Hyposaurus natator, (Troxell, 1925), (Troxell)

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 : 258-266

HYPOSAURUS NATATOR ( TROXELL, 1925)

For measurements, see Tables 7-9 .

Ilium

Similar to that of Congosaurus bequaerti ( MRAC 1806, Fig. 72 ), the ilium of Hyposaurus natator (YPM VP.000753, holotype [ Fig. 66 ]; NJSM 23368 [ Fig. 67 ]) stands out from those of thalattosuchians in possessing a well-developed preacetabular process and acetabular perforation, a mediolaterally deep acetabulum, laterally prominent supraacetabular crest, distinct and laterally prominent peduncles, and a large postacetabular process proportionally to its size.

The ilium of Hyposaurus natator displays a series of intraspecific morphological variations across its two specimens YPM VP.000753 ( Fig. 66 ) and NJSM 23368 ( Figs 67 ; 68 ). Starting with the overall outline of the bone, YPM VP.000753 displays a more pronounced preacetabular process in being anteroposteriorly longer than that of NJSM 23368. The preacetabular crest of YPM VP.000753 also stands out due to the almost right angle its ventral margin forms with the anterior edge of the ilium. The postacetabular process is also quite different between both specimens as it is dorsoventrally taller for NJSM 23368 compared to YPM VP.000753. This variation in size is accompanied by differing dorsal and ventral margins for the postacetabular process, which may presumably explain the difference. Indeed, the dorsal and ventral margins of NJSM 23368 are relatively more convex than those of YPM VP.000753, and their junction with the rest of the ilium is marked by shallow recesses only. In YPM VP.000753, the convex ventral and dorsal margins of the postacetabular process appear shorter due to extended concave recesses bordering them. Other great differing features between YPM VP.000753 and NJSM 23368 include a more prominent supraacetabular crest and a wider acetabular perforation for YPM VP.000753, whereas NJSM 23368 bears relatively larger peduncles with more embossed articular facets.

The anterior margin of the ilium underneath the preacetabular process is relatively straight with the anterior border of the pubic peduncle only dimly protruding, which contrasts with the more prominent one of Congosaurus bequaerti . The preacetabular process of Hyposaurus natator is well developed (it is larger in YPM VP.000753 than in NJSM 23368) and displays a clear anterodorsal orientation, which differs from the more anteriorly aligned process of Congosaurus bequaerti and Acherontisuchus guajiraensis . There is a difference in size of the preacetabular process between YPM VP.000753 and NJSM 23368 that can be partly explained through weathering. The dissimilarity could also possibly reflect intraspecific variations like sexual dimorphism. In extant crocodylians, the preacetabular process is the attachment site for the ilio-costatis and ilio-tibialis 1 muscles ( Romer 1923). In both Hyposaurus natator specimens, the preacetabular process resembles a beak or a hook ( Figs 66 ; 67 ) with the dorsal rim of the process being globally convex whereas its ventral margin is concave. The shape of the dorsal margin of the preacetabular process is dominated by the bend located at about 1/3 of its extension anteriorly; one could argue that the portion posterior to the peak of the bend is actually slightly concave (as impacted by the dorsal recess, see below) whereas the anterior portion is more convex. There is a parallelepiped area laterally emanating from the preacetabular process which culminates ventrally in forming the supraacetabular crest.Whereas this area is smooth and convex for Hyposaurus natator YPM VP. 000753, it is less hollow and slightly pitted in Congosaurus bequaerti . In Hyposaurus natator NJSM 23368 this area is also slightly pitted rather than smooth. Yet, this difference in depth between Congosaurus bequaerti and Hyposaurus natator betrays the greater mediolateral thickness of the preacetabular process of Congosaurus bequaerti . There is also a difference in the size of the supraacetabular crest between both Hyposaurus natator : YPM VP.000753 displays a more pronounced crest than NJSM 23368.

In Hyposaurus natator , the junction between the preacetabular process and the iliac crest is achieved through a marked depression, much clearer than in Congosaurus bequaerti , Acherontisuchus guajiraensis and Dyrosaurus maghribensis , which gives a cosine wave appearance to the dorsal margin of the ilium.

There is a subtle difference between NJSM 23368 and YPM VP.000753: the first one actually displays a shallower hollow which could potentially be linked to the smaller size of the preacetabular process. Unlike Congosaurus bequaerti , the iliac crest of Hyposaurus natator is relatively thin lateromedially and is also globally smoother: the medial and lateral surfaces bordering the crest bear a subtle repousse aspect,´as if it were gently hammered from the inside. The postacetabular process as a whole is longer (anteroposteriorly) than it is high (dorsoventrally), and takes the shape of an acute Lancet arch topped with a pointed peak at its (posterior-most) extremity. Its narrow appearance, which is a feature not present in Congosaurus bequaerti , is namely provoked by the concave recess at the junction between the ischial peduncle and the postacetabular process. The global orientation of the postacetabular process, based on its median, appears strictly anteroposterior, unlike that of Congosaurus bequaerti and Acherontisuchus guajiraensis which possessed a non negligible dorsal component. The postacetabular process of Hyposaurus natator NJSM 23368 appears almost symmetrical dorsally and ventrally like Dyrosaurus maghribensis , but for Hyposaurus natator YPM VP. 000753 the dorsal margin seems more inflated. This difference is probably caused by the relative extension of the ventral recess between the ischial peduncle and the postacetabular process, which is once more limited in the case of Hyposaurus natator NJSM 23368 (i.e. like the dorsal hollow posteriorly to the preacetabular process). In Hyposaurus natator and Acherontisuchus guajiraensis , almost the entire lateral surface of the postacetabular process is concave, whereas for Congosaurus bequaerti and Dyrosaurus maghribensis this area is more restricted. Conversely, in Hyposaurus natator the medial surface of the postacetabular process is nearly entirely convex, whereas in Congosaurus bequaerti the medial surface is not evenly inflated and is rather irregular.

The peduncles appears slightly different between the Hyposaurus natator specimens, with Hyposaurus natator NJSM 23368 displaying more embossed articular facets than Hyposaurus natator YPM VP. 000753, hence giving a stouter appearance to its peduncles. Both peduncles border the acetabulum ventrally and are separated by a wide gap corresponding to the acetabular perforation ( Fig. 68 ). The perforation in Hyposaurus natator NJSM 23368 and YPM VP.000753 is large (seeTable 11) both anteroposteriorly and dorsoventrally (almost as high as it is wide) which greatly contrasts with Congosaurus bequaerti and Dyrosaurus maghribensis . Yet, the size of the acetabular perforation is not even between Hyposaurus natator specimens; indeed, the acetabular perforation of YPM VP.000753 is proportionally larger than than that of NJSM 23368 but the difference is subtle. In both cases, the iliac acetabular perforation appears almost as vast as its counterpart on the ischium.

In both NJSM 23368 and YPM VP.000753, the ischial and pubic peduncles have their ventral margins quite parallel (see Table 10 ), which greatly contrasts with Congosaurus bequaerti and Acherontisuchus guajiraensis . This phenomenon could be caused by more ventrally extended pubic and ischial peduncles for Hyposaurus natator ( Figs 66 ; 67 ) compared to Congosaurus bequaerti . This greater extension also presumably played a role in the more developed height of the acetabular perforation. This hypothesis could also explain what is observed in Acherontisuchus guajiraensis .

In ventral view, both peduncles are distinct, well developed, and equivalent in size. Their outline is wedge-shaped with their concavity oriented towards each other and thereby the acetabulum, similar to Congosaurus bequaerti ( Fig. 74 ). The ischial peduncle resembles that of other dyrosaurids (i.e. Congosaurus bequaerti , Dyrosaurus maghribensis , Acherontisuchus guajiraensis ): in lateral view it displays a globally triangular outline with its ventral margin being slightly concave at its midpoint. The ischial peduncle protrudes laterally from the acetabulum and potentially acted as a barrier which prevented the femur from gliding posteriorly. Hence, the ischial peduncle presumably formed the foundation of the posterior wall of the articular capsule in vivo, which corresponds to the function of the antitrochanter of extant crocodylians ( Tsai & Holliday 2015). On the lateral surface of the bone, the facet of the pubic peduncle draws two adjoined triangles similar to Congosaurus bequaerti , Dyrosaurus maghribensis and Acherontisuchus guajiraensis . Indeed, the posterior-most portion of the pubic peduncle of Hyposaurus natator takes the shape of an isosceles triangle ( Figs 66 ; 67 ). Moving anteriorly, the pubic peduncle then displays another triangular shape whose apex is located on the anterior margin of the ilium and in doing so forms the anterior margin of the peduncle. The junction between those shapes is angular and could be seen as an inverted triangle as in other dyrosaurids (i.e. Congosaurus bequaerti , Dyrosaurus maghribensis , Acherontisuchus guajiraensis ). The maximal height of the lateral facet of the pubic peduncle of Hyposaurus natator does not exceeds that of the ischial peduncle dorsally as it is the case in many other crocodyliforms (e.g. Mecistops cataphractus , Caiman crocodilus , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , Charitomenosuchus leedsi , Congosaurus bequaerti , Dyrosaurus maghribensis , Acherontisuchus guajiraensis ). There is however a slight difference in height between the pubic peduncles of the two specimens of Hyposaurus natator , with YPM VP.000753 displaying a dorsally shorter anterior-most facet compared to NJSM 23368.

As for other dyrosaurids, the acetabulum of Hyposaurus natator is well-developed: it is mediolaterally deep (thanks notably to the orientation of the peduncles, Fig. 74 ), and covers a large area both anteroposteriorly (about half the total length) and dorsoventrally (more than half the height of the bone from the ventral margin of the peduncles). Its deepest point seems relatively centred as in Dyrosaurus maghribensis , whereas for Congosaurus bequaerti and Acherontisuchus guajiraensis it was located near the summit of the ischial peduncle. The supraacetabular crest borders the acetabulum dorsally, but appears to be limited to the anterior half of it. The supraacetabular crest of Hyposaurus natator appears to start more ventrally on the anterior margin of the ilium as in Acherontisuchus guajiraensis and Dyrosaurus maghribensis , making it greater than that of Congosaurus bequaerti . Thereby, the posterior side of the acetabulum, between the supraacetabular crest and the ischial peduncle, is open and directly leads to the concave central part of the postacetabular process by means of a slight bump as it was observed for Congosaurus bequaerti and Dyrosaurus maghribensis . Mediolaterally, the supraacetabular crest represents the thickest portion of the ilium, and was the attachment site for a soft tissue structure probably similar to the acetabular labrum of extant crocodylians.

Medially, the surface of the ilium forms a convex parabola with its vertex (or peak) being located directly dorsally to the sacral rib attachment sites for the sacral ribs, just above the acetabular perforation. Hyposaurus natator displays two main attachment sites, one for each sacral. The anterior attachment site is slightly larger than the posterior one but both are overall equal in size, unlike in Dyrosaurus maghribensis and Congosaurus bequaerti . Each attachment site is located directly medially to a peduncle: the first attachment site is placed medially to the pubic peduncle, whereas the second one is located medially to the ischial peduncle. Within each attachment site, a thin ridge emanating from the dorsal margin of the imprint partially subdivides it into two subareas, giving the impression of four distinct parabolic imprints. Yet, the ridge slowly fades away ventrally, so that each sub-areas are united along the ventral margin of the ilium. There is a difference in the sacral rib attachment sites from NJSM 23368 and YPM VP.000753: in the latter, the subdividing ridge is well developed and creates four distinct indentations with four vertexes (or peaks), whereas in NJSM 23368 the separation between the posterior-most imprints is more tenuous (i.e. less protruding), thus giving the impression of only three imprints. Furthermore, NJSM 23368 shows a large rugged area posteriorly to the second attachment site, whereas this area is much more restricted in YPM VP.000753. This area is shallower than the actual attachment sites, probably because it was merely guiding the posterior extension of the second sacral rib. Indeed, its shallowness presumably prevents it from being used as an anchor point for the sacral rib, so that it probably rather served as an additional stabilizing or locking feature. There is a similar area observed on the ilium of other dyrosaurids that presumably filled the same purpose.

Ischium

The ischium of Hyposaurus natator (YPM VP.000753, holotype [ Fig. 66 ]; NJSM 23368 [ Fig. 67 ]) differs from those of thalattosuchians (e.g. Lemmysuchus obtusidens , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Torvoneustes carpenteri , etc.) in possessing: an enlarged anterior peduncle that exceeds the posterior peduncle in size; a profound acetabular perforation; a twisted distal blade compared to the orientation of the peduncles; a more anteroposteriorly restricted distal blade; a limited anterior process, and a narrow posterior process.

Overall, the ischium of Hyposaurus natator is similar to that of other dyrosaurids (i.e. Dyrosaurus maghribensis and Acherontisuchus guajiraensis ) and extant crocodylians (i.e. Palaeosuchus palpebrosus [ Fig. 7 ], Mecistops cataphractus [ Fig. 8 ], Caiman crocodilus [ Fig. 9 ]) in displaying well-developed peduncles and a rotated distal blade which does not extend posteriorly more than half of the total anteroposterior proximal length (i.e. of both peduncles) of the ischium. In this way, the distal blade of those crocodyliforms resembles the blade of a hatchet or a small axe.

Proximally, the ischium of Hyposaurus natator bears two peduncles separated by a substantial gap. Each articular surface is identifiable thanks to its pitted texture. The anterior peduncle is large and exceeds the size of the posterior peduncle, similar to Acherontisuchus guajiraensis but unlike Dyrosaurus maghribensis , extant crocodylians (e.g. Mecistops cataphractus , Caiman crocodilus ) and thalattosuchians (e.g. Torvoneustes carpenteri , Aeolodon priscus , Neosteneosaurus edwardsi , etc.) where the opposite relation is usually observed. In Hyposaurus natator and other dyrosaurids (i.e. Dyrosaurus maghribensis and Acherontisuchus guajiraensis ), the articular surface of the anterior peduncle is rounded and extends both dorsally and anteriorly, wrapping all the available surface of this extremity of this process. The presence of a rounded and extensive articular surface for the anterior peduncle presumably conveys the existence of a relatively mobile articulation between the pubis and the ischium. Such an articulation could have been made possible through the presence of a thick but flexible cartilaginous coat in vivo, wrapping both the anterior peduncle of the ischium and the peduncle of the pubis, similar to extant crocodylians ( Farmer & Carrier 2000a; Gans & Clark 1976; Munns et al. 2012; Tsai & Holliday 2015; Tsai et al. 2019).

The articular surface of the anterior peduncle is actually divided into two subsurfaces – anterior and dorsal – of differing function. The dorsal share of the articular surface is wedge-shaped, with the concavity facing dorsally. This dorsal portion forms the counterpart of the pubic peduncle of the ilium ( Figs 66 ; 67 ). The bony facets of the anterior peduncle of the ischium and pubic peduncle of the ilium were not in direct contact throughout their whole length, unlike the interlocking ischial peduncle of the ilium and posterior peduncle of the ischium. On the reconstruction of both NJSM 23368 and YPM VP.00753 ( Fig. 68 ) the anterior peduncle of the ischium and pubic peduncle of the ilium appear to have been in contact, but it is possible that some deformation prevents the peduncles of YPM VP.00753 to fully embrace one another. In addition, the entire area was presumably covered with cartilaginous structures ensuring the connection between both bones.

The anterior surface of the peduncle makes up the remaining, and almost the totality, of the articular surface of the anterior peduncle. This surface is larger in NJSM 23368 compared to YPM VP.000753, so that the anterior peduncle of NJSM 23368 appears dorsoventrally taller ( Fig. 68 ). The anterior surface of the peduncle is also wedge-shaped but with the concavity oriented medially; the ventral portion appears like a distinct peduncle – the pubic ‘knob’ – (as in Dyrosaurus maghribensis ( Jouve et al. 2006) and Acherontisuchus guajiraensis ), which corresponds to the area where the peduncle of the pubis theoretically connected to the ischium. The peduncle bridge of the ischium constitutes the thinnest part of the anterior peduncle; it connects the articular extremity of the peduncle with the shaft of the ischium. The peduncle bridge is concave in all orientations, giving a flared appearance to the anterior peduncle of the ischium. The dorsal surface of the peduncle bridge constitutes the ventral border of the acetabular perforation. There is a difference in the length of the peduncle bridge between Hyposaurus natator , with that of NJSM 23368 being slightly shorter that that of YPM VP.000753 ( Figs 66C ; 67D ). The same dissimilarity can be found among the Dyrosaurus maghribensis specimens.

The posterior peduncle is smaller than the anterior peduncle. In dorsal view, the overall outline of the posterior peduncle appears like a thicker version of the wedge-shaped anterior peduncle, with the concavity oriented anteriorly in this case. The articular surface of the posterior peduncle is slightly truncated leading to a strictly dorsal area and a more anterolateral one. The strictly dorsal area was connected to the ventral surface of the ischial peduncle of the ilium (both where interlocked), whereas the anterolateral one follows on from the shape of the ischial peduncle of the ilium. The neck of the posterior peduncle is concave laterally, medially, and anteriorly where it forms the posterior wall of the acetabular perforation. The ischial acetabular perforation is almost as large as its counterpart on the ilium ( Fig. 68 ). The acetabular perforation of dyrosaurids (e.g. Hyposaurus natator [ Figs 66 ; 67 ], Dyrosaurus maghribensis and Acherontisuchus guajiraensis ) is ventrally delimited by the peduncle bridge of the ischium, whereas that of thalattosuchians is laterally deflected and forms a tilted incision on the peduncle bridge (e.g. Lemmysuchus obtusidens [ Fig. 59 ], ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Torvoneustes carpenteri , etc.). Posteriorly, the neck of the posterior peduncle is rather convex, but the concavity is inverted for the remaining of the posterior margin of the ischium (i.e. including the shaft and distal blade). Indeed, the overall anterior and posterior margins of the ischium underneath the peduncles are concave. Ventrally to the peduncles, two consecutive areas can be observed: first is a bottleneck portion corresponding to the shaft or neck of the ischium; and second is a flared out area called the distal blade. The transition between the two parts is set at the start of the distal enlargement and torsion of the neck. Indeed, the distal blade of the ischium gradually shifts from the orientation of the shaft to display a more medial orientation distally, similar to extant crocodylians (i.e. Palaeosuchus palpebrosus [ Fig. 7 ], Mecistops cataphractus [ Fig. 8 ], Caiman crocodilus [ Fig. 9 ]). This shift in orientation is intended for both ischia to meet medially along their distal blade.

In Hyposaurus natator and other dyrosaurids, the anteroposterior flaring of the distal blade is relatively contained, unlike thalattosuchians for which the distal blade protrudes both anteriorly and posteriorly. Still, the shaft and posterior process of Hyposaurus natator are more slender than on the ischium of Dyrosaurus maghribensis , which displays a relatively thick shaft along with a short posterior process. As a consequence, the anterior and posterior extremity of the distal blade of Dyrosaurus maghribensis appear almost on the same level ventrally, whereas in Hyposaurus natator the posterior extremity is located further ventrally than the anterior one.

Similar to many other crocodyliforms (e.g. Palaeosuchus palpebrosus, Mecistops cataphractus , Caiman crocodilus , Lemmysuchus obtusidens , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Torvoneustes carpenteri , etc.), the anterior process of the distal blade of Hyposaurus natator is situated more dorsally than the posterior process ( Fig. 68 ). The curved margin (anterior) leading to the anterior process is thereby shorter, but in return shows a greater concavity than the (posterior) margin leading to the posterior process. The anterior process is pointing anteriorly, whereas the posterior process points strictly ventrally unlike thalattosuchians (e.g. Lemmysuchus obtusidens , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Torvoneustes carpenteri , etc.), extant crocodylians (e.g. Palaeosuchus palpebrosus, Mecistops cataphractus , Caiman crocodilus ), and Acherontisuchus guajiraensis . Each peak (anterior and posterior) of the distal blade is only partially angular as each is composed of a relatively straight margin on one side and a concave one on the other. The ventral border of the distal blade uniting both processes is subtlety concave and its surface is strongly pitted throughout its length. In vivo, both distal blades were ventrally connected, not unlike thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , Lemmysuchus obtusidens , Charitomenosuchus leedsi , etc.).

Pubis

The pubis of Hyposaurus natator (YPM VP.000753, holotype [ Fig. 66 ]; NJSM 23368 [ Fig. 67 ]) differs from those of thalattosuchians (e.g. Lemmysuchus obtusidens , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , etc.) and extant crocodylians (e.g. Palaeosuchus palpebrosus [ Fig. 7 ], Mecistops cataphractus [ Fig. 8 ], Caiman crocodilus [ Fig. 9 ]) in being relatively longer and slender due to an elongated pubic plate (shaft reaches about 36-37% of total length). Similar to thalattosuchians, the pubis of Hyposaurus natator further differs from that of extant crocodylians in possessing a large pubic symphysis.

The global shape of the pubis of Hyposaurus natator (YPM VP.000753, holotype [ Fig. 66 ]; NJSM 23368 [ Fig. 67 ]) is similar to that of other dyrosaurids (i.e. Dyrosaurus maghribensis , Cerrejonisuchus improcerus , Anthracosuchus balrogus ) in being both elongated proximodistally and narrow mediolaterally. Proximally, the pubis of Hyposaurus natator displays a relatively short neck (about 36-37% of total length, see Table 9 ) and an even shorter shaft (about 26% of total length, see Table 9 ) whose sections are elliptic throughout. The shaft and the neck both correspond to transition portions between the peduncle and the pubic apron (Fig. 4). The shaft itself stretches from the peduncle proximally up until the distal widening, which marks the beginning of the pubic apron. The shaft is topped with a slightly concave articular facet called the pubic peduncle. The pubic peduncle displays a sloping articular surface in anteroposterior view, which may actually reflect the curved or wedged anterior surface of the anterior peduncle of the ischium. A similar feature is observed among extant crocodylians (e.g. Palaeosuchus palpebrosus [ Fig. 7 ], Mecistops cataphractus [ Fig. 8 ], Caiman crocodilus [ Fig. 9 ]). The area surrounding the pubic peduncle is scarred for about 1-2cm, which indicates the presence of a cartilage cap in vivo. The junction between the shaft and the pubic plate is identifiable thanks to both the anteroposterior flattening and dorsal bending of the bone (visible in lateral and medial views). There, the pubis starts to gradually widen lateromedially in the distal direction. At the junction between the shaft and the pubic plate, the lateral and medial margins of the shaft are locally parallel then rapidly flare out proximally and distally, creating concave lateral and medial margins. Conversely, the anterodorsal and posteroventral surfaces of the pubis (in lateral view starting from the junction) only dimly flare out proximally, and even become slightly finer distally. The distal portion of the pubis – the pubic plate or apron – comprises five distinct margins: the two lateral ones, the two medial ones, and finally the distal one. The concave medial margin is shorter than the lateral one as it abruptly transitions (through an obtuse bend) into the straight margin constituting the pubic symphysis. In YPM VP.000753 ( Fig. 66 ), the concavity of the medial margin is lesser than that of NJSM 23368 ( Fig. 67 ) and bears almost a straight look. The pubic symphysis was covered by cartilage in vivo as the pitted texture reveals, and served as the connection point with the other pubis. The length of the pubic symphysis is significantly greater in Hyposaurus natator (about 22% of total pubis length for NJSM 23368, whereas it reaches about 18.4% in YPM VP.000753, see Table 9 ) than what is observed in extant crocodylians (e.g. about 7.3% for Caiman crocodilus NHMW 30900 and about 12.9% for Mecistops cataphractus RBINS 18374). Opposite to the pubic symphysis is a short margin relatively straight or subtlety convex which connects the large and concave lateral margin with the distal margin. This short lateral margin is not as protruding as in Cerrejonisuchus , so it cannot be called a protuberance. The distal margin, which connects the lateral and medial margins of the pubis, appears relatively straight but is slightly convex anterodorsally. Indeed, the pubic apron is slightly bulged dorsally starting from the proximal extremity of the pubic symphysis.

The presence of a large pubic symphysis, coupled with rounded respective peduncles on the pubis and ischium ( Fig. 68 ), conveys the relative mobility of the pubes and also presumably the existence of a pelvic aspiration ( Brocklehurst et al. 2020).

Figures

FIG. 72. — Left ilium of Congosaurus bequaerti Dollo,1914, MRAC 1806 (holotype):A, lateral view;B, medial view.Arrow points anteriorly.Scale bar:1 cm.

FIG. 66. — Right pelvic girdle elements of Hyposaurus natator (Troxell,1925),YPM VP.000753, holotype: A, right ilium in medial view; B, right ilium in lateral view; C, right ischium in lateral view;D, right pubis in lateral view;E, left pubis in posteroventral view.Target indicates anterior.Arrow points anteriorly.Scale bar: 1 cm.

FIG. 67. — Right and left pelvic girdle elements of Hyposaurus natator (Troxell,1925), NJSM 23368: A, right pubis in anterior view;B, left ilium in lateral view;C, left pubis in lateral view; D, left ischium in lateral view; E, right ilium in lateral view; F, right ilium in medial view. Target indicates anterior. Arrow points anteriorly. Pictures courtesy of Wayne Callahan. Scale bars: 1 cm.

FIG. 68. — Reconstruction of the pelvic girdle of Hyposaurus natator (Troxell,1925),YPM VP.000753 (B, D, F) and NJSM 23368 (A, C, E): A, B, right lateral views; C, D, anterior views;E, F, ventral views.Target indicates anterior. Arrow points anteriorly. The left ilium, ischium and pubis of YPM VP.000753 are mirrored. The ischium of YPM VP.000753 has been partially reconstructed. The left pubis and right ilium and ischium of NJSM 23368 are mirrored. 3D models of NJSM 23368 courtesy of Candice Stefanic. Scale bars: 5 cm.

FIG. 74. — Crocodyliformes ilia in ventral view: A, left ilium of Congosaurus bequaerti Dollo, 1914, MRAC 1806 (holotype); B, right ilium of Hyposaurus natator (Troxell, 1925),YPM VP.000753 (holotype, mirrored); C, right ilium of Alligator mississippiensis (Daudin, 1801), UF Herp 21461. Blue line corresponds to median of each pubic peduncle; red line corresponds to median of each ischial peduncle. The arrow points anteriorly. The pubic peduncle of Congosaurus bequaerti has been restored. Not to scale.

FIG. 7. — Snapshot of the pelvic girdle of the alligatoroid Palaeosuchus palpebrosus (Cuvier, 1807), RVC-JRH-PP1: A, in anterior view; B, in lateral view. Note the orientation of the pubes. Cross indicates posterior view. Arrow points anteriorly. CT scan courtesy of Professor John Hutchinson (see https://skfb.ly/6ByyV). Scale bars: 1 mm.

FIG. 8. — Right pelvic girdle of Mecistops cataphractus Cuvier, 1825, RBINS 18374: A, ilium in lateral view; B, ischium in lateral view; C, pubis in ventral view; D, ilium in medial view. Cross indicates posterior. Arrow points anteriorly. Scale bar: 1 cm.

FIG. 9. — Left pelvic girdle of Caiman crocodilus (Linnaeus, 1758), NHMW 30900: A, ilium in medial view; B, ilium in lateral view; C, ischium in medial view; D, ischium in lateral view; E, pubis in posterior view. Target indicates anterior. Arrow points anteriorly. Scale bar: 1 cm.

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.