HAYASHI, S. et al., 2011. Ontogenetic Histology of Stegosaurus Plates and Spikes. Palaeontology, pp....

ONTOGENETIC HISTOLOGY OF STEGOSAURUS

PLATES AND SPIKES

by SHOJI HAYASHI1 , KENNETH CARPENTER2 , MAHITO WATABE3 and

LORRIE A. MCWHINNEY4

1Steinmann Institute Division of Paleontology, University of Bonn, 53115 Bonn, Germany; e-mail: [email protected] Museum, 155 East Main Street, Price, UT 84501, USA; e-mail: [email protected] for Paleobiological Research, Hayashibara Biochemical Laboratories, Inc., Okayama 700-0907, Japan; e-mail: [email protected] of Earth Science, Denver Museum Nature and Science, 2001 Colorado Boulevard, Denver, CO 80205, USA; e-mail: [email protected]

Typescript received 17 November 2010; accepted in revised form 17 June 2011

Abstract: The dinosaur Stegosaurus is characterized by

osteoderms of alternating plates and terminal paired spikes.

Previous studies have described the histological features

and possible functions of these osteoderms. However,

ontogenetic changes are poorly documented. In this study,

the ontogenetic changes of the osteoderms are examined

using eight different ontogenetic skeletons (a juvenile, a

subadult, a young adult, and five old adults based on the

cortical histology of their body skeletons). The juvenile

plate and subadult spike show thin cortex and thick can-

cellous bone. The young adult plates have an extensive

vascular network, which is also seen in old adults. Old

adult spikes are different from old adult plates in having a

thick cortex and a large axial channel. The cortical histol-

ogy, in both plates and spikes, show well-vascularized bone

tissue consisting of dense mineralized fibres in young adult

forms. In old adult forms, the bone tissues in the spikes

become more compact and are extensively remodelled. This

might contribute to the structural reinforcement of the

spikes. The plates in old adult forms also show extensive

remodelling and lines of arrested growth, but only limited

signs of compaction. The timing for acquisition of features

seen in old adults is different between plates (an extensive

vascular network in the young adult) and spikes (a thick

cortex with a large axial channel in old adults). The result

suggests that the timing for plate and spike functions is

different. The extensive vascular networks seen in large

plates suggest their function is for display and ⁄ or thermo-

regulation. The thick cortical bone of spikes of old adults

suggests that spikes acquire a weapon function for defence

ontogenetically late.

Key words: Stegosaurus, osteoderms, bone histology,

growth, function, Upper Jurassic, western USA.

T he possible function of plate and spike-shaped osteo-

derms in Stegosaurus has had a long and colourful his-

tory. Based on the position on the body and external

morphology, the plates have been assumed to play a pas-

sive role in defence, whereas the spikes have long been

assumed to be offensive weapons (e.g. Marsh 1877; Lull

1910; Gilmore 1914; Carpenter 1998; McWhinney et al.

2001; Carpenter et al. 2005).

More recently, external and internal morphology have

been used to infer the function of plates for thermal regu-

lation or display (Farlow et al. 1976; de Buffrenil et al.

1986; Main et al. 2005; Hayashi et al. 2009; Farlow et al.

2010). The use of plates in species recognition has been

proposed based on the absence of any phylogenetic trends

in the shape of plates and spikes within Stegosauria

(Main et al. 2005). Plate function has focused mainly on

the presence of large pipe-like vascular canals on both

internal and external surfaces (Farlow et al. 1976; de Buff-

renil et al. 1986; Main et al. 2005; Farlow et al. 2010). In

contrast, it has been assumed that the spikes were used as

a weapon based on the conical morphology and thick

cortical bone (e.g. McWhinney et al. 2001; Carpenter

et al. 2005). A damaged Allosaurus vertebra supported

this hypothesis (Carpenter et al. 2005). Previously, how-

ever, de Buffrenil et al. (1986) argued against a weapon

function because the spike they examined had a cancel-

lous internal structure. All of these studies have been

based on small sample sizes and have not focused on the

ontogenetic changes of plates and spikes. Therefore, the

conflicting interpretations may be due to individual

and ⁄ or ontogenetic variations.

This study approaches the argument on the function of

plates and spikes of Stegosaurus by examining multiple

individuals at different ontogenetic stages. Additionally,

to elucidate the function of the large pipe-like vascular

grooves on the plates, developmental patterns of the vas-

culature are examined in detail. These comparisons will

allow this study to test the cause of the conflicting

[Palaeontology, 2011, pp. 1–17]

ª The Palaeontological Association doi: 10.1111/j.1475-4983.2011.01122.x 1

differences in the previous studies and should lead us to a

better understanding of the functions of the Stegosaurus

osteoderms.

Institutional abbreviations. DMNH, Denver Museum of Nature

and Science, Denver, CO, USA; NSM-PV, National Science

Museum, Tokyo, Japan; HMNS, Hayashibara Museum of Natu-

ral Sciences, Okayama, Japan; UMNH, Utah Museum of Natural

History, Salt Lake City, UT, USA; YPM, Yale Peabody Museum,

Yale University, New Haven, CT, USA.

Other abbreviation. CT, Computed tomography; EFS, external

fundamental system; LAG, line of arrested growth.

MATERIALS AND METHODS

Materials. This study used eight ontogenetically different

Stegosaurus individuals from the Upper Jurassic Morrison

Formation (Table 1). Seven plates and eight spikes from

six skeletons (DMNH 33359, 1483, 2818, YPM 4634,

NSM-PV 20380 and HMNS 14) were examined using CT

scans. Eight plates and five spikes from eight individuals

(DMNH 33359, 1483, 2818, HMNS 14, NSM-PV 20380,

YPM 1853, 4634 and UMNH-VP 13688) were studied

using thin sections and ⁄ or polished sections. The position

of sampled plates and spikes was summarized in Table 1.

The taxonomy of these specimens followed Carpenter

et al. (2001) and Maidment et al. (2008). Most specimens

were previously identified as S.armatus (DMNH 1483,

2818 and YPM 1856; Maidment et al. 2008) or Hespero-

saurus (or Stegosaurus) mjosi (HMNS 14 (HMNH 001 in

previous studies); Carpenter et al. 2001; Maidment et al.

2008). A skeleton of NSM-PV 20380 was identified as

S. armatus because this specimen possesses all diagnostic

characters of S. armatus proposed in Maidment et al.

(2008). The characters are as follows: edentulous portion

of the dentary, anterior to the tooth row and posterior to

the predentary; dorsally elevated postzygapophyses of cer-

vical vertebrae; bifurcated summits of neural spines of the

anterior and middle caudal vertebrae; unexpanded poster-

ior end of the pubis; and the presence of dermal ossicles

embedded in the skin on the underside of the cervical

region (Maidment et al. 2008). Some specimens could

not be identified beyond the level of genus (DMNH

33359, YPM 4634 and UMNH-VP 13688) because they

bear no characters that allow referral to either Stegosaurus

species. Given present uncertainty about the specific and

even generic taxonomy of some of these specimens (Car-

penter et al. 2001; Maidment et al. 2008; Carpenter 2010),

we report the current names without expressing an opin-

ion about their validity.

Five Stegosaurus individuals (DMNH 33359, 1483,

NSM-PV 20380, YPM 4634 and 1856) are from the Salt

Wash Member of the Upper Jurassic Morrison Forma-

tion, Wyoming, USA, while two individuals (DMNH

2818 and UMNH-VP 13688) were collected from the

Brushy Basin Member of the Upper Jurassic Morrison

Formation, Colorado and Utah, USA (see also Carpenter

1998 for DMNH 2818). An individual of Hesperosaurus

(Stegosaurus) mjosi (HMNS 14) was from the Windy Hill

Member of the Upper Jurassic Morrison Formation,

Wyoming, USA (see Carpenter et al. 2001). Localities and

stratigraphic position of the Morrison Formation for

these materials were also noted in Table 1. More strati-

TABLE 1 . Sampled bones and the femur length (mm) of Stegosaurus.

Specimen Taxa Femur

length

(mm)

Observed element Ontogenetic

stage

Locality Stratigraphic

units in

Morrison Fm.

DMNH 33359 Stegosaurus sp. 233* Dorsal plate (1) Juvenile BQ SW

YPM 4634 S. sp. 487 Fore spike (2) Subadult Q13W SW

NSM-PV 20380 S. armatus 734 Dorsal plate (1), caudal plate (1),

fore spike (1), hind spike (1)

Young adult BQ SW

DMNH 1483 S. armatus 950 Caudal plate (1), hind spike (1) Old adult Q13W SW

DMNH 2818 S. armatus 1048 Caudal plate (1), hind spike (1) Old adult GP BM

HMNS 14 S. (Hesperosaurus)

mjosi

NA Plate (position unknown) (1),

cervical plate(1), dorsal plate (1)

Old adult SR WH

YPM 1856 S. armatus NA Fore spike (1), hind spike (1)

dorsal plate (1)

Old adult Q13W SW

UMNH-VP13688 S. sp. NA Plate (position unknown) (1) Old adult CLQ BM

Sample number of scanned and ⁄ or sectioned materials was shown in parentheses. The determination of ontogenetic stages was based

on histological features of body elements (see Hayashi et al. 2009). NA, not applicable; BM, Brushy Basin Member; BQ, Bone Cabin

Quarry; CLQ, Cleveland-Lloyd Quarry; GP, Garden Park; SR, S. B. Smith Ranch, Johnson Country; SW, Salt Wash Member; Q13W,

Reed’s Quarry 13; WH, Windy Hill Member (see Turner and Peterson 1999 and Carpenter et al. 2001 for all of these localities).

*Inferred limb measurements from the length of fibula.

2 P A L A E O N T O L O G Y

graphic data are summarized by Turner and Peterson

(1999) and Carpenter et al. (2001).

Preparations for CT scans and sections. The Stegosaurus

plates and spikes were scanned using a medical helical CT

scanner (CT-W2000; Hitachi Medical Corporation, 1 mm

slice thickness, 120 kV, 175 mA) at the National Institute

of Advanced Industrial Science and Technology, Tsukuba,

Japan. These data revealed that some specimens (a plate

and a spike of NSM-PV 20380) necessitated the use of a

high-resolution helical CT scanner to obtain optimal data

for the vasculature. The CT scanning was performed at

TESCO Corporation, Tokyo, Japan (BIR ACTIS + 3; TE-

SCO Corporation, 200 lm slice thickness, 180 kV,

0.11 mA). The raw scanned data of all specimens were

reconstructed using a bone algorithm. Data were output

from the scanners in DICOM format and then imported

into VG-Studio Max (Volume Graphics) for viewing,

analysis and visualization of vascular networks in plates

and spikes. All CT data, regardless of source, were analy-

sed on 64-bit PC workstations with 4 GB of RAM and

Intel HD Graphics video card and running Microsoft

Windows 7.

To directly examine the internal structures of plates

and spikes, thin sections and ⁄ or sections were made

based on the methodology outlined in Chinsamy and

Raath (1992) and Sander (2000). Because this is a

destructive method, all specimens were first photo-

graphed, had their morphological variations recorded,

and standard measurements taken before sectioning. In

addition, the juvenile plate was moulded and cast before

sectioning. Thin sections, except for a juvenile plate

(DMNH 33359) and fragmentary plates (HMNS 14 and

UMNH-VP 13688), were taken at standardized locations

for each element. The thin sections were taken at the

base, middle and apex because the histology of the plate

varies from the base to the apex (de Buffrenil et al.

1986). This study uses the terms ‘base’ for the most

proximal region of an osteoderm and ‘apex’ for the most

distal part (or apical region) of an osteoderm. ‘Middle’ is

used for the mid-region between the base and apex. After

cutting, both sides of the slice were impregnated with a

synthetic epoxy resin (Petropoxy 154; Maruto Co, Japan),

finely ground and polished to a high gloss. Entire images

of the thin sections were photographed with a digital

film scanner (Canon Pixus Mp 800). Furthermore, the

thin sections were observed by two standard light micro-

scopes (normal transmitted light and transmitted polar-

ized light) with a Leica DMLP and Nikon Optiphot2-pol

microscopes. Microscopic photographs were taken with a

Nikon Coolpix 5000 and Olympus C-5050 Zoom.

Nomenclature and definitions of bone microstructures

are based on Francillon-Vieillot et al. (1990) and Casta-

net et al. (1993).

Determination of ontogenetic stages. The determination of

ontogenetic stages was based on the cortical histology of

body elements as previously discussed by Hayashi et al.

(2009) and Redelstorff and Sander (2009). The results of

Hayashi et al. (2009) and Redelstorff and Sander (2009)

are mostly consistent. Each ontogenetic stage in Hayashi

et al. (2009) is characterized by the following features:

(1) juvenile – a fibro-lamellar tissue with a radial and ⁄ or

reticular vascular network; (2) subadult – fibro-lamellar

tissue with a longitudinal vascular network; (3) young

adult – with lines of arrested growth (LAGs), and (4)

old adult – with an external fundamental system (EFS),

which is tightly packed growth lines that develop

throughout the skeleton as full adult size is attained (Er-

ickson 2005; Erickson et al. 2007). Ontogenetic stages of

most specimens in this study were previously identified

by Hayashi et al. (2009), who classified them into a

juvenile (DMNH 33359), a young adult (NSM-PV

20380) and old adults (DMNH 1483, 2818, UMNH-VP

13693 and YPM 1856). In addition, thin sections were

taken from the femur mid-shaft of YPM 4634 and a rib

mid-shaft for HMNS 14 to determine their ontogenetic

stage. Based on Hayashi et al. (2009), these were deter-

mined to be a subadult (YPM 4634) and an old adult

(HMNS 14).

RESULTS

We start our discussion of the osteoderm histology of

Stegosaurus armatus and Stegosaurus sp. with the descrip-

tion of samples of DMNH 33359, 1483, 2818, NSM-PV

20380, YPM 1853, 4634 and UMNH-VP 13688. Hespero-

saurus (or Stegosaurus) mjosi (HMNS 14) will be dis-

cussed later because it is possibly a different genus (see

Carpenter et al. 2001; Carpenter 2010).

Stegosaurus armatus and Stegosaurus sp.

Plate. Seven plates from a juvenile (DMNH 33359), a

young adult (NSM-PV 20380) and four old adult individ-

uals (DMNH 1483, 2818, UMNH-VP 13688 and YPM

1853) were examined. Each ontogenetic character in his-

tology is described here.

Juvenile. A plate was examined from a juvenile individual

(DMNH 33359), which is the smallest dorsal plate known

to us. The specimen was examined using the medical CT

scanner and thin section. The plate shows the general

plate morphology of Stegosaurus (e.g. Gilmore 1914),

which is transversely flat (Fig. 1A). The surface is uni-

formly flat with some large vascular grooves and pits. The

base is covered in rugosities and pits.

H A Y A S H I E T A L . : P L A T E A N D S P I K E G R O W T H O F S T E G O S A U R U S 3

The CT images and thin section show that the internal

structure consists of only thin cortex and thick cancellous

bone (Fig. 1B–C). The plate lacks ‘pipe-like extensive

large canals’ previously reported in the basal regions of

other plates (de Buffrenil et al. 1986; Main et al. 2005;

Farlow et al. 2010). However, the basal region shows a

more cancellous structure than those of other regions

(Fig. 1B–C). In particularly, the trabeculae are thin and

the cavities between the thin trabeculae are relatively large

in the region where one would expect to find pipe-like

canals in older individuals. The pits on the external sur-

face invade the inside of the plate through nutrient

foramina and connect pores between trabeculae.

The cortical bone consists of well-vascularized tissue

composed of dense ossified collagen fibres (Fig. 1D–G).

Previously, many studies of extant and fossil tetrapods

have suggested that many osteoderms form through

metaplastic ossification, a process in which a pre-existing

fully developed tissue is transformed into bone (Haines

and Mohuiddin 1968). Metaplastic tissue in fossils has

been identified in osteoderms by the presence of interwo-

ven bundles of mineralized collagen fibres (Scheyer and

Sander 2004, 2007; Main et al. 2005; Witzmann and

Soler-Gijon 2008; Scheyer 2009; Cerda and Desojo 2010;

Cerda and Powell 2010). Therefore, we refer to the pri-

mary cortical bone of the juvenile plate as metaplastic

bone. Radial and ⁄ or reticular vascular canals are present

in the cortex (Fig. 1D–G). Many simple primary vascular

canals without surrounding bone lamellae are seen, but

primary osteons are rare in the cortex. There are numer-

ous large osteocyte lacunae, and the outermost vascular

canals open to the surface. Secondary bone tissue is very

rare in the cortex. There are no lines of arrested growth

(LAGs). The cancellous bone comprises mostly primary

bone tissue with many large resorption cavities (Fig. 1E–

G). The resorption cavities exhibit scalloped surfaces and

many Howship’s lacunae, indicating osteoclast activity in

this region (see also Fig. 2L). The primary bone tissue

shows simple primary vascular canals and some primary

osteons embedded in a bone tissue consisting of mineral-

ized collagen fibres. Secondary reconstructions are present

in the cancellous region, but are still not extensively

developed. There is no histological variation from the

base to the apex of the plate except for the orientation

and size of the mineralized collagen fibres. While they are

short and mainly perpendicular to the bone surface in

A

C

B D

E

F

G

F IG . 1 . A plate of a juvenile Stegosaurus (DMNH 33359). A, photograph in lateral view. A plane of sectioning is marked by a dashed

line. B, vertical thin section. C, detail of the basal bone tissue. Note absence of pipe-like large vascular canals in this plate. D–G,

cortical bone tissue of the plate. D–E, detail of the cortex and inner trabecular bone at the apical regions of the plate in normal (D)

and polarized (E) light. F–G, detail of the cortex and inner trabecular bone at the basal regions of the plate in normal (F) and

polarized (G) light. The bone tissues, both in cortical and in cancellous bone, consist of well-vascularized tissue composed of dense

ossified collagen fibres with radial and ⁄ or reticular vascularity. Notably, there are many large resorption cavities in the cancellous

bone. There are no LAGs. Bone surface is to the left and right in D–E and is to the right in F–G.


most places, in the middle and basal regions, there are

large fibre bundles (Sharpey’s fibre in de Buffrenil et al.

1986). These are also directed at high angles to the bone

surface and apparently act as an anchor on the plate in

the skin (de Buffrenil et al. 1986; Scheyer and Sander

2004).

A

F I

G J L

H K

B

C

D

E

F IG . 2 . Young adult plate (NSM-PV 20380). A, photograph of a plate in lateral view. B–E, transverse thin sections of the plate. The

internal structure is composed of thin cortex and thick cancellous bone. Note presence of pipe-like large vascular canals in the base

(E). The sectioning is marked by dashed lines in A. F–L, Cortical bone tissue of the plate. F–G, detail of the cortex and inner

trabecular bone at the apical regions of the plate in normal (F) and polarized (G) light. H, detailed view of the matrix of cortical bone

in figure G in polarized light. Notably, the matrix consists of many ossified fibres. I–J, detailed view of the cortex and inner trabecular

bone at the basal regions of the plate in normal (I) and polarized (J) light. Cortical histology of young adult plates still retains many

primary bone tissues without secondary reconstructions. Many large resorption cavities are present in the cancellous bone. K, detailed

view of the cortical bone at the basal side in normal light. There are many large fibre bundles that apparently served to anchor the

plate in the skin. L, detailed view of the resorption cavities of K in polarized light. There are many Howship’s lacunae, indicating

osteoclast activity. Bone surface is to the left in F–G and is to the top in H–L.


Young adult. Two different-sized plates (a large dorsal

plate situated above the pelvis and a small plate from the

distal caudal region) were sectioned from a skeleton

(NSM-PV 20380). The external surfaces are characterized

by large vascular grooves, pits, frayed texture at the apex

and rugose roughing bone with multiple pits at the base

(Fig. 2A).

Despite their gross size differences, the CT images and

thin sections show no obvious variations in internal

structure between plates. The plates show typical plate

structures, which were previously reported (de Buffrenil

et al. 1986; Main et al. 2005; Farlow et al. 2010). These

are composed of thin cortex and thick cancellous bone,

but significantly differ from a juvenile plate (DMNH

33359) in the presence of large, extensive, pipe-like canals

in the cancellous bone (Figs 2E, 5). These large canals

have previously been interpreted as vascular canals based

on the similarity of vascular canals in living alligator os-

teoderms (Seidel 1979; Farlow et al. 2010). Most of the

thin sections show that these canals outlined by trabecu-

lae pinch out in the middle region as previously reported

in Main et al. (2005). However, a thin section and 3D

analyses using a high-resolution CT scanner show that

the vascular canals connect to the vascular grooves of the

external surface, making extensive vascular networks

(Fig. 3A, B; see also Farlow et al. 2010). The large vascu-

lar canals invade the inside of the plate through pits of

the basal region and connect with the external vascular

grooves through a small number of pits of the middle

region (Fig. 3A).

These cortical bone tissues show well-vascularized bone

that consists almost entirely of coarse fibres oriented in

many directions, suggesting metaplastic bone (Fig. 2F–K).

Histological variations are present in the vascular canal’s

shape and in the degree of remodelling between a large

dorsal plate and small caudal plate. The large dorsal plate

exhibits more reticular vascular canals and less remodel-

ling than those of the small caudal plate. The simple

vascular canals and ⁄ or primary osteons grade into large

resorption cavities from the cortex to cancellous bone.

LAGs are absent. Many osteocyte lacunae are present in

the matrix and are aligned with fibre bundles. Most of

the vascular canals are simple canals, but some primary

osteons are present. The outermost vascular canals are

still open to the surface. Notably, their cortical bone tis-

sues differ from a juvenile plate (DMNH 33359) in show-

ing variations in the vascular canal shape in a single plate

(Fig. 2F, I). In the basal region of plates, vascular canals

A B

F IG . 3 . A, Pattern of inferred pipe-like extensive vascular network in a plate from NSM-PV 20380, a young adult individual of

Stegosaurus armatus. The digital images were created by microfocus CT scanning, with the pipe-like vascular canals, and spaces within

cancellous bone, indicated in black. The sequence of images begins near the sagittal interior, and successive images are progressively

closer to the plate surface, ending with a view of the surface itself. The arrangement of isolated black spaces in the cancellous bone

with respect to the pipes suggests that at least some of the isolated black spaces represent sections through small tributary blood

vessels. The box contains an enlarged image of a portion of the plate surface, showing that some of the internal vascular canals

connect with vascular grooves and foramina on the plate surface. Such connections between interior and surface features of the

inferred vascular system occur on both sides of this plate. B, Portion of Stegosaurus armatus dorsal plate of YPM 1856, showing an

internal pipe-like large vascular canal connecting with a superficial vascular groove.


show a reticular configuration at the inner cortex, but

they change to the longitudinal arrangement at the outer-

most cortex (Fig. 2I–J). In apical regions, most of the vas-

cular canals exhibit a longitudinal arrangement (Fig. 2F–

G). Remodelling occurs at the deepest part of the cortical

bone, although not extensively. Also, the degree of

remodelling in the cancellous bone is different in the sin-

gle plates. While secondary reconstructions are extensively

developed at the middle and upper regions, they are seen

in only a few regions at the basal region. Particularly,

there are many large resorption cavities in the cancellous

bone. Howship’s lacunae are seen in many resorption cav-

ities (Fig. 2L). Numerous osteocyte cell lacunae, which

are aligned with fibre bundles, are present in the cancel-

lous bone matrix. Remodelling is not extensive in the

cancellous bone. Large fibre bundles, known as Sharpey’s

fibres, act as anchors between the plate and skin (de Buff-

renil et al. 1986); these are extensively developed in the

basal to middle region of the plates (Fig. 2J–K). The large

fibre bundles are present in cortical regions, but are lack-

ing in the inner cancellous bone.

Old adult. Four plates were sampled from four individu-

als (DMNH 1483, 2818, YPM 1853 and UMNH-VP

13688). Two isolated plates (YPM 1853 and UMNH-VP

13688) show the multiple growth marks and extensive

remodelling seen in the old adult specimens of this study

and were therefore classified as potentially old adult indi-

viduals. All specimens were examined using thin sections.

One plate (DMNH 1483) was examined using the helical

medical CT scanner as well.

The external surfaces of all plates exhibit similar fea-

tures with other ontogenetic plates, which are large vascu-

lar grooves, pits, a frayed texture at the tip, and rough,

rugose bone with multiple pits at the base. The CT

images and thin sections show that all plates consist of

thin cortex and thick cancellous bone with large pipe-like

vascular canals (Fig. 4B–D). Large vascular canals are

connected to grooves on the external surface and make

extensive vascular networks (Fig. 3B, 5). Cortical thick-

nesses increase from those of young adult (NSM-PV

20380) and juvenile plates (DMNH 33359), but are dis-

tinctly thinner than those of spikes. Cortical bone tissue,

from all plates, consists of almost entirely, coarsely ossi-

fied, collagen fibres, except where it has been replaced in

the cortex by secondary osteons (Fig. 4F). The cancellous

bone in all plates shows extensive remodelling. Notably,

some histological variations in cortical bones are present

among old adult plates.

The cortical bone tissue of a caudal plate from the larg-

est adult individual (DMNH 2818) comprises dense min-

eralized fibres with longitudinal simple vascular canals,

few LAGs and well-developed large collagen fibre bundles

in the basal region. Some primary osteons are seen in the

cortex, but simple vascular canals dominate in this sam-

ple. Secondary reconstructions occur in the inner cortex

and cancellous region (Fig. 4E–F).

A thin section from a large plate fragment from the

middle or apical region (UMNH VP13688) was previ-

ously described as UDSH C-LQ 085 by Reid (1990). The

cortex consists of dense ossified collagen fibres. Multiple

LAGs with decreasing spacing towards the outer surface

are also seen. Notably, the peripheral bone tissue com-

prises a zone of mostly avascular tissue with tightly

packed LAGs, which is known as an external fundamental

system (EFS) in the ossified tendons and long bones of

dinosaurs (e.g. Adams and Organs 2005; Sander et al.

2006). The simple longitudinal vascular canals dominate

in the cortex (Fig. 4G). Remodelling is moderately devel-

oped at the perimedullary to the middle cortex.

The histology of another isolated plate (YPM 1856)

was described by de Buffrenil et al. (1986). The cortex

histology varies from the base to apex of the plate. In the

basal region, the histology consists of dense ossified colla-

gen fibres with strong remodelling towards the interior of

the plate, whereas in the middle region, remodelling is

more extensive towards the basal and apical parts. The

apical region has a similar histology to UMNH VP13688,

that is, bone tissue consisting of dense mineralized colla-

gen fibre bundles with many LAGs.

The cortical bone tissue of a caudal plate (DMNH

1483) is characterized by strong remodelling (Fig. 4H, I).

The tissues are completely or almost completely replaced

by secondary osteons, sometimes by three or fourth gen-

erations from the basal and apical parts of the plate. Sec-

ondary osteons can be seen crosscutting each other. Some

Sharpey’s fibres are present in the outermost cortex, but

most of the fibres are eliminated by secondary reconstruc-

tions. Due to the strong remodelling, it was impossible to

detect the original tissue.

Spike

Six spikes from a subadult (YPM 4634), a young adult

(NSM-PV 20380), and two old adult individuals (DMNH

1483 and 2818) were examined. Each ontogenetic charac-

ter in histology was described here.

Subadult. Two fore spikes were examined from a suba-

dult individual (YPM 4634). All specimens were exam-

ined using the medical CT scanner and sections.

Unfortunately, the cortical bone tissues could not be

observed in this study because thin sections could not be

taken from YPM 4634. The spikes from a subadult indi-

vidual (YPM 4634), previously described by Galton

(1982), were examined for this study. These spikes have

sharp, compressed, anterior and posterior margins with


A

B

C

D

E G H

F I

B

C

D

F IG . 4 . Sections of old adult Stegosaurs plates (YPM 1856, DMNH 2818, UMNH-VP 13688 and DMNH 1483). A, sketch of YPM

1856 plate in lateral view (modified from de Buffrenil et al. 1986). B–D, sections of A. B, section of the apical part. C, section of the

middle part. D, section of the basal part. Pipe-like large vascular canals are seen from the basal to middle part (C, D). The sectioning

is marked by dashed lines in A. E–I, cortical bone tissue of plates (DMNH 2818, UMNH-VP 13688 and DMNH 1483). E, detail of the

cortex of a plate from Stegosaurus armatus (DMNH 2818) in normal light. F, detail of the matrix of cortical bone in figure E in

polarized light. Notably, the bone tissue consists of many ossified fibres. G, detail of the cortex of a plate from Stegosaurus sp.

(UMNH-VP 13688) in polarized light. H–I, detail of the cortex of a plate from Stegosaurus armatus (DMNH 1483) at the middle part

(H) and the basal part (I) in polarized light. The cortical bone tissue of a caudal plate (DMNH 1483) is characterized by strong

remodelling. Bone histologies of old adult plates are characterized by extensive remodelling, multiple LAGs, and EFS. Bone surface is

to the top in E–G and is to the right in H–I.


asymmetrical bases (Fig. 6A). Vascular grooves are present

on the external surface of the distal part but are absent

elsewhere. The base is pierced by large pits and lacks the

rugose roughening structure seen in old adult spikes.

The CT images and sections of the spikes show cancel-

lous internal structures similar to a spike examined in a

previous study (de Buffrenil et al. 1986) and are com-

posed of thin cortex with thick cancellous bone (Fig. 6B,

C). Spikes show small canals at the middle of the shaft,

but lack a large axial channel within the core of the can-

cellous bone (sensu Acharjyo and Bubenik 1983; a large

medullary cavity in McWhinney et al. 2001), and also lack

the thick cortical bone previously reported in other spikes

(McWhinney et al. 2001). The small canals of the mid-

shaft change into cancellous bone at the proximal and

distal regions. They connect pits and apical grooves on

the external surface, although these do not make extensive

networks such as those of adult plates.

Young adult. A fore spike and a hind spike were sampled

from a young adult individual (NSM-PV 20380). The

external morphology of young adult spikes is similar to

that of subadult spikes (YPM 4634): oblique bases with

large pits; few vascular grooves; and sharp, compressed,

anterior and posterior margins (Fig. 7A). The basal mor-

phology of the fore spikes shows a blunter angle than that

of the hind spikes. The young adult spikes show cancel-

lous internal structures like those of the subadult spikes

(YPM 4634). These are composed of a thin cortical bone

and a thick cancellous bone (Fig. 7B–E). The small canals

occur in a hind spike, but are absent in a fore spike.

Histologically, spike cortical bone tissues are similar to

those of a small caudal plate. They exhibit metaplastic bone

composed of dense ossified collagen fibres with a combina-

tion of both radial and reticular vascularity at the basal side

(Fig. 7J–M) and longitudinal vascularity at the apical side

(Fig. 7H–I). The fibres are distributed throughout the

whole spike (Fig. 7F–G). Large fibre bundles (Sharpey’s

fibre in de Buffrenil et al. 1986) are present in the mid- to

basal regions (Fig. 7M). These are directed at high angles to

the bone surface and apparently served to anchor the spikes

in the skin. LAGs do not occur in the cortex. Secondary

reconstruction is not extensive throughout the cortex, but

is extensive in cancellous bone. Osteocytes are aligned with

fibre bundles, which are abundant in the cortex.

Old adult. Two hind spikes were observed from two adult

individuals (DMNH 1483 and DMNH 2818). The exter-

nal morphology of old adult spikes is characterized by

having a more conical shape than those of younger indi-

viduals (Fig. 8A–E). A few pits, and large, longitudinal,

vascular grooves mark the external surfaces.

A

A B A B

A B

A B

A BA B

B

F IG . 5 . Comparison of internal structures of Stegosaurs plates in different ontogenetic stages (DMNH 33359, NSM-PV 20380 and

DMNH 1483). Pipe-like extensive large vascular networks are present from the young adult (NSM-PV 20380) to the old adult

(DMNH 1483). A, vertical cross sections. B, parasagittal cross sections. Black dashed lines indicate cutting planes. Upper row of the

figure, CT images. Lower row of the table, interpretative drawing of large vascular canals in plates. Vascular canals, which connect

with foramina and ⁄ or grooves on the plate surface, indicated by black in the drawings. Indeterminate features, either vascular canals

or cracks, indicated by dashed lines. Broken parts in plates indicated by diagonal lines. Subadult plate was not examined in this study.


All sampled spikes are similar in having thick cortical

bone with a large axial channel (Fig. 8C–E), but distinctly

differ from those of subadult and young adult spikes

(YPM 4634 and NSM-PV 20380) in the absence of thick

cancellous bone. The cancellous bone is more compact

than that of younger spikes because trabeculae are thicker

and the resorption cavities are smaller than those of suba-

dult and young adult spikes (Fig. 8C–E). CT analysis of

spikes detected the presence of some large vascular canals

in the basal region. These canals invade through pits on

the basal surface and then connect with a large axial

channel and some external grooves. However, there are

no indications of pipe-like extensive vascular canals in the

spikes.

The histology of these spikes is identical to that of old

adult plates and is characterized by bone tissue consisting

of dense ossified collagen fibres with longitudinal simple

vascular canals, multiple LAGs and extensive remodelling

(Fig. 8F–I). Extensively developed large collagen fibre

bundles are seen in the basal region. Some primary ost-

eons are observed in the cortex, but simple vascular

canals dominate in these spikes. The cancellous bone is

completely remodelled.

Hesperosaurus (or Stegosaurus in Maidment et al. 2008)

mjosi

Three plates and two spikes were examined using a CT

scanner and a thin section from a holotype of Hespero-

saurus mjosi (HMNS 14; Fig. 9).

Plate. A small plate from the proximal cervical region, a

large plate situated above the dorsal region and a frag-

mentary sample from an apical region of a possible dor-

sal plate were examined. All plates were examined using

a CT scan, but only a fragmentary plate was observed

using thin sections. Plate morphologies differ between

Hesperosaurus (or Stegosaurus) mjosi (HMNS 14) and

Stegosaurus armatus (DMNH 1483, 2818 and YPM

1856). The plate length of Hesperosaurus (or S.) mjosi is

greater than the height, whereas the length of S. armatus

is less than the height (see more descriptions in Carpen-

ter et al. 2001). The external surfaces of these plates

show large vascular grooves, pits, a frayed texture at the

tip, and rough, rugose bone with multiple pits at the

base.

A

B

C

F IG . 6 . Spike section of a subadult

Stegosaurus (YPM 4634). The plane of

sectioning marked by a dashed line. A,

photograph of the fore spike in dorsal

view. B, the transverse section of spike

showing the cancellous internal

structure. C, interpretative drawing of B.

The cortex in the sections is very thin.

Dark grey indicates the regions covered

with resin.


There are no obvious histological variations between

Hesperosaurus mjosi (HMNS 14) and Stegosaurus armatus

(DMNH 1483 and 2818), nor between the different sizes

of plates in a single specimen of Hesperosaurus (HMNS

14). All plates consist of thin compact bone and thick

cancellous bone. Extensive pipe-like large vascular canals

are present in all plates (Fig. 9D). The cortical bone his-

tology in a fragmentary plate shows metaplastic bone

A

B

C

D

E

B D

F

C

H

I

J M

F

G

K

L

E

F IG . 7 . Young adult Stegosaurs spike (NSM-PV 20380). A, photograph of a spike in dorsal view. B–E, transverse thin sections of the

spike. The internal structure is composed of thin cortex and thick cancellous bone without large vascular canals. The sectioning is

marked by dashed lines in A. F–M, cortical bone tissue of the spike. F, the cortex at the middle region (D) of the spike in polarized

light. G, detailed view of the matrix of cortical bone in figure F in polarized light. Notably, the matrix consists of many ossified fibres.

H, detail of the cortex at the apical regions of the spike in normal (H) and polarized (I) light. J–M, detailed view of the cortex at the

basal regions of the spike in normal (J, L) and polarized (K, M) light. Cortical histology of young adult spikes is similar with that of

the plate. Spike shows well-vascularized bone tissue consisting almost entirely of coarse fibres. Large fibre bundles are present at the

basal side (M). Bone surface is to the left in F and is the top in G–M. NA in E indicates reconstructed regions.


consisting of dense ossified collagen fibres, with multiple

growth marks. EFS and few simple vascular canals are

present in the bone tissue (Fig. 9I–J). Remodelling is

extensively developed from the inner to mid-cortex. The

cancellous bone is completely remodelled. Sharpey’s fibres

are not seen in this section. This is possible because the

sample was taken from the apex region of a plate.

Spike. A fore spike and a hind spike were examined using

a CT scanner. Unfortunately, the histological features in

the cortical bones could not be observed because thin sec-

tions could not be taken from these specimens. The CT

images show that all spikes from Hesperosaurus mjosi

consists of a thick cortical bone with a large axial channel

(Fig. 9F–H). The cancellous bone has small cavities

between the trabeculae. These are proportionally small and

are seen in the basal and apex side. These results are con-

sistent with the old adult spikes of Stegosaurus armatus.

Summary of ontogenetic trends in the structure and

histology

Examination of osteoderms from different aged individu-

als of Stegosaurus shows ontogenetic trends in the struc-

ture and the histology through life history. The cortical

A

F H

G I

B C

D

D

E

C

E

F IG . 8 . Sections of old adult Stegosaurs

spikes (DMNH 1483 and 2818). A,

photograph of DMNH 1483 spike in

dorsal view. B, photograph of DMNH

2818 spike in dorsal view. C, E, sections

of A. D, a section of B. The sectioning is

marked by dashed lines in A and B. The

internal structure of the spikes of

DMNH 1483 and 2818 is distinctly

compact and shows a large axial channel

(D). F–I, cortical bone tissue of the

spikes. F–G, detailed view of the cortex

of a spike (DMNH 2818) in normal (F)

and polarized (G) light. H–I, detailed

view of the cortex of a spike (DMNH

1483) in normal (H) and polarized (I)

light. The cortical bone histology shows

multiple LAGs (F–G) and extensive

secondary reconstruction (H–I). Bone

surface is to the top in F–I.


bone histology in both plates and spikes, throughout the

ontogeny, is comprised of dense ossified collagen fibres,

suggesting metaplastic bone. Histological features in each

ontogenetic stage are summarized in Table 2.

In the plate, the ontogenetic change from juvenile

(DMNH 33359) to young adult (NSM-PV 20380) is the

acquisition of the extensive pipe-like large vascular net-

works (Fig. 5). The cortical bone histology in the plate

shows a progressive increase in remodelling, a number of

LAGs, the eventual presence of an EFS and the type of

vascular canals through ontogeny. The juvenile plate

shows reticular vascular canals and only a little remodel-

ling in that cancellous bone. LAGs are absent. In the

young adult plates, vascularity is less extensive than in a

juvenile plate, and longitudinal vascularization dominates

the external cortex. LAGs are still absent. The old adult

plates show the development of LAGs throughout the

cortex, which consists of the metaplastic bone having lon-

gitudinal vascular canals. Spacing between LAGs dimin-

ishes towards the outer surface of the bone. Notably, old

plates are characterized by an EFS in the outermost cortex

and the extensive development of dense secondary tissue.

These tissues have tightly packed LAGs in a lamellar

and ⁄ or parallel fibre bone matrix, indicating attainment

of maximum size of the plate (e.g. Sander 2000; Adams

and Organ 2005; Erickson 2005).

The ontogenetic change of the spikes occurs in the

increasing thickness of cortical bone and trabeculae in the

cancellous bone, and acquisition of a large axial channel

from the young adult (NSM-PV 20380; Fig. 7) to old

A B

C

D

F

G

H

E

I J

F IG . 9 . A plate and spike from old

adult Hesperosaurus mjosi (HMNS 14).

A, photograph of a dorsal plate in lateral

view. B–D, transverse CT images of the

plate. Note presence of pipe-like large

vascular canals in the base (D). The

sectioning is marked by dashed lines in

A. E, photograph of an anterior spike in

dorsal view. F–H, transverse CT images

of the spike. The internal structure of

the spike shows a large axial channel (G,

H). The sectioning is marked by dashed

lines in E. I–J, cortical bone tissue of the

dorsal plate in normal (I) and polarized

(J) light. The cortical bone histology is

characterized by extensive remodelling

and EFS. Bone surface is to the top in

I–J.


adult forms (DMNH 1483 and 2818; Fig. 8). The bone

tissue lacks LAGs in the young adult stage. In old adults,

the bone tissues exhibit multiple LAGs with extensive

remodelling, indicating a decrease in growth rate (e.g.

Sander 2000; Erickson et al. 2007). The vasculature

changes from reticular to longitudinal shape, and vascu-

larization decreases in old adult spikes.

Surprisingly, there are no histological differences in

plates and spikes between Stegosaurus armatus and

Hesperosaurus mjosi, despite differences in the external

morphology of the plates. Histological features of Hesp-

erosaurus osteoderms are identical with those of old adult

Stegosaurus armatus.

DISCUSSION AND CONCLUSIONS

We could not mention whether our growth series of

Stegosaurus represents a single species because three speci-

mens (DMNH 33359, YPM 4634 and UMNH-VP 13688)

could not be identified beyond the level of genus. Addi-

tionally, Hesperosaurus (Stegosaurus) mjosi specimens are

found at the same locality as specimens indentified as

Stegosaurus armatus (Maidment et al. 2008; Siber and

Mockli 2009). However, at least at the stage that is obser-

vable for Hesperosaurus, their histology of plates and

spikes are very similar with those of Stegosaurus armatus.

This might suggest that growth patterns do not vary very

much between these taxa. Therefore, our growth series

would capture important growth changes of Stegosaurus

osteoderms even if it consists of different species.

Development implication of Stegosaurus osteoderms

Previously, Main et al. (2005) suggested that the develop-

ment of Stegosaurus osteoderms may be mainly a meta-

plastic bone process because of the fibrous nature in

Stegosaurus plates. Our observations of the plates and

spikes we studied are largely concordant with their

results. All ontogenetic stages of Stegosaurus osteoderms

show a cortical bone tissue consisting of dense ossified

collagen fibres. Also, plates and spikes from juvenile and

young adult individuals show the bone tissue of dense

ossified fibres both in the cortical and in the cancellous

bone. These results suggest that the main developmental

formation of Stegosaurus osteoderms is metaplastic bone

process, and plates and spikes are mostly formed from

the direct mineralization of an existing dense connective

fibre network in their skin. Additionally, the cancellous

bone in plates and spikes from a juvenile and young adult

comprises many large resorption cavities. This suggests

that osteoclasts produce many resorption cavities in the

inner region of the plates and spikes and then finally

make the cancellous region.

Furthermore, de Buffrenil et al. (1986) proposed a

growth model of a Stegosaurus plate in which growth rate

differs in different regions of a single plate. The base of

the plate was where the highest rates of growth took

place, and because more secondary reworking took place

in the apical regions, the uppermost regions were progres-

sively older ontogenetically than the basal regions. How-

ever, bone continued to be deposited periosteally both on

the outer surfaces of the plate and at its base (de Buffrenil

et al. 1986). In cortical histology, plates and spikes from

our young adult and old adults show growth rate differ-

ences in different regions of a single plate, as in the plate

described by de Buffrenil et al. (1986). While the bases

are well vascularized and have less remodelled bone tissue

with radial or reticular vascular canals, the upper regions

have a more remodelled cortex with longitudinal vascular

canals. On the other hand, our juvenile specimen, which is

possibly less than 1 year old, based on the long bone histol-

TABLE 2 . Histological features of the Stegosaurus osteoderms through ontogeny.

Ontogenetic

stage

Specimen Structure

features of plate

Structure

features of spike

Cortical bone tissue

(plate and spike)

Remodelling

(plate and spike)

Juvenile DMNH 33359 Thin CO and

thick CB

NA Metaplastic bone

and reticular VC

Only a little remodelling

at cancellous bone

Subadult YPM 4634 NA Thin CO and thick CB NA NA

Young adult NSM-PV 20380 Thin CO, thick

CB and Pipes

Thin CO and thick CB Metaplastic bone

Longitudinal and

reticular VC

Remodelling only in

perimedullary region.

Old adult DMNH 1483, 2818,

HMNS 14, YPM

1856 and

UMNH-VP 13688

Thin CO, thick

CB and Pipes

Thick CO, thin CB

and an large axial

channel

Metaplastic bone

LAGs, EFS and

longitudinal VC

Remodelling from

perimedullary region

to mid-cortex or

throughout cortex

The histological features of Stegosaurus osteoderms sampled include structures, cortical bone tissue type, shape of vascularization and

degree of secondary reconstruction. CB, cancellous bone; CO, compact bone; NA, not available; Pipes, pipe-like large vascular canals;

VC, vascular canal.


ogy (see Hayashi et al. 2009), shows no indication of

growth rate difference in the plate. Therefore, our results

suggest that Stegosaurus plates and spikes grew constantly

in all regions of a single plate and spike during an early

ontogenetic stage, after which the basal region continued to

show a fast growth rate, accompanied by a decline in the

growth rate in the upper region. This differential growth

may reflect positive allometric growth of the plates.

Histological variations are present among a single indi-

vidual’s dorsal large plate, caudal small plate and spikes

(NSM-PV 20380). The large dorsal plate exhibits more

reticular vascular canals and less remodelling than those of

the small caudal plate and spikes. Generally, the growth

rate of reticular vascular canals is relatively higher than that

of longitudinal vascular canals in living animals and other

dinosaurs (e.g. Castanet et al. 2000; Sander 2000). There-

fore, this result probably suggests that the relative growth

rate of dorsal large plates is higher than small plates and

spikes. This differential growth may reflect the size differ-

ences of plates in a single individual of Stegosaurus.

Functional implication for Stegosaurus plates and spikes

The plates and spikes we examined show different timing

of development of structural features seen in old adults.

While plates acquire pipe-like large canals in the young

adult, spikes acquire a thick cortex with a large axial

channel in old adults. These results may suggest that the

acquisition timing for the respective functions of the

plates and spikes might be different.

The extensive pipe-like large canals in plates are likely

large vascular canals because of the connectivity with

nutrient foramina and vascular grooves on the plate sur-

face and the structural similarity of large vascular canals

of living alligator osteoderms (detailed discussion in Far-

low et al. 2010). Main et al. (2005) reported that the

pipe-like large vascular canals are not connected with the

outside of the plate. However, our observations on the

pipe-like vascular canals using a high-resolution CT scan

and thin sections revealed that these vascular canals are

apparently connected with the outside of the plate and

make extensive vascular networks (see also Farlow et al.

2010). Because of the complexity in the network of pipe-

like vascular canals, the networks might not have been

detected in the previous study. The large pipe-like vascu-

lar canals of plates might facilitate the creation of large

plates because a large amount of nourishment from these

vascular canals makes a high growth rate possible. Similar

vascularization is seen in antlers of extant deer, permit-

ting high growth rates (Goss 1983). Having large plates

would be useful for enhancing the size of the animal

towards rivals and ⁄ or mates as suggested by Carpenter

(1998). This hypothesis is also supported by the cortical

histologies of plates, which show that relatively fast

growth rates continue long after sexual maturity is

reached (Hayashi et al. 2009). The extensive pipe-like vas-

cular networks are acquired when the large body size of

Stegosaurus is attained (see also Farlow et al. 2010).

Therefore, in addition, the acquisition of the extensive

vascular networks in the young adult might allow plates

to have a secondary thermoregulatory function like that

of the extant Toucan bill (Tattersall et al. 2009), elephant

ears (Phillips and Heath 1992) and possibly alligator os-

teoderms (Farlow et al. 2010). Hence, it is probable that

plates may acquire a display and ⁄ or possible thermo-

regulation function in the young adult form.

The acquisition of thick cortical bone in old adult

spikes probably strengthens the osteoderms. This suggests

that spikes might be used as weapons for defence, but not

until late in ontogeny. Ontogeny may also explain the

apparent contradiction between previous studies (de Buff-

renil et al. 1986 vs. McWhinney et al. 2001 and Carpenter

et al. 2005). Spikes retain small canals in the mid-shaft in

the subadult form, which become a large axial channel.

This developmental process is also seen in some living

tropical deer antlers (Acharjyo and Bubenik 1983). The

enlarged axial channel might provide more nutrients for

spikes enabling them to become larger, similar to the

function of the pipe-like extensive vascular networks in

plates. Bone tissue in both plates and spikes show well-

vascularized bone tissue consisting of dense mineralized

fibres in a young adult. In old adult forms, the bone tis-

sues in the spikes become more compact and are exten-

sively remodelled. Also, they are less vascularized. This

might contribute to structural reinforcement of the

spikes. On the other hand, the plates in old adults also

show extensive remodelling and LAGs, but only limited

signs of compaction.

Soft tissue implication of Stegosaurus osteoderms

Plates and spikes of Stegosaurus and Hesperosaurus show

prominent vascular grooves and obliquely oriented vascu-

lar foramina on bone surfaces (e.g. Fig. 3). Most of speci-

mens also show basal sulcus that describes a saddle-

shaped curve at the base (e.g. Figs 1, 9; see also Main

et al. 2005, fig. 3). Their cortical bone tissues are charac-

terized by dense concentrations of metaplastically ossified

fibres. These features are similar with those of ceratopsian

horns (Hieronymus et al. 2009) and, based on an integu-

ment study of living animals (Hieronymus et al. 2009),

may suggest that plates and spikes were covered by a ker-

atinous sheath. Main et al. (2005) and Christansen and

Tschopp (2010) have also suggested that plates were cov-

ered in keratin. Particularly, Christansen and Tschopp

(2010) noted that they found what was possibly keratin


on a plate. The external and internal vascular canals in

plates and spikes probably contribute to the production

of the keratin sheath.

Acknowledgements. This paper is a portion of the PhD disserta-

tion of the first author (S. Hayashi), who would like to acknowl-

edge his supervisors, namely Y. Kobayashi (Hokkaido University

Museum, Sapporo, Japan), M. Manabe (National Science

Museum, Tokyo, Japan), P. M. Sander (Institut fur Palaontogie,

Universitat Bonn, Bonn, Germany), K. Endo (Nihon University,

Tokyo, Japan) and D. Suzuki (Sapporo Medical University, Hok-

kaido, Japan) for their guidance during this research. We are

grateful to D. Brinkman, W. Joyce and M. Fox (Yale Peabody

Museum, Yale University, New Haven, CT, USA), S. Suzuki and

S. Ishigaki (Hayashibara Natural History Museum, Okayama,

Japan), K. Siber and his staff (Sauriermuseum Aathal, Switzer-

land), M. Getty and S. Sampson (Utah Museum of Natural

History, Salt Lake City, UT, USA), M. Carrano and M. Brett-

Surman (United States National Museum, Washington, DC,

USA) for access to materials in their care and for information

regarding many of the specimens. B. Small, A. Ryan (Denver

Museum of Nature and Science) and B. Ryan (Comcast Com-

pany, Denver, CO, USA) for support in this study. We also wish

to thank M. Okumura (Hokkaido University, Sapporo, Japan)

and T. Takemura (Nihon University, Tokyo, Japan) for permis-

sion to CT-scan the Stegosaurus plates and spikes. The manu-

script benefited greatly from the comments and suggestions of P.

Galton (University of Bridgeport, Bridgeport, CT, USA), K. Stein

and J. Mitchell (Institut fur Palaontogie, Universitat Bonn,

Bonn, Germany) who kindly reviewed earlier versions of the

manuscript. We acknowledge the helpful reviewers by K. Angi-

elczyk (Field Museum of Chicago, Chicago, IL, USA), J. O. Far-

low (Indian-Purde University, Fort Wayne, IN, USA) and S.

Werning (University of California, Berkeley, CA, USA). This

research was funded by JSPS (the Japan Society for the Promo-

tion of Science) the DAAD (German Academic Exchange Ser-

vice) and the Jurassic Foundation.

Editor. Kenneth Angielczyk

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