Marginocephalia (/mär′jə-nō-sə-făl′ē-ən/ Latin: margin-head) is a clade of
ornithischia
Ornithischia () is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek s ...
n dinosaurs that is characterized by a bony shelf or margin at the back of the skull. These fringes were likely used for display. There are two clades included in Marginocephalia: the thick-skulled
Pachycephalosauria
Pachycephalosauria (; from Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Along with Ceratopsia, it makes up the clade Marginocephalia. With the exception of two species, most pachycephal ...
and the horned
Ceratopsia
Ceratopsia or Ceratopia ( or ; Greek: "horned faces") is a group of herbivorous, beaked dinosaurs that thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. ...
. All members of Marginocephalia were primarily
herbivore
A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...
s. They basally used
gastrolith
A gastrolith, also called a stomach stone or gizzard stone, is a rock held inside a gastrointestinal tract. Gastroliths in some species are retained in the muscular gizzard and used to grind food in animals lacking suitable grinding teeth. In othe ...
s to aid in digestion of tough plant matter until they convergently evolved tooth batteries in Neoceratopsia (or "new Ceratopsia") and
Pachycephalosauria
Pachycephalosauria (; from Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Along with Ceratopsia, it makes up the clade Marginocephalia. With the exception of two species, most pachycephal ...
. Marginocephalia first evolved in the
Jurassic Period
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
and became more common in the Cretaceous.
They are basally small facultative
quadrupeds
Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor' ...
while derived members of the group are large obligate quadrupeds.
Primitive marginocephalians are found in Asia, but the group migrated upwards into North America.
Pachycephalosaurs
Pachycephalosauria (; from Greek παχυκεφαλόσαυρος for 'thick headed lizards') is a clade of ornithischian dinosaurs. Along with Ceratopsia, it makes up the clade Marginocephalia. With the exception of two species, most pachycephal ...
, or "thick-headed reptiles", have primitive features that include basally small sized bodies, obligate bipedalism, and simple teeth with one row in operation at a time that are replaced as they are worn down. As they evolved, pachycephalosaurs evolved much thicker and advanced skull roofs including dome forms with horn-like ornamentation. Some research suggests these domes were used like helmets for protection while head-butting members in intraspecific combat. Some research suggests their necks were not strong enough to support such an impact.
Flat-headed pachycephalosaur speciments have been found in Asia, and there is great controversy on the meaning of these flat heads. Recent research suggests the flat heads could be a juvenile state before developing the dome shape in the adult stage. It could also be evidence of sexual dimorphism with the female being more flat-headed.
Ceratopsia
Ceratopsia or Ceratopia ( or ; Greek: "horned faces") is a group of herbivorous, beaked dinosaurs that thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. ...
ns, or "horned-faces", differ from pachycephalosaurs in the presence of a rostral bone, or beak. They are also known for having a jugal horn and a thin parietal-squamosal shelf that extends back and up into a frill. This frill could have been used for anchoring jaw muscles, as well as for display.
The horns were likely used for establishing dominance, or defending territories. It is also possible they were a factor in sexual display and species recognition. One of the basalmost members of this group is ''
Psittacosaurus
''Psittacosaurus'' ( ; "parrot lizard") is a genus of extinct ceratopsian dinosaur from the Early Cretaceous of what is now Asia, existing between 126 and 101 million years ago. It is notable for being the most species-rich non-avian dinosaur gen ...
'', which is one of the most species-rich dinosaur genera from Asia. Ceratopsians later evolved into very large quadrupeds with elaborate facial horns such as ''
Triceratops
''Triceratops'' ( ; ) is a genus of herbivore, herbivorous Chasmosaurinae, chasmosaurine Ceratopsidae, ceratopsid dinosaur that first appeared during the late Maastrichtian stage of the Late Cretaceous Period (geology), period, about 68 m ...
'', ''
Styracosaurus
''Styracosaurus'' ( ; meaning "spiked lizard" from the Ancient Greek / "spike at the butt-end of a spear-shaft" and / "lizard") is a genus of herbivorous ceratopsian dinosaur from the Cretaceous Period (Campanian stage), about 75.5 to 74.5&nbs ...
'', and ''
Centrosaurus
''Centrosaurus'' ( ; ) is a genus of herbivorous ceratopsian dinosaur from the Late Cretaceous of Canada. Their remains have been found in the Dinosaur Park Formation, dating from 76.5 to 75.5 million years ago.
Discovery and naming
The firs ...
''. There was no change in richness of species throughout the
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...
before the
Cretaceous-Paleogene extinction.
Feeding
Marginocephalians have simple, peg-like teeth surrounded by rhamphotheca, a horny sheath of keratin. The teeth are arranged in batteries for easy replacement and have serrations which may have been useful for cutting up vegetation.
Marginocephalia evolved several methods for breaking down vegetation. Pachycephalosaurs had especially large abdomens with broad girths and elongate sacral ribs suggesting the presence of a large stomach. This is presumed to have been useful for breaking down tough vegetation through
bacterial fermentation
Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food p ...
. Another adaptation for advanced vegetation digestion is seen in Ceratopsians, which evolved features to improve their chewing apparatus. Derived ceratopsians have vertical grinding surfaces on their teeth to maximize break-down of tough vegetation. There is also evidence of advanced adductor musculature that extends from a large
coronoid process on the mandible up to the ceratopsian frill, which would increase chewing force. Pachycephalosaurs had
gastrolith
A gastrolith, also called a stomach stone or gizzard stone, is a rock held inside a gastrointestinal tract. Gastroliths in some species are retained in the muscular gizzard and used to grind food in animals lacking suitable grinding teeth. In othe ...
s to help in digestion of food, but only primitive ceratopsians, such as ''
Psittacosaurus
''Psittacosaurus'' ( ; "parrot lizard") is a genus of extinct ceratopsian dinosaur from the Early Cretaceous of what is now Asia, existing between 126 and 101 million years ago. It is notable for being the most species-rich non-avian dinosaur gen ...
'', have been found to have gastroliths.
Margins and social behavior
Marginocephalian remains reveal significant evidence of being social creatures, much of which is related to the many possible functions of the bony skull margins. Some possible functions of the variably shaped and sized margins are to ward off predators, display, ritualized combat, defense of territory, or establishing social ordering.
Both pachycephalosaurs and ceratopsians show evidence of
interspecific communication
Interspecies communication is communication between different species of animals, plants, or microorganisms.
Mutualism
Cooperative interspecies communication implies sharing and understanding information between two or more species that work to ...
, and there may be evidence of intraspecific communication.
Pachycephalosaurs, with their dome-shaped heads, are commonly thought to have used their thick skulls for butting into each other. Their brains and vertebrae are positioned in ways to protect from the impact of head-butting. Also, the columnar structure found in bone remains is consistent with models used to recreate the practice of head-butting. However, some pachycephalosaurs such as ''
Stygimoloch
''Pachycephalosaurus'' (; meaning "thick-headed lizard", from Greek ''pachys-/'' "thick", ''kephale/'' "head" and ''sauros/'' "lizard") is a genus of pachycephalosaurid dinosaurs. The type species, ''P. wyomingensis'', is the only known species, ...
'' have vascularization in the skull cap that would not have supported head-butting behavior. Thus, the heads could have been used as ornamentation or to butt into the softer flank of other pachycephalosaurs for intraspecific communication or as a form of
agonistic behavior
Agonistic behaviour is any social behaviour related to fighting. The term has broader meaning than aggressive behaviour because it includes threats, displays, retreats, placation, and conciliation. The term "agonistic behaviour" was first implemen ...
.
The frills of ceratopsians are incredibly diverse. They may have been used for protective purposes as the frill sometimes splays over the neck. However, some say that the frill would have provided little protection from other large dinosaurs such as ''
Tyrannosaurus
''Tyrannosaurus'' is a genus of large theropoda, theropod dinosaur. The species ''Tyrannosaurus rex'' (''rex'' meaning "king" in Latin), often called ''T. rex'' or colloquially ''T-Rex'', is one of the best represented theropods. ''Tyrannosa ...
''. Other possible functions include intraspecific communication for mating purposes or as a visual display of territorial protection, as seen in many common day organisms such as red-breasted robins. The frills also could have been used for species recognition purposes, as they seem to develop fairly early in life.
There has also been evidence that ceratopsians care for their young, as bone-beds have been found of adult individuals with a nest of juveniles, although some refute this as viable evidence of care for young. There have also been bone-beds found with hundreds of adult ceratopsians, indicating herd activity. A few specimens have been found with puncture wounds, supporting the use of horns as protective or combative weapons. Other research examining juvenile ceratopsians reveals a change in horn morphology over time, suggesting frills and horns could have been used for intraspecific communication of age. Horns also could have been used for thermoregulation as indicated by
isotope analysis
Isotope analysis is the identification of isotopic signature, abundance of certain stable isotopes of chemical elements within organic and inorganic compounds. Isotopic analysis can be used to understand the flow of energy through a food web ...
, as aid in knocking down vegetation, or for horn-locking agonistic behavior.
Sexual dimorphism
The study of
sexual dimorphism in dinosaurs
Sexual dimorphism in dinosaurs refers to the different physical characteristics of male and female dinosaurs of the same species. This means that the male and female dinosaurs of a species may differ in size, color, shape, or they may even look lik ...
is incredibly difficult. The varying size and intricacy of margins in Marginocephalia have shown many signs of sexual dimorphism.
Although the intricate frills of marginocephalians sometimes seem to present with dimorphic features, many doubt the validity of these claims.
''
Stegoceras validum
''Stegoceras'' is a genus of pachycephalosaurid (dome-headed) dinosaur that lived in what is now North America during the Late Cretaceous period, about 77.5 to 74 million years ago (mya). The first specimens from Alberta, Canada, were describe ...
'', a pachycephalosaur, can be segregated into two groups based on the size and shape of their skulls. These two group classifications separated the species population in half, which is highly indicative of sexual dimorphism. However, some report that the two groups may actually represent two separate species. ''
Protoceratops
''Protoceratops'' (; ) is a genus of small protoceratopsid dinosaurs that lived in Asia during the Late Cretaceous, around 75 to 71 million years ago. The genus ''Protoceratops'' includes two species: ''P. andrewsi'' and the larger ''P. hellenik ...
'', a type of ceratopsian, also shows signs of sexual dimorphism. However, their frills don't seem to develop until later in life, and may be coordinated with sexual maturity.
Locomotion
In general, primitive marginocephalians were
bipedal
Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' ...
or
facultative quadrupedal, and derived individuals were obligate quadrupedal.
This is especially prominent in Ceratopsia, where only the primitive ''
Psittacosaurus
''Psittacosaurus'' ( ; "parrot lizard") is a genus of extinct ceratopsian dinosaur from the Early Cretaceous of what is now Asia, existing between 126 and 101 million years ago. It is notable for being the most species-rich non-avian dinosaur gen ...
'' is bipedal. All the derived forms were strong quadrupeds, although their stance is controversial. Some think they were fairly columnar, with front limbs erect under the body, which would have been more efficient for speed. Others think the legs were more sprawling, as evidenced by the shape of the forelimb bones. Although not as fast, this posture would have been efficient for grazing vegetation on the ground.
Less is known about bipedal pachycephalosaur locomotion, although they must have had a fairly broad girth in order to make room for their enlarged gut.
Classification
The
cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...
below follows a 2009 analysis by Zheng and colleagues.
Cladogram after Butler ''et al.'', 2011.
References
{{Taxonbar, from=Q131385
Kimmeridgian first appearances
Maastrichtian extinctions
Taxa named by Paul Sereno