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POSTILLA

PEABODY MUSEUM
YALE UNIVERSITY

NUMBER 165 18 APRIL 1974

THE PECTORAL GIRDLE AND FORELIMB
FUNCTION OF DEINONYCHUS (REPTILIA:
SAURISCHIA): A CORRECTION

JOHN H. OSTROM

POSTILLA

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THE PECTORAL GIRDLE AND FORELIMB FUNCTION OF
DEINONYCHUS (REPTILIA: SAURISCHIA): A CORRECTION

John H. Ostrom

Peabody Museum of Natural History and Department of Geology and
Geophysics, Yale University, New Haven, Connecticut 06520

(Received 8 October 1973)

ABSTRACT

The first reports on Deinonychus antirrhopus Ostrom tentatively identified a
solitary, incomplete bone from the Yale Quarry near Bridger, Montana, as a
right pubis. Subsequent examination and comparison with remains of other
taxa have established that bone to be a right coracoid of surprisingly large
size. The element is redescribed here with a corrected reconstruction of the
pectoral girdle and a revised interpretation of the relevant pectoral
musculature and functions. The unusually large size of the coracoid is
believed to be related to enlarged pectoral muscles that were important in
some predatory activities.

POSTIELA 1653+ 11 p: 18 APRIL 1974

2 POSTILLA 165

INTRODUCTION

During the course of my original analysis of the Deinonychus remains
recovered from the Yale Quarry near Bridger, Montana (Ostrom, 1969a,b),
one incomplete element was described which I was unable to identify with
certainty. Its general configuration was that of a coracoid, but upon
comparison with the scapulocoracoids of other theropods, that identification
was discarded because the bone in question was two to three times larger than
expected for the known scapulae of Deinonychus. Nor did this bone compare
well with the coracoid (or any other element) of Tenontosaurus, the only
other taxon recovered from the Yale Deinonychus Quarry. Finally, with some
reservations, I identified this bone as a pubis, chiefly because it was found in
close association with a right ischium (see fig. 65A, Ostrom, 1969b) in the
Yale Quarry.

Peter Whybrow, now at the British Museum (Natural History), while in the
process of constructing a free-mount of Deinonychus (now being completed
by P.Chatrath), concluded that my tentative identification could not be
correct. Further comparisons with additional theropod material and
re-examination of the scapulae of Deinonychus (AMNH 3015) have
established to my satisfaction that this element actually is a right coracoid,
although of unusual shape and extraordinary size.

In addition to the obvious need to correct the previous error, it also seems
worthwhile to re-examine the morphology and function of the pectoral arch
and forelimb in the light of this new information.

ABBREVIATIONS

Institution names have been abbreviated as follows:

AMNH_ American Museum of Natural History, New York City

BS Bayerische Staatssammlung, Munich

GIMAS Geological Institute, Academy of Sciences of the Mongolian People’s
Republic, Ulan Bator

YPM Yale Peabody Museum of Natural History

DESCRIPTION

Preserved with the left scapula (AMNH 3015) of Deinonychus is the glenoid
portion of the coracoid. Although imperfectly preserved, this fragment

FORELIMB FUNCTION OF DEINONYCHUS 3

FIG. 1. Right scapulocoracoid of Deinonychus antirrhopus as reconstructed from
separate elements. The scapula (AMNH 3015) is from the American Museum Quarry (see
Ostrom, 1969b, fig. 1A); the coracoid (YPM 5236) was recovered from the Yale Quarry
(Ostrom, 1969b, fig. 1B). Although the coracoid seems disproportionately large, the two
bones appear to have come from equivalent-sized individuals, judging from the respective
dimensions of elements in common from both sites, (see tables 7-11, Ostrom, 1969b).
Abbreviations: ac = acromial (deltoid) process; bt = biceps tubercle; cf = supracoracoid
foramen; g/ = glenoid. The scapula and coracoid are illustrated in the same plane, hence
the coracoid is viewed in anterolateral aspect, rather than lateral.

corresponds closely in size and morphology with the corresponding region of
the present bone (YPM 5236) and confirms the present identification.

The coracoid is roughly triangular in outline (the upper part adjacent to the
scapular suture is missing), moderately convexo-concave (externally-
internally), and bears a very prominent anterolateral projection immediately
anterior to the glenoid rim. Walker (1972) has termed a similar coracoid
prominence in Sphenosuchus (traditionally classified as a thecodont, but
currently considered a crocodilomorph by Walker) the ‘biceps tubercle.”
Osmolska etal (1972) noted a comparable feature in the theropod
Gallimimus and labeled it the ‘“‘coracoid tuber.”’ The supracoracoid foramen is
situated immediately dorsoanteriorly to this coracoid prominence, at the
bottom of a broad depression. Its passage through the coracoid from within is
clearly in a forward direction.

4 POSTILLA 165

The glenoid moiety and the adjacent region that articulated with the
scapula are the most robust portions. The preserved part of the scapular
articulation is nearly circular in outline and is strongly digitate. A stout
column, nearly circular in section, extends ventroanteriorly from the
glenoid-scapular region to the coracoid tuber (or biceps tubercle), then veers
ventrally along the posterior margin with diminishing robustness, and finally
disappearing in a caudally-directed hook-like flange at the lower coracoid
extremity. The posterior margin between the glenoid rim and the ventral
“hook” is the most robust of all free coracoid margins. The inferior margin is
moderately thick but is not expanded, and the upper anterior margin is
thin-edged throughout. Presumably, this latter edge thickened near its
junction with the scapula, because the left scapula (AMNH 3015) bears a
thick acromial or deltoid border.

Internally, a sharp concavity occurs at the site of the coracoid tuber
(beneath it), delineated above and below by bony struts or thickenings
extending forward. Anteriorly, this concavity apparently was floored by
extremely thin bone (now lost) or perhaps was fenestrated.

As illustrated in Figure 2, the coracoid of Deinonychus, compared with
that of various other theropods, is much longer (relative to scapular length)
and much deeper (dorsoventrally). Its anteroposterior dimension is 107 mm
(approximately 60% of scapular length), compared with a vertical dimension
that exceeded 100 mm. In addition, the coracoid tuber or biceps tubercle is
much more prominently developed than in any other theropod of which I am
aware. I have not had the opportunity to check a wide variety of theropod
coracoids, but it appears that a correlation may exist between tuber
prominence and relative forelimb length. Gorgosaurus, Albertosaurus and
Tyrannosaurus (with reduced forelimbs) have little or no tuber development,
whereas Allosaurus, Struthiomimus and Gallimimus all have distinct coracoid
tubers. Deinonychus, with what seem to be the longest (relatively) forelimbs
among presently known theropods, is characterized by the most prominent
coracoid tubercle.

It is especially interesting that this correlation extends even to
Archaeopteryx, a structurally similar, obligate biped. The theropod-like
forelimbs (and pectoral arch) of Archaeopteryx are relatively longer than
those of any known theropod, and correspondingly, the biceps tubercle is the
most prominent of all. Furthermore, this tubercle in Archaeopteryx is
situated at almost exactly the same position as in Deinonychus—just anterior
to the glenoid rim and immediately ventral to the supracoracoid foramen. In
my opinion, there can be little doubt that these are homologous structures,
but for a more thorough treatment of the evolutionary relationships between
Archaeopteryx and theropods see Ostrom 1973, 1974.

Inasmuch as the sternum is rarely preserved in theropod specimens, we do
not know the exact position and orientation of the scapulocoracoid. Most
articulated skeletons, however, indicate that the scapular blade ascended up
and backward at an angle of between 20 and 40° to the axis of the dorsal
vertebrae. For example, see the skeletons of Struthiomimus altus, AMNH

FORELIMB FUNCTION OF DEINONYCHUS 5

= Y Gorgosaurus

Allosaurus

aa

Gallimimus

Deinonychus

FIG. 2. Comparison of representative theropod scapulocoracoids to illustrate the
relatively large size of the coracoid of Deinonychus. All scapulae are oriented
horizontally and drawn to unit length for convenient comparison. The horizontal lines
indicate respective scales; each line equals 5 cm. Taxa included represent the major
theropod categories; Deinodontidae (Gorgosaurus), Megalosauridae (Allosaurus),
Ornithomimidae (Gallimimus) and Dromaeosauridae (Deinonychus). All scapulae and
coracoids are illustrated in a single plane; the anteromedial curvature of the coracoids is
eliminated for uniform comparison.

5339 (pl. XXIV; Osborn, 1917); Compsognathus longipes, BS ASI563 (pl. III;
Wagner, 1861); Coelophysis longicollis, AMNH 7223 (pl. 28: Colbert, 1961);

6 POSTILLA 165

Velocirapter mongoliensis, GIMAS (pl. II, 2; Kielan-Jaworowska and
Barsbold, 1972). In that position, the coracoid of Deinonychus would have
been situated entirely anterior (and slightly medial) to the glenoid, with the
supracoracoid foramen immediately in front of the glenoid and the coracoid
tuber in front and slightly below that socket.

The reasons for the shape and unusual size of the coracoid in Deinonychus
can only be guessed at, but there can be little doubt that they were related to
forelimb function and the biomechanics of the shoulder joint. Reconstruction
of the coracoid musculature provides some clues.

CORACOID MUSCULATURE

It is difficult to know which modern species is the best model for
reconstruction of the pectoral and forelimb musculature in a theropod. Since
Deinonychus is classified as reptilian, one is tempted to rely on a generalized
modern reptile, such as Sphenodon or a lizard. But as obligate quadrupeds, in
which the forelimb orientation and function as well as the scapulocoracoid
position and morphology are very different, such analogues are of doubtful
value. On the other hand, the scapula and humerus of Deinonychus (and
those of many other theropods) are distinctly bird-like, even though the
coracoid is not. Flight specializations seemingly disqualify all birds as suitable
models, but birds—like theropods—are obligate bipeds. Hence the modern
bird analogue may not be as inappropriate as it first appears.

If we accept the traditional conclusion that Archaeopteryx was a true bird
(whether or not we accept it as the direct ancestor of modern birds), then we
may assume that the pectoral anatomy of modern carinates evolved from a
stage comparable to that of Archaeopteryx. Elsewhere (Ostrom, 1973; 1974;
MSS), I have shown that the osteology of Archaeopteryx is fundamentally
that of a small theropod. This is especially true of the forelimb and pectoral
arch, including the coracoid! The coracoid of Deinonychus, for example, is
far more similar to that of Archaeopteryx than it is to the coracoid of any
living reptile. Parallelism and convergence notwithstanding, these pronounced
skeletal similarities suggest (but do not establish) that the pectoral muscular
patterns of Archaeopteryx and some theropods were also broadly similar.

In both living reptiles (Sphenodon, lizards and crocodilians) and birds the
number of muscles that arise from the coracoid is few. The most important of
those muscles are the M. coracobrachialis (often bipartite), M. supracora-
coideus, M. subcoracoideus, M. biceps and M. sternocoracoideus. There are
others, but these are the major muscles. With the exception of the
sternocoracoideus, all insert on the forelimb and all but the biceps insert on
the humerus. (In birds, the biceps has a second origin on the proximal medial
crest of the humerus.) The principal function of the coracoid musculature is

FORELIMB FUNCTION OF DEINONYCHUS 7

to move the forelimb. Thus, the location of each muscle origin relative to the
glenoid is of primary importance.

The significance of this last point is dramatically illustrated by discarding
(for the moment) the conventional orientation (living position) of the
scapulocoracoid and adopting instead a standard orientation of the glenoid, as
in Figure 3. In that figure, all scapulocoracoids are oriented with the glenoid
long-axis in a horizontal position. Despite the great morphological differences
among the four species figured, there is surprising uniformity as regards the
location of coracoid muscle origins with respect to the glenoid. In all four
model species, the M. coracobrachialis pulls the humerus toward the coracoid
(as we would expect). In a lizard (also Sphenodon and crocodilians) that
movement is downward and slightly backward. In birds, however, it may be
forward and slightly upward, or down and backward depending upon which
portion of the coracobrachialis is contracting. In all four groups, the biceps is
a primary fore-arm flexor, and the coracoid origin of such flexors obviously
must lie on the “‘flexion’”’ side of the glenoid (i.e., the origin must be situated
between the glenoid and the insertion point on the adducting antebrachium).
The close correspondence of the origin areas of the coracobrachialis (and
supracoracoideus) and the biceps, as shown in Figure 3, assures that the plane
of fore-arm flexion (by the biceps) coincides with the planes of humeral
adduction produced by these other coracoid muscles. Thus, fore-arm flexion
in a lizard adducts the distal limb segments ventromedially, whereas the avian
fore-arm is flexed anteromedially.

From these living models, it is evident that the main function of those
muscles that originate on the coracoid is adduction of the humerus and
adductive flexion of the fore-arm at the elbow. The direction of humeral
adduction and fore-arm flexion is determined by the orientation of the
scapulocoracoid and the location of the muscular origins with respect to the
glenoid. These observations are so elementary that they hardly need mention
here.

However, regardless of what particular anatomical terms happen to be
applied to the muscles that arose from the theropod coracoid, and regardless
of whether they are considered homologous with those of modern reptiles or
birds, it is certain that the same actions and movements must have occurred.
Accepting the scapulocoracoid orientation that is indicated by the several
theropod specimens cited above, the coracoid musculature of Deinonychus
must have produced powerful anteromedial adduction of the humerus,
coupled with strong anteromedial flexion of the fore-arms and hands.
Humeral adduction presumably was accomplished by relatively large
““coracobrachialis” and ‘‘supracoracoideus” muscles that originated on the
large anterior expansion of the coracoid. Anteromedial flexion of the
antebrachium was accomplished by a powerful “biceps” that probably arose
by tendonous attachment on the prominent coracoid tuber (biceps tubercle).
This latter feature is ideally situated for that function, anterior and medial to
the glenoid, and corresponds closely in position and form to the biceps
tubercle of Archaeopteryx (Walker, 1972).

POSTILLA 165

Sphenodon Bi
Bi punctatum
A. cb

Sic Alligator
Bi mississippiensis -
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Bi \
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h\e< Corvus \
———— | americanus

Deinonychus”) antirrhopus

FORELIMB FUNCTION OF DEINONYCHUS 9

APPENDICULAR EVIDENCE

The humerus of Deinonychus has been described previously (Ostrom, 1969b)
and does not require detailed description here. Some review is appropriate,
however, as regards the preceding interpretation of the coracoid and its
musculature.

As noted in the above report, good evidence exists (the large deltopectoral
crest) in the known humeri of Deinonychus that indicates powerful forward
adduction of the forelimbs by large pectoralis and coracobrachialis muscles.
The opposite, recovery movement by the M. deltoideus apparently was not so
powerful, judging from the low profile (Fig. 1) of the incomplete acromial
process of the scapula. While the large deltopectoral crest does indicate a large
pectoralis muscle with considerable leverage, there are no distinct scars
preserved on that structure or on adjacent surfaces to show the relative sizes
of the pectoralis vs. the coracobrachialis. It is presumed, however, on the
basis of the coracobrachialis insertion site in both modern birds and reptiles,
that this muscle inserted proximally along the base of the deltopectoral crest
on the medial (ventral) humeral surface adjacent to the insertion of the M.
pectoralis. Its action, like that of the pectoralis, was anteromedial adduction
of the humerus.

A small but distinct tubercle (marked by the uppermost line labeled “‘PE”’

FIG. 3, opposite page. The scapulocoracoids (in lateral view) of three modern reptiles
and a bird (common crow) showing the attachment areas of three major muscles that
originate on the coracoid. Since these muscles function exclusively to move the humerus
or flex the antebrachium, their positions relative to the glenoid are the critical
mechanical features. For that reason, the scapulocoracoids are illustrated here with the
glenoids oriented in standard position (i.e., the long axis of each glenoid is horizontal,
regardless of its natural position in life). The living position of each example is indicated
by the arrow at the left, which points anteriorly and indicates the horizontal. At the
bottom is the scapulocoracoid of Deinonychus showing the probable areas of origin
proposed here for the coracoid musculature. The clusters of arrows at the right indicate
the direction from the glenoid (indicated by the solid spot) to the center of origin of the
several coracoid muscles (usually, but not always the same as the direction of muscle
pull). In the quadrupedal examples, the coracoid muscle-origin locations are basically
similar. In birds, the locations of muscle origins are different, but the directions of
muscle pull are surprisingly similar to that of the quadrupedal examples. The
supracoracoideus (of birds) pulls the humerus up and forward (dashed arrow), by
virtue of its passage upward from the humerus through the foramen triosseum and
then downward to its origin on the ventral extremity of the coracoid. The
coracobrachialis anterior, with its origin adjacent to that of the biceps on the enlarged
avian biceps tubercle, functions like the coracobrachialis (unmodified) of the quadrupeds
(i.e., adducting the humerus in nearly the same plane as antebrachial flexion by the
biceps), whereas the coracobrachialis posterior appears to be a remnant of the primitive
muscle that retained the primitive origin site adjacent to the sternal border of the
coracoid. The latter now functions to adduct the humerus posteroventrally (folding the
wing). Abbreviations: Bi= M. biceps; Cb= M._ coracobrachialis; Cbh-a= M.
coracobrachialis anterior; Ch-p = M. coracobrachialis posterior; Sc = M. supracora-
coideus. The horizontal lines beneath each scapulocoracoid equal 1.0 cm, all coracoids
being drawn to unit length for easy comparison.

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