Chap 32 SM 23

Chapter 32: The Deuterostomes

Learning Objectives

At the end of this chapter, a student will be able to:

• Name and describe the ancestral and derived features of deuterostomes (including echinoderms and chordates).

• Comprehend the unique characteristics of echinoderms and provide a detailed comparison among their major classes.

• Identify and elaborate on shared derived features of chordates and vertebrates and understand their evolutionary significance.

• Summarize the key characteristics of Urochordata (tunicates) and Cephalochordata (lancelets) in the context of evolutionary development.

• Distinguish between craniate, vertebrate, tetrapod, and amniote groups, detailing significant evolutionary milestones.

• Thoroughly describe various features and examples of living vertebrate clades, focusing on their ecological roles and adaptations.

• Define and provide examples of reproductive strategies in vertebrates: oviparous, viviparous, and ovoviviparous animals.

• Compare and contrast the structural and functional differences between ray-finned and lobe-finned fish, detailing their ecological niches.

• Explain the evolutionary transition from lobe-finned fishes to tetrapods, including anatomical changes that facilitated life on land.

• Compare the two clades of modern birds, delving into their evolutionary adaptations.

• Elaborate on the derived features and general characteristics of protherian, metatherian, and eutherian mammals, examining reproductive and developmental strategies.

New Terms

• Adaptive radiation: The formation of numerous new species from an ancestral species, typically occurring during times of rapid ecological change that allow for diverse adaptations.

• Cloaca: A multifunctional opening that serves as a common exit point for the reproductive, excretory, and digestive systems in some vertebrates, facilitating the efficiency of bodily functions.

• Ectothermic: Animals that utilize environmental sources to regulate body temperature, often referred to as "cold-blooded," requiring behavioral adaptations for temperature control.

• Endothermic: Animals that maintain a constant body temperature independent of external conditions through metabolic processes, commonly seen in mammals and birds.

• Oviparous: Animals that lay eggs which are nourished by the yolk, showing a variety of developmental strategies across different species.

• Viviparous: Animals that give live birth, with nutrients being transferred from the mother to the embryo, seen in most mammals.

• Metamorphosis: A developmental transition involving profound changes in body form across different life stages, common in amphibians and some insects.

Key Concepts in Deuterostome Development

Ancestral Features Leading to Deuterostomes

• Multicellularity: Describes organisms composed of multiple interconnected cells that work together.

• Tissue Layers: The presence of three fundamental germ layers during embryonic development: ectoderm, mesoderm, and endoderm, which give rise to different tissues and organs.

• Bilateral Symmetry: A body plan that is symmetrical along a central axis; however, adult echinoderms exhibit a unique form of pentaradial symmetry, which is a derived trait.

• True Coelom: A coelomate body plan characterized by a fluid-filled body cavity lined by mesodermal tissue, facilitating complex organ systems and bodily functions.

Important Features of Echinoderms

• Feeding and Gas Exchange: The water-vascular system functions effectively to circulate fluids, facilitating locomotion and feeding in echinoderms, serving a role similar to that of a circulatory system.

• Defining Characteristics: Exhibiting pentaradial symmetry and a hydrostatic skeleton, echinoderms use water pressure for movement and support.

Major Classes of Echinoderms:

• Asteroidea: Commonly known as sea stars, which exhibit regeneration capabilities and varied feeding strategies.

• Crinoidea: Includes sea lilies and feather stars that are primarily filter feeders and have a distinct feeding morphology.

• Echinoidea: Encompasses sea urchins and sand dollars, which have evolved rounded and flattened bodies.

Shared Features of Chordates and Vertebrates

• Gill Slits in Pharynx: Present during embryonic stages, gill slits give rise to various structures in different vertebrate lineages, such as gills in aquatic species.

• Notochord: A flexible rod-like structure providing skeletal support, replaced by the vertebral column in many vertebrates.

• Dorsal Tubular Nerve Cord: The precursor to the central nervous system, essential for processing sensory information and coordinating responses.

• Postanal Tail: An extension that provides balance and propulsion in aquatic environments, a feature retained by various vertebrates.

• Endostyle: A feeding groove involved in filter feeding, which later develops into the thyroid gland in vertebrates.

• Cranium: The bony or cartilaginous protective housing for the brain, displaying evolutionary advancements in sensory processing.

• Vertebral Column: Comprising bone or cartilage that forms the backbone in vertebrates, allowing for greater flexibility and protection of the spinal cord.

Subphyla of the Chordata Phylum

• Urochordata: Also known as tunicates, these organisms exhibit chordate features primarily during their larval stages, while adults typically display reduced anatomical structures and are predominantly suspension feeders retaining gill slits.

• Cephalochordata: An example is Amphioxus (Branchiostoma), retaining all foundational chordate features throughout life. They live a benthic lifestyle as filter feeders and lack paired fins or jaws, providing crucial insights into vertebrate evolution.

Vertebrate Classification and Reproduction

Cladogram Analysis

• Craniate: Referring to organisms possessing a cranium, such as hagfish, which are craniates but lack true vertebrates.

• Vertebrates: Defined by a vertebral column, including both jawless and jawed fish.

Major Groups of Living Vertebrates:

• Jawless Fish: Includes hagfishes and lampreys, characterized by their primitive body plan and feeding method.

• Cartilaginous Fish (Class Chondrichthyes): This class includes sharks, skates, and rays, with a skeleton made primarily of cartilage, adapted for swift predation in marine environments.

• Bony Fish (Class Osteichthyes): Representing the largest group of vertebrates, these fish have a bony skeleton and diverse adaptations for survival.

• Amphibians (Class Amphibia): Distinguishing features include a complex life cycle that includes aquatic larvae and terrestrial adults, displaying significant evolutionary adaptations for both environments.

• Reptiles, Birds, and Mammals: Each group exhibiting distinct evolutionary trends that emphasize adaptability and survival.

Reproduction in Vertebrates

• Oviparous: Species that lay eggs, such as birds and most reptiles, showcasing diverse reproductive strategies in the animal kingdom.

• Ovoviviparous: Observed in many shark species, where eggs develop internally yet rely on yolk for sustenance, representing a transitional reproductive mode.

• Viviparous: Commonly seen in most mammals, where placental nourishment supports the developing embryo for an extended gestation period.