Species Resolution in Triceratops

8/10/2019 Species Resolution in Triceratops

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Journal of Vertebrate Paleontology 16(2):259-270, June

1996

1996

by the Society

of

Vertebrate Paleontology

Page charges paid for

THE

INOS UR

SOCIETYCB>

SPECIES

RESOLUTION

IN TRICER TOPS CLADISTIC

AND MORPHOMETRIC

APPROACHES

CATHERINE

A.

FORSTER

Department

of

Organismal Biology and Anatomy, University

of

Chicago, 1025 E. 57th Street, Chicago, Illinois 60637*

ABSTRACT-Sixteen species of Triceratops have been proposed s ince the genus was erected by O. C. Marsh in 1889.

Five

of these species a re

here

considered technically invalid

and a

sixth

is reassigned

to

Diceratops. Based on both

cladistic analysis and morphometric shape analysis,

all

available

skulls

of t he t en remaining species of

Triceratops

are placed in one of two species: horridus and prorsus invalidating eight of the original ten

species.

These two

species

overlap

in geographic

and

stratigraphic

ranges. Because specimens

of

horridus

greatly

outnumber

those of

prorsus these morphotypes may represent d is tinct taxa rather than

a

single sexually dimorphic species.

INTRODUCTION

Triceratops was

one of

the largest and most numerous her

bivorous dinosaurs

in

western

North America

at th e

end

of

the

Cretaceous. Based largely on

well

preserved skulls from Maas

trichtian

formations (Lance, Hel l Creek, Scollard,

Frenchman)

in Wyoming,

Colorado,

Montana,

South

Dakota, Alberta,

and

Saskatchewan, sixteen

species

of

Triceratops have

been named.

Because

of

its

abundance, apparent

diversity, and late

temporal

occurence, Triceratops has

figured

prominently

in

studies of

dinosaur

abundance and diversity near

the Cretaceous-

Tertiary

boundary in

North

America e.g. , Van Valen and

Sloan,

1977;

Sloan et al., 1986; Sheehan et al. , 1991 .

In 986, Ostrom and Wellnhofer redescribed the holotype of

brevicornis

(BSP 1964

1

458)

and

provided

a

historical

ac

count of Triceratops systematics.After a

consideration

of in

dividual

variation,

sexual dimorphism,

and

ontogenetic

effects,

they concluded that Triceratops was monospecific. A complete

analysis

of

this

hypothesis,

however,

was

beyond

the

scope

of

their paper se e also Ostrom and Wellnhofer, 1990 . Their paper

underlined

the

modern biological

species

concept, and called

for

its application

to the

problem

of

species

in

Triceratops.

This s tudy combines cladistic and

morphometric

information

in a species

analysis of

Triceratops. The

novelt ies that

char

acterize species occur by way of continuous shape change and

the appearance of discrete characters. Shape,

or quantitati

ve

,

information

is explored

through

multivariate morphometric

shape

analysis.

Qualitative characters are

examined

by cladistic

analysis. available specimens

of

Triceratops Table 1 were

examined to: 1

determine

the

taxonomic validi ty of named

species, 2 rediagnose

the

species based on

groupings

discov

ered

by analysis, and 3 discuss the diversi ty and

range

of

variation within

the

genus.

Institutional abbreviations include:

AMNH,

American Mu

seum

of Natural History,

New York; BSP, Bayerische Staat

samlung fr PaHiontologie und Historische Geologie, Munich;

NMC,

Canadian

Museum of Nature, Ottawa;

CM, Carnegie

Museum

of Natural

History, Pittsburgh; FMNH, Field Museum

of Natural History, Chicago; MCZ, Museum of

Comparative

Zoology,

Cambridge;

USNM, U.S. National Museum

of

Nat

ural

History,

Washington, D.C.; LACM, Natural

History Mu

seum

of

Los Angeles County, Los Angeles; SMM, Science Mu

seum of Minnesota, St. Paul; SnSM, South Dakota School

of

Mines,

Rapid

City;

UCMP,

University of California at Berke-

Present

address:

Department of Anatomical Sciences, Health

Sci

ences Center, State Universi ty of New

York

at Stony Brook, Stony

Brook, NY

11794.

ley,

Berkeley;

YPM, Peabody

Museum of Natural History,

New Haven.

SYSTEMATIC HISTORY

OF

TRICER TOPS

Named Species

The first specimens attributed to Triceratops

were

a pair

of

partial

supraorbital horns

collected

in

the

Denver Fm.

of Col

orado

and e rroneous ly refer red to the mammalian

genus Bison

by O. C. Marsh (1887). In the following year, J. B. Hatcher

collected

a partial skull from

the

Lance

Formation,

Niobrara

County, Wyoming, which Marsh (1889a) originally

described

as Ceratops horridus. Later

that

year Marsh

(1889b)

erected

the

genus Triceratops

with

horridus as

the type species, and

renamed the bison specimen Triceratops alticornis.

Over t he nex t

eight

years,

Marsh named e ight more species:

galeus

(1889b), flabellatus (1889b),

serratus

(1890a),

prorsus (1890a), sulcatus

(1890b),

elatus (1891),

calicornis

1898 ,

and

obtusus 1898 . Six

additional species

were

described

by subsequent authors:

brevicornis

Hatcher,

1905 hatcheri

(Lull in

Hatcher et al. , 1907 , ingens Lull,

1915 maximus (Brown, 1933 ,

eurycephalus

Schlaikjer,

1935 , and albertensis

(Sternberg,

1949 .

Of t he se s ix teen speci es , four a re based on

inadequate ma

terial:

galeus

(an isolated nasal horn;

USNM

2410), alti-

cornis (a

pair of supraorbital horns;

USNM

4739),

sulcatus

(a pair of supraorbital horns;

USNM

4276), and maximus

(eight dorsa l ver tebrae ;

AMNH

5040).

These

specimens are

nomina dub ia and canno t

be

assigned to Triceratops with cer

tainty. A fifth species,

ingens

(YPM 1828 , is an unpublished

name of Marsh s

mentioned

by

Lull 1915 .

The

type specimen

is a l arg e, par tia l sku ll a ss ignabl e to Triceratops. Lul l men

tioned ingens without

description

or diagnosis, and

it

is

con

sidered here

a

nomen

nudum.

Triceratops hatcheri

was

originally

described

by Lull in

Hatcher et al. , 1907 as Diceratops

hatcheri then later synon

ymized with Triceratops Lull, 1933 . In this study, Diceratops

is considered a

valid

taxon and hatcheri

is

removed

again to

this taxon.

This revis ion is

discussed

in

the

following

section.

Validity

Diceratops

The taxon Diceratops (USNM 2412) consists

of

a single

skull

without lower jaws

or postcrania. Generic characters

were

originally described as: nasal horn absent, squamosals

pierced

by large

fenestra,

parietal

with

small

fenestrae, and a squamosal

lacking

an

inferior jugal notch.

Nasal

horns a re a lso occas ion

ally

missing

in specimens

of

Triceratops due to the la ck

of

fusion

between

the

epinasal

ossification

and

the nasal

boss For-

259


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260 JOURNAL

OF

VERTEBRATE

PALEONTOLOGY

VOL.

16,

NO.

2, 1996

TABLE Specimen

numbers, species assignments, geographic

and

stratigraphic information,

and

basal skull

length

BSL for specimens used

in this study. Abbreviations: H, holotypes; e, estimated measurement; NA, not available skull either incomplete

or

inaccessible .

Original species

Specimen assignment

This study

Location

Formation

BSL

cm

AMNH

5116

elatus

horridus

Niobrara Co., Wyo. Lance 110 e

AMNH 970 serratus horridus western Montana

Hel l Creek NA

BSP 1964 1 456 brevicornis H

prorsus

Niobrara Co., Wyo.

Lance NA

CM

1221 brevicornis

prorsus

western Montana Hel l Creek

116

FMNH

P12003

calicornis

horridus SW Montana

Hel l Creek 110

LACM 7207

sp.

prorsus

western Montana Hell Creek

110

MCZ 1102

eurycephalus H

nomen

dubium

Goshen Co., Wyo.

Lance

NA

NMC 8862 albertensis

H

nomen dubium

southern

Alberta

Scollard NA

SDSM

2760

horridus horridus western Montana

Hel l Creek

NA

SMM P62/1/1

prorsus

horridus western Montana

Hell

Creek

127

UCMP 113697

horridus horridus McCone

Co.,

Montana

Hel l Creek NA

USNM 1201 elatus H horridus Niobrara Co. , Wyo.

Lance

104 e

USNM

2100 elatus horridus Niobrara Co., Wyo.

Lance

NA

USNM 4720 obtusus H

horridus Niobrara Co., Wyo.

Lance NA

USNM 4928 calicornis

H horridus

Niobrara

Co., Wyo. Lance 118

YPM 1820 horridus H horridus H Niobrara Co., Wyo.

Lance

NA

YPM

1821

flabellatus H) horridus Niobrara Co., Wyo.

Lance

106 e

YPM

1822 prorsus H prorsus H)

Niobrara Co., Wyo. Lance

88

YPM 1823 serratus H

horridus

Niobrara Co., Wyo.

Lance 102

ster, 1996 . This likely accounts for the absent nasal horn in

Diceratops.

Squamosal fenestrae commonly occur in all chasmosaurine

ceratopsids

Chasmosaurus, Pentaceratops, Anchiceratops, Ar-

rhinoceratops, Torosaurus except Triceratops.

In

chasmosaur

ines, squamosal fenestrae

are

located pos te rior to the contact

with the paroccipital processes, are

either

uni-

or

bilateral

or

missing

altogether in sorne specimens , a re near ly uniform in

shape,

and possess smoothly rounded

margins.

Squamosals

of

Triceratops are uniformly

thick and

never

exhibit

fenestration

or thinning.

A large fenestra

occurs

in the left

squamosal

of

Diceratops

immediately posterior to

the

paroccipital processes, and a small

oval fenestra

pierces

the

right

squamosal . Sorne authors e.g.,

Lull, 1933

considered these

fenestrae a pathological result of

injuries. Where the

margins

of the fenestrae

are

preserved in

Diceratops

they are

smoothly rounded and lack

any

outward

sign of t rauma. However, there is a pathologica l area of bone

along

t he l ef t squamosal-parietal suture,

immediately posterior

to the left fenestra. The margin of the fenestra near this callosity

is smooth and

even,

apparently uneffected by this injury. The

even

nature of

the squamosal

fenestrae in Diceratops, coupled

with the ubiquitous appearance of fenestrae

of

similar shape,

size,

and position

in all

chasmosaurines exclusive

of

Tricera-

tops, indicate

squamosal

fenestrae are a rea l morphological en

tity

not predicated by

injury.

Diceratops also possesses

parietal fenestrae, a

character

also

shared with all other chasmosaurines exclusive of Triceratops.

The

r ight par ie ta l has a

narrow

16 cm. long opening .

Much

of

the

margin

of thi s fenestra is obscured by plaster, but enough

margin is preserved to discern its approximate size and confirm

its presence. Additionally,

the

parietal

immediately surrounding

the fenestral margin is very thin. The left parietal is too poorly

preserved

to

provide

additional information.

Diceratops

shares with

Torosaurus

a unique configuration of

the frontal fontanelle.

In these two

taxa, a

shallow channel

ex

tends posterolaterally from the frontal fontanelle the frontal

fontanelle is paired in Torosaurus and single in

Diceratops

towards

the anter ior margin

of each

upper temporal

fenestra.

Each channel

terminates

at a foramen in the par ietal the an

terior

temporal

foramen of

Marsh,

1892

medial

to

the upper

temporal

fenestrae. These

anterior temporal

foramina are

found in no o ther ceratopsids.

Dice

ratops

possesses both

parietal

and squamosal

fenestrae,

and sha res a unique frontal fontanelle configuration

with

To-

rosaurus. Additionally, the frilllbasal

skulllength

of Diceratops

i s greater than that

of

all Triceratops specimens. A recent c la

distic analysis of the

Chasmosaurinae by Forster

1990 also

supports the validity of Diceratops. While this is

not

a

complete

reanalysis

of

the systematic position of Diceratops, these char

acters indicate

Diceratops

Hes

outside Triceratops

and validates

i ts removal from this analysis.

Diagnoses of Taxa

The

removal

of

the

taxa listed aboye leaves ten technically

valid species

of Triceratops. Each

of t he se ten specie s were

erected

citing

morphology thought

unique

to

that species.

The

majority

of these

species-level characteristics involve

ornamen

tal

morphology

of

the

supraorbital horn, nasal horn,

and

frill.

Most are poorly diagnosed, and these defining characteristics

are

summarized

below:

horridus

holotype: YPM 1820; Marsh, 1889a . Characters

same as for genus.

flabellatus

holotype:

YPM

1821; Marsh , 1889b . Fan-l ike

frill, large epoccipitals,

very

large size.

serratus holotype: YPM 1823; Marsh , 1890 . Possesses a

series of

bony

projections

a long the median

Hne of t he pa

rietal crest, with a similar

ridge

along the squamosals caudal

to the postorbital .

prorsus

holotype: YPM 1822; Marsh, 1890 .

Massive horn

cores; narrow,

extensive

frill with broadly

convex squamo

sals;

and

a long,

forward projecting

nasal horno

elatus holotype: USNM 1201;

Marsh,

1891 . Moderate sized

nasal horn; long, pointed , and forward

directed supraorbital

horns; and

an elongate

and much

elevated

frill.

calicornis holotype: USNM 4928; Marsh, 1898 . Unusual ,

dorsally-concave nasal

horn with

horseshoe-shaped dorsal

surface.

obtusus

holotype:

USNM 4720;

Marsh , 1898 . Very short,

rounded, and

obtuse

nasal horno

brevicornis holotype: BSP 1964 I 458, formerly YPM 1834;

Hatcher, 1905 .

Short

and

stout

supraorbital horns; short

and

nearly vertical

nasal

horn; ell iptical orbit ; and open frontal

fontanelle.

eurycephalus

holotype: MCZ 1102; Schlaikjer, 1935 . Rel

atively

long

frill;

short den ta ry and

facial region; elevated


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FORSTER SPECIES RESOLUTION

IN TRICERATOPS

261

orbit , small

nasal

horn; very long

and slender

supraorbital

horns;

and separate

exit s for le ft

and right olfactory

nerves.

a.lberte sis holotype:

GSC

8862; Sternberg, 1949).

High

fa

cIal reglon;

large

antorbital fossa; supraorbital

horns mostly

behind orbits and

directed

vertically; frill not strongly up

turned;

squamosal long and

thick.

Generic diagnoses of Triceratops are as general as those of

its species.

Marsh

1889b) originally

diagnosed

Triceratops

as

having

a pair of massive supraorbital horn

cores

on

the

top of

the skull, a

third

horn core on the nose, a rostral forming a

projecting

beak,

a

high

frill

extending

upward and

backward,

and massive lower jaws united by a strong

predentary.

As ad

ditional

ceratopsid

material was collected,

i t became clear tha t

these characters defined a more inclusive group

than

Tricera-

tops.

Hatcher

et al. 1907) emended the original

diagnosis

of Tri-

ceratops: Supraorbital horns

directed

forward and upward a t

an angle

of 45

degrees; nasal

horn

of

moderate length and

di

rec ted nearly straight forward; no parietal fontanelles; squa

mosal

short

and broad. Later

diagnoses of

t he genus

e.g.,

~ u l l

1933; Steel , 1969) added little

or

nothing

to

this descrip

tlon.

The

first

comprehensive

diagnosis

of

Triceratops

was that

of

Ostrom

and Wellnhofer

1986, p. 115), nearly a century

after

the genus

was

first described:

Large ceratopsian

of more

than

6 m.

l ength up to

8

or

more

meters. Skull distinctive bearing elongate supraorbital horn

~ o r e s

plus

a

single

variable nasal

horn

coreo Brow

horns

vary

In taper, stoutness,

curvature

and length, bu t generally

project

up and moderately forward as we ll as laterally. Nasal horn

tapers

from

a

modestl y tapered bl unt bos s to

a

prominent

upward and

forward

directed

projection.

Nasal

horn

always

much shorter than brow horns. Brow horns never longer than

pre-orbital

sku ll l ength and

usually distinctly shorter.

Skull

elongate with

post-orbital

length

always

greater than

pre-or

bital length. Parietal-squamosal fri ll relat ively short com

pared to

sorne

other genera) and

generally

curves

back

and

upward.

The

fril l is

never

fenestrated. Frill

margins may

be

ornamented by blunt, scallop-like epoccipital bones. Horns or

spikes are

never present on

frill

margins or jugal

flanges.

Where known, post-cranial features and

counts

are compa

rable to those of other large Late Cretaceous ceratopsian gen

era.

~ h i s i g ~ o s i s emphasizes variability within Triceratops, par

tIcularly In supraorbit al and nasal horn

size

and orientation

~ h i ~ h historically

formed

much of

the basis

for s p e i f i ~

dlstInctlons. General

characters

defining more

inclusive

groups

among ceratopsids, however, are also included. Triceratops is

diagnosed here as

follows:

Nasal

horn varies f rom small

boss

to modera te ly long horn

p laced over f ront of external

nares.

Frontal

fontanelle

either

s ~ l l

and ~ i r c u l r or absent

due to

closure

of frontals

and pa

rletals. Parletals unfenestrated, extremely thick, and

heavily

vascularized on their upper surface. Vascularized r im present

around perimeter of ventral

surface

of frill.

Squamosals

broad

and

short relative to other

chasmosaurines,

with convex, round

ed

lateral margins.

Epoccipital

spans

midline on

the parietal,

and squamosal-parietal suture. Fri ll

saddle- shaped with up

turned

caudal margin,

and

strong

parietal

midline

ridge.

This diagnosis contains only those characters autapomorphic

for the genus, sorne

previously

identified by

Hatcher

et al.

1907) and

Ostrom

and Wellnhofer 1986).

RESULTS

Cladistic Aoalysis

Discrete characters

for

Triceratops were compi led in

a cla

d is tic analysis. A

recent

cladistic analysis

by For st er

1990),

placing Triceratops within the Chasmosaurinae, formed

the

ba

sis f or the

polarization

of characters within Triceratops. Dicer-

atops and Torosaurus form successive

outgroups.

Five

char

acters were

found to

vary among

Triceratops specimens and

are

discussed below:

1)

Contact of t he squamosal , jugal , and

postorbital

aboye

the lower temporal fenestra.

This

complex suture maintains a

consistent, plesiomorphic pattern in most specimens e.g., CM

1221), bu t exhib it s a

unique pattern

in

YPM

1822

and SMM

P62/1 /1 Fig . 1). Primitively the

jugal

forms the dorsal margin

of

the

lower temporal fenestra and the squamosal extends dorsal

to the j ugal to con tact the postorbital. In

YPM

1822 and

SMM

P62/1/1

the squamosal

forms

the

dorsal

margin of

the

lower

temporal

fenestra

and does not

extend across

the

top of

the

jugal.

2)

Supraorbital horn

length.

The length of the

supraorbital

horns

varies among Triceratops specimens, and

has

been

cited

as a species speci fic

character in

elatus,

brevicornis,

and

eurycephalus.

In relat ion

to

basal

skull length, relat ively

short horns

are

found i n BSP 1964 I 456, CM 1221, LACM

7207, and YPM

1822

horn length/basal

skull

length ranging

f rom 0.42 to 0.61). All other specimens where horn length and

basal

skulllengths

can

be

measured

or

estimated have

relatively

longer

supraorbital

horns

e.g.,

USNM 4928, USNM

1201,

AMNH

5166; horn length/basal skulllength 0.69 to 0.75).

3)

Closing of the

frontal fontanelle.

When

present,

the

fron

tal font anel le is a s ingl e,

circular opening,

often connected or

nearly

connected

to

t he upper t emporal

fenestrae by

shallow

channels or

furrows across

t he sur face

of

the

parietal e.g.,

AMNH 5116, USNM 2100).

The

frontal fontanelle is absent

due

to

the uninterupted suture between the

frontals

and p r i e t ~

als, in sorne specimens

of

Triceratops YPM 1822, LACM

7207, CM

1221; Fig. 2).

This absence of

a frontal fontanelle

in

these

specimens

is

unique

among ceratopsids.

4) Rostrum shape.

The

rostrum exhibits two morphologies

among Triceratops

specimens. Commonly

the rostrum

appears

low and drawn-out, the premaxillae forming a reversed S

shaped

rostral

margin

e.g.,

SDSM

2760,

USNM

4928).

This

form is plesiomorphic among

chasmosaurines.

Sorne Tricera-

tops specimens possess a deep, shorter rostrum with a smoothly

convex

rostral

margin

YPM 1822,

LACM 7207, CM

1221,

BSP 1964 I 458; Fig . 3) .

5)

Nasal horn

length.

The nasal horn assumes

a varie ty

of

shapes and sizes and has been cited as a specific character in

ca

licornis,

prorsus

elatus, brevicornis,

and

ob-

tusus.

Long,

curved,

and

forward

inclined

nasal

horns

angles

range from approximately thirty to fifty degrees f rom the ver

tical),

extending anterior to the external

nares e.g.,

YPM

1822,

M 1221, UCMP 113697), differ discretely f rom the short, up

rlght

nasal horns e.g.,

AMNH 5116, SDSM 2670;

Fig. 4).

Long nasal

horns

have horn

length

to

basal

skull

length

ratios

of

0.23 to 0.30, while short

nasal

horns show ratios of 0 .10 to

0.13. A

small boss

of

undefined

shape

occurs

in

obtusus

USNM 4720) due to the loss of the

epinasal

ossification.

The

unusual nasal horn configuration in calicornis USNM 4928)

results from

the

incomplete fusion of

the epinasal

ossification

onto the

nasal

boss or horncore Forster , 1990, 1996; this mor

phology

is also

clearly

observed in

UCMP

113697).

Nasal

horns are often broken and their original configuration indeter

minable

e.g.,

YPM

1821).

Other

morphologies

which lack discrete distributions, includ

ing frill shape and supraorbital horn orientations, have been

used

to

diagnose

species of Triceratops.

Wide ranging and con

t i ~ u o u s

variation occurs

in

these morphologies, making species

dlagnoses based

on

these

characters

ambiguous. They

are dis

cussed below:

Supraorbital

Doro Orieotat ioo-The supraorbital horns

express

a

range

of

orientation in

Triceratops

and

appear in


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TRICERATOPS

263

FIGURE

4. Nasal horn length character 5 , right lateral view. A,

short, upright horn,

AMNH

51

16;

long, inclined horn,

CM

1221.

FIGURE

5. Consensus tree for character analysis of ricer tops spec

imens showing character distributions. This seven-s tep t ree has a con

sistency index

of

0.714.

Other specimens of horridus

are

AMNH

5116,

FMNH

P12003,

SDSM

2706,

USNM

1201, 2100,4928,

YPM

1820, 1823.

imens.

The

squamosal in

Triceratops

is short and broad relat ive

to that in

other

chasmosaurines, but varies continually in shape

from extremely broad and fan-Iike e.g.,

YPM

1822 to nar

rowed

and tapered posteriorly e.g.,

USNM

4928 . Orientation

of the frill a lso varies

among

specimens, but slight distortions,

notably lateral crushing or dorsoventral ftattening, are evident

in many specimens, making comparisons of subtle orientation

differences inconclusive.

The five characters which vary discretely within

Triceratops

were compiled into a data matrix and analysed using

PAUP

Swofford, 1985; Appendix 1 The resulting 7-step con sensus

tree consistency index

=

0.714, Fig. 5 separated out four spec

imens

YPM

1822, LACM 7207, CM 1221, BSP 1964 1 458;

the prorsus group , united by characters 2, 3, 4, and 5. AII

other

specimens fall within a

horridus

group. Character 5

long nasal horn

occurs

in parallel between the

prorsus

group,

FIGURE 3

Rostrum shape character 4 , lef t lateral view. A, long, S

shaped rostrum,

USNM

4928; short, rounded rostrum,

LACM

7207.


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264

JOURN L

OF VERTEBR TE P LEONTOLOGY

VOL.

16,

NO. 2,1996

SMM

P62/1/1,

and UCMP

113697,

leaving only

characters 2,

3, and 4 to define unequivocably that group. The unusual cheek

suture

shared

by YPM 1822

and SMM

P62/1/1

character

1)

has

an equivocal distribution.

Multivariate Morphometric Shape Analysis

Quantitative analyses

of

dinosaur morphology

have

been

sporadically employed in s tudi es f or

many

years for

a thor

ough review see Chapman, 1990).

These

studies

vary

consid

erably in both

scope and methodology.

Most

notable are

two

ear ly studies by

Dodson

1975, 1976),

which

successfully

used

bivariate

a ll ome tri c ana ly se s to

probe

questions of

growth, sexual dimorphism,

and

systematics in hadrosaurids

and Protoceratops. Chapman et

al.

1981) used

a combination

of bivariate

and

multivariate analyses to study systematics

and

sexual

diorphism

in the

pachycephalosaur

Stegoceras.

A

sim

i lar study was carried

out

by Weishampel and

Chapman

1990)

on Plateosaurus. Other multivariate analyses have used

a

landmark based

shape analysis cal led

RFTRA

Resistance-Fit

Theta-Rho

Analysis; Benson e t

al., 1982) , which

performs

pairwise

comparisons of specimen shapes. This

method

was

used

by

Chapman

and Brett -Surman 1990) to explore system

atics and variation in hadrosaur ids, and by

Dodson 1993)

to

study systematics and morphological trends

in

ceratopsians.

Thi s s tudy

seeks to

discover

species

differences

if any)

among Triceratops specimens based on multivariate discr im

ination

of shape

via

principal components analysis

PCA).

Principal components analysis is designed to analyse sets of

correlated

variables,

in this

case

derived

from

distance

mea

surements t aken f rom Triceratops skulls. Principal compo

nents analysis

doe s not

presume

a

priori

groupings and

thus

allows for their discovery.

Fourteen

skulls of Triceratops

were subjected

to PCA using

varying subsets of thirty-five distance variables. Distance vari

ables were

measured on

the skulls

between predetermined an

atomically

based

landmarks Bookstein

et

al. , 1985; landmark

locations

and

variables

are summarized

in

Appendix

11 .

Dis

tances of less t han 20 cen timete rs were

measured

to the near

es t

millimeter with

dial

calipers, and those

over

20 centimeters

were measured to the nearest half centimeter with sliding cal

ipers.

The d is tance var iables were log

transformed

and sub

jected

to a principal components analysis.

The

resul ts were

examined for g roup ings across the

first

three principal com

ponent

axes.

Postcranial material was not included

in this

analysis. While

skull material is abundant and

well

preserved, postcranial

ma

terial

for Triceratops

is

l arge ly incomplete and scant ily col

lected.

Many

specimens, including

types, lack

any postcranial

material.

Only

skulls not

exhibiting obvious

distortion

were included

in the morphometric

analysis.

Where

distortion occurred, mea

surements were taken

only

of the

undistorted

areas.

Distortion

was assessed through d ir ec t observa tion and the compari son

of bilateral measurements.

Where

good data were obtained

from

both

sides of

a skull,

th e left and r igh t measu rement s

were averaged.

The

distortion

and

incompleteness of many

specimens

rendered

them

completely

or partially unavailable

for analysis. Data missing for

one

or more variables in nearly

all

specimens,

with

missing variables

varying

from

specimen

to specimen, further complicated the construction of data sets.

Regardless,

14

specimens,

including

eight

holotypes, were

analysed using multivariate morphometric methods. A number

of independent analyses were

run

using dif fering subsets of

var iables and specimens. Nine specimens w it h nearl y com

plete

data

sets were analysed

in

the most inclusive runo Spec

imens with less complete

data

sets

were

each

analysed in

sep-

ara te runs using

all

available var iables for that specimen

and

all

o ther specimens for which those var iables were known.

The most inclus ive set

of

Triceratops specimens analysed

Analysis 1)

included nine specimens

YPM 1821, 1822, 1823,

AMNH 5116,

USNM

2100, 4928, SMM P62/1/1,

FMNH

P12003, CM

1221) and twenty-five variables

5-6, v9-15 ,

vI7-20,

v23-35). Numerical results of this and all other anal

yses

are shown in Appendix 111. Principal Component C) I

captured

53.7

of

the

total variation,

PC

II 23.1 , and PC III

6.70/0

for a total

of

83.8 across the first three principal com

ponents.

PC

1

while loading

on

size, also

contains

shape

information

as

evidenced

by

the negative

value for v25, an axial measure

ment along

the

face.

Although

there is a general

trend

to in

crease variable length with general s ize, v25 exhib it s a general

reduction in

length.

All

bu t

two specimens

li ne up

along

the

PC

I axis

according

to ba sa l skull length, again

emphasizing

the

infiuence of size

information within PC 1.

AH residual val

ues

for PC I are

within

0.3 of zero,

and the

percentage of vari

ation captured is only slightly ove r 50 . This is l ikely due to

the narrow

size

range

of the

adult specimens

involved 8 to

127 cm. basal skull length) .

PC

II is bipolar,

containing contrasting

negative and positive

values. Eight of

the

nine negative variables v8-13, v17, v19)

involve transverse measurement across the skull; vIO and vIl

have particularly

high negative

loadings. Principal

Component

II contrasts transverse measurements to other measurements on

the

skul l, tha t is

the width of the

skull is narrowing relat ive

to

other length

measurements.

A

high

amount of variance is ex

plained by PC II 23.1 ), making it a s igni ficant shape axis.

PC 111

capturing

only 6.7

of the total variance, is also bipolar.

Both negative and positive residual values occur for transverse

measurements, measurements along the

side of the face,

and

frill measurements.

Principal component bip lo ts Fig . 6) show a fairly tight clus

tering across

PC 1 PC 11 and PC

111. The

two

specimens of

prorsus YPM 1822, CM 1221) cluster together alongside the

horridus

group,

but do not

separate

completely

from

this

group along any single axis .

An analysis including LACM 7207 with the other

prorsus

group specimens involved

seven

additional

specimens

YPM

1822,

CM

1221, SMM P62/1/1, USNM 2100, 1201, 4928,

AMNH 5116) and eighteen var iables

v3-6,

v23-34;

Fig. 7,

Appendix

111 . The

first Principal

Component

accounted for

48.3 of the var iance, PC II for 24. 8 , and PC I II for 15 .4 ,

capturing a total

of

88.5 of

the

variance

across

the first

three

principal components. Again,

PC

I loads heavily but not ex

clusively on

size,

whi le PC

II

and PC

III

include

a high per

centage of shape information. LACM 7207 clusters with

YPM

1822 and CM

1221,

completely separat ing from the horridus

specimens

along PC 11.

Spec imens YPM 1820 type

of

horridus USNM 1201

type

of

elatus

4720

type

of

obtusus

and

AMNH

970

were also included in separate, less complete analyses. These

analyses, ranging from six to eight total specimens and 7 to 22

variables per analysis, are summarized below.

The YPM 1820 Analysis including YPM 1822, AMNH

5116,

USNM 2100,

USNM 4928, CM

1221, FMNH

P12003;

vl-2, v6-7,

v13,

v18-20,

v35; Fig .

8A, Appendix 111

shows

that YPM

1820 is

morphometrically consistent

with the

hor-

ridus

group. However, since

only nine

variables were employed

the case admittedly is not strong.

Analyses using

USNM

1201 including YPM 1822,

AMNH

5166, USNM 4928, CM 1221, FMNH P12003; vl-2, v4-7,

v14,

v17-20, v23-33) , USNM 4720

including

USNM

1201,

2100,4928,

CM

1221,

YPM

1822,

AMNH

5116; vl-3, v5-7,

v19), and AMNH 970 including YPM 1821, 1822, 1823,

AMNH

5116,

USNM

4928,

CM

1221,

FMNH

P12003;

v5-8,


7/12

FORSTER-SPECIES RESOLUTION IN TRICERATOPS

265

o

O

O

o

o

o

o

o

A

P

8

P

e

P

FIGURE 6. Principal

component

biplots

for

Analysi s 1. A, PC I-PC ; B, PC I-PC

;

C

PC

II-PC Open circles

represent

prorsus

specimens CM 1221

and

YPM 1822,

and closed

circles represent

orri us specimens AMNH 5116, FMNH P12006, SMM P62/1/1, USNM

2100, 4928,

YPM

1821, 1823.

vI2-14 v18, v20, v35 were also mn Fig. 8B-D Appendix

111 These

three

specimens lie within morphospace consistent

with

the

horridus grouping. Available

character data

for

AMNH 970 open frontal fontanelle is also consistent with this

grouping.

Pertinent character data

are

unavailable

for

USNM

4720.

DISCUSSION

Character analysis suggests

that

Trice

ratops specimens

can be divided into two groups: a prorsus group YPM

1822 type

of

prorsus;

CM

1221;

LACM

7207;

BSP 1964

I

458 type

of

brevicornis

and

a

horridus

group

con

taining aH o ther specimens AMNH 5116;

FMNH

P12003;

SDSM 2760; SMM

P62/1/1;

UCMP 113697; USNM 1201

type of elatus; USNM 2100; USNM 4928 type

of

cal

icornis; YPM 1820

type

of

horridus;

YPM 1823

type

of

serratus .

Derived characters indicative of

the

prorsus

group include

a

closed frontal fontaneHe relatively short

su-

praorbital

horns

and

a short

convexly

rounded rostrum.

Specimens within the horridus group retain the p rimi tive

states for these cha racter s but have no autapomorph ic char

acters of

their

own.

Morphometric shape analysis supports the character analysis

by

separating

YPM

1922,

LACM

7207

and CM

1221

f rom the

remainder

of

the specimens

BSP 1964 I

458 was not measured

for this analysis . While the morphometric results

support

the

cladistic analysis, shape cannot delineate groups

in

the absence

of discrete

character

data.

The type

of

flabellatus

YPM 1821

was

excluded from

the character analysis but included in

the

morphometric analysis

where it grouped within the

horridus

morphospace. This

specimen lacks

supraorbital horns, rostral,

and

a

nasal

horn,

and

while the

frontal fontanelle

area

is

poorly preserved

an

opening

appears to

be

presento

Three types were not included

in

either the character analyses

or the morphometric

analyses

due

to

incompleteness of the

specimens. They are

discussed

below.

FIGURE 7. Principal

components

biplots for LACM

7207

Analysis. A PC

I-PC

; B, PC

I-PC

; C PC II-PC Open circles represent

prorsus specimens

CM

1221,

LACM

7202,

and YPM

1822,

and

closed circles

represent horridus

specimens

AMNH 5116 SMM

P62/1/1/,

USNM

2100, 1201, 4928.


8/12

266

JOURN L OF VERTEBR TE

P LEONTOLOGY

VOL 16 NO 2,1996

o

o

P

P

A B

o

o

e

P

D

P

FIGURE 8. Principal

components

biplots

PC

I-PC 11 A

YPM

1820 Analysis.

Open

circles represent

prorsus

specimens CM 1221

and

YPM

1822 closed circles represent horridus specimens AMNH 5116

FMNH

P12006

USNM

2100 4928,

and

cross represents

YPM

1820;

B USNM 1201 Analysis. Open circles represent prorsus specimens CM 1221 and YPM 1822 closed circles represent

horridus

specimens

AMNH 5116, FMNH P12006

USNM

4928, and cross represents

USNM

1201; C,

USNM 4720

Analysis.

Open

circles represent

prorsus

specimens CM 1221 and

YPM

1822 closed circles represent

horridus

specimens AMNH 5116

USNM

1201 2100 4928 and cross represent

USNM 4720; D AMNH 970 Analysis. Open circles represent prorsus specimens CM 1221 and YPM 1822 closed circles represent

horridus

specimens

AMNH 5116 USNM 4928,

YPM

1821 1823

and

cross represents AMNH 970.

obtusus USNM 4720)-The short rounded obtuse na-

sal

horn diagnosing

the species results

from

the absence

of

the

epinasal ossification. The rostrum is

missing

and the frontal

fontanelle region is damaged , but very long supraorbital horns

are presento

albertensis NMC 8862)-This specimen consists primar-

ily of

the

left

cheek and

facial region a partial left supraorbital

horn and a lef t squamosal ; a ll

material

is poorly preserved and

deformed. No character information i s availab le s ince the fron-

tal fontanelle

region and

rostrum are not

preserved.

The verti-

cal caudally placed supraorbital horn used to diagnose the spe-

cies is

the

result of

severe postdepositional

deformation.

Sim

ilarly the antorbital

fossa

region is artificially

enlarged

due to

damage. The morphology of the squamosal Hes within the range

of

that

for

Triceratops

eurycephalus

MCZ

1102)-This

specimen

is heavily

reconstructed and very poorly preserved. I t consists of the

right

cheek region,

two near ly complet e and long supraor

b it al horns , a

partial braincase,

l eft na sa l, le ft squamosa l,

lower jaws, and

portions of

the

parietal

and right squamosal.

However,

the

relatively small

occipital condyle

and nar row

mandibles

of

the specimen do not resemble those

of

Tricer-

atops raising doubts

as

to the generic identity

of

this

frag-

mentary

specimen.

The incomplete

and

fragmentary frill and

lack

of

epoccipitals and

frontal fontanelle

region also make

the

identification of this

specimen as

Triceratops impossible

to

confirmo

There is no

stratigraphic or

geographic

separation

between

horridus and

prorsus

groups. YPM 1822 and BSP 1964

1

456

are

from Niobrara

Co.,

Wyoming, and LACM

7207


9/12

FORSTER SPE lES

RESOLUTION IN

TRICERATOPS

A

FIGURE 9. Representat ive specimens

of

the

two

species of Triceratops A, T horridus

SDSM

2760; B, T prorsus YPM 1922.

267

and CM 1221 f rom western Montana. Specimens

of

T hor-

ridus occur at

both these

sites. General

stratigraphic

data are

available only for

Niobrara

County Wyoming. this region

YPM 1822 was found approximately at m id

section and

BS P

1964 1 45 6

approximately

two thirds of the way up section.

Specimens of T horridus are found both aboye within and

below

the

intervals containing the two

T

prorsus

specimens.

Stratigraphic

and geographic data do

not

preclude

the hy-

pothesis that these two morphotypes represent a single sexually

dimorphic species. However,

T

horridus greatly

outnumber

those

of

T

prorsus

an unlike ly rati o a Ithough cert ainly not

impossible considering possible preservational biases in fossil

taxa for a sexually

dimorphic

species . Basal skul l lengths of

these two

morphs

also broadly overlap, showing no

bimodality

that may indicate a size dif ference between sexes T prorsus

specimens

range from 88 to 116

cm;

T

horridus

specimens


10/12

268 JOURNAL OF VERTEBRATE PALEONTOLOGY VOL

16,

NO

2, 1996

from

102

to

127 cm . Conversely,

the presence of two contem

poraneous

morphs

does

not

negate the

posibility of two separate

species; species part it ioning may have occurred ecologically

rather than temporally or

geographically.

The

preceeding analysis is summarized in

the

following re

vision

of the

systematic hierarchy:

CER TOPSI

Marsh,

1888

NEOCER TOPSI Sereno, 1986

CER TOPSID E

Marsh,

1888

CH SMOS URIN E Lambe, 1915

Genus TRICER TOPS Marsh,

1889

TRICER TOPS HORRIDUS Marsh,

1889

Holotype-YPM

1820

Revised

Species Diagnosis-same as

for genus

see above;

Fig.9A).

Synonyms-T. flabellatus

T

serratus T elatus

T

calicor-

nis

and

T

obtusus

TRICER TOPS

PRORSUS

Marsh, 1890

Holotype-

YPM 1822

Revised

Species Diagnosis-frontal

fontanelle absent, su

praorbital horns relatively short with horn length/basal skull

length

of 0.61

or

less,

rostrum

relatively

deep and short wit h

convexly rounded rostral

margin

Fig. 9B .

Synonym-T. brevicornis

CONCLUSIONS

Triceratops is

here

divided into two species, T horridus and

T

prorsus

The other named species are revised as fol lows: T

flabellatus

T

serratus

T elatus T

calicornis

and T obtusus

are

junior

synonyms of

T

horridus This is due to the absence

of discrete

and consistent morphological

characters

or shape

differences

that

cou ld be

used to

differentiate these

species.

T brevicornis is considered a junior synonym

of

T

prorsus

based on

the

possession

of

the

same shared derived characters

that define

T

prorsus Finally,

T

eurycephalus and

T

alber-

tensis are

considered nomina

dubia.

ACKNOWLEDGMENTS

1 thank Peter Dodson and Paul Sereno for their help through

ou t

this study,

and

Barry Chernoff for assistance

with both

the

cladistic and morphometric analyses. The illustrations were

skillfully prepared by

Carol

Abraczinskas, who 1 also thank for

her assistance and advise.

Access

to

specimens

was

kindly

pro

v ided by J. Ostrom and M. Turner, E. Gaffney and C. Holton,

E Jenkins , Jr.

and

C. Schaff , N.

Hotton and

M.

Brett-Surman,

D. Berman, J. Bolt, B. Er ickson, Bjork, K.

Stadtman,

D.

Russell and

K. Shepherd, K.

Padian and

H. Hutchison,

and

S.

McLeod. Peter Dodson, Paul

Sereno,

and Ralph Chapman

pro

vided valuable

suggestions

on an earlier version of this manu

script

This work was

partially

supported through

grants from

Sigma Xi and the Geological Society

of

America to

the

au

thor ,

and trom t he Dav id and Luci le Packard Foundat ion

to

C. Sereno . This study

formed

part of my

Doctoral

Disser

tation in the Department

of

Geology, University of Pennsyl

vania ; 1

warmly thank

the faculty, staff,

and

students for

their

help and encouragement.

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Chapman, R. E. 1990.

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21-42

in K. Carpenter and J. Currie eds. , Dinosaur

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422.

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and

R. S. Lull. 1907.

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1 9 3 3

A revision of the Ceratopsia or horned dinosaurs. Yale

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1 8 8 8

A

new

family

of

horned Dinosauria

from the Cretaceous.

American Journal of Science, ser ies 3, 36:477-478.

1 8 8 9 a

Notice of new American dinosaurs. American Journal

of


1 8 8 9 b Notice of gigantic horned Dinosauria from the Creta

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1 8 9 0 a Description

of

new

dinosaurian

reptiles. American Jour

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39:81-86.

1 8 9 0 b Additional characters of the Ceratopsia, with not ice

of

new Cretaceous dinosaurs. American Journal of Science, series 3,

39:419-426.

1 8 9 1

Notice of new vertebrate fossils. American Journal of


1 8 9 2

The skull of Torosaurus American Journal

of

Science,

ser ies 3,

42:81-84.

1 8 9 8

New

species of Ceratopsia.

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ser ies 4 , 6:92.

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with a revision of the genus. Zitteliana 14:111-158.

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of the Lance Formation

and a s tudy

of

a new Triceratops Bulletin of the Museum

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Com

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Phylogeny

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11/12


12/12

270 JOURNAL OF VERTEBRATE PALEONTOLOGY,

VOL

16

NO

2 1996

APPENDIX

APPENDIX

Continued

Numerical

results of Analysis 1

Component

n

9

2

3

p

26

Component

Variable

v19 0.125

0.663

0.010

2

3

v20 0.377

0.196

0.190

Variable v35

0.419 0.268 0.502

v5

0.152

0.052

0.018

variance 79.7

10.6

4.5

v6 0.100 0.102

0.123

cumulative

variance

79.7 90.3 94.8

v9

0.185

0.059 0.054

Numerical

results of USNM 1201 Analysis

vIO 0.256

0.086

0.006

n 6

vIl 0.247

0.568

0.078

P

2

v12

0.293

0 .452 0.164

Component

v13

0.251

0.001

0.050

v14

0.188

0 .184 0.106

2

3

v15

0.172

0.088

0.095

vI

0.457 0.492

0.182

v17 0.126 0.060

0.162

v2 0.076 0.187

0.047

v18 0.242 0.125 0.340

v4

0.257

0.059 0.116

v19 0.136

0.217

0.103

v5

0.194

0.070

0.190

v2 0

0.284 0.057 0.009

v6 0.088

0.104 0.090

v23 0.078

0.209

0.086

v7 0.301

0.009 0.057

v24 0.036

0.114

0.093

v14 0.200

0.335 0.203

v25

0.070

0.002

0.236

v17 0.158 0.065

0.065

v26 0.076

0.033 0.398

v18 0.188

0.089 0.416

v27

0.170

0.009 0.107

v19 0.172

0.359

0.020

v28 0.235 0.306

0.384

v20

0.282

0.168 0.178

v29 0.238

0.283

0.201

v23

0.020

0.361

0.148

v30

0.251

0.102

0.234

v24

0.046

0.173

0.070

v31

0.146

0.141

0.099

v25

0.071

0.166

0.261

v32

0.004

0.026

0.465

v26 0.129 0.133

0.408

v33

0.201

0.154 0.124

v27

0.134

0.025

0.095

v34 0.216

0.174

0.208

v28 0.211 0.205 0.485

v35

0.297 0.146

0.031

v29

0.322

0.263

0.200

variance

53.7

23.1 6.7

v30 0.333

0.116

0.234

cumulative variance 53.7 76.8 83.5

v31

0.166 0.180

0.107

Numerical

results

of LACM 7207

Analysis

v32

0.112

0.222 0.131

n 8

variance 59.4

17.9

11.5

P

16

cumulative variance

59.4

77.3

88.8

Component

Numerical results of USNM 4720 Analysis

2 3

n 7

P 7

Variable

Component

v3 0.043

0.408

0.451

2 3

v4

0.156 0.342 0.103

v5

0.082 0.410

0.015

Variable

v6

0.126

0.080 0.115

vI

0.910 0.290

0.176

v23 0.307

0.407

0.048

v2 0.115

0.319

0.422

v24 0.186 0.086 0.071

v3 0.303 0.071

0.560

v25

0.113

0.416 0.156

v5 0.133

0.484

0.304

v26 0.257 0.025

0.536

v6

0.003

0.362

0.031

v27

0.172

0.122

0.170

v7

0.213

0.559

0.179

v28 0.342

0.273

0.554

v19

0.065

0.362

0.593

v29 0.399 0.140 0.106

variance 67.7 15.8 12.9

v30 0.307 0.049

0.233

cumulative

variance 67.7 83.5 96.4

v31

0.247

0.103

0.117

v32

0.328

0.251

0.005

Numerical results of AMNH 970 Analysis

v33

0.327

0.021

0.178

n 8

v34 0.262

0.098

0.061

p

10

variance

48.3

24.8

15.4

Component

cumulative

variance

48.3

73.1 88.5

2

3

Numerical

results of YPM 1820 Analysis

Variable

n 7

v5 0.198

0.149

0.329

p 9

v6 0.131

0.241

0.185

Component

v7 0.334

0.226

0.223

2 3

v8 0.237

0.080 0.376

v12

0.409

0.760

0.279

Variable

v13

0.324

0.147 0.072

vI 0.589 0.297

0.158

v14

0.287

0.266

0.525

v2

0.040

0.445

0.517

v18 0.368

0.032

0.303

v6

0.079 0.242

0.157

v20

0.384

0.096

0.273

v7

0.326 0.246

0.056

v35 0.373

0.427 0.377

v13 0.345

0.004 0.120

variance

72.6 16.0

5.5

v18

0.293

0.214

0.615

cumulative

variance 72.6

88.6 94.1

Documents

Species Resolution in Triceratops