As in other sauropsids, the cleithrum is lost. Turtle pectoral girdles are inside of the rib cage rather than outside of it. A clavicle and interclavicle may be present as the intramembranous component of the girdle. Basal synapsids and monotremes generally have both anterior and posterior coracoids. The single “coracoid” element in therian mammals is the posterior coracoid (simply referred to as the coracoid in this context). The single “coracoid” in amphibians, birds, and reptiles is the anterior coracoid ( procoracoid). Most amniotes possess a scapula and either an anterior coracoid ( procoracoid), a posterior coracoid, or both, and this represents the endochondral component of the amniote pectoral girdle. ![]() The dermal elements that are present in the pectoral girdle are the cleithrum, clavicle, and interclavicle. In early tetrapods and amphibians, the endochondral elements are the anterior coracoid, posterior coracoid, and scapula. The dermal bones are evolutionarily lost to varying degrees across tetrapods. All therian mammals have lost the anterior coracoid but retain the posterior coracoid. However, in synapsids a second “coracoid” evolved: the posterior coracoid. In fishes and early tetrapods, the “coracoid” refers to the anterior coracoid (or procoracoid). Note that the “coracoid” in fishes and the “coracoid” in mammals are not homologous. Where the scapula and the coracoid meet, they form the glenoid fossa-the site of articulation for the forelimb. The two endochondral bones of the pectoral girdle are the coracoid (which is ventral and contacts the sternum) and the scapula (dorsal). In tetrapods, the pectoral girdle attaches to the vertebrae via a muscular sling. Illustrations from Jollie (1962) under CC0 public domain. Pectoral girdles of gar ( Lepisosteus), bowfin ( Amia), and salmon ( Oncorhynchus), clockwise from the top. The dermal bones include the cleithrum, which is always present, and a supracleithrum, posttemporal, and a clavicle, which are absent in some lineages.įigure 7.1. The endochondral elements include the scapula and the anterior coracoid. In bony fish, the pectoral girdle has both endochondral and intramembranous (dermal) components. The area where the fin articulates is called the glenoid region. The ventral portion is called the coracoid bar, the dorsal part is called the suprascapula, and the spine that extends dorsally from the middle region is called the scapular process. On each side of the pectoral girdle of chondrichthyans (e.g., sharks) there is one large scapulocoracoid where the scapula and anterior coracoid form a unit. See the terms list here: Terms 7.1 The Pectoral Girdle Non-Tetrapods ![]() Most vertebrates retain a free-floating pectoral girdle, attached by muscles and ligaments to the axial skeleton. The pelvic girdle, however, articulates with at least one vertebra and provides rigid, non-muscular support, thereby reducing the energy required to elevate the body over the limbs. In tetrapods, the bones of the pectoral girdle lose contact with the skull and attach to the axial skeleton via muscles and ligaments. In fishes, the pectoral girdle attaches to the skull, but the pelvic girdle is not attached to the axial skeleton. The girdles connect the axial and appendicular skeletons. Throughout vertebrate evolution, a diversity of appendage morphologies has emerged, linked to a variety of locomotor modes and lifestyles. Paired appendages are one of the features that distinguish vertebrates from their chordate ancestors. In this lab you will explore the appendicular skeleton of vertebrates, which includes the girdles and the four limbs. The axial skeleton includes the vertebrae, ribs, and sternum, which you became familiar with in the previous lab. The postcranium of the vertebrate skeleton is divided into axial and appendicular components. Fathom the evolutionary changes associated with the transition from the basal synapsids (with a sprawling posture) to modern mammals (with stacked limbs). ![]()
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