Animal diversity 2

Deuterostomes include humans and other chordates, as well as acorn worms and sea stars.

2. The Deuterostomia include three major phyla:

    a. Hemichordata (acorn worms)

    b. Echinodermata (sea urchins and sea stars)

    c. Chordata (vertebrates and closely related invertebrate animals such as sea squirts)

1. Hemichordates include:

     a. Acorn worms: about 75 species of wormlike animals that move through seafloor sediments in search of food particles

    b. Pterobranchs: about a dozen species of animals that attach to the seafloor and use tentacles to filter food from seawater

2. Hemichordates all have a mouth on an elongated protuberance called a proboscis that connects to the digestive tract by a tube called the pharynx, which contains a number of vertical openings called pharyngeal slits separated by stiff rods of protein.

3. They also have a dorsal nerve cord.

4. Hemichordates have body plans quite distinct from those of chordates and echinoderms.

1. Another major group of deuterostomes is the echinoderms, which include sea stars and sea urchins (including sand dollars), as well as brittle stars, sea cucumbers, and sea lilies.

2. All echinoderms have:

    a. A unique fivefold symmetry on top of their basic bilaterian organization

     b. Distinctive skeletons made of interlocking plates of porous calcite, a form of calcium carbonate

    c. A water vascular system, a series of fluid-filled canals that permits bulk transport of oxygen and nutrients d. Tube feet, small projections of the water vascular system that extend outward from the body surface, and that facilitate locomotion, sensory perception, food capture, and gas exchange

3. About 7000 echinoderm species reside in present-day oceans, but the fossil record suggests that this phylum was more diverse in the past.

1. The other great branch of the deuterostome tree is the chordates, the phylum that includes vertebrate animals.

2. Within the phylum Chordata, there are three subphyla: cephalochordates, tunicates, and vertebrates (also called craniates).

3. Common traits:

     a. Like hemichordates, chordates have a pharynx with pharyngeal slits.

        • In fish, these pharyngeal slits form the gills.

         • In terrestrial animals like humans, these slits can be seen only in developing embryos.

    b. The notochord, a stiff rod of collagen and other proteins, runs along the back, providing support for the axis in some chordates.

        • In vertebrates, the notochord is apparent only during early embryogenesis and is replaced by the development of a vertebral column.

    c. Also forming during early development is the neural tube, a cylinder of embryological tissue that develops into a dorsal nerve cord.

    d. Body musculature is organized into a series of segments called myotomes.

1. Amphioxus, a cephalochordate, shares key features of body organization with vertebrates but lacks a well-developed brain and eyes, has no lateral appendages, and does not have a mineralized skeleton.

2. The many similarities of amphioxi to vertebrates suggests that they are closely related, and molecular sequence comparisons confirm this hypothesis.

1. The tunicates share key features of the chordate body plan during early development, but have a unique adult form.

2. Tunicates include about 3000 species of filter-feeding marine animals, such as sea squirts anchored to the seafloor and salps that float in the sea.

3. The adult tunicate body is a basket-like structure that is highly modified for filter-feeding, functionally not unlike the shapes of bivalves and barnacles, which are also adapted to this feeding mode.

4. Larval tunicates have a more typical chordate body plan, including a notochord, neural tube, and a long tail with muscles arranged in myotomes.

5. Molecular data suggest that tunicates are humans’ closest invertebrate relatives.

6. After the divergence of the three major groups of chordates, the tunicates’ adaptation for filter-feeding led to the unique anatomy they have today.

1. Vertebrates are animals named for their jointed skeleton that runs along the main axis of the body, forming a series of hard segments collectively termed vertebrae.

2. In addition to features shared with other chordates, vertebrate animals are distinguished by:

    a. A cranium that protects a well-developed brain

    b. A pair of eyes

    c. A distinctive mouth for food capture and ingestion

    d. An internal skeleton commonly mineralized by calcium phosphate

3. Many of the features that separate vertebrates from invertebrate chordates can be found in the head, including a bony cranium that protects the brain, which is joined in most vertebrates by the mandible (or jawbone) to form a skull.

4. In this figure, anatomical features of vertebrates that are characteristic of all chordates are shown in red, and those that are unique to vertebrates are shown in blue.

1. Fish are the earliest-branching and most diverse vertebrate animals.

2. We sometimes think of fish as one group, but the aquatic animals we commonly call “fish” include four distinct groups of aquatic vertebrates.

3. The earliest branching craniates are the hagfish and lampreys.

    a. These animals have a cranium built of cartilage but lack jaws.

    b. Lampreys also have a vertebral column built of cartilage, but hagfish lack vertebrae.

1. These eel-like organisms feed on soft foods without the aid of jaws because they diverged before jaws evolved from pharyngeal slits.

2. Hagfish feed on marine worms and on dead and dying sea animals, while lampreys live parasitically, sucking body fluids from fish prey.

3. Both hagfish and lampreys have a series of gill slits through which water enters to bring oxygen to the gills.

4. Biologists have long debated the phylogenetic relationships among hagfish, lampreys, and other vertebrate animals.

    a. The presence of a vertebral column in lampreys has convinced many biologists that lampreys are the sister group to other vertebrates and hagfish the sister group to all vertebrates, including lampreys.     b. The debate continues, but several lines of molecular data now favor the view that hagfish and lampreys together form the sister group to all other vertebrates.

1. Chondrichthyes, or cartilaginous fish, form the next deepest branch on the vertebrate tree.

2. This monophyletic group includes about 800 species of sharks, rays, and chimaeras, all of which have jaws and a skeleton made of cartilage.

3. These fish deposit calcium phosphate minerals only in their teeth and in small toothlike structures called denticles embedded in the skin.

4. The best-known cartilaginous fish are the sharks, but the group also includes whale sharks and rays.

5. Nearly all Chondrichthyes occur in the oceans, but bull sharks (Carcharhinus leucas) commonly swim into rivers—they’ve been recorded well up the Mississippi—and there are several freshwater species of rays.

1. Osteichthyes, or bony fish, have a cranium, jaws, and bones mineralized by calcium phosphate.

2. Numbering about 20,000–25,000 fresh and seawater species, these are the fish that we most commonly encounter.

3. Bony fish are by far the most diverse group of vertebrates, possessing several unique features that facilitate their occupation of diverse niches:

    a. A system of moveable elements in their jaws allows them to specialize and diversify their feeding on many different types of food.

    b. A swim bladder, a gas-filled sac, permits them to control their position in the water column through changes in buoyancy.

    c. Kidneys allow them to regulate water balance and occupy waters over a wide range of salinity.

4. Early vertebrates evolved a gut sac that enabled them to gulp air to obtain additional oxygen.

    a. In some fish, this sac evolved into an air bladder.

    b. In one group, the sac was modified to become a lung.

1. Most bony fish have fins that are supported by a raylike array of thin bones.

    a. About a dozen closely related species have paired pectoral and pelvic fins.

    b. These fish are called lobe-finned fish and include the coelacanth and lungfish.

  2. Although these animals resemble other fish, the coelacanth and lungfish are the nearest relatives of tetrapods, four-legged animals.

3. Lungfish can survive periods when their watery habitat dries by burying themselves in moist mud and breathing air.

4. Coelacanths first appeared more than 400 million years ago and were thought to have been extinct for 80 million years until a specimen was discovered in a fisherman’s catch off South Africa in 1938.

1. There are more than 4500 species of Amphibia, ranging in size from tiny frogs a few millimeters in length to the Chinese giant salamander, which is more than 1 m long.

2. Their name reflects their distinctive life cycle.

3. Most species have an aquatic larval form with gills that permit breathing under water as well as an adult form that is terrestrial and usually has lungs for breathing air.

4. Amphibians must reproduce in the water, so they are not completely terrestrial.

5. Most amphibian adults require moist skin for breathing and consequently have small lungs, with toads and red efts being notable exceptions.

1. Some vertebrates evolved an egg adapted to tolerate dry conditions of life on land. Features of the amniotic egg include:

    a. A desiccation-resistant shell

    b. Four membranes that permit gas exchange and management of waste products produced by the embryo

2. Amniotic eggs must be fertilized internally before the eggshell is produced by the female because sperm cannot penetrate the shell.

3. The amniotic egg can exchange gases while retaining water; this permits the group of vertebrates known as amniotes to live in dry terrestrial habitats that amphibian eggs cannot tolerate.

4. Amniotes include lizards, snakes, turtles, and crocodilians (about 6000 species of scaly animals commonly referred to as reptiles), as well as birds and mammals.

5. Most mammals, many lizards, and some snakes have evolved through live birth rather than by laying eggs.

    a. Nonetheless, they retain the specialized membranes characteristic of all amniotes.

    b. Instead of being wrapped around the embryo inside a tough egg shell, these membranes surround and protect the embryo inside the womb.

1. All mammals are covered with hair and feed their young milk from the mammary glands for which the class Mammalia is named.

2. Like many other groups of animals and plants, the early-branching mammals show intermediate stages in the evolution of the body plans that dominate on Earth today.

3. The earliest-branching living mammals, the monotremes, lay eggs like birds or lizards, but their hatched young drink milk secreted from pores in the skin of the mother’s belly.

4. The first mammals that gave birth to live young appeared in the Jurassic Period, about 160 million years ago.

5. These animals gave rise to the two major groups of living mammals, marsupial and placental mammals.

1. Marsupials include kangaroos, koalas, and related groups native to Australia, as well as the opossums found in the Americas.

    a. Their young are born at an early stage of development.

    b. The babies must crawl to a pouch, where mammary glands provide them with milk.

2. The placental mammals are named for a temporary organ called the placenta that develops in the uterus along with the embryo, providing nutrition that will enable the offspring to be larger and more quickly independent when born.

    a. Most living mammals fall into this group.

    b. Placental mammals include the carnivores, such as lions and weasels; the primates, including monkeys, apes, and humans; and the hooved mammals (which include cattle, pigs, and deer), and whales.

1. If we stand back and look at evolution over time, we see a pattern of increasing complexity, which has enabled increasingly specialized function and therefore greater species diversity.

2. We also see a pattern of accumulation through time.

3. Bacteria and Archaea were not replaced by later-evolving eukaryotes.

4. Prokaryotic microorganisms remain at the heart of functioning ecosystems, with complex interactions of protists, plants, animals, and fungi built around them.