Exceptional stegosaur integument impressions from the Upper
Jurassic Morrison Formation of Wyoming
Nicolai A. Christiansen •Emanuel Tschopp
Received: 19 October 2009 / Accepted: 14 July 2010
ÓSwiss Geological Society 2010
Abstract Dinosaur skin impressions are rare in the Upper
Jurassic Morrison Formation, but different sites on the Howe
Ranch in Wyoming (USA), comprising specimens from
diplodocid, camarasaurid, allosaurid and stegosaurian
dinosaurs, have proven to be a treasure-trove for these soft-
tissue remains. Here we describe stegosaurian skin impres-
sions from North America for the ﬁrst time, as well as the ﬁrst
case of preservation of an impression of the integument that
covered the dorsal plates of stegosaurian dinosaurs in life.
Both have been found closely associated with bones of a
specimen of the stegosaurian Hesperosaurus mjosi
CARPENTER,MILES and CLOWARD 2001. The scales of the skin
impression of H.mjosi are very similar in shape and
arrangement to those of Gigantspinosaurus sichuanensis
OUYANG 1992, the only other stegosaurian dinosaur from
which skin impressions have been described. Both taxa show
a ground pattern of small polygonal scales, which in some
places is interrupted by larger oval tubercles surrounded by
the small scales, resulting in rosette-like structures. The
respective phylogenetic positions of G.sichuanensis as a
basal stegosaurian and H.mjosi as a derived form suggest
that most stegosaurians had very similar skin structures,
which also match the most common textures known in
dinosaurs. The integumentary impression from the dorsal
plate brings new data to the long-lasting debate concerning
the function of dorsal plates in stegosaurian dinosaurs.
Unlike usual dinosaur skin impressions, the integument
covering the dorsal plates does not show any scale-like
texture. It is smooth with long and parallel, shallow grooves,
a structure that is interpreted as representing a keratinous
covering of the plates. The presence of such a keratinous
covering has affects on all the existing theories concerning
the function of stegosaurian plates, including defense, ther-
moregulation, and display, but does not permit to rule out any
Keywords Hesperosaurus mjosi Stegosauria
Skin impressions Integument Keratinous covering
BRSM Bristol’s City Museum and Art Gallery, England
SMA Sauriermuseum Aathal, Switzerland
ZDM Zigong Dinosaur Museum, China
Reports of skin impressions from the Upper Jurassic Morri-
son Formation are rare, although it represents one of the most
dinosaur fossil-rich Mesozoic strata and despite the fact that it
has been explored intensely for over a century (Dodson et al.
1980; Foster 2003; Turner and Peterson 2004). To our
knowledge, the only published descriptions of dinosaur skin
impressions from the Morrison Formation are associated with
a juvenile allosaurid from the MeilynQuarry in Southeastern
Wyoming (Pinegar et al. 2003), with the skeletons of juvenile
diplodocids from the Mother’s Day Quarry in South Central
Montana (Myers and Storrs 2007), with Barosaurus lentus
Editorial handling: Jean-Paul Billon-Bruyat & Daniel Marty.
N. A. Christiansen (&)
Museu da Lourinha
˜, Rua Joa
˜o Luis de Moura,
Faculdades de Cie
ˆncias e Tecnologia, Universidade Nova de
Lisboa, 2829-516 Monte de Caparica, Portugal
Swiss J Geosci
MARSH 1890 remains from Dinosaur National Monument in
Northwestern Colorado (White 1967), with sauropod foot-
prints from the Bighorn Basin of North Central Wyoming
(Platt and Hasiotis 2006), as well as loose Dinosauria indet.
skin pieces from the Mygatt-Moore Quarry in central western
Colorado (Kirkland and Carpenter 1994).
However, the quarries at the Howe Ranch near Shell,
Wyoming, USA (Fig. 1), are an exception to this rule in
being remarkably productive for integumentary impres-
sions. Sauropod skin impressions from the Howe Ranch
were ﬁrst reported by Brown (1935) after his excavations at
the Howe Quarry in 1934. During their annual ﬁeld cam-
paigns from 1990 to 2003 at the same locality, the
excavation team of the Sauriermuseum Aathal (SMA)
found a large number of dinosaur skin impressions in dif-
ferent quarries on the Howe Ranch. Most of these were
isolated pieces without any connection to bones. However,
some pieces were found associated with the caudal verte-
brae of a diplodocid sauropod, revealing the presence of
triangular integumentary appendages on the dorsal side of
the animal, and thus inﬂuencing the way diplodocid
sauropods are now reconstructed (Czerkas 1992). Other
skin impressions found by the SMA include skin impres-
sions from various parts of a Camarasaurus COPE 1877
(Tschopp 2008), as well as from an Allosaurus MARSH 1877
(which could not be recovered during excavation; H.-J.
Siber, pers. comm. 2009) and stegosaurian skin impres-
sions (described herein).
Reports of skin impressions from Thyreophora are very
rare. As far as we know, the only thyreophoran specimens
exhibiting such preservation are: Gigantspinosaurus sichu-
anensis ZDM 0019 (Xing et al. 2008), cf. Scelidosaurus
OWEN 1860 (BRSMG CF2781; Martill et al. 2000), an
unpublished juvenile specimen cf. Scelidosaurus sp. (BRSM
12785), an unpublished ﬁnd of a cf. Stegosaurus MARSH 1877
from Bone Cabin Quarry West in Southeastern Wyoming
(H.-J. Siber, pers. comm. 2009; K. Carpenter, pers. comm.
2010), and the present specimen of Hesperosaurus mjosi.
Other thyreophoran skin impressions have been found in
connection with tracks of both stegosaurians (Mateus et al.
2010) and ankylosaurs (McCrea et al. 2001). Furthermore,
osteoderms with a scale-like shape have been reported in a
number of ankylosaurs (e.g., Penkalski 2001) and in Stego-
saurus stenops MARSH 1887 (e.g., Gilmore 1914), but these
do not represent the surface of the skin, although studies
based on extant archosaurs have shown that theyare generally
overlaid by a scale of similar dimensions and morphology
(Vickaryous and Sire 2009).
Geographical and geological context
The specimen with skin impressions (SMA 0018, nick-
named ‘‘Victoria’’) was discovered in 1995 on the Howe
Ranch, approximately 15 km north of Shell in central
northern Wyoming, USA (Fig. 1). It was found at the
Howe-Stephens Quarry (44°390N, 107°490W), 450 m
southwest of the historic Howe Quarry, which was exca-
vated during the Sinclair Dinosaur Expedition of the
American Museum of Natural History in 1934 (Brown
1935; Ayer 2000).
The quarry is located in the middle part of the Upper Jurassic
Morrison Formation below the so-called clay change. Like
the historic Howe Quarry, which is approximately 10 m
lower in the stratigraphical column, the Howe-Stephens
Quarry is among the geologically oldest fossil sites known
from the Morrison Formation (Turner and Peterson 1999;
Foster 2003; Ikejiri 2005; Schwarz et al. 2007).
Sedimentology, palaeoenvironment and taphonomy
The Howe-Stephens Quarry has a horizontal extension of
18 924 m, and a maximal vertical extension of approxi-
mately 8 m (H.-J. Siber, pers. comm. 2003; pers. obs.
2005). The sediments are ﬁne-grained ﬂuvial sandstone
exhibiting cross-bedding in some layers (Schwarz et al.
2007; J. Ayer, pers. comm. 2007). The quarry is an
exception within the Morrison Formation as it contains
abundant plant material, including two very large siliciﬁed
tree logs, plenty of carbonized plant fragments, as well as
numerous carbonized branches (Ayer 2000; pers. obs.
2003). Most of the specimens are preserved in an area of
10 912 m, in a layer that is only 1 m thick, and it has
been hypothesized that the tree logs blocked the dinosaur
carcasses during an interval of heavy ﬂooding and thereby
allowed for a rapid burial of the specimens (Ayer 2000).
Fig. 1 Location of the Howe Ranch quarries (star) in central
northern Wyoming (USA). Scale bar 100 km
N. A. Christiansen, E. Tschopp
The latter is generally thought to be crucial for the pres-
ervation of articulated and almost complete skeletons, as
well as skin impressions.
The reported specimen of Hesperosaurus mjosi was
found lying on its right side (Fig. 2) and the preservation of
that side of the specimen is far superior to that of the left side,
as can be seen from the differential preservation of the mani,
pes and ribcage (Siber and Mo
¨ckli 2009). There was also
variation in the degree of articulation within the skeleton,
with some parts found in full articulation, whereas other
parts were found disarticulated and some lighter elements
were missing (Fig. 2). This seems to indicate that the spec-
imen was only partly buried in a ﬁrst time, while other parts
were still exposed to the elements and that the complete
burial of the specimen only happened in a second instant.
Skin impressions where only found on the specimens under-
side, which is though to have been buried more rapidly.
The dinosaur faunal assemblage found in the Howe-
Stephens Quarry includes all of the most common dino-
saurs of the Morrison Formation. Besides Hesperosaurus
mjosi, it includes a virtually complete articulated skeleton
of Camarasaurus sp. (Tschopp 2008), an articulated post-
cranial skeleton of a juvenile diplodocid (Schwarz et al.
2007), an almost complete skeleton of Allosaurus sp., a
partial skeleton of Othnielosaurus GALTON 2006 and sev-
eral subadult to adult specimens of cf. Diplodocus MARSH
1878 (H.-J. Siber, pers. comm. 2003; pers. obs. 2005).
The reported specimen (SMA 0018, ‘‘Victoria’’) is a well-
preserved skeleton including the skull and most of the post-
cranial skeleton (Fig. 2). SMA 0018 is identiﬁed as
Hesperosaurus mjosi, based on the transverse processes on
anterior caudal vertebrae projecting ventrally rather than
laterally, the postzygapophyses on posterior cervical ver-
tebrae being elongated posteriorly and overhanging the
back of the centrum, the rectangular acromial process of
the scapula, the neural arches of the dorsal vertebrae are
not elongated above the neural canal, the cervical dermal
plates that are longer anteroposteriorly than tall dorsoven-
trally, and the neural spines of the proximal caudal
vertebrae that are enlarged transversely, but not bifurcated
(Carpenter et al. 2001; Maidment et al. 2008; Carpenter
2010). Maidment et al. (2008) also referred the specimen to
Hesperosaurus mjosi, but quoted a wrong institutional
number (SMA V03 instead of SMA 0018).
Maidment et al. (2008) questioned the erection of the
genus Hesperosaurus by Carpenter et al. (2001), arguing
that H.mjosi is congeneric with Stegosaurus armatus
MARSH 1877, based on a high degree of similarity between
the two taxa. However, in the same paper, Maidment et al.
(2008) report seven differences in character states between
S.armatus and H.mjosi and three autapomorphies for
H. mjosi. In our opinion, ten distinguishable differences in
morphology are too numerous to consider the two taxa
congeneric, even though they are sister groups in recent
phylogenetic analyses (Maidment et al. 2008; Mateus et al.
2009). We therefore consider Hesperosaurus to be a valid
genus, in agreement with Carpenter (2010).
Preservation of integument impressions
Integumentary impressions are preserved in several areas
on the underside of SMA 0018 (Fig. 2). Several large
pieces were found during preparation of a ﬁeld jacket
containing the anterior right side of the rib cage (Fig. 3).
Some of them have been preserved in situ, while others
have been removed in order to see the bones (Siber and
Fig. 2 Quarry map of
Hesperosaurus mjosi (SMA
0018). The areas where
integument impressions have
been found are marked in grey.
Drawing by Esther Premru,
modiﬁed from Siber and Mo
(2009). Scale bar 1m
Exceptional stegosaur integument impressions
¨ckli 2009). Another smaller piece was found attached to
a dorsal plate lying under the left ribs. Its preservation as a
negative imprint reveals that it is from the skin covering
the distal part of the left side ribs, which became dislocated
onto the plate during the decomposition of the animal
(Fig. 2). Below this skin impression, a second integument
impression was found, this time pertaining to the structure
that covered the dorsal plate.
The integument impressions are preserved in three dif-
ferent modes: (1) as cross-sections at various angles, (2) as
natural molds, and (3) as natural casts. The natural casts
have the appearance of fossil skin, but they are made up of
sedimentary rock and have no inner structure that could
justify their identiﬁcation as a permineralization of the
actual skin. We therefore interpret them as inﬁlls of natural
molds. The skin impressions are preserved in all the above-
mentioned manners, whereas the plate impressions only
exist as natural molds.
A thin and friable, dark brown to black, layer is found
intercalated between the natural mold and the natural cast,
both on the skin and the plate impressions. Such a dark
layer is associated with various integument impression
found in the Howe Ranch quarries (Brown 1935; Czerkas
1992; Tschopp 2008; pers. obs. 2007), but has never been
observed in association with the bones or isolated within
the sediment (pers. obs. 2007). Both Brown (1935) and
Czerkas (1992) interpreted the layer as the possible
remains of the actual epidermis, but did not study it
thoroughly. However, different ﬁnds of similar thin layers
covering dinosaur integumentary impressions have been
interpreted as microbial mats (e.g., Keller 1992; Briggs
et al. 1997) or as a result of authigenic mineralization, in
which the replication of the tissue morphology in form of a
layer of minerals is the product of decay bacteria (Briggs
2003). Identifying the origin of the dark layer through
microstructural and chemical studies is not the scope of this
paper, but would be a crucial step in understanding the
process that created the fossil and in perceiving the exact
nature of the structures that we are observing.
The skin impressions mainly consist of small, non-imbri-
cating, polygonal (predominantly hexagonal) tuberculate
scales, which make up the ground pattern of the integument
on the anterior part of the rib cage. Their diameters range
from 2 to 7 mm with the majority ranging from 4 to 5 mm
in diameter. The single tubercles are closely spaced, with
shallow and narrow grooves separating them. A relatively
ordered, nearly linear pattern of the scales can be seen on
the ventral part of the ribcage (Fig. 4), but on its dorsal
part, this pattern is lost and the distribution of the scales is
A much rarer and larger second type of scale is present
towards the dorsal side of the animal. These are higher,
domed, ﬁve to ten times larger than the small scales, and
ellipsoid in outline (Fig. 5). Each of them is surrounded by
9–13 small tubercles, which are not distinguishable from
the scales forming the ground pattern. Only two such
structures can be conﬁdently identiﬁed, and thus a speciﬁc
arrangement of these is therefore not discernable. The
larger of the two scales is 20 915 mm, whereas the other
Fig. 3 View of the anterior part of the right side of the ribcage and
the posterior end of the right scapula, showing the position of the skin
impressions. Rectangles mark impressions of small polygonal scales
forming the ground pattern; circles mark rosettes. Scale bar 10 cm
Fig. 4 Skin impression from the ventral part of the body showing a
nearly linear arrangement of the small polygonal scales of the ground
pattern. Preserved as a natural mold. Scale bar 2cm
N. A. Christiansen, E. Tschopp
is 10 98 mm. The surface of both types of scales does not
appear to be sculptured.
Plate cover impression
The plate cover impression is preserved only as a natural
mold. The single largest preserved surface measures
approximately 200 cm
. Together with the other pieces, it
leaves little doubt that most of the upper part of the dorsal
plate was covered by this structure. The plate cover
impression is uniformly ﬂat and smooth with low parallel
ridges running ventrodorsally (Fig. 6). The length of the
ridges is typically 2–3 cm and although they have variable
heights, none of them exceed 0.5 mm. In some areas, the
ridges are as close as 2 mm, whereas they are usually more
separated from each other. No scaly structures are visible.
Given that the plate cover impression is a mold, the ridges
would in reality have been shallow grooves on the surface
of the integument.
Comparison and discussion
Scales of the ground pattern
The small polygonal scales that make up the ground pattern
of the skin impressions in Hesperosaurus mjosi are com-
mon to all known ornithischian skin impressions and are
usually pentagonal or hexagonal (Czerkas 1997). The size
of these scales varies among different taxa, but in many
taxa the small scales building up the ground pattern are
5–7 mm in diameter like those in stegosaurians. A notable
variation from this is the ceratopsian Chasmosaurus belli
LAMBE 1914 (Sternberg 1925; Czerkas 1997) that has much
larger scales with a diameter of approximately 20–25 mm.
Fig. 5 Skin impressions from the dorsal part of the body showing a
rosette. Preserved as a natural mold. Scale bar 1cm
Fig. 6 Detail of the impression of the presumed keratinous covering
of the plate, showing low parallel ridges extending from the upper left
to the lower right corner. Note the remains of the dark layer that cover
the impression in the central lower part of the picture. Preserved as a
natural cast. Scale bar 1cm
Exceptional stegosaur integument impressions
Few skin impressions have been attributed to saurischian
dinosaurs, but these ﬁnds indicate a larger variety of shapes
of the scales that make up the ground pattern of the skin in
this clade. In addition to the classic polygonal pattern
known from brachiosaurids (Hooley 1917) and diplodocids
(Czerkas 1992; Mateus and Mila
`n2009), a ground pattern
of round scales has been reported in both sauropods
(Powell 1980; Chiappe et al. 1998) and theropods (Bona-
parte et al. 1990; Gatesy et al. 2005). Furthermore, linked
sub-rectangular scales forming lines (Chiappe et al. 1998),
as well as large, 20–30 mm broad, polygonal scales
forming a ground pattern in tracks, have also been reported
in sauropods (e.g., Mateus and Mila
Comparison of the skin structures of Hesperosaurus
mjosi (SMA 0018) to the one described from Gigantspi-
nosaurus sichuanensis ZDM 0019 (Xing et al. 2008)
indicates only minor differences. The small polygonal
scales exhibit maximum diameters of about 5.2–7 mm in
the H.mjosi (SMA 0018), whereas Xing et al. (2008) report
5.7–9.2 mm scale diameters in G.sichuanensis (ZDM
0019). The grooves separating the single scales appear to
be broader in some areas in G.sichuanensis (Xing et al.
2008: ﬁg. 2), but this could be the result of local stretching
of the skin, since other areas do not show this difference
(Xing et al. 2008: ﬁg. 1).
The second structure that can be recognized in the skin
impression of Hesperosaurus mjosi are isolated larger
ellipsoid scales surrounded by the small polygonal scales of
the ground pattern, thus forming rosettes. Rosettes con-
sisting of 13–14 smaller scales surrounding a larger one
have also been reported in Gigantspinosaurus sichuanensis
(Xing et al. 2008). This kind of skin pattern is widely dis-
tributed among dinosaurs (e.g., Ornithopoda: Brown 1916;
Ceratopsia: Sternberg 1925; Theropoda: Bonaparte et al.
1990; and Sauropoda: Chiappe et al. 1998), and can
therefore—together with the ground pattern of small, non-
imbricating scales—be hypothesized to represent the ple-
siomorphic state of the structure of the dinosaurian
epidermis. As with the size of the small scales, the dimen-
sion of the larger ones varies among different taxa.
However, the size ratio between the small scales of the
ground pattern and the large scales of the rosettes remains
relatively constant, and in all the dinosaurian taxa for which
rosettes are known, approximately 10–20 smaller scales
pertaining to the ground pattern surround the large scales.
Due to the restricted areas of the skin impression sur-
faces that have been preserved in both H.mjosi and
G.sichuanensis, a speciﬁc arrangement of the rosettes in
stegosaurians cannot be described. However, the two
rosettes identiﬁed in SMA 0018 are found at a similar
height on the ribcage, and it is possible that they were
arranged in irregular longitudinal rows as can be seen in
Carnotaurus sastrei BONAPARTE, 1985 (Bonaparte et al.
1990), Corythosaurus casuarius BROWN 1914 (Brown
1916), Chasmosaurus belli (Sternberg 1925), and Styrac-
osaurus albertensis LAMBE 1913 (Brown 1917). In H. mjosi
(SMA 0018), the rosettes are found on the dorsal part of the
ribcage, and the same is the case for G. sichuanensis (ZDM
0019; Xing et al. 2008), whereas the skin impressions
found on the lower part of the ribcage only contain small
polygonal scales. This distribution is not unique to stego-
saurians, but is according to Czerkas (1997) and Sternberg
(1925) a general pattern found in many dinosaurs. Stern-
berg (1925) noted that rosettes are more common and
larger on the dorsal region of the body of ceratopsian
dinosaurs and decrease in size and frequency ventrally,
with the belly skin being composed only of the small
polygonal scales of the ground pattern.
Integumentary covering of the dorsal plates
Dinosaur skin impression typically consists of tuberculate
scales. Although exceptions are known, no impressions of
skin structures comparable to the impressions found on the
dorsal plates of SMA 0018 have been reported in any
dinosaur. A scale-less skin impression is known from
Pelecanimimus polyodon PEREZ-MORENO,SANZ,BUSCALIONI,
MORATALLA,ORTEGA,&RASSKIN-GUTMAN 1994 (Briggs
et al. 1997), but it shows cross-hatching lines similar to
mammalian skin, rather than long, straight, and parallel
grooves (Fig. 6). Also, the uniformly smooth structure, the
lack of folds or waves and the thin straight grooves in the
integument covering the plates of SMA 0018 seem to
indicate a ﬂat covering with little or no plasticity.
Although we do not have direct proof that this integu-
ment was a keratinous covering, there are indirect lines of
evidence, since a keratinous covering is the only hard
integument that is known to cover such a large surface in
vertebrates and that the neurovascular sulci on the surface
of the bone are highly indicative of a keratinous covering
(Hieronymus et al. 2009).
The function of the dorsal plates in stegosaurians
Previous works have almost exclusively concentrated on
the genus Stegosaurus, but the dorsal plates of Hespero-
saurus have a morphology and distribution that is so
similar to those of Stegosaurus that hypotheses on their
function are likely valid for both genera. These theories
include the use of dorsal plates for display (Carpenter
1998; Main et al. 2005; Hayashi et al. 2009), thermoreg-
ulation (Farlow et al. 1976; Buffre
´nil et al. 1986; Farlow
et al. 2010) and defence (Marsh 1877; Gilmore 1914;
N. A. Christiansen, E. Tschopp
Bakker 1986, Mallison 2010). The fact that the plates had a
keratinous covering has implications on all these hypoth-
eses, and must therefore be considered when evaluating
The function of stegosaurian dorsal plates as defensive
structures has been rejected by several authors, based on
their internal trabecular structure, which is not considered
crush resistant, and would therefore make a defensive use
´nil et al. 1986; Main et al. 2005). The
histological study of Buffre
´nil et al. (1986) also did not ﬁnd
an asymmetrical distribution of Sharpey’s ﬁbres within the
plates that would have indicated the possibility of moving
the plates to a recumbent and therefore more protective
position, as suggested by Bakker (1986). However, the
keratinous covering could have increased their strength
considerably, depending on its thickness, since beta-kera-
tin, which makes up scales, claws, and other corniﬁed
structures in reptiles, is considered to be one of the stron-
gest natural proteins (Baden et al. 1974). The keratinous
covering could also have created much sharper edges on
the plates than what is seen on the osteoderms and thereby
increase their value as defensive structures, even in an
upright position. Although the protection of the upright
plates might not have been needed over the spinal cord in
the pelvic and thoracic regions as pointed out by Bakker
(1986), it would have been important protection for one of
the most exposed regions of the animal, the neck. The
cervical plates would have greatly increased the height of
the neck making a lateral attack more difﬁcult and could
have been an effective defence against a dorsal attack on
the neck, which a reported cervical plate with a possible
bite mark (Carpenter et al. 2005) might prove.
The theory that the dorsal plates of stegosaurians were used
for inter- or intraspeciﬁc display is the most recognized
theory amongst recent authors (Carpenter 1998; Main et al.
2005; Hayashi et al. 2009). The main evidence supporting
this hypothesis are the size of the plates and their
asymmetrical positioning in at least Stegosaurus and
Hesperosaurus, which increases the outline of the animal
considerably. The fact that the plates had a keratinous
covering corroborates this theory, since it is likely to have
increased the surface area of the osteoderms signiﬁcantly,
as seen on horns and claws, thereby enhancing the visual
effect in lateral view. Moreover, keratinous covering in
reptiles and birds, like the beaks of birds, are often col-
oured and interpreted as being used for display (e.g.,
Tattersall et al. 2009).
A thermoregulatory function for the plates has been dis-
cussed extensively (e.g., Farlow et al. 1976; Buffre
´nil et al.
1986; Main et al. 2005; Hayashi et al. 2009; Farlow et al.
2010), with recent studies accepting a possible exaptation
of the plates for this use, although they do not consider it
the main function (Main et al. 2005; Hayashi et al. 2009;
Farlow et al. 2010). As noted by Farlow et al. (1976), a
thermoregulatory use would not be optimized through the
existence of a keratinous covering. The keratinous covering
would reduce the effectiveness of the heat transfer between
the blood and the environment, to an extent that would
depend on its thickness. Moreover, if the use of forced
convection is hypothesized (Farlow et al. 1976), the larger
the surface is, the more efﬁcient is the heat transfer. Thus,
an irregular and pitted surface would be more optimal than
the ﬂat surface present in the integument impressions of
H.mjosi. It must, however, be noted that a thermoregula-
tory function cannot be excluded by the existence of a
keratinous covering, since keratin covered organs have
been shown to have a thermoregulatory function in the case
of bovid horns (Picard et al. 1996,1999; Hoefs 2000)
and bird beaks (ducks: Hagan and Heath 1980; toucans:
Tattersall et al. 2009).
A ground pattern of small, polygonal, non-imbricating
scales and rosettes with larger oval scales at their centre is
observed in both Hesperosaurus mjosi and Gigantspino-
saurus sichuanensis. Given that recent phylogenetic
analyses (Maidment et al. 2008; Mateus et al. 2009) have
shown G.sichuanensis to be one of the most basal and
H.mjosi to be one of the most derived taxa within Steg-
osauria, a similar epidermal structure can be inferred for
other stegosaurians by means of phylogenetic bracketing.
This pattern exhibiting small non-imbricating scales and
rosettes is also hypothesized to be the plesiomorphic state
of the dinosaurian integument, due to its existence in all of
the major higher-level dinosaurian taxa.
The dorsal plates of stegosaurians are shown to have had
a keratinous covering over most of their surface. This
discovery has implications for all the hypotheses that have
been put forward concerning the function of the plates, but
none of them can be rejected on this ground. However,
future knowledge concerning the thickness of the kerati-
nous covering might help to discriminate amongst them, as
a thick covering may favour a defensive use, whereas a thin
covering may favour a thermoregulatory function. Finally,
multiple functions of stegosaurian dorsal plates should not
be excluded nor the possibility that the importance of
Exceptional stegosaur integument impressions
different functions could have varied amongst different
Acknowledgments We thank Hans-Jakob Siber (Sauriermuseum
Aathal) for having invited us and encouraged our study on the
stegosaurian integument impressions of the SMA. We are also
grateful to the ﬁeld team of SMA for having found and preserved the
integument impressions, notably to the preparators Ben Pabst and
Esther Premru. The manuscript beneﬁted greatly from the comments
and suggestions of Octa
´vio Mateus (Universidade Nova de Lisboa;
Museu da Lourinha
˜, Portugal), Rui Castanhinha (Instituto Gulbenkian
ˆncia; Museu da Lourinha
˜, Portugal), James Farlow (Indiana
University, Fort Wayne, Indiana, USA), and Stephen Brusatte
(American Museum of Natural History, New York, USA) who kindly
reviewed earlier versions of the manuscript, as well as of those of the
two reviewers Matt Vickaryous (Department of Biomedical Sciences,
University of Guelph, Ontario, Canada) and Kenneth Carpenter
(College of Eastern Utah Prehistoric Museum, Price, Utah, USA). We
are grateful to James Farlow, Shoji Hayashi, Octa
´vio Mateus, Jesper
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Exceptional stegosaur integument impressions