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PROTOCERATOPSIANS

     Ceratopsians include the giant North American herbivores of the Late Cretaceous that could weigh 6-7 tons.  The Asian genera were smaller, lacked horns, and in the Mid-Cretaceous they migrated from Asia to North America.      Like ornithopods, ceratopsians evolved a dental battery which consisted of many rows of cheek teeth and additional rows of replacement teeth.  Later in ceratopsian evolution, these teeth became more vertical and better at slicing, rather than grinding, plant material.  Ceratopsians possessed fleshy cheeks to hold the plant material while they chewed.  A strong process on the lower jaw (the coronoid process) served for muscle attachment and the large jaw muscles passed through upper temporal openings to attach onto the frill.  The resulting ability to chew meant that they could take advantage of woodier food sources that other herbivores could not digest well (Ostrom, 1966). 

     Ceratopsians browsed low to the ground–even the largest could only reach 2 meters high.  They probably ate shrubby angiosperms (which were spreading rapidly by the Late Cretaceous), ferns, and small conifers.  The largest ceratopsians with the slicing dental battery may have been the only herbivores which could digest the tough leaves of the cycads (a group of gymnosperms).  Some have proposed that they could knock trees down to gain access to their leaves (Ostrom, 1966). 

     The horns not only served as protection against predators, they were also been used in competition with other ceratopsians.  Scars on 5 specimens suggest that they fought with each other, perhaps in intermale combats.

      The obvious frill was composed of elongated parietal and squamosal bones.  Sometimes the margin of the frill bore spikes or additional bones called exoccipitals.  The frill provided jaw muscle attachment for additional jaw strength when chewing.  Since the frill extended beyond area of muscle attachment, it must have served other roles as well.  Blood vessels went to the frill and it is possible that the increased surface area functioned in thermoregulation.  Large frills could also have served as display during mating.  There seems to have been sexual dimorphism in the frills of Protoceratops, Centrosaurus, and Chasmosaurus.  In Pachyrhinosaurus, the males had bigger nose ridges.  In Protoceratops, the dimorphism occurred when adult size was reached (Ostrom, 1966; (Lambert, 1990, Fastovsky, 1996).

     Fossils of a number of species have been found in mass accumulations, suggesting that they traveled in herds.  In some of these sites, the minimum number of individuals present is more than 100.

     The limbs were unique and since few ceratopsian trackways are known, there is some question as to how they held their limbs.  As ceratopisans increased in size and the quadrupedal locomotion became more important, the hands became more robust (Fastovsky, 1996).

PROTO hands
     The first fossil finds of ceratopsians in the 1870s were fragmentary giving little idea of the animal the bones belonged to.  In the 1880s ceratopsian horns were thought to belong to extinct bison.  In the early 1900s many fossils were found and in 1923 an abundance of Protoceratops material was found in Mongolia and China.  Protoceratops was the first dinosaur for which fossils gave a complete life series and also gave the first example of sexual dimorphism in dinosaurs.
psittacosaurus
     Psittacosaurus, the “parrot lizard”, was about 2 meters long and juveniles only measured 23 cm.  Psittacosaurus was bipedal (as were several protoceratopsians such as Leptoceratops and Microceratops).  Psittacosaurus was similar to pachycephalosaurs and ornithopods.  It had a narrow beak in which the upper and lower portions curved together like higher ceratopsians.  Like ceratopsians, the cheekbones form small horns and there were the beginnings of a frill.  One species had a nasal horn.  The 4th and 5th fingers were reduced.  Stenopelix may be the oldest ceratopsian but skull not known (Lambert, 1990, Fastovsky, 1996).
psittacosaurus skull psit and protoceratops
     Some feel that the family Protoceratopsidae is a monophyletic clade separate from the larger ceratopsians but most data suggests that this was an evolutionary line leading to the higher ceratopsians.  Montanoceratops is the protoceratopsian most closely related to the ceratopsians.  Most protoceratopsians were small (1-2.5 meters long) with the exception of Udanoceratops (whose skull measured 1 meter long).   The ages of North American protoceratopsians (which include Leptoceratops and Montanaceratops) approaches that of the oldest Asian species, Archaeoceratops (Ryan, 1998).  Microceratops was an early protoceratopsian and stood only 25 cm tall at the hips (Weishampel).  Leptoceratops was more primitive than Protoceratops, which was more primitive than Montanoceratops.  Some retained the primitive trait of clawed toes rather than hooflike ones. Their derived features include a bony neck frill, cheek hornlets, and three fused neck bones (Ostrom, 1966).
montanaceratops leptoceratops
microceratops
bagaceratops leptoceratops
     Protoceratops, “first horned faces”, was the first dinosaur in which sexual dimorphism was observed and the first in which a complete life series was known.  Specimens of different ages have been found in the vicinity of the nests.  The beak was apparently an effective weapon–one nest preserved a Velociraptor that died from wounds (Lambert, 1990, Fastovsky, 1996).
protoceratops
male female