Osteological correlates of cervical musculature in Aves and Sauropoda
(Dinosauria: Saurischia), with comments on the cervical ribs of Apatosaurus
MATHEW J. WEDEL1 and R. KENT SANDERS2
'University of California Museum of Paleontology, 1101 Valley Life Sciences Building, Berkeley, CA 94720-4780;
sauropod@socrates.berkeley.edu. 2Department of Radiology, University of Utah Medical Center, 50 North Medical
Drive 1A71, Salt Lake City, UT 84132; kentsandersmd@netscape.net
The cervical muscles of birds attach to specific bony features on the vertebrae. Most of these osteological correlates
are also present in the cervical vertebrae of sauropod dinosaurs, which suggests similar cervical musculature in the
two groups. One exception is the processus caroticus, which anchors the long ventral muscles of the bird neck but
has no obvious homolog in the vertebrae of sauropods.
The absence of anterior processes of the cervical ribs has traditionally been regarded as an autapomorphy of
Apatosaurus louisae. However, anterior processes are weakly developed or absent in some specimens of A. ajax and
A. excelsus, so this character is probably less diagnostic than previously assumed.
PaleoBios22(3):l-6, December 15, 2002
© 2002 University of California Museum of Paleontology
INTRODUCTION
Sauropod dinosaurs, the largest terrestrial vertebrates, have
been of continuing interest to paleobiologists because of
their large size and long necks. A great deal of recent work
has focused on sauropod necks and their support and mo-
bility (Martin 1987, Frey and Martin 1997, Martin et al.
1999, Stevens and Parrish 1999). These studies have dealt
mainly with the cervical osteology of sauropods. The cervi-
cal musculature of sauropods has received only limited at-
tention to date.
As the only extant saurischian dinosaurs, birds are the
closest living relatives of sauropods. Many of the osteologi-
cal features of sauropod cervical vertebrae correspond to
muscle attachment points in the cervical vertebrae of birds,
especially ratites such as the ostrich, Struthio camelus
Linnaeus 1758 (see Wedel and Sanders 1999, Sanders et al.
2000). This correspondence suggests that the cervical mus-
culature of sauropods was similar to that of birds. Herein
we briefly compare the cervical osteology of birds and sau-
ropods and discuss the possible musculature of sauropod
necks. We also discuss variation in the cervical ribs of
Apatosaurus Marsh 1877 and the taxonomic utility of cer-
vical ribs in this genus. We follow Baumel et al. (1993) for
avian anatomical terminology, with supplementary informa-
tion drawn from Harvey et al. (1968) and Zweers et al.
(1987).
Abbreviations used in the figures are provided in Table 1.
Institutional Abbreviations: AMNH: American Museum
of Natural History, New York; CM: Carnegie Museum of
Natural History, Pittsburgh; YPM: Yale Peabody Museum,
New Haven.
COMPARATIVE ANATOMY
The origins and insertions of avian neck muscles are listed
in Table 2 and illustrated in Fig. 1. The correspondence of
these features with their presumed homologs in sauropods
is as follows:
1)  The ansa costo-transversaria is the bony loop formed
by the fusion of the cervical rib to the diapophysis and
parapophysis, and it bounds the transverse foramen. This is
present in all sauropods.
2) The tubercula ansae and cristae laterales are tubercles
and crests, respectively, on the lateral faces of the ansae costo-
transversariae. Similar features are present in sauropods, but
less well-developed, and appear as small processes on the
lateral or posterior margins of the diapophyses (see Fig. 3).
Wedel et al. (2000b) referred to these processes as
diapophyseal tubercles.
3)  The processus spinosus is the neural spine. Neural
spines are present in the cervical vertebrae of all sauropods,
although they are extremely low in some taxa, such as
MamenchisaurusYoung 1954 (see Russell and Zheng 1994).
Bifid neural spines are present in some ratites and in several
sauropod clades.
4) The torus dorsalis is a prominent rugosity on the dor-
sal surface of the postzygapophyseal ramus, above the
postzygapophysis itself. Similar rugosities are present above
the postzygapophyses of many sauropods, and have been
Table 1. Abbreviations used in the figures.
act	ansa costotransversana
ant	anterior process
cl	crista lateralis
cto	crista transverso-obliqua
pea	processus caroticus
pco	processus costalis
psp	processus spinosus
spol	spinopostzygapophyseal lamina
ta	tuberculum ansa
td	torus dorsalis
vlp	ventrolateral process
2
PALEOBIOS, VOL. 22, NUMBER 3, DECEMBER 2002
referred to as epipophyses (Sereno et al. 1999) or dorsal
tubercles (Wedel et al. 2000b). These processes are espe-
cially prominent in Jobaria Sereno et al. 1999 and in long-
necked taxa such as Barosaurus Marsh 1890, Brachiosaurus
Riggs 1903, and SauroposeidonWedel et al. 2000a (see Wedel
et al. 2000b).
5)  The crista transverso-obliqua is a low crest that runs
from the processus spinosus to the torus dorsalis. In sauro-
pods the processus spinosus and torus dorsalis are connected
by the spinopostzygapophyseal lamina (Wilson 1999). The
crista transverso-obliqua of birds is a muscle attachment fea-
ture. The spinopostzygapophyseal lamina of sauropods
bounds a pneumatic fossa and is the only cortical surface
available for muscle attachment between the processus
spinosus and torus dorsalis.
6)  The processus caroticus is one of a pair of processes
on the ventral surface of the centrum that bound the ca-
rotid fossa on either side. The processes carotid are ven-
trally directed and sometimes inclined anteriorly. They arise
from the ventral cortex at the junctions of the parapophyses
with the centrum. No feature occupies the equivalent posi-
tion in sauropod vertebrae.
7)  The processus costalis is the cervical rib. The cervical
ribs of birds are typically short, and rarely extend beyond
the ends of their respective centra. Cervical rib length ap-
pears to be related to body size, and the largest taxa, such as
Struthio, have the longest cervical ribs. All sauropods have
cervical ribs. Many taxa have very long cervical ribs that run
beneath several centra and form overlapping bundles (e.g.,
Sauroposeidon; see Wedel et al. 2000b). The cervical ribs of
Apatosaurus are more similar to those of birds in being very
short and robust. In birds, the anterior terminus of each
cervical rib consists of a broad, blunt surface called the caput
of the cervical rib (Zweers et al., 1987). This feature is not
to be confused with the capitulum, which connects the cer-
vical rib to the parapophysis of the centrum. The caput of
the avian cervical rib does not extend forward as a free pro-
cess, as do the anterior processes of the cervical ribs in most
sauropods. In some specimens of Apatosaurus the cervical
ribs lack anterior processes. This absence is unusual among
sauropods and is discussed in more detail below.
8) The processus ventralis corporis is a ventrally-directed
crest descending from the midline of the centrum. This fea-
ture is most prominent in the posterior cervical vertebrae,
and is often formed by the medial fusion of the processes
carotici posterior to the point at which the carotid arteries
diverge from the carotid sulcus as they approach the thorax.
In CM 879, a specimen of HaplocanthosaurusHstchcr 1903,
a low ridge is variably present on the ventral surface of the
centrum between the parapophyses. It is unclear whether
this ridge corresponds to the processus ventralis corporis of
birds, and similar processes are absent in the cervical verte-
brae of all other sauropods that we have examined.
The short muscles of the avian cervical system are the
Mm. intercristales, Mm. interspinales, and Mm.
intertransversarii, which connect the cristae transverso-
obliquae, processes spinosi, and ansae costotransversariae,
respectively, of adjacent vertebrae. The neural spines and
cervical rib loops (ansae costotransverariae) of sauropods
often bear rugosities and tubercles, and probably served to
anchor homologous muscles (Fig. 2). As discussed above,
the avian crista transverso-obliqua occupies the same posi-
tion as the spinopostzygapophyseal lamina of sauropods.
Homologs of the avian Mm. intercristales may have con-
nected the spinopostzygapophyseal laminae of adjacent ver-
tebrae in sauropods.
The long muscles of the avian cervical system consist of a
dorso-lateral group, the M. ascendens cervicalis and M. Ion-
Table 2. Origins and insertions of the cervical musculature of birds. Information drawn from Zweers et al. (1987) and Baumel et
al. (1993). Muscles appear in the order discussed in Baumel et al. (1993); muscles inserting on the skull are not included.
Muscle
M. ascendens cervicalis
M. longus colli dorsalis
Mm. intercristales
Mm. interspinales
Mm. intertransversarii
M. flexor colli medialis
M. flexor colli lateralis
M. longus colli ventralis
Origin
Ansa costotransversaria
Processus spinosus
Crista transverso-obliqua
Processus spinosus
Tubercula ansae
Cristae laterales
Processus caroticus
Processus costalis
Tubercula ansae
Cristae laterales
Processus caroticus
Proc. ventralis corporis
Insertion
Torus dorsalis
Torus dorsalis
Crista transverso-obliqua
Processus spinosus
Tubercula ansae
Cristae laterales
Proc. ventralis corporis
Processus costalis
Processus costalis
Processus costalis
WEDEL & SANDERS-SAUROPOD CERVICAL MUSCULATURE
3
pea
pco
pea
spol
psp
pco
Fig. 1. Cervical vertebrae of Struthio camelus and Apatosaurus louisae, with muscle attachment points labeled (C and D traced from
Gilmore 1936:pl. 24). A. A mid-cervical vertebra of S. camelus in left lateral view. B. A mid-cervical vertebra of S. camelus in ante-
rior view. C. CIO of A. louisae in left lateral view. D. CIO of A. louisae in anterior view.
gus colli dorsalis, and a ventral group, the M. flexor colli
lateralis and medialis and the M. longus colli ventralis. The
dorso-lateral muscles originate on the neural spines and trans-
verse processes and insert on the tori dorsales. The tori
dorsales are therefore the primary anchor points through
which the extensor muscles exert force on the cervical skel-
eton. The prominent tori dorsales of many sauropods, es-
pecially the long-necked taxa, indicate that homologous
extensor muscles were probably well-developed in sauro-
pods (Eig. 2). The ventral muscles of the avian neck origi-
nate on the processes carotici and costales and insert on the
processes costales and the processes ventrales of the centra.
The cervical ribs of sauropods almost certainly served as
insertions for the equivalent muscles.
The absence of processes carotici and processes ventrales
in sauropods is puzzling and warrants further investigation.
This absence is particularly puzzling because all of the other
muscle attachments in avian cervical vertebrae can be readily
identified in the vertebrae of sauropods. The processes
carotici and processes ventrales of birds serve as origins for
the M. flexor colli medialis (in part) and M. longus colli
ventralis. In the absence of similar processes in sauropods,
where did the long ventral muscles of the neck originate?
This question will probably remain unanswered until we have
a better understanding of the evolution of cervical muscula-
ture in archosaurs and can frame the problem in a more
complete phylogenetic context. We are therefore pleased to
note ongoing work by Tsuihiji (2001, 2002) on the evolu-
tion of cervical musculature in diapsids, which promises much
valuable information.
THE CERVICAL RIBS OF APATOSAURUS
The cervical ribs of some specimens of Apatosaurus lack
anterior processes (Fig. 3). This absence of anterior pro-
cesses has traditionally been considered a synapomorphy of
A. /o»h»* Holland 1915 (see Gilmore 1936, Mcintosh 1990).
However, the anterior processes are only weakly developed
on certain cervical vertebrae of YPM 1980, the holotype of
4
PALEOBIOS, VOL. 22, NUMBER 3, DECEMBER 2002
Fig. 2. Possible connections of the cervical musculature of sauropods, based on the neck muscles of birds. The stylized neck seg-
ment is shown in left lateral view. Dashed arrows indicate muscles passing medially behind bony structures. A.-E. Muscles that span
multiple segments. In birds, most of these muscles have multiple slips and they may attach to some vertebrae but not others (see
Zweers et al. 1987:fig. 7). For simplicity, only a single course is illustrated here, but readers are cautioned that the actual muscula-
ture was probably much more complex. A. M. longus colli dorsalis. B. M. cervicalis ascendens. C. M. flexor colli lateralis. D. M.
flexor colli medialis. E. M. longus colli ventralis. In birds, this muscle originates from the processes carotici, which are absent in the
vertebrae of sauropods (see discussion in text). F.-H. Single segment muscles. F. Mm. intercristales. G. Mm. interspinales. H.
Mm. intertransversarii. Vertebrae modified from Gilmore (1936:pl. 24).
A. excelsus Marsh 1879 (see Fig. 3C). Furthermore, ante-
rior processes are present in YPM 1860, the holotype of A.
ajax Marsh 1877, but absent in YPM 1861, a cervical verte-
bra from the same quarry that has been referred to A. ajax
(see Mcintosh 1995). Therefore, the presence or absence
of anterior processes is probably not useful for discriminat-
ing species of Apatosaurus.
Cervical ribs of Apatosaurus that lack anterior processes
typically bear one or more small bony processes on their
ventral margins (Fig. 3). Gilmore (1936:196) described ven-
trolateral processes in the cervical vertebrae of CM 3018,
the holotype of A. louisae, and stated that each of these
processes "doubtless served as the attachment of one of the
powerful intervertebral muscles." They are also present in
cervical vertebra YPM 1861 of A. ajax. Because these ven-
trolateral processes occur on cervical ribs that lack anterior
processes, it is possible that both types of processes corre-
spond to the same muscle and simply represent alternate
attachments. However, the processes in a cervical vertebra
of A. ajax face more posteriorly than anteriorly (Fig. 3E),
and a vertebra of A. excelsus bears processes that face ven-
tromedially rather than ventrolaterally (Fig. 3D). It is doubt-
ful that all of these processes correspond to the same muscle
or muscles. In birds, different muscles attach to the medial
and lateral sides of the cervical ribs, and a single bony fea-
ture may serve as the attachment for more than one muscle.
For example, the M. flexor colli mediales originate from
and insert on the medial surfaces of the cervical ribs, and
the laterally-facing tubercula ansae and cristae laterales serve
as attachments for both the Mm. intertransversarii and the
M. flexor colli laterales. For the present, it may be impos-
sible to determine which of these muscles were associated
with the various processes of Apatosaurus cervical ribs.
CONCLUSIONS
A review of avian cervical musculature reveals that most
of the osteological correlates of specific muscles are also
present in the vertebrae of sauropods. One exception is the
processus caroticus, which is absent in sauropods. The ori-
gins of the long muscles of the ventral neck are therefore
uncertain in sauropods, and more work will be required to
resolve this problem. The absence of anterior processes of
the cervical ribs has traditionally been one of the characters
used to diagnose Apatosaurus lousiae to the exclusion of
other species of Apatosaurus. Anterior processes are vari-
ably absent in A. ajax and A. excelsus, which casts doubt on
the utility of this character. Taxonomists should treat cervi-
cal rib characters with caution, given the variation in cervi-
cal rib morphology in Apatosaurus.
WEDEL & SANDERS-SAUROVOD CERVICAL MUSCULATURE                                       5
A                                   B
Fig. 3. Cervical vertebrae of Apatosaurus showing the variation in cervical rib morphology, scale bar = 20 cm. All vertebrae are
shown in left lateral view, except for D, and at the same scale. A. CIO of A. excelsus (CM 563, modified from Gilmore 1936:pl. 31)
is typical of most sauropod cervical vertebrae in having well developed anterior processes. B. CIO of A buisae (CM 3018, modified
from Gilmore 1936:pl. 24) lacks anterior processes, but it has prominent ventrolateral processes. C. C8? of the A excelsus holotype
(YPM 1980, modified from Ostrom and Mcintosh 1966:pl. 12) has large processes that project ventromedially. These are probably
not equivalent to the ventrolateral processes of A buisae, because different muscles attached to the medial and lateral surfaces of
the cervical ribs. D. The same vertebra in anterior view. E. A posterior cervical vertebra of A ajax (YPM 1861, modified from
Ostrom and Mcintosh 1966:pl. 15) is similar to vertebrae of A buisae in having ventrolateral processes rather than anterior pro-
cesses. Note that at least one of these processes faces posteriorly rather than anteriorly. F. Another posterior cervical vertebra of A
ajax (YPM 1840, modified from Ostrom and Mcintosh 1966:pl. 14) has robust anterior processes. The vertebrae shown in E and F
are from the same quarry and, possibly, the same individual (see Mcintosh 1995). Although anterior processes are consistently ab-
sent in A buisae (e.g., AMNH 460 and CM 3018), they are variable in both A excelsus and A ajax, and we consider the taxo-
nomic utility of anterior processes to be suspect.
6
PALEOBIOS, VOL. 22, NUMBER 3, DECEMBER 2002
ACKNOWLEDGEMENTS
We thank David Berman and Nicholas Czaplewski for
access to specimens, Jeff Person for curatorial assistance,
and Richard Cifelli and Kyle Davies for providing literature.
We are especially grateful to Drs. Kent Stevens, Paul
Upchurch, and Jeff Wilson for helpful review comments.
This is University of California Museum of Paleontology
Contribution No. 1797
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