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Data discussed in this paper can be found in the supplementary materials. All artifacts are stored at the Institute of Archeology and Ethnography, Yerevan, Armenia. We thank the following organizations for their financial support: the University of Connecticut [2008–2014: Norian Armenian Programs Committee, College of Liberal Arts and Sciences (CLAS), Office of Global Affairs, Study Abroad; and CLAS Book Committee]; the UK Natural Environment Research Council (grant IP-1186-0510), the L. S. B. Leakey Foundation (2010 and 2011), the Irish Research Council (2008 and 2009), and the University of Winchester.
We also thank P. Avetisyan and B. Yeritsyan, Institute of Archeology and Ethnography, Republic of Armenia, for their collaboration.
We describe adaptations for a semiaquatic lifestyle in the dinosaur Spinosaurus aegyptiacus. These adaptations include retraction of the fleshy nostrils to a position near the mid-region of the skull and an elongate neck and trunk that shift the center of body mass anterior to the knee joint. Unlike terrestrial theropods, the pelvic girdle is downsized, the hindlimbs are short, and all of the limb bones are solid without an open medullary cavity, for buoyancy control in water. The short, robust femur with hypertrophied flexor attachment and the low, flat-bottomed pedal claws are consistent with aquatic foot-propelled locomotion. Surface striations and bone microstructure suggest that the dorsal “sail” may have been enveloped in skin that functioned primarily for display on land and in water.
S. aegyptiacus from the middle of the Kem Kem sequence (Fig. 1B), which is probably Cenomanian in age (~97 million years ago) (7).
The subadult skeleton, here designated the neotype of S. aegyptiacus (8), preserves portions of the skull, axial column, pelvic girdle, and limbs. It was discovered in fluvial sandstone that has yielded remains of the sauropod Rebbachisaurus (9) and three other medium-to-large theropods (an abelisaurid, Deltadromeus, and Carcharodontosaurus) (7, 10). We regard two additional Kem Kem theropods, Sigilmassasaurus brevicollis and S. maroccanus (11, 12), to be referable to S. aegyptiacus (8).
A concentrated array of neurovascular foramina open on the anterior end of the snout and appear similar to foramina in crocodilians that house pressure receptors that detect water movement (8, 16) (Fig. 2B and fig. S6). The enlarged, procumbent, interlocking anterior teeth are well adapted for snaring fish (5, 6) (Fig. 2B and fig. S4). The fossa for the fleshy nostril is small and, unlike any other nonavian dinosaur, is retracted to a posterior position to inhibit the intake of water (Fig. 2C and figs. S4 and S6) (8).
Most cervical and dorsal centra are elongate compared to the sacral centra, resulting in a proportionately long neck and trunk (Figs. 2A and 3 and tables S1 and S2). The anteriormost dorsal centra, however, are proportionately short, exceptionally broad, and concavoconvex (Fig. 2D). These characteristic vertebrae, the affinity of which has been controversial (7, 11, 12), are referred here to S. aegyptiacus, based on their association with spinosaurid skeletons in Niger (8) and Egypt (2). The horizontal cervicodorsal hinge created by these broad centra would facilitate dorsoventral excursion of the neck and skull in the pursuit of prey underwater.
The distal two-thirds of the tail comprises vertebrae with relatively short centra, diminutive zygapophyses, and anteroposteriorly compressed neural spines (Fig. 2G). The affinity of these caudal elements has been uncertain (17), but comparisons with associated remains from Egypt (2) and more proximal caudals in the neotype (Fig. 2A) allow referral to Spinosaurus. Short centra and reduced neural arch articulations enhance lateral bending during tail propulsion in bony fish (18).
1Department of Organismal Biology and Anatomy, University of Chicago, Chicago, IL 60637, USA. 2Museo di Storia Naturale di Milano, Corso Venezia 55, 20121 Milan, Italy.
139th Avenue Southeast, Bellevue, WA 98005, USA.
7Dipartimento di Scienze della Terra, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Rome, Italy.

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