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Individual variation for the large theropod Tyrannosaurus rex may be seen in the maxilla, dentary and ischium. The maxilla is variable in its depth, the size and shape of the maxillary and antorbital fenestrae, and the size and shape of the lacrimal and jugal processes. Even the left and right maxillae of the same skull show variation. Sexual dimorphism is suggested by the presence of two morphs, one more robust than the other. The angle between the ischia and caudals of the robust morph is greater than in the slender morph, and would provide ample space for the passage of eggs. On this basis, the robust morph is considered the female. -Author
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Variation
in
Tyrannosaurus
rex
Abstract
Individual variation for the large theropod
Tyranno-
. saurus
rex
may be seen in the maxilla, dentary and ischium.
The maxilla
is
variable in its depth, the size and shape
of
the
maxillary and antorbital fenestrae, and the size and shape
of
the
lacrimal and jugal processes. Even the left and right maxillae
of
the same skull show variation. Sexual dimorphism is sug-
gested by the presence
of
two morphs, one more robust than
the other. The angle between the ischia and caudals
of
the
robust morph
is
greater than in the slender morph, and would
provide ample space for the passage of eggs. On this basis, the
robust morph
is
considered the female.
Introduction
The
large
theropod,
Tyrannosaurus
rex, was
named by Osborn in 1905 on the basis of a partial skull
and skeleton from the Hell Creek Formation
of
eastern
Montana. The holotype (AMNH 973) was later trans-
ferred to the Carnegie Museum
of
Natural History (CM
9380) where it is presently on display. A second speci-
men (AMNH 5866) from the Lance Formation
of
east-
ern Wyoming, was named Dynamosaurus imperiosus
by
Osborn (1905) but later synonymized with
T.
rex (Osborn,
1906). This specimen [BM(NH) R7994 and R7995] is
now mounted at the British Museum (Natural History).
Numerous additional specimens have since been recov-
ered from the Scollard and Willow Creek formations
of
Alberta, the Hell Creek Formation
of
North Dakota,
South Dakota, and Montana, and the Laramie Formation
of
Colorado. These specimens are presently under study
by Robert Bakker, Philip Currie, Ralph Molnar, Greg
Paul, and myself. As will be demonstrated below, it
is
doubtful that the specimen reported by Lawson (1976)
from the Tornillo Formation
is
Tyrannosaurus rex.
A detailed osteology
of
T.
rex has not yet been
presented, although Osborn did have a monograph in
In
Dinosaur
Systematics:
Perspectives
and Approaches,
Kenneth
Carpenter and Philip 1. Currie. eds. Copyright © Cambridge University
Press. 1990.
KENNETH
CARPENTER
preparation. Preliminary results
of
Osborn's study were
given in a series
of
short papers (Osborn 1905, 1906,
1912, 1913, 1917). Subsequent papers on Tyrannosaurus,
in whole or in part, include Romer (1923), Newman
(1970),
Russell
(1970),
Molnar
(1973),
Tarsitano
(1983), Bakker et al. (1988), Paul (1988), and Carpenter
(in press). Several popular articles have also appeared
(Anonymous 1910; Brown 1915; Hill 1983). Except for
a brief discussion by Carpenter (in press), none
of
these
papers and articles have examined variation and sexual
dimorphism in Tyrannosaurus.
SkullS
Six partial and complete skulls are now known
for Tyrannosaurus rex, including the holotype CM 9380
(formerly AMNH 973), AMNH 5027, LACM 23844,
MOR 009, SDSM 12047, and TMP 81.6.1. Two addi-
tional specimens, AMNH 5029 and AMNH 5117, are
braincases (Osborn 1912). Five
of
the skulls are shown
in Figure 10.1.
As
may be seen, there
is
a considerable
amount
of
variation in the size and shape
of
all the cra-
nial openings (e.g., orbit and lateral temporal fenestra),
as well as all the individual elements, including the
lacrimal, postorbital, quadratojugal, jugal, and surangu-
lar. In fact, no two specimens are identical.
Seven maxillae were available for comparison.
Five
of
these were overlain in Figure
1O.2A
using the
anterior-most margins
of
the maxilla and maxillary fen-
estra as standard lengths. Most of the maxillae resemble
one another, except for TMM 41436-1, the specimen
reported from the Tornillo Formation
by
Lawson (1976).
Variation can be seen in the depth
of
the maxilla,
size and shape of the maxillary and antorbital fenestrae,
position of the lacrimal processes, and the position and
shape
of
the jugal process (Fig.
1O.2A).
Differences in
the depth
of
the maxilla affect the position
of
the lacri-
mal process, which in
tum
influences the height and
shape
of
the antorbital fenestra. There
is
also a consider-
Kenneth Carpenter
able amount of variation in position
of
the jugal process
that forms the lower rim of the antorbital fenestra, but
this does not seem
to
correlate with the size
of
the antor-
bital process. The shape of the maxillary fenestra ranges
from almost oval (CM 9380)
to
subtriangular (LACM
23844
and
TMP
81.6.1) to
almost
square
(AMNH
5012). There
is
little or
no
difference in the position of
the largest teeth and therefore this feature is not affected
by the depth
of
the maxilla or the shape
of
the antorbital
fenestra. Even the left and right maxilla
of
the same
skull show variation (Figs. 1O.2B,C).
142
The maxilla from the Tornillo Formation lacks
the anterior-most margin and was scaled to the other
maxilla using height and the anterior margin
of
the max-
illary fenestra (Fig.
1O.2A).
The nasal, or dorsal, margin
of
the maxilla arcs sharply down suggesting a face con-
siderably shorter than in
T.
rex. Other differences include
a slightly larger maxillary fenestra in proportion
to
the
size of the maxilla and a much deeper jugal process. The
antorbital fenestra appears
to
be smaller, but the lacrimal
process
is
too incomplete
to
be certain. These differences
are great enough
to
suggest that TMM 41436-1 falls out-
Figure 10.1. A comparison
of
four Tyrannosaurus rex skulls. A, CM 9380 (formerly AMNH 973);
B,
AMNH 5027; C,
LACM 23844; D, TMP 81.6.1; E, SDSM 12047. The reconstruction
of
CM
9380 differs from that given by Osborn (1906)
in the orbital and postorbital regions. Osborn had erroneously used the right ectoptygoid
as
a right postorbital in the recon-
structed skull
at
the Carnegie Museum. As may be seen in the figure, the squamosal no longer extends into the orbit. TMP
81.6.1
is
more complete than shown, but preparation was not complete at the time this figure was made. SDSM 12047 is
crushed dorsolaterally, which has greatly affected the snout, no attempt has been made to compensate for this. (Scale bar =
lOcm.)
Variation in Tyrannosaurus rex
Figure
10.2.
A,
an
overlay
of
maxillae:
CM
9380,
AMNH
5027,
LACM
23844,
TMP
81.6.1,
and
TMM
41436-1.
Teeth
not
shown.
B,
right
(reversed)
and
left
maxillae
ofLACM 23844
and
C,
AMNH
5027.
-----,----
LEFT
a
---·
, .
-
--_/
'!--RIGHT
B
LEFT
C
···---·
-'
--
,,"-RIGHT
143
side the range
of
individual variation for Tyrannosaurus
rex. It may belong to a new genus, but no name should
be
proposed
until
additional
material
is
available.
Because it
is
unusual, it
is
excluded from discussions on
variation.
Variation in the dentary is demonstrated by six
specimens (Fig. 10.3). The distance between the anterior
tip and the highest part
of
the dentary were used for the
standard length. The greatest difference
is
the depth
of
the posteroventral margin
of
the dentary. It is relatively
shallowest
in
SDSM 12047 and deepest in AMNH 5027
(Fig. 10.3). However, AMNH 5027 also has a tooth row
that curves more ventrally than the others and this may
have had an influence on the posteroventral margin. The
posterior margin
of
the dentary is thin and easily dam-
aged, although three
of
the dentaries
have
complete
margins (CM 9380,
AMNH
5027, and
TMP
81.6.1).
The shape
of
the anterior portion of the external mandi-
bular fossa (expressed as a notch at the lower edge
of
the posterior margin, see Fig. 10.1) is variable. The
teeth seem to show more variation in positional size, but
this may be due to how the dentaries were overlain.
Cervicals
Complete cervical series are known for only two
specimens, AMNH 5027 and BM(NH) R7994. There is
a considerable amount
of
variation in the shape
of
the
neural spines. Similar variation appears to be present in
the tyrannosaur Albertosaurus libratus,
so
the use
of
neu-
ral
spine
shape
in
the
diagnosis
of
the
tyrannosaur
Daspletosaurus torosus (Russell 1970)
is
suspect.
The neck is more robust in BM(NH) R7994 than
in AMNH 5027 (Fig. 10.4). This is especially evident in
the atlas and its intercentrum, and in the neural spines
of
cervicals two and three.
Asymmetrical co-ossification
of
the last cervical
and first dorsal in AMNH 5027 has not been observed
in any other specimen, and
is
probably pathological.
Figure
10.3.
An
overlay
of
the
dentaries
of
CM
9380,
AMNH
5027,
BM(NH)
R7994
(formerly
AMNH
5866),
LACM
23844,
SDSM
12047,
and
TMP
81.6.1.
Teeth
not
shown.
Kenneth Carpenter
Ischiae
Three ischia were available for comparison. Length
was standardized and the articular surface
of
the iliac
peduncle was used to determine orientation in Fig. 10.5.
Points
of
variation include the relative size
of
the iliac
peduncle,
the relative
size
and
position
of
the
pubic
Figure 10.4. A comparison of
the
cervicals of
A,
AMNH
5027
and
B,
BM(NH)
R7994. Scale = 10
cm.
Figure 10.5.
An
overlay of
the
ischia of
A,
CM
9380;
B,
TMP
81.61;
and
C,
AMNH
5027.
144
peduncle, the position and size
of
the obturator process,
and
the
development
of
the insertion scar
for
the M.
flexor tibialis internus part 3. This scar is best developed
in
CM
9380 where it forms a prominent ridge.
Of
the two types
of
ischia, one (Fig. 1O.5A) is
oriented
more
ventrally from the horizontal articular
surface
of
the iliac peduncle. I suspect that
CM
9380
and
TMP
81.6.1 are females because the greater angle
between the sacral vertebrae and distal end
of
the ischium
would permit the passage
of
eggs (or live young) more
readily than that
of
AMNH
5027. The
more
divergent
ischia are
associated
with robust skeletons, whereas the
less divergent ischium is from a gracile skeleton.
Conclusions
Variation
and
sexual
dimorphism
in
theropods
has been discussed by Colbert (this volume), Raath (this
volume), and Molnar (this volume). Both Colbert and
Raath noted robust and gracile forms in their mass death
assemblages. The preliminary conclusion that the robust
form
of
Tyrannosaurus rex is female, is similar to that
reached
by
Raath for Syntarsus.
More
specimens and
work are needed to determine when sexual dimorphism
is expressed ontogenetically.
Individual variation in
T.
rex
is
demonstrated by
the maxilla, dentary, and ischium. The importance for
determining the range
of
individual variation in taxon-
omy is illustrated by the maxillary suite. Most
of
them
clustered and overlap showed where variation occurred.
An
isolated maxilla from the Tornillo Formation is dif-
ferent enough from the others to justify its removal from
T.
rex.
Acknowledgments
I would like to thank the following individuals for
access to specimens or data
in
their care:
P.
Bjork (South
Dakota School of Mines),
D.
Berman (Carnegie Museum of
Natural History),
K.
Campbell
(Museum
of Natural History of
Los
Angeles County),
P.
Currie
(Tyrrell
Museum
of Palaeon-
tology),
E.
Gaffney (American Museum of Natural History),
and
Angela Milner [British Museum (Natural History)]. This
paper
is
dedicated
to
the
memory
of
Ken
Sauer,
for
the
hours of
discussions
we
had
on
large
theropods, variation,
and
life.
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... comm.). Instead, ontogenetic variability is better known in other groups of large-sized theropods, such as tyrannosaurids (e.g., Carpenter, 1990;Bakker et al., 1988;Carr, 1999Carr, , 2020Currie, 2003;Carr and Williamson, 2004;Larson, 2008;Witmer and Ridgely, 2010;Fowler et al., 2011;Tsuihiji et al., 2011;Mallon et al., 2019), allosaurids (e.g., Chure and Madsen, 1996;Smith, 1998;Smith et al., 1999;Foth et al., 2016), and carcharodontosaurids (e.g., Canale et al., 2015a). The limited information about patterns of morphological changes that occurred during the ontogeny of abelisaurids complicates comparative studies. ...
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Abelisaurids are among the most abundant and diverse Patagonian Late Cretaceous theropods. Here, we present a new furileusaurian abelisaurid, Llukalkan aliocranianus gen. et sp. nov., represented by cranial remains from the Bajo de la Carpa Formation (Santonian) at La Invernada fossil area, northwestern Patagonia. Features characterizing this taxon include a possible caudal tympanic recess posterior to the columellar recess, a T-shaped lacrimal with jugal ramus lacking a suborbital process, and large foramina for caudal middle cerebral veins widely separated from the median supraoccipital crest. In addition to this, a bulge on the anteromedial border of the supratemporal fossa, tall and posteriorly projected paroccipital processes, basal tubera interconnected distally, a triangular basisphenoid recess, and a single foramen for the sphenoidal artery on the basisphenoid, differentiate Llukalkan from Viavenator exxoni. The latter is the other furileusaurian taxon from the same area and stratigraphic unit. Although the holotype of Llukalkan probably corresponds to a sub-adult—as the lacrimal morphology suggests— the possibility that it represents a juvenile of V. exxoni is discarded based mainly on the presence of a caudal tympanic recess (which is absent in V. exxoni). The probable coexistence of two abelisaurid taxa demonstrates that the abelisaurids were one of the most important—and likely the main—predator component of the ecosystems, not only in this area, but also in all of Patagonia, during the Late Cretaceous.
... In addition, rare fragmentary remains of a large theropod dinosaur, Tyrannosaurus? sp. (Lawson, 1976;Carpenter, 1990) and a hadrosaur, Kritosaurus cf. navajovius have been found (Wagner, 2001). ...
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A bone bed in the middle part of the Javelina Formation (Maastrichtian) in Texas yielded parts of about 37 identifiable ceratopsid dinosaur bones, mostly appendicular and limb girdle elements belonging to one juvenile and two adult individuals of Torosaurus cf. utahensis. The bone bed is a lag assemblage comprising large immobile parts of the skeletons accumulated in an abandoned stream channel. In general form and proportions the postcranial bones are similar to those in Pentaceratops sternbergi and are not as robust as those in Torosaurus latus or Triceratops horridus. A few cranial elements are preserved, including parts of a parietal, squamosal, maxilla, and two dentaries. The form of the parietal fragment is comparable to that of a more nearly complete specimen of Torosaurus cf. utahensis collected nearby at about the same stratigraphic level. The bone bed material provides a basis for the first skeletal reconstruction of this enigmatic horned dinosaur. Most characters used in diagnoses of T. utahensis and T. latus are inadequate. Only the raised bar along the squamosal/parietal suture, present in T. latus; and the midline epiparietal, absent in T. latus, may discriminate the two species.
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Despite reports of sexual dimorphism in extinct taxa, such claims in non-avian dinosaurs have been rare over the last decade and have often been criticized. Since dimorphism is widespread in sexually reproducing organisms today, under-reporting in the literature might suggest either methodological shortcomings or that this diverse group exhibited highly unusual reproductive biology. Univariate significance testing, especially for bimodality, is ineffective and prone to false negatives. Species recognition and mutual sexual selection hypotheses, therefore, may not be required to explain supposed absence of sexual dimorphism across the grade (a type II error). Instead, multiple lines of evidence support sexual selection and variation of structures consistent with secondary sexual characteristics, strongly suggesting sexual dimorphism in non-avian dinosaurs. We propose a framework for studying sexual dimorphism in fossils, focusing on likely secondary sexual traits and testing against all alternate hypotheses for variation in them using multiple lines of evidence. We use effect size statistics appropriate for low sample sizes, rather than significance testing, to analyse potential divergence of growth curves in traits and constrain estimates for dimorphism magnitude. In many cases, estimates of sexual variation can be reasonably accurate, and further developments in methods to improve sex assignments and account for intrasexual variation (e.g. mixture modelling) will improve accuracy. It is better to compare estimates for the magnitude of and support for dimorphism between datasets than to dichotomously reject or fail to reject monomorphism in a single species, enabling the study of sexual selection across phylogenies and time. We defend our approach with simulated and empirical data, including dinosaur data, showing that even simple approaches can yield fairly accurate estimates of sexual variation in many cases, allowing for comparison of species with high and low support for sexual variation.
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Background During the growth of complex multicellular organisms, chronological age, size and morphology change together in a hierarchical and coordinated pattern. Among extinct species, the growth of Tyrannosaurus rex has received repeated attention through quantitative analyses of relative maturity and chronological age. Its growth series shows an extreme transformation from shallow skulls in juveniles to deep skulls in adults along with a reduction in tooth count, and its growth curve shows that T. rex had a high growth rate in contrast to its closest relatives. However, separately, these sets of data provide an incomplete picture of the congruence between age, size, and relative maturity in this exemplar species. The goal of this work is to analyze these data sets together using cladistic analysis to produce a single hypothesis of growth that includes all of the relevant data. Methods The three axes of growth were analyzed together using cladistic analysis, based on a data set of 1,850 morphological characters and 44 specimens. The analysis was run in TNT v.1.5 under a New Technology search followed by a Traditional search. Correlation tests were run in IBM SPSS Statistics v. 24.0.0.0. Results An initial analysis that included all of the specimens recovered 50 multiple most parsimonious ontograms a series of analyses identified 13 wildcard specimens. An analysis run without the wildcard specimens recovered a single most parsimonious tree (i.e., ontogram) of 3,053 steps. The ontogram is composed of 21 growth stages, and all but the first and third are supported by unambiguously optimized synontomorphies. T. rex ontogeny can be divided into five discrete growth categories that are diagnosed by chronological age, morphology, and, in part, size (uninformative among adults). The topology shows that the transition from shallow to deep skull shape occurred between 13 and 15 years of age, and the size of the immediate relatives of T. rex was exceeded between its 15th and 18th years. Although size and maturity are congruent among juveniles and subadults, congruence is not seen among adults; for example, one of the least mature adults (RSM 2523.8) is also the largest and most massive example of the species. The extreme number of changes at the transition between juveniles and subadults shows that the ontogeny of T. rex exhibits secondary metamorphosis, analogous to the abrupt ontogenetic changes that are seen at sexual maturity among teleosts. These results provide a point of comparison for testing the congruence between maturity and chronological age, size, and mass, as well as integrating previous work on functional morphology into a rigorous ontogenetic framework. Comparison of the growth series of T. rex with those of outgroup taxa clarifies the ontogenetic trends that were inherited from the common ancestor of Archosauriformes.
Article
Remains of enigmatic spinosaurs from mid-Cretaceous North African strata have, for over a century, been the subject of taxonomic deliberations. The gigantic Spinosaurus aegyptiacus Stromer, 1915 has gained iconic status in the vertebrate palaeontological community and amongst the general public. Perhaps the largest predatory dinosaur to have lived, this animal exhibits a bizarre range of adaptations consistent with a piscivorous diet and semiaquatic mode of life. Despite its popularity, the systematics of this taxon remains a matter of considerable debate. African spinosaur taxonomy is complex, with up to three separate species proposed for the Cretaceous Kem Kem Group of Morocco: Spinosaurus aegyptiacus, Spinosaurus maroccanus Russell, 1996 and Sigilmassasaurus brevicollis Russell, 1996. Here, the taxonomic status of spinosaurs in the Kem Kem Group is examined, and the morphology of the cervical and dorsal vertebrae re-evaluated in the light of this taxonomic reappraisal. The validity of Spinosaurus maroccanus and Sigilmassasaurus brevicollis are not supported, as all autapomorphies of these taxa are proposed here to be the result of intraspecific variation, or morphological changes through the axial column of a single taxon. Both taxa are junior synonyms of Spinosaurus aegyptiacus. This reanalysis has implications for the taxonomy of spinosaurs from other deposits. Based on the currently available material, the Brazilian spinosaurid Oxalaia quilombensis is determined to fall within the Spinosaurus aegyptiacus hypodigm. The prevalence of spinosaurid heterodonty and limited diagnostic potential of spinosaur teeth necessitates that two spinosaurid tooth taxa: Ostafrikasaurus crassiserratus and Siamosaurus suteethorni, be regarded nomina dubia.
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The theropod family Tyrannosauridae (Dinosauria) is composed of four genera and seven species. All taxa are known from nearly complete skeletons and/ or skulls, thus making it one of the best documented large theropod families. The stratigraphic and palaeobiogeographic distribution of the Tyrannosauridae extends from the lower Campanian to upper Maastrichtian of North America, and to the Campanian-Maastrichtian of Asia.
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A study of the stance and locomotion of Tyrannosaurus was made for the mounting of the partial skeleton at the British Museum (Natural History). This shows that the posture was much more bird-like than is indicated by previous mounts, and also the tail is shorter. During walking the vertebral column was held nearly horizontal with the tail clear of the ground. The fore-limbs acted as struts to stop the body sliding forward as the animal raised its body from the resting position.
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