7-chapter 4 - C h a p t e r 4 : T h e C r a n ia l S k e le...

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Chapter 4: The Cranial Skeleton Purpose Understand the basic material that comprises the cranial skeleton in Craniates. Learn to classify the cranial skeleton based on bone origin and type. Understand the broad evolutionary forces shaping the Craniate braincase in a phylogenetic context. Learn structural differences between the represented classes of Craniates. Understand the basic types of jaw suspension and articulation. Understand the evolution of fenestration in Vertebrates. Learn to identify tooth shape, tooth attachment, and tooth replacement in Vertebrates. Helpful link: www.digimorph.org Introduction Ch. 4 Supplement- Figs. 4-15, 4-16, 4-17 As was mentioned in Chapter 1, the Craniates can be defined by the possession of a braincase. The development of the braincase and cranial skeleton was accompanied by a high degree of cephalization: a tendency in the evolution of organisms to concentrate the sensory and neural organs in the anterior head region. Along with the braincase, the cranial skeleton also includes those cartilages and bones that support the mouth and gills. As Craniates progressively developed more complex sense organs and brains, the braincase evolved to enlarge and better protect the head region. The cranial skeleton is also responsible for assisting in acquiring food and for providing a respiratory passageway for water and/or air. The cranial skeleton is formed from of a combination of three materials. Depending on which class of vertebrates one examines, we find: calcified cartilage , endochondral bone , and dermal bone . Cartilage is the ancestral substance of the Craniate skeleton, while dermal bone is formed within the dermis and most often associated with the skin. Those bones that form deeper within the body are composed of endochondral bone. Endochondral bone is replacement bone that is laid down on a cartilaginous template during development. The cranial skeleton is composed of three components based on its embryonic development: the chondrocranium , splanchnocranium, and dermatocranium . As vertebrates became more advanced these three components became more integrated. As separate entities they are more easily recognizable in ancestral vertebrates and fishes. We will deal with each of these components briefly before examining the cranial skeleton of various craniates. The chondrocranium functions mainly to protect the brain (Figure 4-1). In early vertebrates lacking bone (such as Chondrichthyans) the chondrocranium is cartilage. In most vertebrates the chondrocranium ossifies and is replaced with endochondral bone. The splanchnocranium refers to any element of the cranial skeleton that is derived from a visceral arch. Its ancestral function may have been to aid in respiration, but it soon became integrated as part of the feeding apparatus. The composition of the splanchnocranium is shown in Figure 4-1. In early vertebrates lacking bone (such as Chondrichthyans) the splanchnocranium is cartilage. In most other vertebrates the splanchnocranium ossifies and is
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7-chapter 4 - C h a p t e r 4 : T h e C r a n ia l S k e le...

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