Chapter 8 Outline

INTRODUCTION TO PACHYCEPHALOSAURIA (THICK HEADED LIZARD)

• CHARACTERISTICS
  • 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
  • BIPEDAL LIMB POSTURE
  • SMALL, TRIANGULAR, SERRATED TEETH SUGGEST PACHYCEPHALOSAURS WERE HERBIVORES
  • RIB CAGE IS BROAD AND ABDOMEN PROBABLY HOUSED A LARGE GUT
  • ONE TO THREE METERS LONG, ALTHOUGH PACHYCEPHALOSAURUS REACHED EIGHT METERS IN LENGTH
• GEOLOGICAL RANGE & DIVERSITY
  • EARLY TO LATE CRETACEOUS (~115 - 65 MILLION YEARS)
  • MAXIMUM DIVERSITY - ALL BUT 1 GENERA DURING THE LATE CRETACEOUS
    • At least 13 genera throughout their time on earth
  • FOUND IN NORTH AMERICA, EUROPE, MONGOLIA, CHINA AND MADAGASCAR!

HISTORY OF PACHYCEPHALOSAUR DISCOVERIES

• 19TH CENTURY
  • NORTH AMERICA (1856)
    • Stegoceras from late Cretaceous beds (teeth only; initially misidentified)
• 20TH CENTURY
  • NORTH AMERICA
    • More Stegoceras plus Pachycephalosauru s plus 3 additional genera from late Cretaceous beds (1902, 1920's, 1940's, 1970's, 1980's)
  • MONGOLIA & CHINA
    • Prenocephale and Homalocephale plus 4 additional genera from late Cretaceous beds (1970's, 1980's)
  • EUROPE
    • 1 genus from early Cretaceous beds (1971)
  • MADAGASCAR
    • 1 genus from late Cretaceous beds (1970's)

PACHYCEPHALOSAUR DIVERSITY

• BASAL FLAT-HEADED FORMS, INCLUDING HOMALOCEPHALE AND DOME-HEADED PACHYCEPHALOSAURIDAE

PACHYCEPHALOSAUR PALEOBIOLOGY AND PALEOECOLOGY

• DISTRIBUTION
  • PACHYCEPHALOSAURS PROBABLY ORIGINATED IN ASIA AND DISPERSED TO OTHER CONTINENTS
  • NORTH AMERICA PACHYCEPHALOSAURS ARE MOSTLY KNOWN FROM ISOLATED WATERWORN SKULL CAPS (indicating long transport)
    • 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
    • Asian Pachycephalosaurs may have lived closer to rivers and lakes
• EATING
  • PACHYCEPHALOSAURS WERE LOW-BROWSERS (less than 1 meter)
  • PACHYCEPHALOSAURS, LIKE THYREOPHORANS, HAVE A MIX OF CHARACTERISTICS, SOME OF WHICH SUGGEST SOPHISTICATED ORAL FOOD PROCESSING AND OTHERS OF WHICH SUGGEST SIMPLE ORAL FOOD PROCESSING COMBINED WITH INTERNAL FOOD BREAKUP
    • Sophisticated oral food processing
      • HORNY BEAKS
      • CHEEKS
    • Simple oral food processing combined with internal food breakup
      • TEETH ARE RELATIVELY SMALL AND TRIANGULAR
      • PACHYCEPHALOSAURS HAD A LARGE ABDOMINAL CAVITY
        • May have housed a fermentation vat
• BRAINS
  • PACHYCEPHALOSAURS HAD AVERAGE-SIZED BRAINS THAT WERE MODIFIED TO ACCOMODATE THE DOMING OF THE SKULL
    • Enlarged olfactory bulbs suggest Pachycephalosaurs had a good sense of smell
• BUTTING
  • PACHYCEPHALOSAUR 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 head-butting
  • 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" head-butted; "females" didn't
  • PACHYCEPHALOSAUR VERTEBRAE HAVE TONGUE-AND-GROOVE ARTICULATIONS
    • Prevents rotation of vertebrae and damage to spinal cord during head-butting
  • PACHYCEPHALOSAUR HEAD-BUTTING PROBABLY BEGAN WITH PUSHING ENCOUNTERS AMONG THE EARLIER FLAT-HEADED PACHYCEPHALOSAURS TO ESTABLISH SOCIAL HEIRARCHY
    • Vertebral column would have been held horizontally
  • EVEN DOME-HEADED PACHYCEPHALOSAURS MAY HAVE ONLY HAD PUSHING CONTESTS OR ELSE BUTTED EACH OTHER ALONG THEIR FLANKS
    • 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
  • 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 SELECTION
  • SELECTION AMONG THE INDIVIDUALS OF ONE GENDER, OFTEN MALE, NOT ALL INDIVIDUALS OF A SPECIES
  • PACHYCEPHALOSAUR FEMALES APPARENTLY SELECTED MALES WITH PROGRESSIVELY HIGHER DOMES, AS WELL AS KNOBS AND SPIKES
    

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