GEOL 240 The Dinosaurs: Marginocephalia

Summary zMarginocephalia represent an important group of dinosaurs that became dominant in U. Cretaceous zCharacterized by development of a marginal ridge of bone along the back of the skull zOrnithischian and herbivorous zIncludes two taxa on the Ranch: Triceratops and Pachycephalosauria

Marginocephalia

z Name means "ridge head"  Characterized by a frill: a shelf of bone extending back over occipital region of skull  Plant eating  Absent from fossil record until Middle Jurassic, and all known fossils except one are Cretaceous  “Primitive” forms are bipedal; in one branch “derived” forms become very large and obligate quadrupeds  Marginocephalian skulls capture imagination with lots of suggestions as to functions, mainly display and/or combat

Marginocephalia

z Except for a few fragmentary specimens, all marginocephalians known fall into one or the other of two clades: the thick-skulled Pachycephalosauria or the deep-beaked [and often frilled (and sometimes horned)] Ceratopsia. Marginocephalia

zOldest known ceratopsian fossil from the Middle Jurassic of China (Chaoyangosaurus) zNext oldest, and much better known, is Lower Cretaceous Psittacosaurus, the "parrot dinosaur" zLowest fossil marginocephalians were bipeds (possibly facultative bipeds), with bodies like heavily build hypsilophodonts Marginocephalia

zSynapomorphies of Marginocephalia: oRostral bone, a bone unique to Ceratopsia oA single, non-paired bone attached in front of the premaxillae (and so it is a pre-premaxilla!) oForms the upper part of the beak, mirror image of the predentary oJugals form pointed "cheek bones" oCeratopsians have very deep and powerful jaws, with strong shearing bite Marginocephalia

Rostral bone of Triceratops

Ceratopsia zone of the most diverse groups of the Late Cretaceous zTwo clades yPsittacosaurus yNeoceratopsia

Psittacosaurus

z“parrot dinosaur” zprimitive Ceratopsia zrudimentary frill zshort snout zhigh nostrils ztall rostrum zsuperficially resembles parrot beak

Psittacosaurus

Neoceratopsia z In Upper Cretaceous, the first of the more “advanced” Neoceratopsia appears: zNeoceratopsians characterized by a prominent frill oFrill formed by extensions of the parietal and squamosal oServed in part as increased surface attachment for jaw muscles oCould also have been used as a visual display or for cooling or for defense.

Neoceratopsia zNeoceratopsians also have enlarged heads (that is, their heads are 20% or more the length of the postcranium) zFirst fossil neoceratopsians, such as upper Lower Cretaceous Archaeoceratops may have been bipedal zSubsequent neoceratopsians had larger frills and heads, such that they were doubtless obligate quadrupeds oProtoceratops of the Upper Cretaceous of Mongolia and Leptoceratops of the Upper Cretaceous of western North America represent this class of neoceratopsians.

Neoceratopsia  extremely large head  broad and prominent frill  pointed and sharply keeled rostrum  obligate quadrupeds

zTwo clades yProtoceratopsids and Ceratopsids Neoceratopsia

Protoceratopsids zsmall (1.2 meters) z“primitive” group - intermediate between Pittscosaurus and ceratopsids zrelatively larger skull but still small zmore pronounced frill zfore and hind limbs equal length zmassive limbs and big feet zno horns z small nostrils

Protoceratopsids

Ceratopsidae zmoderately large meters zhabitual quadrupeds zvery large skull ( meters (!) long) zlarge nostrils zprominent frills zvariety of horns

Ceratopsidae ztwo types yPachyrhinosaurines (more “primitive”) yCeratopsines

Ceratopsidae

Pachyrhinosaurines zrelatively short high face zshorter frill zlarge nasal horns zsmaller postorbital horns

Pachyrhinosaurines

Ceratopsines zlong, low faces zlong frills zlarge postorbital horns zsmall nasal horns zTriceratops the most common Upper Cretaceous form; known only from North America zFound on the Hanson Ranch, second in abundance to Edmontosaurus

Ceratopsines Triceratops horridus

Ceratopsines

Pachycephalosauridae

zSometimes called boneheads or domeheads or buttheads zExcept for a possible Lower Cretaceous form from Europe, are known only from the Late Cretaceous of Asia and western North America zApparently were obligate bipeds (why?) zPostcranially resemble heavy hypsilophodonts

Pachycephalosauridae zMost obvious “derived” feature: thickened bone on skull roof oIn “primitive” forms, the skull is still relatively flat oIn “advanced” forms, this thickened roof forms a dome oSuggested behavior uses for these domes: oUsed to butt theropod legs and bellies as defense oUsed like the horns of bighorn sheep in dominance fights for females or territory, etc.

Pachycephalosauridae oSome indication of sexual dimorphism, with bigger domed males and smaller domed females oPachycephalosaurs also have rows of small dermal ossifications around back of skull oSome indication of strengthened cervical and dorsal vertebrae (for absorbing impacts?) oMost were smaller than humans; only last and largest (Pachycephalosaurus) was bigger than a human oRelatively unsophisticated jaws and teeth oRelatively rare as fossils; several have been found on the Ranch

Pachycephalosauridae