Sigilmassasaurus

Taxobox
name = "Sigilmassasaurus"
fossil_range = Late Cretaceous
regnum = Animalia
phylum = Chordata
classis = Sauropsida
superordo = Dinosauria
ordo = Saurischia
subordo = Theropoda
unranked_familia = Tetanurae
familia = Spinosauridae?
genus = "Sigilmassasaurus"
genus_authority= Russell, 1996
subdivision_ranks=Species
subdivision=
*"S. brevicollis" Russell, 1996 (type)

"Sigilmassasaurus" (see-jil-MAH-sah-SAWR-us; "Sijilmassa lizard") is a genus of tetanuran theropod dinosaur from the middle of the Cretaceous Period of northern Africa. Not much is known about this dinosaur, but it was almost definitely a bipedal carnivore like most other theropods.

Fossils of this dinosaur were found in the Tafilalt Oasis region of Morocco, near the site of the ancient city of Sijilmassa, for which it was named. Canadian paleontologist Dale Russell named "Sigilmassasaurus" in 1996, from the ancient city and the Greek word "sauros" ("lizard"). A single species was named, "S. brevicollis", which is derived from the Latin "brevis" ("short") and "collum" ("neck"), because the neck vertebrae are very short from front to back. Russell also described another specimen as a possible second species of "Sigilmassasaurus", although he chose not to name it due to its incomplete nature.

"Sigilmassasaurus" comes from sediments in southern Morocco, which are called by various names, including the Grés rouges infracénomaniens, Continental Red Beds, and lower Kem Kem Beds. These rocks date back to the Cenomanian, the earliest faunal stage within the Late Cretaceous Period, or about 100 to 94 million years ago (Sereno "et al"., 1996).

Disputed validity

The holotype, or original specimen, of "S. brevicollis" is a single neck vertebra, although Russell referred about fifteen other vertebra found in the same formation to the species. Other material was found in Egypt, and is known as "Spinosaurus B" (Stromer, 1934). Russell considered this Egyptian specimen to belong to "Sigilmassasaurus" or a closely related animal, and created the family Sigilmassasauridae for these animals (Russell, 1996). The neck vertebrae of these dinosaurs are wider from side to side than they are long from front to back. The exact position of "Sigilmassasaurus" within the theropod family tree is unknown, but it belongs somewhere inside the theropod subgroup known as Tetanurae.

Some scientists do not believe that "Sigilmassasaurus" is a valid genus. In 1996, Paul Sereno and colleagues described a "Carcharodontosaurus" skull (SGM-Din-1) from Morocco, as well as a neck vertebra (SGM-Din-3) which resembled that of "Spinosaurus B," which they therefore synonymized with "Carcharodontosaurus" (Sereno "et al"., 1996). A later study went further, calling "Sigilmassasaurus" itself a junior synonym of "Carcharodontosaurus" (Sereno "et al"., 1998).

More recently, however, it was revealed that SGM-Din-3, which was used to synonymize "Carcharodontosaurus" and "Spinosaurus B" was not actually associated with SGM-Din-1, the "Carcharodontosaurus" skull described in 1996, and shows clear differences with the holotype of "Carcharodontosaurus". Other features of "Spinosaurus B" also clearly differ from "Carcharodontosaurus", lending support to the notion that it (and therefore "Sigilmassasaurus") is a separate taxon (Novas "et al"., 2005).

Paleobiology

Several large theropods (more than one tonne) are known from the Cenomanian of northern Africa, raising questions about how such animals would have coexisted. Species of "Spinosaurus", the largest known theropod, has been found in both Morocco and Egypt, as has the huge "Carcharodontosaurus". Two smaller theropods, "Deltadromeus" and "Bahariasaurus", have also been found in Morocco and Egypt, respectively, and may be closely related or possibly the same genus. "Sigilmassasaurus", from Morocco, and "Spinosaurus B", from Egypt, represent a fourth type of large predator. This situation resembles that in the Late Jurassic Morrison Formation of North America, which boasts up to five theropod genera over one tonne in weight, as well as several smaller genera (Henderson, 1998; Holtz "et al"., 2004). Differences in head shape and body size among the large North African theropods may have been enough to allow niche partitioning as seen among the many different predator species found today in the African savanna (Farlow & Pianka, 2002).

References

*Farlow, J.O. & Pianka, E.R. 2002. Body size overlap, habitat partitioning and living space requirements of terrestrial vertebrate predators: implications for the paleoecology of large theropod dinosaurs. "Historical Biology" 16(1): 21–40.
*Henderson, D.M. 1998. Skull and tooth morphology as indicators of niche partitioning in sympatric Morrison Formation theropods. "Gaia" 15: 219–226.
*Holtz, T.R., Molnar, R.E., & Currie, P.J. 2004. Basal Tetanurae. In: Weishampel, D.A., Dodson, P., & Osmolska, H. (Eds.). "The Dinosauria" (2nd Edition). Berkeley: University of California Press. Pp. 71-110.
*Novas, F.E., de Valais, S., Vickers-Rich, P., & Rich, T.H. 2005. A large Cretaceous theropod from Patagonia, Argentina, and the evolution of carcharodontosaurids. "Naturwissenschaften" 92: 226–230.
*Russell, D. A. 1996. Isolated dinosaur bones from the middle Cretaceous of the Tafilalt, Morocco. "Bulletin du Muséum National d’Histoire Naturelle, Paris", Série 4 18: 349–402.
*Sereno, P.C., Dutheil, D.B., Iarochene, M., Larsson, H.C.E., Lyon, G.H., Magwene, P.M., Sidor, C.A., Varricchio, D.J., & Wilson, J.A. 1996. Predatory dinosaurs from the Sahara and Late Cretaceous faunal differentiation. "Science" 272: 986–991.
*Sereno, P.C., Beck, A.L., Dutheuil, D.B., Gado, B., Larsson, H.C., Lyon, G.H., Marcot, J.D., Rauhut, O.W.M., Sadleir, R.W., Sidor, C.A., Varricchio, D.J., Wilson, G.P., Wilson, J.A. 1998. A long-snouted predatory dinosaur from Africa and the evolution of spinosaurids. "Science" 282: 1298–1302.
*Stromer, E. 1934. Wirbeltierreste der Baharíje-Stufe (unterstes Cenoman). 13. Dinosauria. "Abh. bayer. Akad. Wissensch., math-naturwiss. Abt. N.F." 22:1–79.