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 PROEXOCHOKEFALOS CF . BOUCHARDI ( SAUVAGE , 1872 ) For measurements, see Tables 7-9 . The bone referred to the pubis of Proexochokefalos cf. bouchardi MJSN SCR010- 374 in Schaefer et al. (2018) is not included in our description. Indeed, the bone displays all the characteristics of a derived teleosauroid humerus: shortening of humeral shaft; posterior deflection of the articular proximal head; distal torsion of the condyles in relation to the coronal plane; continuous surface between the deltopectoral crest and the proximal articular head. Ilium The ilium of Proexochokefalos cf. bouchardi ( Figs 50 ; 51 ) stands out from that of other teleosauroids with the bulged shape of its preacetabular process and dorsal margin as a whole. Indeed, the dorsal margin of the ilium of Proexochokefalos cf. bouchardi is almost entirely convex, with a shallow recess located at 2/3 of its length posteriorly rather than at the base of the preacetabular process as in other teleosauroids (e.g. Platysuchus multiscrobiculatus , Macrospondylus bollensis , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , Lemmysuchus obtusidens ). As a result, the preacetabular process of Proexochokefalos cf. bouchardi is dorsoventrally thick, and the area of the ilium extending from the supraacetabular crest to the dorsal margin is enlarged compared to other teleosauroids. Still, the preacetabular process of Proexochokefalos cf. bouchardi rapidly slims down anteriorly to form a sharp apex, thanks to its strongly concave ventral margin. Like in Charitomenosuchus leedsi , Neosteneosaurus edwardsi , and Lemmysuchus obtusidens , the preacetabular process of Proexochokefalos cf. bouchardi is, in general, oriented anteriorly and is mediolaterally thin. FIG . 49. — Pelvic reconstruction of Charitomenosuchus leedsi ( Andrews, 1909 ) , NHMUK PV R 3806: 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: 5 cm. In comparison, the postacetabular process of Proexochokefalos cf. bouchardi possesses a dorsal component in its orientation, so that its apex is oriented dorsoventrally as in Sericodon jugleri , Charitomenosuchus leedsi , and Neosteneosaurus edwardsi . The postacetabular process of Proexochokefalos cf. bouchardi takes the shape of a Lancet arch as in most teleosauroids (except Macrospondylus bollensis and Platysuchus multiscrobiculatus ), despite possessing a slightly concave dorsal margin. The posterior margin of the ilium of Proexochokefalos cf. bouchardi forms a symmetrical sine curve, with a convex portion dorsally switching to a more concave shape ventrally at about its mid length. The inflexion point marks the base of the postacetabular process of Proexochokefalos cf. bouchardi , which is proportionally short as it only reaches about 36% of the total dorsal length (anteroposteriorly) of the ilium. The concave portion of the posterior margin of the ilium may have presented a short spike around its centre as in Neosteneosaurus edwardsi . Indeed, the centre presents a broken portion whose sides constitute two slightly emerging surfaces. The anterior margin of the ilium of Proexochokefalos cf. bouchardi underneath the preacetabular process is relatively straight throughout its length. Still, the rim of the supraacetabular crest of Proexochokefalos cf. bouchardi , although broken, hangs over the edge of the anterior margin, making it slightly convex as in Pelagosaurus typus and Charitomenosuchus leedsi . In Proexochokefalos cf. bouchardi , the supraacetabular crest forms a thin and convex ridge, which strongly protrudes laterally which contrasts with Sericodon jugleri , Platysuchus multiscrobiculatus , Macrospondylus bollensis , Neosteneosaurus edwardsi and Lemmysuchus obtusidens . In comparison, Charitomenosuchus leedsi also has a markedly convex supraacetabular crest, but instead of laterally protruding, the later is noticeable due to the deep indentation of the acetabulum directly underneath it. The bony acetabulum of Proexochokefalos cf. bouchardi is large (i.e. both dorsoventrally tall and anteroposteriorly long) as in other teleosauroids, but only forms a relatively shallow indentation as in Lemmysuchus obtusidens (as opposed to Sericodon jugleri , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , and possibly Macrospondylus bollensis ). This is partly due to the relatively thin mediolateral thickness, and subsequent small lateral protrusion, of both pubic and ischial peduncles. The pubic peduncle of Proexochokefalos cf. bouchardi only forms a subtle bulge along the anterior margin of the ilium, as in Sericodon jugleri , Indosinosuchus potamosiamensis ( Martin et al. 2019a ) , Platysuchus multiscrobiculatus , Neosteneosaurus edwardsi , and Lemmysuchus obtusidens . Like in other teleosauroids, the pubic peduncle of Proexochokefalos cf. bouchardi is more ventrally located than the ischial peduncle, which gives an undulating aspect to the ventral margin of the ilium. In ventral view, the pubic peduncle of Proexochokefalos cf. bouchardi shows an overall constant thickness throughout its length, and displays the typical lunate or wedge-shaped outline found in other teleosauroids but with a smaller concavity. Like in Sericodon jugleri , the posterior edge of the pubic peduncle of Proexochokefalos cf. bouchardi is concave where is transitions to the acetabular perforation. On the lateral side of the ilium, the outline of the pubic peduncle of Proexochokefalos cf. bouchardi draws two adjacent triangular shapes with the posterior one being the greatest, as in other teleosauroids. However, unlike in Neosteneosaurus edwardsi , Charitomenosuchus leedsi and Sericodon jugleri , the posterior triangular shape of the pubic peduncle of Proexochokefalos cf. bouchardi is comparatively dorsoventrally squashed, and hence is shorter than the ischial peduncle. Laterally, the ischial peduncle of Proexochokefalos cf. bouchardi displays an overall triangular outline, with the dorsal apex posteriorly shifted as in other teleosauroids ( e.g Sericodon jugleri , Indosinosuchus potamosiamensis ( Martin et al. 2019a ) , Macrospondylus bollensis , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , Lemmysuchus obtusidens ). Posteriorly, the ischial peduncle gradually protrudes laterally from the rest of the ilium, forming the posterior barrier of the acetabulum. The ventral surface of the ischial peduncle of Proexochokefalos cf. bouchardi forms a sinusoidal shape which dealigns it with the ventral margin of the pubic peduncle and gives the ventral margin of the ilium its undulating aspect. In ventral view, the ischial peduncle of Proexochokefalos cf. bouchardi also shows a subtly lunate outline like the pubic peduncle. Its mediolateral thickness is overall constant for the first anterior 2/3 of its lengths, then it gradually increases posteriorly to reach 150% of its anterior thickness at its peak. The maximum mediolateral thickness and anteroposterior length of the ischial peduncle of Proexochokefalos cf. bouchardi exceed the dimensions of the pubic peduncle. The acetabular perforation of Proexochokefalos cf. bouchardi forms a shallow notch (about five times longer anteroposteriorly than deep) separating the pubic and ischial peduncles, similarly to most teleosauroids (except Sericodon jugleri in which it is enlarged). The acetabular perforation of Proexochokefalos cf. bouchardi is most similar to Charitomenosuchus leedsi in terms of size (and possibly Macrospondylus bollensis ). However, it contrasts with Neosteneosaurus edwardsi and Lemmysuchus obtusidens in being less extented anteroposteriorly, which brings the pubic and ischial peduncles close to one another. The attachment sites for the sacral ribs form two distinct impressions on the medial side of the ilium of Proexochokefalos cf. bouchardi , as in most teleosauroids (except Lemmysuchus obtusidens and Machimosaurus in which there are three). Their shape is also similar to what is observed in most teleosauroids; the attachment sites for the sacral ribs are bell-shaped (i.e. their dorsal and ventral margins are convex) with their apex meeting around the centre of the ilium and their greatest axis oriented anteroposteriorly. Each print possesses a strongly protruding excrescence near its apex, which was inserted in the hollow tip of the sacral rib. The dorsal margin of the attachment site of the second sacral rib reaches the apex of the postacetabular process and divides it in half, whereas the dorsal margin of the attachment site of the first sacral rib is in line with the ventral margin of the preacetabular process. In comparison, the ventral margin both attachment sites reach each ventral corner of the ilium. FIG . 50. — Pelvic girdle elements of Proexochokefalos cf. bouchardi ( Sauvage, 1872 ), MJSN SCR010-374: A , left ilium in lateral view; B , left ilium in medial view; C , right ischium in medial view; D , right ischium in lateral view. Arrow points anteriorly. Original models courtesy of Dr Jeremy Anquetin (Jurassica MuseumMJSN). Scale bar: 1 cm. Ischium The ischium of Proexochokefalos cf. bouchardi ( Figs 50 ; 51 ) displays a relatively slender shaft, similar to Mycterosuchus nasutus , Teleosaurus sp. , and Charitomenosuchus leedsi . However, the ischium of Proexochokefalos cf. bouchardi drastically differs from these taxa (and other teleosauroids) in displaying a rather gracile outline. The anterior and posterior margins of the ischium of Proexochokefalos cf. bouchardi are both concave with a relatively great amplitude, resulting in a more ventrally stretched ischium. Furthermore, both anterior and posterior margins have their apex located around their mid-length which contributes to their symmetrically concave aspect throughout their length and overall stretched look. The anterior margin of other teleosauroids is often symmetrically concave (i.e. the apex is located at the mid-length of the margin; e.g. Aeolodon priscus , Platysuchus multiscrobiculatus , Macrospondylus bollensis , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , Lemmysuchus obtusidens ), but it is not always the case for the posterior margin. Comparatively, Platysuchus multiscrobiculatus and Macrospondylus bollensis also display a symmetrically concave posterior margin like Proexochokefalos cf. bouchardi . Proximally, the shaft of Proexochokefalos cf. bouchardi widens to form the anterior and posterior peduncles. The latter has an overall circular outline and has a bulged rim around its lateral edge whereas the rim becomes thinner medially. The dorsal surface of the posterior peduncle is separated into a wedgeshaped surface medially and a trapezoid-shaped surface laterally through a faintly protruding ridge. The lateral surface – which is involved in the hip acetabulum– is dorsally facing, which is similar to the Machimosaurinae members Neosteneosaurus edwardsi and Lemmysuchus obtusidens . Another common trait between Proexochokefalos cf. bouchardi and Neosteneosaurus edwardsi and Lemmysuchus obtusidens is the relative size of each dorsal surface of the posterior peduncle: both surfaces appear to have similar dimensions, although the medial one is slightly smaller. The anterior peduncle of Proexochokefalos cf. bouchardi strongly differs from that of other teleosauroids as it presents a sharp extremity pointing strictly anteriorly, which resembles an isosceles triangle in lateral and dorsal views. Furthermore, the articular facet of the anterior peduncle of Proexochokefalos cf. bouchardi is mainly oriented dorsally, with an additional small surface present on the underside, laterally, which presumably met with the peduncle of the pubis. This strongly contrasts with the slightly convex articular surface mainly anteriorly facing found in other teleosauroids (e.g. Aeolodon priscus , Teleosaurus sp. , Indosinosuchus potamosiamensis ( Martin et al. 2019a ) , Macrospondylus bollensis , Charitomenosuchus leedsi , Neosteneosaurus edwardsi and Lemmysuchus obtusidens ). Such a sharp anterior peduncle was found in some metriorhynchoids, like ‘ Metriorhynchus ’ brachyrhynchus LEICT G. 418.1956.13.5 and Thalattosuchus superciliosus . The anterior and posterior peduncles are separated by the acetabular perforation.Like in most teleosauroids, the acetabular perforation of Proexochokefalos cf. bouchardi is partially obstructed laterally by the peduncle bridge and, as a result, it is medially tilted. The dorsal margin of the peduncle bridge is markedly concave as it forms the base of the acetabular perforation. In comparison, the ventral margin of the peduncle bridge is straight. Laterally and medially, the peduncle bridge gradually widens and reaches its maximum width at its junction with the anterior peduncle. Like Platysuchus multiscrobiculatus and Teleosaurus sp. , the posterior process of Proexochokefalos cf. bouchardi displays a tubular to subquadrangular shape, yet less slender. The posterior process of Proexochokefalos cf. bouchardi is dorsoventrally thick throughout its length and its apex is rectangular with rounded corners and a subtly convex posterior edge. The ventral margin of the ischium of Proexochokefalos cf. bouchardi appears to have been relatively straight but is not entirely preserved. The anterior process of Proexochokefalos cf. bouchardi is relatively short and extends as far anteriorly as the anterior peduncle as in other teleosauroids. The anterior process of Proexochokefalos cf. bouchardi points mainly anteriorly and forms an isosceles triangle with its slightly concave dorsal margin and straight ventral margin.