Evolution of Ceratopsian Dental Microstructure David Kay

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2014 Evolution of Ceratopsian Dental Microstructure David Kay

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FLORIDA STATE UNIVERSITY

COLLEGE OF ARTS AND SCIENCES

EVOLUTION OF CERATOPSIAN DENTAL MICROSTRUCTURE

David Kay

A Thesis submitted to the Department of Biological Science in partial fulfillment of the requirements for graduation with Honors in the Major

Degree Awarded:

Summer, 2014 1

The members of the Defense Committee approve the thesis of David Kay defended on April 21st, 2014.

______

Professor Gregory M. Erickson Thesis Director

______

Professor William C. Parker

Outside Committee Member

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Professor Scott J. Steppan

Committee Member

Acknowledgements

To be brutally honest, I almost didn’t write this section because I figured that as an undergraduate thesis that there didn’t need to be one; then I realized that by trivializing this I in effect trivialize all the time and guidance that people have given to me. First and foremost I need to thank Greg Erickson for all that he has done for me, including but not limited to: letting me into his reading group, letting me work in the lab, giving me spectacular advice on how to pursue a career in the paleo world, and being a top-notch mentor to a guy with a few screws loose. I would also like to thank Dr. Steppan, whose comments and advise are always amazing and make whatever I’m writing at least five times better. Dr. Parker for opening the geology department up to me and helping me to understand statistics as well as encouraging me to do what I want to in life. Thanks also to Ken Womble, not only for the figure help and printing out the same poster three times, but for always making me laugh.

I also want to thank my lab family, Aki Watanabe, Matt Kolmann, Bonnie Garcia, Ken Gloeckner, and Stephen Hendricks. Thanks to Aki and Matt for teaching me how to read papers and analyze other people’s work as well as teaching me some ins and outs of academic life and for the input on my work and ideas. Thank you Bonnie, for being the lab mother and releasing the kraken when the going gets tough. Ken, thank you for all the good conversation about science fiction and for buying me my first beers. And thank you to Stephen for all of your collaboration and the math discussions. All of you have taught me a lot and have become what I consider some of my best friends.

I would also like to thank a certain Microsoft millionaire who has shown me as well as hundreds of other young scientists what we need to avoid becoming.

I need to thank my genetic family as well. You have all been there for me with love and support from the beginning before I was cool. Thank you to my parents for instilling in me a thirst for knowledge and all the good life advice. To my siblings old, young and new through marriage, thank you for being there for me, especially my sister Kristen for helping to guide me through my biology degree. I love you guys. 3

Abstract

Throughout vertebrate evolution, a number of lineages evolved dental occlusion, whereby the contact faces of the teeth self-wear to their functional morphology. It has been shown that in mammals, increases in dental complexity accompany such changes. These presumably allowed for modifications in biomechanical form, function and performance relevant to dietary ecology.

Recently, it was shown that a lineage of reptiles, the Hadrosauridae, evolved some of the most architecturally sophisticated teeth known, in association with their acquisition of a grinding dental unit. Independently, another lineage of ornithischian dinosaurs, the Ceratopsia, evolved dental occlusion in the form of slicing cheek teeth. Here, I have tested the hypothesis that ceratopsian teeth increased in complexity in association with their evolution of shearing.

Transverse and occlusal plane histological sections were made using cheek teeth from representative Ornithischia spanning the transformation series leading to the evolution of slicing in ceratopsians. The sections were viewed with dissecting and polarizing light microscopy. The microstructure was described and phylogenetically character-mapped in association with whole tooth and wear facet morphological attributes. My results show that ceratopsian teeth are considerably more complex than those of the outgroup ornithischians in possessing four distinct tissues: enamel, orthodentine, coronal cementum, and vasodentine. Coronal cementum evolved in association with a cohesive dental battery and a shearing masticatory system in the common ancestor of Leptoceratops + Triceratops. Vasodentine appeared in the common ancestor of

Protoceratops + Triceratops with the advent of high-angled slicing. These findings represent the second demonstration of complex dental architecture outside of Mammalia, and show that some reptiles rivaled, if not exceeded, most mammals in dental complexity. It also supports the 4

hypothesis that complex histological attributes in teeth appear in association with precise dental occlusion.

Introduction

Teeth, serve a number of functions including display, defense, prey capture, and relevant to this study, the oral processing of food (. Variation in osseous constituents (hereafter referred to as constituents or histology) that make up teeth varies considerably between groups (Peyer 1968,

Schmidt and Kiel 1971, Hillson 1986). These include: enamel, dentine, and cementum. Enamel is an extremely hard tissue composed of hydroxy-apatite crystals within a protein matrix (Peyer

1968, Schmidt and Kiel 1971). Dentine is a softer, bone-like tissue that often makes up the majority of the tooth structure (Peyer 1968, Schmidt and Kiel 1971, Hillson 1986). Cementum is primitively a root attachment tissue in vertebrates but it is found on the chewing surfaces of some animals such as horses and bison as a crest supporting and basin contributing tissue known as coronal cementum (Peyer 1968, Schmidt and Kiel 1971, Hillson 1986). Because it is composed of more ground tissue and protein matrix than enamel and dentine, cementum is relatively softer

(Peyer 1968, Hillson 1986).

Most gnathostomes, including nearly all reptiles, have non-occluding teeth and the tooth

crowns are composed of just an enamel shell overlying a dentine core. However in some

herbivorous mammals (for which precise dental occlusion is primitive) which have dentitions

composed of folded layers of enamel, several forms of dentine and cementum. Such tissue

complexes self-wear to file or rasp-like surfaces used for the pulverization of tough and often

abrasive plants (Peyer 1968, Schmidt and Kiel 1971, Hillson 1986). 5

Notably, a few groups of Ornithischian dinosaurs, the hadrosaurids and ceratopsians evolved precise dental occlusion (Ostrom 1964, Ostrom 1966, Dodson 1996, Hailu and Dodson

2004, Horner and Weishampel 2004, Erickson et al. 2012). Hadrosaurids in particular were shown to possess up to six different dental tissues in their teeth (Erickson et al. 2012). As in herbivorous mammals with grinding dentitions, the evolution of these complexes was shown to correlate with the evolution of precise dental occlusion and selection for more complex self- wearing architecture to pulverize plants. These findings beg the question; do the other dinosaurs with precise dental occlusion, the Ceratopsia, show histological complexity beyond that typical of reptiles?

Coronosaurians (Dinosauria: Marginocephalia) are a group of non-avialan dinosaurs

(hereafter dinosaurs) which, like hadrosaursids, evolved dental batteries (groups of interlocked developing and functional teeth that form a single chewing surface; Figure 1) and an occluding dentition (Ostrom 1964, Ostrom 1966, Dodson 1996, Hailu and Dodson 2004). ). The teeth of coronosaurians are thought to have worked like a giant pair of shears, whereby the corresponding dental batteries, wore to high-angled slicing faces, (unlike the typically more coarse planar surfaces of hadrosaurids) for the shearing of tough plant matter, (the types of which are unknown) (Ostrom 1964, Ostrom 1966). The teeth in the lower jaw possess hard enamel only on their lingual faces. Conversely, enamel is restricted to the labial face of teeth in the lower jaw

(Figure 2). These configurations allow for self-wear to blades since the enamel is highly wear resistant and the dentine core poorly wear resistant (Figure 3) (Ostrom 1964, Ostrom 1966).

Notably, like all other dinosaurs, with the exception those lacking cheek teeth and possibly derived heterodontosaurs, ceratopsian had polyphyodont dentitions (Hopson 1980, Butler et al. 6

2008). Thus, the functionality of these shearing implements was maintained despite continuous tooth eruption (Ostrom 1964, Ostrom 1966).

Like all reptiles, it has been traditionally assumed that all ceratopsians possessed simple tooth histology composed of just enamel over an orthodentine (dentine possessing von-Ebner’s growth lines) core. Hatcher and colleagues (1907) however reported cementum on the crowns of

Triceratops (Hatcher et al. 1907, Ostrom 1964). Examination of figures from the same publication suggests a fourth tissue, evident in the circumpulpar dentine, may also be present

(Hatcher et al. 1907). In this figure (Figure 4) there is a region on the occlusal face that is differentiated as being darker than the adjacent dentine. This finding, along with the discovery of advanced dental architecture in hadrosaurids led me to ask whether ceratopsians possessed advanced dental histology beyond that typical of reptiles?

Here I survey the dental morphology and histology of tooth crowns in outgroup through derived

Ceratopsia and character map my results in a phylogenetic context. I use the results to test the following hypothesis:

Ceratopsians evolved more complex dental architecture involving 4 or more tissues in

association with the evolution of dental occlusion.

Materials and Methods

Phylogenies, evolutionary grade identification, and selection of specimens

The recent comprehensive global phylogeny of Makovicky (2010) for Ceratopsia were used in conjunction with data from the literature on the dental morphology of taxa to infer the transformation sequence by which coronosaurians evolved precise dental occlusion. The 7

Makovicky phylogeny in particular covers 19 ceratopsians with the least differentiated genus

being Yinlong and the most differentiated genus being Triceratops (Figure 5). The phylogeny were then used to select taxa representing each evolutionary grade from which the histology could be correlated. This analysis revealed an outgroup condition to Ceratopsia with non- occluding (at least not regularly) leaf-shaped cheek teeth. This morphology was represented in my analysis by the thyreophoran dinosaur Euoplocephalus tutus (uncatalogued) (Vickaryous et al. 2004). The next grade crownward shows oblique-angled, intermittent occluding cheek dentition involving all functional teeth (Hailu and Dodson 2004, Tanoue 2008, Tanoue et al.

2009). This grade was represented by the psittacosaurid Psittacosaurus mongoliensis (MAE 97-

15) (Osborn 1923, Tanoue 2008, Tanoue et al. 2009). The next grade exhibits dental cohesion involving all cheek teeth with a low-angled shearing occlusal plane (Ostrom 1964, Tanoue 2008,

Tanoue et al. 2009). This morphotype in my analysis is represented by the leptoceratopsid

Leptoceratops gracilis (Brown 1914, Makovicky et al. 2001). The next more crownward grade exhibit an interlocking cohesive dental battery of cheek teeth with a nearly-vertical slicing occlusal plane (Ostrom 1964, Ostrom 1966, Tanoue 2008, Tanoue et al. 2009). This morphology is represented by the early Cretaceous protoceratopsid, Protoceratops andrewsii (Granger and

Gregory 1923, Mackovicky 2010), and the highly derived neoceratopsian, Triceratops horridus

(Mackovicky 2010).

Preparation, examination, and comparison of specimens

The total length and crown height were measured to the nearest 0.1 mm for each tooth

using dial calipers. For worn teeth the crown height was taken from the worn crown and not

extrapolated. Histological preparations of the teeth were made by first setting the individual teeth

in clear epoxy resin (Epoxyset: Allied High Tech Products, Inc., Compton, CA). The specimens 8

were then cut into 1.4-1.5 mm thick slices using a slow-speed bone saw (Isomet 1000: Buehler,

Lake Bluff, IL) fitted with a diamond-tipped wafering blade. The sections were cut at the in-vivo

angle of wear that was determined from the slightly worn teeth I sampled; except in the case of

Triceratops where an unworn tooth was studied, in which the angle was determined from casts of

naturally worn tooth batteries. This was done to allow study of the tissue constituents naturally

exposed at various wear stages. The lone exception to this practice was the tooth of

Euoplocephalus that was sectioned transversely since it lacks precise dental occlusion.

The resin-embedded tooth slices were mounted onto petrographic slides using either clear epoxy (ITW Devcon, Danvers, MA) or cyanoacrylates (Henkel AG&Co, Rocky Hill, CT) as adhesive. The specimens were initially viewed on the glass slide using a dissecting scope. In

most cases the histological attributes were readily apparent. If not, then further processing was

conducted as follows. The specimens were ground using silicon carbide grinding disks (Allied

High Tech Products, Inc.) in a descending serial grit order (180-1200) on a Rotopol-11 grinding

and polishing machine (Struers Inc., Cleveland, OH) with water as a lubricant. They were ground

down to a thickness that allowed for dissecting light tissue identification (around 100 microns

thickness) and if necessary, transmitted polarizing light microscopy (around 20-60 microns

thickness). The slices were numbered 1-n spanning from the apex towards the root for each

specimen.

After preparation, the histology of each specimen spanning the crown was described

using the terminology of Tomes (1878), Peyer (1968), Schmidt and Keil (1971); Hillson (1986),

Kalthoff (2011), and Erickson and colleagues (2012). Measurements of tissue thicknesses were

made using the digital analysis program ImageJ (National Institute of Health, USA, Bethesda, 9

MD) from digital photographs taken with an Olympus DP 11 (Olympus Corporation, Center

Valley, PA) or from direct measurements using digital calipers.

The entirety of each taxon’s histological suites was character mapped using to weighted

parsimony on the aforementioned composite phylogeny for Ceratopsia and evolutionary patterns

inferred.

Results

Euoplocephalus tutus

External morphology

The specimen (Figure 6A) has a total length of 15.20 mm. Nine slices were produced.

The estimated crown height is 9.12 mm, maximum mesial-distal width of 8.85 mm and labial-

lingual width of 3.50 mm. The tooth is leaf-shaped and bladelike. It is labio-lingually

compressed and has eight denticulate ridges that stem from the fluted crown faces. The carinae

are asymmetrical in size and bend away from the midline of the tooth (in the mesial and distal

directions). The single root has a circular transverse cross-sectional shape.

Enamel

The enamel on this specimen has been weathered so it is absent in some spots on the

tooth; however, it is present around the entire tooth crown beginning at the apex and extending

down 8.44 mm of the tooth. This represents 92.5% of the estimated crown height.

Dentine

The entire core of the tooth (crown and root) is composed of orthodentine with prevalent incremental lines of Von Ebner. Secondary or tertiary dentines were not observed. 10

Walkthrough

At the apex of the tooth, the crown is composed completely of enamel; beneath this

veneer the tooth is made of an enamel shell that is filled in by orthodentine. The exteriorly

visible fluting and denticulate edges are seen as rounded bumps in transverse view, and continue

5.6 mm down the length of the crown. At 3.34 mm (22.2%) down the tooth, the orthodentine is no longer completely filling in the tooth because the pulp cavity is now visible and has been permineralized. The pulp cavity widens in the mesial/distal directions with the tooth crown, and shrinks back down proportionally with the width of the tooth as the crown narrows and connects into the root (Figure 7).

Psittacosaurus mongoliensis

External morphology

MAE 97-15 (Mongolian Academy of Sciences, Ulaan Bataar, Mongolia) is a partial

maxilla bone with two erupted teeth. One is slightly worn while the other shows a remnant of the

tooth crown. There are six other teeth in the bone, but the tooth crowns were broken off prior to

collection. MAE 97-15 has a represented total length of 11.17 mm. A total of four slices were

produced. The teeth have a crown height of 8.37 mm, a mesial-distal width of 4.11 mm, and a

labial-lingual width of 1.75 mm. The teeth are labial-lingually compressed, single rooted and

leaf-shaped. The intact crown has six column-like secondary ridges on the lingual side, with the

most mesial and distal ridges being relatively broader than the other four (Figure 6B).

Enamel

As in the Euoplocephalus teeth crowns, enamel is present on on all faces of the crowns.

The enamel is thicker on the labial side than the lingual side, which helps to form the oblique

wear angle exhibited by Psittacosaurus, as the enamel on the lingual side (in dentary teeth) gets partially worn away.

Orthodentine

The entire core of the tooth (crown and root) is composed of orthodentine with clearly visible incremental lines of Von Ebner surrounding the pulp cavity (Figure 8).

Walkthrough

Beginning at the occlusal face and moving through the tooth in the direction of wear, the tooth has a thin enamel sheet. Beneath it the tooth exhibits an orthodentine core with an enamel

shell on all the faces. As the end of the crown is reached, enamel only occupies the mesial, distal,

and lingual faces. The root is composed completely of orthodentine with a pulp cavity in the

middle that occupies 50% of the width of the root (Figure 8).

Leptoceratops gracilis

External morphology

AMNH # (American Museum of Natural History, New York) is a slightly worn cheek

tooth crown with a broken root. AMNH # has a total height of 13.2 mm, and produced eight

slices. The root has a height of 6.63 mm and the crown a height of 6.57 mm; the crown has a

maximal mesio-distal width of 8.85 mm, and 8.95 mm thick labio-lingual plane (Figure 6C). 12

The crown has a prominent keeled carina (ridge) down the middle of the lingual face, and

two vertically offset accessory carinae, one located mesially and one located distally to the

central carina (Figure 6C). These accessory carinae are at the most distal and mesial ends of the

lingual face (Figure 6C). These three carinae produce the characteristic "serrated" top of the

occlusal face of the tooth which has been hypothesized to have a functional significance in shearing (Ostrom 1966). It appears that the mesial and distal faces show that the enamel does not reach all the way around the tooth.

The root is more lightly colored than the crown, and is a single root, consistent with non- ceratopsid ceratopsians (Ostrom 1964, Dodson 1996, Hailu and Dodson 2004). The root is rounded on the labial face of the tooth, flattening out at the mesial and distal faces, and is missing on the labial face of the tooth.

Enamel

Because AMNH # is a worn tooth, the apex is missing, along with a portion of the crown.

The remaining face of the tooth shows enamel present on the labial face extending around the

two outer carinae, putting it at the juncture of the labial and mesial as well as the labial and distal

faces. This continues for the first 3.3 mm (50.2% of the crown) of the tooth; when 3.3 mm

(50.2% of crown) is reached, the higher offset lateral carinae disappears, leaving only the central

keel and one lateral carina; the enamel at this point is now on the labial face and extends slightly

onto the mesial and distal faces. Beyond this at 5.60 mm (85.2%) the enamel is on the labial side

as both the remaining carinae disappear; the enamel shrinks on the labial face as the depth of the

tooth increases, and by 6.57 mm enamel is absent.

Dentine

The interior of AMNH # is completely composed of orthodentine, making it the most

abundant tissue in the tooth; it follows the shape of the tooth with complete incremental lines of

Von Ebner, and fills in the tooth with no sign of a pulp cavity as seen in other specimens.

Cementum

AMNH # exhibits cementum on all faces of the crown where enamel is absent (i.e. the

lingual face) or on the labial face of the root (Figure 6C). The cementum is present 1.4 mm (21.3

%) down the tooth on the mesial and distal faces of the crown (Figure 9). These two bodies of

cementum occupy about 20% of the mesial and distal faces. Farther down the tooth, the

cementum patches expand along the mesial and distal faces, and at 6.6 mm (50% of tooth height)

are on the lingual face. 9.9 mm (75% of tooth height) down the tooth, the cementum is a

continuous layer all the way around the mesial, lingual and distal faces and remains this way on

the entirety of the root.

Walkthrough

Beginning at the occlusal face that AMNH # presented, the core of the tooth is completely composed of orthodentine with visible incremental lines of Von Ebner, and enamel present on the lingual face, covering the carinae and ending at the junctures of the lingual and the mesial/distal faces. At 1.4 mm down the tooth, cementum appears on the mesial and distal faces, occupying about 20% the length. Over the next 2.8 mm the cementum expands to the entire length of the mesial and distal faces. 2.8 mm into the tooth the lateral carina that is more distal to the central carina disappears, and by 5.6 mm the remaining two carinae disappear. At this point the cementum has migrated from the mesial and distal faces onto the labial face of the tooth. At 14

around 6.7 mm into the tooth, the enamel disappears, and the mesial, distal, and lingual faces are

covered by a continuous layer of cementum; the tooth (now at the root) is now arch-shaped, and continues this way (orthodentine with a cementum layer on the mesial, distal and labial faces)

until the end of the root (Figure 9).

Protoceratops andrewsii

External morphology

This specimen (uncatalogued) from is a single worn cheek tooth that is heavily

weathered, with a height of about 24 mm, producing six slices. The mesial-distal width is 8.10

mm, a labial-lingual width is 8.49 mm, and a crown height is 12.32 mm.

The crown has three carinae: one central and two lateral; these carinae and the crown itself have a somewhat glassy appearance. The lateral carinae are vertically offset from one another, and the central carina is offset from the midline of the lingual face (Figure 6D). The occlusal face shows some bowling out.

Enamel

The enamel is present on lingual face and curls around onto the mesial and distal faces for about 10% the labio-lingual length of the crown. It increases to about 66% the length of the crown over the next 2.8 mm (33% of the crown height) and remains at that length until 5.6 mm down the tooth. At which point the enamel abruptly disappears.

Orthodentine

The specimen has an orthodentine core that fills the majority of the tooth with visible lines of Von Ebner. The vasodentine (below) occupies an area of about 10% of the occlusal face 15 of the tooth about 1.4 mm (16.67%) into the tooth. After this point, the interior of the tooth is composed solely of orthodentine.

Cementum

There is no visible cementum on this specimen. I suspect that this is due to weathering as this is the softest and least wear resistant tissue present (Erickson et al. 2012) and it is fully exposed to the elements after the specimen’s death.

Vascularized Dentine

On the worn face of the tooth, there is a small amount of centrally located vasodentine.

This vasodentine is in a bowled out area, so it there is very little present. However, 1.4 mm into the tooth, the vasodentine is still visible, and because this section is unworn, it is a bit clearer to see (Figure 10). It is located in the lower part of the crown face, and occupies about one third of the width of the tooth at this point. Beyond 1.4 mm there is no vasodentine.

Walkthrough

At the naturally worn occlusal face, the enamel extends over the lingual face and down

10% the length of the mesial and distal faces; the interior has vasodentine present which occupies the lower central area about half of the width of the face. The vasodentine will continue in this manner for 1.4 mm and then disappear. The enamel expands down the mesial and distal faces of the tooth for the next 6 mm and then disappears. The orthodentine filling the tooth exhibits a pulp cavity between 2.8 mm and 7 mm into the crown. (Figure 10)

Triceratops horridus

External morphology

AMNH ---- is an erupted unworn adult tooth crown that measured 33.7 mm in height

(Figure 6E) producing 18 slices. Externally, the lingual face of the crown has three carinae, a deeply keeled central carina and two smaller lateral secondary carinae; the crown-most edge has serrations. These carinae are not vertically offset as in the Leptoceratops specimen, and the central keeled carina is actually centrally located on the lingual face, as opposed to the

Protoceratops and Leptoceratops specimens whose central carinae are offset from the midline of the tooth.

Enamel

The first one and a half millimeters of the tooth have enamel all sides of the tooth except

the lingual face that does not show enamel. The enamel on the mesial and distal sides of the

tooth wrap around from the labial face and only extend partway across the tooth, roughly around to the point of the tissues that become the outer keels on the labial face of the tooth. The enamel

shrinks on the mesial and distal sides of the tooth around the outer keels as they become more

prominent. At about 18.6 mm (roughly the half the length) down the tooth crown, enamel is only

located on the labial face of the tooth. The enamel ceases to exist on the root of the tooth around

30 mm (about 89%) of the length of the tooth. As it passes beyond the carina and keels of the

tooth, the enamel begins to shrink along the surface area of the labial face, giving the enamel (on

the labial face) a shape similar to that of an almond, starting narrow, growing in width , and then

shrinking back down into another point. An interesting observation about the enamel on this

tooth is that it is not smooth like in most other teeth; this enamel is rough on its surface, giving it 17

a unique “warty” appearance. The roughness increases at the labial and distal/mesial sides of the

tooth. This is probably not an artifact of fossilization for this specific sample because similar

textured enamel is visible on a shed Triceratops tooth specimen (AMNH).

Cementum

The tooth exhibits cellular cementum tissue on both its crown and root. The cementum

first occurs on the right and left sides of the crown roughly 8.1 mm (24.0% tooth length) from

the tip of the crown; this is a layer of it about the same thickness of the enamel on either side,

just past the enamel. That cementum occurs for only about 1.9 or 2 millimeters. Cementum is exhibited on the next slice, roughly 0.4 mm from the last cementum, but this occurs in a different part of the tooth, in this case, it is on the right side of the right keel (in reference to the tip of the crown pointing towards the viewer) right next to the enamel on the portion that is curved around from the “labial” side. This cementum continues down that side, starting at 12.9 mm, and ending at 31.9 mm; it expands in length for about 6.1 mm starting at 12.9 mm. After the 6.1 mm of expanding on the labial-lingual axis, the remainder of the cementum expands in the mesial-distal plane until the cementum ends at 31.9 mm. Here it is fractured and so may have extended further down the root.

Orthodentine

Orthodentine is consistently present throughout the tooth, and is the most abundant tissue.

It is present throughout the entire tooth. It follows the shape of the tooth, expanding in the x-y

plane until about slide 5 roughly 9.1 mm (27.00%) down the length of the tooth, and then shrinks

slightly down to slide 9, where the two roots grow off of the tooth. It becomes progressively

slimmer down to slide 18. Where the two roots diverge, there should be a circle of dentine, 18

which might be evident from around 16.7 to 21.25 mm (49.55-63.06% of tooth length) but

mineral infilling extending from the lingual side to the center of the tooth precludes identification. On the second and third slices the Von Ebner’s dentine has complete ovals of dentine, because it is before the vascularized tissue starts to occur, which touches the lingual side of the tooth

Vascularized Dentine

This tissue is found only within a small section of the inside of the tooth, within slides 3-

7, which occupies from 5.3-12.9 mm (15.7%-38.2%). It starts off with a width equivalent to

about 20% of the total width of the more rootward end of the third slice, reaches a maximum

width of about 66% of the rootward side of the fifth slice, or 7.6 mm down the tooth. It ends with certainty at the rootward side of slide 7 a length of 12.9 mm (38.2%). After this, there is mineral infill in the center of the tooth, making it impossible to tell what type of tissue is present, if any.

The vascularization in the tissues starts off as grooves, but farther down the tissue, the grooves

turn, and change more into distinct tubules evident on the occlusal face.

Walkthrough

Beginning at the tip of the crown, the tooth is just enamel, and then beneath it

orthodentine appears, with the enamel just borders the tooth on the labial, mesial and distal sides.

The tooth then becomes ridged with the carina appearing. At this point the orthodentine shows

Von Ebners incremental line layering. On the labial side of the tooth, two crests form on the

outer edges of the tooth. Near this point the vasodentine begins to occur. Note that by this point

the elliptical lines of orthodentine are no longer present. The two outer ridges on the labial side

produce a rounded nob like appearance in the cross section that ends at the mesial-distal fissure, 19

along with the enamel. Both the orthodentine and vasodentine expand along the mesial-distal and

labial-lingual axes for around 4 mm, and then at roughly 9.1 mm (27.78%) the tooth exhibits

small amounts of cementum on both the mesial and distal sides near the lingual side of the tooth.

This is also the largest part of the tooth (when cut at an angle of wear). After this, the pattern of enamel on the most labial face of the tooth and wrapping around the ridges on the edge of the tooth ending at the division of the mesial-distal fissure continues, as well as the orthodentine filling of the interior. The lingual 2/3 of the tooth along the midline shows vasodentine that extends until about 13.3 mm (44% of the crown height). Because mineral infilling has replaced

some vasodentine it is inconclusive where it stops and a pulp cavity may have begun. However,

at around 12.9 mm (38.89%) cementum shows up again on the mesial side of the tooth and

continues with the tooth, expanding as the overall tooth size decreases from 9.1 mm into the

tooth. At 20.9 mm into the tooth, the enamel is reduced to the labial side of the tooth because of the expansion of the cementum. The general shape of the tooth is rounding out now and losing its ridged structure, instead becoming rounder at the root of the tooth. The enamel of the tooth shrinks to only part of the labial face of the tooth and 3.8 mm (11.11%) before the end of the

tooth it ends, with a round dentine area composed of completely of orthodentine with a patch of

cementum about ½ the width of the dentine at this point. The cementum shrinks over the next 1.9

mm and is absent in the last slice, which is composed of just orthodentine (Figure 11).

Discussion

There is an apparent change in masticatory systems from the outgroup condition to the more differentiated ceratopsids. Euoplocephalus, representing the outgroup condition to

Ceratopsia, exhibits non-occlusion. Here the primitive two-tissue dental architecture, enamel and orthodentine, observed in most reptiles is present (Peyer 1968). Non-coronosaurian ceratopsians 20 represented by Psittacosaurus with the more differentiated non-coronosaurian character state of intermittent occlusal dentition (Hailu and Dodson 2004, Tanoue 2008) retains this same histological character suite. However, the coronosaurs and more differentiated ceratopsian clades exhibit precise dental occlusion with cohesive dentitions (Ostrom 1964, Ostrom 1966, Hailu and

Dodson 2004, Dodson et al. 2004, Tanoue 2008, Tanoue et al. 2009). The most basal ceratopsians representatives possessing this anatomy, such as Leptoceratops in the present study, show low-angled shearing dentition. In these animals, “coronal” cementum first appears. The loss of enamel on the lingual faces in the lower dentitions and labial faces in the upper dentitions leads to the self-wear that produces the slicing faces in these teeth. This cementum is on the crown; however it does not contribute to the wear surface, and perhaps functioned to interlock the dental battery so that teeth could erupt in concert and act as single rigid bodies during chewing which probably increased the efficiency of the shearing dentition. The evolution of this unnamed clade composed of the common ancestor of Protoceratops and Triceratops and all of its descendants signaled a shift to the high-angled slicing occlusion in ceratopsians (Ostrom

1964, Ostrom 1966). Both taxa show vasodentine in association with this shift in feeding morphology. The teeth in these animals have a visible bowling out in the region of the tooth where the vasodentine is located, which suggests that the vasodentine was less wear resistant than the surrounding orthodentine. This leads to the inference that the functional significance of this tissue in ceratopsians served to reduce the contact areas between opposing teeth, reducing the friction created between them and decreasing the amount of force needed to carry out a chewing stroke; these biomechanical implications and models are being investigated by members of and collaborators within the Erickson lab. 21

In order to see the trends from the results more clearly, they were phylogenetically

character mapped according to weighted parsimony (Figure 12). There are two broad kinds of

traits that are being mapped, and these are occlusal masticatory systems and dental tissues.

Euoplocephalus exhibits the primitive condition of non-occluding teeth, as do Yinlong

and Choangysaurus (Tanoue 2008). Psittacosaurus is the first to exhibit occlusion (intermittent

occlusion), and because basal neoceratopsians possess the same occlusal pattern (Tanoue 2008)

it is more parsimonious to put intermittent occlusion evolving in the common ancestor of

Psittacosaurus and the neoceratopsians. The next sampled taxon, Leptoceratops, shows the

advent of a new occlusal system: low-angled shearing (Ostrom 1964, Ostrom 1966, Tanoue

2008) with dental cohesion. No data could be found on the occlusal pattern of the Asiaceratops, it cannot be confidently discussed; however, Cerasinops, Montanoceratops, and Udanoceratops all had the same dental occlusion type as Leptoceratops, as well as the dental cohesion (Ostrom

1964, Ostrom 1966, Kurzanov 1992, Chinnery and Horner 2007, Tanoue 2008, Mackovicky

2010). With this in mind, low-angled shearing and dental cohesion can be placed at the common

ancestor of Cerasinops and the rest of the Leptoceratopsids. Protoceratops and Triceratops both

show the masticatory system of a high-angled slicing, as well as the trait of an interlocking

cohesive dental battery so the evolution of that trait can be parsimoniously attributed the

common ancestor of the Protoceratopsids and the Ceratopsids (Ostrom 1964, Ostrom 1966,

Tanoue 2008).

The non-Neoceratopsian taxa sampled (Euoplocephalus and Psittacosaurus) have teeth

composed solely of the simple two tissue suite found in almost all reptiles; all taxa sampled

exhibit these tissues as well. Leptoceratops is the first taxa sampled to show cementum;

Triceratops possesses this tissue as well. As stated, the cementum functioned to create dental 22

cohesion between teeth. The Protoceratops lacked cementum, but that condition could possibly be a result of taphonomic processes or weathering after exposure. As cementum in ceratopsians functions to keep teeth cohered to each other and both the leptoceratopsids and the clade that includes protoceratopids and ceratopsids exhibit a form of dental cohesion (with the latter possessing interlocking teeth as well) it is more parsimonious to hace cementum evolving at the common ancestor of coronosauria than in leptoceratopsids and the ceratopsids separately.

Protoceratops and Triceratops both exhibit vasodentine, suggesting that the tissue evolved at their common ancestor, apparently in association with advent of an interlocking dental battery and high-angled slicing.

My analysis of these data with respect to phylogeny support my hypothesis that increases in histological complexity accompanied the evolution of precise dental occlusion in ceratopsians

(Figure 12).

Conclusions

The hypothesis that the ceratopsians exhibit at least four dental histological types in the crowns of their cheek teeth is confirmed. These include enamel and dentine the two primitive reptilian tissues, coronal cementum and vasodentine. This is the second example of reptilian dental architecture becoming sufficiently complex to rival that of the most sophisticated seen in mammals such as large ungulates with grinding dentitions that also possess four dental tissues.

Figures

Lingual view of a sample of an interlocking dental battery from the dentary of a Triceratops specimen showing the vertical growth of the teeth and the nonfunctional replacement teeth beneath the top functional row.

From Hatcher et al. 1907

Transverse view of a Triceratops dentary, showing the replacement column of teeth and the wear plane.

OS: Occlusal Surface

From Ostrom 1964

Lingual

Transverse view of a Triceratops dentary and maxilla showing the occlusal surfaces of the teeth and how they match up for chewing.

From Ostrom 1964

Drawn dental column from a Triceratops dentary with the top tooth being the functional one with the replacements underneath. The dark line at the top of the tooth is enamel, the cross-hatched area on the inside of the tooth is dentine; the area labeled “a” follows a similar pattern to the nonhomogeneous dentine in ceratopsians.

Taken from Hatcher et al. 1907

Most parsimonious cladogram detailing the phylogenetic relationships within ceratopsia.

Redrawn from Makovicky 2010

A B C

D E

Picture of the whole specimen teeth. A: Euoploephalus B: Psittacosaurus C: Leptoceratops D: Protoceratops E: Triceratops. Scale bars=5 mm. 27

Euoplocephalus

Cross section of Euoplocephalus. Note the daily deposited lines of Von Ebner and the thin mineralized pulp cavity in the middle of the section.

E= enamel, O= orthodentine. Scale bar is 5 mm.

Cross section of the Psittacosaurus tooth, it is only composed of orthodentine and enamel with a pulp cavity present in the middle of the section.

E=enamel O=orthodentine. Scale bar is 5 mm.

Occlusal section from Leptoceratops, it has a solid interior of orthodentine, an enamel shell on the labial face and cementum patches on the mesial and distal faces.

E=enamel

O=orthodentine

C=cementum

Scale bar is 5 mm

10.

Occlusal section of Protoceratops, the interior of the tooth is VD mostly orthodentine with a small patch of vasodentine in the lower central region; the enamel shell on the lingual, mesial and distal faces is weathered, which could be an explanation for the lack of cementum.

E=enamel

O=orthodentine

VD=vasodentine

Scale bar is 5 mm

11.

Occlusal section of the Triceratops specimen, VD it contains orthodentine with a large amount of vasodentine in the middle of the occlusal face, enamel on the lingual, mesial and distal faces, and cementum on the mesial and distal faces.

E=enamel

O=orthodentine

C=cementum

VD=vasodentine

Scale bar is 5 mm 30

12.

are taxa Characteron Makovicky out cladogram based of 2010. Sampled Ceratopsia mapped

lined in red. red. in lined

Ceratopsida

Coronosauri

Neoceratops

Ceratopsia

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