the first phalanx of digit I in Spinosaurus is
the longest nonungual phalanx in the pes (fig.
S1) and would have been in contact with the
substrate in a stationary pose. The pedal unguals are proportionally large, long, low, and
flat-bottomed (Fig. 2K and figs. S1 and S2),
features that differ markedly from the deeper
recurved unguals in other large theropods.
The unguals in Spinosaurus are reminiscent of
the flattened pedal unguals of shorebirds that
do not perch (22). In addition, the toes of some
shorebirds have fleshy lobes and interdigital
webbing that enhance foot-propelled propulsion.
The lengthened digit I and flattened pedal unguals in Spinosaurus suggest that the foot may
have been adapted to traversing soft substrates
or webbed for paddling.
Increases in bone mass and density are common skeletal modifications in terrestrial vertebrates transitioning to a semiaquatic existence (23).
In Spinosaurus, this was achieved by enlarging
midline display structures, eliminating open medullary cavities in the long bones, and increasing
bone density. In subadult Spinosaurus, the dorsal
neural spines are composed primarily of dense
bone with only a narrow central zone of cancellous bone (Fig. 4D), and long bones have solid
shafts (Fig. 4, A and C) with no development of
the open medullary cavity that is present in other
theropods, including early spinosaurids (Fig. 4B).
Bone density within the long bones, in addition,
is 30 to 40% greater in Spinosaurus than in other
We estimated a center-of-body mass for a
flesh rendering of Spinosaurus created over
the digital skeleton (8). Center-of-mass estimates
for several theropods have been expressed as
a percentage of femoral length measured anteriorly from the hip joint (24). The center of
mass in a biped must be located over the middle one-third of the pes to generate a plausible
mid-stance pose (25). In our flesh rendering of
Spinosaurus, the center of body mass is positioned in front of both the hip and knee joints
at a distance greater than femur length (fig. S3),
suggesting that forelimb support was required
during terrestrial locomotion. Spinosaurus appears to have been poorly adapted to bipedal
terrestrial locomotion. The forward position of
the center of mass within the ribcage may have
enhanced balance during foot-propelled locomotion in water.
These adaptations suggest that Spinosaurus
was primarily a piscivore, subsisting on sharks,
sawfish, coelacanths, lungfish, and actinopterygians
that were common in the Kem Kem river system
(5, 7, 11). A long narrow skull and powerful fore-
limbs are also present in earlier spinosaurids, which
like Spinosaurus (26) have been interpreted as
predominantly piscivorous (13, 14, 27, 28).
The locomotor adaptations outlined above,
however, mark a profound departure in form and
function from early spinosaurids. Prominent
among these are the reduced pelvic girdle; short
hindlimb; short femur; and long, low, flat-bottomed
pedal unguals, all of which can be verified in
the second partial skeleton described by Stromer
as “Spinosaurus B” (2, 8). We note here that Spinosaurus must have been an obligate quadruped
on land, the first discovered among theropod
dinosaurs, given the usual horizontal sacroiliac
joint and the anterior location of the estimated
center of body mass (8). Baryonyx was interpreted
as a facultative quadruped, based on its long skull
and neck and robust humerus (27), but this was
not confirmed by the discovery of more complete
hindlimb remains of the related Suchomimus (13).
In Spinosaurus we infer foot-powered paddling
from the relatively short femur with hypertrophied flexor attachment and strong pedal digit
I, as occurs in semiaquatic mammals such as early
cetaceans (19–21). Low, flat-bottomed pedal unguals
are coincident with digital lobes or webbing in
Fig. 3. Ternary morphospace plot comparing
forelimb, hindlimb, and body length. Forelimb
(humerus + radius + metacarpal II), hindlimb (femur +
tibia + metatarsal III), and body length (from snout
tip to posterior extremity of pelvic girdle) are plotted
as percentages of the sum of forelimb, hindlimb,
and body lengths in S. aegyptiacus and other large
tetanuran theropods (data from Table 1). Blue zone
shows the range of forelimb length, from 7% (
Tyrannosaurus) to 12% (Allosaurus). Hindlimb length
(red zone) ranges from 34% (Allosaurus) to 19%
(Spinosaurus). Abbreviations: Ac, Acrocanthosaurus;
Al, Allosaurus; Sp, Spinosaurus; Su, Suchomimus; Ty,
Fig. 4. Bone microstructure and dorsal spine form. (A) Mid-shaft thin section of the right femur of
S. aegyptiacus (FSAC-KK 11888). (B) Mid-shaft thin section of the right femur of Suchomimus tenerensis
(MNN GAD608). (C) Cross-sectional view of right manual II-1 phalanx of S. aegyptiacus (FSAC-KK 11888).
(D) Thin section of a dorsal neural spine (distal section) in S. aegyptiacus (FSAC-KK 11888). (E) Dorsal
vertebrae with tall neural spines and spinal tendons in a cleared and stained specimen of Trioceros
(Chamaeleo) cristatus (FMNH 19886). Abbreviations: cb, cancellous bone; ec, erosional cavities; Hb,
Haversian bone; mc, medullary cavity; ns, neural spine; pb, primary bone; sc, scapula; st, striae; te,
tendon of multisegment spinal muscle. Institutional abbreviations: FMNH, Field Museum of Natural
History. Scale bars, 2 cm in (A) and (C), 3 cm in (B), 5 mm in (D), and 1 cm in (E).