Lemmysuchus obtusidens, (Andrews, 1909), (Andrews)

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 : 244-250

LEMMYSUCHUS OBTUSIDENS ( ANDREWS, 1909)

For measurements, see Tables 7-9 .

Ilium

The ilium of Lemmysuchus obtusidens ( Figs 59 ; 60 ) possesses the typical feature of Teleosauroidea among Thalattosuchia: the presence of a well-developed (about 36 % of the total anteroposterior length of the ilium) postacetabular process. The postacetabular process is an extension of the ilium, posterior and dorsal to the ischial peduncle. In Lemmysuchus obtusidens , it takes the shape of a Lancet arch, similar to several teleosauroids (e.g. Sericodon jugleri , Charitomenosuchus leedsi, Proexochokefalos cf. bouchardi, and Neosteneosaurus edwardsi ), extant crocodylians and dyrosaurids (e.g. Mecistops cataphractus , Congosaurus bequaerti , and Hyposaurus natator ). The presence of a postacetabular process strongly influences the shape of the bone, so that ilia bearing a postacetabular process develop a long dorsal blade. Thereby, the dorsal margin of the ilium forms a convex arch posteriorly to the preacetabular process. The posterior-most portion of the ilium constitutes the peak of the Lancet arch, which points strictly posteriorly unlike in Sericodon jugleri , Charitomenosuchus leedsi, Proexochokefalos cf. bouchardi, and Neosteneosaurus edwardsi . The junction between the posteacetabular process and the ischial peduncle forms a smooth curve.

The ventral margin of the ilium is undulating: it is composed of the ischial peduncle posteriorly and the pubic peduncle anteriorly. The acetabular perforation separating the two peduncles is shallow, unlike that of other teleosauroids (e.g. Macrospondylus bollensis , Neosteneosaurus edwardsi , Charitomenosuchus leedsi , etc.), Mecistops or Congosaurus bequaerti . The small size of the acetabular perforation diminishes the space between the peduncles, and thereby renders the acetabulum of Lemmysuchus obtusidens relatively smaller. The ventral margins of both peduncles are oriented in the same overall direction (like in metriorhynchoids), rather than being tilted at a different angle (as in Mecistops cataphractus or Congosaurus bequaerti ). Yet, the ventral border of the ischial peduncle is situated more dorsally than the pubic peduncle, and is mediolaterally thinner. This difference in relative height between the peduncles is compensated by the ischium and its protruding anterior peduncle (set on the peduncle bridge). The anterior margin of the ilium is straight (as in Dakosaurus maximus or Neosteneosaurus edwardsi ) rather than convex as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Tyrannoneustes lythrodectikos , Suchodus durobrivensis or Charitomenosuchus leedsi , and ends up with the preacetabular process dorsally. The junction between the anterior margin and the preacetabular process forms a rounded acute angle.

The preacetabular process is proportionally small in relation to the overall size of the ilium (about 16% of the anteroposterior length of the bone), unlike what is observed in some metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 and Suchodus durobrivensis ) and teleosauroids (e.g. Charitomenosuchus leedsi , Neosteneosaurus edwardsi ). Moreover, the orientation of the preacetabular process of Lemmysuchus obtusidens further differs from what is observed in metriorhynchoids as it has no dorsal component in its orientation, and is facing strictly anteroventrally in the direct extension of the iliac crest (making it parallel to the ventral margins of the peduncles; Fig. 60 ). The shallow junction between the preacetabular process and the iliac crest emphasizes their relative linearity as in Neosteneosaurus edwardsi , Plagiophthalmosuchus gracilirostris , Macrospondylus bollensis , Platysuchus multiscrobiculatus . The preacetabular process of Lemmysuchus obtusidens is flattened dorsoventrally, and is wide lateromedially as in Platysuchus multiscrobiculatus , Plagiophthalmosuchus gracilirostris , or Thalattosuchus superciliosus . This effect is less pronounced in Charitomenosuchus leedsi and Neosteneosaurus edwardsi so that even though the ventral surface of their preacetabular process is relatively flat, their dorsal surface remains rounded thus giving a cylindrical appearance to their preacetabular process.

The acetabulum of Lemmysuchus obtusidens is large (about 72% of the dorsoventral height of the ilium), and encompasses almost all of the area extending from the preacetabular process through the ventral peduncles. Also, the acetabulum appears centred on the triangular surface defined by the preacetabular process and the ventral peduncles (like in Macrospondylus bollensis ), as opposed to Suchodus durobrivensis , Charitomenosuchus leedsi and Neosteneosaurus edwardsi NHMUK PV R 3898. This phenomenon is imputable to the relative size of the acetabulum, but also to the position, orientation and extension of the supraacetabular crest, which is a parabolic ridge delimiting the acetabulum. Indeed, the supraacetabular crest of Lemmysuchus obtusidens displays a large focal width so that its anterior border closely follows the anterior margin of the ilium, while the posterior border shows a trajectory path aiming at the junction between the ischial peduncle and the base of the postacetabular process. It is also possible that the relative inclination of the anterior margin of the ilium may emphasize this phenomenon.

The configuration of Lemmysuchus obtusidens (i.e. large focal width) is similar to extant crocodylians (e.g. Mecistops cataphractus , Caiman crocodilus and dyrosaurids (e.g. Congosaurus bequaerti , and Hyposaurus natator ), contra what is observed in some metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Tyrannoneustes lythrodectikos , Suchodus durobrivensis , etc.) for which the supraacetabular crest is posteriorly shifted and contact the anterior margin of the ilium relatively ventrally. The supraacetabular crest of Lemmysuchus obtusidens also bears the typical prominent anterior ridge and the smooth posterior one, as it can be seen from many crocodyliforms (e.g. Mecistops cataphractus , Congosaurus bequaerti , Hyposaurus natator , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , etc.).

In Lemmysuchus obtusidens , almost half of the surface of the acetabulum is not hollow, but is obstructed by a laterally protruding hump of bone ( Figs 59 ; 60 ). This seemingly bony growth disrupts the function of the acetabulum as a cavity and presumably happened later in the life of the specimen. It is unknown if this hump is the result of a trauma or the ossification of cartilaginous (e.g. osteoarthritis of the hip) structures through ageing processes. In theory, there could have been a good dorsoventral congruence between the femoral head and acetabulum if the acetabular labrum were at least as extensive as it is for extant crocodylians ( Tsai & Holliday 2015). The existence of a deeply scarred area extending from the pubic peduncle of the ischium through the preacetabular process and the supraacetabular crest in Lemmysuchus obtusidens ( Figs 59 ; 60 ) and also other thalattosuchians (e.g. Thalattosuchus superciliosus , Suchodus durobrivensis , Neosteneosaurus edwardsi , etc.) teases the presence of a widely spread acetabular labrum (but does not inform on the mediolateral breath of this hypothetical structure).

The attachment sites for the sacral processes are observable on the medial side of the ilium, and take up almost half of the entire surface ( Fig. 59 ). In some metriorhynchoids (i.e. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Tyrannoneustes lythrodectikos or Suchodus durobrivensis , etc.), the areas are characterized by being slightly in relief rather than being hollow like extant crocodylians and dyrosaurids (e.g. Mecistops cataphractus , Congosaurus bequaerti , Hyposaurus natator , etc.). There is a thin border made of hollows and bumps drawing the general outline of the sacral rib attachment sites. The delimitation between each attachment site can be deducted from the shape of the dorsal and ventral contours, but the internal demarcation is less clear as in Suchodus durobrivensis . The exact contour of each attachment site is based on the careful observation of both the surface of the ilium and the distal extremities of both sacral as well as the first caudal. The general shape of the sacral rib attachment sites forms three lobes ( Fig. 59 ), with the middle one being the greatest. The anterior lobe (the smallest) corresponds to the attachment site of the first sacral, the middle lobe belonged to the second sacral whereas the posterior one (which is cordiform) was the anchoring site for the first caudal. The anterior and middle lobes are composed of a series of radiate ridges, whereas the posterior lobe is more strongly pitted. The latter also roots deeper in the ilium than the other two, which are more superficially anchored.

Ischium

The ischium of Lemmysuchus obtusidens ( Fig. 59 ) resembles that of other thalattosuchians (e.g. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, ‘ Metriorhynchus ’ brachyrhynchus LEICT G. 418.1956.13.5, Thalattosuchus superciliosus , Cricosaurus suevicus , Dakosaurus maximus , Torvoneustes carpenteri , Aeolodon priscus , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , etc.): the distal blade is elongated anteroposteriorly, giving the impression of an axe head. This shape is not found among Dyrosauridae and Crocodylia, as the shaft of the ischium is usually longer and the distal blade shorter (e.g. Mecistops cataphractus , Caiman crocodilus , Hyposaurus natator , etc.). Proximally, the ischium of Lemmysuchus obtusidens bears two peduncles separated by a notch: the acetabular perforation ( Fig. 59 ). This hollow is actually tilted towards the sagittal plane and is thus entirely borne by the medial surface of the ischium, similar to several other thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Dakosaurus maximus , Torvoneustes carpenteri , Neosteneosaurus edwardsi, Proexochokefalos cf. bouchardi, Teleosaurus sp. , etc.). In extant crocodylians, this gap is covered by a membrane which protects the ligamentum capitis of the femur from compression, and grants it sufficient space during episodes of movements ( Tsai & Holliday 2015).

The anterior peduncle is borne on an anteriorly protruding process called the peduncle bridge.Overall,the anterior peduncle of Lemmysuchus obtusidens does not significantly protrude from the anterior process of the ischium ( Fig. 60 ), compared to what is observed in dyrosaurids or extant crocodylians (e.g. Mecistops cataphractus , Caiman crocodilus , Hyposaurus natator , etc.). Also, similar to other thalattosuchians, the anterior peduncle of Lemmysuchus obtusidens appears markedly reduced in size (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , Torvoneustes carpenteri , Cricosaurus suevicus , Dakosaurus maximus , Aeolodon priscus , Charitomenosuchus leedsi , Macrospondylus bollensis , Neosteneosaurus edwardsi , etc.). Yet, the anterior peduncle of Lemmysuchus obtusidens is more developed than metriorhynchoids, namely in being mediolaterally thicker as in other teleosauroids. The facet of the anterior peduncle is rugged, indicating the presence of a cartilage cap in vivo. In anterior view, the articular surface of the anterior peduncle is wedge-shaped with its concavity dorsally facing and its greatest axis oriented lateromedially. The articular facet of the anterior peduncle is however asymmetrical: its medial corner is sharp whereas its lateral one is more rounded.At about its mid-length lateromedially, the medial half of the articular surface of the anterior peduncle is posteriorly deflected similar to Charitomenosuchus leedsi . The lateral half of the articular surface of the anterior peduncle presumably corresponds to the anchoring site of the peduncle of the pubis ( Fig. 60 ). The peduncle bridge of Lemmysuchus obtusidens ( Fig. 59 ) stems from the shaft of the ischium and bears the anterior peduncle on its extremity. Its ventral surface is relatively straight whereas its dorsal surface is strongly concave and borders the acetabular perforation ventrally. In dorsal view, the peduncle bridge appears to be slightly curved towards the medial side of the bone.Unlike in Charitomenosuchus leedsi , the lateral and medial margins of the dorsal surface of the peduncle bridge of Lemmysuchus obtusidens ( Fig.59 ) are highly asymmetrical, with the lateral margin being almost on the same level as the articular surface of the posterior peduncle. As a result, the acetabular perforation of Lemmysuchus obtusidens ( Fig. 59 ) appears almost non-existent in lateral view as in Neosteneosaurus edwardsi contra Pelagosaurus typus, Macrospondylus bollensis and Charitomenosuchus leedsi ; this makes the ischium of Lemmysuchus obtusidens superficially appear more similar to that of metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, ‘ Metriorhynchus ’ brachyrhynchus LEICT G. 418.1956.13.5, Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Torvoneustes carpenteri , etc.) while still being more developed than the latter (among other traits). Indeed, the acetabular perforation of Lemmysuchus obtusidens is medially deflected like a burrow tilted towards the medial side of the bone which can be followed up until the junction between the peduncle bridge and the base of the posterior peduncle. Still, the peduncle bridge of Lemmysuchus obtusidens possesses a larger concave dorsal surface than in metriorhynchoids, so that the peduncle bridge creates more room for the acetabular perforation compared to that of metriorhynchoids (which almost solely relies on the burrow on the medial surface of the ischium).

The posterior peduncle is both wide lateromedially and long anteroposteriorly, and its articular surface takes the shape of an isosceles trapezoid with its long base positioned anteriorly. Comparatively, the articular surface of the anterior peduncle is downsized with its dorsoventral height reaching about 39 % of the anteroposterior length of the posterior peduncle. Indeed, the articular surface of the anterior peduncle is not dorsally facing like the posterior peduncle, but is oriented anteriorly (with dorsal and ventral components) as in many crocodyliformes, such as Dyrosauridae and Crocodylia (e.g. Mecistops cataphractus , Caiman crocodilus , Hyposaurus natator ).

The anterior process of the ischium forms anteriorly an acute peak which is slightly pointing dorsally, and is shaped by the concave anterior margin of the ischium underneath the anterior peduncle, and the relatively straight distal blade. Both the medial and lateral surfaces of the distal blade present a rugged texture near the distal border, hinting at the existence of a cartilage cap in vivo. Only the medial surface of the distal blade is deeply scarred though, as it indicates the area where both ischia were joined. The ischial suture of Lemmysuchus obtusidens extends over the entire length of the distal blade but is the thickest around the anterior third quarter. This area also presents the deepest indentations whereas the surrounding portions are less textured. The first quarter anteriorly starts at the base of the anterior process and is entirely smooth, similar to other thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Thalattosuchus superciliosus , Torvoneustes carpenteri , Neosteneosaurus edwardsi , etc.). The ischial suture and the lateral surface of the distal blade form an acute angle of approximately 35°, unlike in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763 and Neosteneosaurus edwardsi .

The posterior margin of the ischium is undulated: starting from the posterior peduncle the surface is concave; at about the mid-length of the whole posterior margin, the concavity switches to convex, thus creating the rounded posterior distal extremity of the bone (which meets with the distal blade). The area extending posteriorly to the shaft is called the posterior process of the ischium. The shaft represent the thinnest portion of the ischium anteroposteriorly since it is formed by two concave margins anteriorly and posteriorly. Similar to Macrospondylus bollensis and Neosteneosaurus edwardsi , the shaft of the ischium of Lemmysuchus obtusidens appears relatively squared as its dorsoventral height and anteroposterior thickness reach about the same length.

When superimposed, the outline of the right and left ischia of Lemmysuchus obtusidens do not coincide ( Fig. 59G ). Indeed, there are dissimilarities in the size of the posterior peduncle, thickness of the shaft, extension of the posterior process, and inclination between the ventral margin of the distal blade and the dorsal surface of the posterior peduncle. These differences reveal an important level of intraspecific variation for Lemmysuchus obtusidens .

Pubis

The pubis of Lemmysuchus obtusidens ( Fig. 59 ) shows an overall triangular outline, with a truncated distal extremity. In this way, the pubis of Lemmysuchus obtusidens resembles that of other thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Cricosaurus araucanensis , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , etc.) and dyrosaurids (e.g. Hyposaurus natator , Anthracosuchus balrogus Hastings, Bloch & Jaramillo, 2014 , Cerrejonisuchus improcerus , etc.), but strongly differs from that of extant crocodylians (e.g. Mecistops cataphractus , Caiman crocodilus , etc.). Indeed, this difference is due to the existence of a long pubic symphysis for Lemmysuchus obtusidens (and other members of Thalattosuchia and Dyrosauridae ), whereas it is greatly reduced in extant crocodylians.

In Lemmysuchus obtusidens , the pubic symphysis is a straight margin which forms an angle of approximately 27° with the axis of the shaft, similar to Charitomenosuchus leedsi and Mycterosuchus nasutus . The angular relation in Neosteneosaurus edwardsi is slightly greater but overall falls within the same range. In vivo, the pubic symphysis of both pubes were medially connected presumably using soft tissues ( Fig. 60 ). The pubic symphysis is proximally connected with the medial margin of the pubis, and distally with the pubic blade. The junction between the concave medial margin of the pubis and the pubic symphysis forms a right angle as in other thalattosuchians displaying a long pubic symphysis (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Machimosaurus , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , Mycterosuchus nasutus ). In comparison, the intersection between the pubic symphysis and the pubic blade is achieved through an obtuse angle of approximately 140-150°, which is closer to Charitomenosuchus leedsi and Mycterosuchus nasutus than Neosteneosaurus edwardsi .

The pubic blade, which corresponds to the distal margin of the pubis, is relatively convex with its turning point slightly offset to the lateral side. The transition to the lateral margin of the pubis is smooth and rounded, but the angle between the distal and lateral margin approximates 90°. Like several other thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Dakosaurus maximus , Platysuchus multiscrobiculatus , Charitomenosuchus leedsi , Mycterosuchus nasutus , etc.), the pubic plate of Lemmysuchus obtusidens is large, especially compared to the relatively short shaft. The extend of the pubic symphysis and distal blade contribute for the most part to the breadth of the pubis. The lateral margin of the pubis, which connects the pubic peduncle and the ventral margin of the distal blade, is lightly undulating: even though the whole lateral margin appears concave, it is actually subtly convex, with the inflexion point observable at about the mid-length of the lateral margin of the pubic plate, similar to Machimosaurus . The truly concave portion remains the lateral margin of the shaft. Proximally, the shaft flares out to form the peduncle of the pubis, whose articular surface presumably takes the shape of an ellipse in section (but is unfortunately not preserved; Fig. 59 ).

In Lemmysuchus obtusidens ( Fig. 59 ), the anteroposterior thickness of the pubic apron increases laterally so that the distal blade is thicker than the pubic symphysis similar to ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Neosteneosaurus edwardsi and Mycterosuchus nasutus . In comparison, the taxa Suchodus durobrivensis and Thalattosuchus superciliosus display a thinner pubis.

The pubis of Lemmysuchus obtusidens ( Fig. 59 ) is deeply arched anteroposteriorly possibly as a consequence of diagenetic deformation (whereas its ilium is acutely flattened lateromedially). In vivo, the pubis of Lemmysuchus obtusidens was presumably more gently arched, in a fashion similar to Crocodylia (e.g. see Palaeosuchus palpebrosus).

Figures

FIG. 59. — Pelvic girdle elements of Lemmysuchus obtusidens (Andrews, 1909), NHMUK PV R 3168 (holotype): A, left ilium in lateral view; B, left ilium in medial view; C, left pubis in lateral view; D, left ischium in lateral view; E, left pubis in anterodorsal view; F, right ischium in lateral view; G, surimposition of both ischia (right and left) of Lemmysuchus obtusidens, NHMUK PV R 3168. Target indicates anterior. Arrow points anteriorly. Scale bar: 1 cm.

FIG. 60. — Pelvic reconstruction of Lemmysuchus obtusidens (Andrews, 1909), NHMUK PV R 3168 (holotype): A, lateral view; B, anterior view; C, ventral view; D, dorsal view. Arrow points anteriorly. Target indicates anterior. The left ischium and right ilium and pubis are mirrored. Scale bar: 5 cm.