the pubis marking the end of the shaft is located at about 36% of the total length of the pubis proximally. Comparatively, this value reaches around 40% in Cricosaurus suevicus and Cricosaurus bambergensis . Also, the mediolateral width at the constriction of the shaft of Cricosaurus araucanensis is lesser than that of the proximal peduncle. However, the latter does not account for about twice the length of the constriction, which differs from Cricosaurus bambergensis and Cricosaurus suevicus but to a lesser extent. The pubis of Cricosaurus araucanensis ( Fig. 17 ) drastically stands out from that of other Cricosaurus species in displaying a well-developed pubic symphysis and distal blade (i.e. Cricosaurus suevicus , Cricosaurus albersdoerferi , Cricosaurus bambergensis ). Indeed, the length of the pubic symphysis of Cricosaurus araucanensis constitutes about 34% of the total proximodistal height of the pubis, whereas this number reaches about 30% for Cricosaurus bambergensis and this number is estimated to be even less for Cricosaurus suevicus and Cricosaurus albersdoerferi ( Fig. 87 ). However, the pubic symphysis forms an angle of approximately 45° with the median of the shaft in both Cricosaurus araucanensis and Cricosaurus bambergensis . The junction between the pubic symphysis and the medial margin of the bone forms almost a right angle (about 100°), whereas the transition to the distal margin is achieved through an angle of approximately 144°. The overall shape of the pubic apron of Cricosaurus araucanensis also differs from that of other thalattosuchians due to the shape of the distal blade. Indeed, the distal blade is strongly convex with a relatively long focal length resulting in an almost hemispherical outline. As a result, the apex of the distal blade is set further distally than the lateral corner assuring the junction between the distal blade and the lateral margin, similar to ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Geosaurus giganteus , and Rhacheosaurus gracilis among metriorhynchoids ( Fig. 87 ). Comparatively, other Cricosaurus species show a more parabolic distal blade with the lateral corner being the more distal element ( Fig. 87 ). The medial margin of the pubis of Cricosaurus araucanensis is concave throughout whereas the lateral margin appears relatively straight for most of its length. The monotony of the lateral margin is ruptured around the mid-length of the pubis which marks the beginning of the mediolateral flaring of the pubic apron. From this point the lateral margin forms a sinusoidal wave and is first concave then convex distally. The sudden curvature of the lateral margin almost appears like a bent, and this feature is only found in Cricosaurus suevicus and Cricosaurus albersdoerferi among metriorhynchoids.