Acherontisuchus guajiraensis, Hastings, Bloch & Jaramillo, 2011

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 : 271-274

ACHERONTISUCHUS GUAJIRAENSIS HASTINGS, BLOCH & JARAMILLO, 2011

For measurements, see Tables 7-9 .

Ilium

Overall, the ilium of Acherontisuchus guajiraensis ( Figs 70 ; 71 ) is similar to those of other dyrosaurids in possessing large peduncles and a well-developed postacetabular process greater or equal to the anteroposterior length of the acetabulum (i.e. Congosaurus bequaerti , Hyposaurus natator , and Dyrosaurus maghribensis ).

The preacetabular process of the ilium of Acherontisuchus guajiraensis is longer anteroposteriorly than it is high dorsoventrally. Its dorsal margin is slightly concave whereas its ventral one is subtlety convex, leading to a pointed and drooping hook shape unlike any dyrosaurids. Hence, the orientation of the preacetabular process is anterodorsally, which contrasts with Hyposaurus natator and Dyrosaurus maghribensis . Even though the overall shape is different, the preacetabular process of Congosaurus bequaerti was similarly oriented.

The area enclosed within the margins of the preacetabular process and the supraacetabular crest is strongly pitted, and is also more elevated laterally along the side of the supraacetabular crest than near the preacetabular process. Like in other dyrosaurids (especially visible in Congosaurus bequaerti and Hyposaurus natator ), the supraacetabular crest of Acherontisuchus guajiraensis is formed of a wide anterior portion (laterally prominent) and a less marked posterior rim. Overall, the supraacetabular crest is both strongly apparent and greatly arched with its vertex located at the posterior-most point of the bordering pitted area. Hence, the dorsal and posterior margins of the acetabulum are well defined. Similar to other dyrosaurids, the acetabulum of Acherontisuchus guajiraensis forms a relatively deep hollow medially (i.e. Congosaurus bequaerti , Hyposaurus natator , and Dyrosaurus maghribensis ). However, the acetabulum of Acherontisuchus guajiraensis appears squeezed anteroposteriorly, posteriorly tilted (i.e.the concavity opens anteriorly and ventrally), and restricted to the anterior-most portion of the ilium. This effect is emphasized by the strong convexity of the supraacetabular crest, along with the posterior component in the orientation of its main axis. In other dyrosaurids (i.e. Congosaurus bequaerti , Hyposaurus natator , and Dyrosaurus maghribensis ), the supraacetabular crest is less convex, and the acetabulum is more open and proportionally wider.

The junction between the iliac crest and the preacetabular process is achieved through a slightly concave hollow ( Fig. 70 ), similar to Dyrosaurus maghribensis but contrasting with Congosaurus bequaerti and Hyposaurus natator . The iliac crest, which forms the dorsal border of the postacetabular process, shows an important convexity with its apex located at about 2/3 of its total length posteriorly. However, the convexity on both sides of the apex is of different intensity, with the anterior portion being the weakest. Such a marked convexity for the iliac crest is not found in other dyrosaurids, and thus gives a unique appearance to the ilium of Acherontisuchus guajiraensis . The entire lateral surface of the iliac crest is scarred with perpendicular ridges, marking the presence of a cartilage cap in vivo. Posteriorly, the postacetabular process culminates in the shape of a Lancet arch whose apex is located at about the mid-height of the postacetabular process, similar to other dyrosaurids. The ventral margin of the postacetabular process is also markedly convex, which resembles the ilia of other derived dyrosaurids (i.e. Congosaurus bequaerti , Hyposaurus natator ), but differs from the more subtlety arched postacetabular processes of Dyrosaurus maghribensis . However, in Acherontisuchus guajiraensis , the convex portion of the ventral margin of the postacetabular process stops at about its mid distance with the ischial peduncle. From there, the ventral margin of the postacetabular process becomes moderately concave, forming the posterior border of the ischial peduncle at this anterior extremity. Hyposaurus natator is the only dyrosaurid bearing a similar configuration (more specifically YPM VP.000753, but NJSM 23368 does not depart too much).

The lateral surface of the postacetabular process of Acherontisuchus guajiraensis is evenly concave over almost its entire surface, like Hyposaurus natator but unlike Congosaurus bequaerti or Dyrosaurus maghribensis for which only a localized portion appears concave.

In Acherontisuchus guajiraensis ( Figs 70 ; 71 ), the ischial and pubic peduncles bear consequent differences in their respective size and orientation of their ventral surface. In contrast, in other dyrosaurids (i.e. Congosaurus bequaerti , Hyposaurus natator and Dyrosaurus maghribensis ), the difference is more subtle. The ischial peduncle is dorsoventrally higher than it is anteroposteriorly long. Similar to other many other crocodyliforms (e.g. Palaeosuchus palpebrosus, Mecistops cataphractus , Caiman crocodilus , Suchodus durobrivensis , Thalattosuchus superciliosus , Congosaurus bequaerti , Hyposaurus natator , Dyrosaurus maghribensis , etc.), the ischial peduncle takes the global shape of a triangle.Similar to Congosaurus bequaerti and Hyposaurus natator NJSM 23368, the ischial peduncle greatly protrudes from the ilium with two noticeable facets: one is triangular and borders the acetabulum posteriorly, and the other one is more wedge-shaped. The former was presumably the anchoring site for a structure similar to the antitrochanter of extant crocodylians ( Tsai & Holliday 2015), whereas the latter assured the connection to the posterior peduncle of the ischium. Unlike any other dyrosaurids, the pubic peduncle of the ilium of Acherontisuchus guajiraensis is large and its ventral margin forms a bent at about 1/3 of its length posteriorly; presumably, only the posterior-most portion of the peduncle met with that of the ischium. Also, none of the two tilted portions of the ventral surface of the pubic peduncle are parallel to that of the ischial peduncle. Comparatively, Congosaurus bequaerti is the only other dyrosaurid displaying pubic and ischial peduncles with differing orientations of their ventral surfaces. The lateral surface of the pubic peduncle is poorly preserved ( Hastings et al. 2011), so the exact shape of its articular facets is not identifiable.

The important size of the pubic peduncle could be a consequence of the position, orientation and dimensions of the bony acetabulum, especially since most of the bony acetabulum is borne on the pubic peduncle. Looking on the medial side of the ilium, it also appears that most of the area belonging to the pubic peduncle is scarred with attachment sites for the first sacral. Hence the important size of the sacral ribs could have had an influence on the relative size of the pubic peduncle. The anterior margin of the ilium connecting the preacetabular process to the pubic peduncle is short and relatively straight. The important size of the pubic peduncle also brings it closer to the preacetabular process.

Both peduncles are separated by a gap – the acetabular perforation – similar to other dyrosaurids and extant crocodylians (thalattosuchians bear a significantly smaller perforation). The acetabular perforation of Acherontisuchus guajiraensis ( Fig. 70 ; Table 11 ) is taller dorsoventrally than it is long anteroposteriorly, which differs from all other dyrosaurids (i.e. Congosaurus bequaerti , Hyposaurus natator and Dyrosaurus maghribensis ) for which the acetabular perforation is usually longer than it is tall. In Acherontisuchus guajiraensis , the acetabular perforation is relatively smaller than in other dyrosaurids as its base length anteroposteriorly only reaches a fraction of that of the pubic peduncle. To that regard, the acetabular perforation of Acherontisuchus guajiraensis is closer to that of Congosaurus bequaerti than other dyrosaurids. The narrow appearance of the acetabular perforation of Acherontisuchus guajiraensis mirrors that of the bony acetabulum. The latter is provoked by notably a different inclination of the ischial peduncle whose largest width is oriented more lateromedially as in Congosaurus bequaerti than anteroposteriorly as in Hyposaurus natator and Dyrosaurus maghribensis . This placement of the ischial peduncle reduces the overall anteroposterior length of the bony acetabulum. In Acherontisuchus guajiraensis the small size of the acetabular perforation on the ilium is presumably counterbalanced by the relatively larger size of the acetabular perforation on the ischium.

Medially, the ilium of Acherontisuchus guajiraensis stands out by possessing wide and deep attachment sites for both sacrals. Comparatively, for Congosaurus bequaerti , Hyposaurus natator YPM VP. 000753, and for the Dyrosaurus maghribensis specimens ( Jouve et al. 2006) the anterior attachment site is the largest, whereas for Hyposaurus natator NJSM 23368 ( Fig. 67 ) the posterior one is the largest. In Acherontisuchus guajiraensis , the anterior attachment site is composed of two distinct triangular indentations separated by a prominent ridge. Hence, the anterior-most portion is the widest whereas the posterior-most one is the tallest. The anterior attachment site as a whole stretches from the preacetabular process (at its mid-height) dorsally, up to the acetabular perforation posteriorly (almost encompassing the entire area of the pubic peduncle). Comparatively, the posterior attachment site is also well-developed, occupying a similar area in size than the anterior attachment site. A distinctive feature of Acherontisuchus guajiraensis in relation to other dyrosaurids is the close proximity of both attachment sites which, along with their large sizes, entails a sound support of the pelvic girdle and a potentially better allocation of load. Overall, the posterior attachment site takes the shape of a blunt and dorsally oriented hook ( Fig. 70 ). It spreads posteriorly from the point along the ventral margin of the postacetabular process where the concavity inverts, up until the anterior attachment site anteriorly. Also, it extends as high dorsally as the posterior-most portion of the anterior attachment site. Dorsally to the posterior attachment site is a rough area along which the posterior winglet of the second sacral presumably pressed ( Fig. 70 ). It runs from the apex of the postacetabular process to near the top of the posterior attachment site.

Ischium

The ischium of Acherontisuchus guajiraensis ( Fig. 70 ) is in poor preservation state.Notably, the distal blade is broken anteriorly, the anterior peduncle is missing its dorsal portion, and the posterior peduncle is almost entirely hollowed out. Still, the ischium of Acherontisuchus guajiraensis appears to stand out from those of Hyposaurus natator and Dyrosaurus maghribensis in possessing the combination of a short peduncle bridge and a large pubic knob on its anterior peduncle ( Figs 70 ; 71 ).

The anterior peduncle of Acherontisuchus guajiraensis appears to be missing portions medially and dorsally ( Fig. 70B, E ). Presumably, most of the anterior and anteroventral (i.e. the pubic ’knob’) articular facets are present whereas the dorsal portion of the articular surface is absent ( Fig. 70E ). Hence, the medial portion of the anterior peduncle of Acherontisuchus guajiraensis has been tentatively reconstructed ( Fig. 70E ) based on those of Hyposaurus natator (YPM VP.000753, YPM VP.000985, and NJSM 23368). Overall, the articular surface of the anterior peduncle of Acherontisuchus guajiraensis displays a mediolaterally wide pubic ‘knob’ in relation to the mediolateral width of the peduncle bridge compared to other dyrosaurids (i.e. Hyposaurus natator and Dyrosaurus maghribensis ). It is possible that the dorsal expansion of the articular surface of Acherontisuchus guajiraensis was larger than what is preserved.

The dorsal margin of the peduncle bridge is broken, so its actual shape remains unknown. Still, based on the relative position of the pubic peduncle of the ilium of Acherontisuchus guajiraensis and also other dyrosaurids (i.e. Hyposaurus natator and Dyrosaurus maghribensis ), it is possible that the missing portion of the peduncle bridge and anterior peduncle rapidly rose dorsally to meet the ilium ( Fig. 70 ). As in other dyrosaurids, the mediolateral thickness of the peduncle bridge of Acherontisuchus guajiraensis drastically reduces from about its mid-height, which implies a relatively thin breadth for the broken portion of the dorsal margin (and presumably accounts for its preservation state). This hollow borne by the peduncle bridge and anterior peduncle forms the ventral portion of the acetabulum. There is a prominent rim separating the peduncle bridge in two asymmetrical halves, from which the thickness of the bone decreases dorsally.The rim stems from the anterior extremity of the posterior peduncle, and can be followed up until the dorsal extremity of the anterior peduncle. Overall, the rim takes the shape of a wide and concave parabola, and marks the ventral border of the bony acetabulum. In other dyrosaurids (i.e. Hyposaurus natator and Dyrosaurus maghribensis ), the rim on the peduncle bridge underlining the ventral border of the acetabulum is less prominent than in Acherontisuchus guajiraensis , and is also located more dorsally. Indeed, in Hyposaurus natator (YPM VP.000753, YPM VP.000985, and NJSM 23368) and Dyrosaurus maghribensis , the rim is almost on the same level as the dorsomedial corner of the peduncle bridge (medial side of the acetabular perforation of the ischium) and thus appears like a dorsolateral corner. For this reason, the peduncle bridge of Hyposaurus natator and Dyrosaurus maghribensis has the appearance of a strictly dorsal area, whereas Acherontisuchus guajiraensis seems to possess a lateral component in the orientation of the surface positioned dorsally to the rim. It is possible that some deformations have provoked this orientation in the hollow dorsal surface of the ischium of Acherontisuchus guajiraensis . The combination of the extension of the hollow surface, added to the relative height of the bony acetabulum on the ilium conveys the existence of an enlarged acetabulum for Acherontisuchus guajiraensis compared to other dyrosaurids (i.e. Hyposaurus natator and Dyrosaurus maghribensis ). The large and oval shape of the entire acetabulum (i.e. ilium plus ischium) of Acherontisuchus guajiraensis presumably reflect the unique shape its femur (i.e. strong sigmoid shape due to prominent anterior deflection of femoral head and likewise posterior deflection of distal condyles), and the subsequent articular capsule.

The anterior peduncle of Acherontisuchus guajiraensis presumably extended as far dorsally as the posterior peduncle ( Figs 70 ; 71 ), similar to all other dyrosaurids (i.e. Hyposaurus natator and Dyrosaurus maghribensis ). The overall outline of the articular facet of the posterior peduncle is preserved, revealing its relatively triangular outline which appears almost as long dorsoventrally (or mediolaterally) than it is anteroposteriorly. In parallel, the preservation state of the posterior peduncle allow us to take a peek at its junction with the ilium and the orientation of the articular facets involved: thus, the articular facet of the posterior peduncle seems tilted towards the ilium rather than dorsally oriented as in other dyrosaurids (i.e. Hyposaurus natator and Dyrosaurus maghribensis ). This orientation of the posterior articular facet in Acherontisuchus guajiraensis plays a role in its greater dorsal extension (along with the ventral deflection of the anterior peduncle). The neck of the posterior peduncle is composed of a concave anterior margin and a strongly convex posterior one, resembling that of Dyrosaurus maghribensis OCP DEK-GE 252 but more accentuated. Lateromedially, the convex posterior portion of the neck is also the thinnest portion, which gradually thickens anteriorly.

Underneath both peduncles, the anterior and posterior margins of the ischium display each a concave shape of differing intensity. In their convergent area, they form the shaft or neck of the ischium, then they diverge to create the distal blade. At its thinnest point, the shaft of Acherontisuchus guajiraensis ( Fig. 70 ) displays the smallest anteroposterior length among dyrosaurids (i.e. Hyposaurus natator and Dyrosaurus maghribensis ) ( Hastings et al. 2011). Also, when looking at the ischial length starting from the base of the peduncles up until the tip of the distal blade, the thinnest portion of the shaft of Acherontisuchus guajiraensis is located more proximally than in other dyrosaurids. The posterior margin of the ischium of Acherontisuchus guajiraensis is weakly concave and thus appears almost straight, like Hyposaurus natator NJSM 23368 and Dyrosaurus maghribensis OCP DEK-GE 254 ( Jouve et al. 2006), as opposed to Hyposaurus natator YPM VP. 000753 and Dyrosaurus maghribensis OCP DEK-GE 252. Potentially, the shape of the posterior margin of the ischium of Acherontisuchus guajiraensis could also be subjected to intraspecific variations. The anterior margin of the ischium of Acherontisuchus guajiraensis has not been recovered. Since the anterior margin of the ischium seems less subject to variations among Dyrosauridae (i.e. Hyposaurus natator and Dyrosaurus maghribensis ), the ischium of Acherontisuchus guajiraensis and has been reconstructed based on that of Dyrosaurus maghribensis OCP DEK-GE 255. Yet, the distal margin of the position of the anterior process of the ilium, and its shape, remain unknown.

Lateromedially, the surface of the ischium underneath the peduncles varies in thickness, with the thicker portion of the bone forming a beam connecting the base of the anterior peduncle and the distal corner of the ischial blade, similar to other dyrosaurids (i.e. Hyposaurus natator and Dyrosaurus maghribensis ). The lateral surface of the ischium anterior to the beam is strongly concave in the region of the distal blade, but in the area of the shaft it is still slightly convex. The gradual transition between the two areas takes place underneath (i.e. ventrally to) the constriction of the shaft.

The distal corner of the ischial blade of Acherontisuchus guajiraensis is rounded and does not appear to form a protruding posterior process as in Dyrosaurus maghribensis . Also, the distal corner of Acherontisuchus guajiraensis appears to flatten lateromedially at the same time as the beam turns outward, unlike Hyposaurus natator which displays an anteriorly curled corner. From this point, the distal (or ventral) margin of Acherontisuchus guajiraensis bends medially to connect with the overall concavity of the ischial blade. The lateral surface of the distal corner is strongly pitted, indicating the presence of a cartilage cap in vivo.

Figures

FIG. 70. — Left pelvic girdle elements of Acherontisuchus guajiraensis Hastings, Bloch & Jaramillo, 2011, UF/IGM 38: A, left ilium in medial view; B, detail of left ischium in medial view; C, left ilium in lateral view; D, left ischium in lateral view; E, detail of dyrosaurid ischia in anterior view. Arrow points anteriorly. Scale bars: 1 cm.

FIG. 71. — Pelvic reconstruction of Acherontisuchus guajiraensis Hastings, Bloch & Jaramillo, 2011, UF/IGM 38; ischium has been repaired based on Hyposaurus natator (Troxell, 1925), NJSM 23368; pubis have been recreated based on Hyposaurus natator, YPM VP.000753; A, lateral view;B, anterior view; C, ventral view; D, dorsal view. Arrow points anteriorly. Target indicates anterior. Original 3D models of YPM VP.000753 courtesy of Narimane Chatar; original 3D models of NJSM 23368 courtesy of Candice Stephanic. Reconstructed bones only serve as a qualitative representation of the pelvic girdle of Acherontisuchus guajiraensis. Scale bar: 5 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.