Carcharodontosauridae, Stromer, 1931

Carcharodontosauridae gen. et sp. indet.

( Fig. 14)

Referred specimen

A partial left ischium (NHMUK PV R 16437).

Morphological description

The left ischium NHMUK PV R 16437 has its proximal portion preserved, missing the distalmost portion of the shaft and the ventralmost portion of the obturator process (Fig. 14). The iliac and pubic peduncles are separated by a concave acetabular rim that is shallow, wide in anterior view with a middle constriction, giving it an ‘hourglass-shape’ in proximal view (Fig. 14). The peduncles are subequal in size. In the proximal region, the iliac articular surface is triangular and deeply concave suggesting a peg-and-socket articulation (Fig. 14). In the anteriormost region of the iliac peduncle, anterior to the articular surface with the ilium, the ischium is thick mediolaterally, forming the ischial antitrochanter, which is parallelogram-shaped in anterior view. Although the antitrochanter is thick, it is not well projected, being a reduced ridge. In the posterior part of the iliac joint, there is a posterior flange that rises, and it is broken in the dorsalmost portion. The pubic peduncle is subtriangular and medially concave with the articular surface being laterally oriented.

The lateral and medial surfaces of the ischium are concave between the peduncles, with the concavity displaced dorsally in the lateral side and ventrally in the medial side. Posteroventral to the flange in the iliac peduncle, a deep and rugose sulcus, homologous to the ischial tuberosity (e.g. Hutchinson 2001b, Brusatte et al. 2008, Cuesta et al. 2018), runs in the lateral surface becoming shallower posteriorly (Fig. 14). The most proximal part of this sulcus, somewhat elliptical in shape, represents the osteological correlate of the origin of the muscle flexor tibialis internus 3 (FTI3), which is delimited slightly more distally. The most distal part of the sulcus in the posterodorsal region of the ischial shaft, which is more linear, less elliptical than the proximal part, becomes shallower distally, and represents the osteological correlate of the origin of the muscle adductor femoris 2 (ADD2) (Fig. 14A, B). Both of these muscle origins are level II inferences by Witmer’s (1995) systematization and are topologically compatible with other theropods (e.g. ceratosaurs— Cerroni et al. 2024, early tetanurans— Lacerda et al. 2024, and derived coelurosaurs— Carrano and Hutchinson 2002).

In the preserved portion of the ischium, the shaft is dorsoventrally flattened, giving it a subrectangular shape, lacking the distalmost part (Fig. 14). Ventral to the shaft and posterior to the pubic peduncle, the obturator process is separated from the pubic peduncle by a shallow and anteroposteriorly notch. Posterior to this notch, the obturator process is twisted medially from the pubic peduncle and broken in its ventralmost portion (however, a notch ventral to obturator process can be noted) and seems to be confluent with the shaft (Fig. 14). Although the ischial shaft is not completely preserved, the preserved part is straight, suggesting that the orientation of the main axis of the ischium was straight in NHMUK PV R 16437.

Morphological comparisons

The overall shape of the NHMUK PV R 16437 partial ischium resembles that of carcharodontosaurid theropods rather than of other dinosaurs. The acetabular rim is shallow in lateral view with a weak ‘U-shape’ due to the ventral position of the pubic peduncle, as seen in allosauroids (sensu Rauhut and Pol 2019), including the carcharodontosaurids Acrocanthosaurus , Concavenator , Giganotosaurus , Mapusaurus , and Neovenator . In some neovenatorids, such as Siats, the acetabular rim is shallow, not forming a ‘U-shaped’ border, being straighter than in other allosauroids (Zanno and Makovicky 2013). The ‘U-shape’ of the acetabular rim is more pronounced in megalosauroids such as Piatnitzkysaurus and also in the spinosaurids Baryonyx , Ichthyovenator , Vallibonavenatrix, FSAC-KK 11888, and possibly in Suchomimus due to a dorsal projection of the pubic peduncle ( Allain et al. 2012, Malafaia et al. 2020, Sereno et al. 2022, Lacerda et al. 2024). This condition differs from that observed in NHMUK PV R 16437.

The ilioischiatic articulation of NHMUK PV R 16437 has a deep peg-and-socket (or ball-and-socket) configuration, as seen in Acrocanthosaurus , Concavenator , Giganotosaurus , Mapusaurus , and Siats (Stovall and Langston 1950, Coria and Currie 2006, Carrano et al. 2012, Zanno and Makovicky 2013, Cuesta et al. 2018, Rauhut and Pol 2019, Lacerda et al. 2023). In other theropods, this articulation has a concave plane configuration (e.g. Carrano et al. 2012, Lacerda et al. 2023, Isasmendi et al. 2024).

The ischial antitrochanter is a well-developed, notch-shaped structure in coelophysoids, some ceratosaurs, the early tetanuran Sinosaurus , the spinosaurid Ichthyovenator , and the neovenatorid Siats ; it is a reduced notch in the ischium of other theropods (e.g. Allain et al. 2012, Carrano et al. 2012, Zanno and Makovicky 2013, Cuesta et al. 2018, Lacerda et al. 2023). In NHMUK PV R 16437, although the ischial antitrochanter is a reduced ridge, it represents a thick structure at its base, and this is more comparable with forms such as Acrocanthosaurus , Giganotosaurus , and Sinraptor , than Siats and other contemporary theropods such as spinosaurids.

The presence of the sulcus in the dorsolateral shaft of the ischium, which is posterior to the flange, is similar in NHMUK PV R 16437 and other theropods (e.g. Zanno and Makovicky 2013, Cuesta et al. 2018); however, in the ischium described here it is deeper, similar to carcharodontosaurids and neovenatorids rather than spinosaurids. Consequently, the osteological correlates of the origins of the muscles FTI3 (proximal) and ADD2 (distal) in NHMUK PV R 16437 are deeper than those noted in other non-carcharodontosaurid theropods (e.g. Carrano and Hutchinson 2002, Cerroni et al. 2024, Lacerda et al. 2024).

The obturator process of NHMUK PV R 16437 presents a notch that is shared with other theropods such as the ceratosaur Ceratosaurus , piatnitzkysaurids, spinosaurids (except Ichthyovenator ), and allosauroids such as Acrocanthosaurus, Allosaurus , Giganotosaurus , and Sinraptor (Stovall and Langston 1950, Carrano et al. 2012, Lacerda et al. 2023). However, NHMUK PV R 16437 has a lamina, part of the dorsalmost portion of the obturator process, which is immediately ventral to the pubic peduncle. This feature is also shared with the metriacanthosaurid Sinraptor and the carcharodontosaurid Giganotosaurus .

Furthermore, if the ischial shaft in NHMUK PV R 16437 is indeed straight, as the preserved portion suggests, this would be another feature shared between this individual and carcharodontosaurids (also seem in some allosauroids— Madsen 1976), but not with some early diverging tetanurans and also metriacanthosaurids. Thus, based on the set of characteristics shared between NHMUK PV R 16437 and carcharodontosaurid theropods, as well as the differences between this ischium and those of spinosaurids, we assign this material to an indeterminate Carcharodontosauridae .

Brief survey of theropod dinosaurs from the Kem Kem Group

Several records of theropod dinosaurs are known from the North Africa, especially those that derive from Middle Cretaceous rocks from the Kem Kem Group region, therefore, suggesting a high theropod diversity. However, much of the diversity known for the region is represented by shed teeth, which are more likely to be fossilized as they are more resistant to weathering and taphonomic alterations ( Benyoucef et al. 2015, Hendrickx et al. 2024). There is also a good and broad ichnological record that helps confirm faunal occurrences and serves as a proxy for more reliable palaeoenvironmental reconstructions ( Belvedere et al. 2013, Ibrahim et al. 2014b). Regarding skeletal remains, records are scarcer ( Hendrickx et al. 2024); however, they are still of broad relevance for understanding both biogeographical and evolutionary issues in different clades. Below, a brief nonexhaustive survey of the body fossil occurrences is presented, as well as integration with our findings and their relevance to current knowledge.