Theropod dinosaur facial reconstruction and the importance of soft tissues in paleobiology

Abstract

Large theropod dinosaurs are often reconstructed with their marginal dentition exposed because of the enormous size of their teeth and their phylogenetic association to crocodylians. We tested this hypothesis using a multiproxy approach. Regressions of skull length and tooth size for a range of theropods and extant varanid lizards confirm that complete coverage of theropod dinosaur teeth with extraoral tissues (gingiva and labial scales) is both plausible and consistent with patterns observed in living ziphodont amniotes. Analyses of dental histology from crocodylians and theropod dinosaurs, including Tyrannosaurus rex, further indicate that the most likely condition was complete coverage of the marginal dentition with extraoral tissue when the mouth was closed. This changes our perceptions about the appearance and oral configuration of these iconic predators and has broad implications for our interpretations of other terrestrial animals with large teeth.

Full text

PALEONTOLOGY
Theropod dinosaur facial reconstruction and the
importance of soft tissues in paleobiology
Thomas M. Cullen
1,2,3
, Derek W. Larson
4,5
, Mark P. Witton
6
, Diane Scott
7,8
, Tea Maho
7,8
,
Kirstin S. Brink
9
, David C. Evans
5,10
, Robert Reisz
7,8
*
Large theropod dinosaurs are often reconstructed with their marginal dentition exposed because of the
enormous size of their teeth and their phylogenetic association to crocodylians. We tested this hypothesis using
a multiproxy approach. Regressions of skull length and tooth size for a range of theropods and extant varanid
lizards confirm that complete coverage of theropod dinosaurteethwithextraoraltissues (gingiva and labial
scales) is both plausible and consistent with patterns observed in living ziphodont amniotes. Analyses of dental
histology from crocodylians and theropod dinosaurs, including Tyrannosaurus rex, further indicate that the
most likely condition was complete coverage of the marginal dentition with extraoral tissue when the mouth
wasclosed.Thischangesourperceptionsabouttheappearance and oral configuration of these iconic
predators and has broad implications for our interpretations of other terrestrial animals with large teeth.
The antorbital region of the cranium plays
a number of important roles in the biology
of terrestrial vertebrates, including res-
piration, olfaction, and food capture and
manipulation. Most known dinosaurs are
herbivorous, and some (ornithischians) show
evidence for an expanded rictus that formed a
superficially cheek-like structure that covered
their relatively small dentition externally, with
this being particularly relevant for hadrosaurs
and ceratopsians (1–3). By contrast, many non-
avian theropod dinosaurs are renowned for
possessing very large teeth, which has led to
reconstructions in both scientific and popular
literature since the 1980s that show m axil lary
dentition protruding from their closed mouths
rather than covered by extraoral tissues, as
in most terrestrial vertebrates (Fig. 1 and fig.
S2) (4,5). Among the arguments in favor of
this interpretation are the relatively large
sizes of some theropod teeth and evidence
from the dinosaur phylogenetic bracket, where
crocodylians, the closest extant dentigerous
relatives of dinosaurs, lack extensive extra-
oral tissues (6–8). Some recent research on
theropod rostral neurovasculature has ar-
gued that direct data and evidence are lack-
ing for extraoral tissue reconstructions (9), but
rigorous reconstructions of these tissues are
important for biological inferences for dino-
saurs. Theropod dinosaur teeth have relatively
thin enamel, and large theropods retained
their dentition for prolonged periods of time
(10,11), potentially exposing them to dam-
aging desiccation and wear (12). Here, we
use multiple lines of evidence, including
dental histology, skull and tooth size regres-
sions, and morphological comparisons, to
test alternate hypotheses of theropod facial
reconstruction.
In extant reptiles, two major anatomical
patterns occur with respect to dentition and
extraoral tissues. In crocodylians, about one-
quarter of the tooth crown height that extends
beyond the labial edge of the maxillary bone
is covered by a fleshy gingiva, and the enamel-
covered crowns are not covered by labial scales
(“lips”) (Fig. 2). In extant lepidosaurs, which
are more distant reptilian relatives to dino-
saurs than crocodiles, the base of the teeth is
similarly covered in gingiva; however, the
enamel-covered crowns of the teeth are cov-
ered externally by labial scales when the mouth
RESEARCH
Cullen et al., Science 379, 1348–1351 (2023) 31 March 2023 1of4
Fig. 1. Comparisons of the reconstructions of
T. rex. (A) Skull, based on Field Museum of Natural
History specimen FMNH PR 2081. (Bto E) Two
hypothetical flesh reconstructions, one with exposed
teeth (B) and an associated cross section of the
snout (C) and one with extraoral tissues covering
the teeth (D) and an associated cross section
of the snout (E).
Downloaded from https://www.science.org at Auburn University on March 30, 2023

is closed (Fig. 2). This applies even in large-
toothed taxa such as predatory varanid lizards.
Notably, in both these lizards and theropod
dinosaurs, the teeth are parasagital ly ali gn ed
with the vertical plane of the skull and do not
lean outward as in extant crocodiles (13).
Phylogenetic bracketing, in the absence of
evidence from birds and fossils, could support
the hypothesis that the large teeth of thero-
poddinosaurswouldshowthesamepattern
as that of extant crocodiles, with the upper
marginal dentition being exposed when the
mouth is closed (8). However, such narrow
applications of extant phylogenetic bracket-
ing can be problematic when considering
dinosaur facial tissues (3,14,15), especially
given recent studies into the derived facial
integument of crocodylians and its relation to
their aquatic lifestyles and sensory adapta-
tions (16–18). The foramina that are present
along the jaw margins of reptiles facilitate the
passage of blood vessels and branches of the
trigeminal nerve to the extraoral tissues and,
in derived crocodylians, house sensory organs
that were more widely distributed across the
snout (9). Extinct terrestrial crocodylomorphs
(e.g., the Late Triassic taxon Hesperosuchus;
Fig. 2) possess a more theropod-like pattern
of linearly arranged jaw foramina, as well
as ziphodont dentition (Fig. 2) (7). Indeed, a
broader extant comparison (Fig. 2) demon-
strates that the lower-density, linear pattern
of foramina on the face and jaws of theropods,
such as tyrannosaurids, is as or more similar
in structure to that of many extant squamates,
such as Varanus or Amblyrhynchus,thanto
the pattern observed in extant crocodylians
such as Alligator. This is concordant with
other work suggesting that similarly low den-
sities of linearly arranged facial foramina are
a widespread feature in tetrapods that pos-
sess extraoral soft tissues (1,9).
Dentition in reptiles, including dinosaurs,
is characterized by the presence of a relatively
thin enamel layer that covers the crown of the
tooth. Enamel is formed during tooth devel-
opment through amelogenesis, is not repa-
rable or replaceable, and is invariably thin in
most carnivorous reptiles, both fossil and extant
(19,20).In theropod dinosaurs, the thickness of
the enamel is similar on the lingual and labial
sides of the tooth crown and is somewhat size
dependent, with the largest theropod dino-
saurs having the thickest enamel (20,21).
Crocodylians generally have overall thicker
enamel than dinosaurs, with thicker regions
toward the apex of the crown (21). In addi-
tion, dentine exposure is common in teeth and
tusks that are exposed to the environment (22).
To investigate the dental histology of large
theropods in detail, we removed a functional
upper maxillary tooth from a large individual
of the tyrannosaurid Daspletosaurus and ex-
amined it for age and enamel ultrastructure in
histological thin section under plane-polarized
Cullen et al., Science 379, 1348–1351 (2023) 31 March 2023 2of4
1
Department of Geosciences, Auburn University, 2050 Beard
Eaves Coliseum, Auburn, AL 36849, USA.
2
Ottawa-Carleton
Geoscience Centre, Department of Earth Sciences,
Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S
5B6, Canada.
3
Nagaunee Integrative Research Center,
Field Museum of Natural History, 1400 S. Lake Shore Drive,
Chicago, IL 60605, USA.
4
Collections Care, Royal BC
Museum, 675 Belleville Street, Victoria, BC V8V 9W2,
Canada.
5
Department of Ecology and Evolutionary Biology,
University of Toronto, 25 Willcocks Street, Toronto, ON M5S
3B2, Canada.
6
School of the Environment, Geography and
Geosciences, University of Portsmouth, Burnaby Building,
Burnaby Road, PO1 3QL Portsmouth, UK.
7
College of Earth
Science, Dinosaur Evolution Research Centre and
International Centre of Future Science, Jilin University,
Changchun, China.
8
Department of Biology, University of
Toronto Mississauga, Mississauga, ON L5L 1C6, Canada.
9
Department of Earth Sciences, University of Manitoba,
125 Dysart Road, Winnipeg, MB R3T 2N2, Canada.
10
Department of Natural History, Royal Ontario Museum,
100 Queen’s Park, Toronto, ON M5S 2C6, Canada.
*Corresponding author. Email: robert.reisz@utoronto.ca
Fig. 2. Comparisons of life appearance and reconstructions, skull shape, and maxillary morphology
in lepidosaurs and archosaurs. (A)V. salvadorii.(B)Amblyrhynchus cristatus.(C) Extant crocodylian
A. mississippiensis.(D) Extinct crocodylomorph Hesperosuchus agilis.(E) Extinct theropod T. rex. Note the
linear pattern of foramina (LF) along the extraoral margin in sampled lepidosaurs and theropods in contrast
to the broadly distributed pattern of foramina and dome pressure sensor pores (DFDP) in Alligator. Also
note the ziphodont tooth condition (zc) in the inset image of Hesperosuchus (D) compared with the condition
present in extant crocodylians. [Image credits: V. salvadorii,A. cristatus, and A. mississippiensis in-life
photographs from Wikimedia Commons (public domain); A. cristatus skull photograph from E. Graslie (used
with permission); A. mississippiensis skull photograph from D. Descouens (CC-ASA-4.0); T. rex skull
photograph from J. Weinstein at FMNH (used with permission); remaining images are from the authors]
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and cross-polarized light using a petrographic
microscope (Fig. 3 and fig. S1; see supplemen-
tary text). The thin section confirmed that this
tooth was fully developed, with an estimated
512 von Ebner lines being present, consistent
with tooth development and replacement
rates of well over 1 year that have been esti-
mated in other large tyrannosaurids, including
Tyrannosaurus rex (11). The enamel was found
to be of similar thickness on both the lingual
and labial sides, with no evidence of any sub-
stantial wear (Fig. 3, A to E). Despite its advanced
age, the tooth still carried well-formed mesial
and distal cutting edges (carinae) with delicate
serrations [ziphodont (23)orincrassate(24)].
Wear on tyrannosaurid teeth occurs rarely and
primarily on the medial surface of the maxillary
dentition because of tooth-on-tooth contact
with the opposing dentary teeth (25). By con-
trast, the enamel of the largest teeth of Alligator
mississippiensis,liketheoneintoothposition4
(Fig.3,FtoH),frequentlybecomeserodedon
the exposed labial side, with even a substantial
portion of the dentine occasionally worn away.
Enamel has a relatively low water content
but is still hydrated and maintained in extant
terrestrial vertebrates by glandular secretions
in the mouth (12,26), which arrest detrimen-
tal changes in enamel hardness and elasticity
(12,27). Dry enamel has a higher nanohard-
ness and elastic modulus, resulting in stiffer
tissue (12,27), whereas wet enamel is better
Cullen et al., Science 379, 1348–1351 (2023) 31 March 2023 3of4
Fig. 3. Histological thin sections of teeth from
the large theropod Daspletosaurus and Alligator.
(Ato E)Daspletosaurus tooth (Royal Tyrrell Museum
of Palaeontology specimen TMP 2003.010.0003)
(A) showing relatively unworn enamel of equal
thickness on the lingual (B) and labial (C) surfaces of
this functional tooth, as well as a reduction of
enamel at the base of the crown (D) and cementum
present along the root (E). (Fand G)Alligator
tooth (Royal Ontario Museum specimen ROM R600)
(F) showing highly uneven wear patterns between
the labial and lingual surfaces (G), with all enamel
and some dentine worn away along the labial
surface and thick enamel still present on the lingual
surface. (H) Unerupted Alligator tooth without
any wear and with the presence of even enamel
thickness. See fig. S1 for images of the maxilla
of TMP 2003.010.0003 and additional information
on the sampled tooth. Images in (B) to (E)
and (G) and (H) are thin sections photographed
using a petrographic microscope, under plane-
polarized [(E) and (H)] and cross-polarized
[(B), (D), and (G)] light.
Fig. 4. Plot of log
10
skull length to log
10
tooth height for a range of extant varanids and extinct
theropods. Model II major axis (MA) regressions run on extant varanids (all of which have extraoral tissues
covering teeth) (blue points, line, and shaded confidence intervals) and extinct theropods (orange points,
line, and shaded confidence intervals). Also plotted are a phylogenetic generalized least squares (PGLS) line
for the same extant varanids (green) and the line of isometry (dashed gray). Goodness-of-fit for Varanus
data is as follows: coefficient of determination (r
2
) = 0.9285, and p< 0.001. The slope of the Varanus
MA regression line is 1.215, the slope of the PGLS line is 1.140, and the slope of the theropod MA regression
line is 1.218. The Varanus and theropod lines are not significantly different (p= 0.97).
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at resisting wear and abrasion (12). Given the
relationship between hydration and wear re-
sistance, and the difficulty of maintaining hy-
dration if a tooth is exposed to air for long
periods of time, it is unlikely for functional
teeth to remain relatively unworn if exposed,
unless the enamel structure and thickness are
considerably modified. The comparative lack of
wear and abrasion in theropod teeth (Fig. 3, A
to E) (23), in contrast to the extensive and asym-
metric wear [Fig. 3, F to H; see also (28–30)]
and breakage (31) observed through ontogeny
in crocodylians, suggests that theropod teeth
existed under hydrated conditions consistent
with the possession of extraoral tissues.
Although the skulls and teeth of theropod
dinosaurs, such as Daspletosaurus and
Tyrannosaurus, are indeed very large com-
pared to those of extant reptiles, major-axis
regression analyses demonstrate that the slope
of the tooth-skull size relationship in theropods
closely matches that observed for extant vara-
nids (Fig. 4), thus refuting interpretations
that their teeth were unusually large to the
extent that tooth size could preclude extraoral
tissue coverage. Even the varanid with the
largest relative tooth size (Varanus salvadorii)
does not have exposed dentition (Figs. 2A and
4), and it possesses greater tooth height–to–
skull length ratios (0.096) than the largest
sampled theropod, T. rex (0.074). These data
indicate that theropod teeth were not too
large to be covered with extraoral tissues when
the mouth was closed and that such a condi-
tion would be consistent with what is observed
in living amniotes.
The scaling relationships of tooth to skull
size between varanids and theropods (Fig. 4)
provide further support for the potential in-
ference of soft tissue coverings of the marginal
dentition in theropods. Although the relation-
ship between tooth and skull size is weakly
positively allometric, the relationship does
not greatly affect expected tooth size over the
scales represented (dashed versus solid lines
in Fig. 4), and data comparing tooth crown
height to extraoral tissue height, where avail-
able, suggest that a weakly negative allometric-
to-isometric relationship exists between these
measures (i.e., crown height increases at a
slower rate than extraoral tissue height with
increasing body size; fig. S4). No change in the
presence of complete coverage of teeth with
extraoral tissues is noted over a 12-fold in-
crease in size between the smallest and largest
Varanus skulls in the dataset, despite the in-
clusion of Varanus species with teeth that are
proportionally larger (relative to skull size)
than those observed in most theropods (e.g.,
V. salvadorii). It would therefore be inconsistent
with the data to expect the extraoral tissues to
deviate from this pattern over the sixfold size
increase between Varanus komodoensis and
T. rex. Given the close fit of multiple lineages
of small theropods to the tooth-to-skull size
relationship documented in varanids, well-
developed extraoral tissues appear likely in
smaller members of all major theropod groups,
and it is unlikely for tooth height to have ex-
ceeded facial soft tissue growth, even in larger
theropods.
These comparisons show that extraoral tis-
sues of nonavian theropods (Fig. 1 and fig. S2;
seesupplementarytext)weremorelikethose
of extant lepidosaurs and other tetrapods than
those of birds or crocodylians and that the
faces of extant archosaurs do not accurately
reflect the ancestral condition of the archo-
saurian clade. The results of this study strongly
support “lipped”facial reconstructions in the-
ropods with wide-reaching implications for their
portrayal in science and popular culture. More
importantly, the presence of extensive extra-
oral tissues has implications for tooth strength,
feeding ecology, and biomechanics and there-
foremayhaveplayedanimportantroleinhow
carnivorous theropod dinosaur teeth resisted
forces associated with feeding close to the bone
and even may have permitted carcass dis-
memberment while reducing spalling in large
tyrannosaurids. Finally, we posit a lepidosaur-
like plesiomorphic condition for extraoral tis-
sues in Dinosauria and expect that our results
not only will provide a deeper understanding
of the evolution of buccal soft tissues generally
and advanced oral processing in ornithischians
in particular but also, more broadly, will open
new directions of research into the relationships
between oral soft tissues and feeding behav-
ior in terrestrial vertebrates with large teeth.
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We thank J. Weinstein (FMNH) for permission to use photographs
of FMNH PR 2081 in this study. Funding: Support for this work
was provided by Jilin University, the University of Toronto
Mississauga, and Natural Sciences and Engineering Research
Council of Canada (NSERC) Discovery Grant 2020-04959 (R.R.); a
NSERC Canada Graduate Scholarship, the Kenneth C. Griffin Fund,
and NSERC Postdoctoral Fellowship PDF-545802-2020 (T.M.C.);
NSERC Discovery Grant 2021-00364 (K.S.B.); and Dinosaur
Research Institute 2011 and 2015 Student Project Grants (D.W.L.).
Author contributions: Conceptualization: R.R., D.S., D.C.E.;
Methodology: T.M.C., D.W.L., T.M., D.C.E.; Investigation: All authors;
Visualization: T.M.C., M.P.W., D.W.L., D.S., T.M.; Writing –original
draft: R.R., T.M.C., K.S.B., M.P.W., D.W.L.; Writing –review and
editing: T.M.C., K.S.B., D.C.E., M.P.W., R.R. Competing interests:
The authors declare no competing interests. Data and materials
availability: All data are available in the main text or
the supplementary materials. License information: Copyright ©
2023 the authors, some rights reserved; exclusive licensee
American Association for the Advancement of Science. No claim to
original US government works. https://www.science.org/about/
science-licenses-journal-article-reuse
SUPPLEMENTARY MATERIALS
science.org/doi/10.1126/science.abo7877
Supplementary Text
Materials and Methods
Figs. S1 to S4
Tables S1 to S3
References (32–68)
MDAR Reproducibility Checklist
Data S1 and S2
Submitted 6 March 2022; accepted 3 March 2023
10.1126/science.abo7877
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Theropod dinosaur facial reconstruction and the importance of soft tissues in
paleobiology
Thomas M. Cullen, Derek W. Larson, Mark P. Witton, Diane Scott, Tea Maho, Kirstin S. Brink, David C. Evans, and Robert
Reisz
Science, 379 (6639), .
DOI: 10.1126/science.abo7877
Not a toothy grin
Theropod dinosaurs such as the iconic Tyrannosaurus rex have long been portrayed with their teeth fully visible,
similar to extant crocodilians. This pattern of portrayal largely had to do with relatedness between dinosaurs and
crocodilians and the relationship between tooth and jaw size. Cullen et al. tested hypothesized facial reconstruction
in this group using histological analysis of tooth wear patterns and quantitative relationships between skull length
and tooth size in both extinct and extant reptiles. Contrary to depictions that have dominated for more than a century,
they found that theropods, including T. rex, had lips that covered their teeth, leaving them looking more like modern
Komodo dragons than crocodiles. —SNV
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