CERATOPSIA

A sketch of Triceratops made by E. L. Crisp, 1995 (reconstruction based on a 1:40 model from The Carnegie Museum).

INDRODUCTION

     We will now turn our attention to the other clade of the Marginocephalia, the Ceratopsia.

Cladogram of the Ornithischia.  The Ceratopsia are outlined in blue (Slightly modified from Dingus and Rowe, 1997).

     The horned dinosaurs are some of the ones that are most familiar to us.  For example, Triceratops in combat with Tyrannosaurus rex is a favorite scene in movies.

A Late Cretaceous scene showing Tyrannosaurus rex with his bounty, a Triceratops that he has "bagged". This picture was photographed by E. L. Crisp at The Carnegie Museum of Natural History.
 

CERATOPSIA

Lecture 18: Early Cretaceous II
From: http://rainbow.ldeo.columbia.edu/courses/v1001/liaoning16.html
     1. cheeks
     2. armor
     3. armor as plates covering back
     4. armor as spikes or plates
     5. uneven enamel
     6. back part of skull makes shelf
     7. rostral bone
     8. frill
     9. reduced hands
     10. domed head
     11. jaw joint below tooth row
     12. elongate hands
     13. prepubic process well developed
     14. ridge on teeth
     15. reduction in manus digits I and V (and large nares)
     16. manus digit 1 spike-like
 

     The Ceratopsia are exclusively Cretaceous in age and have a very large fossil record.  Ceratopsians are one of the most diverse groups of plant eating dinosaurs of the Cretaceous.  They were very abundant during the last 35 million years of the Cretaceous and only from Asia and North America.  The fossil evidence indicates that Ceratopsia arose in Asia during the early Cretaceous and later migrated to North America.

     The key derived characters of Ceratopsia include the following:

• A rostral bone (extra bone) at the tip of the snout (in front of the premaxilla)
• A narrow beak with flaring jugal (cheek) bones, which resulted in a triangular shape for the head.
• Development of a frill (at least an incipient one) (from the squamasal and parietal bones) at the back of the skull, that caused the posterior top edge of the skull to overhang the central back part of the skull.
• 
• Cladogram of the Ceratopsia (Modified from Dingus and Rowe, 1997)

GENUS Psittacosaurus

      The most primitive (least derived) member of the Ceratopsia is the genus Psittacosaurus of the Lower Cretaceous of eastern Asia.  Psittacosaurus is often referred to as the "Parrot dinosaur".  It is known from many complete skeletons from the Lower Cretaceous of eastern Asia.

Psittacosaurus skull and skeleton (From Lucas, 1997)

     The skull had only a rudimentary frill.  This was a small dinosaur of less than 2 meters in length.  The skeleton is of the primitive ornithischian type with hind limbs longer than fore limbs, fore limbs with only three functional digits (5 gone and 4 greatly reduced), short neck, and a long tail.  It probably walked on two legs most of the time (bipedal), but when eating may have moved on all four limbs.  Psittacosaurus was widespread and fairly common in Asia during the Early Cretaceous.  It is perhaps very similar to the ancestral stock for all of the Ceratopsia.
    Psittacosaurus (The Natural History Museum, London)
 

NEOCERATOPSIA

     The distribution of the Neoceratopsia suggests that the group originated in Asia and then spread to North America.  This group includes some of the most diverse and abundant dinosaurs of the Late Cretaceous of North America.

     The following derived characters separate Neoceratopsia from Psittacosaurus:

• 1) Very large head relative to the body.
• 2) Broad, prominent frill.
• 3) Pointed and sharpely keeled rostrum.
• 4) Obligate quadraped limb structure (except for the primitive Neoceratopsians, Leptoceratops and Microceratops).  The return to the tetrapod primitive condition of quadrapedality (even though the primitive condition for dinosaurs and Ceratopsia is bipedality) is secondary in nature.

PROTOCERATOPSIDAE

     With Sereno's objection to a monophyletic Protoceratopsidae aptly noted, we will continue with a discussion of the primitive ceratopsians that have traditionally been placed in the family Protoceratopsidae.

     Perhaps the best known of the Protoceratopsidae is the small dinosaur Protoceratops from Upper Cretaceous rocks of the Gobi Desert in Mongolia.  In addition to Protoceratops,Bagaceratops is also known from the Upper Cretaceous of Mongolia, and Leptoceratops and Montanoceratops are known from Upper Cretaceous rocks of western North America.  The protoceratops are all of Late Cretaceous age and represent a stage of evolution intermediate between Psittacosaurus and Ceratopsidae (Lucas, 1997).

     The first specimens of Protoceratops (also Psittacosaurus) were found in the Gobi desert as a result of the Central Asiatic Expeditions led by Roy Chapman Andrews ( American Museum of Natural History: Roy Chapman Andrews at http://www.amnh.org/Exhibition/Fossil_Halls/Personalities/andrews.html) of the American Museum of Natural History in the early 1920s.

Discovery Online -- Dinosaurs in the Dunes, Protoceratops andrewsi (From: http://icarus.online.discovery.com/area/specials/gobi/bonezonepages/dino.06.html).

     Protoceratopsids had a large skull and much larger frill than Psittacosaurus (which had only an incipient frill).  The protoceratopsids had no horns, as in later neoceratopsians.

LIFESTYLE OF PROTOCERATOPSIDS

     The Late Cretaceous of eastern Asia was characterized by an arid climate with desert dune fields and intermittant lakes and streams.  Of course, this type of environment is ideal for preserving fossils.  It is thought that protoceratopsids lived in small herds and possible nested communally (somewhat speculative).  Although large numbers of Protoceratops skeletons (dozens) have been found, no eggs of Protoceratops are known.  The large number of eggs found by the Central Asiatic Expeditions (an latter expeditions) as ring-shaped clusters in shallow depressions in dune sand are now known to be eggs of the small, strange looking theropod dinosaur, Oviraptor (Click this site:AMNH Dino News | Headlines | The Baby That Changed a Theory (http://www.amnh.org/enews/headl/e2_h3.html).

     Well documented growth sequences, from young to adult, have been found for Protoceratops.  Onset of the frill growth is associated with sexual maturity.  Thus, the frill may have been important for display.  The frill is highly vascularized and perhaps may have served some function in thermoregulation.  For adults, there are two sizes of skulls, suggesting sexual dimorphism.  The larger skull with more prominent nose bumps have been interpreted as males.

 From Lucas, 1997.

CERATOPSIDAE

     Members of the Ceratopsidae are relatively large (4 to 8 meters long) and are obligate quadrapeds.
Derived characters include:

• 1) Very large skulls (relative to body), 1-2.4 meters.
• 2) Large nostrils
• 3) Prominent frills.
• 4) Facial horns (in a variety of styles).

     Cladogram showing the relaionships among the Ceratopsia (From: Dodson, Peter and Philip J. Currie; 1990; Neoceratopsia, p. 609; in The Dinosauria; Weishampel, David B., Peter Dodson, and Halszda Osmolska (eds.); 1990; University of California Press; Berkely, Los Angeles, London; 733 p.

CENTROSAURINAE

     The Centrosaurinae are the more primitive (least derived) of the Ceratopsidae.  They are known primarily from Upper Cretaceous rocks of Montana, Alberta, and Alaska.

• Short, high faces, and short frills
• Nasal horns larger than the paired horns behind the eyes.
• Examples are Centrosaurus and Styracosaurus.
• 
• Centrosaurus (From: Menefee Marvels (http://www.nmmnh-abq.mus.nm.us/nmmnh/menefee.html)
• 
• Bones of Centrosaurus found in Alberta, Canada.  These bones probably represent
• bones that were washed onto a sand bar after the drowning of a herd of Centrosaurus.
• The dinosaurs probably tried to cross a flood swollen stream.  From: Royal Tyrrell Museum: Field Station (http://tyrrell.magtech.ab.ca/fieldstation/)

     This is the group that contains Triceratops.  These dinosaurs have very large skulls, some as large as 2.8 meters.  They are also fairly long of body, some with bodies as long as 8 meters (about 26 feet).  A typical Triceratops would weigh from 5 to 7 tons.  As with the Centrosaurinae, they are only known from western North America.  The Chasmosaurinae ranged across western North America from Alberta to Texas.

• Long, low faces.
• Long frills.
• Postorbital horns larger than nasal horns.
• Examples are Torosaurus, Triceratops, and Pentaceratops.
• 
•  Triceratops (From: http://itc.novi.k12.mi.us/ms/leap/triceratops.html)

TRICERATOPS

     Triceratops is a typical representative of the Chasmosaurinae, so let's look at the characteristics of this dinosaur.

• 1) Long, low skull and large postorbital horns.
• 2) Relatively short frill (compared to other chasomsaurines) that lacked openings (fenestrae).
• 3) The edge of the frill is lined with small horns.
• 4) Teeth - complex dental battery, locked together in longitudinal rows and vertical columns; similar to hadrosaurian (duck-billed dinosaurs) dental batteries.
• 5) Large jaw muscles anchored to the frill.
• 6) Three forward pointing horns, two large ones above orbits and a small nasal horn.
• 7) Numerous grooves and channels for blood vessels on surface of horns and frill.
• 8) Probably had keratinous sheaths over the horns, as in cows.
• 9) Fore limbs and hind limbs nearly equal.
• 10. Ossified tendons present only in the hip region.
• 11) The pelvis is fused to the backbone.
• 12) Ten sacral vertebrae, 4 or 5 more than for most dinosaurs.  This suggests great stability when walking or running.
• 13) Massive hind limbs with 4 toes, and the toes are hooved.
• 14) Massive shoulder girdle and fore limbs (some have suggested a sprawling posture for the fore limbs).
• 15) First four vertebrae of the neck are fused (to support the very large head).

LIFESTYLE OF CERATOPSIDAE

• Evidence suggests that they lived in herds.
• Centrosaurus drownings in Alberta.  Mass accumulation of bones for nine species.
• Brain size small suggesting slow and uncomplicated life.
• Frill anchored jaw muscles, so great chewing power.  In long frilled ceratopsids, the jaw musculature must have been very extensive and poweraful.
• Frill and horns also for display, to recognize their own species.  Intraspecific ritualized combat and establishment of dominance in defense of territories may be highly linked to reproductive success.  The frill and horns were probably important tools to this end.  Horns may have served some defensive function, but display seems more likely.

CERATOPSIAN EVOLUTION AND BIOGEOGRAPHY

• The Ceratopsia first appeared in Early Cretaceous of Asia, about 100 million years ago.
• In Asia, known primarily from China and Mongolia.
• Migrated from Asia to North America about 75 million years ago.
• Known throughout most of the western U.S. and Canada, ranging from Alberta to New Mexico.
• Became highly successful in North America.
• Most common and most diverse of Late Cretaceous dinosaurs.
• Triceratops is the last known dinosaur in the fossil record.