A reappraisal of the morphology and systematic position of the theropod dinosaurSigilmassasaurusfrom the “middle” Cretaceous of Morocco

Abstract

Sigilmassasaurus brevicollis is an enigmatic theropod dinosaur from the early Late Cretaceous (Cenomanian) of Morocco, originally based on a few isolated cervical vertebrae. Ever since its original description, both its taxonomic validity and systematic affinities were contentious. Originally considered to represent its own family, Sigilmassasauridae, the genus has variously been suggested to represent a carcharodontosaurid, an ornithischian, and, more recently, a spinosaurid. Here we describe new remains referrable to this taxon and re-evaluate its taxonomic status and systematic affinities. Based on the new remains, a re-evaluation of the original materials, and comparisons with other spinosaurids, the holotype of Sigilmassasaurus brevicollis is identified as an anterior dorsal, rather than a cervical vertebra, and differences between elements referred to this taxon can be explained by different positions of the elements in question within the vertebral column. Many characters used previously to diagnose the genus and species are found to be more widespread among basal tetanurans, and specifically spinosaurids. However, the taxon shows several autapomorphies that support its validity, including the presence of a strongly rugose, ventrally offset triangular platform that is confluent with a ventral keel anteriorly in the mid-cervical vertebral centra and a strongly reduced lateral neural arch lamination, with no or an incomplete distinction between anterior and posterior centrodiapophyseal laminae in the posterior cervical and anterior dorsal vertebrae. We argue furthermore that Spinosaurus maroccanus, also described on the basis of isolated cervical vertebrae from the same stratigraphic unit and in the same paper as Sigilmassasaurus brevicollis, is a subjective synonym of the latter. Both a detailed comparison of this taxon with other theropods and a formal phylogenetic analysis support spinosaurid affintities for Sigilmassasaurus. However, we reject the recently proposed synonymy of both Spinosaurus maroccanus and Sigilmassasurus brevicollis with Spinosaurus aegyptiacus from the Cenomanian of Egypt, as there are clear differences between the vertebrae of these taxa, and they do not share any derived character that is not found in other spinosaurids. Together with a comparison with other spinosaurid vertebral material from the Kem Kem, this suggests that more than one taxon of spinosaurid was present in the Kem Kem assemblage of Morocco, so the referral of non-overlapping material from this unit to a single taxon should be regarded with caution.

Spinosaurid Dinosaurs from the Early Cretaceous of North Africa and Europe: Fossil Record, Biogeography and Extinction

“…More recently, several new specimens of S. aegyptiacus have been reported from Northern Africa, which confirm many of Stromer's observations and provide a more complete picture of what this strange animal would have looked like (e.g., Dal Sasso et al, 2005;Ibrahim et al, 2014). However, a complete or nearcomplete skeleton of S. aegyptiacus has yet to be found, meaning that many ideas about the body size, posture, locomotion, and behaviors of this famous dinosaur are still uncertain, despite much attention surrounding recent discoveries (see discussion in Evers et al, 2015). Buffetaut (1989b) described a fragment of maxilla with circular alveoli from the Kem Kem Formation.…”

“…Sereno et al (1998) considered this species a nomen dubium due the lack of diagnostic features that confirm the occurrence of more than one Spinosaurus species in the Albian and Cenomanian strata of North Africa. This idea has been followed by some later authors, who considered S. maroccanus to be the same species as the earlier named S. aegyptiacus (e.g., Ibrahim et al, 2014), but other workers have recently argued that S. maroccanus may indeed be valid (e.g., Evers et al, 2015). Taquet & Russell, 1998, from the Elrhaz Formation (Aptian) of Niger, was described based on two separate premaxillae, a fragment of the right maxilla and dentary, and teeth.…”

Biotic and environmental dynamics through the Late Jurassic–Early Cretaceous transition: evidence for protracted faunal and ecological turnover

“…Late Jurassic (Oxfordian) carbonate platforms experienced severe growth crises (Weissert & Mohr, 1996), and calcareous nannoplankton underwent a significant global radiation in the Tithonian-Berriasian (Weissert et al, 1998;Bornemann et al, 2003;Falkowski et al, 2004;Weissert & Erba, 2004). However, the Early Cretaceous also saw a dramatic reduction in carbonate production, with a series of repeated 'biocalcification crises', notably in the Valanginian and Aptian (Weissert & Erba, 2004).…”

“…In the Panboreal realm, this negative trend is coincident with a high abundance of spores and prasinophytes (unicellular green algae), the latter of which might relate to an algal bloom driven by disturbances to marine ecosystems and/or shifts in oceanic productivity (Zakharov et al, 2014). However, the global radiation of calcareous plankton in the Tithonian-Berriasian (Weissert et al, 1998;Bornemann et al, 2003;Falkowski et al, 2004;Weissert & Erba, 2004) is not fully expressed within the δ 13 C record. Within the Boreal-Tethyan region, a positive carbon isotope excursion has been identified (Dzyuba et al, 2013).…”

“…Hu et al, 2009;Rauhut, Milner & Moore-Fay, 2010), but remain largely absent from Early to 'middle' Cretaceous Gondwanan theropod faunas, which instead comprise a diverse array of small-and large-bodied taxa (e.g. carcharodontosaurids and spinosaurids) (Sereno et al, 1996;Novas et al, 2005;Brusatte & Sereno, 2007;Evers et al, 2015). Eumaniraptora, or the more inclusive Paraves, underwent a significant acceleration in diversification rates across the J/K boundary (Lloyd et al, 2008).…”