Tyrannoneustes lythrodectikos, YOUNG, DE ANDRADE, BRUSATTE, SAKAMOTO & LISTON, 2012

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 : 188-190

TYRANNONEUSTES LYTHRODECTIKOS YOUNG, DE ANDRADE, BRUSATTE, SAKAMOTO & LISTON, 2012

For measurements, see Tables 7-9 .

Ilium

The ilium of Tyrannoneustes lythrodectikos GLAHM V 972 and GLAHM V1145 ( Figs 26 ; 27 ) is similar to that of other derived metriorhynchoids in lacking the presence of postacetabular process (e.g. Suchodus durobrivensis , Thalattosuchus superciliosus , Cricosaurus araucanensis , Geosaurus giganteus , etc.). The ilium of Tyrannoneustes lythrodectikos GLAHM V 972 and GLAHM V1145 is strongly similar to that of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 both in overall shape and in details (see below).

The ilium of Tyrannoneustes lythrodectikos GLAHM V 972 and GLAHM V1145 possesses a short and stout preacetabular process similar to ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 or Thalattosuchus superciliosus NHMUK PV R 2054 . Indeed, the anteroposterior length of the preacetabular process of the ilium of Tyrannoneustes lythrodectikos almost reaches the total dorsoventral height of its base. In addition, the preacetabular process does not drastically slim down from its base thickness, and is topped with a rounded apex (as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 or Thalattosuchus superciliosus NHMUK PV R 2054 ). The preacetabular process of Tyrannoneustes lythrodectikos is directly in line with the dorsal margin of the ilium. The latter is only slightly longer than the preacetabular process and also subtly convex (similar to ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and Thalattosuchus superciliosus NHMUK PV R 2054 ). The junction between the slightly convex dorsal margin and strongly concave posterior margin of the ilium of Tyrannoneustes lythrodectikos is achieved through a rounded obtuse angle as in Suchodus durobrivensis , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and Thalattosuchus superciliosus NHMUK PV R 2054 . As in other metriorhycnhoids, the dorsal margin of the ilium of Tyrannoneustes lythrodectikos is shorter than the anterior margin, and ends ventrally to form the posterior corner of the ischial peduncle. The ischial peduncle of the ilium of Tyrannoneustes lythrodectikos displays an overall triangular outline as in other thalattosuchians, crocodylians and dyrosaurids (e.g. Suchodus durobrivensis , Macrospondylus bollensis , Mecistops cataphractus , Congosaurus bequaerti , etc.). The anterior and posterior margins of the facet of the ischial peduncle of Tyrannoneustes lythrodectikos are overall similar in length, which gives the ischial peduncle the aspect of an isosceles triangle like for many other metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Thalattosuchus superciliosus NHMUK PV R 2054 , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Suchodus durobrivensis , etc.). The ischial peduncle is oriented both anteriorly and laterally, with its posterior corner laterally protruding compared to its anterior corner. In this way, the ischial peduncle forms a bony barrier posteriorly to the bony acetabulum as in other crocodyliformes. The articular facet of the ischial peduncle where it connects to the ischium is not located on the same level as the ventral facet of the pubic peduncle, resulting in an undulating ventral margin for the ilium of Tyrannoneustes lythrodectikos . On the lateral side of the ilium, the outline of the ischial peduncle gradually changes to the pubic peduncle without any ruptures unlike in other crocodyliformes bearing a postacetabular process (i.e. teleosauroids, dyrosaurids, crocodylians, etc.). The pubic peduncle of the ilium of Tyrannoneustes lythrodectikos GLAHM V1145 reaches threequarters of the ischial peduncle’s total dorsoventral height but is slightly longer anteroposteriorly. The outline of the pubic peduncle is undulating: its overall shape is that of a dome with a peak present on each side and a corresponding hollow in between. There is another peak formed by the pubic peduncle situated on the anterior margin of the ilium which is separated from the rest by the ‘root’ of the supraacetabular crest as in Mecistops cataphractus , Thalattosuchus superciliosus , and ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804. The ventral surface of the pubic and ischial peduncles are lunate as in other thalattosuchians due to their lateral surface displaying a component oriented towards the bony acetabulum. The ventral surfaces of the pubic and ischial peduncles are separated by a subtle recess or notch corresponding to the acetabular perforation. The latter is strongly reduced and almost imperceptible, like for ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804. For Tyrannoneustes lythrodectikos (and ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804), most of the acetabular perforation is formed by the curved peduncle bridge of the ischium ( Fig. 27 ). The pubic peduncle of the ilium of Tyrannoneustes lythrodectikos forms a rounded corner anteriorly which does not protrude, unlike in some metriorhynchoids (e.g. Pelagosaurus typus, Thalattosuchus superciliosus NMI F21731, Cricosaurus araucanensis ). The anterior margin of the ilium of Tyrannoneustes lythrodectikos is gently convex and forms a smooth rounded re-entrant at its junction with the preacetabular process.

The supraacetabular crest originates on the anterior edge of the ilium, near the lateral facet of the pubic peduncle. It takes the shape of an arch which protrudes laterally to border the bony acetabulum dorsally and anteriorly. The supraacetabular crest of Tyrannoneustes lythrodectikos is deeply scarred as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, Thalattosuchus superciliosus , and Suchodus durobrivensis . The rugged area is not limited to the supraacetabular crest (like in aforementioned taxa): it extends anterodorsally up until the base of the preacetabular process, and it also covers the entire bony acetabulum. Unlike ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, the supraacetabular crest of Tyrannoneustes lythrodectikos does not follow the anterior margin of the ilium but rather differs from it dorsally.

The attachment sites for the sacral ribs form bilobate areas on the medial side of the ilium, as in Suchodus durobrivensis , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763. The sacral rib attachment sites share a margin mesially and are located towards the dorsal margin of the ilium. The dorsal lobe of the sacral rib attachment sites are imprinted on the ilium and display a dorsal bony wall as a result. On the contrary, the ventral lobe appears to be slightly raised. Underneath the sacral rib attachment sites, the medial surface of the ilium shows concentric ridges as seen in other metriorhynchoids (e.g. ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Suchodus durobrivensis , Thalattosuchus superciliosus , etc.).

The ilium of Tyrannoneustes lythrodectikos GLAHM V 972 and GLAHM V1145 resembles that of Thalattosuchus superciliosus (all morphotypes) but with key differences. First, the acetabular perforation of the ilium of Tyrannoneustes lythrodectikos GLAHM V 972andGLAHM V1145 is strongly reduced (and appears almost absent as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and Suchodus durobrivensis ) whereas all Thalattosuchus superciliosus bear a small but characteristic notch at the junction of their peduncle.Then, the preacetabular process of Tyrannoneustes lythrodectikos GLAHM V 972 and GLAHM V1145 is proportionally shorter than in Thalattosuchus superciliosus NHMUK PV R 2054, GLAHM V1146 (which we would call the ’de Blainville’ morphotype) but still presents the same rounded tip aspect. Compared to Thalattosuchus superciliosus SMNS 10116, NHMUK PV R 1530, NMI F21731 or GLAHM V960 (which we would call the ’Leeds’ morphotype), the tip of the preacetabular process of of Tyrannoneustes lythrodectikos is shorter and more rounded like that of ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804.

Ischium

The ischium of Tyrannoneustes lythrodectikos GLAHM V972 ( Figs 26 ; 27 ) stands out with its short and thick shaft whose proximodistal length accounts for about 3/4 of its anteroposterior constriction. Other metriorhynchoids with short and thick ischial shaft include ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and Dakosaurus maximus . Underneath the shaft, the ischium of Tyrannoneustes lythrodectikos appears to rapidly flare out distally. This effect is accentuated by the strongly concave and almost hemispherical anterior margin of the ischium as in ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804 and Dakosaurus maximus . Comparatively, the posterior margin of the ischium of Tyrannoneustes lythrodectikos is concave but with a lesser degree of curvature. The posterior peduncle of the ischium of Tyrannoneustes lythrodectikos is large: its anteroposterior length almost reaches that of the shaft constriction. However, the posterior peduncle is mediolaterally slender with a thickness reaching half of its total anteroposterior length. Laterally, the peduncle bridge of the ischium stems from the corner of the posterior peduncle resulting in the complete obstruction of the acetabular perforation on this side of the bone, similar to several derived metriorhynchoids (e.g. Cricosaurus suevicus , Cricosaurus albersdoerferi , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Geosaurus giganteus ). Starting from its base, the peduncle bridge of Tyrannoneustes lythrodectikos bends towards the medial side of the bone to bring the anterior peduncle (not preserved) near the ilium. In this way, the peduncle bridge creates a gap for the acetabular perforation. The latter is visible on the medial side of the ischium, where it forms an oblique notch or burrow tilted towards the medial side of the bone as in most metriorhynchoisd (e.g. Cricosaurus suevicus , Cricosaurus albersdoerferi , ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 4763, ‘ Metriorhynchus ’ brachyrhynchus NHMUK PV R 3804, Geosaurus giganteus ).

Figures

FIG. 26. — Ilium and ischium of Tyrannoneustes lythrodectikos Young, de Andrade, Brusatte, Sakamoto & Liston, 2012, GLAHM V972 (holotype) and ilium of GLAHM V1145 (referred): A, left ilium of GLAHM V972 in lateral view; B, left ischium of GLAHM V972 in lateral view; C, left ilium of GLAHM V972 in lateral view; D, left ischium of GLAHM V972 in medial view; E, right ilium of GLAHM V1145 in lateral view; F, right ilium of GLAHM V1145 in medial view. Arrow points anteriorly. Target indicates anterior. The right ischium and left pubis are mirrored. Scale bars: 1 cm.

FIG. 27. — Pelvic reconstruction of Tyrannoneustes lythrodectikos Young, Andrade, Brusatte, Sakamoto & Liston, 2013, GLAHM V972 (holotype), based on the ilium of Tyrannoneustes lythrodectikos, GLAHM V1145 (referred),and the ischium and pubis of ‘Metriorhynchus’ brachyrhynchus (Eudes-Deslongchamps, 1868), NHMUK PV R 3804: A, lateral view of reconstructed portion; B, lateral view of original ilium and ischium; C, anterior view; D, posterior view; E, ventral view; F, dorsal view. Arrow points anteriorly. Target indicates anterior. Scale bar: 5 cm.