Endocranial morphology of Apatosaurus (Dinosauria: Sauropoda) based on computed tomography of a previously undescribed braincase with comments on variation and evolution in sauropod neuroanatomy

Abstract

We describe a previously unreported braincase of the sauropod dinosaur Apatosaurus from the Cactus Park Quarry, Morrison Formation of western Colorado using high-resolution X-ray computed tomography. The digital nature of these data allowed us to prepare and describe the first three-dimensional rendering of the endocranial space in this historically important dinosaur species. Results are compared with a range of taxa drawn from across the sauropod tree revealing previously underappreciated variation in the sauropod neurocranium. Examples of variable characters include the degree of cerebral and pontine flexure, the morphology of the parietal body and superior sagittal sinus and their relationship with the overlying dermal roof, and the conformation of several cranial nerve foramina. We provide preliminary evolutionary hypotheses and discussion for many of these features. The recognition that considerable variation is present in the sauropod neurocranium hopefully will encourage more detailed descriptions of this anatomically complex region, as well as facilitating a synthetic review of the sauropodomorph braincase as these descriptions become available.

Full text

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3677, 29 pp., 10 figures, 1 table March 4, 2010
The Braincase of Apatosaurus (Dinosauria:
Sauropoda) Based on Computed Tomography of a
New Specimen with Comments on Variation and
Evolution in Sauropod Neuroanatomy
AMY M. BALANOFF,
1,2
GABE S. BEVER,
3
* AND TAKEHITO IKEJIRI
4
ABSTRACT
We describe a previously unreported braincase of the sauropod dinosaur Apatosaurus from the
Cactus Park Quarry, Morrison Formation of western Colorado using high-resolution X-ray
computed tomography. The digital nature of these data allowed us to prepare and describe the first
three-dimensional rendering of the endocranial space in this historically important dinosaur
species. Results are compared with a range of taxa drawn from across the sauropod tree revealing
previously underappreciated variation in the sauropod neurocranium. Examples of variable
characters include the degree of cerebral and pontine flexure, the morphology of the parietal body
and superior sagittal sinus and their relationship with the overlying dermal roof, and the
conformation of several cranial nerve foramina. We provide preliminary evolutionary hypotheses
and discussion for many of these features. The recognition that considerable variation is present in
the sauropod neurocranium hopefully will encourage more detailed descriptions of this
anatomically complex region, as well as facilitating a synthetic review of the sauropodomorph
braincase as these descriptions become available.
Copyright EAmerican Museum of Natural History 2010 ISSN 0003-0082
1
Division of Paleontology, American Museum of Natural History.
2
Department of Earth and Environmental Sciences, Columbia University.
3
Division of Paleontology, American Museum of Natural History.
* Current address: Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT
06520.
4
Museum of Paleontology and Department of Geological Sciences, University of Michigan, 1109 Geddes Road, Ann
Arbor, MI 48109-1079.

INTRODUCTION
The complex nature of the vertebrate brain-
case makes this region a rich source of
phylogenetically informative data. The brain-
case and neuroanatomy of sauropod dinosaurs,
however, historically have played a relatively
minor role in shaping hypotheses with regards
to the phylogenetic and evolutionary history of
this unique and interesting group. This relative
lack of influence undoubtedly stems from the
rarity with which well-preserved cranial mate-
rial is recovered, with the postcranial elements
traditionally considered diagnostic for refined
taxonomic levels within this group (Tidwell and
Carpenter, 2003; see Wilson, 2002: table 8).
This preservational gap translates directly to
the existing disparity in our understanding of
morphological variation for these respective
anatomical systems across Sauropoda and
reinforces the need for detailed anatomical
description and comparison of whatever crani-
al data are available. One way to increase the
return of anatomical data from a particular
specimen is through the use of high-resolution
X-ray computed tomography (HRCT). HRCT
is a nondestructive tool for visualizing internal
structures and is particularly useful for study-
ing the complex internal anatomy of a fossilized
braincase (Carlson et al., 2003). In addition to
providing access to internal cranial features
such as the presence/absence of bony contacts
and processes, HRCT enhances our ability to
visualize and interpret soft-tissue structures of
the central nervous, circulatory, and endocrine
systems that are housed within the braincase.
The purpose of this study is to provide a
detailed description of a previously unreported
braincase of Apatosaurus that includes the first
observations on the internal braincase anatomy
of this taxon. Observed morphologies are
compared with selected sauropod and outgroup
taxa allowing preliminary assessments of neu-
rocranial variation and evolution within
Sauropoda. We review the published literature
and explicitly establish baseline evolutionary
hypotheses for a number of selected neurocra-
nial features within Sauropoda. The hope is that
these hypotheses will elicit future testing and
provide the groundwork for more synthetic
studies examining broad patterns of neurocra-
nial evolution within Sauropodomorpha.
Institutional abbreviations used in this study:
AMNH (American Museum of Natural His-
tory, New York, New York); BYU (Brigham
Young University, Earth Science Museum,
Provo, Utah); CM (Carnegie Museum of
Natural History, Pittsburgh, Pennsylvania);
HMS (Houston Museum of Natural Science,
Houston, Texas); YPM (Yale University,
Peabody Museum of Natural History, New
Haven, Connecticut).
MATERIALS AND METHODS
SPECIMEN
We describe a fairly complete, well-pre-
served braincase of Apatosaurus (BYU 17096;
figs. 1, 2), using HRCT. BYU 17096 was
collected from the Jurassic Morrison
Formation of the Cactus Park BYU Quarry
in western Colorado. The endocranial space is
completely infilled with a fine-grained sand-
stone matrix. The allocation of BYU 17096 to
Apatosaurus is based in part on the hundreds
of disarticulated and semiarticulated postcra-
nial bones diagnosable to Apatosaurus (espe-
cially cervicals, dorsals, anterior caudals, and
relatively robust limb bones) that were found
in the same quarry (Curtice and Wilhite, 1996;
Foster, 2003). None of the other genera
common in the Morrison Formation, such as
Diplodocus and Camarasaurus, were recovered
from the Cactus Park Quarry. The diagnosis
of BYU 17096 to Apatosaurus also is based on
observed braincase apomorphies discussed in
the Phylogenetic Diagnosis section below.
SCANNING
BYU 17096 was scanned at the University of
Texas High-Resolution X-ray Computed
Tomography Facility on 14 May 2004 using
their high-energy system. Scanning was per-
formed using a brass filter and air wedge, a
voltage of 420 kV, and an amperage of 4.8 mA.
The resulting images were then processed for
the removal of ring and streak artifacts using
programs written by Richard Ketcham. The
specimen was scanned along the coronal axis
for a total of 127 slices at an image resolution of
1024 31024 pixels. The interslice spacing is
1.0 mm, and the slice thickness is 0.8 mm. Each
image has a reconstructed field of view of
2 AMERICAN MUSEUM NOVITATES NO. 3677

265 mm. The reconstruction of the endocranial
endocast was done with the original 16-bit
imagery in the volumetric rendering program
VGStudioMax
E
1.2.1. Contrast of the images
was increased until the infilled endocranial
space and bone were easily distinguishable from
each other. The endocranial cavity was selected
using the segmentation tools available in this
program, separated into its own volume, and
exported as an isosurface. All measurements of
the braincase and endocast (including volume)
were taken in VGStudioMax
E
.Endocastvol-
ume measurements were taken by calculating
the volume of negative space of the endocranial
cavity. For ease of description, features of the
endocranial cast are referred to by the names of
the soft tissues of the brain that they reflect
(e.g., cerebrum rather than cast of cerebrum). It
is important to note, however, that what
actually is preserved is a cast of the endocranial
space, which also reflects structures other than
the brain, such as meninges and sinuses. This
cast, however, is useful in determining relative
size and shape of different regions of the brain
as well as recognizing the morphology of the
cranial nerve roots. The original slice data and
movies showing the endocranial cast are
available at the DigiMorph website (www.
digimorph.org/specimens/Apatosaurus_sp).
TAXONOMIC COMPARISONS
The inclusion of cranial material into phylo-
genetic analyses of sauropods (e.g., Wilson,
2002; Upchurch et al., 2004; Rauhut et al.,
2005) makes the diagnosis of newly discovered
material more confident. Previous descriptions
of cranial material allocated to Apatosaurus
include that of Berman and McIntosh (1978),
who described a fairly complete skull (CM
11162) of a ‘‘probable’’ Apatosaurus (Berman
and McIntosh, 1998: 21) and a partial braincase
(YPM 1860) from the upper Jurassic Morrison
Formation of Colorado. A partly disarticulat-
ed braincase is known from Como Bluff,
Wyoming (Connely and Hawley, 1998). In
addition to describing a new braincase of
Apatosaurus, we present the only endocast that
is known for this taxon.
The number of endocasts that exist for
sauropodomorph taxa is surprisingly large,
although the majority of these were only
briefly described and illustrated in the late
19th and early 20th centuries (Marsh, 1880,
1884; Holland, 1906; Osborn and Mook, 1921;
Ostrom and McIntosh, 1966). An endocast of
Diplodocus (AMNH 694) was described by
Marsh (1884, 1896) and reillustrated by
Hopson (1979). Janensch (1935–36) described
an endocast of Brachiosaurus as well as two
diplodocoid taxa, Dicraeosaurus and Tornieria
(Tornieria is referred to as Barosaurus in
Janensch [1935–36]; however, see Remes
[2006] for updated taxonomy). Many of the
more recent descriptions are based on synthetic
endocasts. These include Plateosaurus (Galton,
1985), Shunosaurus (Chatterjee and Zheng,
2002), an early Cretaceous titanosauriform
(TMM 40435; Tidwell and Carpenter, 2003),
and Camarasaurus (Chatterjee and Zheng,
2005; Sereno et al., 2007). A digital endocast
was extracted from HMS 175 and described as
Diplodocus hayi (Franzosa, 2004), although the
allocation of this specimen to Diplodocus is
questionable (see Harris, 2006). Comparative
illustrations of a digital endocast of Diplodocus
also are available in Sereno et al. (2007).
DESCRIPTION
GENERAL FEATURES
Figures 1, 2
BYU 17096 is an articulated braincase that
is complete in that it includes the right and left
orbitosphenoid, laterosphenoid, exoccipital-
opisthotic, prootic, and the midline basisphe-
noid, basioccipital, and supraoccipital. The
paired frontal, parietal, squamosal, and post-
orbital also are present as components of the
dermal roof fused to the endochondral ele-
ments of the braincase. The braincase is well
preserved overall, although the distal extrem-
ities of several bones and processes are
broken. The dermal roofing components are
better preserved on the right side of the skull.
The specimen is large overall (see table 1) and
the cranial sutures are so tightly fused as to be
indistinguishable in many cases in external
view (as is common in sauropods; e.g., Tidwell
and Carpenter [2003], Wilson et al. [2005],
Remes [2006]). This advanced state of fusion is
present in both the dermal roofing and
endochondral elements, indicating skeletal
2010 BALANOFF ET AL.: BRAINCASE OF APATOSAURUS 3

Fig. 1. Braincase of BYU 17096 in right lateral (A), left lateral (B), dorsal (C), and ventral (D) views.
4 AMERICAN MUSEUM NOVITATES NO. 3677

Fig. 2. Braincase of BYU 17096 in anterior (A) and posterior (B) views.
2010 BALANOFF ET AL.: BRAINCASE OF APATOSAURUS 5

maturity for these cranial partitions (but not
necessarily indicative of sexual maturation or
maturation of other skeletal regions). The
HRCT slices did help considerably in locating
at least some of these sutures by revealing their
location deep to the external surface (fig. 3).
Their three-dimensional paths could then be
traced superficially to the external surface of the
specimen. Our ability to do this indicates that
the degree of fusion is not uniform along the
three-dimensional path of these sutures and that
cranial fusion often occurs first at the external
surface of the skull and progresses deeper.
DERMAL ROOF
FRONTOPARIETAL FENESTRA: The dorsal sur-
face of BYU 17096 is formed largely by the
dermal roofing elements, which overlie the
anterior two-thirds of the endocranial cavity.
The dorsal surface is slightly concave up in
overall shape and is dominated by a large
central opening that lies at the junction of the
paired frontals and parietals. This opening
constitutes a broad communication (table 1)
between the external space dorsal to the cranial
roof and the endocranial cavity. This commu-
nication often is referred to as the parietal
foramen (e.g., Salgado and Calvo, 1992;
Chatterjee and Zheng, 2002), presumably be-
cause of an inferred homology with the parietal
foramen that houses the pineal body in other
reptiles (Janensch, 1935–36; Edinger, 1955).
Hopson (1979) noted this opening may not
represent the parietal foramen but rather was
possibly filled with part of the cartilaginous
endocranium or with a portion of the superior
sagittal sinus (or a combination of the dorsal
longitudinal and transverse sinuses; Witmer and
Ridgely, 2009). The opening in BYU 17096 is
confluent with the superior sagittal sinus that
otherwise is enclosed within the paired parietals
(see description of endocast below). The open-
ing also resides at a position homologous to the
frontoparietal fontanelle of amniote embryos.
Therefore, it is possible that an anterodorsal
expansion of the superior sagittal sinus into the
space between the developing elements of the
dermal roof influenced the developmental
dynamics of that region and thus resulted in
the paedomorphic retention of the frontopari-
etal fontanelle in the adult skull as the
frontoparietal fenestra.
The shape of the frontoparietal fenestra in
BYU 17096 is not perfectly circular but rather
exhibits a brief constriction near its posterior
margin due to a slight medially directed cur-
vature of the parietals (fig. 1C). This constric-
tion results in an incomplete partitioning of the
fenestra into a relatively large anterior section
and a much smaller posterior opening. This
partitioning suggests that the frontoparietal
fenestra may have housed two separate soft-
bodied structures (positioned anteriorly and
posteriorly, respectively). The anterior compo-
nent may have housed the parietal (pineal)
body, with a dorsal peak of the superior sagittal
sinus filling the posterior partition (see Witmer
and Ridgely, 2009). This medial constriction of
the parietals and associated partitioning of the
frontoparietal fenestra is common in sauropods
that exhibit this dorsal opening, and a complete
division of the fenestra through medial contact
of the parietals is known within this group (e.g.,
Amargasaurus cazaui; Salgado and Calvo, 1992;
Salgado, 1999). This posterior opening gener-
ally is described as the postparietal foramen
(Janensch, 1935–36; Salgado and Calvo, 1992;
Salgado, 1999), which was an unambiguous
synapomorphy of Dicraeosauridae (Wilson,
2002) but may be derived for the more inclusive
Flagellicaudata (Harris and Dodson, 2004;
Harris, 2006).
FRONTALS: The frontals are wedge-shaped
bones in dorsal view whose posterior margin is
TABLE 1
Measurements taken from BYU 17096
All measurements are in mm.
Transverse width of parietals 169.6
Anteroposterior length of parietals 30.2
Dorsoventral height of supraoccipital 52.0
Transverse width of supraoccipital 60.8
Transverse width of foramen magnum 24.6
Dorsoventral height of foramen magnum 27.6
Anteroposterior width of supratemporal fenestra 21.2
Transverse length of supratemporal fenestra 39.7
Anteroposterior length of frontoparietal fenestra 33.6
Transverse width of frontoparietal fenestra 25.6
Anteroposterior length of skull 94.0
Transverse width of skull across metotic foramina 31.5
Transverse width of skull across paroccipital
processes 169.2
Anteroposterior length of endocast 72.5
Transverse width of endocast at widest point 49.4
6 AMERICAN MUSEUM NOVITATES NO. 3677

tapered due to the presence of the frontopa-
rietal fenestra to which they contribute (along
with the parietals; fig. 1C). The frontals are
relatively shorter anteroposteriorly with their
length being less than twice their transverse
breadth (Wilson, 2002). The frontals contact
the orbitosphenoid and laterosphenoid ven-
trolaterally, parietal posteriorly, postorbital
posterolaterally, and each other medially. The
frontal-parietal suture, which is clearly visible
in the HRCT slices (fig. 3A), begins lateral to
the frontoparietal fenestra and extends to the
supratemporal fenestra along a slightly sinu-
ous trajectory. This suture turns anteriorly
and somewhat ventrally at the lateral edge of
the braincase resulting in the dorsal surface of
the postorbital process being comprised en-
tirely by the frontal (the lateral ends of this
suture lie completely rostral to the supratem-
poral fenestra unlike Suuwassea; Harris,
2006). The frontal-parietal contact lies near
the suture between the laterosphenoid and
Fig. 3. Two-dimensional HRCT slices through the coronal plane of BYU 17096. Arrows denote the
frontal-parietal suture (A), frontal-frontal suture (B), parietal-supraoccipital suture (C), and orbitosphenoid-
laterosphenoid suture (D).
2010 BALANOFF ET AL.: BRAINCASE OF APATOSAURUS 7

prootic (as in most sauropods; Wilson et al.,
2005). The frontal-parietal suture is overlap-
ping for much of its breadth (with the frontals
overlying the parietals) before pinching out
posterolaterally (fig. 3A). The frontal-frontal
suture is present and, as revealed in the CT
slices, distinctly interdigitating (fig. 3B). This
medial contact is sutured but not fully fused as
in dicraeosaurids (Salgado and Calvo, 1992;
Wilson, 2002) and possibly Tornieria (Harris,
2006). The frontals form the dorsal margin of
the large anterior fenestra for the olfactory
tracts (see Orbitosphenoid below). A pair of
bilaterally symmetrical fossae, whose origin is
unclear, penetrates the lateral margin of the
frontals from within the supratemporal fenestra
(fig. 4). These fossae extend ventromedially
within the frontals before terminating at or
near the frontal-parietal suture lateral to the
frontoparietal fenestra. The frontals fail to
contribute to the supratemporal fossa (Wilson,
2002).
PARIETALS: The parietals are smaller than
the frontals. The parietals contact the frontals
anteriorly, postorbital anterolaterally, parietal
and squamosal posterolaterally, prootic ante-
roventrally, exoccipital-opisthotic posteroven-
trally, and supraoccipital posteriorly. Their
median contact is negated by the diameter of
the frontoparietal fenestra, whose posterior
margin is formed by the supraoccipital (see
below). The medial margins of the parietals do
extend a short distance towards the cranial
midline thereby constricting (slightly) the
frontoparietal fenestra. The parietal along
with the postorbital contributes to the supra-
temporal fenestra, which is wider than long
(table 1) and preserved only on the left side of
the skull. The diameter of the supratemporal
fenestra (in either direction) is distinctly larger
than that of the foramen magnum (table 1;
Wilson, 2002). The distance separating the
right and left supratemporal fenestrae is
greater than the largest diameter of the
supratemporal fenestra (Wilson, 2002). The
supratemporal fenestra faces more laterally
than dorsally (as in Apatosaurus and Suu-
wassea; Harris, 2006). The parietals of BYU
Fig. 4. Two-dimensional HRCT slice through a coronal plane of the braincase of BYU 17096. Dotted
line delineates a paired fossa formed within the frontals.
8 AMERICAN MUSEUM NOVITATES NO. 3677

17096 contain low arcuate ridges on either side
of the supraoccipital that mark the posterior
margin of the skull (Wilson et al., 2005). The
parietals overlap the supraoccipital and extend
laterally to overlay the exoccipital-opisthotic.
The parietal-exoccipital suture is sinuous as
described for Apatosaurus (Berman and
McIntosh, 1978) and Tornieria (Remes, 2006)
and unlike the linear suture of Suuwassea
(Harris, 2006). A squamosal-supraoccipital
contact excludes a parietal participation in the
dorsal margin of the posttemporal fenestra (a
diplodocoid synapomorphy; Calvo and
Salgado, 1995; Upchurch, 1998; Remes, 2006).
The occipital process of the parietal is deep,
being approximately twice the diameter of the
foramen magnum (Wilson, 2002).
POSTORBITALS: The left postorbital is more
completely preserved than the right (fig. 1B,
C). The postorbital contacts the frontal
anteromedially, parietal dorsomedially, and
squamosal medially. The postorbital contrib-
utes to the formation of the supratemporal
fenestra through its significant contribution to
the postorbital process where it contacts the
frontal along its medial surface. The distal
ends of both the jugal and squamosal pro-
cesses are broken resulting in the postorbital
lacking the triradiate shape present in
Apatosaurus and other sauropods (Berman
and McIntosh, 1978). The preserved lengths of
these processes both indicate that they were
broader transversely than anteroposteriorly
(Wilson, 2002). The postorbital does possess
a distinct but short posterior process (Wilson,
2002). The temporal bar is longer anteropos-
teriorly than transversely, and is shifted
ventrally exposing the supratemporal fossa in
lateral view (Wilson, 2002).
SQUAMOSALS: The right and left squamo-
sals are present and contact the parietal
medially, postorbital laterally, and exoccipi-
tal-opisthotic posteromedially. The squamosal
lies in the posterolateral corner of the skull
directly anterior to the paroccipital process of
the exocccipital-opisthotic (structure to which
the squamosal is firmly fused). The squamosal
contacts the lateral margin of the parietal
along a relatively complex suture that includes
both overlapping (with the squamosal over-
lapping the parietal) and interdigitating re-
gions.
BRAINCASE
SUPRAOCCIPITAL: The supraoccipital is a
midline ossification that roofs the posterior
portion of the endocranial cavity. The supra-
occipital contacts the parietals anterodorsally
and laterally, and the exoccipital-opisthotic
ventrolaterally. In dorsal view, the supraoc-
cipital underlies the paired parietals along its
lateral margins and forms the posterior
margin of the frontoparietal fenestra
(fig. 2B). The anterodorsal margin of the
supraoccipital that borders the frontoparietal
fenestra is slightly concave in posterior view
(fig. 1C). The anterior surface of the supraoc-
cipital that forms the caudal wall of the
posterior portion of the frontoparietal fenestra
is concave, forming a vertical groove that
would have housed a dorsal extension of the
dural venous sinus system.
The occiput overall is flat to slightly concave
(posteriorly) and therefore lacks the convex
‘‘supraoccipital wedge’’ described in some
sauropod braincases (fig. 2B; e.g., eusauropod
from India; Wilson et al., 2005). In posterior
view, the occiput is rectangular in shape and
compares closely with the high and vaulted
occiput described for Apatosaurus (Berman and
McIntosh, 1978). The height of the supraoccip-
ital in posterior view is greater than twice the
height of the foramen magnum, which appears
to be the plesiomorphic condition for sauro-
pods (Wilson, 2002). A distinct nuchal crest
extends along the midline of the supraoccipital
beginning at the posterior margin of the
frontoparietal fenestra and ending at or just
above the dorsal margin of the foramen
magnum. A nuchal fossa lies lateral to the
nuchal crest on either side in the posterior
occipital plate.
The supraoccipital forms a contact with the
exoccipital-opisthotic that begins laterally,
just dorsal to the base of the paroccipital
process, and curves ventromedially to reach
the dorsolateral margin of the foramen mag-
num. The angle of this ventromedial curvature
is steeper in BYU 17096 than in YPM 1860
(Berman and McIntosh, 1978). The supraoc-
cipital forms the dorsal margin of the circular
foramen magnum (slight mediolateral com-
pression). The external occipital fenestra lies
dorsal and lateral to the foramen magnum and
is clearly visible in posterior view (fig. 2B).
2010 BALANOFF ET AL.: BRAINCASE OF APATOSAURUS 9

This fenestra pierces the occipital plate at the
supraoccipital-parietal suture, where it would
have transmitted the external occipital vein
(caudal middle cerebral vein; Witmer and
Ridgely, 2009) out of the endocranial cavity
and onto the external surface of the occipital
plate. The external path of this vein is marked
by a distinct groove that runs ventrolaterally
from the external occipital fenestra, following
the general trajectory of the supraoccipital-
parietal suture (this external groove often is
present in birds; Baumel and Witmer, 1993).
EXOCCIPITAL-OPISTHOTIC: The exoccipital
and opisthotic are fused completely and
therefore described as a single complex. The
exoccipital-opisthotic contacts the parietal
dorsally, supraoccipital medially, prootic and
squamosal anteriorly, basioccipital postero-
ventrally, and basisphenoid anteroventrally.
The exoccipital-opisthotic contributes to the
lateral and ventral margins of the foramen
magnum. The suture with the overlying
supraoccipital is visible and positioned near
the dorsoventral midline of the foramen
magnum. A distinct prominence lies lateral
to the foramen magnum and encompasses
portions of both the supraoccipital and
exoccipital-opisthotic. It is unclear whether
this structure represents a proatlantal facet,
which is rare in sauropods (Wilson et al.,
2005).
The ventral suture with the basioccipital
cannot be discerned so it is unclear to what
degree the exoccipital contributes to the massive
occipital condyle and whether the basioccipital
is completely excluded from the ventral margin
of the foramen magnum. The exoccipitals do
form a large proportion of the occipital condyle
in YPM 1860 and completely exclude the
basioccipital from the ventral margin of the
foramen magnum (Berman and McIntosh,
1978). In most sauropods where the relative
contribution can be discerned (e.g., Shunosau-
rus; Chatterjee and Zheng, 2002), the basioc-
cipital forms the majority of the occipital
condyle. The occipital condyle of BYU 17096
as a whole is hemispherical in shape with a
dorsal surface that is concave up, differing from
Diplodocus that has a rounded dorsal margin
(Berman and McIntosh, 1978). The peduncu-
late occipital condyle is deflected posteroven-
trally, projecting from the main body of the
basicranium at an angle of approximately 80u
from the horizontal plane of the skull roof.
The exoccipital-opisthotic forms a promi-
nent paroccipital process that extends ventro-
laterally to contact the squamosal. Rather
than extending at a distinct posterolateral
Fig. 5. Stereo rendering of the braincase of BYU 17096 in ventrolateral view. The squamosal and
postorbital have been digitally removed from the braincase.
10 AMERICAN MUSEUM NOVITATES NO. 3677

orientation, these processes lie approximately
in the same coronal plane as the flattened
occipital plate (a diagnostic character of
Eusauropoda; Wilson, 2002). The shaft of the
paroccipital process is thin relative to its distal
end, which widens distinctly where it meets the
squamosal along a broad, flat contact that is
oriented dorsoventrally. The shaft of the
paroccipital process in BYU 17096 forms the
ventral margin of the posttemporal foramen
and lacks a ventral nonarticular process (as
scored for Apatosaurus by Wilson, 2002).
A large foramen pierces the lateral surface
of the braincase at a position posterior to, and
slightly below, the trigeminal fenestra. The
fenestra is delineated by the prootic anteriorly,
exoccipital-opisthotic dorsally, and basioccip-
ital ventrally (fig. 5). A slight dorsoventral
constriction at midlength partly divides the
opening into anterior and posterior compo-
Fig. 5. Continued. Three-dimensional volume rendering of the braincase of BYU 17096 in ventrolateral
view showing the spatial distribution of the major braincase foramina. The squamosal and postorbital have
been digitally removed from the braincase.
2010 BALANOFF ET AL.: BRAINCASE OF APATOSAURUS 11

nents. The anterior component is the fenestra
vestibuli and would have accepted the colu-
mella auris. The posterior component is the
postnatal remnant of the metotic fissure (here
referred to as the cavum metoticum) and
would have housed the jugular vein, cranial
nerves IX–XI, and the distal end of the
perilymphatic duct. The cavum metoticum is
thus undivided in the traditional sense (De
Beer, 1937; Rieppel, 1985) and its lateral
opening is referred to as the metotic fenestra
following the majority of sauropod workers
(jugular foramen of White, 1958; Madsen et
al., 1995; see below). The extensive confluence
between the vestibular fenestra and the meto-
tic fenestra results from the lateral margin of
the crista interfenestralis being either broken
away or unossified. The bilateral symmetry of
this morphology supports a late postnatal
ossification of this margin in BYU 17096. The
absence of this margin exposes the foramen
perilymphaticum, which opens posteroven-
trally into the cavum metoticum. The metotic
fenestra lacks the foramen lacerum posterior
described in Thecodontosaurus antiquus
(Benton et al., 2000). Directly caudal to the
metotic fenestra on the posterolateral surface
of the left exoccipital-opisthotic is a relatively
small foramen. The foramen leads into a canal
that extends posteromedially before opening
into the posterolateral floor of the endocranial
space. We interpret this foramen as a third
hypoglossal foramen that is not present on the
right side. A pair of bilaterally symmetrical
hypoglossal (XII) foramina is positioned
posterior and ventral to the metotic fenestra
near the base of the peduncle of the occipital
condyle. We interpret these hypoglossal canals
as lying completely within the basioccipital,
although it is difficult to discern whether the
external foramina lie completely within the
basioccipital or between the basioccipital and
exoccipital-opisthotic. The asymmetric third
hypoglossal canal exits the endocranial space
through the basioccipital but exits the skull
through the exoccipital-opisthotic.
The extremely close proximity of the third
hypoglossal foramen to the metotic fenestra in
BYU 17096 forces the consideration that it
may represent a division of the cavum
metoticum, and instead of housing a third
branch of XII, it would have transmitted one
or more of the structures that plesiomorphi-
cally traverse that space. One possibility under
this scenario would be that the opening
referred to here as the metotic fenestra is the
fenestra pseudorotundum of birds and croco-
diles, which is the lateral opening of the
recessus scalae tympani (i.e., the anterior
division of the cavum metoticum) and there-
fore would have housed the extracapsular
length of the perilymphatic duct and possibly
the glossopharyngeal (IX) nerve (Currie,
1997). This would make the small posterior
foramen the jugular foramen, which is the
external opening of the posterior division of
the cavum metoticum that transmits the
jugular vein, vagus (X) and accessory (XI)
nerves. The small size of this foramen seems to
preclude the possibility that it transmitted all
of these structures. It might be possible that
this small asymmetric foramen reflects an
internal ramification of the vagal complex
such that the accessory (XI) nerve ends up
with its own bony canal. If this is the case, the
cavum metoticum still is not divided in the
traditional sense and the lateral opening
should be referred to as the metotic fenestra.
If this foramen did transmit the accessory
nerve, then its medial confluence with the
endocranial space would be expected to lie
directly behind the point of exit of cranial
nerve X rather than adjacent to the foramen
magnum (which is where it is located). The
asymmetry of this feature also suggests that it
is more likely to have housed a branch of the
hypoglossal nerve, as intraspecific variation in
the number and symmetry of hypoglossal
foramina is well documented in extant reptiles
(Bellairs and Kamal, 1981).
BASIOCCIPITAL: The basioccipital floors the
posterior half of the endocranial cavity in-
cluding the rhombencephalon portion of the
hindbrain (fig. 1D). The basioccipital contacts
the parabasisphenoid anteriorly, prootic ante-
rodorsally, and exoccipital-opisthotic poster-
odorsally. As noted above, the basioccipital
participates in the fenestra pseudorotundum,
metotic fenestra, and hypoglossal foramina
and contributes to the ventral portion of the
occipital condyle.
A pair of prominent and robust basal
tubera projects from the basicranium as
ventrolateral extensions of the basioccipital
12 AMERICAN MUSEUM NOVITATES NO. 3677

(fig. 1D). These tubera do not appear to
receive a contribution from the parabasi-
sphenoid as described in some sauropods
(e.g., Wilson et al., 2005). The anteroposterior
depth of each tuber is approximately one-half
the dorsoventral height of the process. The
basal tubera are separated from the more
anteriorly positioned basipterygoid processes
of the parabasisphenoid by a low rounded
ridge, the crista ventrolateralis, which delin-
eates a shallow recess (in lateral view). This
recess results in the basal tubera being
separated from the basipterygoid processes
rather than closely associated and forming a
single large complex (e.g., Tornieria; Remes,
2006). There is no distinct depression between
the foramen magnum and the basal tubera.
PARABASISPHENOID: The parabasisphenoid
contacts the laterosphenoid and prootic dor-
solaterally, basioccipital posteriorly, and the
exoccipital-opisthotic posterodorsally. There
is no evidence of a contact between the
parabasisphenoid and the missing quadrates,
which is a derived condition in some sauro-
pods (Wilson, 2002). The parabasisphenoid
contributes to the posterior margin of the
oculomotor (III) foramen on the lateral
surface of the braincase (fig. 5; see latero-
sphenoid below). The external abducens fora-
men lies just ventral to the oculomoter
foramen and is approximately the same size
as this opening (see Discussion). The inferior
opening of the craniopharyngeal canal is
present as a foramen on the ventral surface
of the parabasisphenoid along its sagittal
midline (see Endocranial Cast below).
The distal end of both the right and left
basipterygoid process is broken with the right
process more completely preserved. The pre-
served proportions of these processes compare
closely with those of YPM 1860 (Berman and
McIntosh, 1978), suggesting that their length
was at least four times greater than the diameter
of their base. The basipterygoid processes
extend anterolaterally (a diplodocoid synapo-
morphy; fig. 1D; Calvo and Salgado, 1995;
Upchurch, 1998; Wilson, 2002; Remes, 2006)
and are separated from each other by an angle
of approximately 60u–70u. This angle was
somewhat difficult to assess due to some
potential postmortem distortion. Between these
processes lies a distinct recess, but there is no
evidence of a medial, rounded shelf as described
for YPM 1860 (Berman and McIntosh, 1978).
A shallow groove in the external surface of the
basipterygoid process marks the path of the
internal carotid artery as it approaches the
internal carotid foramen, which is positioned
laterally near the base of the process (see
Endocranial Cast below). The mediolaterally
flattened cultriform process is broken.
ORBITOSPHENOID: The paired orbitosphe-
noids form the dorsolateral walls of the rostral
end of the endocranial cavity (fig. 2A). These
bones are oriented largely along a transverse
plane. The orbitosphenoids contact each other
anteromedially, the frontals dorsolaterally,
laterosphenoid posterolaterally, and parabasi-
sphenoid ventrally. The orbitosphenoid forms
the lateral and ventral margin of the large
median opening that transmits the olfactory
tracts into the endocranial cavity (the roof of
this opening is formed by the frontals).
Ventral and slightly anterior to the olfactory
fenestra, the orbitosphenoids make a small
contribution to the anterior margin of the
optic foramina (see Laterosphenoid below).
PROOTIC: The prootic (fig. 1A, B) is a large,
paired element that contacts the laterosphenoid
anteriorly, parietal dorsally, parabasisphenoid
anteroventrally, basioccipital posteroventrally,
and exoccipital-opisthotic posteriorly. The dor-
sal suture between the prootic anteriorly and the
opisthotic posteriorly is difficult to discern but
can be approximated based on the position of
the anteroposteriorly elongate fenestra vestibuli.
The prootic does not contribute to the metotic
fenestra, as described for a eusauropod brain-
case (metotic foramen of Wilson et al., 2005).
The prootic forms the posterior margin of the
trochlear (IV) and trigeminal (V) foramina (see
Laterosphenoid). A relatively small foramen
transmitting the facial (VII) nerve out of the
endocranial space penetrates the prootic poste-
rior to the trigeminal fenestra and directly
anterodorsal to the metotic fenestra. The facial
foramen is visible only on the right side.
A crista prootica extends posterodorsally
along the lateral surface of the prootic
separating the vestibular fenestra (posteriorly)
from the facial foramen (anteriorly; the facial
foramen lies nearly within the crista prootica).
The crista, which is better preserved on the
right side, is strongly developed and heavily
2010 BALANOFF ET AL.: BRAINCASE OF APATOSAURUS 13

compressed along its long axis. A crista
antotica, which is much less developed than
the crista prootica, also is present and separ-
ates the facial foramen from the foramina
transmitting cranial nerves III–V (fig. 5). This
crista begins on the prootic and extends onto
the laterosphenoid (see below).
LATEROSPHENOID: The paired laterosphe-
noid is a platelike bone that helps to enclose
the anterolateral portion of the endocranial
space (see Clark et al., 1993). The latero-
sphenoid contacts the orbitosphenoid antero-
medially, frontal dorsally, parabasisphenoid
ventrally, and prootic posteriorly. The latero-
sphenoid-orbitosphenoid suture is overlapping
for most of its length (with the laterosphenoid
positioned lateral to the orbitosphenoid). The
suture between these elements is clearly visible
anteriorly in the HRCT slices (fig. 3D) but
becomes obscured in the coronal slices posteri-
orly. A facet marking the point of contact with
the epipterygoid is present (Wilson, 2002).
The laterosphenoid forms the majority of
the large optic fenestra, which transmits the
optic (II) nerve into the orbital cavity. (There
is also a small contribution from the orbito-
sphenoid, see above.) The laterosphenoid
contributes the anterior margin of the oculo-
motor (III), trochlear (IV), and trigeminal (V)
foramina. The large trigeminal fenestra is
positioned posterior to the optic fenestra
(approximately within the same horizontal
plane), dorsal to the oculomotor foramen,
and ventral to the trochlear foramen. The
presence of a single trigeminal fenestra indi-
cates that the ophthalmic and maxillomandib-
ular branches of the trigeminal nerve split
external to the endocranial cavity. The oculo-
motor foramen is the smallest of the three
foramina and is positioned dorsal to the
external foramen of the abducens (VI) nerve.
The trochlear foramen is relatively large and is
the dorsalmost foramen on the lateral surface
of the braincase.
ENDOCRANIAL CAST
Figures 6–8
GENERAL: The cranial endocast of BYU
17096 is complete in that it terminates
anteriorly at the opening for the olfactory
tracts and posteriorly at the foramen magnum
(fig. 6). A small amount of bilateral asymme-
try is present in the shape of the endocranial
cast reflecting postmortem distortion of the
braincase. The volume of the endocranial cast
in BYU 17096 is 125.14 cm
3
. This value may
be slightly inflated due to a large fracture on
the right side of the braincase. The endocast is
72.5 mm in total anteroposterior length and
49.4 mm in dorsoventral depth, making it
relatively short and deep (as in most sauro-
pods; Hopson, 1979; table 1). Cerebral and
pontine flexures are prominent, giving the
endocast a sigmoid shape (in lateral view;
fig. 7) that differs from the tubular casts of
some theropods and ornithischians (e.g.,
Marsh, 1877; Hopson, 1979; Brochu, 2000;
Larsson et al., 2000; Franzosa, 2004; Sampson
and Witmer, 2007).
In ventral and dorsal views, the outline of the
endocranial cast exhibits regions of lateral
expansion and medial constriction rather than
a more uniform tubular shape (figs. 7, 8). The
greatest mediolateral breadth of the endocranial
space lies across the cerebral hemispheres of the
forebrain, whereas the narrowest point lies in
the hindbrain just posterior to the cerebellum.
Tidwell and Carpenter (2003) described the
‘‘midbrain’’ as the widest portion of the brain in
sauropods. Presumably they were referring to
the anteroposterior middle of the endocast
rather than the midbrain proper (i.e., meten-
cephalon), as they also note that the sauropod
endocast is narrowest across the optic lobes
(which probably would be the widest portion of
the mesencephalon, but the optic lobes are not
defined in sauropod endocasts: see below;
Hopson, 1979). The frontals, whose ventral
surfaces reflect the morphology of the anterior
portion of the cerebral hemispheres, are absent
in the titanosauriform described by Tidwell and
Carpenter (2003), which may account for the
disparate observations.
FOREBRAIN: The visible features of the
forebrain and its associated anatomy include
the olfactory tracts, cerebral hemispheres,
optic (II) nerves, and pituitary body. A short
length of the paired olfactory tract is visible
anteriorly as a single midline structure extend-
ing posteroventrally from the large medial
opening between the orbitosphenoids and
frontals to the point where they communicate
with the main body of the telencephalon. The
14 AMERICAN MUSEUM NOVITATES NO. 3677

path of these tracts is reflected on the medial
surface of the orbitosphenoids. The combined
width of the olfactory tracts is nearly equal to
that of the cerebral hemispheres. The olfactory
(I) nerves and bulbs are not preserved on the
endocast as they are anterior to the braincase.
The cerebral hemispheres are anteroposterior-
ly shortened and somewhat expanded lateral-
ly. Their posterior margin is delineated by an
indentation that runs dorsoventrally and
separates the cerebral hemispheres from the
inferred position of the optic tectum.
A small, paired canal is present in the space
around the main body of the telencephalon
near the base of the olfactory tracts (figs. 7, 8).
These canals extend lateral and slightly
Fig. 6. Three-dimensional digital reconstruction of BYU 17096 in anterior (A) and dorsal (B) views. The
endocranial space is rendered opaque (white) and the surrounding bone is rendered transparent.
2010 BALANOFF ET AL.: BRAINCASE OF APATOSAURUS 15

Fig. 7. Three-dimensional isosurface rendering and line drawings of the endocranial cast of BYU 17096
in right lateral (A) and left lateral (B) views. The second branch of the hypoglossal (XII) nerve, which could
not be isolated digitally (see text), is marked by XII
b
.
16 AMERICAN MUSEUM NOVITATES NO. 3677

anterior through the frontal to exit the skull in
the dorsomedial wall of the orbit. These
structures are not described in previous studies
of sauropod endocasts, but they may reflect
the path of the supraorbital artery (a branch
of the ophthalmic artery; Baumel and Witmer,
1993) along the lateral wall of the endocranial
space through the frontal to the supraorbital
foramen and into the orbit. The optic (II)
nerves enter the endocranial cavity through a
pair of large, circular foramina in the latero-
sphenoid (with a small contribution from the
orbitosphenoid). These nerves extend postero-
medially to converge at the midline of the
anteroventral margin of the main body of the
diencephalon. Also part of the diencephalon,
the pituitary body descends from the ventral
surface of the endocast and exhibits extreme
hyperdevelopment. The infundibular stalk is
directed slightly posteroventrally, whereas the
pituitary body overall is oriented anteroven-
trally (fig. 7).
The sella turcica, which houses the pituitary
body (combined hypophysis and infundibu-
lum), is pierced by at least six distinct canals.
Two of these canals represent the paired
cranial carotid canals that pass through the
parabasisphenoid to enter the lateral margins
Fig. 8. Three-dimensional isosurface rendering and line drawings of the endocranial cast of BYU 17096
in dorsal (A) and ventral (B) views.
2010 BALANOFF ET AL.: BRAINCASE OF APATOSAURUS 17

of the sella turcica near the distal end of the
pituitary body (fig. 8). The third canal is a
midline structure that opens onto the ventral
surface of the parabasisphenoid and extends
dorsally to enter the sella turcica at the distal
end of the pituitary body. The canal narrows
as it approaches the sella turcica. This
structure is the craniopharyngeal canal
(fig. 1D; Edinger, 1942), which is an adult
remnant of the embryonic hypophyseal fenes-
tra—the space separating the posterior ends of
the cartilaginous trabeculae in the developing
skull (Edinger, 1942; Bellairs and Kamal,
1981). The craniopharyngeal canal is variably
present in sauropods and a variety of other
vertebrates (Edinger, 1942); however, the
phylogenetic distribution of this canal as an
adult structure has never been comprehensive-
ly surveyed. The craniopharyngeal canal is
known to be polymorphic in the only extant
species where it has been studied explicitly
(i.e., humans; Hauser and De Stefano, 1989).
In humans, the canal normally is closed during
prenatal development and its retention in
adults often is linked to pathology and large
adult body size (Chen, 2001). Tidwell and
Carpenter (2003) described this opening in a
titanosauriform braincase as the hypophyseal
fenestra and suggested that it housed the
pituitary vein. The pituitary vein is a trans-
verse structure connecting the venous sinuses
of the left and right orbit (Goodrich, 1930;
Romer, 1956), and therefore is not likely to
have traversed the craniopharyngeal canal to
exit onto the ventral surface of the braincase
in sauropods. There is no evidence in BYU
17096 of an interorbital canal (by which the
pituitary vein passes through the parabasphe-
noid between the orbits).
The other three canals that enter the sella
turcica all penetrate the dorsum sellae. Two of
these canals represent the paths of the bilater-
ally symmetrical abducens (VI) nerves (figs. 5,
7), which penetrate the dorsum sellae along its
lateral margins (see hindbrain below). A third
canal extends through the dorsum sellae along
the sagittal midline at a position dorsal to the
abducens canal (approximately two-thirds the
height of the dorsum sellae; fig. 9). The
diameter of this canal is slightly larger than
those of the abducens canals. It is unclear what
structure, if any, was transmitted through this
opening, but it appears to represent a true
anatomical feature. A similar midline penetra-
tion of the dorsum sellae that is distinct from
the abducens canal is present in Plateosaurus
(personal observation of AMNH 6810). This
shared opening may represent a derived onto-
genetic trajectory in which the posterior wall of
the sella turcica does not fully ossify, however,
the lateral surface of this opening in
Plateosaurus is finished in a way that suggests
it housed a soft-bodied structure.
The shape of the dorsum sellae, as reflected
by the space immediately posterior to the
pituitary body in the endocast, is relatively tall
and compressed along its long axis with a
distal (anterodorsal) margin that tapers to a
finely rounded edge. We assessed the com-
pression of the dorsum sellae by measuring the
angle formed between the posterior margin of
Fig. 9. Two-dimensional HRCT slice through a
sagittal plane of the braincase of BYU 17096
showing two of the six canals opening into the
sella turcica.
18 AMERICAN MUSEUM NOVITATES NO. 3677

Fig. 10. Cladogram showing the phylogenetic relationships and higher taxonomy (based on Wilson, 2002) of selected sauropod and outgroup taxa
for which endocranial casts are available in the literature. Endocasts were redrawn from the following sources: Tyrannosaurus (Brochu, 2000),
Plateosaurus (Galton, 1985), Shunosaurus (Chatterjee and Zheng, 2002), Diplodocus (Hopson, 1979), Tornieria (Janensch, 1935–36), Apatosaurus (this
study), Dicraeosaurus (Janensch, 1935–36), Camarasaurus (Chatterjee and Zheng, 2005), Brachiosaurus (Janensch, 1935–36), indeterminate
titanosauriform TMM 40435 (Tidwell and Carpenter, 2003). Each of the endocasts was scaled to the same size to facilitate comparison and oriented
so that the olfactory tracts lie horizontally.

the pituitary at the base of the infundibular
stalk and the foramen of the abducens canal
on the posterior margin of the dorsum sellae
(i.e., on the ventral margin of the endocast).
This angle in BYU 17096 is 18u.
A large dorsal convexity, lying on top of
the cerebral hemispheres along the sagittal
midline, is interpreted to reflect an expanded
dural venous sinus (superior sagittal or com-
bined dorsal longitudinal-transverse sinus)
and parietal body (Hopson, 1979; Witmer
and Ridgely, 2009). This sinus largely fills the
space formed by the frontoparietal fenestra in
the dermal skull roof (between the frontals
and parietals, see above) and therefore does
not have a well-defined shape over most of its
surface area. The superior sagittal sinus does
exhibit at its posterolateral margins a bilater-
ally symmetrical pair of rounded ridges that
reflect its enclosure within the paired parietals
(fig. 7).
MIDBRAIN: Visible structures of the mid-
brain include the oculomotor (III) and troch-
lear (IV) nerves. The inferior and superior
colliculi, as well as the optic lobes, are not
clearly discernible. The oculomotor (III) nerve
is positioned on the ventrolateral surface of
the mesencephalon directly posterior to the
optic nerve and directly anterior to the larger
trigeminal nerve. The oculomotor canal is
large, comparable in size to that of the optic
nerve, and not fully divided from the external
opening of the trigeminal fenestra laterally.
The position of the trochlear nerve is antero-
posteriorly intermediate to the optic and
oculomotor nerves (anterior and posterior,
respectively) and dorsal to both.
The rostral middle cerebral vein is visible as
a ridge running dorsoventrally along the
lateral surface of the endocast just anterior
to the midbrain (fig. 7B). The path of this vein
does not extend along a straight dorsoventral
trajectory but rather follows the anterior
margin of the midbrain whose shape is slightly
convex anteriorly (in lateral view). The rostral
middle cerebral vein fully contacts the surface
of the midbrain only at the apex of this
anterior convexity. This vein is most easily
seen on the right side of the specimen. This
asymmetry probably reflects differential pres-
ervation rather than a true anatomical asym-
metry, although the latter is a possibility.
HINDBRAIN: The features in the region of
the hindbrain that are visible on the endocra-
nial cast include the cerebellum, trigeminal
(V), abducens (VI), and facial (VII) nerves
(metencephalon), as well as the medulla
oblongata, glossopharyngeal (IX), vagus (X),
accessory (XI), and hypoglossal (XII) nerves
(myencephalon). Although its general shape is
visible, details of the cerebellum are difficult to
discern, probably because of the presence of
an occipital sinus that likely covered this
region of the brain (Hopson, 1979; Sedlmayr,
2002). The flocculus is not clearly present,
although this region of the braincase is
difficult to interpret due to poor differentia-
tion in the HRCT data.
The paired trigeminal (V) nerve passes
between the laterosphenoid and prootic from
a position on the rhombencephalon dorsal to
the pituitary body (in lateral view) and directly
posterior to and in the same horizontal plane as
the oculomotor (III) nerve. As noted above, the
presence of a single trigeminal fenestra indicates
that the gasserian ganglion lies extracranially
(Hopson, 1979). A slender abducens (VI) nerve
is visible on the ventrolateral surface of the
endocast, passing medial and slightly ventral to
the trigeminal. The abducens nerve enters the
pituitary fossa after passing through the dor-
sum sellae along a straight, posterodorsal
trajectory. There is no evidence that the
abducens nerve passes lateral to the sella turcica
in any sauropod, as it does (as a derived
condition) in some theropod dinosaurs
(Franzosa, 2004; Witmer and Ridgely, 2009).
The facial (VII) nerve is reflected on the
endocast as a structure relatively small in
diameter and positioned posterior to the
trigeminal nerve and anterodorsal to the
metotic fenestra. This nerve can be traced only
on the right side of the endocast.
The posterior region of the endocranial cast
contains a relatively large structure that is the
endocast of the cavum metoticum. This space
transmitted the jugular vein and cranial nerves
IX–XI out of the skull and housed the
extracapsular length of the perilymphatic
duct. The anterior portion of this structure
includes the space homologous to the fenestra
vestibuli; their division is obscured, probably
reflecting a delayed ossification of at least the
lateral margin of the crista interfenestralis.
20 AMERICAN MUSEUM NOVITATES NO. 3677

This delayed ossification also results in a
broad path for the perilymphatic duct between
the otic capsule and cavum metoticum. The
general position and extent of the vestibular
cavity of the inner ear is visible in the HRCT
images, although we were not able to discern
details of the semicircular canals.
The medullary portion of the hindbrain is
relatively short anteroposteriorly (fig. 8B). Its
mediolateral width is constricted relative to the
more anterior portions of the brain, but its
cross-sectional shape is slightly compressed
dorsoventrally. Two hypoglossal (XII) nerves
extend laterally and exit through the basioccip-
ital at a position posterior and slightly ventral to
the accessory canal. The second, more posterior
canal was traced with difficulty in the HRCT
slices because of the large number of fractures
that are present in this region (on both sides of
the skull). The second hypoglossal nerve
therefore is not represented on the endocranial
cast (but its position is marked as ‘‘XII
b
’’;
fig. 7); however, the second hypoglossal fora-
men is easily distinguished on the external,
ventral surface of the basioccipital. A third
hypoglossal canal is present on the left side of
the endocast and is unique in its close distal
proximity to the cavum metoticum (see above).
PHYLOGENETIC DIAGNOSIS OF BYU 17096
A total of 23 characters from the phyloge-
netic analysis of Wilson (2002) for Sauropoda
are observable on the braincase and associated
dermal roofing elements of BYU 17096. Eleven
of these characters exhibit the derived state for
Sauropoda or more exclusive clades nested
within Sauropoda, thereby allowing BYU
17096 to be diagnosed based on published
apomorphic features. A frontal-frontal contact
that is sutured but not fully fused, a temporal
bar whose anteroposterior length is less than its
transverse width, lateral exposure of the supra-
temporal fossa reflecting a ventral shift in the
position of the temporal bar, and an occipital
region that is flattened (rather than convex)
with paroccipital processes that are oriented
transversely (rather than posterolaterally) are
synapomorphies that support BYU 17096 as
nested within Sauropoda (or Eusauropoda
based on a DELTRAN [delayed transforma-
tion] optimization because Vulcanodon lacks a
skull; Raath, 1972; Wilson, 2002). The absence
of a contribution from the frontal to the
supratemporal fossa, the presence of a ventral
process of the postorbital that is broader
transversely than anteroposteriorly, an occipital
process of the parietal that is twice the diameter
of the foramen magnum, and a supratemporal
fenestra that is broader transversely than
anteroposteriorly diagnoses BYU 17096 to a
more inclusive clade within Eusauropoda that
based on ACCTRAN (accelerated transforma-
tion) optimization excludes only Shunosaurus
(DELTRAN optimization of the shape of the
ventral process of the postorbital is diagnostic
of the slightly more exclusive Neosauropoda +
Jobaria clade; Wilson, 2002: tables 9, 10). A
single character—distance separating the supra-
temporal fenestrae twice the length of the
supratemporal fenestrae along their long
axis—places BYU 17096 within Neosauropoda.
Two synapomorphies diagnose BYU 17096
to Diplodocoidea: the presence of a parietal
contribution to the posttemporal fenestra and
orientation of the basal tubera at approximately
a45uangle to the skull roof (rather than
perpendicular to it). The presence of a ventrally
directed occipital condyle was considered to be
diagnostic of Flagellicaudata—a slightly more
exclusive clade within Diplodocoidea (Harris
and Dodson, 2004; see also Wilson, 2002;
Harris, 2006; Remes, 2006). A third character,
basipterygoid processes at least four times
longer than the diameter of their base, also
appears to support this specimen as a diplodo-
coid, but because the basipterygoid processes
are partially broken, we refrain from scoring
this character for BYU 17096.
The eleventh synapomorphy is the presence
of a distinct recess positioned between the
paired basipterygoid processes. The presence
of this recess is plesiomorphic for Sauropoda
with a transformation of this recess into a
rounded shelf diagnosing Neosauropoda. A
reversed condition within Neosauropoda, in
which the recess is present, is found in the
titanosaurid Nemegtosaurus (Wilson, 2002,
2005) and in the diplodocid Apatosaurus.
Because BYU 17096 can be diagnosed to
Diplodocoidea based on at least two synapo-
morphies, the presence of the basipterygoid
recess unambiguously allocates this specimen
to Apatosaurus. Additional support for a
2010 BALANOFF ET AL.: BRAINCASE OF APATOSAURUS 21

diagnosis to Apatosaurus may be drawn from
the angle separating the basipterygoid process-
es. We approximated this angle in BYU 17096
to be roughly 60u–70u. Upchurch et al. (2004)
concluded that a 60uangle, rather than the 40u–
45uangle found in most sauropods, is diagnos-
tic of Apatosaurus. An angle of approximately
45uwas considered the plesiomorphic condition
for Sauropoda by Wilson (2002), with a smaller
angle of less than 30ua derived condition that
diagnoses Dicraeosauridae and is convergently
present in the titanosaur Rapetosaurus (Wilson,
2002). An angle significantly larger than 45uwas
not defined as a derived state by Wilson (2002).
An angle of approximately 90uwas reported for
Shunosaurus (Chatterjee and Zheng, 2002),
suggesting that the plesiomorphic condition
may not be 40u–45u. This angle seemingly needs
reassessment for Sauropoda, probably as a
continuous character with well-defined land-
marks and taking into account the potential for
significant postnatal repatterning and variation
due to diagenetic distortion.
Robust basitubera, the massive nature of the
occipital condyle, and basipterygoid processes
that are relatively short and stout are characters
shared between BYU 17096 and the well-
preserved braincases of Apatosaurus (YPM
1860 and CM 11162) that differ with known
specimens of Diplodocus, and therefore repre-
sent potential autapomorphies of Apatosaurus.
These characters need to be formally defined
and surveyed across a broadened taxonomic
spectrum and in the context of a specific
phylogenetic hypothesis.
DISCUSSION
A number of features described in BYU
17096 appear to vary within Sauropoda and
therefore present the possibility of establishing
new phylogenetically informative characters
from the sauropod braincase, and in some
cases, suggest that established characters are
in need of critical review. Although the
comprehensive review of sauropod braincase
morphology needed to meaningfully establish
the evolutionary history of these characters or
adequately revise them is outside the scope of
this project, a number of these characters do
provide points of discussion that hopefully
will act as a catalyst for future research. It is
important to emphasize that the purpose of
the following discussion is to establish baseline
hypotheses of evolutionary transformations in
the sauropod braincase based on current
understanding of the morphology and taxo-
nomic distribution of these features as estab-
lished from the published literature, and to
point out areas of apparent variation that may
be phylogenetically informative but in need of
further research. The discussion is not meant
as an endorsement of the literature from which
it is derived or as a critical review of published
observations—but rather as a starting point
for such a review.
CEREBRAL AND PONTINE FLEXURES
The sigmoid shape of the endocast in BYU
17096 is due to the development of both the
cerebral and pontine flexures such that they
form nearly 90uangles with the horizontal and
vertical planes respectively (fig. 7). As noted
above, this shape differs markedly from the
tube-shaped, linear endocranial casts of basally
diverging theropods (e.g., Brochu, 2000;
Sampson and Witmer, 2004) and ornithischians
(e.g., Stegosaurus;Marsh,1877;Galton,2001;
Galton and Upchurch, 2004), indicating that
the evolution of these flexures is independently
derived along the prosauropod-sauropod
branch of the saurischian tree. The endocranial
casts of Plateosaurus,Shunosaurus,andCama-
rasaurus exhibit marked cerebral and pontine
flexures relative to the above outgroups
(Galton, 1985; Chatterjee and Zheng, 2002,
2005; Ostrom and McIntosh, 1966). This dis-
tribution indicates that a distinctly flexed
endocranial space is plesiomorphic for
Sauropodomorpha (fig. 10). The flexure of this
space in Plateosaurus,Shunosaurus,andCama-
rasaurus is not as highly developed as that
found in Apatosaurus,Tornieria,Diplodocus,
Dicraeosaurus,Brachiosaurus, and the TMM
titanosauriform. The taxonomic distribution of
this more extreme flexure indicates that either
this condition evolved once as a transformation
diagnostic of Neosauropoda and was subse-
quently reversed in Camarasaurus, or the rela-
tively flattened endocranial space of Camara-
saurus is plesiomorphic and the highly devel-
oped flexures evolved twice within Neosauro-
poda—once in the diplodocoid lineage and
22 AMERICAN MUSEUM NOVITATES NO. 3677

once in the titansauriform lineage (fig. 10). The
subtle nature of these differences obviously
makes distinguishing discrete character states
difficult and uncomfortably subjective. These
flexures therefore should be reexamined mor-
phometrically once an expanded taxonomic
sample is available.
PITUITARY BODY AND DORSUM SELLAE
The dorsum sellae (fig. 9), as reflected by
the space immediately posterior to the pitui-
tary body in the endocast (fig. 7), is a
relatively elongate structure in BYU 17096
that is distinctly compressed along its long axis
and exhibits a finely rounded distal margin.
This anteroposteriorly compressed shape also
is present in Tornieria (Janensch, 1935–36),
Diplodocus (Ostrom and McIntosh, 1966;
Hopson, 1979; Galton, 1985; Franzosa,
2004), Dicraeosaurus (Janensch, 1935–36),
and Brachiosaurus (Janensch, 1935–36; see
also Tidwell and Carpenter, 2003), and
contrasts with the more broadly rounded
dorsum sellae of Shunosaurus (Chatterjee and
Zheng, 2002), Camarasaurus (Ostrom and
McIntosh, 1966; see also fig. 2 of Tidwell
and Carpenter, 2003), and the TMM titano-
sauriform described by Tidwell and Carpenter
(2003). We attempted to quantify these
differences by measuring the angle between
the posterior margin of the pituitary at the
base of the infundibular stalk and the foramen
of the abducens canal on the posterior margin
of the dorsum sellae (i.e., ventral margin of the
endocast; fig. 7). This angle in BYU 17096,
Tornieria,Diplodocus,Dicraeosaurus,and
Brachiosaurus is approximately 18u,16u,32u,
38u,and39urespectively, whereas the angle in
Plateosaurus,Shunosaurus, the titanosauri-
form, and Camarasaurus is 72u,52u, and 75u
respectively (fig. 10).
The infundibular stalk of BYU 17096 is
directed slightly posteroventrally (fig. 7); how-
ever, overall the pituitary body is oriented
anteroventrally, differing from the posteroven-
tral angle of Diplodocus. The pituitary also is
larger and more bulbous than in Diplodocus
(Franzosa, 2004). This overall anteroventral
orientation differs from that of all other
sauropod taxa for which endocranial casts are
available. In other sauropods, as well as in
theropods (e.g., Tyrannosaurus, Brochu, 2000;
Acrocanthasaurus, Franzosa and Rowe, 2004),
the pituitary body as a whole is angled postero-
ventrally rather than anteroventrally. The
accuracy of the perceived differences in pitui-
tary orientation suffers from the lack of a
completely reliable comparator for orienting
the endocasts involved in these comparisons.
The best comparator might be the lateral
semicircular canal (e.g., Alonso et al., 2004),
but this structure is not reconstructed for many
of the taxa in this comparison including BYU
17096. We used the general orientation of the
olfactory tracts as our reference, which is
problematic (see Sereno et al, 2007). The point
we stress here is that variation in the orientation
of the pituitary body does appear to occur
within Sauropoda and therefore is potentially
informative—phylogenetically or otherwise.
Whereas in Diplodocus a craniopharyngeal
canal extends from the base of the pituitary
fossa through the ventral surface of the
basisphenoid (Edinger, 1942; Franzosa, 2004),
the craniopharyngeal canal in BYU 17096
extends ventrally to open between the basal
tubera. This feature likely is variable intraspe-
cifically (Edinger, 1942).
FRONTOPARIETAL FENESTRA
Figures 2, 8
The terms parietal and postparietal foramen
are historically used to describe the openings in
the dermal roof of the braincase that form
communications with the underlying endocra-
nial space. The term ‘‘parietal foramen’’ gener-
ally is assigned when a single dorsal opening is
present (e.g., Chatterjee and Zheng, 2002). As
noted above, this terminology implies a primary
homology between this opening in sauropods
and the parietal foramen of other reptile taxa,
such as some lepidosaurs, in which the foramen
houses the photoreceptive parietal (pineal)
organ (Quay, 1979; the parietal opening gener-
ally is lost in archosaurs; Roth and Roth, 1980).
‘‘Parietal foramen’’ is applied whether this
opening closely resembles that of other reptile
groups in which it is relatively small, as in
Shunosaurus (Chatterjee and Zheng, 2002), or it
is distinctively large, as in BYU 17096 (although
see Hopson, 1979). The presence of a second,
posteriorly positioned ‘‘postparietal’’ foramen is
2010 BALANOFF ET AL.: BRAINCASE OF APATOSAURUS 23

a derived character within Sauropoda that
unambiguously diagnoses Dicraeosauridae
(Amargasaurus +Dicraeosaurus;Wilson,2002).
A postparietal foramen also is present in
Suuwassea (Harris, 2006), which currently is
placed in an unresolved trichotomy with
Diplodocidae and Dicraeosauridae (Harris and
Dodson, 2004). This means that the postparietal
foramen may be apomorphic for this more
inclusive clade and therefore lost in
Diplodocidae. The postparietal foramen in
Amargasaurus is fully distinct from the parietal
foramen because of a broad medial contact
between the paired parietals between these
openings. In Dicraeosaurus, the parietal and
postparietal foramina retain a small degree of
confluence because the parietals closely ap-
proach each other along the sagittal midline
but fail to actually contact.
The phylogenetic distribution of the ex-
tremely large ‘‘parietal foramen,’’ which is
such a unique feature in the sauropod speci-
mens that contain it, has never been explicitly
surveyed (the most detailed discussion of these
features remains that of Janensch, 1935–36);
however, it does appear to be relatively
common and taxonomically widespread within
Sauropoda. In specimens that contain this
large opening, the medial margins of the
parietals forming the posterolateral margin of
the ‘‘foramen’’ often turn slightly inward,
resulting in a subtle but partial division of this
expanded opening into a large anterior portion
and a much smaller posterior opening (this is
true of BYU 17096, see above). This division is
not as clearly developed as that exhibited by
Dicraeosaurus, and whereas the resultant
anterior foramen in these specimens remains
distinctively large, the posterior foramen is
approximately the same relative size as the
postparietal foramen of Dicraeasaurus and
Amargasaurus. These observations suggest
that, although the degree to which the ‘‘post-
parietal’’ foramen is closed off anteriorly by
the parietals is unique in Dicraeosauridae, a
space homologous to the postparietal foramen
of dicraeosaurids may be more broadly dis-
tributed in Sauropoda than reflected in the
character matrix of Wilson (2002).
The presence of a postparietal foramen,
whether fully or nearly closed as in dicraeosaur-
ids, or broadly confluent anteriorly as in BYU
17096, suggests that at least two soft structures
pass through the dermal skull roof from the
endocranial space (or at least filled these
foramina in life). The anterior margin of the
supraoccipital, which forms the posterior mar-
gin of the ‘‘postparietal’’ foramen, is concave
forming a distinct vertical groove in BYU 17096
(see above). A similar groove is present in the
caudal margin of the postparietal foramen of
the dicraeosaurids (see Remes, 2006) and
presumably reflects the shape of the structure
housed within this space. The relative size of the
postparietal foramen, even in specimens where
it is broadly confluent anteriorly (e.g., BYU
17096), closely compares in size with the
parietal foramen of Shunosaurus and most
other reptiles (e.g., squamates). The anterior
opening likely housed in part the pineal body,
whereas the postparietal foramen is likely
associated with a dural venous sinus (Witmer
and Ridgely, 2009). The nature and polarity of
the morphological transformations involved in
the evolutionary history of these structures
within sauropods cannot be interpreted mean-
ingfully until a more comprehensive survey of
their presence/absence is available.
CRANIAL NERVES
Figure 5
The optic (II) nerve of BYU 17096 passes
laterally out of the endocranial space into the
orbital fossa at the anterior margin of the
laterosphenoid (with a small contribution to
this canal from the orbitosphenoid). This
morphology is the most common condition
in sauropods (and archosaurs in general;
Goodrich, 1930; Baumel and Witmer, 1993)
and therefore likely represents the plesio-
morphic condition within Sauropoda. The
optic nerves of Shunosaurus are described as
exiting between the orbitosphenoid and latero-
sphenoid (Zheng, 1991; Chatterjee and Zheng,
2002), but are figured as exiting the endocra-
nial space anteriorly through a single medial
opening between the paired orbitosphenoids
directly below the midline opening for the
olfactory tracts (see fig. 7, Chatterjee and
Zheng, 2002). A third morphology in which
the optic fenestra opens laterally but the canal
and associated foramen lie completely within
the orbitosphenoid was described for Apato-
24 AMERICAN MUSEUM NOVITATES NO. 3677

saurus (see fig. 11D of Berman and McIntosh,
1978). The latter two conditions are interpret-
ed as derived within sauropods.
The trigeminal nerve passes through the
laterosphenoid-prootic margin within a single
fenestra in Apatosaurus (this study; Berman
and McIntosh, 1978), Diplodocus (Holland,
1906; Berman and McIntosh, 1978; Franzosa,
2004; Sereno et al., 2007), Brachiosaurus
(Janensch, 1935–36; Tidwell and Carpenter,
2003), Camarasaurus (Sereno et al., 2007),
Plateosaurus (Galton, 1985), and most other
sauropods (e.g., titanosauriform of Tidwell
and Carpenter, 2003) indicating that within
these taxa the gasserian ganglion is positioned
extracranially. Hopson (1979) described an
extracranial position of this ganglion as the
condition in all sauropods. An intracranial
position for this ganglion is described in
Shunosaurus and Camarasaurus based on the
presence of multiple canals and foramina
inferred to transmit the ophthalmic, maxillary,
and mandibular branches of the trigeminal
nerve (Chatterjee and Zheng, 2002, 2005).
Sereno et al. (2007), however, found an
extracranial split of the trigeminal nerve (i.e.,
only one trigeminal fenestra in the skull). If
the interpretations of Chatterjee and Zheng
are correct for Camarasaurus, then there is at
least one reversal of this morphology in
Sauropoda (along the Camarasaurus lineage).
The glossopharyngeal (IX), vagus (X), and
accessory (XI) nerves and the jugular vein are
interpreted to pass through an undivided cavum
metoticum and exit the braincase through the
metotic fenestra. An undivided cavum metoti-
cum (inferred from a single described external
opening) also is present in Plateosaurus
(Galton, 1985), Brachiosaurus (Tidwell and
Carpenter, 2003), and Diplodocus (Berman
and McIntosh, 1978)—a taxonomic distribu-
tion that suggests an undivided metotic fissure
is the plesiomorphic condition for Sauropoda.
Franzosa (2004) describes a fenestra pseudoro-
tundum in HMS 175. This term refers to the
lateral aperture of the recessus scalae tympani
and implies a divided cavum metoticum (De
Beer, 1937). This probably reflects the wide-
spread confusion regarding the anatomical
nomenclature applied to this area of the
basicranium, rather than an interpretation of
a divided cavum metoticum. (Franzosa [2004]
explicitly interprets cranial nerves IX–XI as
exiting through that opening.)
The undivided metotic fissure of sauropo-
domorphs represents a plesiomorphic reten-
tion of the basal archosaurian condition
(Gower and Weber, 1998; Gower, 2002) that
is transformed at some point in early theropod
evolution (Sampson and Witmer, 2007). A
separate path of the accessory nerve—which
would represent at least a division of the
lateral metotic fenestra if not the entire cavum
metoticum—is reported for Shunosaurus
(Chatterjee and Zheng, 2002), Camarasaurus
(Chatterjee and Zheng, 2005), the titanosauri-
form of Tidwell and Carpenter (2003), and
potentially the titanosauriform braincase de-
scribed by Berman and Jain (1982). The
identity of this foramen as transmitting the
accessory nerve in these taxa might be
questionable, especially considering the possi-
bility of a variable third branch of the
hypoglossal nerve (see above). A separate
path of the accessory nerve, if one does exist,
would be a derived variation in the basicrani-
um of Sauropoda and therefore deserves
further attention and scrutiny. Many descrip-
tions of sauropod braincases fail to account
for the accessory nerve (e.g., Hopson, 1979;
Tidwell and Carpenter, 2003 for
Brachiosaurus), which is not surprising as it
is part of the larger vagal complex, but one
could infer that this reflects the absence of a
separate accessory foramen. A comprehensive
and explicit survey for this foramen is needed.
A single hypoglossal foramen that exits
completely within the basioccipital may be the
most common condition in Sauropoda
(Wilson et al., 2005). A single foramen was
described on the endocast of Diplodocus
(Franzosa, 2004), whereas two foramina are
described for BYU 17096 (with an asymmetric
third foramen on the left side; fig. 5) and the
Diplodocus specimen described by Berman and
McIntosh (1978). Chatterjee and Zheng (2002)
described the hypoglossal in Camarasaurus as
splitting into two major branches as it emerges
from the braincase. Tidwell and Carpenter
(2003) note that in Camarasaurus and
Brachiosaurus the hypoglossal foramina are
absent, and suggest that the hypoglossal
nerve(s) in those specimens may have exited
the skull through the metotic fenestra. Such a
2010 BALANOFF ET AL.: BRAINCASE OF APATOSAURUS 25

configuration would require a considerable
reorganization of the neuro- and basicranial
anatomy, as it presumably would require that
the hypoglossal nerves branch from the
myencephalon at a much more anterior
position so that they are ‘‘captured’’ by the
cavum metoticum—a configuration not
known to occur in any described extant
amniote of which we are aware. Support for
the lateral capture of a hypoglossal nerve by
the metotic fenestra, however, might be drawn
from the close proximity of the metotic
fenestra with lateral opening of the asymmet-
ric third hypoglossal canal in BYU 17096. The
number of distinct foramina transmitting
branches of the hypoglossal nerve certainly
exhibits variation within Sauropoda and
therefore is potentially informative. The po-
tential for ontogenetic and individual varia-
tion in the number and position of these
foramina, however, may require relatively
large sample sizes to establish the presence of
a phylogenetically informative signal—sam-
ples that may not be attainable for sauropods.
FLOCCULUS (CEREBELLAR AURICULAR FOSSA)
Figure 7
Chatterjee and Zheng (2002) note that the
flocculus is not discernible in Shunosaurus and
that this absence is probably due to a reduced
need for balance and orientation control in
sauropods relative to the condition in bipedal
theropods. A distinct flocculus was not found in
BYU 17096 or Diplodocus (Franzosa, 2004)—
although the reality of these absences cannot be
considered definitive. If the flocculus indeed is
absent in these taxa, then the loss of this
structure, as well as any functional correlates, is
a derived condition within Sauropodomorpha
and needs to be surveyed comprehensively.
CONCLUSIONS
The endocranial morphology of sauropod
dinosaurs historically was considered to exhibit
little variation (Hopson 1979). Close observa-
tion of external and internal braincase features
in an expanded taxonomic sample reveals the
presence of considerable variation that un-
doubtedly is informative at a range of phylo-
genetic and taxonomic levels. Many of the
observed differences are related to changes in
proportion and shape whose meaningful delin-
eation may require comparative morphomet-
rics. Discrete variation in sauropod neurocra-
nial anatomy, however, certainly is present.
Recognizing the presence of variation and thus
the potential for new characters is the necessary
first step in acquiring a better understanding of
this complex anatomical partition. Hopefully
this recognition will serve as a catalyst for
acquiring more data, not only from additional
sauropod taxa, but also from additional
specimens of taxa for which endocasts already
exist. The expansion of samples that allow
documenting both inter- and intraspecific
variation is the only way that meaningful
conclusions can be drawn regarding the evolu-
tion of the braincase and neuroanatomy in this
derived group of vertebrates.
ACKNOWLEDGMENTS
We thank Kenneth Stadtman (Museum of
Western Colorado, formerly of Brigham Young
University Earth Science Museum) for permis-
sion to scan and study the specimen. Matt
Colbert and Jessie Maisano (University of
Texas at Austin) respectively HRCT scanned
the specimen and helped us with posting the
associated digital data for which we are
grateful. We gratefully acknowledge Brooks
Britt (Brigham Young University) for his
photographs of the specimen. The thoughtful
comments of Peter Galton, Mark Norell, and
Larry Witmer significantly improved the man-
uscript and are greatly appreciated. John
Whitlock provided valuable discussion on
general braincase morphology of diplodocids.
Financial assistance for this project was pro-
vided by a grant from the Jurassic Foundation
(to TI and GSB), the Geology Foundation at
the University of Texas at Austin, and the
Division of Paleontology, American Museum
of Natural History.
REFERENCES
Alonso, P.D., A.C. Milner, R.A. Ketcham, M.J.
Cookson, and T.B. Rowe. 2004. The avian
nature of the brain and inner ear of Archaeop-
teryx. Nature 430: 666–669.
Baumel, J.J., and L.M. Witmer. 1993. Osteologia.
In J.J. Baumel, A.S. King, J.E. Breazile, H.E.
26 AMERICAN MUSEUM NOVITATES NO. 3677

Evans and J.C. Vanden Berge (editors), Hand-
book of avian anatomy: Nomina Anatomica
Avium. 2nd ed. Publications of the Nuttall
Ornithological Club 23: 45–132. Cambridge:
Nuttall Ornithological Club.
Bellairs, A.d’A., and A.M. Kamal. 1981. The
chondrocranium and the development of the
skull in recent reptiles. In C. Gans and T.S.
Parsons (editors), Biology of the Reptilia.
Vol. 11. Morphology F: 1–263. London:
Academic Press.
Benton, M.J., L. Juul, G.W. Storrs, and P.M.
Galton. 2000. Anatomy and systematics of the
prosauropod dinosaur Thecodontosaurus anti-
quus from the Upper Triassic of southwest
England. Journal of Vertebrate Paleontology
20: 77–108.
Berman, D.S., and S.L. Jain. 1982. The braincase of
a small sauropod dinosaur (Reptilia:
Saurischia) from the Upper Cretaceous
Lameta Group, Central India, with review of
Lameta group localities. Annals of Carnegie
Museum 51: 405–422.
Berman, D.S., and J.S. McIntosh. 1978. Skull and
relationships of the Upper Jurassic sauropod
Apatosaurus (Reptilia, Saurischia). Bulletin of
Carnegie Museum of Natural History 8: 1–35.
Brochu, C.A. 2000. A digitally rendered endocast
for Tyrannosaurus rex. Journal of Vertebrate
Paleontology 20: 1–6.
Butler, A.B., and W. Hodos. 1996. Comparative
vertebrate neuroanatomy. New York: Wiley-
Liss, 514 pp.
Calvo, J.O., and L. Salgado. 1995. Rebbachisaurus
tessonei sp. nov. A new Sauropoda from
Albian–Cenomanian of Argentina; new evi-
dence on the origin of the Diplodocidae. Gaia
11: 13–33.
Carlson, W.D., T. Rowe, R.A. Ketcham, and M.W.
Colbert. 2003. Geological applications of high-
resolution X-ray computed tomography in
petrology, meteoritics and paleontology. In F.
Mees, R. Swennen, M. Van Geet and P. Jacobs
(editors), Applications of X-ray computed
tomography in the geosciences 215: 7–22.
London: Geological Society.
Chatterjee, S., and Z. Zheng. 2002. Cranial
anatomy of Shunosaurus, a basal sauropod
dinosaur from the Middle Jurassic of China.
Zoological Journal of the Linnean Society 136:
145–169.
Chatterjee, S., and Z. Zheng. 2005. Neuroanatomy
and dentition of Camarasaurus lentus.In V.
Tidwell and K. Carpenter (editors), Thunder-
lizards: the sauropodomorph dinosaurs: 199–211.
Bloomington: Indiana University Press.
Chen, C.J. 2001. Suprasellar and infrasellar cranio-
pharyngioma with a persistent craniopharyn-
geal canal: case report and review of the
literature. Neuroradiology 43: 760–762.
Clark, J.M., J. Welman, J.A. Gauthier, and J.M.
Parrish. 1993. The laterosphenoid of early archo-
sauriforms. Journal of Vertebrate Paleontology
13: 48–57.
Connely, M.V., and R. Hawley. 1998. A proposed
reconstruction of the jaw musculature and
other soft cranial tissues of Apatosaurus.
Journal of Vertebrate Paleontology 18(Suppl.
to 3): 35A.
Currie, P.J. 1997. Braincase anatomy. In P.J. Currie
and K. Padian (editors), Encyclopedia of dino-
saurs: 81–85. San Diego, CA: Academic Press.
Curtice, B.D., and D.R. Wilhite. 1996. A re-
evaluation of the Dry Mesa Dinosaur Quarry
sauropod fauna with a description of juvenile
sauropod elements. In A.C. Huffman, Jr., W.R.
Lund and L.H. Godwin (editors), Geology and
resources of the Paradox Basin. Utah Geological
Association Guidebook 25: 325–338. Salt Lake
City: Utah Geological Association.
De Beer, G. 1937 (1985). The development of the
vertebrate skull. Chicago: University of
Chicago Press, 744 pp.
Edinger, T. 1942. The pituitary body in giant animals
fossil and living: a survey and a suggestion.
Quarterly Review of Biology 17: 31–45.
Edinger, T. 1955. The size of the parietal foramen
and organ in reptiles. Bulletin of the Museum
of Comparative Zoology 114: 3–34.
Foster, J.R. 2003. Paleoecological analysis of the
vertebrate fauna of the Morrison Formation
(Upper Jurassic), Rocky Mountain region,
U.S.A. Bulletin of New Mexico Museum of
Natural History 23: 1–95.
Franzosa, J., and T. Rowe. 2005. Cranial endocast
of the Cretaceous theropod dinosaur Acrocan-
thosaurus atokensis. Journal of Vertebrate
Paleontology 25: 859–864.
Franzosa, J.W. 2004. Evolution of the brain in
Theropoda (Dinosauria). Ph.D. dissertation,
University of Texas at Austin, 357 pp.
Galton, P.M. 1985. Cranial anatomy of the prosau-
ropod dinosaur Plateosaurus from the Knollen-
mergel (Middle Keuper, Upper Triassic) of
Germany. Geologica et Palaeontologica 19:
119–159.
Galton, P.M. 2001. Endocranial casts of the plated
dinosaur Stegosaurus (Upper Jurassic, western
USA): a complete undistorted cast and the
original specimens of Othniel Charles Marsh. In
K. Carpenter (editor), The armored dinosaurs:
103–129. Bloomington: Indiana University Press.
Galton, P.M., and P. Upchurch. 2004. Stegosauria. In
D.B. Weishampel, P. Dodson and H. Osmo´ lska
(editors), The Dinosauria. 2nd ed: 343–362.
Berkeley: University of California Press.
2010 BALANOFF ET AL.: BRAINCASE OF APATOSAURUS 27

Goodrich, E.S. 1930 (1986). Studies on the struc-
ture and development of vertebrates. Chicago:
University of Chicago Press, 837 pp.
Gower, D.J. 2002. Braincase evolution in suchian
archosaurs (Reptilia: Diapsida): evidence from
the rauisuchian Batrachotomus kupferzellensis.
Zoological Journal of the Linnean Society 136:
49–76.
Gower, D.J., and E. Weber. 1998. The braincase of
Euparkeria, and the evolutionary relationships
of birds and crocodilians. Biological Reviews
73: 367–411.
Harris, J.D. 2006. Cranial osteology of Suuwassea
emilieae (Sauropoda: Diplodocoidea: Flagel-
licaudata) from the upper Jurassic Morrrison
Formation of Montana, USA. Journal of
Vertebrate Paleontology 26: 88–102.
Harris, J.D., and P. Dodson. 2004. A new diplodo-
coid sauropod dinosaur from the upper Jurassic
Morrison Formation of Montana, USA. Acta
Palaeontologica Polonica 49: 197–210.
Hauser, G., and G.F. De Stefano. 1989. Epigenetic
variants of the human skull. Stuttgart:
Schweizerbart, 301 pp.
Holland, W.J. 1906. The osteology of Diplodocus
Marsh. Memoirs of the Carnegie Museum 2:
225–278.
Hopson, J.A. 1979. Paleoneurology. In C. Gans,
R.G. Northcutt and P. Ulinski (editors),
Biology of the Reptilia 9: 39–146. London:
Academic Press.
Janensch, W. 1935–36. Die Scha¨ del der Sauropoden
Brachiosaurus,Barosaurus und Dicraeosaurus aus
den Tendaguru-Schichten Deutsch-Ostafrikas.
Sonder-Abdruck aus Palaeontographica Bei-
tra¨ ge zur Naturgeschichte der Vorzeit herausge-
geben von F. Broili in Mu¨ nchen. Suppl. 7. Vol. 2:
147–297, 13 pls.
Larsson, H.C.E., P.C. Sereno, and J.A. Wilson.
2000. Forebrain enlargement among nonavian
theropod dinosaurs. Journal of Vertebrate
Paleontology 20: 615–618.
Madsen, J., J.S. McIntosh, and D.S. Berman. 1995.
Skull and atlas-axis complex of the upper
Jurassic sauropod Camarasaurus Cope (Reptilia:
Saurischia). Bulletin Carnegie Museum of
Natural History 31: 1–115.
Marsh, O.C. 1877. New order of extinct Reptilia
(Stegosauria) from the Jurassic of the Rocky
Mountains. American Journal of Science (3)
14: 513–514.
Marsh, O.C. 1880. Principal characters of American
Jurassic dinosaurs. Part 3. American Journal of
Science (3) 19: 253–259.
Marsh, O.C. 1884. Principle characters of American
Jurassic dinosaurs. Part VII. On the Diplodo-
cidae, a new family of the Sauropoda. American
Journal of Science (3) 27: 161–167.
Marsh, O.C. 1896. The dinosaurs of North America.
Annual Report of the U. S. Geological Survey
16: 133–244.
Osborn, H.F., and C.C. Mook. 1921. Camara-
saurus,Amphicoelias, and other sauropods of
Cope. Memoirs of the American Museum of
Natural History. new ser. v. 3. pt. 3: 247–287.
Ostrom, J.H., and J.S. McIntosh. 1966 (1999
reprint). Marsh’s Dinosaurs: the collections
from Como Bluff. New Haven, CT: Yale
University Press, 388 pp.
Quay, W.B. 1979. The parietal eye-pineal complex.
In C. Gans, R.G. Northcutt and P. Ulinski
(editors), Biology of the Reptilia. Vol. 9.
Neurology: 245–406. London: Academic Press.
Raath, M.A. 1972. Fossil vertebrate studies in
Rhodesia: a new dinosaur (Reptilia: Saurischia)
from near the Triassic-Jurassic boundary.
Arnoldia 5: 1–37.
Rauhut, O.W.M., K. Remes, R. Fechner, G. Cladera,
and P. Puerta. 2005. Discovery of a short-necked
sauropod dinosaur from the Late Jurassic period
of Patagonia. Nature 435: 670–672.
Remes, K. 2006. Revision of the Tendaguru sauro-
pod dinosaur Tornieria africana (Fraas) and its
relevance for sauropod paleobiogeography.
Journal of Vertebrate Paleontology 26: 651–669.
Rieppel, O. 1985. The recessus scalae tympani and
its bearing on the classification of reptiles.
Journal of Herpetology 19: 373–384.
Romer, A.S. 1956. Osteology of the reptiles.
Chicago: University of Chicago Press, 772 pp.
Roth, J.J., and E.C. Roth. 1980. The parietal-pineal
complex among paleovertebrates: evidence for
temperature regulation. In R.D.K. Thomas and
E.C. Olsen (editors), A cold look at the warm-
blooded dinosaurs: 189–250. Boulder, CO:
Westview Press.
Salgado, L. 1999. The macroevolution of the
Diplodocimorpha (Dinosauria; Sauropoda): a
developmental model. Ameghiniana 36:
203–216.
Salgado, L., and J.O. Calvo. 1992. Cranial osteology
of Amargosaurus cazaui Salgado and Bonaparte
(Sauropoda, Dicraeosauridae) from the Neoco-
mian of Patagonia. Ameghiniana 29: 337–346.
Sampson, S.D., and L.M. Witmer. 2007. Cranio-
facial anatomy of Majungasaurus crenatissimus
(Theropoda: Abelisauridae) from the Late
Cretaceous of Madagascar. In:S.D.Sampson
and D.W. Krause (editors), Majungasaurus
crenatissimus (Theropoda: Abelisauridae) from
the Late Cretaceous of Madagascar. Society of
Vertebrate Paleontology Memoir 8: 225–253.
Sedlmayr, J.C. 2002. Anatomy, evolution, and
functional significance of cephalic vasculature
in Archosauria. Ph.D. dissertation, Ohio
University, Athens, Ohio. 398 pp.
28 AMERICAN MUSEUM NOVITATES NO. 3677

Sereno, P.C., J.A. Wilson, L.M. Witmer, J.A.
White, A. Maga, O. Ide, and T. Rowe. 2007.
Structural extremes in a Cretaceous dinosaur.
PloS ONE 2 (11): e1230, doi:10.1371/journal.
pone.0001230.
Tidwell, V., and K. Carpenter. 2003. Braincase of an
early Cretaceous titanosauriform sauropod from
Texas. Journal of Vertebrate Paleontology 23:
176–180.
Upchurch, P. 1998. The phylogenetic relationships
of sauropod dinosaurs. Zoological Journal of
the Linnean Society 124: 43–103.
Upchurch, P., P.M. Barrett, and P. Dodson. 2004.
Sauropoda. In D.B. Weishampel, P. Dodson and
H. Osmo´lska (editors), The Dinosauria. 2nd ed:
259–322. Berkeley: University of California
Press.
White, T.E. 1958. The braincase of Camarasaurus
lentus (Marsh). Journal of Paleontology 32:
477–494.
Wilson, J.A. 2002. Sauropod dinosaur phylogeny
critique and cladistic analysis. Zoological
Journal of the Linnean Society 136: 217–276.
Wilson, J.A. 2005. Redescription of the Mongolian
sauropod Nemegtosaurus mongoliensis Nowinski
(Dinosauria: Saurischia) and comments on Late
Cretaceous sauropod diversity. Journal of
Systematic Palaeontology 3: 283–318.
Wilson, J.A., M.S. Malkani, and P.D. Gingerich.
2005. A sauropod braincase from the Pab
Formation (Upper Cretaceous, Maastrichtian)
of Balochistan, Pakistan. Gondwana Geological
Magazine 8: 101–109.
Witmer, L.M., and R.C. Ridgely. 2009. New insights
into the brain, braincase, and ear region of
tyrannosaurs (Dinosauria, Theropoda), with
implications for sensory organization and be-
havior. The Anatomical Record 292: 1266–1296.
Zheng, Z. 1991. Morphology of the braincase of
Shunosaurus. Vertebrata Palasiatica 29: 108–118.
2010 BALANOFF ET AL.: BRAINCASE OF APATOSAURUS 29