HORN-HEADS AND DOME-HEADS
World of Dinosaurs Lecture 12
24 February 2009
MARGINOCEPHALIA: PACHYCEPHALOSAURS AND CERATOPSIANS
-The marginocephalians are a group of ornithischian dinosaurs that includes two well-known clades: the Pachycephalosauria, or bone-headed dinosaurs, and the Ceratopsia, or horned dinosaurs. The Ceratopsia includes small-bodied psittacosaurs, mid-sized “protoceratopsians", and large-bodied ceratopsids.
-Like all other dinosaur groups, the ancestral marginocephalian was small-bodied and bipedal. Marginocephalian means “margin-headed”, in reference to the rear extension of the skull roof behind the rest of the skull.
-Pachycephalosaurs are known from the Cretaceous of Europe, Asia and N. America, although they are quite rare wherever they are found.
- Ceratopsians, in contrast, were a very successful group, known from abundant remains in the Late Cretaceous of N. America and Asia.
MARGINOCEPHALIAN ANATOMY
Pachycephalosaurs
-The name means “thick-headed lizards”, in obvious reference to the greatly thickened skull domes. This thickening is composed of two bones present in almost all vertebrates: the frontals and parietals.
-The most specialized forms had the highest domes. Many had additional ornamentations such as bony spikes or knobs arranged around the base of the dome.
-Although varying greatly in size, all pachycephalosaurs were bipedal.
-Their teeth are typically quite primitive, being relatively weak with leaf-shaped crowns bearing coarsely serrated margins for slicing up plants.
Ceratopsians
-The name means “horned heads”, although only the ceratopsids tend to have true horns, typically occurring over the nose opening and eye sockets (orbits).
-More characteristic of the group is the expanded neck frill, which is an extension of the bones at the back of the skull. In this case, the bones involved are the parietals and squamosals.
-Psittacosaurs are the most primitive, all being small-bodied and bipedal, with only a modest expansion of the back of the skull roof. The name means “parrot-lizard” because of the parrot-like beak. The teeth are relatively simple.
- “Protoceratopsians” are a group of mid-sized ceratopsians.
-Some are thought to have been bipedal whereas others were quadrupedal at least some of the time.
-They have longer frills than psittacosaurs, but most were relatively short compared to ceratopsids. Some had a nasal bump on the nose, but not a true horn
-The teeth are more advanced, being packed closer together.
-Ceratopsids are the largest and most derived ceratopsians, including such well-known forms as Triceratops. All were quadrupedal.
-Ceratopsids had elongate frills. The group includes at least two genera (Torosaurus, Pentaceratops) with the largest skulls of any terrestrial vertebrates (up to 10 feet long).
-Members of the group also have horns over the nose and orbits, some reaching lengths of several feet. The outer nose opening is also greatly expanded.
-Ceratopsids have complex dental batteries composed of closely packed masses of teeth that formed a continuous cutting surface. The teeth occur in columns as well as rows, so that new teeth were constantly present to replace those that wore out.
MARGINOCEPHALIAN PHYLOGENY
- Psittacosauridae and Ceratopsidae are regarded as natural groups or clades. In contrast, protoceratopsians are a group of mid-sized ceratopsids previously thought to be a monophyletic clade, but now recognized as a series of separate branches.
Ornithischia
Marginocephalia
Pachycephalosauria
Homalocephale
Pachycephalosauridae
Stegoceras
Pachycephalosaurus
Ceratopsia
Psittacosauridae (e.g., Psittacosaurus)
Neoceratopsia
Leptoceratops
Protoceratops
Montanoceratops
Ceratopsidae
Centrosaurinae (e.g., Centrosaurus, Styracosaurus)
Chasmosaurinae (e.g., Chasmosaurus, Triceratops)
MARGINOCEPHALIAN PALEOBIOLOGY AND PALEOECOLOGY
Pachycephalosaurs
-The thickened dome of pachycephalosaurs has most often been regarded as an adaptation for head-butting. According to this view, the head is employed as a battering ram to take on members of the same species in contests for dominance.
-In recent years, this view has been questioned. Among other things, head butting is an extremely specialized behavior exhibited by only a few derived groups of mammals.
-The main alternative is flank butting. Evidence in support of this view includes the shape of the domed head and the presence of broken/rehealed ribs in some specimens.
Ceratopsians
-With regard to diet, some psittacosaurs have been found with gastroliths, indicating that ceratopsians used gizzards to help process food.
-The somewhat more advanced teeth of "protoceratopsians" (e.g., Protoceratops) indicate a new means of processing food. These teeth may have allowed for the consumption of a tougher, more fibrous diet.
-Ceratopsids have by far the most advanced teeth and jaws among ceratopsians, consisting of a dental battery of hundreds of teeth adapted for slicing poor quality fodder. It is likely that the job of the beak was to break off plant matter. Then the teeth would slice it up into bite sized chunks before swallowing. There may or may not have been a gizzard with gastroliths in the ceratopsids. Finally, the gut, aided by bacteria, would have completed the digestion process, removing the nutrients and passing along the waste.
- There has been considerable controversy over the posture and gait of ceratopsids. Depending on whether they are reconstructed with the forelimbs sprawled or tucked in directly beneath the body, ceratopsids are either pitifully slow and awkward, or fleet-footed and rhinoceros-like. The answer is almost certainly a compromise of these two extremes. There has also been much question as to the function ofhorns and frills, and several hypotheses have been put forward. For the frill alone, these include: 1) an expanded surface for attachment of large jaw muscles; 2) a structure for protection against predators; 3) a means of controlling body temperature (thermoregulation); and 4) a display device for attracting mates and intimidating rivals.
-Evidence is strong that the frill did not serve as an attachment site for muscles, nor was used primarily in predator defense. However, it likely served as a display structure, and perhaps in thermoregulation as well. Evidence for the display hypothesis includes the delayed appearance of adult horn and frill shapes; that is, the animals were virtually adult size before they sprouted the horn and frill characters.
- Large bonebeds of ceratopsids provide circumstantial evidence of herding. Elaborate horns and frills provide additional evidence. Specifically, it is possible that the horns and frills of ceratopsids were associated with dominance hierarchies, which allow animals to live in large, multi-male groups without need for continuous contests of strength. It is likely that herding originally evolved as means of predator defense.
Lecture 12—Horn-Heads and Dome-Heads—1