9-chapter 6 - Chapter 6 Appendicular Skeleton Purpose Understand the basic components that comprise the appendicular skeleton in Craniates

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Chapter 6: Appendicular Skeleton Purpose Understand the basic components that comprise the appendicular 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 skeletal elements of the appendicular region consist anteriorly of paired pectoral appendages (fins or limbs) and an associated pectoral girdle, and posteriorly of pelvic appendages and pelvic girdles in the hip region. The earliest craniates lacked paired appendages and their associated girdles in both the pectoral and pelvic regions. This body form is also characteristic of modern jawless craniates (hagfishes and lampreys). In early gnathostomes, the pectoral appendages first appeared as small fins that likely aided in maneuverability and stability of the body. As vertebrates made the transition to land, these fins evolved into limbs, which were necessary for both body support and movement. The associated girdles of the shoulder and hip region, while small in fishes, are large supportive structures that help transfer body weight from the axial skeleton to the appendicular skeleton. Chondrichthyes (Use figure 5-2 for identification) The pectoral and pelvic girdles of chondrichthyans are composed entirely of cartilage. The girdles are enlarged, extending dorsally and merging ventrally to form U-shaped bridges. The well- developed basal cartilages of the fins are not dermal, but the ceratotrichia are considered dermal derivatives. Examine the plastic mounts in the lab; notice the homologous elements of both girdles with those of other vertebrates. Make sure you can identify the following structures: scapular process, suprascapula, radial cartilage, ceratotrichia, coracoid bar, metapterygium, mesopterygium, propterygium, pubioischiac bar, iliac process. Aquatic Osteichthyes (Use figure 5-3 for identification) Among extant actinopterygians, the detailed structure of the pectoral and pelvic girdles is highly diverse. Examine the bowfin ( Amia calva ), a basal actinopterygian fish, and the perch, a fairly typical derived actinopterygian. The endoskeletal cartilages remain as the point of appendage attachment; however, dermal elements have fused to the pectoral girdle to add support. The cleithrum complex (“shoulder” area) is directed dorsally from the fin, and is variably composed of several dermal bones. Some of these bones form a connection with the skull and superficially appear as skull elements. Locate these on the skeletal preparations. The pelvic girdle is endochondral , as in all other gnathostomes. Note that in actinopterygians, unlike in tetrapods, the pelvic girdle does not attach to the vertebral column. The structure of the fin itself deserves some attention, and is usually the basis for differentiating
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9-chapter 6 - Chapter 6 Appendicular Skeleton Purpose Understand the basic components that comprise the appendicular skeleton in Craniates

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