Cretaceous Period Summary

This chapter explored the evolution and lifestyles of the Margino-cephalia.

1. The Marginocephalia were ornithischian dinosaurs that included the Ceratopsia ("horned faces") and Pachycephalosauria ("thick-headed lizards'). They were a subgroup of the Cerapoda, the same group that included the ornithopods.

2. The name Marginocephalia refers to a bony shelf at the back of the skull that is characteristic of all members of the clade.

3. Marginocephalia are found only in the Northern Hemisphere and probably had origins in Asia.

4. The roots of the ceratopsians can be traced to the Late Jurassic and Early Cretaceous and their small, bipedal early representatives such as Yinlong, Chaoyangsaurus, and the Psittaco-sauridae. All other ceratopsians were part of the larger group Neoceratopsia, which is further divided into the Protocera-topsidae, a group of basal neoceratopsians, and Ceratopsidae, the large-bodied horned dinosaurs.

5. The most primitive members of the Ceratopsia are represented by Yinlong, Chaoyangsaurus, the Psittacosauridae, and a number of basal members of the Neoceratopsia, most notably the protoceratopsids.

6. Ceratopsids—the horned dinosaurs—included the Centro-saurinae, horned dinosaurs with short frills, and Chasmo-saurinae, horned dinosaurs with long frills. Ceratopsids were large and heavy, the largest being about 30 feet (9 meters) in length.

7. The ceratopsid skull, including the frill, made it one of the largest and heaviest known for any terrestrial vertebrate.

8. Ceratopsian frills likely provided several functions: as a defensive shield against predators or rivals, an attachment point for jaw muscles, and a visual display device for recognizing individuals, attracting a mate, or intimidating rivals.

9. The Pachycephalosauria consisted of small to medium-sized bipedal ornithischian herbivores measuring from about 2 to 15 feet (0.6 to 4.5 meters) in length. They are the only dinosaur group distinguished by a thick, bony skull cap.

Cretaceous Summary

SECTION FOUR:

Mesozoic Seas and Skies k7J

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