Pelagosaurus TYPUS BRONN 1841
Creators: name: Scavezzoni, Isaure
name: Fischer, Valentin
name: Johnson, Michela M.
name: Jouve, Stéphane
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Reptilia
Crocodylia
Metriorhynchidae
Pelagosaurus

description: PELAGOSAURUS TYPUS BRONN, 1841   
For measurements, see Tables 7-9.   Ilium   
 Overall, the ilium of Pelagosaurus typus (Figs 10; 11) appears more similar to that of extant crocodylians (e.g. Palaeosuchus palpebrosus Cuvier, 1807 [Fig. 7], Mecistops cataphractus Cuvier, 1824 [Fig. 8], Caiman crocodilus [Fig. 9]) than other thalattosuchians. The ilium of Pelagosaurus typus differs from that of metriorhynchoids (e.g. Tyrannoneustes lythrodectikos Young, de Andrade, Brusatte, Sakamoto & Liston, 2012, Suchodus durobrivensis Lydekker, 1890, Dakosaurus maximus (Plieninger, 1846)) in possessing a developed postacetabular process and a protruding pubic peduncle (anteriorly and ventrally). Moreover, the ilium of Pelagosaurus typus further differs from that of teleosauroids in displaying a protruding pubic peduncle. In SMNS 17758, the ventral margin of the pubic peduncle is not parallel to that of the ischial peduncle, unlike in BRLSI M.1417.1, Sericodon jugleri Von Meyer, 1845, Charitomenosuchus leedsi (Andrews, 1909), Proexochokefalos cf. bouchardi (Sauvage, 1872), Neosteneosaurus edwardsi (Eudes-Deslongchamps, 1868), and Lemmysuchus obtusidens (Andrews, 1909). In comparison, extant crocodylians possess both a developed postacetabular process as well as a protruding and leaning pubic peduncle whose ventral margin is not parallel to neither the ventral margin of the ischial peduncle nor the dorsal margin of the iliac crest. There are at least two dyrosaurids which display non-parallel peduncles on their ilia, Congosaurus bequaerti Dollo, 1914 and Acherontisuchus guajiraensis, but only the latter shows an angular relation between their peduncles similar to Pelagosaurus typus (Fig. 10D).   
 The anterior margin of the ilium of Pelagosaurus typus (Fig. 10D) forms a straight surface connecting the preacetabular process to the pubic peduncle. The junction between the anterior margin and the ventral border of the preacetabular process is achieved through an acute angle, due to the relative inclination of the anterior margin of the ilium. Indeed, by prolonging both surfaces until they meet (thus ignoring the preacetabular process), the anterior margin of the ilium appears to form an angle of approximately 120° with the dorsal margin. Overall, the preacetabular process of Pelagosaurus typus is well developed as in other metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus (Eudes-Deslongchamps, 1867)NHMUK PV R 4763, Suchodus durobrivensis, Tyrannoneustes lythrodectikos, etc.) and also teleosauroids (e.g. Macrospondylus bollensis (Jäger, 1828), Sericodon jugleri, Charitomenosuchus leedsi, etc.) compared to extant crocodylians (e.g. Palaeosuchus palpebrosus, Mecistops cataphractus, Caiman crocodilus). The preacetabular process of Pelagosaurus typus reaches about half of the anteroposterior length of the pubic (or even ischial) peduncle, as in some metriorhynchoids (e.g. Suchodus durobrivensis or Thalattosuchus superciliosus (Blainville in Eudes-Deslongchamps & Blainville, 1852)). However, several other thalattosuchian taxa display a larger preacetabular process, both proportionally and in absolute values (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus NHMUK PV R 2054, Cricosaurus suevicus (Fraas, 1901), Lemmysuchus obtusidens, etc.). The preacetabular process of Pelagosaurus typus is proportionally long, sharp, and in line with the iliac crest. The junction between the preacetabular process and the iliac crest is achieved through a subtle recess, and the iliac crest is faintly convex. Some teleosauroids also display a relatively straight to subtly convex dorsal margin, due to the alignment of the preacetabular and postacetabular processes without a marked depression at their junction (e.g. namely Macrospondylus bollensis and Platysuchus multiscrobiculatus (Berckhemer, 1929), but also the more derived Neosteneosaurus edwardsi and Lemmysuchus obtusidens).   
 As mentioned, the dorsal margin of the ilium of Pelagosaurus typus (Fig. 10D) corresponding to the iliac crest is slightly convex over most of its surface, with only the subtle recess marking the junction with the preacetabular process. The lateral surface of the iliac crest is scarred with a series of ridges perpendicular to the margin of the bone. Posteriorly, the dorsal margin of the ilium culminates in a relatively sharp apex pointing strictly posteriorly which corresponds to the extremity of the postacetabular process. The latter takes the overall shape of a Lancet arch since its dorsal and ventral borders are slightly convex. The postacetabular process of Pelagosaurus typus is slender with a subtlety convex ventral margin. The latter stems from the main body of the ilium at around half of its dorsoventral height, which results in a relatively narrow postacetabular process, similar to some teleosauroids (e.g. Macrospondylus bollensis and Platysuchus multiscrobiculatus) although not as tubular. The postacetabular process of Pelagosaurus typus appears shorter than that of teleosauroids as its anteroposterior length does not constitute half of the total anteroposterior length of the iliac crest (unlike in Lemmysuchus obtusidens, Platysuchus multiscrobiculatus, Charitomenosuchus leedsi, Neosteneosaurus edwardsi). In addition, the anteroposterior length of the ilium at the peduncles almost equals that of the entire iliac crest dorsally in teleosauroids. The transition between the postacetabular process and the posterior margin of the ischial peduncle is achieved through a smooth obtuse angle, as in the teleosauroids Macrospondylus bollensis and Platysuchus multiscrobiculatus.   
 The entire ventral margin of the ilium of Pelagosaurus typus (Fig. 10D) is strongly undulating due to a major difference in the orientation of both peduncles. Hence, along the ventral margin of the bone, there is a small notch marking the changeover to the pubic peduncle. This small indentation corresponds to the acetabular perforation, which is usually not pronounced on the ilium of Thalattosuchia regardless of the clade (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Thalattosuchus superciliosus, Dakosaurus maximus, Macrospondylus bollensis, Lemmysuchus obtusidens, Proexochokefalos cf. bouchardi, etc.). In more derived metriorhynchoids, the acetabular perforation does not separate the peduncles (which form a continuous surface laterally), as a consequence of the reduction of the dorsal elements of their girdles (e.g. Thalattosuchus superciliosus, Suchodus durobrivensis, Cricosaurus araucanensis (Gasparini & Dellape, 1976), etc.). In Pelagosaurus typus, the acetabular perforation forms a concave notch, which is more than twice as long as it is deep. The corners of the ischial and pubic peduncles bordering the acetabular perforation are prominent, which increases the concave aspect of their respective ventral margins.   
 The pubic peduncle of the ilium of Pelagosaurus typus SMNS 17758 (Fig. 10) stands out from the rest of the bone as it is shaped as an anteroventrally protruding rectangle, whereas in BRLSI M.1417.1 the pubic peduncle is not as titled anteroventrally. Anteriorly, the pubic peduncle of BRLSI M.1417.1 strongly protrudes from the anterior margin of the ilium, forming a bump. In SMNS 17758, the pubic peduncle is not as prominent. Ventrally, the surface of the pubic peduncle is a slightly concave, similar to the ischial peduncle. The subtle inclination of the pubic peduncle of Pelagosaurus typus conveys the idea that only the posterior corner of the said peduncle was close to the anterior peduncle of the ischium (Fig. 12), as in extant crocodylians (e.g. Palaeosuchus palpebrosus [Fig. 7], Mecistops cataphractus [Fig. 8], Caiman crocodilus [Fig. 9]) and teleosauroids (e.g. Lemmysuchus obtusidens, Charitomenosuchus leedsi, Neosteneosaurus edwardsi). Comparatively, in metriorhynchoids, the anterior peduncle of the ischium extends more anteriorly proportionally to the ilium and either meets with the middle portion of the pubic peduncle of the ilium (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763) or is located underneath the anterior corner the pubic peduncle of the ilium (e.g. Thalattosuchus superciliosus NHMUK PV R 2054). Laterally, the facet of the pubic peduncle of BRLSI M.1417.1 forms a strong sinusoidal shape: its anterior half is dorsally concave whereas its posterior half displays an overall elliptical outline, with the apex pointing posteriorly. A similar shape of the pubic peduncle is found in the teleosauroids Sericodon jugleri, Proexochokefalos cf. bouchardi, and Neosteneosaurus edwardsi. Dorsally, the highest portion of the pubic peduncle of BRLSI M.1417.1 towers the ischial peduncle, whereas the ischial peduncle of Pelagosaurus typus SMNS 17758 shows a size and shape similar to the pubic peduncle.   
 On the lateral surface of the ilium, the facet of the ischial peduncle appears relatively reduced compared to other metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Thalattosuchus superciliosus, Cricosaurus suevicus, etc). This relation is usually inverted in extant crocodylians, metriorhynchoids and dyrosaurids (e.g. Mecistops cataphractus [Fig.8], ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Congosaurus bequaerti). In comparison, teleosauroids tend to have an ischial peduncle that is shorter or subequal to the pubic peduncle on the lateral surface of the ilium (e.g. Macrospondylus bollensis, Charitomenosuchus leedsi, Sericodon jugleri, Neosteneosaurus edwardsi, Lemmysuchus obtusidens), similar to Pelagosaurus typus. The lateral facet of the ischial peduncle is lunate-shaped and is therefore anteroposteriorly longer than it is dorsoventrally high, similar to some teleosauroids (e.g. Macrospondylus bollensis, Charitomenosuchus leedsi, Sericodon jugleri, Neosteneosaurus edwardsi, Lemmysuchus obtusidens, etc.). The apex of the elliptic outline of the ischial peduncle is not centred but is rather posteriorly deflected. Compared to the pubic peduncle, the ischial peduncle is dorsoventrally shorter (as mentioned previously). Hence, in Pelagosaurus typus the ischial peduncle does not appear to form a salient posterior barrier due to its short size. It is however still markedly protruding laterally.   
 The bony acetabulum is located on the lateral surface of the ilium of Pelagosaurus typus (Fig. 10D). It is limited by the supraacetabular crest anteriorly and dorsally, and by the ischial peduncle posteriorly. The supraacetabular crest extends over about 2/3 of the entire length of the ilium. Anteriorly, the supraacetabular crest does not influence the shape of the anterior margin of the bone, similar to Geosaurus giganteus (Von Sommerring, 1816) sensu Von Quenstedt (1852), Lemmysuchus obtusidens, and Platysuchus multiscrobiculatus but unlike in more derived metriorhynchoids where the supraacetabular crest forms a more extensive rugged area encompassing the anterior margin (e.g.‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis, or Thalattosuchus superciliosus).   
 Pelagosaurus typus bears two distinct attachment sites for the sacral ribs on the medial side of the ilium (Fig. 11B). The sacral rib attachment sites are located around the mid-height of the bone and along its anterior and posterior margins, unlike more derived metriorhynchoids (e.g. Thalattosuchus superciliosus, Cricosaurus araucanensis, Dakosaurus maximus, etc.). Yet, Pelagosaurus typus displays the long and pendent sacral ribs characteristic of Metriorhynchoidea, as opposed to the relatively short and horizontal ones of Teleosauroidea (e.g. Lemmysuchus obtusidens, Macrospondylus bollensis, Neosteneosaurus edwardsi, etc.). The posterior attachment site is overall larger than the anterior one and is located more ventrally which indicates a slight dorsal orientation for the ilium (Fig. 12). The shape and position of the sacral rib attachment sites of Pelagosaurus typus is similar to that of other unreduced ilia which still display a postacetabular process. This reflects the impact of the girdle reduction on its position and orientation in more derived members of Metriorhynchoidea.    Ischium   
 The ischium of Pelagosaurus typus (Figs 10-12) strongly differs from that of other metriorhynchoids and teleosauroids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus, Geosaurus giganteus, Aeolodon priscus (Von Sömmerring, 1814) Von Meyer, 1832, Charitomenosuchus leedsi, Proexochokefalos cf. bouchardi, Lemmysuchus obtusidens, etc.) in displaying a thick distal suture whose surface is perpendicular to that of the distal blade. The main consequence is a different orientation of the ischium of Pelagosaurus typus compared to that of other thalattosuchians, where both ischia of Pelagosaurus typus are set at a greater angle medially (Fig. 12).   
 Overall, the ischium of Pelagosaurus typus displays a proportionally long and thick shaft. Indeed, the narrowing of the shaft (which delimits it) is located at about half of the total dorsoventral height of the bone and the anteroposterior length of this narrowing exceeds that of the posterior peduncle. Several thalattosuchians display a relatively thick shaft (e.g. Cricosaurus suevicus, Dakosaurus maximus, Aeolodon priscus NHMUK PV R 1086, Lemmysuchus obtusidens, Macrospondylus bollensis), however a proportionally elongated shaft is less common (e.g. Thalattosuchus superciliosus NHMUK PV R 2054). In addition, the anterior and posterior margins of the shaft of Pelagosaurus typus (i.e. underneath the peduncles) are both markedly concave as seen in several thalattosuchians (e.g. Cricosaurus suevicus, Cricosaurus araucanensis, Dakosaurus maximus, Torvoneustes carpenteri (Wilkinson, Young & Benton, 2008), Aeolodon priscus, Proexochokefalos cf. bouchardi, Neosteneosaurus edwardsi, Lemmysuchus obtusidens, etc.), resulting in a significant flaring of the bone proximally and distally.   
 The anterior peduncle of the ischium of Pelagosaurus typus (Fig. 10F) appears to have an ovoid shape and is located further dorsally than the posterior peduncle as in other thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus NHMUK PV R 2054, Geosaurus giganteus, Aeolodon priscus, Lemmysuchus obtusidens, etc.). Indeed, the anterior peduncle is borne on the extremity of the peduncle bridge, which constitutes an elongated cylindrical structure emanating from the dorsal-most portion of the shaft. The latter is almost as long proximodistally as the posterior peduncle is anteroposteriorly long. The thickness of the peduncle bridge in lateral view seems constant throughout its length, and is equal to that of the anterior peduncle. In Pelagosaurus typus, the anterior and posterior peduncles are set further apart than in most thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus NHMUK PV R 2054, Cricosaurus suevicus, Aeolodon priscus, Lemmysuchus obtusidens, etc.). The junction between the anterior and posterior peduncles appears forms a wide hemispherical indentation on the dorsal margin of the ischium. This large gap constitutes the ventral border of the acetabular perforation, and thus makes up for the more reduced counterpart of the ilium. The posterior peduncle of the ischium of  Pelagosaurus  typus dorsally stands out from the acetabular perforation. Its articular surface appears slightly undulating and uneven so that the posterior-most border is located more dorsally (Fig. 10A, F). The posterior corner of the ischium connecting the posterior peduncle with the shaft is smooth and rounded.   
 The distal blade of the ischium (Figs 10A, F; 12) is relatively slender as in ‘ Metriorhynchus ’ brachyrhynchus LEICT G.418.1956.13.5, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Cricosaurus bambergensis (Sachs et al. 2019) Wagner, 1858, Macrospondylus bollensis, and Platysuchus multiscrobiculatus. Indeed, the dorsoventral height of the posterior process of the distal blade is smaller than the constriction of the shaft, giving it a tapered look. The dorsal margin of the posterior process is relatively straight, like the ventral margin of the distal blade. The extremity of the posterior process is hemispherical similar to ‘  Metriorhynchus ’ brachyrhynchus LEICT G.418.1956.13.5, Cricosaurus bambergensis, and Platysuchus multiscrobiculatus . Anteriorly, the distal blade culminates in a sharp apex mainly pointing anteriorly but with a small dorsal component. The dorsoanterior margin of the anterior process follows the continuity of the anterior margin of the shaft: i.e. the margin is concave with the same degree of concavity.   
 Pubis   
 The pubis of Pelagosaurus typus (Figs 10-12) differs from other thalattosuchians in displaying the combination of a drastically slender pubic apron (almost three times longer than wide) with a proportionally well-developed pubic symphysis regarding the width of its pubic apron (e.g. ‘  Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis,  Thalattosuchus  superciliosus  NHMUK PV R 2054, Cricosaurus suevicus, Cricosaurus bambergensis,  Lemmysuchus obtusidens, Machimosaurus Von Meyer, 1837, Platysuchus multiscrobiculatus, Charitomenosuchus leedsi , etc.). Indeed, the pubic symphysis of Pelagosaurus typus reaches almost 1/3 of the total dimension of the distal blade, and the shaft only accounts for about 12.79% of the entire proximodistal height of the pubis. Still, the pubic symphysis of Pelagosaurus typus is overall shorter than in most thalattosuchians. Other thalattosuchians possessing a globally shorter pubic symphysis include notably Geosaurus giganteus and Macrospondylus bollensis.  ABLET 8. — Continuation. Overall, the pubis of Pelagosaurus typus has a very distinctive appearance as it is strongly arched laterally, with its median margin being concave throughout and its lateral margin being entirely convex (Fig. 12). The specimens NHMUK PV OR 32605 and SMNS 17758 display a stronger curvature than in BRLSI M.1420. The outline of the distal margin of the pubic apron of Pelagosaurus typus (including the pubic symphysis) is also characteristic: it is slightly convex as in extant crocodylians, but with a more developed symphysis. Comparatively, few thalattosuchian taxa display a similarly curved distal margin, namely rhacheosaurines (i.e. Rhacheosaurus gracilis Von Meyer, 1831, Cricosaurus suevicus, Cricosaurus albersdoerferi (Sachs, Young, Abel & Mallison, 2021), and Cricosaurus bambergensis), Geosaurus giganteus, and  Macrospondylus bollensis. The latter displays the overall closest shape to Pelagosaurus typus, but with a wider and shorter pubic apron and thicker shaft. The pubic symphysis of Pelagosaurus typus  is strongly reduced and corresponds to the area forming the junction between the medial and distal margins, similar to extant crocodylians (e.g. Palaeosuchus palpebrosus, Mecistops cataphractus, Caiman crocodilus, etc.). . h AP. h AP. w BL.lw h Ll Wa BR . l w h l h S C 47 Charitomenosuchus leedsi NHMUK PV R 3806127.3427.6529.05––20.0711.9424.0626.9928.8310.1517.51––36.3928.7648Proexochokefalos cf. bouchardiMJSN SCR 010 - 374131.6641.22––136.5817.616.5734.3025.6528.9910.5320.9987.8129.2940.1933.3649Neosteneosaurus edwardsiNHMUK PV R 3701125.5916.55–––21.574.8625.6625.6826.888.8112.69––28.2637.7350Neosteneosaurus edwardsiNHMUK PV R 2865165.8234.5948.8233.86173.2127.249.9337.8235.7131.6715.7721.4489.2535.5943.7849.3051Neosteneosaurus edwardsiNHMUK PV R 3898234.1427.2460.04–256.40––54.4353.94–––––53.3566.3152 Lemmysuchus Proximally, the shaft of Pelagosaurus typus displays a significant widening to form the pubic peduncle. Indeed, the mediolateral width of the latter is 2.5 times greater than the constriction of the shaft. The pubis of SMNS 17758 is anteroposteriorly flattened and shows breakage proximally, which presumably emphasizes the width of its proximal extremity compared to BRLSI M.1420. obtusidens NHMUK PV R 3168203.0053.2251.9147.02206.5028.7311.3052.1751.6732.7020.41 31.33 87.0043.5355.2654.59–54.4353.94–––––53.3566.3152 Lemmysuchus Proximally, the shaft of Pelagosaurus typus displays a significant widening to form the pubic peduncle. Indeed, the mediolateral width of the latter is 2.5 times greater than the constriction of the shaft. The pubis of SMNS 17758 is anteroposteriorly flattened and shows breakage proximally, which presumably emphasizes the width of its proximal extremity compared to BRLSI M.1420. obtusidens NHMUK PV R 3168203.0053.2251.9147.02206.5028.7311.3052.1751.6732.7020.41 31.33 87.0043.5355.2654.59 Proximally, the shaft of Pelagosaurus typus displays a significant widening to form the pubic peduncle. Indeed, the mediolateral width of the latter is 2.5 times greater than the constriction of the shaft. The pubis of SMNS 17758 is anteroposteriorly flattened and shows breakage proximally, which presumably emphasizes the width of its proximal extremity compared to BRLSI M.1420. obtusidens NHMUK PV R 3168203.0053.2251.9147.02206.5028.7311.3052.1751.6732.7020.41 31.33 87.0043.5355.2654.59  
 Several other thalattosuchians show a proximal flaring (e.g. Suchodus durobrivensis,  Thalattosuchus  superciliosus NHMUK PV R 2054, Cricosaurus suevicus, Cricosaurus bambergensis, Dakosaurus maximus, Geosaurus giganteus,  Platysuchus multiscrobiculatus, Machimosaurus, Macrospondylus bollensis , but few display a comparable intensity (e.g. Cricosaurus suevicus, Geosaurus giganteus, Platysuchus multiscrobiculatus, Macrospondylus bollensis).   
 The shape of the peduncle of the pubis of Pelagosaurus typus does not differ from what is observed in most thalattosuchians: the articular surface is oval in shape, is slightly convex, and has rounded corners laterally and medially defining the extremities of the peduncle.
description: Published as part of Scavezzoni, Isaure, Fischer, Valentin, Johnson, Michela M. & Jouve, Stéphane, 2024, Form and function of the pelvic girdle of Thalattosuchia and Dyrosauridae (Crocodyliformes), pp. 135-326 in Geodiversitas 46 (6) on pages 150-159, DOI: 10.5252/geodiversitas2024v46a6, http://zenodo.org/record/11106598