Monday, March 7, 2016

[Paleontology • 2016] A Large Abelisaurid (Dinosauria, Theropoda) from Morocco and comments on the Cenomanian Theropods from North Africa

  Large Moroccan abelisaurid [Abelisauridae indet. femur OLPH 025]
Illustration: Davide Bonadonna


We describe the partially preserved femur of a large-bodied theropod dinosaur from the Cenomanian “Kem Kem Compound Assemblage” (KKCA) of Morocco. The fossil is housed in the Museo Geologico e Paleontologico “Gaetano Giorgio Gemmellaro” in Palermo (Italy). The specimen is compared with the theropod fossil record from the KKCA and coeval assemblages from North Africa. The combination of a distally reclined head, a not prominent trochanteric shelf, distally placed lesser trochanter of stout, alariform shape, a stocky shaft with the fourth trochanter placed proximally, and rugose muscular insertion areas in the specimen distinguishes it from CarcharodontosaurusDeltadromeus and Spinosaurus and supports referral to an abelisaurid. The estimated body size for the individual from which this femur was derived is comparable to Carnotaurus and Ekrixinatosaurus (up to 9 meters in length and 2 tons in body mass). This find confirms that abelisaurids had reached their largest body size in the “middle Cretaceous,” and that large abelisaurids coexisted with other giant theropods in Africa. We review the taxonomic status of the theropods from the Cenomanian of North Africa, and provisionally restrict the Linnean binomina Carcharodontosaurus iguidensis and Spinosaurus aegyptiacus to the type specimens. Based on comparisons among the theropod records from the Aptian-Cenomanian of South America and Africa, a partial explanation for the so-called “Stromer’s riddle” (namely, the coexistence of many large predatory dinosaurs in the “middle Cretaceous” record from North Africa) is offered in term of taphonomic artifacts among lineage records that were ecologically and environmentally non-overlapping. Although morphofunctional and stratigraphic evidence supports an ecological segregation between spinosaurids and the other lineages, the co-occurrence of abelisaurids and carcharodontosaurids, two groups showing several craniodental convergences that suggest direct resource competition, remains to be explained.

Systematic Palaeontology

Dinosauria Owen (1842).
Theropoda Marsh (1881).
Abelisauridae Bonaparte (1991).

Locality and age: Based on the registry of the OLPH, the specimen was collected nearby the Moroccan-Algerian boundary just south of Taouz (Errachidia Province, Meknès−Tafilalet Region), Morocco. Following Cavin et al. (2010), the age of this fossil is considered as Late Cretaceous (Cenomanian).

Material: OLPH 025, partial proximal portion of a right femur (Fig. 1).

Figure 1: Abelisauridae indet. femur OLPH 025.
(A) proximal view, (B) anterior view, (C) medial view, (D) posterior view, (E) lateral view, (F) distal view (not at same scale as other views). Scale bars, 5 cm.
Abbreviations: gt, greater trochanter; iMie, insertion for the M. iliofemoralis externus; fn, femoral neck; s, shallow sulcus.   DOI: 10.7717/peerj.1754

The taxonomy and inclusiveness of the theropod clades from the “middle” Cretaceous of North Africa is complex and problematic. Since Stromer (1931) and Stromer (1934) the minimum number of taxa recovered from these fossil associations has been considered controversial, in particular due to the fragmentary nature of most of the specimens found. Stromer himself (1934) was aware of this as one of the main problems in North African dinosaur palaeontology. Several factors, both biological and geological, may bias the taxonomic composition of the North African theropod faunas. Most North African units are poorly constrained stratigraphically (see Cavin et al., 2010; Fanti et al., 2014), thus preventing detailed correlations between the various localities. For example, the age of the KKCA has been alternatively placed between the Aptian and the Cenomanian (Russell, 1996; Cavin et al., 2010), and both number of and relationships among the units represented by that assemblage remain controversial (Sereno et al., 1996; Cavin et al., 2010). The temporal extent of these assemblages is uncertain, possibly spanning several million years (Cavin et al., 2010). Therefore, the application of biological (neontological) “rules,” based on ecological models and data from modern ecosystems (in order to constrain the number of carnivorous taxa included in a fossil assemblage) is often not adequately justified or not testable. This is particularly problematic for fossil assemblages, like the KKCA, that lack present-day analogues and where an unusually unbalanced ecological web has been suggested (e.g., Läng et al., 2013). Since the co-occurrence in the same North African theropod associations of distinct species belonging to the same clade has been documented (e.g., spinosaurids, Fanti et al., 2014; Hendrickx, Mateus & Buffetaut, 2016; carcharodontosaurids, Cau, Dalla Vecchia & Fabbri, 2012; Cau, Dalla Vecchia & Fabbri, 2013), the referral of all isolated elements of one lineage to a single species cannot be justified. Furthermore, the referral of isolated and non-overlapping material to the same species is a phylogenetic hypothesis itself that needs to be explicitly tested by numerical analyses. In absence of positive evidence supporting the referral of such material to a particular species, the inclusion of non-overlapping elements into a single taxon may led to the creation of a potential chimera, with unpredictable effects on the phylogenetic and palaeoecological interpretation of these faunas.

We have described the fragmentary femur of a large-bodied theropod from the “Kem Kem Compound Assemblage” of Morocco. The specimen lacks tetanuran synapomorphies and is referred to Abelisauridae as it shares the overall morphology of the femora of ceratosaurians and the stocky robust proportions of some Late Cretaceous abelisaurids (e.g., EkrixinatosaurusMajungasaurusCarrano, 2007; Juarez-Valieri, Porfiri & Calvo, 2011). The large size of the preserved femur suggests an individual comparable in body size with the type specimens of Carnotaurus sastrei and Ekrixinatosaurus novasi, both estimated to reach 9 meters in length and approaching two tons in body mass (Juarez-Valieri, Porfiri & Calvo, 2011). This discovery further supports that abelisaurids had evolved their largest size no later than the “mid-Cretaceous” (Smith et al., 2010; Juarez-Valieri, Porfiri & Calvo, 2011) and that abelisaurids and carcharodontosaurids co-existed and ecologically overlapped in both North Africa and South America during the Aptian-Turonian. Based on comparison with other “middle Cretaceous” units (Juarez-Valieri, Porfiri & Calvo, 2011; Fanti et al., 2014), we suggest that the co-occurrence of spinosaurids and other large theropods (abelisaurids and carcharodontosaurids) in the KKCA may be mainly an artefact due to poor stratigraphic resolution rather than genuine evidence of  ecological and environmental overlap. Given the convergent evolution of several craniodental features among abelisaurids and carcharodontosaurids (Lamanna, Martinez & Smith, 2002; Sampson & Witmer, 2007; Carrano & Sampson, 2008; Cau, Dalla Vecchia & Fabbri, 2013), suggesting similar ecological adaptations in these clades, how these apparently competing groups co-existed for at least 30 million years in both Africa and South America remains to be resolved.

Alfio Alessandro Chiarenza and Andrea Cau. 2016. A Large Abelisaurid (Dinosauria, Theropoda) from Morocco and comments on the Cenomanian Theropods from North Africa.  PeerJ. 4:e1754; DOI: 10.7717/peerj.1754

Fossil find reveals just how big carnivorous dinosaur may have grown