Chapter 15 – Overview of the Clade Marginocephalia

Why Study Marginocephalia (margin head)

Diversity rivals Theropoda and Ornithopoda in the Late Cretaceous, because of the Ceratopsians

Lots of Ceratopsian skeletal fossils

More than 50% of species are named from complete or nearly complete skeletons

Ceratopsian tracks only recently recognized

Among the most endemic dinosaur clade

Found only in Western North America, Asia and Europe

Found only in Cretaceous strata

Known for their HEADS

Thickness or size

Ceratopsians had among the largest heads of any known land-dwelling animal

Elaborate and gaudy accessories, such as horns, spikes, bosses and frills

Many ceratopsians travelled in herds

Use of heads in intraspecific competitions

Definition and Unique Characteristics of Marginocephalia

Sister clade to Ornithopoda, comprising Cerapoda

Characteristics of Marginocephalians - SEE Figure 15.2

A narrow shelf of bone on the parietal and posterior part of the squarmosal

Abbreviated premaxillary

Shortened pubis and wide hips

Characteristics of Ceratopsia (horn face)

Impressive dental battery for slicing plant material

Sharp, parrotlike beak (formed by Rostral Bone at the tip of the snout)

Quadrupedal limb posture among later and larger ceratopsians

Bipedal stance for earlier and smaller ceratopsians

Forelimbs shorter than hindlimbs; both pairs massive (among quadrupedal forms)

Toes ended in broad hooves (5 on manus, 4 on pes)

Large sheet of bone called the frill extending from the back of the skull

Epoccipitals, long spikes or lump of bones, ornament the margin of the frill in some genera

Cheeks extending laterally and posteriorally

Large, aggressive-looking horn cores

Some ceratopsians had 2, one over each eye, & a third over the nose; some had a nose horn and no eye horns; several had roughened bone over the nose and eyes; some had no horns at all

Trace Fossils

Ceratopsian trackways in Late Cretaceous strata of the western U. S.

Psittacosaurus gastroliths from several specimens

No nests or eggs

Five to eight meters in length; up to 8000 kilograms (larger Ceratopsians)

Smaller Ceratopsians were about 2 meters long or less

Ceratopsian Geological range & diversity

Early to Late Cretaceous (~130 - 65 million years)

One genera reported from the Middle-Late Jurassic of China

Maximum diversity - 23 genera during the Late Cretaceous

At least 26 genera (and many more species) throughout their time on earth

Three genera, including Psittacosaurus from Early Cretaceous of eastern Asia (China, Mongolia [also Thailand])

Found in North America, Mongolia, China and Kazakhstan

Characteristics of Pachycephalosauria (thick head)

Small, triangular, cheek teeth with denticles and canines in front (that is, Pachycephalosaurs were heterodont)

Bipedal limb posture

No pes recovered for any species

Thickened skull roof with prominent domes in many species

Back of skull rotated forward beneath skull roof

Specialized articulations in back & tail vertebrae and strengthening of pelvis

Ossified tendons stiffened back half of tail

Rib cage is broad and abdomen probably housed a large gut

Less than one to three meters long, although Pachycephalosaurus reached eight meters in length

Pachycephalosaur Geological range & diversity

Early to Late Cretaceous (~115 - 65 million years)

Maximum diversity - all but 1 genera during the Late Cretaceous

At least 11 genera throughout their time on earth

Found only in Asia (Mongolia, China) and western North America, except for one species from Europe (England)

Clades and Species of Marginocephalia

SEE Figure 15.1 and Table 15.1

Two Clades of Marginochephalians - Pachycephalosauria and Ceratopsia

Ceratopsia

Introduction

Marsh named Triceratops horridus in 1889 and also the Ceratopsidae in 1890

First named ceratopsian – Monoclonius – by Cope in 1876

John Bell Hatcher the most important single contributor to ceratopsian studies

Two Clades of Ceratopsians - Psittacosauridae and Neoceratopsia

Psittacosauridae

Bipedal dinosaurs, less than 2 m long

One genus, at least 7 species

Classified as ceratopsian based on rostral bone, toothless beak and triangular skull shape

No frill and no horns

Only ceratopsian with gastroliths

Have close affinities with bipedal pachycephalosaurs and ornithopods

Implies that quadrupedal stance in most Ceratopsians is a return to the ancestral condition for Tetrapoda as a whole

Neoceratopsia

Small, hornless (or small, blunt nasal horn) “primitive” genera (including Protoceratops & Bagaceratops among others, but excluding Leptoceratops) form a lineage, not a clade called “Protoceratopsidae”

Protoceratops andrewsi – one of the most abundant dinosaurs, discovered during Central Asiatic Expeditions of the 1920’s to the Gobi, credited as egg layer of Oviraptor eggs, one of the dinosaurs in the “Fighting Dinosaurs” specimen and exhibits sexual dimorphism

Ceratopsidae

Large (up to 8 meters and 7 to 8 tons), horned ceratopsians found only in Late Cretaceous strata of western North America

Two clades - Chasmosaurinae & Centrosaurinae

Chasmosaurinae

Long frills with horns over the eyes & nose

Includes Chasmosaurus, Pentaceratops, Arrhinoceratops, Triceratops & Torosaurus

Centrosaurinae

Short frills without horns over the eyes, but with nose horns or bosses

Includes Centrosaurus, Styracosaurus, & Pachyrhinosaurus

Pachycephalosaur Diversity

Flat-headed Homalocephalidae and dome-headed Pachycephalosauridae

Pachycephalosaurs had average-sized dinosaur brains that were modified to accomodate the doming of the skull

Enlarged olfactory bulbs suggest Pachycephalosaurs had a good sense of smell

Paleobiogeography and Evolutionary History of Marginocephalia

Main Points

Limited distribution in time and space

Ceratopsians and Pachycephalosaurs probably originated in Asia and dispersed to North America

Early, smaller Asian “protoceratopsids” lived in arid to semi-arid environments, while later, larger North American ceratopsids lived in forested lowland areas with fluvial and deltaic environments

North America Pachycephalosaurs are mostly known from isolated waterworn skull caps (indicating long transport) and probably live in upland areas

Only single specimens of Stegoceras and Pachycephalosaurus are represented by more than skull caps (skull and partial skeleton for Stegoceras and nearly complete skull for Pachycephalosaurus)

Asian Pachycephalosaurs are better preserved with nearly complete skulls and associated skeletons and lived in the same areas as Asian “protoceratopsids”

Marginocephalian Evolution

Divergence into Ceratopsia and Pachycephalosauria is problematic

Best candidate for common ancestor, Stenoplexis, is from the Lower Cretaceous of England

Excellent example of evolutionary change in Ceratopsia (and other dinosaurs clades), driven by changing sea level that resulted in broad-scale environmental changes, from the Upper Cretaceous Judith River and Two Medicine Formations of Montana

Sea level rise constricted lowland habitats, putting environmental pressures on dinosaur faunas

Evolutionary sequence: Styracosaurus albertensis to Styracosaurus sp. to Einosaurus procurvicornis to Achelousaurus horneri to Pachyrhinosaurus canadensis

Marginocephalians as Living Animals

Ceratopsians

Reproduction and Growth

Horns and frills probably used for sexual display, to establish territory, or to assert dominance

Horns occasionally used for defense against predators

Puncture wounds on faces, frills and bodies of ceratopsians (Triceratops, Diceratops, Pentaceratops, and Torosaurus) from intraspecific combat

Frills especially useful for display with inclination of the head and nodding or shaking the head side-to-side in chasmosaurines

short-frilled centrosaurines may have used their nasal horns more readily

exceptions: Styracosaurus is a centrosaurine but has long frill margin spikes & may have depended more on frill display; Triceratops is a chasmosaurine with a secondarily shortened frill & combat may have taken place without extensive frill display

Growth series available for Psittacosaurus, Protoceratops, Centrosaurus, Chasmosaurus & Triceratops

Sexual dimorphism in size and shape of adult frills – larger frills are male

Frill growth occurs at sexual maturity for Protoceratops

Locomotion

Orientation of the forelimbs is problematic

Modern reconstructions pose ceratopsian forelimbs as fully erect

Other reconstruction pose ceratopsian forelimbs with a more sprawling posture

Trackways suggest a slightly sprawling forelimb posture

Walking speed was about 4 kph (2.5 mph)

Feeding

Fleshy cheeks

Dental battery and beak (rostral and predentary bones)

vertical occlusion for slicing and dicing of food

A sturdy coronoid process

Jaw muscles may also have been attached to the large frills of ceratopsians

Psittacosaurus is the only Ceratopsian (or Ornithischian) with gastroliths

Note: All of these attributes are similar to those of hadrosaurids and of modern herbivorous mammals (like sheep, cows & horses)

Ceratopsians probably did not feed on cycads and palms, but instead on a variety of shrubby angiosperms (flowering plants), ferns, and perhaps small conifers

The browsing of Ceratopsians and other large, low-browsing herbivorous dinosaurs may have contributed to the extraordinary rise of flowering plants during the Late Cretaceous

Health

Active animals with some injuries

Puncture wounds and healed ribs (Pachyrhinosaurus & Centrosaurus) from intraspecific conflict

Stress fractures in toes of Centrosaurus, Pachyrhinosaurus, Styracosaurus & Triceratops

Prey animals

Protoceratops & Velociraptor

Triceratops & Tyrannosaurus

Social Life

Many, if not all, Ceratopsians lived in herds at least part, if not all, of the year

Bonebeds for Achelousaurus, Centrosaurus, Einosaurus, Pachyrhinosaurus & Styracosaurus and for Anchiceratops & Chasmosaurus

At least 300 individuals in the Centrosaurus bone bed in Alberta

Lots of specimens for Protoceratops & Psittacosaurus

Trackways show many individuals walking through the same areas

Brains less than size expected of a similarly sized crocodilian or lizard

larger than sauropods, ankylosaurs and stegosarus

Pachycephalosaurs

Reproduction and Growth

Pachycephalosaurs butted each other with their thickened skulls

Pachycephalosaurid domes are dense with fine bony columns radiating so as to be approximately perpendicular to the external surface of the dome

This columnar bone is oriented in the same direction as stresses induced by butting

Pachycephalosaur occipital condyles are robust

Cushions skull at its articulation with the neck during butting

Pachycephalosaur vertebrae have tongue-and-groove articulations

Prevents rotation of horizontally-held vertebrae and damage to spinal cord during butting

Pachycephalosaurs butted each other’s flanks, rather than head-to-head

Without self-correcting mechanisms of the kind seen in modern head-to-head butters like goats and bighorn sheep, precision head-butting would have been difficult and serious injury to the brain or spinal cord could have resulted

Impact into flesh would have generated 2 orders of magnitude less force

Pachycephalosaur butting probably began with pushing encounters among the earlier flat-headed pachycephalosaurs to establish social heirarchy

Pachycephalosaurs had other features related to display

Canine-like teeth

Knobby and spinous osteoderms that cover the snout and the side of the face

Sexual Dimorphism

Pachycephalosaur domes come in 2 forms - larger & thicker and flatter & thinner, interpreted as male and female respectively

“Female” domes looked very similar to those of juveniles or young adult “males”

“Males” butted; “females” didn’t

Locomotion

Problematic because no pes recovered for any pachycephalosaur

Feeding

Fleshy cheeks

Beaks

Teeth are relatively small and triangular

Pachycephalosaurs had a large abdominal cavity

May have housed a fermentation vat

Social Life

Virtually nothing known

Ceratopsians and Pachycephalosaurs

Extinction

Marginocephalians persisted until the end of the Cretaceous