Histology and Ontogeny of Pachyrhinosaurus Nasal Bosses
byElizabeth Kruk

A thesis submitted in partial fulfillment of the requirements for the degree ofMaster of Sciencein Systematics and Evolution

Department of Biological Sciences
University of Alberta© Elizabeth Kruk, 2015

Abstract
Pachyrhinosaurus is a peculiar ceratopsian known only from Upper Cretaceous strata ofAlberta and the North Slope of Alaska. The genus consists of three described species
Pachyrhinosaurus canadensis, Pachyrhinosaurus lakustai, and Pachyrhinosaurus perotorum
that are distinguishable by cranial characteristics, including parietal horn shape and orientation,
absence/presence of a rostral comb, median parietal bar horns, and profile of the nasal boss. A
fourth species of Pachyrhinosaurus is described herein and placed into its phylogenetic context
within Centrosaurinae. This new species forms a polytomy at the crown with Pachyrhinosaurus
canadensis and Pachyrhinosaurus perotorum, with Pachyrhinosaurus lakustai falling basal tothat polytomy. The diagnostic features of this new species are an apomorphic, laterally curved
Process 3 horns and a thick longitudinal ridge separating the supraorbital bosses.
Another focus is investigating the ontogeny of Pachyrhinosaurus nasal bosses in ahistological context. Previously, little work has been done on cranial histology in ceratopsians,
focusing instead on potential integumentary structures, the parietals of Triceratops, and how
surface texture relates to underlying histological structures. An ontogenetic series is established
for the nasal bosses of Pachyrhinosaurus at both relative (subadult versus adult) and fine scale(Stages 1-5). It was demonstrated that histology alone can indicate relative ontogenetic level, but
not stages of a finer scale. Through Pachyrhinosaurus ontogeny the nasal boss undergoes
increased vascularity and secondary remodeling with a reduction in osteocyte lacunar density.
A histological study of ceratopsian cranial elaborations was also performed to better
understand the functional and developmental implications of these structures, as well to place thenasal boss of Pachyrhinosaurus into the context of ceratopsian elaborations. In centrosaurines,
parietal spikes and hooks, postorbital horncores, bosses, and nasal horns are formed asii

outgrowths of the dermatocranium. Although the bone tissue type remains fibrolamellar across
any given specimen, organization of the bone varies (cortex versus core, compact versus spongy)
across cranial elaborations. However, epoccipitals (epiparietals and episquamosals) form via
metaplasia. This may also be the case for epinasals in chasmosaurines, which do not form fromthe nasal bone, but instead fuse onto the nasal-rostral complex later in life, indicating that they
are not outgrowths of the dermatocranium.
Historically, cranial histology is a poorly studied component of paleohistology and is
expanded upon in this thesis. The ontogeny of Pachyrhinosaurus nasal bosses is explored in ahistological perspective, which gives deeper understanding to how these atypical nasal
ornamentations form. New fossil reports have expanded our understanding of Pachyrhinosaurus
diversity, although their relationship to each other is not entirely clear. This increased
understanding has revealed interesting evolutionary patterns, such as the replacement of nasal
horns with nasal bosses by the last of the centrosaurines, the Pachyrostra (Achelousaurus +Pachyrhinosaurus). Here I provide new phylogenetic, historical, and histological research that
informs these topics.

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Preface
This thesis is an original work by Elizabeth Kruk. No part of this thesis has been previously
published.

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Acknowledgements
I would like to thank Dr. Philip Currie for supervising my thesis and Dr. Michael
Caldwell for all the help and feedback they have given me over the years. I want to thankBrandon Strilisky and Don Brinkman from the Royal Tyrrell Museum of Palaeontology forallowing me to visit their collections and thin section material. Also, Dr. Jordan Mallon,
Margaret Currie, and Kieran Shepard from the Canadian Museum of Nature for allowing me tovisit collections. Dr. David Evans and all the graduate students associated with paleontology at
the University of Toronto, Toronto-Mississauga, and the Royal Ontario Museum for hosting apaleohistology workshop. Thanks to Dr. Martin Sander and associated students at Universität
Bonn for hosting a histology workshop. John Horner and Ellen-Therese Lamm from the Museumof the Rockies for guidance on my thin sections and allowing observation of their thin sectioning
techniques. Dr. Tobin Hieronymus for pachyrhinosaur guidance and sending me histologicalslides to use for my studies. I would like to thank all the graduate students of the University ofAlberta Laboratory for Vertebrate Paleontology, both past and present, for their guidance andfriendship over the years; especially Michael Burns who taught me the basics of histology andthin sectioning for without him this project would not have been entirely possible. Also, thanks
to my friends who helped me even from far away, from bouncing around ideas to editing mywork, it’s all been appreciated. A special thanks to Sydney Mohr for drawing the reconstructionin Chapter 3. I want to thank the Faculty of Graduate Studies and Research, the Graduate Student
Association, and the Dinosaur Research Institute for providing funding.

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Table of Contents
Introduction ……………………………………………………………………………… 1
Chapter 1 ……………………………………………………………………………….. 11“Histological and morphological ontogeny of Pachyrhinosaurus nasal bosses”
Chapter 2 ……………………………………………………………………………….. 28“Histological study of cranial elaboration in ceratopsian dinosaurs: Functionaland developmental implications”
Chapter 3 ……………………………………………………………………………….. 54“A new species of Pachyrhinosaurus from the Wapiti Formation (LateCampanian) of Alberta”
Conclusions …………………………………………………………………………….. 78
References ……………………………………………………………………………… 81
Appendix ……………………………………………………………………………….. 87vi

List of Tables
Chapter 1
Table 1.1 ……………………………………………………………………….. 23
Table 1.2 ……………………………………………………………………….. 24
Chapter 2
Table 2.1 ……………………………………………………………………….. 31
Chapter 3
Table 3.1 ……………………………………………………………………….. 60

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List of Figures
Chapter 1
Figure 1.1 ………………………………………………………………………. 17
Figure 1.2 ………………………………………………………………………. 19
Figure 1.3 ………………………………………………………………………. 21
Figure 1.4 ………………………………………………………………………. 23
Figure 1.5 ………………………………………………………………………. 25
Chapter 2
Figure 2.1 ………………………………………………………………………. 33
Figure 2.2 ………………………………………………………………………. 35
Figure 2.3 ………………………………………………………………………. 37
Figure 2.4 ………………………………………………………………………. 39
Figure 2.5 ………………………………………………………………………. 41
Figure 2.6 ………………………………………………………………………. 43
Figure 2.7 ………………………………………………………………………. 45
Figure 2.8 ………………………………………………………………………. 47
Figure 2.9 ………………………………………………………………………. 49
Figure 2.10 ……………………………………………………………………... 51
Chapter 3
Figure 3.1 ………………………………………………………………………. 57
Figure 3.2 ………………………………………………………………………. 59
Figure 3.3 ………………………………………………………………………. 62
Figure 3.4 ………………………………………………………………………. 65
Figure 3.5 ………………………………………………………………………. 66
Figure 3.6 ………………………………………………………………………. 69
Figure 3.7 ………………………………………………………………………. 72
Figure 3.8 ………………………………………………………………………. 73

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Abbreviations
GPRC, Grande Prairie Regional College, Grande Prairie, Alberta, Canada; TMP,
Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada; UALVP,
University of Alberta Laboratory for Vertebrate Paleontology, Edmonton, Alberta,
Canadaix

INTRODUCTION
Pachyrhinosaurus is a genus of centrosaurine ceratopsians found only in Upper
Cretaceous strata of Alberta and Alaska of North America. The Pachyrostra (Achelousaurus +Pachyrhinosaurus sensu Fiorillo and Tykoski, 2012) is the youngest centrosaurine clade toappear in the fossil record (Ryan, et al., 2010). Pachyrhinosaurus contains three described
species: Pachyrhinosaurus lakustai from the Wapiti Formation of northern Alberta (upper
Campanian); (Currie et al., 2008); Pachyrhinosaurus canadensis from the Horseshoe Canyon
Formation of southern Alberta (upper Campanian) (Sternberg, 1950; Langston, 1967, Langston1975); and Pachyrhinosaurus perotorum from the Prince Creek Formation from the North Slopeof Alaska (upper Campanian to lower Maastrichtian; Fiorillo and Tykoski, 2013). Additionally,
Ryan et al. (2010) tentatively refers to material recovered from the upper Dinosaur Park
Formation of Alberta as Pachyrhinosaurus. Pachyrhinosaurus lakustai is stratigraphically older
than Pachyrhinosaurus canadensis and is different based on the presence of a rostral comb, an
upturned “spout” (in some specimens) on the anterior end of the nasal boss, and an epiparietal
Process 3 that twists anterolaterally, as well as horns on the median parietal bar.
Pachyrhinosaurus perotorum differs from the other named species by the presence of a medianhump on the nasal boss, a rostral comb, and an anterior facing parietal horn not seen in any other
Pachyrhinosaurus specimens (Fiorillo and Tykoski, 2012). Pachyrhinosaurus canadensis andPachyrhinosaurus perotorum both have nasal and supraorbital bosses that are nearly in contact,
only separated by a narrow groove, whereas Pachyrhinosaurus lakustai and the pachyrhinosaur
from Dinosaur Park have widely separated nasal and supraorbital bosses. The growth andvariation of nasal bosses has never been investigated in a histological context.
Gross morphology is not the only valuable source of information from fossils; histology,
the study of tissues, can provide evidence to further our understanding of dinosaurs. The
microstructure of bone records the effects of various factors including rate of bone formation,
environmental conditions, biomechanics, and lifestyle adaptations (Chinsamy-Turan, 2005).
Studies on the histology of dinosaur bone histology show that limb bones often preserve a
growth record for the individual (Erickson and Tumanova, 2000; Horner et al., 2000; Padian etal., 2001; Horner and Padian, 2004; Erickson 2005; Chinsamy-Turan, 2005; Klein and Sander,
2007; Erickson et al., 2009; Erickson and Druckenmiller, 2011). Lines of arrested growth areoften used with confidence in extant animals to estimate age and the corresponding structures in1

fossil bones convey similar information (Levitt, 2013). Chinsamy (1994) estimates the age of
specimens that span various developmental stages by combining the growth-line counts withmeasurements of size to produce quantified age-versus-size growth curves for dinosaurs. There
has been work done on the long bone histology of ceratopsian dinosaurs (Erickson and
Tumanova, 2000; Lee, 2007; Reizner, 2010; Erickson and Druckenmiller, 2011; Levitt, 2011;
Levitt, 2013; Zhao et al., 2013). However, long bone histology is still better understood than theskull elements of ceratopsians and their characteristic cranial elaborations (Hieronymus, Witmer,
Tanke, Currie, 2009; Scannella and Horner, 2010; Tumarkin-Deratzian, 2010; Horner andLamm, 2011; Kruk et al., 2013; Kruk et al., 2014).
The first known paleohistological study transpired when Richard Owen took a thinsection of a bone from Scelidosaurus when he described it in 1861. The study of the microscopic
structure of fossil bone was never a large field, but it has steadily been on the rise since the1960s. Armand de Ricqlès published a series of works based on his dissertation, which were thefirst systematic studies into the histology of fossil bone. His studies were based on fragmentarybones that often were not identified, but nevertheless Armand de Ricqlès was able to introduce aseries of generalizations about patterns of bone growth that still hold up to this day (FrancillonVieillot et al., 1990). In the 1980s and 1990s Anusuya Chinsamy, Armand de Ricqlès, John R.
Horner and their students and colleagues were able to standardize their bone samples anddetermine what paleohistology could determine about the ages and growth rates of dinosaurs.
This increased acceptance and interest in the field of paleohistology also led to a renewedinterest in the subject.
Bone is composed of three types of cells: osteoblasts, osteocytes, and osteoclasts.
Osteoblasts are the bone-forming cells that cover most surfaces of bones and line the bloodchannels as well. They also deposit osteoid, which is a collagenous matrix that mineralizes later.
Osteocytes are osteoblasts that have become embedded in the bone within small spaces known as
lacunae. These osteocytes communicate with other osteocytes/osteoblasts through little channels
called canaliculi. Osteocytes are responsible for bone maintenance, mechano-sensory reception,and calcium homeostasis. On the other hand, osteoclasts are large, multi-nucleated bone
resorbing cells. They occur on the bone surface in resorption pits and move through the bonesecreting enzymes that breakup and resorb organic as well as inorganic structures.

2There are three different types of ossification: endochondral, intramembranous, andmetaplasia. Endochondral ossification begins from a cartilage model where the primaryossification center develops in the mid-diaphysis. Here blood vessels invade, the cartilaginous
model calcifies, and osteoblasts secrete osteoid along the calcified matrix. In intramembranous
ossification there is no cartilage precursor, this is typical of flat bones such as skull bones.
Within the fibrous connective tissue, mesenchymal stem cells differentiate into osteoblasts
forming an ossification center where osteoid is deposited and the periosteum forms; the bone
continues to form along the periosteum. Metaplasia differs from the other two forms because it isthe direct transformation of one fully formed tissue, such as cartilage or tendon, into another,such as bone. It occurs in areas where no periosteum forms, such as attachment sites of tendons,
as well as dense dermis and cartilage can ossify via metaplasia. Metaplastic bone forms without
osteoblasts. It can also be defined as a poorly vascularized, amorphous dermal bone tissue inwhich fibers are numerous and oriented in various directions (Main et al., 2005).
Lamellar, parallel fibered, and woven bone are the types of bone matrix. Lamellar bone
has an one-dimensional structure, flat osteocytes with few canaliculi, and tends to be found in
vascular canals. Parallel fibered bone is two-dimensional and has an interbraided structure.
Woven bone is three-dimensional, well vascularized, with plump osteocytes, and lots ofcanaliculi. This is indicative of fast growth and a high metabolic rate. Bone tissue type is acombination of a given bone matrix and vascularity (e.g. radial, laminar, longitudinal, etc.). The
two types are lamellar zonal and fibrolamellar bone. Lamellar zonal bone is slow growing,
poorly vascularized, and also forms growth marks. It is typically found in ectotherms such asamphibians and reptiles, as well as small mammals. Fibrolamellar bone is fast growing, highly
vascularized, with growth marks, and is typical of modern birds, mammals, and also dinosaurs.
In the cortex of the bone, osteoclasts tunnel through bone while osteoblasts centripetally
deposit layers of lamellar bone, in which a vascular canal forms. An osteon is the fundamental
functional unit of bone; it consists of concentric layers of bone that surround a central vascular
canal. The boundary of an osteon is the cement line. With secondary osteons, collagen fibers get
deposited in various directions, and at the center of the secondary osteon a vascular canal known
as a Haversian canal forms. Another cement line forms as a boundary between the secondary
osteon and primary or secondary bone. Haversian bone is defined as bone that is covered entirely
by many generations of secondary osteons. Typically, secondary osteons form first within the3inner core and over time spread out toward the outer cortex. Bone continues to be remodeledwith secondary osteons until death.
Nasal Boss Histology
The crania of Pachyrhinosaurus have not been examined histologically in acomprehensive manner. Previous studies have focused on histological changes in the nasal boss
during ontogeny (Edwards and Russell, 1994) and possible integumentary structures(Hieronymus et al., 2009. All other cranial histological studies have focused on otherceratopsians such as Centrosaurus (Tumarkin-Deratzian, 2010) and Triceratops (Horner andGoodwin, 2008; Horner and Lamm, 2011). As such, one focus of this thesis aims to expand thehistological understanding of Pachyrhinosaurus, describe the morphological and histologicaldifferences between juvenile and adult nasal bosses, and use these differences to determine finer
ontogenetic stages (e.g. subadult versus adult) than morphology can by itself.
The bosses on Pachyrhinosaurus lakustai are polymorphic, ranging from inflated to aflattened mushroom shapes, to elongate, broadly dished out bosses with anterodorsally oriented“spouts” rising from the anterior margins, to thick-walled cups (Currie et al., 2008; Ryan et al.,2010). However, it is unclear whether these extremes are due to taphonomic deformation,
possible sexual dimorphism, or ontogenetic variation. Similar to other centrosaurines, juveniles
of Pachyrhinosaurus have long-based nasal horncores that only differ in the absence of pointed
distal ends, instead possessing a broad flattened appearance in lateral view (Sampson et al.,
1997; Currie et al., 2008; Fiorillo and Tykoski, 2013). The ontogenetic series of pachyrhinosaurs
shows a progression from small, sagittally divided horncores in juveniles to full-fledged
pachyostotic bosses in adults (Sampson et al., 1997; Fiorillo and Tykoski, 2013). Furthermore,
the highly rugose texture of the boss indicates rapid deposition of bone in combination with theattainment of adult size (Sampson et al., 1997).
Hieronymus et al. (2009) proposes two hypotheses for the integumentary structure on the
cranial bosses of Pachyrhinosaurus: 1) heavily keratinized skin over the rugose surfaces of theboss; or 2) large, rhinoceros-like keratinous horns on the nasal and supraorbital bosses. Some
specimens of adult Pachyrhinosaurus lakustai have defined apical notches, pitting, and gradationfrom dorsal pitting to lateral neurovascular grooves. All of these things indicate that the skin has
become a thick pad of keratinized epidermis (Hieronymus et al., 2009). Furthermore,4

examination of a histological specimen from the dorsal surface of the nasal boss reveals bony
spicules deep within the grooves. Hieronymus et al. (2009) attribute this feature to be mostsimilar to that of a frontal boss of muskoxen because the spicules of both are composed ofosteonal bone tissue, instead of metaplastically ossified deep dermis as in the case of Rhinoceros.
Also, on the lateral surface the palisaded texture of ridges and grooves suggests a cornified
sheath projecting at a shallow angle from the boss (Hieronymus et al., 2009; Fiorillo andTykoski, 2013). Therefore, the most likely integumentary structure supported by this evidence
suggests that the nasal boss of Pachyrhinosaurus likely supported a thick cornified pad with athinner cornified sheath on the lateral surface.
In extant animals, such as muskoxen, African buffalo, and helmeted hornbills, thepresence of a thickened cornified pad may be correlated with head-butting behavior; therefore, asimilar function could be inferred for the nasal boss in Pachyrhinosaurus based on its proposed
associated integumentary structure (Hieronymus et al., 2009). Several authors also suggest thatthe nasal boss was used as a battering ram, with sexual or social selection as the main function(Sternberg, 1950; Farlow and Dodson, 1975). However, Currie et al. (2008) propose that thenasal and supraorbital bosses could have functioned mainly as a visual display structures topotentially attract mates, deter predators, or for species recognition.
Histological Study of Ceratopsian Cranial Elaborations
The basic elaborations of a ceratopsid skull include posteriorly projecting frills comprisedof the parietal and squamosal bones, along with horns above the orbits and the nares (Dodson etal., 2004). Although the function of ceratopsian horns remains unclear, however the mostcommon hypothesis is that the horns were used in intraspecific combat (Lull, 1933; Farlow andDodson, 1975; Sampson et al., 1997; Farke, 2004; Farke, Wolff, Tanke, 2009). Other possiblefunctions include defense against predators (Hatcher et al., 1907; Lull, 1933), thermoregulation(Barrick et al., 1998), and as aids in knocking down vegetation (Tait and Brown, 1928).
Padian and Horner (2011) test the possibility of facial horns as a mechanism for species
recognition using two approaches: (1) the group would need to demonstrate non-directional
evolutionary variation of these structures, and (2) include several sympatric species. Most recent
phylogenetic analyses of neoceratopsians do not reveal any trends in the morphology of the
facial horns that would indicate any directionality in the phylogenetic sense (Xu et al., 2002;

5

Dodson et al., 2004). Also, the geographical and temporal distribution of ceratopsian clades
indicate that bizarre structures existed in groups that lived at around the same time in geographic
proximity (Padian and Horner, 2011). Therefore, ceratopsian cranial elaborations pass the two
tests for the Species Recognition Hypothesis but not for adaptation or sexual display, due tolacking evidence of sexual dimorphism.
Nasal horns of Pachyrhinosaurus increase in size and mass ontogenetically but indifferent patterns than Triceratops (Horner and Goodwin, 2006). Juvenile and sub-adultcentrosaurines possess nasal horncores that are largely triangular, laterally compressed, havemedian sutures, and are either erect or curve somewhat posteriorly (Sampson et al., 1997). Prior
to attaining the adult condition, nasal horncore halves fuse entirely, starting from the tip andmoving proximally, likely between or within the juvenile and sub-adult growth stages (Sampson
et al., 1997; Horner and Goodwin, 2006). Furthermore, the horncores in juvenile centrosaurines
overlie most of the narial openings; in adults, they only overlie the posterior margin of thisopening (Sampson et al., 1997). In contrast, the base of the Triceratops horncore becomes larger
than the span of the paired nasal bones beneath it (Horner and Goodwin, 2006). Additionally, theepinasal fusion of the horn to the skull anterodorsally begins during the sub-adult growth stage.
Generally, the nasal horn extends anteriorly over the premaxillae but is highly variable in sizeand profile in adults (Horner and Goodwin, 2006).
Overall, juvenile and sub-adult centrosaurines possess roughly the same form ofsupraorbital horncore as each other, and only display adult horncore morphology once adult bodysize is reached (Sampson et al., 1997). Generally, taxonomically/phylogenetically informative
characters are thought to be most prominent in somatically mature animals (Sampson et al.,1997). Juvenile supraorbital horncores are long, low rounded processes that are generally evenwith the external surface of the face whereas the medial surfaces slope sharply toward the
midline (Sampson et al., 1997). However, some centrosaurine specimens lack true supraorbital
horncores and instead have low masses, simple raised ridges, or concavities that could haveresulted from periodic resorption and regrowth, resorption related to age, periodic loss, or
pathology (Sampson et al., 1997). In contrast, the postorbital horns of Triceratops begin asstraight stubs in early ontogeny, which curve posteriorly in juveniles, straighten in subadults, andcurve anteriorly in adults (Horner & Goodwin, 2006).

6

Across ceratopsians, nearly two-thirds of the frill is comprised of the parietals with theremaining portion made up of the squamosals (Sampson et al., 1997). There are generally five toseven parietal processes per side, although there is known variation between species, within
species, and within individuals across developmental time. In a centrosaurine, the squamosal isroughly rectangular in outline and short in comparison to that of a chasmosaurine squamosal,
which is triangular and longer (Sampson et al., 1997). Fusion of the parietals probably occurs at
an early ontogenetic stage because there is no indication of any sutural contacts in juvenile
specimens (Sampson et al., 1997). Furthermore, only with adult size did the hooks and spikes of
the ceratopsian frill develop. Epoccipitals are accessory elements that fuse onto the frill margin
during ontogeny and possession of these has been assumed for all chasmosaurines andcentrosaurines (Sampson et al., 1997; Horner and Goodwin, 2006). Furthermore, histologicalstudies of Centrosaurus (Tumarkin-Deratzian, 2010) and Triceratops (Horner and Lamm, 2011)
demonstrate similar growth within the parietals of these two genera. In juvenile Triceratops
parietals consist of cancellous bone with primary osteons and an abundance of radially oriented
canals that progresses to heavily remodeled bone (Horner and Lamm, 2011). The epoccipitals
undergo considerable change during ontogeny; they begin as triangular bones that fuse
sequentially from posterior to anterior to the frill margin creating radiating scallops. Withmaturity these scallops flatten and take on a sine-wave-like patterns (Sampson et al., 1997;
Horner and Goodwin, 2006; Horner and Lamm 2011). It has been suggested that epoccipitals
form via metaplasia (Horner and Lamm, 2011).
Hieronymus et al. (2009) completed a comprehensive study on the possible
integumentary structures of centrosaurine skulls and explored their possible extant analogs.
Overall, the centrosaurines examined with horncores present in adults (Centrosaurus andEiniosaurus) demonstrated the necessary criteria for cornified sheaths (i.e. prominent
neurovascular grooves and obliquely-oriented neurovascular foramina). However,Achelousaurus, the sister taxon of Pachyrhinosaurus, instead has osteological correlates for a
thick epidermal pad (dorsal pitting and lateral neurovascular grooves).
Description of the new taxon from the Wapiti River Bonebed
In the past sixty-five years, the understanding of pachyrhinosaurian diversity has greatly
increased. The discovery of Achelousaurus horneri in the Two Medicine Formation of Montana

7

increased the temporal and taxonomic range of pachyrhinosaurs (Sampson, 1995). The oldest
pachyrhinosaur was found in the Dinosaur Park Formation (75 Ma) of Dinosaur Provincial Park;
it bears similarities to Achelousaurus and Pachyrhinosaurus, and may represent a new taxon(Ryan et al., 2010). Pachyrhinosaurus canadensis was described by C.M. Sternberg (1950), andwas collected from the St. Mary River Formation (Upper Cretaceous) of southern Alberta. It was
the first ceratopsian known to have enlarged nasal and supraorbital bosses instead of facial horns.
Additional material was collected from Scabby Butte by Wann Langston, Jr. (1975) and in the
Horseshoe Canyon Formation, along the Red Deer River. Material from a monodominantbonebed in the Wapiti River Formation near Grande Prairie, Alberta led to the establishment ofPachyrhinosaurus lakustai as a new species of pachyrhinosaur (Currie et al., 2008). The mostrecently described pachyrhinosaur is Pachyrhinosaurus perotorum from the Prince Creek
Formation of the North Slope of Alaska (71-68 million years) (Fiorillo and Tykoski, 2012).
The Pipestone Creek Pachyrhinosaurus Bonebed has been known since 1975, it islocated around 800 m from its confluence with the Wapiti River, and is about 25 km to thesouthwest of the city of Grande Prairie (Fanti, et al., in press). A second ceratopsian-dominatedbonebed occurs along the Wapiti River and is 40 km to the southwest of Grande Prairie. The
Wapiti River Bonebed contains at least eight individuals of the genus Pachyrhinosaurus. The
Pipestone Creek Bonebed has an estimated age of 73.25±0.25 million years, while the Wapiti
River Bonebed has an age of 71.89±0.14, meaning there is 1.36 million years and 150 meters ofsection separating the two bonebeds (Currie et al., 2008; Fanti et al., in press). The age of the
Wapiti River Bonebed is equivalent to the Drumheller Member of the lower Horseshow Canyon
Formation, and coeval with Pachyrhinosaurus canadensis (Fanti et al., in press; Eberth et al.,2013).
In this thesis, a new pachyrhinosaurian ceratopsian dinosaur is described from the Wapiti
River Bonebed. The diagnostic features of this pachyrhinosaur include a thick longitudinal ridge
between the supraorbital bosses and laterally projecting parietal horns (P3) that sharply curve
ventrally, whereas Pachyrhinosaurus lakustai parietal twists anterolaterally, andPachyrhinosaurus canadensis points dorsolaterally. The new pachyrhinosaurian lacks the rostralcomb and anteriorly projecting pommel that is seen in Pachyrhinosaurus lakustai; the lack of a“supranasal boss” and or a “fist sized knob” distinguishes it from Pachyrhinosaurus canadensis.

8

It also lacks the median hump on the nasal boss seen in Pachyrhinosaurus perotorum as well asanteromedially projecting horns on the parietal transverse bar.
Summary
Overall, there are several main hypotheses to be tested. With regards toPachyrhinosaurus nasal boss ontogeny will be tested to see if ontogenetic stage (subadult vs.
adult) can be determined by histology as well as morphology. The ontogenetic stages of nasal
boss development of Pachyrhinosaurus were outlined in Table 1 of Fiorillo and Tykoski (2013),
and form the basis for comparison. Hopefully relative ontogenetic stage can be determined byhistology alone, but can also be conscripted into specific stages of nasal boss development.
Expanding upon this, ceratopsian cranial elaborations will be tested to determine if theygrow as extensions of the dermatocranium or are formed metaplastically through mineralizationof other tissues. A comparison will be made across various taxonomic groups and will
demonstrate that all cranial elaborations, with the exception of epoccipitals and possibly
Triceratops nasal horns, form in the same way. To do this, samples were selected fromchasmosaurine and centrosaurine ceratopsian ornamentations, namely bosses, supraorbital
horncores, parietals, squamosals, parietal horns, and an epoccipital. Then a histological analysis
was performed where each of the specimens were embedded in epoxy, glued to a plexiglass
slides, cut, and ground until they reached the optimal thickness (around 50 µm) to be viewed
under a microscope. This was the same technique used in the nasal boss ontogeny study.
Lastly, a description of the new Pachyrhinosaurus species from the Wapiti River
Bonebed is presented within the framework of ontogenetic, stratigraphic, and taphonomic
variability established in the earlier chapters of the thesis. Two partial skulls, a partial parietal
bar, and a parietal horn will be used to diagnose this new species. Following the description is aphylogenetic analysis placing the new species into context within the Centrosaurinae.
Overall, ceratopsian long bone histology has not been as well studied as other groups
such as the Theropoda. Ceratopsian studies have focused on Psittacosaurus (Erickson and
Tumanova, 2000; Zhao et al., 2013) and another on Pachyrhinosaurus (Erickson andDruckenmiller, 2011). There appears to be a disparity between low and high latitude ceratopsians
in terms of number of lines of arrested growth (Levitt, 2011).

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As previously mentioned, there has been little work previously on the histology of ceratopsiancranial elaborations; studies on cranial elaborations have focused determining their function andmorphological ontogeny. Pachyrhinosaurus is a strange and curious ceratopsian that represents
the last of the centrosaurines. It is intriguing to note that the species within the crown group of
the Centrosaurinae share nasal bosses as a common feature. It raises the question of why there
was a deviation from nasal horns to nasal bosses. These structures are poorly understood from amorphological point of view, and are even less understood from a histological perspective.
Pachyrhinosaurus has some of the most peculiar cranial elaborations seen within ceratopsians
and the variety within the genus is increasing as more localities are discovered.

10Chapter 1“Histological and morphological ontogeny of Pachyrhinosaurus nasal
bosses”
This chapter will be submitted to the Canadian Journal of Earth Sciences for publication.

11

Introduction
Pachyrhinosaurus is a centrosaurine ceratopsian found only in Alberta and Alaska ofNorth America. Unlike other ceratopsians, which have bony horncores, Pachyrhinosaurus has
cranial bosses. The specimens examined in this study are juvenile, subadult, and adult nasal
bosses of Pachyrhinosaurus lakustai from the Pipestone Creek Bonebed. Additional subadultand adult bosses of Pachyrhinosaurus sp. nov. from the stratigraphically higher (by 150m)
Wapiti Bonebed. Both bonebeds are located near Grande Prairie, Alberta. Superficially, thejuvenile nasal boss bears almost no resemblance to those of the adult specimens and moreclosely resembles a horncore rather than a boss. The juvenile nasal boss is ridge-like in dorsal
view and the surface texture includes neurovascular grooves extending predominantly
dorsoventrally and foramina for nerves and/or blood vessels. In contrast, the adult bosses arelarge, coalesced masses that are spongy with resorption pits and are roughly triangular in dorsal
view. Histologically, the juvenile nasal boss is composed of mostly fibrolamellar bone. The adultnasal bosses from Pipestone have a combination of spongy bone with thick trabeculae andcompact bone consisting of several generations of secondary osteons.
Although Pachyrhinosaurus nasal bosses differ superficially in juvenile versus adultspecimens, histologically similar tissues may be found in both. It is possible, based on the
differences observed in these specimens, that nasal boss microstructure may be useful indetermining ontogenetic stages (e.g. subadult versus adult) than morphology alone.
Abbreviations
TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada; UALVP,
University of Alberta Laboratory for Vertebrate Paleontology, Edmonton, Alberta, Canada.
Materials and Methods
Several partial nasal bosses described by Currie et al. (2008) were identified as juvenile
based on size, surface bone textures, and cranial ornamentation. The remaining specimens wereidentified as adults based on their relative size. Histological samples from several juvenile andadult specimens (Table 1) of Pachyrhinosaurus lakustai from the Pipestone Creek Bonebed andPachyrhinosaurus sp. from the Wapiti River Bonebed (both representing theCampanian/Maastrichtian Wapiti Formation) near Grande Prairie, Alberta, Canada were

12

examined. All specimens were embedded in epoxy resin (EpoThin™, Low Viscocity Resin andHardener Buehler, Lake Bluff, IL), vacuum impregnated (Welch Duo-Seal), then left to cure for24 hours. The embedded samples were rough-cut on a tile saw then stabilized with PaleoBond
Penetrant Stabilizer. The billets were ground on a lapidary wheel and glass plate using 240, 400,
and 600 grit SiC powder (E.T. Enterprises, Calgary, AB) until the majority of scratches had been
removed. The billets were glued to glass or plexiglass slides using EpoThin™. The section was
cut and rough-ground on a Hillquist, stabilized further (PaleoBond, Penetrant Stabilizer), groundby hand on a glass plate using 600 and 800 grit SiC, and polished against denim with CeO2 Grit
GPP Tumble Polish for 2-3 minutes.
Sections were examined and photographed on a Nikon Eclipse E600POL trinocularpolarizing microscope with an attached Nikon DXM 1200F digital camera. Whole slide scans
were taken with a Nikon Super Coolscan 5000 ED.

13

Table 1.1. List of specimens.
Specimen No.

Histological No.

Ontogenetic Stage

Locality

Taxon

TMP 1989.55.894

TMP 1993.55.2

Juvenile (1)Pipestone Creek Bonebed

P. lakustai

TMP 1989.55.174

TMP 1993.55.11

Juvenile (1)Pipestone Creek Bonebed

P. lakustai

TMP 1986.55.048

TMP 1993.55.12

Juvenile (1)Pipestone Creek Bonebed

P. lakustai

TMP 1987.55.161

TMP 1993.55.13

Juvenile (1)Pipestone Creek Bonebed

P. lakustai

Juvenile (2)Pipestone Creek Bonebed

P. lakustai

Subadult (3)Pipestone Creek Bonebed

P. lakustai

Subadult (4)Wapiti River Bonebed

P. sp. nov.

Subadult (4)Pipestone Creek Bonebed

P. lakustai

UALVP 53300

Adult (5)

Wapiti River Bonebed

P. sp. nov.

TMP 1985.112.119

Adult (5)Pipestone Creek Bonebed

P. lakustai

TMP 1989.55.1240
TMP 1989.55.1038

TMP 1993.55.8

UALVP 55138
TMP 1989.55.1342

TMP
1993.55.16/17/18/20

14

Table 1.2. Ontogenetic stages of nasal boss development in Pachyrhinosaurus
Growth Stage Characteristics
Stage 1
Low, paired demihorns with minimal lateral texturing in form ofneurovascular grooves
Demihorn essentially covers nasal from anterior process to posterior edge
Posterior surface of demihorn not strongly deflected medially and only
slightly sculptured
Stage 2Isometric growth of paired (not fused) demihorns, that extend most of thelength of nasal
Lateral surface texturing with larger neurovascular grooves with variable
orientation
Posterior sculptured surface larger, more distinctly sculpture, anddeflected medially
Stage 3At least” two-thirds” adult size
Fusion of nasals begins anteriorly and progress posteriorly?
Thickening of lateral surfaces, “broad tumid expansion” of demihorns
into an incipient boss
Boss precursor enlarges posteriorly but not yet to posterior edge of nasal
Posterior sculpted area apparently coalesced into expanding incipientboss
Dorsal margins with lateral ridges bounding medial depression
Stage 4Lateral surfaces of boss migrate rapidly away from midline, accompanied
by rapid deposition of spongy bone between them to form obvious boss
structure
Boss gross rapidly posteriorly to or nearly to posterior edge of nasals
Boss defined ventrolaterally by distinct basal sulcus
Lateral surfaces show signs of palisade texture comprised of parallel
dorsoventrally-oriented ridges and grooves
Stage 5
Boss mediolaterally as wide or wider dorsally than ventrally along basal
sulcusFull nasal boss extends from premaxillary process to posterior edge ofnasals, may contact supraorbital bosses in P. canadensis and P.
perotorum
Lateral surfaces extensively marked by palisade texture of dorsoventrally
oriented grooves and ridges
Remodeling of boss dorsal surface

15

Results
Nasal bosses were sorted into more specific stages of growth using the descriptions inTable 1.2 (modified from Fiorillo & Tykoski, 2013; originally derived from Currie et al. 2008).
Stage 1
Stage 1 is characterized by the smallest and most immature demihorns, which closely
resemble the small nasal horncores of juvenile centrosaurines. However, the demihorns take onan anteroposteriorly elongated fin or ridge appearance. Overall, these are composed offibrolamellar bone, with dense osteocyte lacunae, and the bone is somewhat trabecular (Figure1.1). Also, the osteons are primary and there is no evidence of secondary remodeling. Each of theStage 1 demihorns is relatively flat and low in lateral profile. Within the scans of the thinsections there does not appear to be much differentiation within a single slide, except for Figure
1.1 B. The more vascular region is located in the middle of the demihorn, so it is not surprising
to find a slight differentiation before core and cortex.

16

17

Stage 2
Bigger in size than the Stage 1 demihorns, there are more prominent neurovasculargrooves visible at Stage 2 and it is more sculpted with a slight medial curvature. However, withregard to histology not much has changed since Stage 1. Primary osteons are numerous whereas
the osteocyte lacunar density is quite high (Figure 1.2). For the most part there is not much of adifferentiation between core and cortex except for a slight increase in trabecular bone towards
the center of the section.

18

19

Stage 3
TMP 1989.55.8 is categorized into Stage 3 based upon the thickening of the lateralsurfaces into an incipient boss (Figure 1.3). An interesting note on the histology of this section,
there is some division of tissue textures, the tissues closer to the dorsal surface appear morecompact and not as trabecular as tissue positioned ventrally. There is evidence of zones; dorsallythe vascular canals are oriented transversely then ventrally large vascular spaces occur andcrosscut the primary tissues. Closer in, the texture resembles the rope-like appearance of moremature nasal bosses however there is still a large number of osteocyte lacunae.

20

21

Stage 4
TMP 1989.55.1342 is classified as Stage 4 based on the distinct basal sulcus and overall
size (Fig. 1.4 A). However, UALVP 55138 (Fig. 1.4) is a well-worn nasal boss that is placed into
Stage 4 based only on histological evidence. Both specimens have a mixture of primary andsecondary osteons but are not as trabecular as Stage 5 nasal bosses. Two interesting points tonote with UALVP 55138, there is a resorption pit with a clear resorption line (Fig. 1.4 G, I) as
well as differing orientations of vascular canals in the coronal section (Figure 1.4 H). Towards
the outside of the slide (Fig. 1.4 H) the canals are circular and oriented perpendicular to the fieldof view, but towards the center the canals are longitudinal. The slides of TMP 1989.55.1342 (B,C, D) exhibit differentiation between the cortex and core. The tissues at the cortex are morecompact than the interior and the canals are oriented dorsoventrally at the dorsal most edge.
Moving ventrally the tissue becomes more trabecular and looks most similar to an adult boss at
its most ventral edge. It should also be noted that the difference in shading across slides B, C,
and D is due to the slide being polished too thin, the lighter areas represented thinner portions of
the section. In both TMP 1989.55.1240 and UALVP 55138 the density of osteocyte lacunae isless than that of the previous stages but denser than the Stage 5 slides.

22

23

Stage 5
Stage 5 sections were taken from a fully developed nasal boss from a partial skull(UALVP 53300) and an isolated nasal boss (TMP 1985.112.119) (Fig. 1.5). Microscopicallythere are several generations of secondary remodeling and the amount of vascular space isgreater than the earlier stages. Also, the density of osteocyte lacunae has decreased as well. The
fibrolamellar bone has taken on a rope-like appearance. Several generations of secondary osteonsare visible in the thin sections (Fig. 1.4 C, E, F). In Figure 1.4 D, a resorption cavity has resorbed
part of several osteons.

24

25

Discussion
There is an overall increase in vascularity (sponginess), as the Pachyrhinosaurus getsolder. Secondarily remodeling increases as well but it is not seen until Stage 3 at the earliest.
Also osteocyte lacunar density decreases with age as well, tapers off at Stage 5. The presence ofosteoblasts in all stages of ontogeny indicates that these structures are ossified
intramembranously, are outgrowths of the nasal bone, and are not formed through metaplasia.
Based on these results, I am proposing that the ontogenetic stages based on nasal boss
development from Fiorillo and Tykoski (2013) can be divided into three traditional stages of
ontogeny. Stage 1 and 2 can be considered juvenile due to dense osteocyte lacunae, extensive
primary tissue, and minimal amounts of differentiation between core and cortex. The demihorns
are laterally compressed and take on a ridge-like appearance when viewed dorsally. Stage 1
maintains a fairly linear shape dorsally, while Stage 2 curves medially in a concave fashion.
Neurovascular grooves are distinctly visible in lateral view on TMP 1989.55.1240, a feature notseen in the Stage 1 demihorns. All specimens in Stage 1 and 2 do not resemble the future nasal
boss it would eventually progress to.
Stage 3 and 4 are described as subadults because there is an increase in vascular space aswell as secondary remodeling. In Figure 1.3 B, TMP 1989.55.1038’s dorsal surface haslatitudinal vascular canals that display the “broad tumid expansion” of the incipient nasal boss
described by Currie et al. (2008). Furthermore, in Figure 1.4 H the coronal section of UALVP
55138 shows a difference in orientation of vascular canals between the outside and middle of thesection. This difference indicates that the nasal boss does not have a point of origination withregards to growth and shows that the vascular canals do not have a particular orientation. Also
the resorption pit seen in Figure 1.4 (G, I) is a true resorption pit because it shows a distinct
resorption line around the edge, cutting across the surrounding osteons, and it is further evidenceof secondary remodeling.
Finally, Stage 5 represents adulthood due to the extensive amount of remodeling,
multiple generations of secondary osteons, rope-like texture, and large vascular spaces.
Macroscopically an adult Pachyrhinosaurus nasal boss’s lateral surface has extensive,
dorsoventrally oriented grooves and ridges, as well as remodeling on the dorsal surface of theboss including resorption pits or even entirely “scooped out” bosses that have undergone
extensive remodeling. Extreme amounts of resorption are an indicator of senescence.

26

Pachyrhinosaurus has one of the more extreme ontogenetic sequences with regards to thedevelopment of its nasal ornamentation. It begins with a horn that resembles those of closely
related centrosaurine species but morphs into a structure that is unlike any other found outside
the Pachyrostra. It is curious that the last of the centrosaurines evolved their nasal ornamentation
to be a boss instead of a horn core.
Conclusions
Nasal boss histology is poorly understood, but our understanding was improved by thinsectioning an ontogenetic series of nasal bosses, as well as placing the specimens into finer
ontogenetic stages (Table 1.1, Table 1.2) than just juvenile, subadult, and adult based on grossmorphology. As the pachyrhinosaurs progress through ontogeny secondary remodeling andvascularity increase while osteocyte lacunar density decreases. It was determined that relative
ontogenetic stage (subadult vs. adult) can be determined by histology alone, but not into finer
ontogenetic stages (Fiorillo and Tykoski, 2013). However, further subdivision of ontogenetic
stages (Stage 1-5, Table 1.2) leads to three traditional stages of ontogeny --juvenile, subadult,
and adult-- based on the histology of individual specimens. All ontogenetic stages are composedof fibrolamellar bone and progress to have many generations of secondary osteons, which causes
the heavily remodeled and rope-like appearance.
Prior to this study, the only systematic histological study of ceratopsian nasal
ornamentation ontogeny was performed on Triceratops (Horner and Goodwin, 2008). Now it has
been expanded to include Pachyrhinosaurus. However, more histological studies need to be
performed on the ontogeny of ceratopsian cranial elaborations. There is an immense amount of
information that can be found by using histology that adds to our overall knowledge ofdinosaurs.

27

Chapter 2“Histological study of cranial elaboration in ceratopsian dinosaurs:
Functional and developmental implications”

28

Introduction
Hypotheses for the developmental processes of cranial elaborations in ceratopsians haverelied upon gross morphology and studies of morphological changes in taxa with relatively
complete growth series. To date, these hypotheses have not been tested using histology. A study
on the cranial epidermal coverings of centrosaurine skulls, however, did investigate the nasal
boss histology of Pachyrhinosaurus as a correlate for the keratinized covering of that uniquestructure (Hieronymus et al., 2009). Here the histological data available is expanded for the
cranium of Pachyrhinosaurus, a centrosaurine ceratopsian dinosaur that has cranial bosses inplace of bony horn cores. In addition, other cranial material from Late Cretaceous Albertan
ceratopsians of various ontogenetic stages is examined.
The focus of this study is to test whether morphologically diverse cranial elaborations inceratopsians demonstrate similarly diverse developmental processes via paleohistologicalanalysis. Overall, cranial elaborations in ceratopsians are observed to be intramembranous in
origin, not metaplastic, with the exception of epoccipitals. This is in contrast to some otherdinosaurs, such as the ankylosaurs, whose cranial elaborations form via combinations ofmetaplastic mineralization of the dermis and intramembranous outgrowth of the cranial vaultbones.
In the specimens examined, these structures generally contain trabecular bone atmaturity, but the nasal bosses show thicker trabeculae than the other cranial material sectioned.
Secondarily remodeled bone is common among all elements examined, with occasional
interstitial primary fibrolamellar bone preserved. Periosteal bone was observed in only two horncore specimens (UALVP 49545 and UALVP 52658). Trabecular bone in the examined
specimens is mostly associated with mature structures, commonly found with resorption pits andspicules. An immature horn core examined exhibited radial fibrolamellar bone, which suggeststhat the bone was growing quickly at the time of the animal’s death. Secondary remodeling inthis specimen (UALVP 52658) is restricted to the core and retains a high proportion of the
primary fibrolamellar bone. This study proposes that all ceratopsian cranial elaborations growrapidly as primary, fibrolamellar outgrowths of the dermatocranium and remodel, later in
ontogeny, into mature, largely trabecular structures.

29Abbreviations
TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada; UALVP,
University of Alberta Laboratory for Vertebrate Paleontology, Edmonton, Alberta, Canada.
Materials and Methods
Chasmosaurine and centrosaurine specimens from Upper Cretaceous strata of Alberta
were chosen to cover a range of cranial elaborations. All specimens were embedded in epoxy
resin (EpoThin™, Low Viscocity Resin and Hardener Buehler, Lake Bluff, IL), vacuum
impregnated (Welch Duo-Seal), then left to cure for 24 hours. The embedded samples wererough-cut on a tile saw then stabilized with PaleoBond Penetrant Stabilizer. The billets wereground on a lapidary wheel and glass plate using 240, 400, and 600 grit SiC powder (E.T.
Enterprises, Calgary, AB) until the majority of scratches had been removed. The billets wereglued to glass or plexiglass slides using EpoThin™. The section was cut and rough-ground on a
Hillquist, stabilized further (PaleoBond, Penetrant Stabilizer), ground by hand on a glass plateusing 600 and 800 grit SiC, and polished against denim with CeO2 Grit GPP Tumble Polish for
2-3 minutes.
Sections were examined and photographed on a Nikon Eclipse E600POL trinocularpolarizing microscope with an attached Nikon DXM 1200F digital camera. Whole slide scans
were taken with a Nikon Super Coolscan 5000 ED.

30

Table 2.1. List of specimens
Specimen Number

SpeciesElement

TMP 1981.16.499

Chasmosaurus sp.

Orbital horncore

TMP 1982.16.215

Chasmosaurus sp.

Parietal

TMP 1985.112.119

Pachyrhinosaurus lakustai

Nasal boss

TMP 1989.55.1240

Pachyrhinosaurus lakustai

Horncore (juvenile)

TMP 1998.93.034

Styracosaurus albertensis

Postorbital/Supraorbital

TMP 2000.57.009

Anchiceratops sp.

Squamosal/Frill

UALVP 49545

CeratopsianHorncore

UALVP 52526

Pachyrhinosaurus sp.

Frill

UALVP 52658

Centrosaurus sp.

Orbit & Orbital horncore

UALVP 53300

Pachyrhinosaurus sp.

Nasal boss

UALVP 54165

Pachyrhinosaurus sp.

Parietal horn (P3)

UALVP 54208

Pachyrhinosaurus sp.

Postorbital

UALVP 54894

CeratopsianEpoccipital

UALVP 55138

Pachyrhinosaurus sp.

Nasal boss

31

Results
TMP 1998.93.034
The Styracosaurus albertensis postorbital/supraorbital has fairly dense osteocyte lacunaewith a few secondary osteons, showing at least one generation of remodeling (Figure 2.1). It is
composed of fibrolamellar bone and is not greatly vascularized.

32

33

UALVP 54208
The Pachyrhinosaurus postorbital (Figure 2.2) is riddled with microfractures and cracks(most likely acquired postmortem, due to no evidence of remodeling). The bone is heavily
remodeled through fossilization processes, however at least one secondary osteon was visible.

34

35

UALVP 52658
This specimen is a juvenile Centrosaurus (Figure 2.3). The orbital region is composed ofmostly woven bone with some secondary remodeling. However it is still primary bone in theinterstitial spaces (Figure 2.3 A,C). Also it has a fine-grained texture, not many vascular
openings, and periosteal bone is found on the external edges. The orbital region of bone shows
histology expected of a developing skull bone. The orbital horncore primarily consists of wovenbone with little fibrolamellar bone and has slightly higher relative vascularity with some radial
canals, in comparison to some of the larger specimens (Figure 2.3 B, D). Dense osteocyte
lacunae are visible in patches; more would be seen if the slide was not as thin.

36

37

UALVP 49545
The cortical area of the ceratopsian horncore (UALVP 49545, Fig. 2.4) consists ofcompact fibrolamellar bone, which is radial out to the surface along with periosteal bone at its
external edge. In the core exist secondary remodeling resorption cavities that form spongy or
trabecular bone. Overall, ceratopsian horncores have a compact cortex and a spongy core. There
are dense osteocyte lacunae, indicative of rapid growth.

38

39

TMP 1981.16.499
Similar to UALVP 49545, the orbital horncore of TMP 1981.16.499 (Figure 2.5) showsdifferentiation between the core and cortex. The specimen is composed of fibrolamellar bone andalso has dense osteocyte lacunae. The core is composed of more trabecular bone than the cortex,
which is compact with smaller vascular spaces.

40

41

UALVP 52526
The Pachyrhinosaurus frill (UALVP 52526, Fig. 2.6) is poorly vascularized incomparison to horncores, but is also composed of fibrolamellar bone that secondarily remodels
as the animal ages. There is secondary remodeling seen in the core of the frill.

42

43

TMP 2000.57.009
The Anchiceratops squamosal (TMP 2000.57.09, Fig 2.7) is a slightly more vascular incomparison to UALVP 52526. However, it is still composed of fibrolamellar bone and has
evidence of secondary remodeling.

44

45

UALVP 54165
The Pachyrhinosaurus parietal horn (UALVP 54165, Fig. 2.8) has large trabeculae and ismostly secondarily remodeled fibrolamellar bone, with several generations of secondary osteons.
It also have a spongy appearance similar to nasal bosses. No metaplastic bone is visible.

46

47

TMP 1989.55.1240, UALVP 55138, UALVP 53300, TMP 1985.112.119
Pachyrhinosaurus nasal bosses are exceptionally spongy with rather thick trabeculae andall specimens show several generations of secondary remodeling (Figure 2.9). However, unlike
other ceratopsian horncores there is no differentiation of bone compactness between the core andcortex. Vascularity and the relative amount of remodeling increases with age same with theappearance of absorption pits. Juvenile nasal bosses (TMP 1989.55.1240, Fig. 2.9 A; see also
Chapter 1) initially look more like nasal horns than bosses before they morph into their morerugose counterparts.

48

49

UALVP 54894
In contrast to the other cranial elaborations, the epoccipital (UALVP 54894, Fig. 2.10)
demonstrates metaplastic bone, which is described as interwoven structural fiber bundles. The
bone is not particularly vascular and there are several generations of secondary osteons. Near theanterior portion of the specimen, is a pit representing a neurovascular groove running across the
specimen.

50

51

Discussion
With the exception of epoccipitals, the cranial elaborations of ceratopsians are formed
intramembranously, and thus are outgrowths of the dermatocranium. It is interesting to see how
the formation of various elaborations on the frill differ so much. Because we often find isolated
epoccipitals that have a sutural-like surface instead of a break, it would be plausible thatepoccipitals form within the skin on the frill and fuse to the frill bones later in ontogeny. Before
fusing to the frill margins, epoccipitals remodel after initially laying down woven bone, whichforms quickly but is more disorganized than Haversian bone. The one ceratopsian that may be an
exception to this hypothesis is Triceratops, where its epinasal and epoccipitals can be found inisolation before fusing to the skull later in ontogeny. Furthermore, nasal bosses look unlike anyother cranial elaboration sampled here. For a greater histological analysis of Pachyrhinosaurus
nasal bosses see Chapter 1. However, across the taxa that were sampled in this study, it can be
postulated that the cranial elaborations of ceratopsians all form in similar manners.
The only other comparable histological study focuses on the ontogeny of the parietal of
Triceratops (Horner and Lamm, 2011). The results of Horner and Lamm (2011) agree with thosepresented here, in the sense that parietals are composed of fibrolamellar bone. This suggests that
all ceratopsian parietals are composed of fibrolamellar bone. Otherwise there has been nocomprehensive study of ceratopsian cranial elaborations.
This study does not have wider implications for all of Marginocephalia, which possess
bizarre cranial ornamentations that form as outgrowths of the skull, pachycephalosaurs form
their elaborations through metaplasia. Horner and Goodwin (2009) sectioned the cranial dome
and squamosal horns of various pachycephalosaurs and determined that they formed throughmetaplasia. The sweeping conclusion states that marginocephalian dinosaurs utilize metaplasia togrow their cranial ornamentations, which this study disagrees with that assumption. Based on thepresence of osteocytes in many of these specimens, it is shown that these ornamentations growout of the skull, with the exception of the epoccipitals, instead of forming through metaplasia.
How the formation of cranial ornamentation differs between the two major groups of
Marginocephalia is interesting, and further investigation of this could prove to be enlightening.
Five different species were sampled in this study (Anchiceratops, Centrosaurus, Chasmosaurus,
Pachyrhinosaurus, and Styracosaurus), two chasmosaurines and three centrosaurines. Future52research would include a more comprehensive study of ceratopsian cranial elaborations of more
species and across a greater span of time.
Conclusions
Prior to this study, it was unknown if the horns and spikes on the frills of ceratopsians
were formed through metaplasia (similar to ankylosaur osteoderms) or as outgrowths of thedermatocranium. This chapter demonstrates that in centrosaurine and chasmosaurine (with the
exception of Triceratops) cranial elaborations are outgrowths of the skull, with the exception of
epoccipitals (i.e. epiparietals and episquamosals), which are formed through metaplasia.
Triceratops differs partially in this aspect. Its nasal horn is not formed from the nasals, butinstead derives from a bone known as the epinasal that fuses onto the nasal bones during
ontogeny; it is derived from a separate center of ossification (Horner and Goodwin, 2008). It isquite possible that the epinasal of Triceratops is a trait found only in chasmosaurines. However,
little has been done on the cranial ontogeny of chasmosaurines other than Triceratops, so it isuncertain whether the epinasal is a feature found just in Triceratops. Additionally, if the epinasal
is a feature found only in chasmosaurines that could be used as a character to further distinguishchasmosaurines from centrosaurines. Conversely, a mostly complete juvenile Chasmosaurus(UALVP 52613) from the Dinosaur Park Formation has been found and has a nasal bump withno evidence of an epinasal. Instead the epinasal could be a feature unique to Triceratops.
Overall, a more comprehensive histological study of ceratopsian cranial elaborations
needs to be performed. There is still much to be discovered about ceratopsians and their obscureornamentations.

53

Chapter 3“A new species of Pachyrhinosaurus from the Wapiti Formation (upper
Campanian) of Alberta”
This chapter will be submitted to PLoS ONE for publication.

54

Introduction
The upper Cretaceous strata of the Grande Prairie region (Alberta, Canada) are
exceptionally rich in their record of prehistoric Alberta. Here the taxon from the Wapiti River
Bonebed, a recently described multi-taxic ceratopsian bonebed, is described as a new species ofhorned dinosaur. The bonebed is located 150 meters stratigraphically higher than the well-known
Pipestone Creek Bonebed that produces Pachyrhinosaurus lakustai specimens.
Pachyrhinosaurus youngi sp. nov. exhibits an apomorphic parietal horn and wide, raised
separation between its supraorbital bosses. A phylogenetic analysis of seventeen taxa of
ceratopsians produced a weakly supported resolution of the relationships between the
Pachyrhinosaurus species, resulting in a polytomic crown group composed of Pachyrhinosaurus
canadensis, Pachyrhinosaurus perotorum, and Pachyrhinosaurus youngi. The Wapiti River
Bonebed represents one of the most inland occurrences of centrosaurines in North America.
Geologic Setting
The Wapiti River Bonebed is a multi-taxic ceratopsian dominated bonebed within the
Wapiti Formation (Fanti, Currie, and Burns, in press). The bonebed can be traced linearly 107 m
and has been systematically excavated over an area of 40 m2 during annual field expeditions
from 2007 to 2014. The Wapiti River Bonebed occurs 660 meters above the base of the Wapiti
Formation and 300 meters above a coaly interval, that is correlative to the lower tongue
maximum flooding surface of the Bearpaw (Fanti and Catuenau, 2010; Fanti et al., in press). The
lithography of the bonebed is composed of a lower mudstone layer (up to 40 cm thick)comprised of a large amount of oxides that form a light-colored ironstone layer. The overlying
mudstone layer exhibits disorganized and folded laminae and is then overlain by parallel andundisturbed layers (Fanti et al., in press). Larger bones are encased within the ironstone layer,
whereas the upper laminated layer preserves smaller elements. The depositional setting was an
oxbow lake or marshy/swampy, water-saturated area, or a lacustrine setting within an alluvial
plain (Fanti et al., in press). Other identified fossils have been identified as toissible
ornithomimid elements and dromeosaurid, troodontid, and tyrannosaurid teeth; there are several
elements from hadrosaurids (Fanti et al., in press).
Several elements demonstrate evidence of pre-burial exposure (fragmentation andbreakage) that indicate some degree of sub-aerial exposure and fluvial transport prior to burial

55

(Fanti et al., in press). Tooth marks are only visible on five specimens. No bones have beenfound articulated. The minimum number of Pachyrhinosaurus individuals is eight and is basedon partial skulls and occipital condyles. 40Ar/39Ar dating provides an age of 71.89±0.14 million
years, based on a 25 cm thick, altered volcanic ash 180 cm below the bonebed layer (Figure 3.1;
Fanti et al., in press). The Wapiti River Bonebed represents one of the most inland occurrencesof centrosaurines in North America.

56

57

Abbreviations: UALVP, University of Alberta Laboratory for Vertebrate Paleontology;
Edmonton, Alberta, Canada.
Systematic Paleontology
Dinosauria Owen, 1842
Ornithischia Seeley, 1888
Ceratopsidae Marsh, 1888
Centrosaurinae Lambe, 1915
Pachyrhinosaurus Sternberg, 1950
Type Species: Pachyrhinosaurus canadensis Sternberg, 1950, north side of Little Bow
River, east of Carrmongay, Alberta, Canada, Late Cretaceous, Campanian
Pachyrhinosaurus youngi sp. nov.
Figs. 3.2-3.5
Etymology: In recognition of the Young family who have allowed the University ofAlberta to excavate the Wapiti River Bonebed on their land.
Type Material: Holotype: UALVP 53300, partial skull missing parietals, squamosals, andright cheek region. Paratypes: UALVP 54165, automorphic parietal P3 horn; UALVP 50652,
parietal bar and horn (P2); UALVP 55805 (originally catalogued as GPRC/RH
03.06.03.01.02.015) partial skull missing rostrum, parietals, squamosals, and cheek regions.
Type Locality: All specimens collected from the Wapiti River Bonebed along the Wapiti
River, near Grande Prairie, Alberta, Canada.
Type Horizon: The specimens were collected from upper Campanian rocks (71.89±0.14Ma) within the Wapiti River Formation (Fanti et al., in press).
Diagnosis: Pachyrhinosaurus youngi is distinguished from other pachyrhinosaurs by its
dorsolaterally curved parietal horns and a distinctive tall separation between supraorbital bosses.
It differs from Pachyrhinosaurus lakustai by lack of a rostral comb and an anteriorly protruding“pommel.” Pachyrhinosaurus youngi is also differentiated from Pachyrhinosaurus canadensis
by the absence of a “supranasal boss” and “fist-sized knobs;” however the new species is united
with Pachyrhinosaurus canadensis and Pachyrhinosaurus perotorum based on the nasal andsupraorbital bosses that are nearly in contact and only separated from each other by a narrowgrooves.

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Table 3.1 Measurements (in mm) of UALVP 53300, Pachyrhinosaurus youngi holotype
skull. Landmarks used by Langston (1975) and Currie et al. (2008). Lowercase “e” after
number signifies an estimate.
A

Top of nasal boss to ventral limit of maxilla

487

B

Vertical height of narial opening in lateral view

101

C

Maximum vertical height

487

D

Vertical height of narial fossa

198

E

Top of orbit to tip of supraorbital boss

69

intrF Rostrum to posterior edge of naris

331e

G

Posterior edge of naris to anterior margin of orbit

298

H

Anteroposterior length of orbit

157

I

Posterior edge of orbit to supratemporal fenestra?

J

Antorbital length

582e

K

Height of orbit

127

L

Posterior edge of naris to posterior edge of nasal boss

444

M

Bottom of orbit to bottom of jugal?

N

Greatest width of expansions behind nares

255

O

Width across lowest flange of premaxilla

126

P

Least transverse width of narial bridge

85

Q

Greatest width of narial aperture

190

R

Greatest width of nasal boss

264

S

Least transverse width of face between nares and orbit

229

T

Length of nasal boss

607

Length of skull roof (tip of nasal boss to posterior edge supraorbital

744e

boss
Skull length (rostrum to occipital condyle)

875e

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Description
RostralWhat remains of the rostral bone (Fig. 3.2) is fused to the premaxillae and is missing theanterior point or beak and posteroventral portions. The rostral bone is rugose in texture. The
degree of curvature of the beak could not be determined. There is no evidence for pre-mortem
breakage of the rostral bone.
Premaxilla
The premaxillae (Fig 3.2) are fused together with the right side being slightly eroded andsmooth, whereas the left side is well preserved and pitted. The premaxillae form the broad narial
septum typical of a centrosaurine. The narial septum is mostly smooth; however, there are small
cracks in it, most likely due to postmortem compaction. The septum composes the anterior twothirds of the narial fossa. There is no rostral comb present in Pachyrhinosaurus youngi, unlike
Pachyrhinosaurus lakustai and Pachyrhinosaurus perotorum.
MaxillaThe maxilla (Fig. 3.2) has a broad triangular profile, a medial ascending process that
contacts the premaxillae anterodorsally, and an ascending lateral process that contacts the jugal.
The floor of the antorbital fenestra separates these processes. In left lateral view, the ventral
surface of the maxilla has been partially eroded, whereas in the right lateral view the ventral
surface has eroded up to the overhanging portion of the maxilla. There are 20 alveoli preservedin the paratype skull (UALVP 55805) (Fig. 3.3), although the tooth count may have had a few
more tooth positions. This number is close if not comparable to other similarly sized ceratopsids(25-28 in Achelousaurus, 18-30 in Chasmosaurus, 16-35 in Centrosaurus, 24-28 in Einiosaurus)(Currie et al., 2008). The maxilla contacts the ectopterygoid, jugal, lacrimal, palatine,premaxillae, pterygoid, and vomer; however, these contacts are either not visible or notpreserved in this specimen.

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NasalThe skull of UALVP 53300 (Fig. 3.2, 3.3) is dominated by a massive nasal boss that
extends from the anterodorsal part of the narial fossa to above the middle of the orbit. The boss
has no anterior pommel, such as is found in specimens of Pachyrhinosaurus lakustai, but instead
has several large resorption pits (Fig. 3.3). Resorption pits have not been found on the anterior
portion of the nasal boss in any other pachyrhinosaur species, but are common on the
supraorbital bosses. There are at least three large pits visible on the anterior nasal boss with two
being best seen in anterior view (Fig. 3.2 E) and the other from the left lateral view (Fig. 3.2 A).
There is another possible resorption pit seen in right lateral view, but it is well worn from erosion(Fig. 3.2 B). These resorption pits could be considered pathological; however, there is evidence
that small resorption pits can form in subadults (Chapter 1) indicating that these pits may not be
pathological in origin but are instead common in this species of Pachyrhinosaurus. The basal
sulcus (Hieronymus et al., 2009) is best seen angling anteroventrally from at the posterior edgeof the nasal boss just dorsal to the orbit. Just anterior to the orbit, the basal sulcus becomesdifficult to see but most likely continues toward the dorsal rim of the narial fossa. The lateralsides of the nasal boss bear vague dorsoventrally oriented grooves and ridges giving the palisade
texture seen in other Pachyrhinosaurus bosses. The dorsal edges are relative jagged, instead ofsmooth.
The nasal boss is roughly rectangular in dorsal view (Fig. 3.2 C), with rounded anterior
and posterior margins. The edges of the nasal boss are raised whereas the middle of the dorsal
surface has a dished out concavity, similar to specimens of Pachyrhinosaurus lakustai (Currie etal., 2008). There is only sediment visible in the medial portion of the nasal boss preparation was
not completed because the nature of the preservation makes it difficult to determine where theironstone stops and bone begins. In dorsal view (Fig. 3.2 C) the widest point of the boss is above
the center of the maxilla, which is normal in pachyrhinosaurs. Along the posterior andposterolateral edges of the dorsal surface of the boss are a series of short grooves oriented
anteroposteriorly. The median ridge that defines the fusion of the two nasals is not visible in thisspecimen. The nasal boss continues posteriorly where it stops dorsal to the middle of the orbit asa ridge. Behind this ridge is a wide transverse groove.
Within the narial fossa, the narial process is only preserved on the left side (Fig. 3.2 A,
3.3 A). The process is sizeable and is triangular in profile with a slight hook oriented ventrally.

63

The bulbous anterior portion of the nasal boss where the resorption pits are located is in the same
position as the “supranasal boss” of Pachyrhinosaurus canadensis (Langston 1967, 1975).
However, there is no “fist-sized knob” posterodorsal to the narial fossa also seen in
Pachyrhinosaurus canadensis (Sternberg 1950). Neither the “supranasal boss” nor the “fist-sizedknob” is seen in Pachyrhinosaurus lakustai (Currie et al., 2008).
Supraorbital bosses
The supraorbital bosses of UALVP 53300 are mostly eroded (Fig. 3.2); however, part of
the right boss is preserved as a nondescript lump (Fig. 3.2 B). There is a discernable transverse
groove between the nasal and supraorbital bosses. Although the supraorbital bosses are eroded,
based on the left lateral view (Fig. 3.2 A) it can be seen that the nasal and supraorbital bosses
were not separated as much as they are in Pachyrhinosaurus lakustai, due to the continuation of
the rugosity along the dorsal edge of the skull.
The supraorbital bosses are preserved in the paratype skull (UALVP 55805) (Fig. 3.4).
These are deeply excavated bosses that may connect with the sinus cavity below; however, smallbones were deposited in these pits during burial, and they obscure the extent of the pits (Fig. 3.4
C). These pits are a result of resorption that occurs at an advanced age and are also seen in othercentrosaurines such as Centrosaurus and Styracosaurus (Sampson et al., 1997). Separating thetwo supraorbital bosses is a thick longitudinal ridge (Fig. 3.4 C) that sits flush with the walls of
the excavated boss and is a feature not seen in other pachyrhinosaurs where the bone typicallydips down between the supraorbital bosses even when they are exceptionally resorbed.
Therefore, this can be considered a diagnostic character.

64

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Parietal
The paratype parietals of Pachyrhinosaurus youngi consist of a single right parietal horn
(UALVP 54165) (Fig. 3.5 A), and a partial parietal bar and horn (UALVP 50652) (Fig. 3.5 B).
The convention of referencing parietal horns and processes laid out in Sampson (1995) (Fig. 3.6)
will be used here. The median paratype parietal bar (UALVP 50652) (Fig. 3.5 B) is generally
triangular in section and its ventral side is vaulted. The bar flattens dorsoventrally towards theback and the ventral surface becomes relatively flat. This vaulting is similar to that of
Pachyrhinosaurus lakustai (Currie et al., 2008) that also differs from other large ceratopsians inthat their median parietal bars are broadly convex in ventral view. Visible on the dorsal surfaceof the median parietal bar are numerous neurovascular grooves. Near the posterior end of theparietal bar, the bar widens laterally and a deeply excavated pit is visible on the left lateral sideof the bar; (Fig. 3.5 B) there is a shallower pit on the right lateral side of the bar in line with theother pit. These pits are of unknown origin and could represent resorption pits due to advanced
age or pathology.
At the posterior edge of the parietal bar is the dorsomedially hooked P2.
Pachyrhinosaurus youngi lacks Process 1 near the midline, which is consistent with
Achelousaurus, Einiosaurus, Pachyrhinosaurus canadensis, and Pachyrhinosaurus lakustai. The
posterodorsal region experienced some erosion that allows the inner striations of the bone texture
to become visible. There is a drop in topography moving from the resorption pits on the parietal
bar toward the Process 2 horn, this is also most likely due to erosion. The dorsal and ventral
surface textures alternate between smooth and striated bone.
Neurovascular grooves are visible on both sides of the paratype parietal horn (UALVP
54165) (Fig. 3.5 A). This specimen has been designated as a P3 horn based on its similarity to
the P3 horns of Pachyrhinosaurus lakustai. However, unlike Pachyrhinosaurus lakustai, the P3
horn of this specimen is not twisted dorsally, but instead it is curves laterally. The specimen also
does not have the grooves extending along its length like those seen in Pachyrhinosaurus
lakustai (Currie et al., 2008). The distal tip of the P3 horn is broken and thus would have beenslightly longer in life. Although the P3 horns of Pachyrhinosaurus lakustai can be quite variable,
and the horns of Pachyrhinosaurus canadensis are mostly straight, the curvature of the process 3
horn of Pachyrhinosaurus youngi is distinct enough to be considered a diagnostic feature.
Furthermore Currie et al. (2008) are uncertain as to the origin of the hooks and spikes on the67pachyrhinosaur frill, whether they are osteodermal or outgrowths of the parietal. Based on
preliminary studies (Kruk et al., 2013) it has been determined that parietal horns are simply
outgrowths of the parietal as they do not show metaplastic bone typically found in osteoderms(Chapter 2).

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Neurocranium
The braincase is only seen in the paratype skull (UALVP 55805) (Fig 3.4 D). The
articular surfaces of the occipital condyle in both the holotype and paratype skulls are nearlyround, which is common in ceratopsids. The exoccipitals surround the foramen magnum and arebroken laterally. Two foramina exit near the base of the condyle and the exoccipitals, the large
one is the jugular foramen carrying cranial nerves IX-XI and the smaller one is the hypoglossal
formen (XII). The basioccipital and basipterygoid processes are visible and intact.
Phylogenetic Analysis
To determine the relationship between Pachyrhinosaurus youngi and other centrosaurines
a phylogenetic analysis was performed. This analysis was based on that of Fiorillo and Tykoski(2012), which derived from Currie et al. (2008) based on previous works by Sampson (1995),
Dodson et al. (2004), and Ryan (2007). The entire character list and data matrix can be found in
Appendix 1. Non-applicable characters were coded as missing data, all multistate characters
were considered unordered, and two taxa had two polymorphic characters that were treated assuch. The taxon-character matrix was created in Mesquite 2.75 (Maddison and Maddison, 2011)
and analyzed in TNT 1.1 (Goloboff et al., 2008). Bremer support, Bootstrap (1000 replicates),and Jackknife (1000 replicates) values were calculated for the resulting trees.
The analysis initially produced three equally parsimonious trees. These trees differed in
the position of Pachyrhinosaurus lakustai, which is determined as either the sister taxon to aclade consisting of Pachyrhinosaurus canadensis, Pachyrhinosaurus perotorum, andPachyrhinosaurus youngi; or Pachyrhinosaurus is a pair of sister species relationships with
Pachyrhinosaurus lakustai being more closely related to Pachyrhinosaurus perotorum with
Pachyrhinosaurus canadensis and Pachyrhinosaurus youngi being more closely related to eachother; or Pachyrhinosaurus lakustai forms a clade with TMP 2002.76.1 and is sister taxon to aclade comprising of Pachyrhinosaurus canadensis, Pachyrhinosaurus perotorum, andPachyrhinosaurus youngi. A majority rule consensus produced one parsimonious tree (Fig 3.7)
with a length of 110, consistency index (C.I.) of 0.7909, and a retention index (R.I.) of 0.8231.
Diabloceratops eatoni is the most basal of the centrosaurine monophyly and is followed
by Albertaceratops nesmoi. A weakly supported clade of Coronosaurus brinkmani,
Centrosaurus apertus, and Styracosaurus albertensis is the sister clade to the group consisting of70

Rubeosaurus ovatus and Einiosaurus procurvicornis, and the Pachyrostra (Achelousaurus +Pachyrhinosaurus). These results are consistent with prior analyses (Sampson, 1995; Dodson etal., 2004; McDonald and Horner, 2010).
Achelousaurus horneri forms the base of the nasal-boss bearing taxa; it is a clade
supported by the presence of nasal and supraorbital bosses in place of more typical nasal
ornamentations. There is weak support for the placement of TMP 2002.76.1 as basal to the clade
composed of Pachyrhinosaurus lakustai, Pachyrhinosaurus canadensis, Pachyrhinosaurus
perotorum, and Pachyrhinosaurus youngi due to the presence of a reduced antorbital fenestra, or
its outright loss.
There is also weak support for Pachyrhinosaurus lakustai falling basal to the clade
formed by Pachyrhinosaurus canadensis, Pachyrhinosaurus perotorum, and Pachyrhinosaurus
youngi (Fig. 3.7). These three Pachyrhinosaurus species are most likely united by having nasal
and supraorbital bosses that are nearly in contact, separated by a narrow groove as well as theloss of the anterior protruding “pommel” typically seen in specimens of Pachyrhinosaurus
lakustai and TMP 2002.76.1.

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73

Discussion
The phylogenetic analysis was unable to fully resolve the polytomy between
Pachyrhinosaurus canadensis, Pachyrhinosaurus perotorum, and Pachyrhinosaurus youngi.
There is very weak support for Pachyrhinosaurus lakustai being basal to that group and even
weaker support for TMP 2002.76.1 being basal to the other Pachyrhinosaurus. Some features
uniting TMP 2002.76.1 and Pachyrhinosaurus lakustai are an anteriorly directed pommel and aseparation between the nasal and supraorbital bosses. Uniting Pachyrhinosaurus lakustai andPachyrhinosaurus perotorum is the presence of a rostral comb; however, this is not seen in either
Pachyrhinosaurus canadensis or Pachyrhinosaurus youngi. Having nasal and supraorbital bosses
that are nearly in contact unite Pachyrhinosaurus canadensis, Pachyrhinosaurus perotorum, andPachyrhinosaurus youngi. The diagnostic P3 process of Pachyrhinosaurus youngi resembles
those of Pachyrhinosaurus lakustai, but it lacks the anterolateral twist seen in the latter, and the
new species P3 is not as long or large as it is in Pachyrhinosaurus lakustai. Furthermore, thedifferences in degree of separation between the supraorbital bosses are unique among thepachyrhinosaurs.
The diversity of ceratopsians, especially centrosaurines, exploded in the Late Cretaceousand the group also had a fairly rapid turnover. The Pachyrostra (Achelousaurus +Pachyrhinosaurus) are the last centrosaurine clade to appear in the fossil record (Ryan et al.,2010) and recently have become much more diverse than previously thought. The temporal
ranges of Pachyrhinosaurus youngi and Pachyrhinosaurus canadensis overlapped for the fullexistence of the former. However the new species is not Pachyrhinosaurus canadensis based on
their differing frill ornamentations and overall skull size, with Pachyrhinosaurus canadensis
being significantly bigger.
A possible fifth species of Pachyrhinosaurus has been reported from Dinosaur Park
(Alberta, Canada) (Ryan et al., 2010). The authors describe a Pachyrhinosaurus-like ceratopsianfrom the top of the Dinosaur Park Formation (71.5 million years). However, it lacks thediagnostic parietal and therefore has not been named yet as a new species. When this fossil isplaced into a phylogenetic matrix larger than performed by Ryan et al. (2010), it is recovered asbasal to the other described Pachyrhinosaurus species but as more derived than Achelousaurus(Fiorillo and Tykoski, 2012). This placement at the base of Pachyrhinosaurus is supported by alarge separation between the nasal and supraorbital bosses and a reduced antorbital fenestra. It isexcluded from Achelousaurus based on the presence of an overhanging pommel on the anterior
portion of the nasal boss similar to Pachyrhinosaurus lakustai, and a nasal boss without the finlike texture, which is characteristic of the dorsal view of Achelousaurus.
Characters shared between TMP 2002.076.0001 and Pachyrhinosaurus lakustai include
an anteriorly directed pommel on the nasal boss and a distinct separation between the nasal andsupraorbital bosses. The presence of a rostral comb is shared by Pachyrhinosaurus lakustai andPachyrhinosaurus perotorum, but is absent in Pachyrhinosaurus canadensis or
Pachyrhinosaurus youngi. Furthermore, nasal and supraorbital bosses that are practically incontact unite Pachyrhinosaurus canadensis, Pachyrhinosaurus perotorum, andPachyrhinosaurus youngi. The distinct parietal Process 3 of Pachyrhinosaurus youngi resembles
that of Pachyrhinosaurus lakustai, although it lacks the anterolateral twist seen in the latter, and
the P3 process of Pachyrhinosaurus youngi is not as long or large as Pachyrhinosaurus lakustai.
Also, the thick longitudinal ridge separating the supraorbital bosses in Pachyrhinosaurus youngi
is unique among pachyrhinosaurs.
In their phylogenetic analysis including the proposed fifth pachyrhinosaur, Ryan et al.
(2010) modifies Character 3 from Sampson (1995); nasal horncore base (subadult): short-based,
restricted in length anteroposteriorly (0); long-based, extends over virtually entire length of nasal(1). Ryan et al. (2010) modifies this character to: nasal ornamentation, basal length (adult): shortbased, less than 5% basal skull length (0); long-based, between 10% and 20% basal skull length(1); long-based, greater than 25% basal skull length. However, these percentages do not seem towork, assuming that basal skull length is measured from rostrum to occipital condyle.
Achelousaurus species and Pachyrhinosaurus lakustai are given a character state of 1 by Ryan etal. (2010), but when comparing the nasal ornamentation to basal skull length, their average
percentages are 27% and 58% respectively. Einiosaurus is given a character state of 0 but has abase ornamentation length to basal skull length percentage of 36%. However, there may be
phylogenetic value to this character. Nasal ornamentation length should be plotted against basal
skull length and on the resulting graph there may be natural groupings, which could be used to
improve this character.

75

Conclusions
With the addition of a fourth, and possibly fifth, described species of Pachyrhinosaurus,
this genus is now the most diverse of any of the ceratopsians. All other ceratopsian genera
typically have one and at most two species representing them This potentially could be due topreservational biases, the fact that morphological differences are not pronounced or because only
fragmentary remains represent most species. With each of the Pachyrhinosaurus species(excluding TMP 2002.76.1) their remains have been discovered in bonebeds from which
multiple specimens were recovered. It is curious to note that the last of the centrosaurines werethe most specious and that all had nasal bosses. New species of ceratopsids are continuing to be
described across western North America, and it is evident that this group was diversifying at an
explosive rate.
It is entirely plausible for there to be multiple species of the same genus coexisting in thesame temporal range, such as Pachyrhinosaurus canadensis and Pachyrhinosaurus youngi, as
there is modern evidence to support this situation. The genus Equus is comprised of seven extant
species: horse, Equus ferus; ass or donkey, Equus asinus; mountain zebra, Equus zebra; plains
zebra, Equus quagga; Grévy’s zebra, Equus grevyi; the kiang, Equus kiang; and the onager,
Equus hemionus (Rubenstein, 2001). There is little difference in the skeletal morphology of eachof these extant species, although they occupy slightly different ecological niches. The diversityof the genus Pachyrhinosaurus within any single bonebed can be explained by ontogenetic or
sexual dimorphism, but the taxonomic diversity of synchronous species (P. canadensis,
perotorum, and youngi) may represent geographically or ecologically separated genetic
populations.
The amount of speciation, rate of faunal turnover, and diversity in cranial ornamentationin the ceratopsians is unlike any other group within the Ornithischia. Within the Hadrosauridae,
the lambosaurines have the elaborate crests but there is little other variation between them. For
the most part, the differences are either a posteriorly elongated crest or a more dorsally oriented
crest. In the Thyreophora, the Stegosauria only differ slightly in the placement of their dorsal
osteoderms, shoulder spikes, and neural spines (Kentrosaurus; Mallison, 2011). The only groupthat comes close to the diversity of ornamentation seen in ceratopsids is the Ankylosauridae
displaying a wide variety of dermal armor with spikes and tail clubs (Hayashi et al., 2010). The
other group within Marginocephalia is the Pachycephalosauria, which is the least specious of the76

groups previously mentioned. While all members have cranial ornamentations and do exhibit
extreme ontogenetic changes (Horner and Goodwin, 2009), the variation between species is not
as dramatic as the ceratopsids. Overall, ceratopsians are amongst some of the most interesting
dinosaurs and their diversity only continues to grow as more species are discovered.

77

CONCLUSIONS
Pachyrhinosaurus is a peculiar clade of ceratopsians, known only from the Upper
Cretaceous (upper Campanian to lower Maastrichtian) of North America. Three species havebeen reported only from Alberta, Canada and one from Alaska. These species include thepreviously described Pachyrhinosaurus canadensis, Pachyrhinosaurus lakustai,
Pachyrhinosaurus perotorum, as well as Pachyrhinosaurus youngi first described here. These
species are distinguished from one another based on characteristics of the skull, including
parietal horn shape and orientation, absence/presence of a rostral comb, median parietal bar
horns, and profile of the nasal boss. The ceratopsian diversity in the Late Cretaceous is surprisingconsidering the high and fast rate of faunal turnover (Ryan et al., 2010).
The description of the new species Pachyrhinosaurus youngi has increased the diversityof ceratopsians. The genus Pachyrhinosaurus has more individual species than any otherceratopsian genus. Furthermore, this implies that it is completely possible for there to be multiple
species of the same genus often coexisting at the same time, such as Pachyrhinosaurus
canadensis and Pachyrhinosaurus youngi. This is fitting because there are modern examples ofmultiple species of the same genus with overlapping temporal, ecological, or geographic ranges;
one example is the genus Equus (Mallison, 2001). Ceratopsians had quite possibly one of themost explosive radiations seen during the Mesozoic; they were able to diversify immensely in ashort time within a limited geographic range (namely Laramidia).
One focus of this thesis was to investigate the ontogeny of Pachyrhinosaurus from ahistological perspective. Historically, the focus of histological workers has been on long bone
histology and understanding the growth of an individual species of dinosaurs. Studies
investigating cranial histology are rare but have included studies of Triceratops (Horner andGoodwin, 2008; Horner and Lamm, 2011), possible centrosaurine integumentary structures(Hieronymus, et al., 2009), and surface textures of Centrosaurus frills (Tumarkin-Deratzian,2010). These last two studies are the only histological investigations of centrosaurines.
This is the second comprehensive histological study of the ontogeny of a ceratopsiancranial elaborations and is the first one to examine the growth of the nasal ornamentations; theprevious study was an investigation of the parietal of Triceratops (Horner and Lamm, 2011). The
Pachyrhinosaurus nasal bosses starts off as a horn-like structure that is more similar to the nasal
horns of other juvenile centrosaurines than it is to an the adult nasal boss. These “demihorns” are78

composed of compact fibrolamellar bone that continues to resorb and remodel through out the
lifetime of a Pachyrhinosaurus unti it has a spongy microstructure. No other cranial elaboration
examined in this thesis resembles the histology of a nasal boss. Furthermore, Pachyrhinosaurus
undergoes one of the more extreme ontogenetic changes with regards to cranial ornamentationsof the frill.
Studying the ontogeny of nasal bosses has added to the histological work on ceratopsians
and has provided a better understanding of the growth of their elaborations. Nasal boss histology
can provide an ontogenetic stage (juvenile, subadult, adult) to a given specimen; this will be
helpful when only a partial nasal boss is found because it will allow the specimen to be “aged.”
This discovery can be extrapolated to other ceratopsians whose nasal ornamentation derives fromthe nasal bones themselves. Calculated osteocyte lacunar density for each stage will be
performed prior to the publication of the chapter, it will give a quantitative value to the study andbetter aid in inferring the ontogenetic stage of the nasal ornamentations in other ceratopsians. It
is my belief that more than just long bones deserve to be thin sectioned, there is much more
information to be gained from these animals.
Chapter 2 represents the first study comparing the histology of ceratopsian cranialelaborations. Prior to this study, other authors focused on only single species (TumarkinDeratzian, 2010; Horner and Lamm, 2011) and this study expanded to include ceratopsians
found in Alberta. At least across the species examined all cranial elaborations (except
epoccipitals) were found to have been derived from the dermatocranium. Epoccipitals, on theother hand, were formed initially through metaplasia and later fused onto the edges of the frill.
Triceratops is strange in that its nasal horn derived from an epinasal instead of from the nasal
horns. It is uncertain whether this feature is unique to chasmosaurines or Triceratops alone.
Further investigation into the histology of these structures would clarify this discrepancy. At thevery least it can be said that all centrosaurine cranial elaborations, minus epoccipitals, form
intramembranously and are composed of fibrolamellar bone that remodels throughout ontogeny.
Epoccipitals are composed of metaplastic bone that remodels and eventually fuses onto the frill
margin.
Pachyrhinosaurus is the most derived of the centrosaurines, which raises the question ofwhat advantage nasal bosses had over nasal horns and what drove the replacement of the latterby the former. Regardless, we now have a better understanding of the histological ontogeny of79

Pachyrhinosaurus nasal bosses, as well as the diversity in Pachyrhinosaurus. Furthermore, there
is a deeper understanding of the microstructure of ceratopsian cranial elaborations, which has
implications on their development. Overall, future work will include expanding the study ofceratopsian cranial elaborations as well as a revision of the ceratopsian character matrix.

80References
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D.L. Wolberg. Department of Geological Sciences, University of Tennessee. pp. 213227.
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Appendix
Character List – List of characters used in the phylogenetic analysis. Characters 47 through 54
are intended to clarify pachyrhinosaur relationships (Fiorillo and Tykoski, 2012). Characters 55
through 57 are for further pachyrhinosaur clarification (Ryan et al., 2010). Character list is based
mainly upon that of Currie, et al. (2008).
1. Rostral, size and shape: triangular in lateral view with short dorsal and ventral processes (0);elongate with deeply concave posterior margin and hypertrophied processes (1) (Sereno, 1986).
2. Premaxilla, septum shape: absent (0); present and subcircular (1); present and anteriorly
elongate (2) (Ryan, 2007).
3. Premaxilla, septum within narial chamber: absent (0); thick and has simple, plate-likeconstruction (1); thin, often with transverse perforations (2) (Langston, 1967; Forster, 1996;
Holmes et al., 2001).
4. Premaxilla, premaxillary (narial) process extending into the external naris from thecaudoventral margin of the premaxillary septum: absent (0); present (1) (Forster, 1990).
5. Premaxilla, thickened narial strut (separating fenestra through septum from narial opening)along posterior border of premaxillary septum: absent (0); present, anteriorly inclined (1);present, caudally inclined (2) (Forster et al., 1993; Holmes et a
6. Premaxilla, ventral expansion of posteroventral margin: absent, posteroventral margin ofpremaxilla unexpanded and level with alveolar margin of maxilla (1); present, expanded
ventrally well below alveolar margin of maxilla (1) (Sereno, 1986; Penkas
7. Premaxilla, posterior tip of posteroventral process inserts into embayment in nasal and issurrounded by the nasal: yes (0); no (1) (Forster et al. 1993; Holmes et al. 2001)
8. Premaxilla contact with lacrimal: separated by nasal and maxilla (0); in contact (1) (Lull,
1933; Makovicky 2002).
9. External antorbital fenestra size: large, 20% or more length of body of maxilla (0); greatly87reduced to <10% length of body of maxilla (1); reduced to a foramen or absent (2) (Granger andGregory 1923; Chinnery and Weishampel 1998).
10. Nasal, (subadult) basal length of horn or boss: short based, restricted in length
anteroposteriorly (0); long-based, ornamentation covers almost entire length of nasal (1)(Sampson et al. 1997).
11. Nasal, (adult) ornamentation type: absent or poorly developed, <15% basal skull length (0);elongated horn >20% basal skull length (1); long based, low thickened ridge (2); boss (3)(Forster et al. 1993; Sampson 1995; Ryan 2007).
12. Nasal, posterior margin of external naris: concave (0); pronounced tab-like process projecting
anteriorly into nasal vestibule (1) (Langston 1975; Sereno 1986).
13. Jugal, infratemporal flange (adult): absent (0); present, contacts jugal flange of squamosalunder infratemproal fenestra (1) (Brown and Schlaikjer 1940; Lehman 1996; Forster 1996; Ryan
2007).
14. Prefrontal, separated by frontals and excluded from margins of frontal fontenalle (0); contact
each other on midline, separate nasals from frontals and form anterior margin of frontal
fontenalle (1) (Lambe 1915; Forster 19990; Ryan 2003).
15. Prefrontal and lacrimal: form prominent antorbital buttress (0); do not form antorbital
buttress (1) (Currie et al. 2008).
16. Postorbital, (subadult) postorbital ornamentation horn core: conical, at least 3X taller thananteroposterior basal length, rounded base and pointed apex (0); pyramidal, approx. 1:1 ratio
length to height (1); longer than high, rounded (2) (Sampson 199
17. Supraorbital (adult) ornamentation type: absent (0); present, horn (1); present, boss (2)(Sampson 1995).
18. Postorbital, horn core shape (unmodified adult): elongate, pointed apex, rounded base (0);pyramidal, rounded apex, at least as tall as base length (1); rounded apex, base longer than horntall (2) (Sampson 1995; Ryan 2007).

8819. Postorbital, horn core height (unmodified adult): long, >60% length of face (0); short, <40%length of face (1) (Forster 1990; Holmes et al. 2001; Ryan 2007).
20. Postorbital, position of horn core (adult): posterior to orbit (0); over or anterior to orbit (1)(Lehman 1996).
21. Postorbital horn core curvature (adult): no horn, absent (0); straight, dorsally, anteriorly,
anterodorsally, or anterolaterally curved (1); posteriorly curved (2) (modified from Forster et al.,
1993; Lehman, 1996; Holmes et al., 2001)
22. Parietosquamosal frill, length relative to basal skull length: elongate, 0.80 or more (0);shortened, 0.70 or less (1) (Hatcher et al. 1907; Lehman 1996).
23. Squamosal, length relative to parietal: equal or sub-equal in lenth (0); squamosal <60% total
parietal length (1) (Sereno 1986).
24. Squamosal, shape of posterodorsal (medial) margin: straight (0); posterior portion stepped-up
relative to anterior portion, transition where quadrate groove passes from ventral dorsal surface
of bone (1) (Dodson 1986; Penklaski and Dodson 19999; Ryan
25. Squamosal, anteromedial lamina forming posterolateral floor of dorsotemproal fossa: absent(0); present (1) (Dodson 1986).
26. Parietal, dorsal surface of medial bar: smooth and flat (0); small rounded midline bumps (1);large spikes (2) (Sampson 1995).
27. Parietal, posterior surface on midline: posteriorly convex or straight (0); deep, U- shaped
emargination (1) (Currie et al. 2008)
28. Epoccipital, profile shape of epoccipitals on squamosal: not present (0); crescentic to lozenge
shaped (1); triangular (2) (Holmes et al. 2001).
29. Epoccipital, number of loci on parietal rami lateral to midline margin: none (0); three to five(1); six to eight (2) (Ryan 2007).
30. Epoccipital, most medial process (P1): absent (0); unelaborated, posterior margin (1); short89

(length=base diameter) procurving hook on dorsal margin (2); long (length 2x base diameter)
procurving hook on dorsal side (3); triangular on dorsal side of parietal (4) (Currie et al., 2008).
31. Epoccipital, P1 orientation: absent (0); posteriorly directed (1); dorsally directed (2);anteriorly directed in pronounced anterior curl (3) (Sampson 1995; Ryan 2007).
32. Epoccipital, P2: absent (0); unelaborated on posterior margin (1); small medially directedhook (2); large medially curled hook (3); multipronged posteriorly directed (4); large triangularprofile (5) (Sampson 1995; Ryan and Russell 2005; Ryan 2007)
33. Epoccipital, P3 length compared to base width: absent (0); small unelaborated on posterior
margin (1); length 1-3x base diameter (2); length >4X basal diameter (3) (modified from
Sampson 1995; Ryan and Russel1, 2005)
34. Epoccipital, P3 orientation: absent or small (0); posteriorly directed (1); posterolaterally
directed (2); laterally or anterolaterally directed (3); dorsolaterally directed (4); posteromediallydirected (5) (modified from Sampson, 1995; Ryan and Russell, 2005; Ryan, 2007).
35. Epoccipitals, pattern of fusion to frill margin: occurs from rostral to caudal (0); occurs from
caudal to rostral (1) (Lehman 1996).
36. Parietal process P4-P6: absent (0); present (1) (modified from Sampson, 1995).
37. Epoccipital, imbrication of lateral marginal undulations of parietal: absent (0); present (1)(Sampson et al. 1997).
38. Predentary, orientation of triturating surface relative to horizontal plane of element: nearlyhorizontal (0); steeply inclined laterally (1) (Lehman 1990; Forster 1996).
39. Dentary, coronoid process: incipient process with gently convex apex and no neck (0); welldeveloped but lacks anterior extension distally (1); high, powerful and expands anteriorly at the
distal end (2) (Lull, 1933).
40. Dentary, posterior extent of tooth row: terminates medial to coronoid process (0); termiantes
posterior to coronoid process (1) (Brown and Schlaikjer 1940).

9041. Teeth, roots: single (0); double (1) (Brown and Schlaikjer 1940).
42. Teeth, number of vertically stacked replacement teeth per tooth family: one or two (0); morethan two (1) (Brown and Schlaikjer 1940).
43. Tooth ornamentation: subsidiary ridges present, extending from margin to base of tooh (0);subsidiary ridges reduced, present only at margin of teeth (1) (Dodson et al. 2004).
44. Sacrum, deep longitudinal channel on ventral surface: present (0); absent (1) (Lambe 1915;
Lehman 1990).
45. Ischium, cross-sectional shape of shaft: ovoid (0); laterally compressed and blade like,narrow along dorsal margin (1) (Dodson et al. 2004).
46. Ischium, orientation of shaft: nearly stright (0); slightly decurved (1)l broad and continuously
curved (2) (Brown and Schlaikjer 1940).
47. Premaxilla, anterior surface of narial bar between nasal and rostral bears: shallow transverse
grooves and ridges (0); two or more large transversely-oriented protrusions that contribute to a
rostral comb (1).
48. Nasal ornamentation or boss anterior end in mature individuals: a nearly continuous curved
profile from nasal to premaxilla (0); nasal with distinct anteriorly protruding, overhanging'pommel' in some individuals (1).
49. Nasal ornamentation or boss posterior edge: stops anterior to orbit (0); stops dorsal to orbit(1).
50. Nasal ornamentation or boss mediolateral width in mature individuals: much narrower thanunderlying rostrum (0); approximately equal width to underlying rostrum (1); widest part ofantorbital region of skull (2).
51. Nasal and supraorbital ornamentation separation on dorsal skull surface of matureindividuals: widely separated (0); nearly in contact or contacting, separated only by narrowgroove (1).

9152. Elongate parietal process P3 lateral and medial margins: curved (0); straight (1) (McDonald
and Horner, 2010).
53. Parietal process P4: absent (0); small, unmodified epoccipital (1); small spike (2); elongate
spike (3) (derived from Sampson, 1995; McDonald and Horner, 2010).
54. Parietal process P5: absent (0); small, unmodified epoccipital (1); small spike (2); elongate
spike (3) (derived from Sampson, 1995; McDonald and Horner, 2010).

92

Character Matrix