The lifestyle of spinosaurid dinosaurs has been a topic of lively debate ever since the unveiling of important new skeletal parts for Spinosaurus aegyptiacus in 2014 and 2020. Disparate lifestyles for this taxon have been proposed in the literature; some have argued that it was semiaquatic to varying degrees, hunting fish from the margins of water bodies, or perhaps while wading or swimming on the surface; others suggest that it was a fully aquatic underwater pursuit predator. The various proposals are based on equally disparate lines of evidence. A recent study by Fabbri and coworkers sought to resolve this matter by applying the statistical method of phylogenetic flexible discriminant analysis to femur and rib bone diameters and a bone microanatomy metric called global bone compactness. From their statistical analyses of datasets based on a wide range of extant and extinct taxa, they concluded that two spinosaurid dinosaurs ( S . aegyptiacus , Baryonyx walkeri ) were fully submerged “subaqueous foragers,” whereas a third spinosaurid ( Suchomimus tenerensis ) remained a terrestrial predator. We performed a thorough reexamination of the datasets, analyses, and methodological assumptions on which those conclusions were based, which reveals substantial problems in each of these areas. In the datasets of exemplar taxa, we found unsupported categorization of taxon lifestyle, inconsistent inclusion and exclusion of taxa, and inappropriate choice of taxa and independent variables. We also explored the effects of uncontrolled sources of variation in estimates of bone compactness that arise from biological factors and measurement error. We found that the ability to draw quantitative conclusions is limited when taxa are represented by single data points with potentially large intrinsic variability. The results of our analysis of the statistical method show that it has low accuracy when applied to these datasets and that the data distributions do not meet fundamental assumptions of the method. These findings not only invalidate the conclusions of the particular analysis of Fabbri et al . but also have important implications for future quantitative uses of bone compactness and discriminant analysis in paleontology.
Abstract Fabbri et al. 1 claim that the huge sail-backed dinosaur Spinosaurus aegyptiacus and the spinosaurid Baryonyx were “subaqueous foragers,” diving underwater in pursuit of prey, based on their measure of bone “compactness.” Using thin-sections and computed tomographic (CT) scans of thigh bone (femur) and trunk rib from various living and extinct vertebrates, they claim to be able to distinguish taxa with “aquatic habits” from others. Their conclusions are undermined by selective bone sampling, inaccuracies concerning spinosaurid bone structure, faulty statistical inferences, and novel redefinition of the term “aquatic.”
Alvarez et al. (1) proposed assessing the relative climate benefits of alternative energy technologies for policy purposes by comparing a time-integrated approximation to the radiative forcing produced by each alternative. In contrast, Myhrvold and Caldeira (2) propose comparing the change in global mean temperature that each alternative technology would produce under various schedules of deployment.
Abstract Measures of bone compactness in amniote tetrapods of varying lifestyle were used to infer that two spinosaurid dinosaurs ( Spinosaurus aegyptiacus , Baryonyx walkeri ) were diving “subaqueous foragers,” whereas a third spinosaurid ( Suchomimus tenerensis ) and other sampled nonavian dinosaurs were non-diving terrestrial feeders entering water only as waders. We outline shortcomings in this analysis that involve bone compactness sampling and measurement, lifestyle categorization, the inclusion and exclusion of taxa in the dataset, and flawed statistical methods and inferences. These many shortcomings undermine the evidence used to conclude that two spinosaurid taxa were avid divers. Bone compactness indices remain a valuable tool for interpretation of lifestyle in extinct species when based on sound dataset composition, robust statistical analysis, and consilience with evidence from functional, biomechanical, or paleoenvironmental considerations.
Article Figures and data Abstract Editor's evaluation Introduction Results Discussion Materials and methods Appendix 1 Appendix 2 Appendix 3 Appendix 4 Appendix 5 Data availability References Decision letter Author response Article and author information Metrics 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. Editor's evaluation This article evaluates the hypothesis that Spinosaurus was a specialized aquatic dinosaur, by developing a CT-based skeletal restoration and examining its hydrodynamic properties. In this reappraisal of the "aquatic hypothesis", new results support the alternative "semi-aquatic hypothesis". This article will be of interest to vertebrate paleontologists and functional morphologists, as well as wider academic and non-academic audiences. https://doi.org/10.7554/eLife.80092.sa0 Decision letter eLife's review process Introduction In 1915, Ernst von Stromer announced the discovery in Egypt’s Western Desert of the elongate jaws and partial skeleton of a large sail-backed predator Spinosaurus aegyptiacus (Stromer, 1915). Other bones found nearby (Stromer, 1934) contributed to his initial reconstruction of S. aegyptiacus as a sail-backed, piscivorous biped (Stromer, 1936), shortly before all of these bones were destroyed in World War II (Nothdurft et al., 2002; Smith et al., 2006). Over the last 30 years, additional skull and postcranial bones came to light in western Morocco in beds of similar age to those in Egypt (Russell, 1996; Dal Sasso et al., 2005; Smyth et al., 2020; Ibrahim et al., 2020a). Central among these finds was a partial skeleton (designated the neotype) that allowed a more complete reconstruction, confirming its interpretation as a semiaquatic piscivore (Ibrahim et al., 2014). As skeletal information on the unusual predator improved, so has speculation as to whether S. aegyptiacus was better adapted to life in water as an aquatic predator, based on inferences from oxygen isotopes in enamel (Amiot et al., 2010), the dental rosette likened to the jaws of a conger eel (Vullo et al., 2016), the alleged elevated positioning of the orbits in the skull for visibility while largely submerged (Arden et al., 2019), the hypothetical underwater role of the trunk sail (Gimsa et al., 2016), and the infilling of the medullary cavities of hind limb bones that may have functioned as ballast (Ibrahim et al., 2014; Aureliano et al., 2018). The aquatic hypothesis Recent discovery of the tall-spined tail bones of the neotypic skeleton reinvigorated the interpretation of S. aegyptiacus as the first fully aquatic dinosaur (Ibrahim et al., 2020b), here dubbed the ‘aquatic hypothesis,’ which makes three basic propositions. Unlike any other theropod, according to the hypothesis, S. aegyptiacus: reverted to a quadrupedal stance on land, as shown by a trunk-positioned center of mass (Ibrahim et al., 2014; Ibrahim et al., 2020b), ostensibly knuckle-walking with long-fingered, long-clawed forelimbs; functioned in water as a capable, diving pursuit predator using an expanded tail as a ‘novel propulsor organ’ (Ibrahim et al., 2020b) or as a ‘subaqueous forager’ (Fabbri et al., 2022); and fossils would be found exclusively in coastal or deep-water marine habitats, like all large-bodied secondarily aquatic vertebrates, and would not be expected to be found in freshwater inland environments. We test these three central propositions. Critique of the aquatic hypothesis thus far has focused on an alternative functional explanation for the high-spined tail as a display structure and largely qualitative functional interpretations of its skeletal anatomy (Hone and Holtz, 2021). Biomechanical evaluation of the aquatic functionality of S. aegyptiacus remains rudimentary. The propulsive capacity of the tail in water was judged to be better than terrestrial counterparts by oscillating miniature plastic tail cutouts in water (Ibrahim et al., 2020b), a limited approximation of the biomechanical properties of an anguilliform tail (Lighthill, 1969; van Rees et al., 2013; Gutarra and Rahman, 2022) that failed to take account of the bizarre anterior half of the animal. The center of body mass, a critical functional parameter, has been estimated for S. aegyptiacus three times, each estimate pointing to a different location ranging from the middle of the trunk (Ibrahim et al., 2014; Ibrahim et al., 2020b) to a position over the hind limbs (Henderson, 2018). Quantitative comparisons have not been made regarding the size or surface area of the limbs, hind feet, and tail of S. aegyptiacus to counterparts in extant primary or secondary swimmers. Thus, adequate evaluation of the aquatic hypothesis requires more realistic biomechanical tests, quantitative body, axial and limb comparisons between S. aegyptiacus and extant primary and secondary swimmers, and a survey of bone structure beyond the femur and shaft of a dorsal rib. Such tests and comparisons require an accurate 3D digital flesh model of S. aegyptiacus, which, in turn, requires an accurate skeletal model. Hence, we began this study by assembling a complete set of CT scans of the fossil bones for S. aegyptiacus and its African forerunner, Suchomimus tenerensis (Sereno et al., 1998). Aquaphilic terminology Aquatic status is central to the ‘aquatic hypothesis.’ The hypothesis holds that S. aegyptiacus is the first non-avian dinosaur bearing skeletal adaptations devoted to lifestyle and locomotion in water, some of which inhibited terrestrial function. The contention is that S. aegyptiacus was not only a diving pursuit predator in the open-water column, but also a quadruped on land with long-clawed forelimbs poorly adapted for weight support. A later publication seemed to downgrade that central claim by suggesting that any vertebrate with ‘aquatic habits,’ such as wading, submergence, or diving, had an ‘aquatic lifestyle’ (Fabbri et al., 2022). That broadened usage of ‘aquatic lifestyle,’ however, blurs the long-standing use of aquatic as applied to lifestyle (Pacini and Harper, 2008). We outline below the traditional usage of aquaphilic terms, which we follow. The adjective ‘aquatic’ is used either as a broad categorization of lifestyle or, in more limited capacity, in reference to an adaptation of a species or group. In the former case, a vertebrate with an ‘aquatic lifestyle’ or ‘aquatic ecology’ is adapted for life primarily, or solely, in water with severely reduced functional capacity on land (Pacini and Harper, 2008). Aquatic vertebrates (e.g., bony fish, sea turtles, whales) live exclusively or primarily in water and exhibit profound cranial, axial, or appendicular modifications for life in water, especially at larger body sizes (Webb, 1984; Webb and De Buffrénil, 1990; Hood, 2020). For example, extant whales are secondarily aquatic mammals that spend all of their lives at sea and exhibit profound skeletal modifications for aquatic sensory and locomotor function. A marine turtle, similarly, is considered an aquatic reptile, regardless of whether it ventures ashore briefly to lay eggs, because the vast majority of its life is spent in water using profoundly modified limbs for aquatic locomotion (flippers) that function poorly on land. An aquaphilic animal with less profound adaptations to an aqueous arena is said to be semiaquatic (or semi-aquatic), no matter the proportion of aquatic foodstuffs in its diet, the proportion of time spent in water, or the proficiency of swimming or diving. Nearly all semiaquatic vertebrates are secondarily aquaphilic, having acquired aquatic adaptations over time to enhance functional capacity in water without seriously compromising terrestrial function (Howell, 1930; Hood, 2020). Indeed, semiaquatic animals are also semiterrestrial (Fish, 2016). For example, freshwater turtles are regarded as semiaquatic reptiles because they frequent water rather than live exclusively within an aqueous habitat, are sometimes found in inland habitats, and exhibit an array of less profound modifications (e.g., interdigital webbing) for locomotion in water (Pacini and Harper, 2008). Likewise, extant crocodylians and many waterbirds are capable swimmers and divers but retain excellent functional capacity on land. Auks (Alcidae), among the most water-adapted of semiaquatic avians, are agile wing-propelled, pursuit divers with an awkward upright posture on land resembling penguins, but they retain the ability to fly and inhabit land for extended periods (Nettleship, 1996). On the other hand, the flightless penguins (Sphenisciformes) are considered aquatic due to their more profound skeletal modifications for swimming and deep diving and more limited terrestrial functionality, although still retaining the capacity to trek inland and stand for considerable durations while brooding. As nearly all semiaquatic vertebrates have an aquatic diet and the ability to swim or dive, more profound functional allegiance to water is requisite for an ‘aquatic’ appellation (Pacini and Harper, 2008). An aquatic adaptation of an organism refers to the function of a particular feature, not the overall lifestyle of an organism. That feature should have current utility and primary function in water (Houssaye and Fish, 2016). Aquatic adaptations are presumed to have evolved their functionality in response to water and cannot also have special functional utility in a subaerial setting. For example, the downsized, retracted external nares in spinosaurids would inhibit water intake through the nostrils while feeding with the snout submerged (Sereno et al., 1998; Dal Sasso et al., 2005; Ibrahim et al., 2014; Hone and Holtz, 2021). There is at present no plausible alternative explanation involving terrestrial function for the downsizing and retraction of the external nares in spinosaurids, a unique condition among non-avian theropods. In contrast, the hypertrophied neural spines of the tail in S. aegyptiacus are ambiguous as an ‘aquatic adaptation’ because expanded tails can function both as aquatic propulsors and terrestrial display structures. For the expanded tail to be an ‘aquatic adaptation,’ its morphological construction and biomechanical function must unequivocally show primary utility and capability in water, as is the case with extant tail-powered primary or secondarily aquatic vertebrates (e.g., newts, crocodylians, beavers, otters; Fish et al., 2021). The same must be shown or inferred to be the case in extinct secondarily aquatic vertebrates (Gutarra and Rahman, 2022). Using various comparative and biomechanical approaches (below), we have not found such substantiating evidence to interpret the heightened tail in S. aegyptiacus or other spinosaurids as an aquatic adaptation, confirming similar conclusions reached recently by Hone and Holtz, 2021. Our approach To test the aquatic hypothesis for S. aegyptiacus, we began with CT scans of spinosaurid fossils from sites in Africa to build high-resolution 3D skeletal models of S. aegyptiacus (Figure 1A) and its forerunner, S. tenerensis (Figure 1F). Many vertebrae and long bones in both genera show significant internal pneumatic (air) or medullary (marrow) space, which has ramifications for buoyancy. When compared to the 2D silhouette drawing used in the aquatic hypothesis (Ibrahim et al., 2020b), our CT-based 3D skeletal model of S. aegyptiacus differs significantly in skeletal proportions. We enveloped the skeletal model in flesh informed by CT scans revealing the muscle volume and air spaces in extant reptilian and avian analogs. To create a 3D flesh model for S. aegyptiacus (Figure 2A and B), internal air spaces (trachea, lungs, air sacs) were shaped and positioned as in extant analogs. We created three options for internal air volume based on extant squamate, crocodilian, and avian conditionsand assigned densities to body partitions based on local tissue types and air space. We calculated the surface area and volume of the flesh model as well as its component body parts. We posed this integrated flesh model in bipedal, hybrid- and axial-powered poses, the latter two based on the swimming postures of extant semiaquatic reptiles (Grigg and Kirshner, 2015; Figure 2B). We calculated center of mass (CM) and center of buoyancy (CB) to evaluate the habitual two- or four-legged stance of S. aegyptiacus on land (Figure 1A), the depth of water at the point of flotation (Figure 2D), and the neutral position of the flesh model in deeper water (Figure 2A and B). Using biomechanical formulae (Lighthill, 1969) and data from extant alligators (Fish, 1984), we estimated the maximum force output of its tail, which was used to calculate maximum swimming velocity at the surface and underwater(Figure 3A). We also evaluated its stability, maneuverability, and diving potential in water (Figure 3B), with all of these functional capacities compared to extant large-bodied aquatic vertebrates. We turned to extant analogs to consider the structure and function of similar spine-supported sails over the trunk and tail in lizards and the form of tail vertebrae in tail-powered secondary swimmers (Figure 4). We also considered the relative size (surface area) of appendages in a range of secondary swimmers (Figure 5), and how the surface area of foot paddles and tail scale in crocodylians (Figure 6). Lastly, we turned to the spinosaurid fossil record to look at the habitats where spinosaurid fossils have been found. We reviewed their distributionto determine whether spinosaurids, and S. aegyptiacus in particular, were restricted to coastal, marine habitats like all large secondarily aquatic vertebrates. We updated spinosaurid phylogeny in order to discern major stages in the evolution of spinosaurid piscivorous adaptations and sail structures (Figure 8), incorporating the latest finds including new fossils of Spinosaurus from Niger. Institutional abbreviations BSPG, Bayerische Staatssammlung für Paläontologie und Geologie, Munich, Germany; FMNH, Field Museum of Natural History, Chicago, IL, USA; FSAC, Faculté des Sciences Aïn Chock, University of Casablanca, Casablanca, Morocco; KU, The University of Kansas, Natural History Museum, Lawrence, KS, USA; MNBH, Musée National de Boubou Hama, Niamey, Niger; MNHN, Muséum national d’Histoire naturelle, Paris, France; NMC, Canadian Museum of Nature, Ottawa, Canada; UCMP, University of California, Museum of Paleontology, Berkeley, CA, USA; UCRC, University of Chicago Research Collection, Chicago, IL, USA; UF, University of Florida, University of Florida Collections, Gainesville, FL, USA; UMMZ, University of Michigan, Museum of Zoology, Ann Arbor, MI, USA; WDC, Wildlife Discovery Center, Lake Forest, IL, USA. Results Spinosaurid skeletal models Our skeletal reconstruction of an adult S. aegyptiacus is just under 14 m long (Figure 1A), which is more than 1 m shorter than previously reported (Ibrahim et al., 2014). Major differences are apparent when compared to the 2D graphical reconstruction of the aquatic hypothesis (Ibrahim et al., 2020b). The length of the presacral column, depth of the ribcage, and length of the forelimb in that reconstruction were overestimated by ~10, 25, and 30%, respectively, over dimensions based on CT-scanned fossils. When translated to a flesh model, all of these proportional overestimates (heavier neck, trunk, forelimb) shift the center of mass anteriorly (see ‘Materials and methods’). Figure 1 Download asset Open asset 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). The hind limb long bones (femur, tibia, fibula, metatarsals) in S. aegyptiacus lack the medullary cavity common to most dinosaurs and theropods in particular. When first discovered, the infilled hind limb bones in S. aegyptiacus were interpreted as ballast for swimming (Ibrahim et al., 2014). However, the infilled condition is variable as shown by the narrow medullary cavity in a femur of another individual slightly larger than the neotype (Russell, 1996; NMC 41869). Furthermore, the bone infilling is fibrolamellar and cancellous, similar to the infilled medullary cavities of other large-bodied terrestrial dinosaurs (Vanderven et al., 2014) and mammals (Houssaye et al., 2016). In contrast, dense pachystotic bone composes the solid and sometimes swollen bones of many secondarily aquatic vertebrates that use increased skeletal density as ballast (Houssaye, 2009). Medullary space is present in most forelimb bones in both S. aegyptiacus and S. tenerensis (Figure 1D and H). The centra of anterior caudal vertebrae are occupied by a large medullary space (Figure 1C and J), and large air-filled pneumatic spaces are present in the centra and neural arches of cervical vertebrae (Evers et al., 2015; Figure 1B). Collectively, these less dense, internal marrow- and air-filled spaces in S. aegyptiacus more than offset the added density of infilled medullary space in the relatively reduced hind limb long bones (Figure 1A). Hind limb bone infilling is better explained as compensation for the reduced size of the hind limb long bones that must support a body mass at the upper end of the range for theropods. Bending strength increases by as much as 35% when the medullary cavity is infilled (see Appendix 1). S. aegyptiacus flesh model form and function We added flesh to the adult skeletal model and divided the flesh model into body partitions adjusted for density. Muscle volume was guided by CT cross-sections from extant lizards, crocodylians, and birds (Figure 2B), and internal air space (pharynx-trachea, lungs, paraxial air sacs) was modeled on lizard, crocodilian, and avian conditions (Figure 2C; see ‘Materials and methods,’ Appendix 2). Whole-body and body part surface area and volume were calculated, and body partitions were assigned density comparable to that in extant analogs (see ‘Materials and methods’). For biomechanical analysis, we positioned the integrated flesh model in bipedal stance (Figure 1A) as well as hybrid- and axial-powered swimming poses (Grigg and Kirshner, 2015; Figure 2A and B). Figure 2 Download asset Open asset 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. The CM and CB of the flesh model were determined to evaluate habitual stance on land and in shallow water (Figure 1A), the water depth at the point of flotation (Figure 2D), and its swimming velocity, stability, maneuverability, and diving potential in deeper water (Figure 3). No matter the included volume of internal air space, CM is positioned over the ground contact of symmetrically positioned hind feet (Figure 1A, red cross). Thus, S. aegyptiacus had a bipedal stance on land as previously suggested (Henderson, 2018), contrary to trunk-centered CM of the aquatic hypothesis (Ibrahim et al., 2020b). Consistent with a bipedal stance, the manus is adapted for prey capture and manipulation (elongate hollow phalanges, scythe-shaped unguals) rather than weight support (Figure 1A and D). Figure 3 Download asset Open asset 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). Adult S. aegyptiacus can feed while standing in water with flotation occurring in water deeper than ~2.6 m (Figure 2D). In hybrid or axial swimming poses, trunk air space tilts the anterior end of the model upward (Figure 2A and B). With density-adjusted body partitions and avian-like internal air space, the flesh model of S. aegyptiacus has a body mass of 7390 kg and an average density of 833 kg/m3 (see ‘Materials and methods’), which is considerably less than the density of freshwater (1000 kg/m3) and saltwater (1026 kg/m3) or the average density of living crocodylians (1080 kg/m3; Grigg and Kirshner, 2015). Swimming velocity at the surface and underwater in extant lizards and crocodylians is powered by foot paddling and axial undulation (hybrid swimming; Frey and Salisbury, 2001) and at moderate to maximum (critical) speeds by axial undulation alone (axial swimming) (Fish, 1984; Grigg and Kirshner, 2015). We used Lighthill’s bulk momentum formula to estimate maximum surface and underwater swimming velocity for the flesh model of S. aegyptiacus (Lighthill, 1969). Assuming a fully compliant Alligator-like tail (tail amplitude 0.24/body length, tail wavelength 0.57/body length, and tailbeat frequency 0.25 Hz; Fish, 1984; Sato et al., 2007), tail thrust (Pt) and maximum velocity (U) can be determined (Pt = –164.93 + 1899.1U – 896.35U2). Assuming turbulent conditions, a body drag coefficient of 0.0035 was estimated for a Reynolds number of 752,400 at a swimming speed of 1.0 m/s. The total power from estimates of drag increased three- to fivefold to account for undulation of the tail, near-surface wave formation, and increased sail drag when underwater (Figure 3A). The addition of the sail increases the drag on the body of S. aegyptiacus by 33.4%. The intersection of the thrust power curve and drag power curves, where the animal would be swimming at a constant velocity, indicates slow maximum velocity at the surface (~0.8 m/s) and only slightly greater when submerged (~1.4 m/s) (Figure 3A). Maximum tail thrust in S. aegyptiacus is 820 Watts (683 N or 154 lbs), a relatively low value for the considerable caudal muscle mass in this large theropod (Snively and Russell, 2007; Mallison et al., 2015). Only a minor amount of caudal muscle power, however, is imparted to the water as thrust during undulation. As a result, maximum velocity is only 1.2 m/s, an order of magnitude less than extant large-bodied (>1 m) pursuit predators. These species (mackerel sharks, billfish, dolphins, and killer whales) are capable of maximum velocities of 10–33 m/s (Tinsley, 1984; Fish, 1998; Fish and Rohr, 1999; Iosilevskii and Weihs, 2008). Stability and the capacity to right are important in water. When positioned upright in water, the trunk sail of S. aegyptiacus is emergent (Figure 3B, position 1). The flesh model, however, is particularly susceptible to long-axis rotation given the proximity of CM and CB, with stable equilibrium attained when floating on its side (Figure 3B, position 3). Righting requires substantial torque (~5000 Nm) that is impossible to generate with vertical limbs and a tail with far less maximum force output (~700 N). This stability predicament remains even with the smallest internal air space. The absence of vertical stability and righting potential in water stands in stark contrast to the condition in extant crocodylians and marine mammals (Fish, 1998; Grigg and Kirshner, 2015). Maneuverability in water (acceleration, turning radius, and speed) wanes as body length increases (Domenici, 2001; Parson et al., 2011; Domenici et al., 2014; Hirt et al., 2017; Gutarra and Rahman, 2022), which is further compromised in S. aegyptiacus by its rigid trunk (see below) and expansive, unretractable sail. In contrast, large-bodied secondary swimmers capable of pursuit predation in open water have fusiform body forms with a narrow caudal peduncle for efficient tail propulsion (ichthyosaurs, cetaceans; Motani, 2009), control surfaces for reorientation, and narrow extensions (bills) to enhance velocity in close encounters with smaller more maneuverable prey (Maresh et al., 2004; Domenici et al., 2014). Besides some waterbirds, semiaquatic pursuit predators are rare and include only the small-bodied (<2 m), exceptionally maneuverable otters that employ undulatory swimming (Fish, 1994). Diving with an incompressible trunk requires a propulsive force (Fg) greater than buoyancy. For S. aegyptiacus, in addition, a depth of ~10 m is needed to avoid wave drag (Figure 3A, bottom). The propulsive force required to dive is ~17,000 N: (Vbody×ρSaltwater-ρ-FleshModel×g; 8.94 m3 [1026–833 kg/m3] 9.8 m/s = 16,909 N), or ~25 times the maximum force output of the tail. Even with lizard-like internal air space, diving still requires ~15 times maximum force output of the tail. To initiate a dive, furthermore, the tail would be lifted into the air as the body rotates about CB (Figure 2D), significantly reducing tail thrust. The now common depictions of S. aegyptiacus as a diving underwater pursuit predator contradict a range of physical parameters and calculations, which collectively characterize this dinosaur as a slow, unstable, and awkward surface swimmer incapable of submergence. Axial comparisons to aquatic vertebrates and sail-backed reptiles Axial flexibility is requisite for axial-propulsion in primary or secondary swimmers. However, in S. aegyptiacus, trunk and sacral vertebrae are immobilized by interlocking articulations (hyposphene-hypantrum), an expansive rigid dorsal sail composed of closely spaced neural spines, and fused sacral centra (Figure 1A). The caudal neural spines in S. aegyptiacus stiffen a bone-supported tail sail by an echelon of neural spines that cross several vertebral segments, which effectively resist bending at vertebral joints (Figure 4A). The caudal centra in S. aegyptiacus have nearly uniform subquadrate proportions along the majority of the tail in lateral view, rather than narrowing, spool-shaped centra in crocodylians and other secondarily aquatic squamates (Figure 4D), which increases distal flexibility during tail undulation. These salient structural features of the tail suggest that it functioned more as a pliant billboard than flexible fluke. Figure 4 Download asset Open asset Skeletal comparisons between Spinosaurus aegypt
Griebeler and Werner offer a formal comment on Myhrvold, 2016 defending the conclusions of Werner and Griebeler, 2014. Although the comment criticizes several aspects of methodology in Myhrvold, 2016, all three papers concur on a key conclusion: the metabolism of extant endotherms and ectotherms cannot be reliably classified using growth-rate allometry, because the growth rates of extant endotherms and ectotherms overlap. A key point of disagreement is that the 2014 paper concluded that despite this general case, one can nevertheless classify dinosaurs as ectotherms from their growth rate allometry. The 2014 conclusion is based on two factors: the assertion (made without any supporting arguments) that the comparison with dinosaurs must be restricted only to extant sauropsids, ignoring other vertebrate groups, and that extant sauropsid endotherm and ectotherm growth rates in a data set studied in the 2014 work do not overlap. The Griebeler and Werner formal comment presents their first arguments in support of the restriction proposition. In this response I show that this restriction is unsupported by established principles of phylogenetic comparison. In addition, I show that the data set studied in their 2014 work does show overlap, and that this is visible in one of its figures. I explain how either point effectively invalidates the conclusion of their 2014 paper. I also address the other methodological criticisms of Myhrvold 2016, and find them unsupported.
Computer models of the tail of Apatosaurus louisae show it could reach supersonic velocities, producing a noise analogous to the “crack” of a bullwhip. Similarity in tail structure suggests this was feasible for other diplodocids, and possibly for unrelated sauropods like Mamenchisaurus and the dicraeosaurids. Lengthening of caudal vertebrae centra between positions 18 and 25 is consistent with adaptation to the stresses generated by such tail motion, as is coossification of vertebrae via diffuse idiopathic skeletal hyperostosis (DISH), which occurs in the same region in about half the specimens. The noise produced may have been used for defense, communication, intraspecific rivalry, or courtship, in which case supersonic “cracking” may have been a sexually dimorphic feature. Comparisons with the club-bearing tails of the sauropods Shunosaurus lii and Omeisaurus tianfuensis show the diplodocid whiplash tail was not well adapted as a direct-impact weapon, bringing the tail-as-weapon hypothesis into doubt.
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 (<1m/s) too buoyant to dive. Living reptiles with similar spine-supported sails over trunk and tail in living reptiles 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.
This dataset contains the digitized treatments in Plazi based on the original journal article Holly N. Woodward, Katie Tremaine, Scott A. Williams, Lindsay E. Zanno, John R. Horner, Nathan Myhrvold (2020): Growing up Tyrannosaurus rex: Osteohistology refutes the pygmy “ Nanotyrannus ” and supports ontogenetic niche partitioning in juvenile Tyrannosaurus. Science Advances 6: 6250, DOI: 10.1126/sciadv.aax6250
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.
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