Report

A Long-Snouted Predatory Dinosaur from Africa and the Evolution of Spinosaurids

Science  13 Nov 1998:
Vol. 282, Issue 5392, pp. 1298-1302
DOI: 10.1126/science.282.5392.1298

Abstract

Fossils discovered in Lower Cretaceous (Aptian) rocks in the Ténéré Desert of central Niger provide new information about spinosaurids, a peculiar group of piscivorous theropod dinosaurs. The remains, which represent a new genus and species, reveal the extreme elongation and transverse compression of the spinosaurid snout. The postcranial bones include blade-shaped vertebral spines that form a low sail over the hips. Phylogenetic analysis suggests that the enlarged thumb claw and robust forelimb evolved during the Jurassic, before the elongated snout and other fish-eating adaptations in the skull. The close phylogenetic relationship between the new African spinosaurid andBaryonyx from Europe provides evidence of dispersal across the Tethys seaway during the Early Cretaceous.

In 1912, a series of extremely high-spined vertebrae and a peculiar lower jaw with subconical crocodilelike teeth were discovered in the Baharı̂ya oasis in central Egypt (1). These fossils provided evidence that a large, piscivorous, sail-backed predator roamed the northern shores of Africa during the Late Cretaceous (Cenomanian). This partial skeleton,Spinosaurus aegyptiacus, was destroyed during World War II, and few remains that are attributable to this taxon have since been recovered in these horizons (2–5).

Additional bones of Spinosaurus-like predators have been discovered in Lower Cretaceous (Aptian or Albian) deposits in Niger and Brazil and in somewhat older (Barremian) rocks in Europe. The Nigerien fossils include peculiar arched snout tips and enormous manual unguals (6–8); the Brazilian remains consist of a single partial skull, Irritator (9, 10); and the European fossils, Baryonyx, include the first relatively complete spinosaurid skeleton (11–13).

Fossils were recovered recently from the Elrhaz Formation in Niger (Fig. 1). These Aptian-age rocks consist predominantly of fluvial channel deposits and are exposed in low outcrops amid dune fields in the Ténéré Desert (6, 14). The fossils found here include plant, invertebrate, and vertebrate remains; the vertebrate remains consist mainly of disarticulated bones and teeth in basal channel lag deposits. Dinosaurs are represented by at least three theropods, two sauropods, and three ornithopods (15).

Figure 1

Mid-Cretaceous paleogeography and principal exposures of fossiliferous beds in the region of Gadoufaoua, Niger. (A) Mid-Cretaceous (Aptian, 120 million years ago) paleogeographic map (Mollweide projection) with latitude and longitude lines spaced at 30° intervals (longitude greater than 120° is not shown) (31). White cross, fossil locality. (B) Maps showing Niger, the exposures of the GAD 5 beds (12) (white, above; black, below), and the location of the holotypic skeleton of Suchomimus tenerensis (16°25′N, 9°7′E).

The newly discovered fossils include a partial skull and skeleton of a new spinosaurid, Suchomimus tenerensis gen. nov. sp. nov. (16), which can be distinguished from other spinosaurids (17). An articulated snout (Fig. 2A and B) reveals its remarkably long, low, and narrow proportions. The elongation of the snout is the result of the hypertrophy of both the premaxilla and the anterior ramus of the maxilla. The premaxillae, which fuse early in growth, each contain alveoli for seven teeth. The subconical crowns are slightly recurved and have fine marginal serrations and textured enamel surfaces (Fig. 2E). The external nares are retracted posterior to the premaxillary teeth (Fig. 2, A and D), as in Baryonyx(14). The unusually long, plate-shaped anteromedial processes of the maxillae (18) are firmly held by the premaxillae (Fig. 2B). The medial wall of the antorbital fossa is confined to the anterior end of the antorbital fenestra, and a simple conical pneumatocoel extends anteriorly into the body of the maxilla. As in Baryonyx, the quadrate foramen is very large, and the distal condyles are very broad.

Figure 2

Skull of the spinosauridSuchomimus tenerensis. Articulated premaxillae and maxillae (MNN GDF501) in left (A) lateral (reversed from right) and (B) ventral views. Skull reconstruction of S. tenerensis based on remains from Niger and Baryonyx walkeri in (C) dorsal and (D) lateral views. Shaded portions are not currently known in any spinosaurid. (E) Scanning electron micrograph of the crown margin of an isolated tooth of S. tenerensis showing the small marginal serrations and textured enamel. Scale bar in (A) through (D), 10 cm; in (E), 1 mm. Abbreviations: a, angular; aj, articular surface for jugal; an, articular surface for nasal; antfe, antorbital fenestra; antfo, antorbital fossa; ar, articular; bo, basioccipital; bs, basisphenoid; d, dentary; emf, external mandibular fenestra; en, external naris; eo, exoccipital; f, frontal; j, jugal; l, lacrimal; lh, lacrimal horn; m, maxilla; n, nasal; nh, nasal horn; p, parietal; pm, premaxilla; po, postorbital; popr, paroccipital process; pra, prearticular; prf, prefrontal; pt, pterygoid; q, quadrate; qf, quadrate foramen; qj, quadratojugal; sa, surangular; saf, surangular foramen; so, supraoccipital; sq, squamosal; stf, supratemporial fossa; and 1 through 17, tooth positions.

The new cranial bones indicate that the spinosaurid skull is considerably lower, narrower, and longer (Fig. 2D) than previously reconstructed (13, 19). In dorsal view, the snout is extremely narrow (Fig. 2C). In ventral view, the maxillae meet along the midline, displacing the internal nares and palatal complex (pterygoid, palatine, and ectopterygoid) toward the rear of the skull.

In the postcranial skeleton, the cervical series arches upward (18) and has prominent epipophyses for muscle attachments. The neural spines increase in height rapidly in the middorsal vertebrae, forming a low median sail that is deepest over the sacral vertebrae (Fig. 3). This vertebral morphology, incipiently developed inBaryonyx (13), is distinct from that inSpinosaurus (1), in which the much deeper sail arches to an apex over the middorsal vertebrae (Fig. 4B).

Figure 3

Skeletal reconstruction of the spinosauridSuchomimus tenerensis showing preserved bones (total length is ∼11 m). Height of human silhouette, 1.68 m (5 feet 6 inches); scale bar, 1 m.

Figure 4

Phylogenetic and temporal relationships among spinosauroids. (A) Single most-parsimonious cladogram based on phylogenetic analysis of 45 characters (Table 2) (47 steps: consistency index, 0.98; retention index, 0.98) (32). The cladogram remains stable three steps above minimum length. (B) Phylogram based on the cladogram, recorded temporal ranges, and a recent time scale (33). On the right is depicted the sequential evolution of (in circles) 1, the robust forelimb with sickle-shaped thumb claw among ancestral spinosauroids; 2, the elongate piscivorous snout with terminal rosette, posteriorly displaced internal and external nares, and depressed basipterygoid articulation among ancestral spinosaurids; 3, spaced and nearly straight crowns in spinosaurines; and 4, hypertrophied neural spines in Spinosaurus. Body icons show the relative size of the holotypic skeletons of Baryonyx,Suchomimus, and Spinosaurus.

Complete pectoral and pelvic bones show a deep subrectangular acromion on the scapula and a low obturator flange on the ischium. The distal end of the pubis is distinctive. Although there is a short, sagittal, symphyseal flange that may correspond with the pubic foot in other theropods, the flattened, subrectangular distal end of the pubis faces anteriorly, perpendicular to the sagittal plane (Fig. 3). The forearm is remarkably stout, and manual digit III is robust, as seen in the size of metacarpal III and the third ungual (Fig. 3 and Table 1). The femur has a blade-shaped anterior trochanter, and the ascending process of the astragalus is taller than that in Allosaurus(20).

Table 1

Length measurements (millimeters) and ratios inSuchomimus tenerensis (MNN GDF500), Baryonyx walkeri (13), and Allosaurus fragilis(20). Unguals are measured perpendicular to the articular end. Question mark indicates lack of preservation; I, II, and III are unguals of manual digits I, II, and III, respectively.

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Phylogenetic analysis (21, 22) links spinosaurids with torvosaurids (23) and places this clade (Spinosauroidea) as the sister group to Neotetanurae (Fig. 4A and Table 2). The derived features that are shared between spinosaurids and torvosaurids, including the short forearm and the enlarged manual digit I ungual (22), appear to have evolved by the Middle Jurassic (Fig. 4B). A hook-shaped coracoid characterizes Suchominus, Baryonyx, and neotetanurans but not torvosaurids (24). Thus, the hook-shaped coracoid either evolved convergently in neotetanurans and spinosaurids or originated as a tetanuran synapomorphy that was subsequently lost in torvosaurids (21, 22).

Table 2

Character-state matrix for two outgroups (Ceratosauria and Neotetanurae), six ingroups, and 45 characters (22) used in a phylogenetic analysis of spinosauroids (Fig. 4A). The holotypic specimens of Angaturama and Irritator were scored as one taxon (Irritator). X, unknown as a result of transformation; ?, not preserved.

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Spinosaurids are characterized by numerous derived features, many of which are related to piscivory [including an unusually long snout with a long secondary palate; a terminal rosette of teeth in the upper and lower jaws (25); subcylindrical, spaced crowns; posteriorly displaced external nares; ventrally positioned basipterygoid articulation; and other features (8, 13, 22)]. Our analysis suggests that spinosaurids can be divided into two clades, the Baryonychinae and the Spinosaurinae (26), which diverged before the Barremian (Fig. 4B). The baryonychines Suchominusand Baryonyx are distinct (16, 27) but closely related, as evidenced by several derived features that include the small size and increased number of dentary teeth posterior to the terminal rosette and the deeply keeled anterior dorsal vertebrae (22). Many other similarities between these two taxa are ambiguous because they are not preserved in other spinosaurids. The spinosaurines Irritator and Spinosaurus are united on the basis of the straight unserrated crowns, the small first premaxillary tooth (4, 8), and the increased spacing of the teeth in the upper and lower jaws (8, 13, 22) (Fig. 4B). The posterior displacement of the external nares in Irritator(9) and the deep sail in Spinosaurus (Fig. 4B) may eventually characterize the Spinosaurinae, but these features are currently known in only one member.

Before the discovery of Suchomimus, the geographic distribution and relationships of spinosaurids matched the general pattern of continental fragmentation during the latter half of the Mesozoic and thus could be explained by large-scale vicariance. The split between the northern Baryonyx and the southern spinosaurines, in this hypothesis, could be attributed to the opening of the Tethyan seaway between Laurasia and Gondwanaland, and the divergence among spinosaurines could be the result of the subsequent opening of the Atlantic Ocean between South America and Africa. The discovery of Suchomimus on Africa in the mid-Cretaceous, however, complicates this scenario. Its closest relative is the European Baryonyx rather than the AfricanSpinosaurus—a pattern of relationships that is inconsistent with the large-scale sequence of continental rifting described above.

One biogeographic hypothesis accounts most parsimoniously for the distribution of the four spinosaurids (28), assuming that we have correctly ascertained their phylogenetic relationships and accept the rifting sequence between the continental areas outlined above. Initially, spinosaurids may have had a distribution across Pangaea that was split by the opening of the Tethys; baryonychines evolved to the north (Europe, or Laurasia), and spinosaurines evolved on the southern landmass (South America and Africa, or Gondwanaland). A single dispersal event from Europe to Africa during the Early Cretaceous would account for the presence of Suchomimus in Africa. Alternative scenarios involve additional dispersal or extinction events to account for recorded distributions. The phylogenetic and biogeographic relationships of Suchomimus and other spinosaurids provide further evidence of dispersal across the Tethyan seaway during the Early Cretaceous (21, 29).

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