4-chapter 1 - Chapter 1: Origin and Evolution of...

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Chapter 1: Origin and Evolution of Vertebrates Purpose Familiarize yourself with the relationships among Urochordates, Cephalochordates, and Craniates. Understand the defining features of both Chordates and Craniates. Understand current theories regarding the evolution of Craniates. Become familiar with the basic vertebrate body plan and the classification of craniates. Introduction Ch. 1 supplement-larval/adult tunicate, amphioxus, and lamprey images and reading Figure 1-1. Major groups of living deuterostomes ( Adapted from Liem et al 2001 .) However, recent molecular evidence has indicated that Tunicates are more closely related to Craninates than are Amphioxi. See “Craniate Evolution” below. Craniata is comprised of both vertebrates and hagfishes and is contained within the larger clade deuterostomata (Figure 1-1). There is considerable scientific debate surrounding the phylogenetic placement of hagfishes within the vertebrates and thus we use Craniata to refer to both vertebrates and hagfish. When this text refers to vertebrates it is to the exclusion of hagfish. As the focus of this laboratory is the functional anatomy of the vertebrates, we must begin with an understanding of related metazoans to allow us to characterize what defines a vertebrate. Thus our investigation must begin within the phylum Chordata, to which vertebrates belong. We first discuss two groups of soft-
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bodied marine organisms: Urochordata (tunicates) and Cephalochordata (amphioxus), to gain insight into the evolutionary history of the basic vertebrate body plan. Upon first examination it would seem that Craniates, Cephalochordates, and Urochordates are hardly related. Craniates possess well-developed heads and are generally active animals, while adult tunicates are sessile, sack-shaped animals that live attached to ocean substrates. Adult amphioxi (cephalochordates) are filter feeders that live within shallow burrows on the ocean floor. Yet while superficially different, all three taxa possess five unique features at some point during development that defines them as Chordates. Thus all members of Chordata possess: (1) pharyngeal pouches or slits ; (2) an endostyle (or thyroid gland derived from part of the endostyle)*; (3) notochord , (4) dorsal, tubular nerve cord ; and (5) larva or embryo with postanal tail . To better understand vertebrate origins from a chordate ancestor it is important to understand the above-mentioned groups in greater detail. * Specialized cells within the endostyle have been shown to concentrate iodine into an organic molecule, thyroxine. Thyroxine is produced by the thyroid gland in mammals. This suggests homology (common evolutionary origin) of the cells of the endostyle and thyroid gland. The endostyle also secretes mucus into the buccal cavity to aid in filter feeding (in Urochordates and Cephalochordates). Urochordata (
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4-chapter 1 - Chapter 1: Origin and Evolution of...

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