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Journal of Vertebrate Paleontology
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/ujvp20
Endocranial anatomy of Allosaurus supports neural
trends among non-avian theropod dinosaurs
Emily J. Lessner, Corrine Cranor, Rebecca Hunt-Foster & Casey M. Holliday
To cite this article: Emily J. Lessner, Corrine Cranor, Rebecca Hunt-Foster & Casey M. Holliday
(2023): Endocranial anatomy of Allosaurus supports neural trends among non-avian theropod
dinosaurs, Journal of Vertebrate Paleontology, DOI: 10.1080/02724634.2023.2236161
To link to this article: https://doi.org/10.1080/02724634.2023.2236161
Published online: 21 Aug 2023.
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ARTICLE
ENDOCRANIAL ANATOMY OF ALLOSAURUS SUPPORTS NEURAL TRENDS
AMONG NON-AVIAN THEROPOD DINOSAURS
EMILY J. LESSNER,
1
†
*CORRINE CRANOR,
2
REBECCA HUNT-FOSTER,
3
and CASEY M. HOLLIDAY
1
1
Department of Pathology and Anatomical Sciences, 1 Hospital Drive, University of Missouri, Columbia, MI, 65212 U.S.A.,
emily.lessner@dmns.org; hollidayca@missouri.edu;
2
Department of Geology and Geologic Engineering, 501 E. St. Joseph St., South Dakota School of Mines and Technology,
Rapid City, SD, 57701 U.S.A., corrine.cranor@gmail.com;
3
Dinosaur National Monument, 11625 E 1500 S, Jensen, UT, 84035 U.S.A., Rebecca_Hunt-Foster@nps.gov.
ABSTRACT—Endocranial cavities preserve a record of neural anatomy often used for hypotheses of behavior in extinct
organisms. Two reconstructions of cranial endocasts of Allosaurus fragilis and A. jimmadseni from computed tomography
data expand understanding of theropod endocranial anatomy including endocranial volume, inner ear shape, and
trigeminal ganglion size. Endocranial and trigeminal ganglion volumes are compared with a sample of birds, crocodylians,
and non-avian theropod dinosaurs. Allosaurus is found to have a relatively small trigeminal foramen for its body size
when compared with foramina of birds and crocodylians. The inner ear is fragmentary but similar in shape to semicircular
canals of other large-bodied theropod dinosaurs. These findings suggest Allosaurus had generalist neural structures
relative to other non-avian theropod dinosaurs. Like other large-bodied theropod dinosaurs, Allosaurus likely had a large
dural venous sinus, potentially important for brain cooling. Allosaurus did not have a derived sense of facial
somatosensation akin to that found in crocodylians or some birds. Additional data like these, collected from other
tetanuran dinosaurs, will help clarify the evolution of neurosensory systems in the lineage.
Citation for this article: Lessner, E. J., Cranor, C., Hunt-Foster, R., & Holliday, C. M. (2023) Endocranial anatomy of
Allosaurus supports neural trends among non-avian theropod dinosaurs. Journal of Vertebrate Paleontology.https://doi.org/
10.1080/02724634.2023.2236161
Submitted: February 28, 2023
Revisions Received: June 18, 2023
Accepted: July 6, 2023
INTRODUCTION
Here, we present a complete, anatomical description of the
cranial endocast of Allosaurus fragilis and Allosaurus jimmad-
seni reconstructed in three dimensions from computed tomogra-
phy scan data. Several works (Franzosa, 2004; Hopson, 1979;
Osborn, 1912; Rogers, 1999; Witmer & Ridgely, 2009) detailed
Allosaurus endocasts previously, but contributions from other
exemplary specimens are lacking. Though Hopson (1979) and
Rogers (1999)figure and give some description of a physical
Allosaurus endocast, in a dissertation Franzosa (2004) briefly
describes a digital Allosaurus endocast, and Witmer and
Ridgely (2009)figure a detailed Allosaurus endocast, no work
provides a detailed written description.
Despite there being dozens of preserved braincases and other
cranial elements of Allosaurus, few are as completely preserved
in situ as those in A. fragilis (DINO 2560) and A. jimmadseni
(DINO 11541) expanding possibilities for analyses based on
body size and skeletal material. Providing descriptive and quan-
titative data is necessary to further the field of comparative
paleoneurology and expand our understanding of the evolution
of behavior. In particular, we explore the relationship between
body size and endocranial volume, expanding the study of Balan-
off et al. (2013) to include Allosaurus. We also introduce novel
comparisons of an osteological correlate of the trigeminal
nerve with body mass estimations, which support hypotheses of
sensory ability (Dumont et al., 2022; George and Holliday, 2013).
MATERIALS AND METHODS
Two Allosaurus skulls were scanned in July 2020 at the Ashley
Valley Regional Medical Center in Vernal, Utah. Both were
scanned on a General Electric LightSpeed VCT XT 64-slice
CT scanner. The completely articulated skull of A. fragilis
(DINO 2560), discovered in the 1920s and long used as the
basis for Allosaurus cranial anatomy (Madsen, 1976), was
scanned in three parts; the left mandible at a voxel size of
0.793 × 0.793 × 0.625 mm, right mandible at 0.586 × 0.586 ×
0.625 mm, and cranium at 0.977 × 0.977 × 0.627 mm. The articu-
lated skull of A. jimmadseni was scanned at a voxel size of 0.977 ×
0.977 × 0.627 mm. Though the skull of DINO 11541
(A. jimmadseni) is incomplete, it has a complete braincase, and
†
Current address: Denver Museum of Nature and Science, 2001 Color-
ado Blvd., Denver, CO, 80220 U.S.A.
*Corresponding author.
The work of ReBecca Hunt-Foster was authored as part of her official
duties as an Employee of the United States Government and is therefore
a work of the United States Government. In accordance with 17 USC.
105, no copyright protection is available for such works under US Law.
Emily Lessner, Corrine Cranor and Casey Holliday hereby waive their
right to assert copyright, but not their right to be named as co-authors
in the article.
Color versions of one or more of the figures in the article can be found
online at www.tandfonline.com/ujvp.
Journal of Vertebrate Paleontology e2236161 (9 pages)
Published with license by the Society of Vertebrate Paleontology
DOI: 10.1080/02724634.2023.2236161
Published online 21 Aug 2023
its cranial anatomy is well detailed by Chure and Loewen (2020).
DICOMs were imported into Avizo 2021.2 and endocranial cav-
ities segmented manually using the brush, lasso, and magic wand
tools. Scan data may be found at https://www.morphosource.org/
projects/000498875?locale=en.
For scaling analyses, femur length for DINO 2650 was reported
as 870 mm by Bybee et al. (2006), but re-measurement yielded a
length of 828.6 mm and therefore this value was used in this
study. Femur length for DINO 11541 was measured as 610 mm.
Body mass for the Allosaurus specimens and additional avian
specimens (Table 1) was calculated using the regression equation
from Christiansen and Fariña (2004). Endocast volumes were
collected using the Material Statistics module in Avizo
(Thermo Fisher Scientific, Waltham, MA) and exclude cranial
nerves and olfactory bulbs. Linear measurements of foramina
and avian femur lengths were taken using the line measurement
tool in Avizo. We compared endocranial volume and body size
relationships within the dataset and in the manner of Balanoff
et al. (2013). We compared trigeminal foramina maximum diam-
eter and body size relationships as well. The trigeminal foramina
are osteological correlates for the branches of the trigeminal
nerve that innervate the facial integument, tongue, and jaw
musculature, therefore serving as a proxy for facial and oral sen-
sation and muscular innervation (Dumont et al., 2022; George &
Holliday, 2013). There is variation in the anatomy of the trigem-
inal foramen in that some taxa represented in this study have a
single trigeminal foramen through which the ophthalmic, maxil-
lary, and mandibular divisions of the trigeminal nerve pass and
others possess both ophthalmic and maxillomandibular foramina
(see Table 1 for details and citations). In the case where multiple
foramina were present, foramen maximum diameters were
added together and compared with values from single foramina.
Reduced-major-axis regressions were performed using the stat-
istical program R (R Core Team, 2020) and the package
‘lmodel2’(Legendre, 2018). One-way ANCOVA was employed
to compare group means using the package ‘rstatix’(Kassam-
bara, 2021).
Four crocodylian specimens (all juvenile) were included to
provide a complete extant phylogenetic bracket of the fossil dino-
saurs (Tab l e 1 ); these are in addition to the avian specimens pub-
lished by Balanoff et al. (2013) and some additional birds
downloaded from Morphosource or from the University of Mis-
souri Vertebrate Collections. These four specimens (FMNH
11085, Tomistoma,99.3μm; FMNH 22817, Paleosuchus, 58.9 μm;
TABLE 1. Study specimens, measurements, and citations.
Taxon Specimen Number Body Mass (kg) Endocranial Volume (cm
3
)
Maximum Foramen
Diameter (mm)
V
2–3
V
1
V
1–3
Anas platyrhynchus
1
N/A 0.17
^
6.76 - - -
Brotogeris chrysopteris
1
N/A 0.02
^
1.98 - - -
Bucorvus abyssinicus
1
N/A 1.26
^
22.0 - - -
Chauna chavaria
1
N/A 1.20
^
7.69 - - -
Chordeiles minor
1
N/A 0.01
^
0.87 - - -
Coragyps atratus
1
N/A 0.90
^
8.68 - - -
Eudyptes chrysocome
1
N/A 2.33
^
11.4 - - -
Fregata magnificens
1
N/A 0.17
^
9.11 - - -
Gavia immer
1
N/A 0.30
^
9.87 - - -
Grus canadensis
1
N/A 2.18
^
15.1 - - -
Haliaeetus leucocephalus
1
N/A 2.06
^
17.69 - - -
Melanerpes aurifron
1
N/A 0.01
^
2.20 - - -
Phaethon rubricada
1
N/A 0.03
^
4.02 - - -
Phalacrocorax penicillatus
1
N/A 0.26
^
12.28 - - -
Phoebastria immutabilis
1
N/A 0.43
^
14.3 - - -
Podilymbus podiceps
1
N/A 0.05
^
2.48 - - -
Ptilinopus melanospilus
1
N/A 0.10
^
1.11 - - -
Struthio camelus
1
N/A 59.3
^
56.3 - - -
Conchoraptor gracilis
1
IGM 100/3006 5.02
^
9.44 - - -
Citipati osmolskae
1,2, a
IGM 100/978 86.0
^
22.6 - - 5.50
Khaan mckennai
1
IGM 100/973 12.6
^
8.83 - - -
Zanabazar junior
1,2, b
IGM 100/1 49.3
^
25.1 - - 5.40
Unnamed troodontid
1
IGM 100/1126 0.92
^
3.11 - - -
Tsaagan mangas
1,2,b
IGM 100/1015 16.0
^
3.07 - - 5.20
Archaeopteryx lithographica
1
BMNH 37001 0.50
^
1.44 - - -
Shuvuuia deserti
1,2,c
IGM 100/977 0.25
^
0.83 1.60 0.65 -
Alioramus altai
1,2,d
IGM 100/1844 359
^
73.2 15.2 6.10 -
Tyrannosaurus rex
1
AMNH 5029 5840
^
343 - - -
Acrocanthosaurus atokensis
1,2,e
OMNH 10146 3770
^
191 - - 15.7
Allosaurus fragilis
3,f
DINO2560 1504* 163 8.90 2.20 -
Allosaurus jimmadseni
3,f
DINO11541 496* 93.2 10.6 2.04 -
Paleosuchus palpebrosus
4,g
FMNH 22817 0.07
%
0.82 2.18 1.41 -
Osteolaemus tetraspis
4,g
FMNH 53632 0.08
%
1.11 2.49 1.23 -
Tomistoma schlegelii
4,g
FMNH 11085 0.28
%
2.07 3.60 1.59 -
Crocodylus niloticus
4,g
FMNH 16162 0.03
%
0.91 2.25 0.70 -
Coeligena violifer
3,h
FMNH 288711 0.003* - 0.53 0.23 -
Sturnus vulgaris
5,h
MUVC AV069 0.00001* - 0.07 0.05 -
Aeronautes saxatalis
3,h
KU B 68887 0.006* - 0.83 0.20 -
Calocitta formosa
3,h
KU B 56830 0.01* - 1.04 0.40 -
Data source/availability: (1) Balanoff et al. (2013); (2) http://digimorph.org/; (3) http://morphosource.org/; (4) OSF Crocnet: https://osf.io/jmpck/;
(5) MUVC Collections.
Trigeminal foramen citations: (a) Clark et al. (2002); (b) Norell et al. (2006); (c) Personal observation; (d) Bever et al. (2013); (e) Franzosa and
Rowe (2005); (f) McClelland (1990); (g) Jollie (1962); (h) Baumel and Witmer (1993).
Body mass estimation citations: (^) Balanoff et al. (2013); (*) Christiansen and Fariña (2004); (%) Farlow et al. (2005).
Lessner et al.—Endocranial anatomy of Allosaurus (e2236161-2)
FMNH 53632, Osteolaemus,60.8μm; FMNH 16161, Crocodylus,
24.7 μm) were CT scanned at the University of Texas High-Resol-
ution X-Ray Computed Tomography Facility on an NSI scanner.
TIFF files were imported into Avizo 2022.3 and endocranial cav-
ities segmented manually using the magic wand tool and interp-
olate function. Linear measurements of foramina maximum
diameters, skull widths, and femora lengths were taken using the
line measurement tool and endocranial volumes collected using
the Material Statistics module. Body mass for the crocodylian
specimens was calculated using the regression equations of
Farlow et al. (2005; based on femur length) and O’Brien et al.
(2019; based on skull width) though only results from the Farlow
et al. (2005) equations were used in comparison because the calcu-
lations resulted in very similar values.
Institutional Abbreviations—AMNH, American Museum of
Natural History, New York, New York, U.S.A.; BMNH, British
Museum of Natural History, London, U.K.; DINO,Dinosaur
National Monument, Jensen, UT, U.S.A.; FMNH,FieldMuseum
of Natural History, Chicago, IL, U.S.A.; IGM, Mongolian Institute
of Geology, Ulaan Bataar, Mongolia; KU, University of Kansas
Biodiversity Institute and Natural History Museum, Lawrence,
KS, U.S.A.; MUVC, University of Missouri Vertebrate Collections,
Columbia, MI, U.S.A.; OMNH, Sam Noble Oklahoma Museum of
Natural History, Norman, OK, U.S.A. UUVP/UMNH,Utah
Museum of Natural History, Salt Lake City, UT, U.S.A.
COMPARATIVE DESCRIPTION AND RESULTS
Endocast
The bony braincases completely enclose the endocranial cav-
ities except for the rostral extension of the olfactory bulbs there-
fore preserving two complete, largely similar, symmetrical
endocasts (Figs. 1,2). The slight mediolateral compression of
the skull of A. jimmadseni suggests equal mediolateral com-
pression of the endocast and therefore, description is largely
based on the endocast of A. fragilis. The CT scan of A. fragilis
also provided a more detailed endocast. We describe any major
differences in morphology between the two species with com-
parisons to other known Allosaurus fragilis endocasts (UMNH
VP 18055 [formerly UUVP 5961]: Franzosa, 2004; UMNH VP
7435 [formerly UUVP 294]: Hopson, 1979 and Rogers, 1999;
UMNH VP 18050 [formerly UUVP 3304]: Witmer & Ridgely,
2009) and other theropod endocasts. The endocranial volumes
not including the endosseous labyrinth and cranial nerve trunk
volumes are 163 (A. fragilis) and 93.2 (A. jimmadseni)cm
3
.
Overall, the endocast is dorsoventrally tall and mediolaterally
narrow and possesses features indicative of fore-, mid-, and hind-
brain structures. The cephalic flexure (angle between fore- and
midbrain) and the pontine flexure (angle between the mid- and
hindbrain) are similar at ∼135° (Lautenschlager & Hübner,
2013) resulting in an endocast that is dorsoventrally tall as
noted for UMNH VP 7435, 18055, and 18050 (Franzosa, 2004;
Hopson, 1979; Witmer & Ridgely, 2009). The dorsoventral
height is visually comparable to Ceratosaurus,Majungasaurus,
Giganotosaurus, and Viavenator (Paulina-Carabajal & Canale,
2010; Paulina-Carabajal & Filippi, 2017; Sampson & Witmer,
2007; Sanders & Smith, 2005) and in contrast to the more rostro-
caudally elongate endocast of Tyrannosaurus and Alioramus
(Bever et al., 2013; Witmer & Ridgely, 2009).
Forebrain—Extending rostrally from its junction with the
endocast of the cerebral hemispheres, the cylindrical passage
for the olfactory tract narrows in all dimensions then expands
mediolaterally to the impressions of the olfactory bulbs (Fig.
1). Only the dorsal-most borders of the olfactory bulbs are
visible as impressions on the ventral surface of the frontal bone
as in other Allosaurus and theropods (Franzosa, 2004; Hopson,
1979; Rogers, 1999; Witmer & Ridgely, 2009). The olfactory
bulb impressions are ovate with a rostrolaterally oriented long
axis. The olfactory tract is bounded ventrally by the laterosphe-
noids and orbitosphenoids and dorsally by the frontals (McClel-
land, 1990). The tract is slightly convex on the dorsal surface as
noted in Giganotosaurus (Paulina-Carabajal & Canale, 2010).
At its caudal junction with the endocast of the cerebral hemi-
spheres, the passage for the tract also expands, though more dor-
soventrally than mediolaterally. The endocast of the cerebral
hemispheres marks the portion of the brain with the largest med-
iolateral width and narrow mediolaterally both rostrally and
caudally. Bilateral venous canals extend rostrolaterally from
the rostral ventrolateral border of the cerebral hemispheres as
in Majungasaurus and A. fragilis (Hopson, 1979; Sampson &
Witmer, 2007). These pass through the laterosphenoid and this
foramen is described in error as the passage for the trochlear
nerve by Franzosa (2004). Ventral to the endocast of the cerebral
hemispheres, the oval, mediolaterally elongated foramen for the
optic nerve (CN II) extends rostrally, passing through the orbito-
sphenoids (Fig. 1D, E, G, H). Ventral to the optic foramen, and
endocast of the cerebral hemispheres, the hypophyseal fossa,
which contained the pituitary gland, extends ventrally into the
basisphenoid, though its anterolateral extent is not bounded in
bone. Similar to the case in UMNH VP18055 (Franzosa, 2004),
the endocast of the pituitary fossa is not entirely represented ven-
trally though is likely larger than reconstructed, as indicated by
the ventrally extensive pituitary fossa in the endocast of
UMNH VP 18050 (Witmer & Ridgely, 2009). As noted in Majun-
gasaurus,Tyrannosaurus,Alioramus,Viavenator,Giganoto-
saurus, and other specimens of Allosaurus (Bever et al., 2013;
Franzosa, 2004; Hopson, 1979; Paulina-Carabajal & Canale,
2010; Paulina-Carabajal & Filippi, 2017; Sampson & Witmer,
2007; Sanders & Smith, 2005; Witmer & Ridgely, 2009), there is
a dorsally expanded peak, likely representative of a dural
venous sinus, present on the dorsal surface of the endocast of
the cerebral hemispheres (Figs. 1,2).
Midbrain—There is no obvious representation of the optic
tectum (optic lobes) on the endocast. The canals for the oculo-
motor (CN III) and trochlear (CN IV) nerves extend rostrally
from the lateral border of the orbitosphenoid. The oculomotor
nerve foramen is present lateral to the optic nerve foramen,
and the trochlear nerve foramen is located just dorsal and poster-
olateral to the optic nerve foramen (Fig. 1D–I). Distinct fora-
mina are present in A. fragilis unlike the case detailed for
UMNH VP 7435 (Rogers 1999). The endocast of the dural
venous sinus (dorsal sagittal sinus of UMNH VP 18055; Fran-
zosa, 2004) is present dorsal to the forebrain extending caudally
as a ridge on the dorsal surface of the midbrain.
Hindbrain—The endocast of the dural venous sinus continues
caudally as a ridge on the dorsal surface of the hindbrain as well,
narrowing dorsal to the endocast of the medulla. It is difficult to
define boundaries between the cerebellum, medulla, and pons in
the endocast of the hindbrain. No impressions of floccular pro-
cesses are apparent on the endocast of the cerebellum of A.fra-
gilis. There are dorsoventrally oriented ridges extending laterally
from the cerebellar region of A. jimmadseni that may represent
the endocast of the flocculus (Fig. 2D). All other Allosaurus
endocasts (UMNH VP 7435, 18055, and 18050) have large endo-
casts of flocculi preserved (Franzosa, 2004; Hopson, 1979;
Rogers, 1999; Witmer & Ridgely, 2009) as do the endocasts of
Majungasaurus,Tyrannosaurus,Giganotosaurus,Alioramus,
and Viavenator (Bever et al., 2013; Paulina-Carabajal &
Canale, 2010; Paulina-Carabajal & Filippi, 2017; Sampson &
Witmer, 2007; Witmer & Ridgely, 2009) so the absence in these
specimens is likely a result of incomplete preservation or low res-
olution. The endocast is also slightly expanded laterally near the
exit for the glossopharyngeal (CN IX) and vagus nerves (CN X).
The canal for the trigeminal nerve (CN V) emerges from the
level of the pons on the ventrolateral aspect of the endocast as
Lessner et al.—Endocranial anatomy of Allosaurus (e2236161-3)
FIGURE 1. Reconstruction of cranial endocast of Allosaurus fragilis (DINO 2560). A, skull and mandible, B, left lateral and C, dorsal views of
the endocast within the reconstructed skull. D, right lateral, E, left lateral, F, dorsal, G, ventral, H, rostral, and I, caudal views of the
A. fragilis endocast with venous canals, endosseous labyrinth, and nerve canals. Abbreviations:cer, cerebral hemisphere; dp, dural
peak; el, endosseous labyrinth; ob, olfactory bulb; otc, olfactory tract cavity; pfo, pituitary fossa; vc, venous canal; II, optic nerve;
III, ophthalmic nerve; IV, trochlear nerve; V
1
, trigeminal nerve first division; V
2–3
, trigeminal second and third divisions; VI, abducens
nerve; VII, facial nerve; IX&X, combined canals for glossopharyngeal and vagus nerves; XII, hypoglossal nerve.
Lessner et al.—Endocranial anatomy of Allosaurus (e2236161-4)
a small expansion representative of the trigeminal ganglion (Fig.
3). Two canals extend from the space for the ganglion (Figs. 1,2).
One passes rostrally through the laterosphenoid and would have
housed the ophthalmic division of the trigeminal nerve (CN V
1
).
The other passes ventrolaterally, bounded by the laterosphenoid
rostrally and the prootic caudally and would have housed the
maxillomandibular division of the trigeminal nerve (CN V
2–3
).
Dual trigeminal canals are present in other specimens of Allo-
saurus and many theropods as well (Franzosa, 2004; Hopson,
1979; Witmer & Ridgely, 2009) though not always preserved as
an enclosed canal (e.g., groove in Viavenator; Paulina-Carabajal
& Filippi, 2017). The canal for the abducens nerve (CN VI)
extends rostrally from the ventral aspect of the pons, passing
through the basisphenoid into a fissure between the laterosphe-
noid and orbitosphenoid, approaching the hypophyseal fossa
and entering the fossa in UMNH VP 7435, 18055, and 18050
(Franzosa, 2004; Hopson, 1979; Witmer & Ridgely 2009;Fig.
1D, E, G, H). The small canal for the facial nerve (CN VII) is
present just caudal to the opening for CN V
2–3
and passes ventro-
laterally through the prootic (Fig. 1). Preservation and resolution
make it difficult to distinguish the opening for the vestibuloco-
chlear nerve (CN VIII), which is expected ventral to the endoss-
eous labyrinth and rostral to the passageway for CN IX and CN
X. The glossopharyngeal and vagus nerves (CN IX and CN X)
extend laterally in the jugular/vagal canal (metotic fissure)
(Franzosa, 2004; Paulina-Carabajal & Canale, 2010; Sampson &
Witmer, 2007; Witmer & Ridgely, 2009;Figs. 1–3). The canal
for the hypoglossal nerve (CN XII) extends ventrolaterally
from the caudoventral aspect of the medulla, passing through
the exoccipital (Fig. 1).
Endosseous Labyrinths—The endocasts of the semicircular
canals are partially preserved in A.fragilis (Figs. 3B,4). Struc-
tures poorly preserved or not discernible from the CT data
include the cochleae, vestibules, and fenestrae vestibuli and
cochleae of the middle ear. The endocasts of the semicircular
canals are subtriangular and resemble the endocast of the
endosseous labyrinths of UMNH VP 7435 (Hopson, 1979;
Rogers, 1999) and UMNH VP 18050 (Witmer & Ridgely,
2009). The rostral and caudal canals are slightly thinner than
the lateral canal. All canal junctions are suborthogonal. The
crus communis of the left endosseous labyrinth is large in diam-
eter, though this is likely a result of preservation. On the right
endosseous labyrinth, there is a thickening preserved at the
rostral part of the lateral semicircular canal that likely represents
the ampulla of the lateral semicircular canal (Fig. 4H). Notably,
the endocast of the caudal semicircular canal is more linear
and the overall structure more dorsoventrally elongate than
that of Majungasaurus,Tyrannosaurus,Alioramus, and Viavena-
tor (Bever et al., 2013; Paulina-Carabajal & Filippi, 2017;
Sampson & Witmer, 2007; Witmer & Ridgely, 2009).
FIGURE 2. Reconstruction of cranial endocast of Allosaurus jimmadseni (DINO 11541). A, skull and mandible, B, left lateral and C, dorsal views of
the endocast within the reconstructed skull. D, right lateral, E, ventral, and F, rostral views of the endocast and nerve canals. Abbreviations:cer, cer-
ebral hemisphere; dp, dural peak; fl?, potential flocculus; otc, olfactory tract cavity; pfo, pituitary fossa; V, combined trigeminal nerve canals; V
1
, canal
for the ophthalmic division of the trigeminal nerve; V
2-3
, canal for the maxillomandibular divisions of the trigeminal nerve; IX&X, combined canals for
glossopharyngeal and vagus nerves.
Lessner et al.—Endocranial anatomy of Allosaurus (e2236161-5)
FIGURE 3. Cross sections of Allosaurus CT data. Cross sections of A. fragilis (DINO 2560; A,B) and A. jimmadseni (DINO 11541; C,D)inA,C,
coronal and B,D, axial views with section locations indicated in insets. Abbreviations:br, braincase; el, endosseous labyrinth; V, trigeminal nerve canal;
VII, facial nerve canal; IX&X, combined canals for glossopharyngeal and vagus nerves.
FIGURE 4. Reconstructions of endosseous labyrinths of Allosaurus fragilis (DINO 2560). A-D, left and E-H, right labyrinths in A,E, lateral, B,F,
rostral, C,G, caudal, and D,H, dorsal views. Abbreviations:csc, caudal semicircular canal; crc, crus communis; lsc, lateral semicircular canal; lsca,
ampulla of lateral semicircular canal; rsc, rostral semicircular canal.
Lessner et al.—Endocranial anatomy of Allosaurus (e2236161-6)
Scaling Analysis
Our regression of endocranial volume on body mass and
general results resemble those of Balanoff et al. (2013) in that
non-avian dinosaurs have lower relative endocranial volumes
than birds (Fig. 5). We also find crocodylians to have lower rela-
tive endocranial volumes than birds. The two Allosaurus speci-
mens fit well among non-maniraptoran theropods. The best-fit
line for crown birds has an equation: y = 0.49x + 1.10 (R
2
=
0.78). The best-fit line for all datapoints has an equation y =
0.45x + 0.81 (R
2
= 0.77). The best-fit line for the paraphyletic
‘non-avian theropods,’representative of the ancestral condition
(Balanoff et al., 2013) has an equation of y = 0.59x + 0.31 (R
2
=
0.95).
In comparison of trigeminal foramen maximum diameter with
body mass, we find that theropods have relatively smaller fora-
mina than those of crocodylians (Fig. 6). The best-fit line for
crown birds has an equation y = 0.34x + 0.77 (R
2
= 1), and the
best-fit line for crocodylians is y = 0.25x + 0.85 (R
2
= 1).
ANCOVA of extant groups reveals significant difference
between crown birds and crocodylians (P< 0.005).
DISCUSSION
Though this is the first detailed, published description of Allo-
saurus cranial endocasts revealed by CT scanning, the endocasts
and semicircular canals of A. fragilis (DINO 2560) and
A. jimmadseni (DINO 11541) described here do not present
any surprising or unexpected morphologies. Besides differences
in preservation and size, they are largely similar. They resemble
other known Allosaurus endocrania (Franzosa, 2004: UMNH VP
18055; Hopson, 1979; Osborn, 1912: AMNH 5753; Rogers, 1999:
UMNH VP 7435; Witmer & Ridgely, 2009: UMNH VP 18050)
and compare favorably with other basally branching theropods
such as Ceratosaurus,Majungasaurus, and Viavenator (Paulina-
Carabajal & Filippi, 2017; Sampson & Witmer, 2007; Sanders
& Smith, 2005) supporting previously proposed endocranial con-
servatism among non-coelurosaurian theropods (Sampson &
Witmer, 2007). General anatomical differences from more
derived theropods include: a dorsoventrally oriented endocast,
elongate olfactory tract, no representation of optic tecta,
present but minimally extensive dorsal venous sinuses, and a
straight caudal semicircular canal (as opposed to rostrocaudally
oriented endocasts with short olfactory tracts, laterally placed
optic tecta, extensive dorsal venous sinuses, and bowed caudal
semicircular canals in coelurosaurs: Bever et al., 2013; Paulina-
Carabajal & Canale, 2010; Sampson & Witmer, 2007; Witmer
& Ridgely, 2009).
For the first time, completeness of the skulls and associated
skeletons allows for additional analyses even though the large
size of the specimens limits resolution and therefore interpret-
ation of morphology. Scaling analyses reveal that Allosaurus
fits the trend of increasing endocranial volume with increasing
body size (Fig. 5). Adding additional large-bodied theropods to
an existing dataset (Balanoff et al., 2013) revealed that this
relationship may exhibit some positive allometry in comparison
with endocranium-body size relationships among both birds
and reptiles as a whole. The trendline indicates larger non-
avian theropods have relatively larger endocrania than smaller-
bodied theropods. While initial instinct may be to assign behav-
ioral significance to this trend, assuming the larger endocranium
is because of a larger brain, other possibilities are more likely.
FIGURE 5. Bivariate plot of log-transformed body-mass data after Balanoff et al. (2013). Body mass (kg) vs. endocranial volume (cm
3
). Crown birds
(black circles), non-maniraptoran theropods (squares), Shuvuuia (star), oviraptorosaurs (diamonds), deinonychosaurs (upturned triangles), Archae-
opteryx (downturned triangle), crocodylians (white circles). Reduced major-axis regression line for entire sample (solid), crown birds (large dashes),
and non-avian theropods (small dashes). Black squares represent Allosaurus additions.
Lessner et al.—Endocranial anatomy of Allosaurus (e2236161-7)
For instance, it is well known that the brain in reptiles is smaller
than the endocranial volume because of space occupied by dural
venous sinuses (Jirak & Janacek, 2017; Watanabe et al., 2019).
Larger-bodied organisms such as theropods have relatively
small surface-area-to-volume-ratios, lose heat to the environ-
ment slowly, and thus would have to combat high body tempera-
tures (Ganse et al. 2011; Spotila et al., 1973), which can be
damaging to brain tissue. Increased intracranial space for dural
venous sinuses provides a potential solution allowing for brain
cooling via countercurrent heat exchange (Holliday et al., 2020;
Porter & Witmer, 2015,2020; Sedlmayr, 2002). Additionally, it
is important to mention disparities in growth rate between the
brain and braincase; among archosaurs, brain size and shape cor-
responds variably with endocranial size and shape through onto-
geny (Watanabe et al., 2019), which means knowledge of
ontogenetic stage is necessary for well-founded volumetric and
descriptive comparisons.
Further scaling analyses of trigeminal osteological correlates
reveal relatively smaller foramina among theropods (Fig. 6).
Because motor nerves make up a small fraction of total
neurons and neuron volume in the trigeminal system (Pennisi
et al., 1991), differences in motor function are likely not indicated
by these data. Instead, this indicates Allosaurus and other non-
avian theropods possessed relatively smaller trigeminal sensory
tissues than crocodylians, and thus likely did not have a heigh-
tened sense of cranial somatosensory abilities akin to crocody-
lians (Leitch & Catania, 2012). Small trigeminal tissues in
Allosaurus follow ecological trends as the system in crocodylians
is highly specialized to the semi-aquatic environment, whereas
Allosaurus occupied terrestrial habitats. Thus, phylogenetic
bracketing by crown archosaurs supports a generalist trigeminal
sensory system among non-avian theropods.
ACKNOWLEDGMENTS
We thank The Jurassic Foundation (to EJL) and the National
Science Foundation (to CMH; NSF EAR 1631684, IOS 1457319)
for funding. Also thanks to C. Levitt-Bussian, Morphosource, the
Ashley Regional Medical Center Staff including A. Batty,
J. Nickells, and J. Hanberg, and the University of Texas High-
Resolution X-Ray CT Facility, M. Colbert, and J. Maisano
(NSF EAR 1762458). The conclusions presented here are those
of the authors and do not represent the views of the United
States Government.
DATA AVAILABILITY
The data that support the findings of this study are openly
available in the digital repository Morphosource at https://
www.morphosource.org/projects/000498875?locale=en.
AUTHOR CONTRIBUTIONS
EJL designed the project, gathered and analyzed data, and
drafted the manuscript, CC gathered and analyzed data, RHF
gathered data, and CMH analyzed data and drafted the manu-
script. All authors edited the manuscript.
FIGURE 6. Bivariate plot of log-transformed body-mass against trigeminal osteological correlate. Body mass (kg) vs. trigeminal foramen maximum
diameter (mm). Crown birds (black circles), non-maniraptoran theropods (squares), Shuvuuia (star), Citipati (diamond), deinonychosaurs (upturned
triangles), crocodylians (white circles). Reduced major-axis regression line for crocodylians (solid) and crown birds (dashes). Black squares represent
Allosaurus.
Lessner et al.—Endocranial anatomy of Allosaurus (e2236161-8)
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