Editors' choice

Two new stegosaur specimens from the Upper Jurassic
Morrison Formation of Montana, USA
D. CARY WOODRUFF, DAVID TREXLER, and SUSANNAH C.R. MAIDMENT
Woodruff, D.C., Trexler, D., and Maidment, S.C.R. 2019. Two new stegosaur specimens from the Upper Jurassic
Morrison Formation of Montana, USA. Acta Palaeontologica Polonica 64 (3): 461–480.
Two partial skeletons from Montana represent the northernmost occurrences of Stegosauria within North America. Oneof these specimens represents the northernmost dinosaur fossil ever recovered from the Morrison Formation. Consistingof fragmentary cranial and postcranial remains, these specimens are contributing to our knowledge of the record anddistribution of dinosaurs within the Morrison Formation from Montana. While the stegosaurs of the Morrison Formationconsist of Alcovasaurus, Hesperosaurus, and Stegosaurus, the only positively identified stegosaur from Montana thus
far is Hesperosaurus. Unfortunately, neither of these new specimens exhibit diagnostic autapomorphies. Nonetheless,these specimens are important data points due to their geographic significance, and some aspects of their morphologies
are striking. In one specimen, the teeth express a high degree of wear usually unobserved within this clade—potentially
illuminating the progression of the chewing motion in derived stegosaurs. Other morphologies, though not histologicallyexamined in this analysis, have the potential to be important indicators for maturational inferences. In suite with otherspecimens from the northern extent of the formation, these specimens contribute to the ongoing discussion that bodysize may be latitudinally significant for stegosaurs—an intriguing geographical hypothesis which further emphasizes
that size is not an undeviating proxy for maturity in dinosaurs.
Ke y w o rd s : Dinosauria, Thyreophora, Stegosauria, Jurassic, Morrison Formation, USA, Montana.
D. Cary Woodruff [sauropod4@gmail.com], Great Plains Dinosaur Museum and Field Station, Malta, MT, USA; andDepartment of Ecology and Evolutionary Biology, University of Toronto; Royal Ontario Museum, 405 N 1st Ave E,
Malta, MT 59538, Toronto, ON, Canada.
David Trexler [Dinoguy10@yahoo.com], Two Medicine Dinosaur Center, 120 2nd Ave S, Bynum, MT 59419, Bynum,
MT, USA.
Susannah C. R. Maidment [susannah.maidment@nhm.ac.uk], Department of Earth Sciences, Natural History Museum,
Cromwell Rd, South Kensington, London SW7 5BD, UK.
Received 18 December 2018, accepted 13 April 2019, available online 5 August 2019.
Copyright © 2019 D.C. Woodruff et al. This is an open-access article distributed under the terms of the Creative
Commons Attribution License (for details please see http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Introduction
Stegosauria is a clade of ornithischian (“bird-hipped”) dinosaurs that is characterized by the possession of a seriesof plates and spikes that extend from the neck to the tail in
two parasagittal rows. The best-known genus of the clade,
Stegosaurus, is from the Upper Jurassic of the western
USA, but members of the clade are known from all continents except for Antarctica and Australia, and their temporal range extends from the Middle Jurassic to the uppermost Lower Cretaceous (Maidment et al. 2008, Tumanova
and Alifanov 2018). Despite the iconic status of stegosaurs
among Dinosauria, they are poorly known as fossils, withmost genera being known from single partial skeletons and/or fragmentary remains. Consequently, they have received
Acta Palaeontol. Pol. 64 (3): 461–480, 2019

relatively little attention, and much remains unknown about
their paleobiology.
The Upper Jurassic Morrison Formation of the western
USA comprises the deposits of rivers and floodplains thatwere laid down in a broad shallow basin over about 10 million years in the Kimmeridgian and Tithonian. The deposits
host a diverse and well-preserved dinosaurian fauna that
includes such well-known taxa as Apatosaurus, Diplodocus,
and Stegosaurus. The dinosaurs of the southern part of theMorrison basin, from Colorado, Utah, and southern Wyoming, have been intensively studied for about 150 years,
but those from northern outcrops in northern Wyoming andMontana are much less exploited (Foster 2007). An increasing body of evidence suggests that northern Morrison faunas were somewhat different to their southern counterparts(Maidment et al. 2018; Whitlock et al. 2018), but further exhttps://doi.org/10.4202/app.00585.2018

462

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ACTA PALAEONTOLOGICA POLONICA 64 (3), 2019

B
N
Flathead
Lake

Fort Peck Lake

Great Falls

GDPM 178
GDPM 205

Missoula
HelenaMiles City
Butte
Billings
BozemanFig. 1. A. Map of the USA with indicated the state of Montana, were the stegosaur specimens GPDM 178 and GPDM 205 were found. B. Map of Montana
with the extent of the Morrison Formation in green, and the general locations of GPDM 178 and GPDM 205 indicated by the Stegosaurus skull silhouettes(modified from Maidment et al. 2018).

ploration in this part of the basin is needed. Here, we report
two stegosaurian dinosaur specimens from the most northerly outcrops of the Morrison Formation in Montana, one ofwhich is the most northerly occurrence of a dinosaur from
the Morrison Formation. The specimens, which comprisefragmentary cranial and postcranial material, are therefore
important data points in our understanding of the fauna anddiversity of the Morrison Formation.
Institutional abbreviations.—BYU, Brigham Young University Museum of Paleontology, Provo, USA; CM, Carnegie
Museum, Pittsburgh, USA; DMN, Dinosaur National Monument, Vernal, USA; FPDM, Fukui Prefectural Dinosaur
Museum, Japan; GPDM, Great Plains Dinosaur Museum andField Station, Malta, USA; HMNH, Hayashibara Museum of
Natural History, Okayama, Japan; IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China;
ML, Museum of Lourinhã, Portugal; MOR, Museum of theRockies, Bozeman, USA; NHMUK, Natural History Museum, London, UK; USNM, United States National Museum,Washington D.C., USA; SMA (VFSMA), Saurier museum
Aathal, Switzerland; YPM, Yale Peabody Museum of Natural History, New Haven, USA; ZDM, Zigong Dinosaur
Museum, China.

Systematic palaeontology
Dinosauria (Owen, 1842)
Ornithischia (Seeley, 1887)
Stegosauria (Marsh, 1877)
Stegosauria indet.
Material.—GPDM 205, a partial postcranial skeleton fromthe Little Snowy Mountains, Fergus County, Montana,
USA and GPDM 178, a partial skeleton including skull andpartial postcrania from Giffen, Cascade County, Montana,
USA (Fig. 1).
GPDM 205 (“Gates”) was collected in 2004 by the Judith
River Dinosaur Institute from the 5E Ranch owned by the
Hein family, Little Snowy Mountains, Fergus County,
Montana. The specimen is now housed at the Great Plains
Dinosaur Museum in partnership with the Judith River
Foundation. GPDM 205 was collected within the undivided
Morrison Formation, and this specimen was found in close
proximity to Camarasaurus sp. GPDM 220 (Woodruff
and Foster 2017). Unfortunately, neither field notebooks
nor quarry maps exist for the excavation of this specimen,
so we do not possess any documentation regarding taphonomy. However, SCRM visited the 5E Ranch quarry in2005 and was told by the excavation crew that the specimen was found disarticulated, stratigraphically above theCamarasaurus remains (GPDM 220). During the 2005 fieldseason, at the time of SCRM’s visit, a carpal and somepoorly preserved dermal plates from this specimen werealso recovered. Several other stegosaur specimens, referred
to Hesperosaurus mjosi (= Stegosaurus mjosi) by Saitta(2015), were discovered during the 2005 field season at
the 5E Ranch quarry, and were excavated over subsequent
years. These specimens currently reside in a private collection, and thus are unavailable for study (sensu K. Padianand K. Carpenter in Chen 2015). At the current time, aside
from a prepared humerus, ulna, and radius, only an unprepared coracoid, fragmentary scapula, humerus, and ulna
have been relocated within the collections of the GPDM.
Only those elements prepared to-date are described, while
the skeletal reconstruction for this specimen (Fig. 2) denotes
elements within the GPDM collections.
GPDM 178 (“Giffen”) was collected in 2003 and 2004 by
the Judith River Dinosaur Institute near the town of Giffen,
Cascade County, Montana, on land belonging to the Yurek
family. Elements were initially found eroding out of a hill
adjacent to the Yurek’s house. The distal end of the rightWOODRUFF ET AL.—LATE JURASSIC STEGOSAURS FROM MONTANA, USA

463

B

A

1m

Fig. 2. Skeletal reconstruction of the stegosaurs GPDM 178 (A) and GPDM 205 (B) scaled together based on humeral lengths. Black, elements preservedin each specimen; gray, missing elements. Hesperosaurus reconstruction by Genya Masukawa (Ibaraki University, Japan). Human scale is the Artemision
Bronze featuring the deity of Zeus as 1.83 m in height.

tibia was the first element recovered, and it was not immediately recognized as a fossil (Linda Yurek, personal communication, 2016). For a period of time, this “black rock”(as described by Linda Yurek), served as a door stop until
construction of a pole barn on the hill started, and further
elements were discovered below the surface (Linda Yurek
personal communication, 2016). At this time the Yurek
family contacted the Judith River Dinosaur Institute, andthe excavation commenced. The specimen is now housed
at the Great Plains Dinosaur Museum in partnership with
the Judith River Foundation Collected within the undivided
Morrison Formation, GPDM 178 consists of a partial skeleton including both crania and post-crania. Several fieldjackets that are labeled as dorsal ribs or vertebrae await
preparation. Only those elements prepared to-date are described, while the skeletal reconstruction for this specimen(Fig. 2) denotes elements collected via the quarry log.
Description.—GPDM 205: For measurements of all elements, see Table 1. Humerus: The left humerus is well preserved (Fig. 3). Large cracks and the parts of the diaphysis
are filled and/or restored with a form of epoxy clay. In
anterior view, the humerus has a dumb bell-shaped profile,as in other stegosaurs (Maidment et al. 2015, 2018), with astout, cylindrical diaphysis. The deltopectoral crest appears
to be larger and project far more medially than in someother stegosaur specimens (e.g., Stegosaurus, NHMUK PV
Table 1. Greatest measurements (in mm) for GPDM 205. 1 anterior-posterior length 2 lateral width; r reconstructed.

Forelimb
Humerus
Length (dorsal-ventral)
516
length121.531
Proximal end
width
1512
length
115.631
Distal end
width
2172
Deltopectoral crest length
186.61
Diaphysis mimimum circumference
278

Ulna
517
136.83
1152
292
59.04
–
192

Radius384
110.2691.752
93.18
92.09 r–168

R36730, Maidment et al. 2015; Hesperosaurus, MOR 9728,
Maidment et al. 2018; Dacentrurus, NHMUK PV OR 46013,
Loricatosaurus, NHMUK PV R3167, Miragaia ML 433,
Mateus et al. 2009; Kentrosaurus, NHMUK PV R37143,
Henning 1925) but is similar to the condition in other specimens such as YPM 1853, YPM 1856 (Stegosaurus), MB
R.4804, and MB R.4805 (Kentrosaurus). This variation appears to be intraspecific and is likely to be taphonomic or
related to restoration. Transversely, the deltopectoral crest isfairly uniform in thickness throughout, only tapering at theanteromedial end. The anterolateral margin of the deltopectoral crest, particularly the anteriormost aspect, has a rugose
surface texture. This rugose margin forms a “lip” along thedeltopectoral crest, and in combination with the mediallyconvex curvature of the crest, produces a prominent bowlshaped crest. Unlike the deltopectoral crest in Stegosaurus
stenops (NHMUK PV R36730; Maidment et al. 2015), whichmerges without a break in slope into the proximal aspect of
the humerus, the deltopectoral crest of GPDM 205 has adip, or a break in slope, along the proximal contact between
the crest and diaphysis. In posterior orientation a prominent
dorsomedial, posterolateral angled triceps ridge traverses
along the entire length of the deltopectoral crest; this is asynapomorphy of Stegosauria (Raven and Maidment 2017).
The large humeral head is very bulbous, and 68% the transverse width of the distal condyles. In lateral and medial
orientations, the bulbous humeral head is offset posteriorly.
Along the medial edge of the humerus, just distal to thehumeral head is a symmetrical, triangular-shaped process.
Though nowhere near in size, this medial triangular process
opposes the deltopectoral crest.
The diaphysis, though largely restored, (see above), appears to be circular to sub-circular in cross section. Before
sharply transversely expanding into the proximal and distal
ends, this sub-circular diaphysis is uniform in thickness.
The straight diaphysis, together with the slightly posteriorly
oriented proximal end and the slightly anteriorly oriented

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ACTA PALAEONTOLOGICA POLONICA 64 (3), 2019

A1

A2

A3

humeralcrest

A4

humeralhead

humeralhead

deltopectoral
crest

A5

deltopectoral
crest

deltopectoral
crest
latissimus
tuberclelateralcondyleA6deltopectoral
crest

medialcondylelateralcondylemedialcondylelateralcondyle100 mmFig. 3. Left humerus of stegosaur GPDM 205 from the 5E Ranch, Little Snowy Mountains, Fergus County, Montana, USA; in anterior (A1), posterior (A2),lateral (A3), medial (A4), dorsal (A5), and ventral (A6) views.
olecranon process
olecranon
process

anterior
process
medialprocess

anterior
process

olecranon
process
medialprocess

olecranon
process

medialprocess

A5
anterior
process

A6

radialfacet
100 mm

A1

A2

A3

A4

Fig. 4. Left ulna of stegosaur GPDM 205 from the 5E Ranch, Little Snowy Mountains, Fergus County, Montana, USA; in anterior (A1), posterior (A2),lateral (A3), medial (A4), dorsal (A5), and ventral (A6) views.

distal end, gives the humerus a slightly sigmoidal lateraland medial profile. The distal portion of the humerus terminates with the transversely widened and bulbous articular condyles. The distal aspect of the humerus is 73%
the width of the proximal end—contributing to the dumb
bell profile. The medial epicondyle in most orientations isfairly spherical in overall morphology. The lateral epicon-dyle is larger than its counterpart. In anterior and posteriorview, the lateral epicondyle has a sub-triangular profile thatprojects anterolaterally. While this condyle projects slightly
anteriorly, it does not possess the triangular anterior eminence. In ventral view the lateral epicondyle is 1.3 times astransversely wide and 1.08 times anteroposteriorly elongate
than the medial epicondyle.

WOODRUFF ET AL.—LATE JURASSIC STEGOSAURS FROM MONTANA, USA

A1

A2

A3

465

A4

A5

intermuscular
ridge

A6

100 mmFig. 5. Left radius of stegosaur GPDM 205 from the 5E Ranch, Little Snowy Mountains, Fergus County, Montana, USA; in lateral (A1), medial (A2),
anterior (A3), posterior (A4), dorsal (A5), and ventral (A6) views.

Ulna: The left ulna is quite well preserved; a few large
cracks are filled in with epoxy clay (Fig. 4). In anterior
and posterior views, the right ulna has a slightly sinusoidal profile. The olecranon process consists of a prominent
triangular bony projection which extends dorsally from thediaphysis. In GPDM 205, the olecranon process is a very
prominent scalene-like process, comparable in development
to that of USNM 4937 (“Stegosaurus sulcatus”), YPM 1854
(Stegosauria indet.), and MB R.4800 (Kentrosaurus). In someother specimens, e.g., BYU 12290, DNM 2438, and NHMUKPV R36730 (Stegosaurus stenops), the olecranon process isless well-developed. The olecranon process of GPDM 205 is
45% the total dorsoventral length, and has a transverse thickness that is over 1.5 times that of the thinnest portion of the diaphysis. The development of the olecranon process has been
related to ontogenetic stage in stegosaurs (Galton 1982a), andthe well-developed olecranon process in this specimen may
indicate the individual was a mature adult at time of death.
This hypothesis is supported by the observation that all surfaces of the olecranon process have a very rugose bone texture. Ossification of cartilage to give a rugose bone texture
in mature individuals has also been noted in other stegosaurs(Mallison 2010) and ossification of the triceps tendon to givean “olecranon horn” has been described in the stegosaurs
Miragaia and Dacentrurus (Mateus et al. 2009).
In anterior view, the anterior and medial processes
are more parallel and similarly oriented to one another incontrast in Stegosaurus stenops (NHMUK PV R36730;
Maidment et al. 2015), in which the medial process appears
nearly perpendicularly oriented to the anterior process. In
Stegosaurus stenops (NHMUK PV R36730; Maidment etal. 2015), there are gently curved regions transitioning fromthe three proximal processes (the olecranon, anterior, andmedial processes), yet in GPDM 205 the transition fromeach of these processes is more abrupt. For example, in
GPDM 205 the medial process appears nearly flat with asharp incline of 126° to the olecranon process. Along theentire outer length of the olecranon process is a transversely
thickened margin. With this thickened outer margin and aslight curvature of this process the medial aspect is slightly
concave.
In dorsal view, the olecranon process is significantly
larger than the medial and anterior processes. Distal to theolecranon process, the diaphysis is fairly straight and ovoid
to sub-circular in cross section, before abruptly expanding
and terminating. In distal view, the distalmost region of the466

ACTA PALAEONTOLOGICA POLONICA 64 (3), 2019

A1

A2

100 mm

R36730), but in contrast to the basal taxa Huayangosaurus(ZDM T7001), and Isaberrysaura (Salgado et al. 2017). The
element has an anteroposteriorly elongate profile, beingnearly two times the anteroposterior length than dorsoventral height. The narial opening is similarly anteroposteriorly
elongate. The anteriormost region is well preserved, andpossesses a crenulated and slightly rugose border (denticles;David C. Evans personal communication 2018) that would
have served as an attachment for the rhamphotheca. The nasal process projects posterodorsally from the anterior margin of the bone and is triangular in cross-section with the
apex pointed laterally, forming a ridge on the lateral surfaceof the process. It is broken at its dorsal extremity. The process is delicate and gracile, as in Stegosaurus (NHMUK PV
R36730) and Miragaia (ML 433) but differs from the condition in Huayangosaurus (ZDM T7001) where it is morerobust. The maxillary process forms the ventral margin of
the external naris and then curves dorsally proceeding posteriorly. There is a broad, flat to slightly concave articular
facet for the maxilla.

A1
Fig. 6. Cranial elements of stegosaur GPDM 178, near Giffen, Cascade
County, Montana, USA; in left (A1) and right (A2) lateral views; gray,missing elements.

ulna is gently convex, and is tear dropped shaped with thetapered apex pointing anteriorly. A large, flat facet for the
distal end of the radius is present, and as in Stegosaurus
stenops (NHMUK PV R36730; Maidment et al. 2015),
medial to this facet is a prominent dorsoventrally oriented
ridge. Observable in posterior, lateral, and medial views in
Stegosaurus stenops (NHMUK PV R36730; Maidment etal. 2015), the distal region of the ulna has a slight laterally
oriented curvature (opposed to the largely straight rest of
the ulna). Yet in GPDM 205, this region is more kinked
than curved, and more similar to the corresponding regionof Kentrosaurus aethiopicus (Galton 1982b) and Miragaia
longicollum (ML433; Mateus et al. 2009).
Radius: Like the humerus, the existing portion of the leftradius appears to be well-preserved (Fig. 5). Large cracks inthe diaphysis are filled in with epoxy clay, and a significant
portion of the proximal region is entirely sculpted. While
Stegosaurus stenops (NHMUK PV R36730; Maidment et al.
2015), has a dorsoventrally short, almost rectangular radiusin lateral view, the radius of GPDM 205 is more like that of
Kentrosaurus aethiopicus (Galton 1982b) in that it is very
dorsoventrally elongate, with a much more transversely widened distal region.
GPDM 178: For measurements of all elements, see
Table 2. Skull: The skull consist of a partial braincase, bothmaxillae, both dentaries, and the left premaxilla (Fig. 6).
Premaxilla: Given its size and delicacy, the left premaxilla is in excellent condition (Fig. 7). The premaxilla is edentulous, and as in some other stegosaurs such as Miragaia(ML 433; Mateus et al. 2009) and Stegosaurus (NHMUK PV

denticle

naris

naris

A2

denticle

A3

50 mm

naris

A4
denticledenticle

Fig. 7. Left premaxilla of stegosaur GPDM 178, near Giffen, Cascade
County, Montana; in dorsal (A1), lateral (A2), medial (A3), and ventral(A4) views.

WOODRUFF ET AL.—LATE JURASSIC STEGOSAURS FROM MONTANA, USA

467

Table 2. Greatest measurements (in mm) for GPDM 178. 1 anterior-posterior length; 2 lateral width; 3 anterior-posterior height; 4 length of ventral
margin; d, damaged; r, reconstructed.
Shoulder girdle

Coracoid Scapula

anterior-posterior
dorsal-ventral
Width (lateral)
Anterior end length(proximal-distal)Mid-length scapular blade length(proximal-distal)
Acromion (?)
length (anterior-posterior)
length (dorsal-ventral)
Glenoidwidth (lateral)
Length

228 r
285 r

Cervical vertebra

CentrumLength (anterior-posterior)
Width (lateral)
Height (dorsal-ventral)

80.27

Articular
connection
with scapula

638 r
130.2590.72
346
146.31Forelimb
Humerus Ulna Radius
Length (dorsal-ventral)
454
374
364
length
78.511
165
116.21
Proximal end
width
153
118.34 71.43
length114.171 63.55 109.43
Distal end
width
2042
83.59 87.98
Deltopectoral crest length
125.05––(anterior-posterior )Mimimum circum-ference of diaphysis
257
184
139

234 r
118.54
86.16
Anterior Posterior Neural
face
face
arch

Neural canalTransverse
anterior posterior Prezygapophyses Postzygapophyses
processes
view
view

68.09
78.30
65.33

Skull
Length (anterior-posterior)
Width (lateral)
Height (dorsal-ventral)
Occicipital condyle height (dorsal-ventral)
Tooth row length (anterior-posterior)

67.94
68.25

71.43

Craniumposteriorforamencraniummagnum117.97
81.26
21.99
98.78
26.73
28.36

Maxilla: Neither maxilla is complete, but as preserved
they are both triangular in lateral or medial view with gently
upwardly convex ventral, tooth-bearing margins (Fig. 8).
The anteriormost ~2–3 cm of both the left and right element is edentulous, as in Stegosaurus (USNM 4934), thisdiastema is transversely thin, but thickens at the tooth row.
In the left maxilla 27 alveoli are present, and at least 23
in the right. There are 27 or 28 maxillary teeth present in
Huayangosaurus (ZDM T7001), at least 22 are present in
Stegosaurus (USNM 4934; Gilmore 1914) and 20 are present in Hesperosaurus (HMNH 001; Carpenter et al. 2001).
The anterodorsal and posterodorsal margins of neither maxillae are preserved, so that relationships with other cranial
elements remain unknown and the presence or absence of anantorbital fossa cannot be demonstrated.
Dentary: Both dentaries are largely complete, except forthe posteriormost parts (Fig. 9). In lateral and medial views,
the dentary has a gentle sigmoidal profile. This curved profile is slight along the dorsal margin, yet more pronounced
along the ventral margin, especially the anterior aspect.
Overall the dentaries are transversely thin, except for thesymphysis and the tooth row. The anterior is the laterally
thickest portion of this element. The tooth row occupies themedial side of a medially projecting thickened ridge of bone.

34.11
34.80368.67d45.16
25.743

Maxilla

Dentary105.99d52.45

Premaxilla

left

right

left

right

left

Dorsalplate

155.52
13.03
65.99

170.1
17.32
65.96

190.8
27.30
57.77

204
26.93
72.38

123.10
17.76
53.72

4554
86.894550.2

116.6293.81

124.02

127.59As in Stegosaurus (NHMUK PV R36730), but in contrast to
Huayangosaurus (ZDM T7001), Gigantspinosaurus (ZDM0019), and Jiangjunosaurus (Jia et al. 2007), the tooth row
is angled slightly medially and obscured in lateral view by athin ridge of bone.
The anterior one third of the dentary is edentulous,
as in Stegosaurus (NHMUK PV R36730) and Jiangjunosaurus (Jia et al. 2007), but in contrast to the condition in Huayangosaurus (ZDM T7001), Kentrosaurus(MB.R.3806.1), and Gigantspinosaurus (ZDM 0019),
where the tooth row starts immediately posterior to the predentary articular facet. The tooth row occupies nearly theentirety of the remaining length of the dentary. The tooth
row is curved to gently sigmoidal, as in Huayangosaurus(ZDM T7001), Gigantspinosaurus (ZDM 0019), and Jiangjunosaurus (Jia et al. 2007) but in contrast to the condition
usually seen in Stegosaurus, where it is straighter (e.g.,
NHMUK PV R36730). The left dentary possesses at least
24 alveoli, and the right possesses perhaps up to 26 alveoli. There are 21 dentary teeth in Huayangosaurus (ZDM
T7001) and Jiangjunosaurus (Jia et al. 2007), more than 30
in Gigantspinosaurus (Ouyang 1992), and the number in
Stegosaurus appears to vary from around 17 to 23 (Berman
and McIntosh 1986)

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ACTA PALAEONTOLOGICA POLONICA 64 (3), 2019

A1

B1

A2

B2
tooth row
tooth row
50 mm

A3

B3

tooth row

A4

tooth row

B4

Fig. 8. Stegosaur GPDM 178, near Giffen, Cascade County, Montana. A. Left maxilla in dorsal (A1), lateral (A2), medial (A3), and ventral (A4) views.
B. Right maxilla in dorsal (B1), lateral (B2), medial (B3), and ventral (B4) views.
tooth rowA 1 dentary
symphysis

tooth row

B1
B2

tooth row

dentarysymphysis

A2

B3

dentary symphysis

dentary symphysis

A3

50 mm
dentary symphysis

A4dentarysymphysis

dentary symphysis

B4
tooth row

tooth row

Fig. 9. Stegosaur GPDM 178, near Giffen, Cascade County, Montana. A. Left dentary in dorsal (A1), medial (A2), lateral (A3), and ventral (A4) views.
B. Right dentary in dorsal (B1), medial (B2), lateral (B3), and ventral (B4) views.

WOODRUFF ET AL.—LATE JURASSIC STEGOSAURS FROM MONTANA, USA

Teeth: The maxillary and dentary teeth of GPDM 178
are similar to other stegosaurs, being leaf-shaped (Galtonand Upchurch 2004). In lingual view the teeth are fairly
symmetric with a ringed cingulum observed in all teeth. In
labial view all teeth have a fairly prominent midline central
denticle with several denticles of varying size and number(approximately four to eight) on either side of the midline
denticle. In labial, lingual, and apical views, these denticles
are strongly convex, and with the ringed cingulum, the teeth
have rounded profiles.
Perhaps the most interesting aspect of the teeth of GPDM
178 are the wear facets (Fig. 10). Wear facets are known inmany stegosaurian teeth (Gigantspinosaurus, Hao et al. 2018;
Huayangosaurus, Sereno and Dong 1992; Kentrosaurus,
Hennig 1925; and Stegosaurus, Galton and Upchurch 2004),
and these wear facets are largely sub-horizontal to gently inclined posteriorly and labially (Galton and Upchurch 2004;
Reichel 2010). Stegosaurian tooth wear facets led Galtonand Upchurch (2004) to suggest they were formed by toothfood contact, which would indicate chewing; but the rarityand general weak degree of said facets would suggest that
this was not a common occurrence. However, Reichel (2010)
noted that some stegosaurian teeth have extensive wear facets, indicating prolonged chewing and/or a longer intervalthat the tooth was active. Interestingly, nearly all of the maxillary and dentary teeth of GPDM 178 have extensive wear
facets. The majority of these wear facets are sub-horizontal,
and the size of the facets vary from approximately ¼ to ½
of the crown height. The teeth of GPDM 178 have not been
sectioned to document the Lines of Von Ebner correlating
to tooth age in dinosaurs was studied in the Erickson (1996)
analysis, so at this time we cannot say if the extensive wear
facets are a direct result of the teeth being active and worn
down over a prolonged period, or if they indicate a greater
degree of chewing. However, contra to Hao et al. (2018),
while the degree of wear may indicate that the tooth itself ismore mature (i.e., it has been an erupted functional tooth fora longer period of time), as dinosaurs continually shed theirteeth throughout their lives, tooth wear is not an acceptableindicator of an individual’s maturational status.
Within the wear facets of GPDM 178 are smaller featuresthat may hint at the cause of these facets. In a majority of the
sub-horizontal wear facets are numerous small-scale gouges(Fig. 10C). These gouges appear entirely restricted to thewear facets, and they uniformly consist of a series of anteroposteriorly oriented gouges with another sets of gouges
cross-cutting roughly perpendicular. If the marks were infrequent or an isolated occurrence, they could be associatedwith taphonomic wear or even preparation damage. However,given the prevalence and uniformity of these gouges, we hypothesize that they are in fact signs of microwear.
Posterior cranium: A co-ossified unit consists of thebraincase, partial frontals, and the parietal (Fig. 11). In lateralview the frontal region slopes gently anteroventrally. The
parietal is not well preserved dorsally, but is roughly squarein outline, and flat, as in Stegosaurus (USNM 4934, DMNH

A

469

B

denticles

C1

wear facet

C2

cross-cutting
micro gouges

D

Fig. 10. Maxillary and dentary teeth of stegosaur GPDM 178 (A–D), near
Giffen, Cascade County, Montana; C2, explanatory drawing showing thewear facet and cross-cutting gouges. Not to scale, anteroposterior wear
facet length of approximately 2–2.5 mm.

2818), Huayangosaurus (ZDM T7001), and Hesperosaurus(MOR 9728; Maidment et al. 2018). The frontals are flat
dorsally, and incomplete anteriorly. Additionally, the sutures between the frontals and parietal is not clear, and theleft and right frontals cannot be observed because the bone
surface is poorly preserved in this area.
In posterior view, the supraoccipital is not well preserved, and the braincase is damaged dorsal to the foramen magnum. The paroccipital processes have broken
off and are not preserved. The occipital condyle projects
posteriorly, as in other stegosaurs (Hesperosaurus mjosi,
Carpenter et al. 2001; Huayangosaurus taibaii, ZDM T7001;
Kentrosaurus aethiopicus, Hennig 1925; and Stegosaurus
stenops, NHMUK PV R 36730). In posterior view, theoccipital condyle is ovoid (laterally wider than dorsoventrally tall) as in Huayangosaurus taibaii (ZDM T7001),
Kentrosaurus aethiopicus (Hennig 1925), and Stegosaurus
stenops (NHMUK PV R 36730). The foramen magnum
is circular, and is approximately similar in dorsoventral
height as the occipital condyle, but is transversely muchsmaller, unlike that of Hesperosaurus mjosi (Carpenter et al.
2001), but similar to Huayangosaurus taibaii (ZDM T7001),

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ACTA PALAEONTOLOGICA POLONICA 64 (3), 2019

A1

A2

A3

parietalparietal

occipital
condylefrontalfrontalparietal

A4

50 mm

basal tubera

A5

frontalfrontalA6

foramenmagnumoccipital
condyleoccipital condylebasal tubera

Fig. 11. Braincase of stegosaur GPDM 178, near Giffen, Cascade County, Montana; in anterior (A1), left (A2) and right (A5) lateral, dorsal (A3), posterior(A4), and ventral (A6) views.

Kentrosaurus aethiopicus (Hennig 1925), and Stegosaurus
stenops (Carpenter et al. 2001). In ventral view, the suture between the basisphenoid and basioccipital cannot be
observed, as in other stegosaurs (Maidment et al. 2018).
Anterior to the occipital condyle, the basioccipital-basisphenoid narrows transversely into a short neck, which is anteroposteriorly shorter than the occipital condyle. The fused
elements then flare transversely to form the basal tubera,
which are separated by a midline groove. In lateral view thebasal tubera are triangular with the apex pointing ventrally,
and in ventral view they are ovoid in shape, being wider
transversely than they are anteroposteriorly. The basal tubera extend further ventrally than the occipital condyle, as
in Stegosaurus stenops (USNM 4934), and Hesperosaurus
mjosi (MOR 9728), but in contrast to the condition in thebasal thyreophoran Scelidosaurus (NHMUK PV R1111).
Anterior to the basal tubera, the basipterygoid processes arebroken, and anterior to this area, fused to the ventral surfaceof the frontal, the orbitosphenoid is present as a ventral convex, hour glass-shaped element opening anteriorly. Ossified
orbitosphenoids are observed in specimens of Stegosaurus(CM 106; Gilmore 1914) and are also present in the basal
thyreophoran Scelidosaurus (NHMUK PV R1111).
In lateral view, the elements of the braincase are co-ossified and sutures cannot be observed. On the left side, thecrista prootica of the prootic lies dorsal to a large, heartshaped fenestra, which is the combined opening of the fe-

nestra ovalis and fenestra pseudorotunda. The crista prootica is not as well preserved on the right side. On both sides,
posterior to the fenestra ovalis, are two small foramina thatlikely represent the exits of the hypoglossal nerve (XII),
as in the basal ornithischian Lesothosaurus (Sereno 1991).
Anterior to the fenestra ovalis, a groove is present extending anteroventrally. A foramen situated in the groove at thesame level as the fenestra ovalis is probably the exit for thefacial nerve (VII). Ventral to this foramen, on the left side
the groove is weathered and opens into a large concavity, thesella turcica. On the right side, close to the anterior marginof the braincase, the trigeminal foramen, for the exit of themaxillary and mandibular branches of the trigeminal nerve(V), is present. Dorsal to these fenestrae and foramina thelateral wall of the braincase is composed of the prootic,
which is smooth and featureless. The suture between the
prootic and parietal on the right side is marked by an irregular concavity that has probably been accentuated by erosion. The laterosphenoid extends anteriorly from the dorsal
margin of the trigeminal foramen to contact the frontal, andappears to be roughly triangular in shape, as in the basal ornithischian Lesothosaurus, although sutures cannot be seen.
Postcranial material: Cervical vertebra: The one fully
prepared cervical vertebra is generally well-preserved (Fig.
12). By comparison with Stegosaurus stenops (NHMUKPV R36730), this vertebra appears to be an anterior cervical,
perhaps cervical three or four. In lateral view, the overall

WOODRUFF ET AL.—LATE JURASSIC STEGOSAURS FROM MONTANA, USA

A1

prezygapophysis
diapophysis

neuralcanalneural spine

471

A 2 neural spine

A 3 diapophysis

postzygapophysis
post
zygapophysis
postzygodiapophyseal
laminaparapophysis

prezygapophysis

postzygapophysis

neural spine

A4
postspinal
fossa

A5

neural spine

postzygapophysis

A6

parapophysis

spinopostzygopophyseal
laminapostzygodiapophyseal
laminapostzygapophysis

neural canaldiapophysis

50 mm
diapophysis

Fig. 12. Cervical vertebra of stegosaur GPDM 178, near Giffen, Cascade County, Montana; in anterior (A1), left (A2) and right (A5) lateral, dorsal (A3),posterior (A4), and ventral (A6) views.

vertebral profile (centrum + neural arch) is rhomboid. The
anterior and posterior articular facets of the centrum arenearly flat to slightly concave. In lateral view, the centrum isroughly square; the anterior and posterior facets are slightly
offset, producing a slightly kinked centrum profile. In anterior view, the centrum facet is largely circular in outline.
Along the ~mid anterior centrum facet, the parapophyseal
facets abut onto the rim of the centrum producing laterally
projecting subtriangular profiles at this margin. In posteriorview the posterior articular facet, like the anterior one, iscircular in outline, except for the anterior margin with theneural canal that is rather horizontal. In lateral and ventral
views, the ventral profile of the centrum is upwardly concave. This concave ventral margin does not exhibit a ventral
keel or ridge. As observed to a lesser degree in Stegosaurus
stenops (NHMUK PV R36730; Maidment et al. 2015), in
lateral view, slightly posteroventral to the parapophyseal
facets is a single, small foramen (approximately 3.8 mm
anteroposteriorly long by 2.9 mm dorsoventrally tall) which
is probably vascular.
While the neural arch is fully fused to the centrum, theneurocentral suture is clearly visible on all sides. The neural
canal of this vertebra is very large, being 43% dorsoven-trally tall and 44% laterally wide as the centrum. In anteriorview, except for the curved dorsal margin, the neural canalis almost square in anterior profile, being 1.09 times dorsoventrally tall than laterally wide. In both anterodorsal andposterodorsal oblique views, the ventral margin of the neural canal is strongly curved and pervades into the midlineof the centrum. In posterior view, the profile of the neural
canal is triangular with the apex pointing dorsally. This triangular neural canal profile is laterally wide (approximately
1.75 times wider than tall). The left prezygapophysis is broken, and the right is damaged. The articular facet of the right
prezygapophysis is strongly dorsoventrally inclined as in
Stegosaurus stenops (NHMUK PV R36730; Maidment etal. 2015). The neural spine, though damaged, is slightly posteriorly inclined. The dorsal margin of the postzygapophyses are anteroposteriorly elongate and nearly horizontal. The
ventral margin is gently anteriorly curved, and this curved
profile seamlessly traverses ventrally to the neurocentral suture. The articular facet of the right postzygapophysis is notpreserved, but the left is elongate, being 74% the length ofthe centrum. Anteroposteriorly elongate articular facet are
observed in the Morrison stegosaurs Hesperosaurus mjosi

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ACTA PALAEONTOLOGICA POLONICA 64 (3), 2019

A1

scapula proximal plate

ridge

A2

scapula blade

A3

scapula proximal plate

A4

acromial ridge

scapulaproximal
plate

ridge

glenoid fossa

scapula blade

A5

A6

groove
100 mmFig. 13. Left scapula of stegosaur GPDM 178, near Giffen, Cascade
County, Montana; in dorsal (A1), ventral (A2), lateral (A3), anterior (A4),
medial (A5), and posterior (A6) views.

(Carpenter et al. 2001) and Stegosaurus stenops (NHMUKPV R36730; Maidment et al. 2015).
Scapula: The left scapula is preserved (Fig. 13). The
blade is distally incomplete and the whole of the medial
border has been reconstructed with epoxy clay. The dorsal
surface of the scapular blade is nearly straight, and flares
slightly distally. In lateral view, a ridge extends anteroposteriorly along the scapula blade, as in Stegosaurus stenops(NHMUK PV R36730). The proximal plate is square in
outline with a prominent dorsoventrally oriented acromial
ridge, as in Stegosaurus stenops (Maidment et al. 2015).
In both dorsal and ventral view, the scapula has a gentle
laterally oriented curvature. The apex of this curvature isapproximately at mid length. In anterior view, the proximal
plate is triangular in cross section. The ventral aspect of theplate is the transversely widest portion, and both lateral andmedial borders taper proximally. The scapular portion of the
glenoid fossa is transversely widened and oval in anteriorview. In ventral orientation, the overall profile is triangular,
the anterior region is transversely widened and strongly
tapers posteriorly. The anterior aspect is over 6 times astransversely wide than the posteriormost region.

Coracoid: The left coracoid is not fused with the scapula(Fig. 14). Fusion of the scapula and coracoid in stegosaurs is
intraspecifically variable and may be related to ontogenetic
stage (Maidment et al. 2015). The anterior and dorsal marginof the coracoid is missing and has been reconstructed in epoxy clay. This reconstruction, likely not an accurate representation, gives the coracoid a pentagonal profile. In other stegosaurs the coracoid is sub-circular (Miragaia longicollum,
ML 433, Mateus et al. 2009; Stegosaurus stenops, NHMUKPV R36730, Maidment et al. 2015) to ovoid (Kentrosaurus
aethiopicus, MB R.4802 and MB R.4803, Hennig 1925).
However, while the anterior and dorsal margins are incomplete, the ventral, glenoid, and posterior margins merge with
distinct corners or breaks of slope, unlike that of Stegosaurus
stenops (NHMUK PV R36730; Maidment et al. 2015).
Humerus: The left humerus of GPDM 178 is largely complete, except for some large cracks filled with epoxy clay
and a reconstructed apex on the deltopectoral crest (Fig. 15).
In anterior view, the humerus has the dumb-bell profile observed in GPDM 205 and other stegosaurs (Maidment et al.
2015, 2018). The overall morphology of deltopectoral crestof GPDM 178 is very similar to that of the Stegosaurus
stenops (NHMUK PV R36730) and the Hesperosaurus
mjosi (MOR 9728; Maidment et al. 2015, 2018). The deltopectoral crest occupies approximately the proximal half ofthe element. Transversely, the deltopectoral crest is fairly
uniform in thickness, with a gentle taper along the distal
aspect. The anterolateral margin of the deltopectoral crest
is rugose and bears a slight rugose “lip” similar to GPDM
205; however, this rugosity and lipped region are much lesspronounced in GPDM 178, and more akin to the morphologies of Stegosaurus stenops (NHMUK PV R36730) andHesperosaurus mjosi (MOR 9728; Maidment et al. 2015,2018). While the deltopectoral crest of GPDM 178 does havea slight anteromedial curve, it is nowhere near as curved
or bowl-shaped as GPDM 205 or Miragaia longicollum(ML 433; Mateus et al. 2009). As in Stegosaurus stenops(NHMUK PV R36730) and Hesperosaurus mjosi (MOR
9728; Maidment et al. 2015, 2018), in anterior view the deltopectoral crest of GPDM 178 is somewhat proximodistallystraight with the distal end oriented more medially. However,in lateral or medial view, the deltopectoral crest of GPDM
178 is like that of GPDM 205 in that there is a dip in the slope
along the proximal contact between the crest and humeralbody. This break in slope is weaker than that of GPDM
205, but it is not the continuous, undisrupted contact seen in
the humerus of Stegosaurus stenops (NHMUK PV R36730;
Maidment et al. 2015) or Miragaia longicollum (ML 433;
Mateus et al. 2009). The humeral head is bulbous, roundedin outline, and offset posteriorly. The humeral head is 61% of
the transverse width of the distal condyles. As in GPDM 205,
distal to the humeral head on the medial side of the humerus
is a symmetrical, triangular-shaped process. This triangularprocess (which is opposed the deltopectoral crest) projects
anteromedially, and like the deltopectoral crest is proportionally smaller in GPDM 178 compared to that of GPDM 205. In

WOODRUFF ET AL.—LATE JURASSIC STEGOSAURS FROM MONTANA, USA

A1

A2

A3

473

A4

100 mmglenoid fossa

coracoid foramen

Fig. 14. Left coracoid of stegosaur GPDM 178, near Giffen, Cascade County, Montana; in anterior (A1), lateral (A2), posterior (A3), and anterior (A4)views.

humeralhead

A5deltopectoral
crest

deltopectoral
crest

humeralhead

deltopectoral
crest

deltopectoral
crest

latissimus
tubercle

medialcondylemedialcondylehumeralhead

A6

lateralcondylemedialcondylelateral condyle

A1

A2

A3

A4

100 mmFig. 15. Left humerus of stegosaur GPDM 178, near Giffen, Cascade County, Montana; in anterior (A1), posterior (A2), lateral (A3), medial (A4), dorsal(A5), and ventral (A6) views.

lateral view the lateral dorsal process is slightly anteroposteriorly inclined with a slight anterior oriented concavity.
The nearly intact diaphysis appears to be circular to
sub-circular in cross-section. The sub-rounded diaphysistransversely expands into the proximal and distal ends. This
straight diaphysis, in combination with the slightly posteriorly angled proximal end and the slightly anteriorly angled
distal end, gives the humerus a slightly sigmoidal profile
in lateral view (as in GPDM 205, the Hesperosaurus mjosi,
MOR 9728, Maidment et al. 2018, and to a much lesser
degree in the Stegosaurus stenops NHMUK PV R36730,
Maidment et al. 2015; versus the practically straight humeral profile of Miragaia longicollum ML433, Mateus et al.
2009). The distal portion of the humerus terminates with thetransversely widened and bulbous articular condyles. In all
views the medial epicondyle is ovoid in overall morphology;
but as in GPDM 205, in lateral and medial views, there is aslight triangular, anteriorly oriented eminence. The lateralepicondyle is by far the larger of the two condyles. In anterior and posterior view, the lateral epicondyle has a sub-circular profile and is 1.16 times as proximodistally long as
the lateral epicondyle. In ventral view, both the lateral andmedial epicondyles are ovoid and have similar orientations.
In ventral view the lateral epicondyle is 1.5 times as transversely wide and ~1.3 times as anteroposteriorly elongate
than the medial epicondyle.
Ulna: Like the humerus, the left ulna of GPDM 178 islargely complete, and well preserved (Fig. 16). Likewise, allof the forelimb material of GPDM 178 appears to be much
more “gracile” than those counterparts of GPDM 205 (andthese differences are addressed in the Discussion section).
In all views, the majority of the element is dorsoventrally
straight, with only a slight kink in the distalmost end. The
olecranon is the most prominent of the ulnar processes—

474

A1

ACTA PALAEONTOLOGICA POLONICA 64 (3), 2019

A2

A3

A4
olecranon
processA5

olecranon
process

medialprocess

olecranon
process

anterior
process

medialprocess

medial process
anterior process

A6

100 mmFig. 16. Left ulna of stegosaur GPDM 178, near Giffen, Cascade County, Montana; in medial (A1), posterior (A2), lateral (A3), dorsal (A4), and ventral(A5) views.

being approximately twice the dorsoventral height of themedial process, and rather bulbous anteroposteriorly andlateromedially.
In anterior view, the anterior and medial processes aresubparallel to one another (with the anterior process being slightly more proximal). The transition between thesethree proximal processes (the olecranon, anterior, and medial processes) is gentle and curved, more akin to that of
Stegosaurus stenops (NHMUK PV R36730; Maidment etal. 2015) than to GPDM 205.
Distal to the proximal processes, there is a slightly flared“ring” or region that demarks these processes from the diaphysis. The diaphysis distally tapers and is sub-circular
to ovoid at mid diaphysis cross section. The distal end of
the ulna is slightly expanded and kinked relative to thelong axis. In distal view, the distalmost aspect of the ulna
is convexed and shaped like an isosceles triangle, with the
apex pointing anteriorly. A flat facet for the distal end of
the radius is present, but the size of this facet and the adjacent dorsoventrally oriented ridge is more reduced thanthese features seen in GPDM 205 and Stegosaurus stenops(NHMUK PV R36730, Maidment et al. 2015).
Radius: The radius, like the other forelimb elements of
GPDM 178 is well preserved (Fig. 17). Large cracks in thediaphysis are filled in with epoxy clay, but no portions of
the element are sculpted or reconstructed. Like the radius of
GPDM 205, the radius of GPDM 178 is dorsoventrally elongate, with a transversely widened distal region. More akinto that of Kentrosaurus aethiopicus (Galton 1982b), then thedorsoventrally short and rectangular radius of Stegosaurus
stenops (NHMUK PV R36730, Maidment et al. 2015).
Tibia: The right tibia of GPDM 178 is represented by the
distal and proximal ends (Fig. 18). The proximal end was thefirst element recovered from the specimen (see the Material
section above). While there was no diaphysis recovered, nor
do we know approximately where the distal section was
found in relation, given the similar states of preservation it
is most parsimonious to assume these ends are parts of thesame element.
In proximal view, the proximal end is broadly oval, being anteroposteriorly longer than it is transversely wide.
The medial surface is convex, while the lateral surface
bears a strongly laterally projecting process for articulation of the fibula. The fibula process is very well-developed and is rounded in outline in lateral and anterior view.
A very well-developed fibula process is also present in
Hesperosaurus mjosi (MOR 9728; Maidment et al. 2018).
The cnemial crest is poorly preserved. The distal end isheart-shaped in distal view, with the apex pointing posteriorly. In anterior view, the distal end is separated into two
distinct surfaces, the lateral malleolus and the medial malleolus, separated from each other by a groove. The anterior
surfaces of the malleoli are flattened; the lateral malleolus
is somewhat posteriorly offset by postmortem deformation.
The fibula articulated with the flattened surface of the lateral malleolus. The lateral malleolus projects further ventrally than the medial malleolus, onto which the astragalus
would have articulated distally. In posterior view, a verticalridge extends dorsally along up the midline of the bone.
Dermal armor: While four plates are recorded in thequarry log for GPDM 178 (three are identified as cervicalplates and one as a dorsal plate), only one, a dorsal plate, isat this time fully prepared and available for study.
Dorsal plate: In notes and drawings made during excavation, this plate is located near the pelvic girdle, suggesting that it may be a large dorsal plate situated over the

WOODRUFF ET AL.—LATE JURASSIC STEGOSAURS FROM MONTANA, USA

A1

A2

A3

475

A4

A5

intermuscular
ridge

A6

100 mmFig. 17. Left radius of stegosaur GPDM 178, near Giffen, Cascade County, Montana; in anterior (A1), posterior (A2), medial (A3), lateral (A4), dorsal (A5),and ventral (A6) views.

A1

lateral process

cnemial crest

A2

A3

A4

cnemialcrest

100 mm

A5

A6
medialmalleous

A7
A8

lateralmalleous

Fig. 18. Partial right tibia of stegosaur GPDM 178, near Giffen, Cascade County, Montana; in medial (A1, A6), lateral (A2, A7), anterior (A3, A5), dorsal(A4), and ventral (A8) views. A1–A4, proximal end, A5–A8, distal end.

476

ACTA PALAEONTOLOGICA POLONICA 64 (3), 2019

100 mmFig. 19. Dorsal plate of stegosaur GPDM 178, near Giffen, Cascade County,
Montana.

pelvis, by comparison with Stegosaurus stenops (NHMUKPV R36730; Maidment et al. 2015). This incomplete plate,
superficially triangular in shape, has a gentle convex ventral
margin (Fig. 19). The entire ventral margin of the dorsal plateof GPDM 178 is transversely widened (86.9 mm). Unlike
other stegosaur plates that dorsally or anteroposteriorly taper
towards the margins (as Saitta 2015 showed via CT scans),
aside from the transversely widened ventral margin, the restof the plate is a fairly uniform thickness of nearly 2 cm. Thisrather uniform plate thickness coupled with the widened
ventral margin gives the plate more of an inverted T-shaped
cross-sectional profile; again, differing from the usually observed dorsally tapering profile. The non-tapering aspectof the plate could be attributed to taphomonic processes
where the thinning margins were damaged. Likewise, due todamage it is not possible to ascertain if the plate possessed
the anteriormost “stepped” profile seen in plates 13–15 in
Stegosaurus stenops (NHMUK PV R36730; Maidment etal. 2015). Given the plate size and the convex ventral margin, assuming similar plate homology corresponds seriallywithin stegosaurs, we can only hypothesize that the dorsal
plate of GPDM 178 corresponds to plate 13–16; potentiallyserially placing it more posteriorly than previously believed.

Discussion
Taxonomy.—Within the Morrison Formation there are threeknown stegosaur genera: Alcovasaurus, Hesperosaurus,
and Stegosaurus.

According to Galton and Carpenter (2016) Alcovasaurus
bears the following autapomorphies: (1) all caudal vertebrae
bear transverse processes, (2) distal caudal centra short, soheight greater than length, (3) femoral condylar articularsurface confined almost exclusively to the distal surface, (4)two pairs of greatly elongate distal dermal tail spines (~90%of femoral length) with subequal bases and slender diaphysis, (5) posterior pair widest at ~25% of length. It is further
distinguishable from other Morrison Formation stegosaursby (6) having six pairs of sacral ribs.
Maidment et al. (2018) diagnosed Hesperosaurus using
the following combination of autapomorphies (marked by an*) and plesiomorphic characters: (1) no contact between the
lacrimal and prefrontal on the lateral surface of the skull, (2)ventral margin of axis upwardly concave in lateral view*,(3) postzygapophyses not elevated significantly on posteriorcervical vertebrae, (4) prezygapophyses join ventrally andface dorsomedially in anterior dorsal vertebrae, (5) dorsal
neural arch pedicels not elongated above the neural canal in
mid-dorsal vertebrae, (6) dorsal ribs distally expanded, (7)ossified epaxial tendons present, (8) caudal neural spines
not bifurcated, (9) dorsal dermal plates longer anteroposteriorly than tall dorsoventrally*, (10) 11 dorsal vertebrae*, and(11) fourth sacral vertebra not fused to the sacrum*.
Finally, Stegosaurus stenops can be diagnosed by the following autapomorphies (Maidment et al. 2015): (1) portion of
the dentary anterior to the tooth row and posterior to the predentary edentulous; (2) dorsally elevated postzygapophysesof cervical vertebrae; (3) bifurcated summits of neural spinesof the anterior and middle caudal vertebrae; (4) presence ofdermal ossicles embedded in the skin on the underside of thecervical region.
As GPDM 205 is only represented by a prepared humerus,
ulna, and radius, and no Alcovasaurus, Hesperosaurus, orStegosaurus forelimb autapomorphies exist, we can only
tentatively refer to it as Stegosauria indeterminate.
None of the autapomorphy-bearing elements of Alcovasaurus are preserved in GPDM 178, and thus we are unable
to compare the specimen to that taxon. In comparison to
Hesperosaurus, in GPDM 178 it is only possible to assesscharacters (3) and (9) of Maidment et al. (2018) based on theavailable material. Character (9) cannot be assessed because
the prepared dorsal plate is incomplete, so its profile is unclear. Finally, regarding Stegosaurus stenops, only characters (1) and (2) can be assessed based on the available material. Character (1) is present in GPDM 178. However, it is nota robust autapomorphy of Stegosaurus because a diastema
between the predentary facet and dentary tooth row is also
known in Jiangjunosaurus (Jia et al. 2007). Additionally, because the dentary is unknown in Hesperosaurus (Carpenter
et al. 2001; Maidment et al. 2018) and Alcovasaurus (Galton
and Carpenter 2016), it is difficult to assess the importanceof the feature for Morrison Formation stegosaurs. Character(3) from the Hesperosaurus diagnosis and character (2)from the Stegosaurus diagnosis serve to differentiate thetwo taxa. GPDM 178 has elevated postzygapophyses, an

WOODRUFF ET AL.—LATE JURASSIC STEGOSAURS FROM MONTANA, USA

Ontogeny.—Though GPDM 178 and 205 have not yet been
histologically sampled to document their maturational
states, their relative sizes contribute to the ongoing discussion of latitudinal body size amongst Morrison Formation
stegosaurs proposed by Maidment et al. (2018). In comparing humeral length, GPDM 178 and 205 appear to fall justwithin the upper half of humeral lengths (Fig. 20), yet, asshown by Maidment et al. (2018), humeral length does notadequately correlate to maturational age in stegosaurs. The
smallest and largest Hesperosaurus mjosi humeri (MOR
9728, 390 mm; SMA0018, 545 mm) are also the histologically most mature specimens, indicating skeletal plasticity
in this genus. Histologically aging GPDM 178 and 205 will
help to determine if these specimens contribute to our understanding of body size versus maturity in stegosaurs (i.e.,does the relative size and morphology of these specimens
denote maturity, or morphologies consistent with the biomechanics of their overall size).
Additionally, Maidment et al. (2018) proposed that bodysize difference between Hesperosaurus and Stegosaurus
might relate to ecologic partitioning where the larger
Stegosaurus occupied the more arid environments, while thesmaller Hesperosaurus occupied the wetter, more vegetatedenvironments. Unfortunately, with the current available
data, the relative size of these two specimens does not aidein elucidating taxomony (both GPDM 178 and 205 could
be “larger” Hesperosaurus, or “mid-sized” Stegosaurus).
However, it would be very interesting if these specimens,
particularly GPDM 205, were Stegosaurus. Given the environmental partitioning theory of Maidment et al. (2018) andthe high northern latitude of these specimens (even in the
Late Jurassic), if they were Stegosaurus, this could indicate
the change to a more arid climate following the northward
regression of the Sundance Seaway.
Though histologic analysis of either GPDM 178 or 205
has yet to occur, some postcranial morphologies could one
day substantiate generic level maturational states. In a review of immature Stegosaurus sp. remains from Utah andWyoming, Galton (1982a) noted 14 postcranial morphologies attributable to ontogenetic development. While none ofthese “ontogenetic indicators” of Galton (1982a) have been
histologically tested, we subscribe that many have the poten-

humerus length (mm)
DNM 2438 (Stegosaurus sp.)

164

DNM 2438 (Stegosaurus sp.)168

YPM 4634 (Stegosaurus sp.)290

USNM 7409 (Stegosaurus sp.)

452

USNM 4929 (Stegosaurus stenops)

506

USNM 4929 (Stegosaurus stenops)

512

USNM 4934 (Stegosaurus stenops)

572

comparisonof stegosaur
humeral lengths
150

250

350

USNM 4937 (Stegosaurus “sulcatus”) 609
USNM 6646 (Stegosaurus “ungulatus”) 610
MOR 9728 (Hesperosaurus mjosi)390

SMA 0092 (Hesperosaurus mjosi)

403

VFSMA 001 (Hesperosaurus mjosi)

438

SMA 0018 (Hesperosaurus mjosi)

545

GPDM 205 (“Stegosauria indet.”)454

GPDM 178 (“Stegosauria indet.”)

516

450

humerus length (mm)autapomorphy of Stegosaurus, but comparisons with MOR
9728 (Hesperosaurus; Maidment et al. 2018) indicate little
difference in the character states between the two, suggesting this character needs reassessment in the light of new
discoveries. Due to the lack of robust autapomorphies of any
genus of stegosaur in GPDM 178, we consider the specimen
as Stegosauria indeterminate.
It is clear that the fragmentary nature of many stegosaur specimens and the paucity of material for most genera
means that the taxonomy of stegosaurs continues to be problematic, and each new discovery leads to re-diagnosis. Onlyby the discovery and description of new specimens will webe able to clearly understand the diversity of Stegosauria.

477

550

650

Fig. 20. Stegosaur humeral lengths (measurement from Gilmore 1914,
Galton 1982a, and Maidment et al. 2018). The grading colors represent thetransitions from the smallest to largest specimens. Circles, Stegosaurus;
triangles, Stegosauria indeterminate; squares, Hesperosaurus.

tial to be legitimate. Among Galton’s (1982a) “ontogenetic
indicators”, some are fusion (open vertebral synostosis, lackof a fused scapula-coracoid; Galton 1982a) or proportion(slender and elongate scapulae, more gracile metacarpals;
Galton 1982a) related, and four are directly related to longbone soft-tissue scarring (Galton 1982a: characters 4 and11c) and proportion of long bone processes (Galton 1982a:
characters 5, 6, 11b). Galton (1982a) noted that in immature animals, the surface of the long bones are relatively
smooth, compared to the highly rugose and irregular articular end in mature animals. Likewise, in immature animals
the triceps ridge and the olecranon process are much lessprominent than their mature counterparts (Galton 1982a).
Galton (1982a) noted that since of these traits were present
within other taxa (Dacentrurus and Kentrosaurus), they
were likely general stegosaur growth patterns.
In comparing GPDM 178 and 205, the deltopectoral creston the humerus of GPDM 205 is greatly enlarged and robust
compared to that of GPDM 178. An enlarged deltopectoral
would correspond to the observations of Galton (1982a).
In discussing the enlarged olecranon process of Miragaia
longicollum, therein referred to as the “olecranon horn”,
a similar consensus was reached by Mateus et al. (2009)
in suggesting that the growth of this process developed
throughout ontogeny. These ridges, processes, and condylesof the limb elements are all associated with connective tissues, muscles, ligaments, tendons, and cartilage. Schwarz et478

al. (2007) noted calcified cartilage along the distal marginof the humerus from the sauropod Cetiosauriscus greppini,
while Wilson et al. (2016) noted the biomineralization of thevertebral ligaments throughout body size and ontogeny intheropods, and Cerda et al. (2015) likewise documented theoccurrence of biomineralized vertebral ligaments and their
possible ontogenetic development in nine sauropods. Thus,
biomineralization of connective tissues, and their development throughout ontogeny, is not a dinosaurian precedent.
We must reiterate that a histologic analysis is the only way
to substantiate such, but in agreement with Galton (1982a),perhaps in stegosaurs, as the individuals increase in age
and body mass, the cartilaginous covered ends of the ulnae(the “olecranon horn” of Mateus et al. 2009) and the ligamentous/tendinous attachment sites along the deltopectoral
crests biomineralize (through metaplasia; sensu Horner et
al. 2016) through ontogeny. If true, then we could hypothesize that GPDM 178 is less mature than GPDM 205. Whilemorphology would seem to substantiate such development,
currently this is a speculative hypothesis, and we await ahistological analysis to test for such.
Tooth wear.—The high prevalence of macro- and microwear on the teeth of GPDM 178 would seem to indicate that(i) this animal was possibly feeding on coarser vegetation(the possibility of a mixed vegetation diet was also proposedby Lautenschlager et al. 2016), and/or (ii) these combinations of extensive tooth wear and cross-cutting micro features may indicate a complex chewing motion. Although
Lautenschlager et al. (2016), via cranial biomechanics, suggested that complex chewing in stegosaurs was unlikely,
Nabavizadeh (2016) and Nabavizadeh and Weishampel(2016) discussed and reviewed stegosaur feeding motions.
Possessing a cropping rhampotheca, the orthal and minor
long-axis rotation of the dentaries would likely produce an
initial orthal jaw motion possibly followed by a minor palinal
motion (i.e., the jaws vertically come together, followed bya posterior motion while in occlusion; Barrett 2001; Galtonand Upchurch 2004; Nabavizadeh 2016; Nabavizadeh andWeishampel 2016).
The extensive wear facets of GPDM 178 would seemto indicate in part, a more complex chewing motion or jaw
mechanics than previously assessed. Both Barrett (2001)
and Nabavizadeh (2016) note that in Stegosaurus, the equal
rostrocaudal lengths in the glenoid fossa of the jaw joint andthe ventral condyles of the quadrate would primarily produce an orthal jaw motion. Additionally, with a mesially focused bite force, the relative bite force (Nabavizadeh 2016)of Stegosaurus stenops (USNM 4934) is low compared tothat of hadrosaurs and ceratopsians (0.6471 vs. 1.5595 and
2.7, respectively; Nabavizadeh 2016). This lower relative
bite force could in part explain the “typical” lack of prominent wear facets observed in stegosaurs. While missing the
portions of the jaw joint, the sub-horizontal wear facets of
GPDM 178 would seem to corroborate a primarily orthal
occlusion (sensu Barrett 2001 and Nabavizadeh 2016). The

ACTA PALAEONTOLOGICA POLONICA 64 (3), 2019“flatness” could also have been formed in part by a secondary palinal motion. This is noteworthy because Nabavizadeh
and Weishampel (2016) proposed that stegosaurs may haveincorporated a palinal jaw motion when chewing; however, they stated that dental microwear would be neededfor verification. The horizontal facets of GPDM 178 appear
to verify the jaw motion hypothesis of Nabavizadeh andWeishampel (2016).
Additionally, the micro-gouges within the wear facets
offers added jaw-motion insight. Following cross-cutting relationships, we hypothesize that in GPDM 178 the maxillary
and dentary teeth orthally occluded, followed by a palinal
occlusal motion indicated by the ~anteroposterior gouges.
Finally, the cross-cutting, laterally oblique micro gouges
could have been produced from the rotation of the dentaries;
which we presume would have been caused by the forward
rotational movement of the dentaries. Although we do notknow the role of vegetation types regarding stegosaur wear
facets, such prominent facets in GPDM 178 would indicatethat extensive chewing was required when this animal was
feeding.

Conclusions
The specimens GPDM 178 and 205 represent the northernmost stegosaurs found to-date, and GPDM 178 is the northernmost dinosaur fossil thus far recovered from the entire
Morrison Formation. Interestingly, according to a Works
Progress Administration report from 1938, a “Stegosaurus”
was found at Sheep Creek, 25 miles north of Great Falls,
MT (Syverud 1938). If true, then this specimen is the northernmost Morrison Formation dinosaur, as GPDM 178 was
found near equivalent distance southeast of Great Falls, MT.
Unfortunately, this report does not mention what material
was collected or where it was sent. Searching for this material in Montana institutions with historic paleontologicalcollections and the USNM has yielded no results. Therefore
we are left to believe that these remains, if true, have subsequently been lost or discarded. Thus by default, GPDM 178
shall remain the northernmost record.
These two new specimens, consisting of rare cranial
as well as post-cranial material, increase the number ofMorrison Formation dinosaur specimens recovered fromMontana. Saitta (2015) reported five specimens from the
JRDI 5ES Quarry, the same locality as GPDM 205. These
specimens have always resided in a private collection, andare thus unavailable to study, despite the declaration madeby Saitta (2015) to the contrary. One stegosaur (unknown genus) is being collected by the Bighorn Basin Paleontological
Institute in Carbon County, Montana, and another stegosaur specimen is awaiting excavation in the vicinity ofLewistown, Montana (DCW personal observations). Finally,
combining MOR 9728 and two other fragmentary specimens from a single locality in Livingston, Montana, thisbrings the number of stegosaur specimens from Montana

WOODRUFF ET AL.—LATE JURASSIC STEGOSAURS FROM MONTANA, USA

to 12. The Paleobiology Database (www.paleobiodb.org)
lists 72 occurrences of stegosaurs from North America (67Stegosaurus, four Hesperosaurus, and one Alcovasaurus)
meaning that the Montana specimens account for 16% of the
known Morrison Formation record. However, only publishedfossil remains are recorded in the Paleobiology Database,and it is likely that further unpublished specimens reside inmuseum collections across the western USA. Nonetheless,considering the relative paucity of research on the Morrison
Formation in Montana in comparison to better-known areas in Colorado and Utah, the fossil record of stegosaurs in
Montana indicates that these thyreophorans were a common
faunal component in the north of the Morrison basin.

Acknowledgements
First and foremost, we would like to thank all of the board members of
the Judith River Foundation for recognizing the significance of GPDM
178 and GPDM 205 and allowing us the opportunity to share these
specimens with the scientific community. Thanks to the Hein and Yurek
families of Grass Range and Great Falls, MT for donating these specimens to the Great Plains Dinosaur Museum and Field Station. ReBecca
Hunt-Foster (Dinosaur National Monument, Vernal, Utah, USA) andAli Nabavizadeh (Rowan University, Glassboro, New Jersey, USA)provided invaluable comments and suggestions that greatly improved
this manuscript. Jack Wilson (Badlands Dinosaur Museum, Dickinson,
North Dakota, USA) provided many of the photographs of GPDM
178 used in this manuscript. Cory Coverdell (Two Medicine Dinosaur
Center, Bynum, Montana, USA), Jack Wilson, and the Two Medicine
Dinosaur Center prepared many of the GPDM 178 elements examinedherein. Genya Masukawa (Ibaraki University, Mito, Japan) graciously
provided the skeletal reconstructions of Hesperosaurus mjosi used inFig. 2. Thanks to the Hein family for allowing DCW to visit the GPDM
205 locality in 2016 and for personal accounts. Dixie Stordahl (Malta,Montana, USA), Sue Frary (Helena, Montana, USA), and Linda Yurek(Great Falls, Montana, USA) provided invaluable firsthand accounts
on the history and excavation of GPDM 178.

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