Sericodon jugleri, Von Meyer, 1845

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 : 208-212

SERICODON JUGLERI VON MEYER, 1845

For measurements, see Tables 7-9 .

The bone referred to the pubis of Sericodon jugleri MJSN SCR 010- 312 in Schaefer et al. (2018) is not included in our description. Indeed, the bone displays all the characteristics of a derived teleosauroid humerus: shortening of humeral shaft; posterior deflection of the articular proximal head; distal torsion of the condyles in relation to the coronal plane; continuous surface between the deltopectoral crest and the proximal articular head.

Ilium

The ilium of Sericodon jugleri ( Figs 39 ; 40 ) stand out with its combination of a long and mediolaterally thick preacetabular process with a proportionally short and broad postacetabular process. Indeed, the preacetabular process of Sericodon jugleri slightly flares out anteriorly as it increases in thickness mediolaterally, which contrasts with other teleosauroids (e.g. Platysuchus multiscrobiculatus , Macrospondylus bollensis , Charitomenosuchus leedsi, Proexochokefalos cf. bouchardi, Neosteneosaurus edwardsi , and Lemmysuchus obtusidens ). The apex of the preacetabular process of Sericodon jugleri is semi-circular in dorsal view. In lateral view, the preacetabular process forms a relatively long (about 26% of total dorsal length) and sharp apex: its ventral margin is concave whereas its dorsal margin is slightly convex, similar to other teleosauroids.However, in Sericodon jugleri , the preacetabular process is oriented anterodorsally which further contrasts with other teleosauroids (e.g. Platysuchus multiscrobiculatus , Macrospondylus bollensis , Charitomenosuchus leedsi, Proexochokefalos cf. bouchardi, Neosteneosaurus edwardsi , and Lemmysuchus obtusidens ). Posteriorly, the junction between the preacetabular process and the dorsal margin of the ilium – the iliac crest – forms a shallow recess.

Overall, the postacetabular process of Sericodon jugleri stands out from that of other teleosauroids in displaying the following combined traits: 1) two convex margins forming a Lancet arch; 2) two apparent hollows marking its base dorsally and posteriorly; and 3) a posterodorsal orientation. Similarly to the preacetabular process, the postacetabular process of Sericodon jugleri possesses a dorsal component in its orientation so that its apex points posterodorsally, as in Charitomenosuchus leedsi, Proexochokefalos cf. bouchardi, and Neosteneosaurus edwardsi . The dorsal and posterior margins of the postacetabular process of Sericodon jugleri are both slightly convex with the same degree, giving the posterior process the shape of a Lancet arch. In comparison, Charitomenosuchus leedsi, Proexochokefalos cf. bouchardi, Neosteneosaurus edwardsi , Lemmysuchus obtusidens and Machimosaurus also have a posterior process in the overall shape of a Lancet arch, although with relatively differing degrees of convexity. Anteriorly, the first half of the lateral surface of the iliac crest is finely striated and so is the apex of the posterior process, indicating the presence of a cartilage cap in vivo. Ventrally, around 2/3 of its length, the concave posterior margin of the ischium becomes slightly concave for the remaining of its length. The inflection point marks the base of the postacetabular process, like the recess along the dorsal margin of the ischium. In comparison, only Machimosaurus and Lemmysuchus obtusidens appear to display two shallow recesses bordering the base of the postacetabular process, whereas other teleosauroids either displays none (e.g. Platysuchus multiscrobiculatus and Macrospondylus bollensis ) or only one (e.g. Charitomenosuchus leedsi , Neosteneosaurus edwardsi, Proexochokefalos cf. bouchardi).

The anterior margin underneath the preacetabular process is straight throughout most of its length. Indeed, the supraacetabular crest follows the outline of the anterior margin of the ilium without brimming over the edge (like in Pelagosaurus typus). Ventrally, the anterior margin of the ilium of Sericodon jugleri forms a slight bulge corresponding to the anterior edge of the pubic peduncle. The ventral margin of the ilium of Sericodon jugleri is strongly undulating as the ventral surfaces of the peduncles are not aligned, as in other teleosauroids. However, Sericodon jugleri differs from most teleosauroids in displaying a strongly ventrally protruding pubic peduncle as well as a marked acetabular perforation. In comparison, the pubic peduncle of Neosteneosaurus edwardsi and Lemmysuchus obtusidens is also ventrally prominent but their acetabular perforation is shallower. In Sericodon jugleri , the acetabular perforation forms a relatively deep notch with a semi-circular outline.

Laterally, the bony acetabulum of the ilium of Sericodon jugleri forms a relatively large and deep recess almost spherical in shape and the bony acetabulum is almost as long anteroposteriorly as dorsoventrally tall as in most teleosauroids. The bony acetabulum is bordered anteriorly and dorsally by an arched structure: the supracetabular crest. The latter forms a thin ridge anteriorly which gradually widens dorsally to form a wedged-shape scarred area, which contrasts with the thinner supraacetabular crest of Charitomenosuchus leedsi, Proexochokefalos cf. bouchardi, Neosteneosaurus edwardsi , and Lemmysuchus obtusidens . Laterally, the supraacetabular crest of Sericodon jugleri faintly protrudes throughout its length, as in most teleosauroids (except Proexochokefalos cf. bouchardi).

On the lateral surface of the ilium, the outline of the pubic and ischial peduncle of Sericodon jugleri are similar to Charitomenosuchus leedsi, Proexochokefalos cf. bouchardi, Neosteneosaurus edwardsi , and Lemmysuchus obtusidens . The pubic peduncle of Sericodon jugleri forms two major dorsal peaks slightly posteriorly titled and separated by a deep depression. The anterior peak is isosceles-shaped whereas the posterior one possesses a circular top. In addition, the pubic peduncle of Sericodon jugleri is dorsoventrally taller than the ischial peduncle as in Charitomenosuchus leedsi and Neosteneosaurus edwardsi . Still, in Sericodon jugleri , both peaks of the pubic peduncle are similarly sized which contrasts with Charitomenosuchus leedsi, Proexochokefalos cf. bouchardi, Neosteneosaurus edwardsi , and Lemmysuchus obtusidens . Ventrally, the surface of the pubic peduncle of Sericodon jugleri displays a wedged outline similar to other teleosauroids: the lateral margin is concave as it borders the acetabulum, whereas the medial margin is convex. However, unlike in Neosteneosaurus edwardsi , and Lemmysuchus obtusidens , the posterior margin of the pubic peduncle of Sericodon jugleri does not form a pointed tip but is rather concave (which is similar to Charitomenosuchus leedsi , and Proexochokefalos cf. bouchardi). Like in Neosteneosaurus edwardsi and Proexochokefalos cf. bouchardi, the mediolateral thickness of the pubic peduncle is relatively constant throughout its length.

Laterally, the outline of the ischial peduncle of Sericodon jugleri has a more common shape as in other teleosauroids: it is crescent-shaped with its dorsal apex being posteriorly shifted. As it borders the acetabulum posteriorly, the ischial peduncle protrudes laterally and hence is mediolaterally thicker posteriorly. In ventral view, the ischial peduncle displays a triangular outline with its longest axis anteroposteriorly oriented. Like for the pubic peduncle, the lateral margin of the ischial peduncle of Sericodon jugleri is slightly concave whereas its medial margin is slightly convex. Like in Charitomenosuchus leedsi and Proexochokefalos cf. bouchardi, the ischial peduncle of Sericodon jugleri is markedly longer anteroposteriorly than the pubic peduncle (about 1/3 of the pubic peduncle’s length). This contrasts with Macrospondylus bollensis , Neosteneosaurus edwardsi , and Lemmysuchus obtusidens in which the difference is more subtle.

The attachment sites for the sacral ribs of Sericodon jugleri are found on the medial side of the ilium, and are overall similar to most teleosauroids (except Lemmysuchus obtusidens and Machimosaurus which possess a third attachment site for the first caudal). Indeed, the attachment sites for the sacral ribs form two distinct imprints located at the mid-height of the bone. They share a margin around the centre of the ilium, then gradually flare out in opposing directions (i.e. anteriorly and posteriorly) like a pair of almost symmetrical wings. The attachment site for the second sacral rib is the largest. The dorsal edge of the attachment site for the first sacral rib splits the preacetabular process in two along its greatest length, and extends up to its apex. Similarly, the dorsal edge of the attachment site for the second sacral rib reaches the apex of the postacetabular process through its mid-line. However, the ventral edge of the attachment site of the second sacral rib reaches the posterovental corner of the ilium whereas the ventral edge of the attachment site for the first sacral rib forms a semi-circular shape which stretches to the base of the preacetabular process. The medial surface of both attachment sites are deeply scarred with radiating ridges, and also possess large medial excrescences located around their top (for the second) and centre (for the first). The attachment site for the second sacral rib also forms a small bump along the edge of the posterior margin of the ischium, which is partly visible in anterior view.This small bump differs from the marked pointed corner found along the posterior margin of Neosteneosaurus edwardsi (underneath the postacetabular process).

Ischium

Proximally, the ischium of Sericodon jugleri ( Figs 39 ; 40 ) bears the distinctive combination of a proportionally short peduncle bridge as well as a marked acetabular perforation. The former trait – short peduncle bridge – is also found in Aeolodon priscus in which it is proportionally smaller in relation to the posterior peduncle. The peduncle bridge of Sericodon jugleri appears distantly set from the posterior peduncle, as in Macrospondylus bollensis and Charitomenosuchus leedsi (which also have a large acetabular perforation). The dorsal margin of the peduncle bridge is slightly concave whereas the anterior margin is relatively straight. Anterodorsally, the peduncle bridge of Sericodon jugleri is proportionally thick – a trait emphasized by its short length – but still decreases in dorsoventral thickness towards the anterior peduncle. Mediolaterally, the peduncle bridge increases in width towards the articular facet of the anterior peduncle. The junction between the articular facet of the anterior peduncle and the peduncle bridge is marked dorsally and ventrally by a small depression, whereas mediolaterally it is marked by a peak in width. The articular facet of the anterior peduncle is convex and displays the typical crescent-shape found in other teleosauroids: dorsally concave, ventrally convex, and pointed mediolaterally. Like in other thalattosuchians, the anterior peduncle of Sericodon jugleri appears laterally offset when looking at the posterior peduncle as a whole, but it is actually centred in relation to the lateral articular facet.

In Sericodon jugleri , the shape of the acetabular perforation is uncommon among teleosauroids: it is large, is sub-circular in shape and is mediolaterally oriented as in extant crocodylians and dyrosaurids. In comparison, the acetabular perforation of Aeolodon priscus and Macrospondylus bollensis display the same mediolateral orientation, but in Charitomenosuchus leedsi it is slightly tilted medially (see below). In Sericodon jugleri , the pronounced depth of the acetabular perforation coupled with the shortness of the peduncle bridge make the posterior peduncle stand out dorsally. The overall shape of the posterior peduncle of Sericodon jugleri is similar to other teleosauroids. About 1/3 of the posterior peduncle is oriented medially and displays a strongly scarred surface as it was connected to the ischial peduncle of the ilium. The remaining portion of the posterior peduncle is laterally oriented, has a subquadrangular outline and a slightly concave surface. The junction between the two surfaces forms a slightly protruding ridge also concave. The lateral edge of the posterior peduncle forms a thick rounded rim (e.g. Teleosaurus sp. , Macrospondylus bollensis, Proexochokefalos cf. bouchardi, Neosteneosaurus edwardsi , Lemmysuchus obtusidens and presumably Machimosaurus ), which becomes more tenuous on the other sides. In comparison, Mycterosuchus nasutus , Indosinosuchus potamosiamensis Martin, Suteethorn, Lauprasert,Tong, Buffetaut, Liard, Salaviale,Deesri, Suteethorn & Claude, 2019 , and Platysuchus multiscrobiculatus display a thin rim marking the edge of the posterior peduncle.

Distally, the ischium of Sericodon jugleri is distinct due to its bulky posterior process; it bears a dorsoventrally thick and rounded posterior process so that its entire posterior margin presumably displayed a strong sinusoidal shape (with the inflection point located around its mid-length). Other teleosauroids with a similarly undulating posterior margin include Aeolodon priscus, Proexochokefalos cf. bouchardi, Machimosaurus , and Lemmysuchus obtusidens , all of which display a relatively thick posterior process. Compared to the more derived teleosauroids Proexochokefalos cf. bouchardi, Machimosaurus , and Lemmysuchus obtusidens , the aeolodontines Sericodon jugleri and Aeolodon priscus display an anteroposteriorly shorter posterior process which contributes to their bulkier aspect. The ischial blade of Sericodon jugleri is relatively straight, and the ischial suture surface is restricted to its ventral margin (i.e. does not expand on the medial surface of the bone) as in the other aeolodontines Aeolodon priscus and Mycterosuchus nasutus . This feature is also present in in Pelagosaurus typus, Proexochokefalos cf. bouchardi, and Neosteneosaurus edwardsi .

Figures

FIG. 39. — Pelvic girdle elements of Sericodon jugleri Von Meyer,1845, MJSN SCR010-312: A, left ilium in lateral view;B, left ilium in medial view;C, left ischium in lateral view;D, left ischium in medial view.Cross indicates posterior.Original models courtesy of Dr.Jeremy Anquetin (Jurassica Museum-MJSN).Scale bar:1 cm.

FIG. 40. — Reconstruction of the pelvic girdle of Sericodon jugleri Von Meyer, 1845, MJSN SCR010-312: A, right lateral view; B, anterior view; C, ventral view. Target indicates anterior. Arrow points anteriorly. The right ilium and ischium are mirrored. Original models courtesy of Dr. Jeremy Anquetin (Jurassica Museum- MJSN). Scale bar: 5 cm.