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
- 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