Macrospondylus bollensis, (Jager, 1828), (Jager)

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 : 220-224

MACROSPONDYLUS BOLLENSIS ( JÄGER, 1828)

For measurements, see Tables 7-9 .

Ilium

The ilium of Macrospondylus bollensis SMNS 17484a ( Fig. 45 ) resembles those of basal teleosauroids Platysuchus multiscrobiculatus and Plagiophthalmosuchus gracilirostris in displaying a slender postacetabulum process. Indeed, the posterior margin of the ilium of Macrospondylus bollensis joining the ischial peduncle to the ventral base of the postacetabulum is relatively long as it almost reaches the length of the anterior margin of the ilium. This trait contrasts with the more derived Charitomenosuchus leedsi , Neosteneosaurus edwardsi and Lemmysuchus obtusidens . In addition, the posterior and anterior margins of the ilium of Macrospondylus bollensis are parallel to one another as in Platysuchus multiscrobiculatus and supposedly Plagiophthalmosuchus gracilirostris . The latter condition is also recovered in Charitomenosuchus leedsi although both margins are curved and of differing length. As a consequence, the dorsal and ventral margins of the postacetabular process of the ilium of Macrospondylus bollensis are located close to one another as in Platysuchus multiscrobiculatus and Plagiophthalmosuchus gracilirostris , but unlike the other more derived teleosauroids (i.e. Charitomenosuchus leedsi , Neosteneosaurus edwardsi and Lemmysuchus obtusidens ). Still, Charitomenosuchus leedsi displays a proportionally narrower postacetabular process compared to Neosteneosaurus edwardsi and Lemmysuchus obtusidens .

The preacetabular process of the ilium of Macrospondylus bollensis displays a relatively stout aspect as in Platysuchus multiscrobiculatus : its base height almost reaches its total anteroposterior length, and its thickness only slightly decreases anteriorly. The preactebular process of Macrospondylus bollensis culminates in a rounded tip similar to Platysuchus multiscrobiculatus . Comparatively, more derived teleosauroids display a slender preacetabular process that appears hook-shaped. The junction between the preacetabular process and the dorsal margin of the ilium forms a shallow recces as in other teleosauroids (i.e. Plagiophthalmosuchus gracilirostris , Platysuchus multiscrobiculatus , Charitomenosuchus leedsi , Neosteneosaurus edwardsi and Lemmysuchus obtusidens ). The dorsal margin of the ilium of Macrospondylus bollensis appears to have been convex like that of other teleosauroids. Comparatively, the anterior margin of the ilium of Macrospondylus bollensis seems relatively straight as in Platysuchus multiscrobiculatus and Plagiophthalmosuchus gracilirostris , but it is possible that it were slightly convex following the arc of the supraacetabular crest similar to what is observed in Lemmysuchus obtusidens . The junction between the preacetabular process and the anterior margin of the ilium of Macrospondylus bollensis is achieved through an acute angle as in Platysuchus multiscrobiculatus and Lemmysuchus obtusidens only.

Ventrally, the margin of the ilium of Macrospondylus bollensis is slightly undulating similar to other teleosauroids (i.e. Charitomenosuchus leedsi , Neosteneosaurus edwardsi and Lemmysuchus obtusidens ): the ventral surface of each peduncle are slightly concave and the junction between them is achieved through a small notch (i.e. the acetabular perforation). Both peduncle are titled mesially towards the centre of the ilium to form the borders of the bony acetabulum ( Fig. 45 ). The ischial peduncle is anteroposteriorly longer than the pubic peduncle, but their lateral facet shows a similar dorsal extension unlike in Charitomenosuchus leedsi and Neosteneosaurus edwardsi (in Lemmysuchus obtusidens the extension of the ischial peduncle is unknown). The bony acetabulum of Macrospondylus bollensis proportionally occupies most of the lateral surface of the ilium, notably due to the small size of the postacetabular process. It also extends as far anteriorly and posteriorly as the margins of the bone, and occupies about 2/3 of the dorsoventral height of the ilium. The bony acetabulum of the ilium is ventrally bordered by the peduncles (of which the ischial peduncle also forms a portion of the posterior border), and anterodorsally by the supraacetabular crest. The supraacetabular crest of the ilium of Macrospondylus bollensis appears relatively shallow and smooth (i.e. not markedly pitted) as in Platysuchus multiscrobiculatus and Lemmysuchus obtusidens . The acetabular perforation of the ilium of Macrospondylus bollensis is strongly reduced as in other thalattosuchians (e.g. Pelagosaurus typus, Thalattosuchus superciliosus , Charitomenosuchus leedsi , Neosteneosaurus edwardsi , Lemmysuchus obtusidens , etc.). However, the counterpart of the acetabular perforation on the ischium of Macrospondylus bollensis is one of the largest among teleosauroidea, along with that of Charitomenosuchus leedsi .

Ischium

Overall, the ischium of Macrospondylus bollensis ( Figs 46 ; 47 ) displays a proportionally thick shaft as well as a deep acetabular perforation in all specimens (NHMUK PV R 5703, SMNS 17484a and SMNS 16848). In this way, it resembles the proximal portion of the ischium of Pelagosaurus typus. Indeed, in Macrospondylus bollensis , the anteroposterior thickness of the shaft at its constriction is greater than that of the posterior peduncle (similar to Cricosaurus suevicus , Dakosaurus maximus , Aeolodon priscus , and Lemmysuchus obtusidens ) of about 1/ 4 in this case. In addition, the shaft of Macrospondylus bollensis is relatively short with its dorsoventral height reaching the same length as anteroposterior constriction, as in Dakosaurus maximus or Lemmysuchus obtusidens . The anterior and posterior margins of the ischium of Macrospondylus bollensis are strongly concave with their apex parallel, similar to most thalattosuchians (with the exception of Thalattosuchus superciliosus NHMUK PV R 2054 and Cricosaurus suevicus ). The posterior margin of the ischium of Macrospondylus bollensis shows a strong posterior bulge located underneath the posterior peduncle. This bulge is not found in other teleosauroids.

The anterior peduncle of the ischium of Macrospondylus bollensis NHMUK PV R 5703 and SMNS 17484a is relatively ovoid in shape with its greatest axis being oriented lateromedially rather than dorsoventrally (SMNS 16848 is flattened mediolaterally). The dorsal-most surface of the anterior peduncle is not significantly taller than the dorsal-most surface of the posterior peduncle and thus appears on the same level, unlike in metriorhynchoids (e.g. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Tyrannoneustes lythrodectikos , Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Geosaurus giganteus , etc.) but similar to other teleosauroids (i.e. Aeolodon priscus , Lemmysuchus obtusidens , Charitomenosuchus leedsi , Neosteneosaurus edwardsi ). The anterior peduncle is borne by the peduncle bridge, which is an anteriorly protruding process. The latter displays a concave dorsal margin and a convex ventral one which slightly diverge posteriorly thus widening the process as it connects with the main body of the ischium. Similar to other thalattosuchians, the dorsal surface of the peduncle bridge is uneven and leans medially (e.g. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , Lemmysuchus obtusidens , Neosteneosaurus edwardsi , etc.). This surface corresponds to the ventral border of the acetabular perforation.

The posterior peduncle of Macrospondylus bollensis (NHMUK PV R 5703, SMNS 17484a and SMNS 16848) differs from that of most thalattosuchians in being markedly prominent dorsally (i.e. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus NHMUK PV R 2054 , Cricosaurus suevicus , Geosaurus giganteus , Dakosaurus maximus , Tyrannoneustes lythrodectikos , Torvoneustes carpenteri , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Aeolodon priscus , Lemmysuchus obtusidens , Neosteneosaurus edwardsi ). This unusual shape can partly be explained through a deeper and larger indentation for the acetabular perforation at the junction between the peduncle. Indeed, the acetabular perforation of Macrospondylus bollensis ( Figs 46 ; 47 ) is almost as deep as the peduncle bridge is anteroposteriorly long. This trait is not usually recovered in thalattosuchians, but rather in dyrosaurids (e.g. Hyposaurus natator , or Dyrosaurus maghribensis ). The articular surface of the posterior peduncle of Macrospondylus bollensis is wedge-shaped, with the iliac suture occupying about 1/3 of the surface medially. The second facet of the posterior peduncle of Macrospondylus bollensis (SMNS 17484a) corresponding to the ventral border of the bony acetabulum displays a triangular shape and is slightly concave as in other teleosauroids.

The distal blade of the ischium of Macrospondylus bollensis SMNS 17484a, SMNS 16848 and PMU 35980 is relatively slender, especially compared to the broadness of the overhanging shaft. Indeed, the slender aspect of the distal blade is mainly caused by the shape of the posterior process, whose anteroposterior length greatly exceeds its dorsoventral height (and accounts for about 150% of its base height).

In addition, the posterior process appears to retain most of its thickness throughout its length. The tip of the posterior process is rather rounded or slightly elliptic. In this way, the ischium of Macrospondylus bollensis strongly resembles that of Platysuchus multiscrobiculatus and Pelagosaurus typus. The anterior process of the ischium of Macrospondylus bollensis forms a short and thin hook with a slightly undulating ventral margin and a convex dorsal one. The anterior process extends anteriorly as further as the anterior peduncle similar to other thalattosuchians (i.e. Pelagosaurus typus, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Thalattosuchus superciliosus , Aeolodon priscus , Lemmysuchus obtusidens , Neosteneosaurus edwardsi , etc.). The ventral margin of the distal blade is overall straight but forms a wave around its mid-length.

Pubis

The pubis of Macrospondylus bollensis NHMUK PV R 5703, SMNS 17484a, and PMU 35980 ( Figs 46 ; 47 ) displays an overall singular shape highly resembling that of Pelagosaurus typus, with the main difference residing in the relative size of the pubic apron (which is proportionally shorter and larger for Macrospondylus bollensis ). Indeed, the pubis of Macrospondylus bollensis is relatively slender due to an elongated but narrow pubic apron whose maximum proximodistal height reaches about 67% of the total height of the pubis.

The shaft of Macrospondylus bollensis is relatively straight, as opposed to Pelagosaurus typus. The lateral and medial margins of the pubic apron of Macrospondylus bollensis follow the same curve, with the medial one being concave and the lateral one convex. Distally, the lateral and medial margins connect to the distal blade, which is slightly convex like that of Pelagosaurus typus. The pubic symphysis is found along this convex margin, near the medial corner of the pubic apron. The pubic symphysis of Macrospondylus bollensis is not as well developed as in some other thalattosuchians (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus NHMUK PV R 2054 , Lemmysuchus obtusidens , Machimosaurus , etc.), but reaches about 35% of the total perimeter of the distal blade ( Fig. 47 ). In addition, the pubic apron of Macrospondylus bollensis does not display a wide flaring, which is similar to Pelagosaurus typus and Plagiophthalmosuchus gracilirostris .

Like other thalattosuchians (except rhacheosaurines and Geosaurus giganteus ), the pubic apron of Macrospondylus bollensis ( Fig. 46 ) is strongly asymmetrical (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus , Lemmysuchus obtusidens , Machimosaurus , etc.). Indeed, the majority of the surface of the pubic apron lies on the medial side of the median of the shaft.

The constriction of the pubis, delimiting the shaft distally, is relatively narrow leading to impressive proximal and distal widening of the bone. Indeed, the thickness of the constriction represents about 1/3 of the mediolateral width of the peduncle. The latter is composed of a convex margin laterally and a concave one medially. The articular surface of the peduncle appears to be slightly convex as well, but also medially tilted as in Suchodus durobrivensis or Thalattosuchus superciliosus .

Figures

FIG. 45. — Pelvic girdle elements of Macrospondylus bollensis (Jäger, 1828): A, left pubis in posterior view of SMNS 17484a; B, left ilium in lateral view of SMNS 17484a; C, left ischium in lateral view of SMNS 17484a; D, left ischium in lateral view of SMNS 16848; E, overview of the pelvic girdle of PMU 35980.Arrow points anteriorly. Target indicates anterior. Scale bars: 1 cm.

FIG. 46. — Pelvic girdle elements of Macrospondylus bollensis (Jäger, 1828), NHMUK PV R 5703: A, left pubis in posterior view; B, left ischium in medial view; C, overview of the pelvic girdle elements. Arrow points anteriorly. Target indicates anterior. Scale bars: 1 cm.

FIG. 47. — Pelvic reconstruction of Macrospondylus bollensis (Jäger, 1828), NHMUK PV R 5703 (holotype): A, lateral view; B, anterior view; C, ventral view; D, dorsal view. Arrow points anteriorly. Target indicates anterior.The right ischium and left pubis are mirrored. Reconstruction of the ischium based on Macrospondylus bollensis, SMNS 16848. Scale bar: 5 cm.