8-chapter 5 - C h a p t e r 5 T h e A x ia l S k e le t o n...

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Chapter 5: The Axial Skeleton Purpose Understand the basic components that comprise the axial skeleton in Craniates. Understand the broad evolutionary forces shaping these elements in a phylogenetic context. Learn structural differences between the represented classes of Craniates. Introduction The post-cranial skeleton is comprised of both axial and appendicular elements. The axial skeleton includes the notochord, vertebral column, median fins, ribs, and the sternum, while the appendicular skeleton includes the paired fins and limbs and the girdles to which they attach. The axial and appendicular elements of the body, together with the associated muscles, work to support and move the craniate body. Due to very different forces at work in the water and on land, the craniate skeleton has undergone drastic changes to deal with this transition. In this lab we will identify the key skeletal elements of these regions and the evolutionary forces that have shaped them over the course of craniate evolution. The shape and arrangement of axial elements in craniates is closely linked to an animal’s ability to support itself. Aquatic craniates tend to have straighter spines in association with the support of hydrostatic buoyancy they receive from their surrounding aquatic medium. Terrestrial vertebrates encounter a greater effect of gravity and show a variety of modifications to their spines in order to support their weight while on land. All adult gnathostomes possess at least traces of calcified vertebrae. The shape and components of vertebrae vary considerably among species and throughout regions of the body. In basal vertebrates such as sharks, the notochord persists and runs through the center of the vertebral column. In most other vertebrates, the notochord is replaced with disk-like vertebral bodies called centra . Successive vertebrae are joined together to form the vertebral column with articulations occurring at these centra. The shape of the centrum has functional implications for movement of the vertebral column (Figure 5-1 below). Centra that are concave on both ends are termed amphicoelous . The strength of the vertebral column is considerably increased if the union between successive vertebrae resembles a ball and socket. One end of the vertebrae forms a bump that fits into the concavity of the adjacent vertebrae. If the anterior (cranial) surface of the centrum is concave, the centrum is procoelous ; if the posterior end is concave, the centrum is opisthocoelous . If both surfaces of the centrum are flat it is termed acoelous . Birds display a unique centrum shape termed heterocoelous , where the ends are saddled-shaped. The vertebrae of some teleost fishes and most tetrapods have additional processes called zygapophyses that increase the resistance to twisting. These extend both backwards and forwards from the neural arches of the individual vertebrae. In this way, the posterior postzygapophyses of one vertebra articulate with the anterior
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8-chapter 5 - C h a p t e r 5 T h e A x ia l S k e le t o n...

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