Spinosaurus is not an aquatic dinosaur

Abstract

A predominantly fish-eating diet was envisioned for the sail-backed theropod dinosaur Spinosaurus aegyptiacus when its elongate jaws with subconical teeth were unearthed a century ago in Egypt. Recent discovery of the high-spined tail of that skeleton, however, led to a bolder conjecture that S. aegyptiacus was the first fully aquatic dinosaur. The 'aquatic hypothesis' posits that S. aegyptiacus was a slow quadruped on land but a capable pursuit predator in coastal waters, powered by an expanded tail. We test these functional claims with skeletal and flesh models of S. aegyptiacus. We assembled a CT-based skeletal reconstruction based on the fossils, to which we added internal air and muscle to create a posable flesh model. That model shows that on land S. aegyptiacus was bipedal and in deep water was an unstable, slow-surface swimmer (<1 m/s) too buoyant to dive. Living reptiles with similar spine-supported sails over trunk and tail are used for display rather than aquatic propulsion, and nearly all extant secondary swimmers have reduced limbs and fleshy tail flukes. New fossils also show that Spinosaurus ranged far inland. Two stages are clarified in the evolution of Spinosaurus, which is best understood as a semiaquatic bipedal ambush piscivore that frequented the margins of coastal and inland waterways.

Keywords: Spinosaurus; ambush predator; aquatic; dinosaur; evolution; evolutionary biology; spinosaurid.

Figure 1.. Digital skeletal reconstructions of the African spinosaurids Spinosaurus aegyptiacus and Suchomimus tenerensis.

(A) S. aegyptiacus (early Late Cretaceous, Cenomanian, ca. 95 Ma) showing known bones based on the holotype (BSPG 1912 VIII 19, red), neotype (FSAC-KK 11888, blue), and referred specimens (yellow) and the center of mass (red cross) of the flesh model in bipedal stance (overlap priority: neotype, holotype, referred bones). (B) Cervical 9 (BSPG 2011 I 115) in lateral view and coronal cross-section showing internal air space. (C) Caudal 1 centrum (FSAC-KK 11888) in anterolateral view and coronal CT cross-section. (D) Right manual phalanx I-1 (UCRC PV8) in dorsal, lateral, and sagittal CT cross-sectional views. (E) Pedal phalanges IV-4, IV-ungual (FSAC-KK 11888) in dorsal, lateral, and sagittal CT. (F) S. tenerensis (mid Cretaceous, Aptian-Albian, ca. 110 Ma) showing known bones based on the holotype (MNBH GAD500, red), a partial skeleton (MNBH GAD70, blue), and other referred specimens (yellow) (overlap priority: holotype, MNBH GAD70, referred bones). (G) Dorsal 3 in lateral view (MNBH GAD70). (H) Left manual phalanx I-1 (MNBH GAD503) in dorsal, lateral, and sagittal CT cross-sectional views. (I) Caudal 1 vertebra in lateral view (MNBH GAD71). (J) Caudal ~3 vertebra in lateral view (MNBH GAD85). (K) Caudal ~13 vertebra in lateral view with CT cross-sections (coronal, horizontal) of the hollow centrum and neural spine (MNBH GAD70). ag, attachment groove; C2, 7, 9, cervical vertebra 2, 7, 9; CA1, 10, 20, 30, 40, caudal vertebra 1, 10, 20, 30, 40; clp, collateral ligament pit; D4, 13, dorsal vertebra 4, 13; dip, dorsal intercondylar process; k, keel; mc, medullary cavity; nc, neural canal; ns, neural spine; pc, pneumatic cavity; pl, pleurocoel; r, ridge; S1, 5, sacral vertebra 1, 5. Dashed lines indicate contour of missing bone, arrows indicate plane of CT-sectional views, and scale bars equal 1 m (A, F), 5 cm (B, C), 3 cm (D, E, H–K) with human skeletons 1.8 m tall (A, F).

Figure 2.. Digital flesh model of Spinosaurus aegyptiacus.

(A) Translucent flesh model in hybrid swimming pose showing centers of mass (red cross) and buoyancy (white diamond). (B) Opaque flesh model in axial swimming pose with adducted limbs. (C) Modeled air spaces (‘medium’ option) include pharynx-trachea, lungs and paraxial air sacs. (D) Wading-strike pose at the point of flotation (2.6 m water depth) showing center of mass (red cross) and buoyancy (white diamond). lu, lungs; pas, paraxial air sacs; tr, trachea.

Figure 3.. Biomechanical evaluation of Spinosaurus aegyptiacus in water.

(A) Tail thrust (yellow curve) and opposing drag forces as a function of swimming velocity at the surface (blue) and submerged (green), with drag during undulation estimated at three and five times stationary drag. (B) Stability curve for the flesh model of S. aegyptiacus in water showing torque between the centers of mass (red cross) and buoyancy (white diamond), unstable equilibria when upright or upside down (positions 1, 5), and a stable equilibrium on its side (position 3) irrespective of the volume of internal air space. Curves are shown for flesh models with minimum (magenta) and maximum (green) air spaces with a dashed line showing the vertical body axis and vector arrows for buoyancy (up) and center of mass (down).

Figure 4.. Skeletal comparisons between Spinosaurus aegyptiacus, a basilisk lizard and secondarily aquatic vertebrates.

(A) Tail in S. aegyptiacus showing overlap of individual neural spines (red) with more posterior vertebral segments. (B) Sail structure in the green basilisk (CT-scan enlargement) and in vivo form and coloration of the median head crest and sail (Basiliscus plumifrons FMNH 112993). (C) Structure of the tail fluke in a urodele, mosasaur, crocodilian, and whale. (D) Centrum proportions along the tail in the northern crested newt (Triturus cristatus FMNH 48926), semiaquatic lizards (marine iguana Amblyrhynchus cristatus UF 41558, common basilisk Basiliscus basiliscus UMMZ 121461, Australian water dragon Intellagama lesueurii FMNH 57512, sailfin lizard Hydrosaurus amboinensis KU 314941), an extinct mosasaurid (Mosasaurus sp. UCMP 61221; Lindgren et al., 2013), an alligator (Alligator mississippiensis UF 21461), and Spinosaurus (S. aegyptiacus FSAC-KK 11888). Data in Appendix 2.

Figure 5.. Appendage versus total body surface area in aquatic and semiaquatic vertebrates.

Spinosaurus aegyptiacus and other non-avian theropods (green polygon, centroid large diamond) have appendages with considerable surface area compared to aquatic and semiaquatic vertebrates (blue polygon, centroid large dot). Underwater fliers (1–7 circled), which propel themselves with lift-based wings, also have less overall appendage surface area than in S. aegyptiacus and other non-avian theropods. Underwater fliers: 1, plesiosaur Cryptoclidus oxoniensis; 2, leatherback sea turtle Dermochelys coriacea; 3, emperor penguin Aptenodytes forsteri; 4, sea lion Zalophus californianus; 5, elasmosaur Albertonectes vanderveldei; 6, nothosaur Ceresiosaurus calcagnii; 7, pliosaur Liopleurodon ferox.

Figure 6.. Appendage surface area and scaling of paddle surface areas in crocodylians compared to S. aegyptiacus.

(A) Right hind foot of Spinosaurus aegyptiacus (FSAC-KK 11888) showing the outlines of digital flesh based on the living ostrich (Struthio camelus) as well as partial (pink) and full (blue) interdigital webbing. (B) Hind foot of an adult Alligator mississippiensis (WDC) in ventral view. (C) Forefoot of an adult A. mississippiensis (WDC) in ventral view. (D) Tail of an adult A. mississippiensis (WDC) in lateral view with CT visualization of vertebrae within the fleshy tail fluke. (E) Log-log plot of surface areas of webbed hind foot and side of the tail as a function of total body area in a growth series for A. mississippiensis (hind foot, green dots; tail, blue diamonds) and adult S. aegyptiacus (hind foot, purple-blue dots; tail, yellow diamond). I, IV, V, digits I, IV, V; un, ungual. Scale bars are 10 cm (A) and 3 cm (B–D).

Figure 7.. Paleogeographic location of spinosaurid fossils.

(A) Paleogeographic map (early Albian, ~110 Mya; Scotese, 2014). showing the circum-Tethyan fossil localities for baryonychines (Baryonyx, Suchomimus) and spinosaurines (Ichthyovenator, Vallibonavenatrix, Oxalaia, Irritator/Angaturama, Spinosaurus). Spinosaurus localities (yellow asterisks) range across northern Africa from coastal (sites 1, 2) to inland (site 3) sites. (B) Spinosaurus sp. right maxilla (MNBH EGA1) from Égaro North (central Niger) in medial (top) and ventral (bottom) views and shown (red) superposed on the snout of Spinosaurus aegyptiacus. 1, S. aegyptiacus holotype (Bahariya, Egypt); 2, S. aegyptiacus neotype (Zrigat, Morocco); 3, Spinosaurus sp. (Égaro North, Niger); am, articular rugosities for opposing maxilla; aofe, antorbital fenestra; Ba, Baryonyx walkeri; en, external naris; Ic, Ichthyovenator laosensis; Ir, Irritator challengeri/Angaturama limai; m3, 12, maxillary alveolus 3, 12; Ox, Oxalaia quilombensis; Su, Suchomimus tenerensis; t, tooth; Va, Vallibonavenatrix cani. Scale bar is 10 cm.

Figure 8.. Calibrated phylogeny of spinosaurids (Barremian to Cenomanian, ~35 My).

Updated phylogenetic analysis of spinosaurids resolves two stages in the evolution of piscivory and display. We show key cranial adaptations in the skull and highlight changes at the anterior end of the trunk to enhance neck ventroflexion (second dorsal vertebra in lateral and anterior views). Bottom, the fully terrestrial theropod Allosaurus fragilis (Madsen, 1976); middle, the baryonychine spinosaurid Suchomimus tenerensis (MNBH GAD70); top, the spinosaurine Spinosaurus aegyptiacus (BSPG 1912 VIII 19). con, condyle; dr, dental rosette; en, external naris; hy, hypopophysis; k, keel; ncr, nasal crest; ns, neural spine; pa, parapophysis; tp, transverse process.

Figure 9.. Comparison of skeletal reconstructions for Spinosaurus aegyptiacus in left lateral view.

(A) Digital skeletal reconstruction from this study in left lateral view. (B) Pelvic girdle. (C) Cervical column (C1–10). (D) Pectoral girdle and forelimb. (E) Hind limb. (F) Anterior trunk. (G) Silhouette skeletal drawing from the aquatic hypothesis (from Ibrahim et al., 2020b). On one side of each length comparison, one or two blue lines are shown that register the alternative reconstructions. The opposing end of each length comparison either has a single blue line (when comparisons match, both 100%) or a red line as well for the shorter one (<100%): A blue line on the right or top sides of each comparison is used for registration. The opposing side has a blue line if reconstructions agree on length (100%), or a blue line for the length estimate in this study and a red line for that of the aquatic hypothesis. In all disparate comparisons, the reconstruction in this study is shorter (percentage given). Skeletal reconstructions (A, G) are aligned by the anterior and posterior margins of the ilium and measured to the cervicodorsal junction (C10-D1); the pelvic girdle (B) is aligned along the ventral edge of the sacral centra and base of the neural spines and measured to the distal ends of the pubis and ischium; the cervical column (C) is aligned at the cervicodorsal junction (C10-D1) and measured to the anterior end of the axis (C2); the scapula and components of the forelimb (humerus, ulna, manual digit II, manual phalanx II-1) (D) are aligned at the distal end of the blade and their proximal ends, respectively, and measured to the opposing end of the bone; the components of the hind limb (femur, tibia, pedal digits I, III) (E) are aligned at their proximal ends and measured to the opposing end of the bone; and anterior trunk depth (F) is aligned along the ventral edge of the centrum and neck of the spine of D6 and measured to the ventral edge of the coracoid. II-1, manual phalanx II-1; Ili, ilium; F, femur; H, humerus; Ish, ischium; l, left; Pe, pes; Pu, pubis; Ma, manus; r, right; RaU, radius-ulna; Sc, scapula; TF, tibia-fibula.

Figure 10.. CT scans inform cross-sectional muscle mass in Spinosaurus aegyptiacus.

Muscle mass reconstructions of the axial column at five points (A-E) in S. aegyptiacus are compared to CT scan cross-sections of Struthio (Snively and Russell, 2007) and Alligator (Wedel, 2003; Mallison et al., 2015).

Figure 11.. Cross-sections from a CT scan of Basiliscus plumifrons FMNH 112993.

(A) Skeleton showing position of CT sections of the axial column. (B) Anterior cervical region (C2). (C) Mid cervical region (C3). (D) Posterior cervical and anterior dorsal region (C4-D1). (E) Mid dorsal region (D12). (F) Anterior caudal region (CA4). (G) Caudal region at most posterior transverse process (CA10). (H) Mid caudal region (CA15). CT scan data available on Morphosource.org. Scale bars, 5 mm.

Appendix 4—figure 1.. Anterior dorsal centrum referable to Spinosaurus sp. (from Brusatte and Sereno, 2007: Fig. 9).

Photographs and line drawings of MNBH IGU11 from the Echkar Formation (Cenomanian) of Niger in ventral (A) dorsal (B), right lateral (C), and posterior (D) views. Cross-hatching indicates broken bone. ana, articular surface for the neural arch; hy, hypapophysis; k, ventral keel; nc, neural canal; pa, parapophysis; pc, pleurocoel. Scale bar, 5 cm.

Appendix 5—figure 1.. Phylogenetic tree for Spinosauroidea.

Single most-parsimonious tree for 12 spinosauroid terminal taxa (9 spinosaurids) and 120 characters split evenly between the cranium (49%) and postcranium (51%), showing decay values (Consistency index = 0.81, Retention Index = 0.85).

Appendix 5—figure 2.. Posterior skull roof of the baryonychine spinosaurid Suchomimus tenerensis (field no. GAD302).

Composite restoration of the posterior skull roof of Suchomimus tenerensis in (A) lateral and (B) dorsal views showing a swollen postorbital brow and narrow orbital notch limiting the frontal orbital margin. Based on original bone (blue), reflected original bone (gray), and reconstructed paroccipital processes.