are probably referable to Spinosaurus (11) and
were possibly used in gaffing and slicing aquatic
prey suggest that the manus is proportionately
longer than in earlier spinosaurids (13, 14).
The pelvic girdle and hindlimb are considerably
reduced in Spinosaurus (Fig. 2A). The surface
area of the iliac blade is approximately one-half
that in most other theropods (table S1), and the
supraacetabular crest that supports the hindlimb
is low (Fig. 2F). Hindlimb length is just over 25%
of body length (table S1). In a plot of forelimb,
hindlimb, and body length (Fig. 3), Spinosaurus
and other large theropods maintain fairly similar
forelimb lengths. Relative hindlimb length, however,
is noticeably less in the spinosaurid Suchomimus
(25%) and especially in Spinosaurus (19%) than
in other large tetanuran theropods.
Unlike other mid- or large-sized dinosaurs, the
femur in Spinosaurus is substantially shorter
than the tibia (Fig. 2, I and J, and table S1). In
smaller-bodied bipedal dinosaurs, short femoral
proportions indicate increased stride length and
enhanced speed. In Spinosaurus this is clearly
not the case, given the short hindlimb. The femur
in Spinosaurus has an unusually robust attach-
ment for the caudofemoral musculature, which
is anchored along nearly one-third of the femoral
shaft (Fig. 2I), suggesting powerful posterior
flexion of the hindlimb. The articulation at the
knee joint for vertical limb support, in contrast,
is reduced. The distal condyles of the femur are
narrow, and the cnemial crest of the tibia is only
moderately expanded (Fig. 2, I and J). Together
these features recall the shortened condition of
the femur in early cetaceans (19, 20) and in extant
semiaquatic mammals that use their hindlimbs
in foot-propelled paddling (21).
Pedal digit I is unusually robust and long in
Spinosaurus: Unlike Allosaurus or Tyrannosaurus,
1614 26 SEPTEMBER 2014 • VOL 345 ISSUE 6204
Fig. 1. Geographic location and stratigraphic
position of the neotype skeleton of S. aegyptiacus.
(A) Locality (X), situated 18 km northeast of Erfoud
in southeastern Morocco. (B) Stratigraphic position
at the base of the upper unit of the Kem Kem beds,
with correlative positions of associated remains
of contemporary dinosaurs. Abbreviations: c, clay;
CT, Cenomanian-Turonian limestone; p, pebbles; P,
Paleozoic; sd, sandstone; st, siltstone.
Fig. 2. Semiaquatic skeletal adaptations in S. aegyptiacus. (A) Skeletal reconstruction in swimming
pose showing known bones (red) based on size-adjusted, computed tomographic scans of the neotype
(FSAC-KK 11888), referred specimens, and drawings of original bones (1). (B) Rostral neurovascular
foramina in lateral view (MSNM V4047 and a digital restoration of the holotypic lower jaw). (C) Narial
fossa in lateral view (MSNM V4047). (D) Anterior dorsal vertebra (~D1) in lateral, anterior, and posterior
views (UCRC PV601). (E) Dorsal neural spine (D8) in left lateral view (FSAC-KK 11888). (F) Left ilium in
lateral view (FSAC-KK 11888). (G) Mid-caudal vertebra (~CA30, reversed) in anterior and left lateral
views (UCRC PV5). (H) Right manual II-1 phalanx in proximal, lateral, and dorsal views (FSAC-KK 11888).
(I) Left femur in lateral view (FSAC-KK 11888). (J) Right tibia (reversed) in lateral view (FSAC-KK 11888).
(K) Right pedal digit III ungual in dorsal, lateral, and proximal views (FSAC-KK 11888). Abbreviations: af,
articular facet; ag, attachment groove; at, anterior trochanter; C2, 10, cervical vertebra 2, 10; CA1, caudal
vertebra 1; cc, cnemial crest; ce, centrum; clp, collateral ligament pit; D13, dorsal vertebra 13; ded, dorsal
extensor depression; dip, dorsal intercondylar process; fl, flange; ft, fourth trochanter; ftu, flexor tubercle;
lco, lateral condyle; nf, narial fossa; ns, neural spine; nvfo, neurovascular foramina; poz, postzygapophysis;
prz, prezygapophysis; S1, 5, sacral vertebra 1, 5; sac, supraacetabular crest; tp, transverse process.
Institutional abbreviations: FSAC, Faculté des Sciences Aïn Chock, Casablanca; MSNM, Museo di
Storia Naturale di Milano; UCRC, University of Chicago Research Collection, Chicago. Scale bars, 10 cm
in (B) to (D), (G), (H), and (K); and 20 cm in (E), (F), (I), and (J).