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Taphonomic Evidence Supports an Aquatic Lifestyle for Spinosaurus

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Taphonomic Evidence Supports an Aquatic Lifestyle for Spinosaurus Cretaceous Research 117 (2021) 104627 Contents lists available at ScienceDirect Cretaceous Research journal homepage: www.elsevier.com/locate/CretRes Taphonomic evidence supports an aquatic lifestyle for Spinosaurus Thomas Beevor a, Aaron Quigley a, Roy E. Smith a, Robert S.H. Smyth a, Nizar Ibrahim a, b, * Samir Zouhri c, David M. Martill a, a School of the Environment, Geography and Geoscience, University of Portsmouth, Burnaby Road, Portsmouth, PO1 3QA, UK b Department of Biology, University of Detroit Mercy, Detroit, MI, USA c Laboratoire de Biodiversite et Sante, Faculte des Sciences Aïn Chock, Universite Hassan II, Casablanca, Morocco article info abstract Article history: A new locality near Tarda on the northern margin of the Tafilalt, south eastern Morocco exposes Received 17 May 2020 extensive sequences of the Ifezouane and Aoufous formations of the fluvial Kem Kem Group (Cretaceous, Received in revised form ?Albian-Cenomanian) on the south western flank of Ikfh n’Oufza escarpment of the Hamada du Meski. 18 August 2020 The stratigraphic sequence here differs significantly from better known exposures of the Kem Kem Group Accepted in revised form 20 August 2020 in the southern Tafilalt, and includes a heterolithic sequence of alternating grey mudstones and fine Available online 28 August 2020 sandstones and a thin (~1.5 m) marine limestone. The locality is noteworthy for three vertebrate-bearing horizons within the upper part of the Ifezouane Formation. The upper two (Sites 1 and 2 in ascending Keywords: fi Cretaceous order) are dominated by dental remains of the saw sh Onchopristis and the semi-aquatic theropod fi Dinosauria dinosaur Spinosaurus. Signi cantly, the remains of terrestrial dinosaurs constitute less than 1% of the Spinosaurus total dental assemblage at Site 1 and 5.6% at Site 2. At Site 2 teeth of Spinosaurus outnumber the rostral Taphonomy “teeth” of Onchopristis. The remarkably high abundance of spinosaur teeth compared to remains of Palaeoecology terrestrial dinosaurs, and even some aquatic animals strongly supports Spinosaurus being a largely Morocco aquatic animal spending much of its life in water where its teeth were shed and preserved. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction disproportionately short femur and osteoclerosis/pachyostosis of the skeleton (Ibrahim et al., 2020a), as well as circumstantial evi- The enigmatic theropod dinosaur Spinosaurus aegyptiacus dence for specialized sensory structures to detect prey in water Stromer, 1915 has been interpreted as a piscivorous and semi- (Ibrahim et al., 2014). Possession of a narrow, somewhat crocodile- aquatic animal (e.g. Stromer, 1936; Taquet, 1984; Ibrahim et al., like overall skull morphology and a dentition of simple conical 2014; Arden et al., 2019), and more recently shown to have teeth, often with pronounced apicobasal ridges, has also been possessed a highly modified tail suited for propelling the animal interpreted as indicating a piscivorous diet (Stromer, 1936; Taquet, through water (Ibrahim et al., 2020a). The hypothesis that this 1984; Ibrahim et al., 2014; McCurry et al., 2019). Comparisons with spectacular dinosaur was semi-aquatic - or even perhaps fully more distantly related forms e large piscivorous fish e have also aquatic e has met with some opposition (e.g. Henderson, 2018; been used to support piscivory in Spinosaurus (Vullo et al., 2016). Hone and Holtz, 2019), not least because it challenges decade-old Additional evidence from phylogenetic analysis suggest a very close ideas on dinosaur ecology and evolution (Ibrahim et al., 2020a relationship to the European Baryonyx walkeri, a spinosaurid in and papers therein). Evidence for an aquatic lifestyle hinges which possibly acid-etched lepidotid fish scales were discovered in mainly on skeletal data, which provides compelling evidence, the ribcage, providing potential evidence for a degree of piscivory including a rearward position of the external nares, flattened pedal in these animals (Charig and Milner, 1997). Isotopic evidence was unguals and considerable divergence of pedal digits, a series of also used to suggest an aquatic lifestyle for Spinosaurus, although elongate neural spines on the caudal vertebrae giving the tail a fin- the data was somewhat inconclusive (Amiot et al., 2010). Taken like morphology, a reduced pelvic girdle and hindlimb, with a together, these diverse and independent lines of evidence all point toward a largely piscivorous e and aquatic e lifestyle. Here we present evidence from a taphonomic analysis of a new locality of the Kem Kem Group strata of south east Morocco that further * Corresponding author. E-mail address: [email protected] (D.M. Martill). strengthens a predominantly aquatic lifestyle for Spinosaurus. https://doi.org/10.1016/j.cretres.2020.104627 0195-6671/© 2020 Elsevier Ltd. All rights reserved. 2 T. Beevor et al. / Cretaceous Research 117 (2021) 104627 Most museum specimens of Spinosaurus from Morocco have At Tarda the base of the Kem Kem Group is not seen and the been obtained from commercial sources, and generally lack precise sequence commences with a series of greyish mudstones with thin locality or horizon data (Ibrahim et al., 2020a,b). Our field studies beds of gypsum and thin (~20e50 mm thick) siltstones with halite over the last 15 years in the Tafilalt region and Hamada du Kem pseudomorphs. These pass upwards into a sequence of massively Kem of south east Morocco reveal that a majority of this material bedded and cross bedded fine sandstones with occasional clay comes from the top of the Ifezouane Formation of the Kem Kem partings a few tens of centimetres thick. The sequence continues to Group. Many isolated teeth, occasional jaw fragments and, more be dominated by fine sandstones upwards until it passes into a rarely, other cranial and postcranial bones are dug by artisan miners series of variegated mudstones capped by a yellow weathering from three or four event horizons characterised by mud-flake well-bedded limestone with Thalassinoides. This represents a brief conglomerates with chaotic bedding, and sharp basal contacts. marine intercalation but has only yielded a shelly fauna of very They are rarely more than 1 m thick, and often just a few centi- small bivalves resembling modiolids and a heterodont near Zrigat, a metres, but may be extensive and traceable over several kilometres. few kilometres to the east. This limestone is prominent in the The excavation of an associated skeleton by Ibrahim et al. (2020a) is landscape for its colour and also for producing the cap to a smaller an exception, as this specimen is one of only three associated di- plateau sitting lower than the dominating Hamada de Meski that nosaurs reported from the Kem Kem Group (Ibrahim et al., forms the northern margin of the Tafilalt Basin. Above the thin 2020a,b), and it occurred at a slightly higher level in the sequence. yellowish limestone is a series of variegated (grey, blue-grey, or- ange, ochreous and red) sandstones that form the upper part of the Ifezouane Formation. These sandstones pass up into variegated 2. Locality and geological context mudstones with thin sandstones of the Aoufous Formation and are in turn capped by limestones of the Akrabou Formation that form 2.1. Locality the top of the Hamada du Meski. A preliminary sedimentary log for the exposure is provided in Fig. 2. The data reported here was obtained from natural exposures at The Akrabou Formation limestones are dated as mid Cen- the foot of the Ikfh n’Oufza escarpment, the western extension of omanian to Turonian on the basis of a diverse ammonite assem- the Hamada du Meski between Goulmima and Errachidia on the blage (Kennedy et al., 2008), but the age of the Kem Kem Group is northern margin of the Tafilalt Basin in south east Morocco. This not securely determined. Several authors, using the fossil verte- locality lies within Errachidia Province near the village of Tarda on brates, have suggested a lower Cenomanian age for the Kem Kem the eastern side of the Oued Tarda (Fig. 1). Exposure is extensive, Group by comparing it with the Bahariya Formation of the Egyptian patchily continuous for ~2.4 km, and has been quarried and mined Western Desert (Sereno et al., 1996; Ibrahim et al., 2020b), but it in several places specifically for the extraction of fossils. may well be somewhat older, as there is a considerable thickness of The Kem Kem Group strata represent an extensive fluvial system strata and a number of disconformities between the vertebrate- that can be traced in Morocco for at least 200 km from Zguilma in bearing Ifezouane Formation and the base of the Akrabou the south west to Aoufous and beyond to the Anoual Basin and the Formation. frontier with Algeria. In Algeria, coeval strata yield remarkably similar vertebrate assemblages both in terms of faunal diversity and 2.3. Taphonomic context taphonomy (Benyoucef et al., 2015; Ibrahim et al., 2020b). Verte- brate fossils occur at event horizons (likely flash-flood deposits and The two vertebrate-bearing horizons examined (Sites 1 and 2 viscous sediment flows, as channel lags and as exceptionally rare [red and yellow stars respectively in Fig. 2]) differ sedimentologi- isolated associated skeletons (Ibrahim et al., 2014, 2020a,b). The cally and taphonomically. At Site 1 the vertebrate remains occur at vertebrate assemblage is diverse (for a full faunal list see Cavin the base of a brown weathering fine-grained lenticular sandstone et al., 2010; Ibrahim et al., 2020b) and includes a wide variety of channel that can be traced laterally for only a few tens of metres. cartilaginous and bony fishes, archosaurs (dinosaurs, pterosaurs The vertebrate remains often occur as broken fragments, are and crocodiles), turtles and snakes and lizards. There are anomalies randomly orientated and brittle. Bone and dentine are white while such as a near absence of ornithischian dinosaurs and a lack of enamel is pale yellow to orange.
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