Natural History of Vertebrates Lecture Notes Chapter 2 - Vertebrate Relationships and Basic Structure

Transcription

1 Natural History of Vertebrates Lecture Notes Chapter 2 - Vertebrate Relationships and Basic Structure These notes are provided to help direct your study from the textbook. They are not designed to explain all aspects of the material in great detail; they are a supplement to the discussion in class and the textbook. If you were to study only these notes, you would not learn enough to do well in the course. List of Terms Vertebrate Relationships Phylum Chordata Though theories are numerous, the sister taxon to the Chordata is probably the Ambulacraria. The Ambulacraria consists of the Echinodermata and their sister taxon, the Hemichordata (formerly a subphylum within Chordata). The sister taxon of those two is the Xenoturbellida (figure 2-1). These three phyla collectively are the sister taxon to the Chordata and along with the Chordata form the deuterostomes. This is based primarily on the fact that in the Deuterostomata the blastopore develops into the anus, as opposed to the mouth in other groups of invertebrates (protostomes), and the cleavage of the zygote is radial (successive cell divisions at right angles to each other) instead of spiral (not at right angles). Pharyngeal slits, which are often thought to be a derived character for the Chordata are now thought to be a primitive character as some extinct echinoderms seem to have these slits. (shared derived characters of the chordates) 1. notochord 2. dorsal hollow nerve chord 3. muscular postanal tail 4. endostyle (groove with cilia for feeding, homologous to the thyroid gland in advanced chordates) Subphylum Vertebrata Basically, a vertebrate is a chordate with a cartilaginous or bony endoskeleton. (shared derived characters, table 2-1) 1. serially arranged vertebrae (for all but hagfishes) 2. anterior skeletal elements form a cranium 3. three part brain (forebrain, midbrain, hindbrain) 4. duplication of all the Hox gene complex (7-13 homeobox genes), duplicated again in jawed vertebrates 5. presence of neural crest cells (quadroblastic) in addition to ectoderm, endoderm, and mesoderm

2 Relatives of the vertebrates within the chordates Subphylum Cephalochordata (amphioxus) Basic chordate plan is similar to that of a larval lamprey (figures 2-2 and 2-3) 1. Fish like movements, body divided into myomeres 2. Support provided by an elastic, incompressible notochord 3. Food is filtered from the water as it enters the mouth. Food is trapped in mucus as the water passes over the cilia and through the pharyngeal slits. The trapped food is then carried into the digestive system. 4. Closed circulatory system 5. Nervous system is very different: no real brain; no homologues to eyes, ears, nose or cranially clustered senses. The Cephalochordata is probably the sister taxon to the Vertebrata, with shared derived characters: 1. Segmented myomeres 2. Lateral plate mesoderm 3. Role of the notochord in development 4. Caudal fin fold (vertebrate like tail fin) 5. Ventral to dorsal pattern of blood flow through the gills (ventral heart, dorsal aorta) 6. Excretory tissues formed from podocytes Subphylum Urochordata (tunicates) The larva of urochordates have chordate features, however, the adults are sessile and do not resemble any chordate (figure 2-2). Several authors have argued for the Urochordata to be the sister taxon for the Vertebrata. Early vertebrates could possibly originated from a tunicate-like larva that became sexually mature without metamorphosis to the adult body plan through a process called paedomorphosis. This hypothesis is not well supported and has fallen out of favor. Paedomorphosis is divided into two processes; neoteny, which is the retention of a few larval characteristics into the adult stage and progenesis, which is the development of sexually mature gonads in the larval stage. Earliest Chordates The earliest date from the Early Cambrian. One of these is Haikouella. It has the standard derived characters for the Chordata as well as several other derived characters that place it as the sister taxon to the Craniata/Vertebrata (large brain, eyes, thickened branchial bars (muscular pharynx and neural crest cells). Filter feeder. All of the early chordates (Pikia, Xidazoon, Haikouella) were very small, filter feeders, with soft bodies. Several species have been described. Most are not clearly assignable to one of the three subphyla within the chordates and may represent separate extinct subphyla. Basic Vertebrate Structure

3 Embryology -- A study of embryonic development is often very helpful in determining the phylogeny of various organisms. To this end, you should be familiar with the basic beginning of embryonic development. We will cover specific aspects in much more detail later on in the course. There are three germ layers and they are first seen during gastrulation and a fourth germ layers appears later. ectoderm forms the epidermis, lining of the anterior and posterior ends of the gut, and the nervous system. endoderm forms the lining of the gut and the glands associated with the gut and the lining of the respiratory structures. mesoderm forms the muscles, skeleton, connective tissue, circulatory system, urogenital system, and splits to form a coelom. neural crest is a fourth type of developmental tissue that forms various tissues in the head region and the peripheral nervous tissue Basic organ systems I. Integument This is the covering of the body Divided into three parts 1. epidermis 2. dermis 3. hypodermis The functions of the integument are: 1. protection from pathogens, predators, or chemicals 2. exchange of compounds 3. sensation (input to the nervous system 4. secretory (mucus, poison glands, sweat glands) 5. prevention of water loss The dermis is the layer of connective tissue that supports the epidermis. The dermis is flexible,and contains vascular and nervous tissue as well as melanocytes. II. Skeletal Functions for: 1. support 2. movement 3. protection 4. storage center for calcium and phosphate Original elements were a notochord that has for the most part given way to vertebrae. Divided into two parts: 1. axial, which includes the cranium, vertebrae, and ribs 2. appendicular, which includes the front and hind limbs and the associated girdles.

4 Bone is divided into two basic types: dermal which is formed in the skin and endochondral which is formed within cartilage. III. Muscular The function of muscular tissue is movement, either of the body itself or organs within the body. Three types of muscle tissue: 1. smooth, which is involuntary muscle that lacks striations. 2. striated or skeletal, which is voluntary muscle that has striations caused by the arrangement of actin and myosin. 3. cardiac, which is involuntary muscle that has striations like skeletal muscle (only found in the heart). Muscles are often arranged in myomeres leading to segmented body musculature. The myomeres extend backward and forward over several vertebrae in order to give better control and make the contractions smoother and more efficient (figure 2-10). IV. Digestive Typically divided into mouth, esophagus, stomach, intestine, cecum, large intestine, anus. The size or even presence of these structures depends upon the feeding habits of the organism. In general, parasitic forms have greatly reduced digestive systems, while herbivorous forms have large and elaborate digestive systems. V. Respiratory The function is to allow the exchange of gases between the body and the environment. This occurs in different ways depending upon habitat and size of the organism. 1. gills - aquatic 2. skin - aquatic 3. lungs - aquatic to terrestrial Ancestral vertebrates were small and used cutaneous respiration, while larger more active forms must rely on gills or lungs. In part, it was this more efficient exchange of gases that allowed vertebrates to increase in size as a group. VI. Circulatory The function is to transport gases (oxygen and carbon dioxide), nutrients, hormones, and heat throughout the body. Certain cells also participate in immunological response to foreign substances or pathogens. Vertebrates have a closed circulatory system in that blood is contained in specialized vessel that are lined by epithelial tissue (figure 2-11). Capillaries are the site of gas exchange. Blood is pumped by a heart that can have from 2 to 4 chambers. There is a dorsal aorta to carry blood into the posterior regions of the body. Blood vessels that are between two capillary beds are called portal vessels (hepatic portal system). Formed elements in the blood are erythrocytes (transport oxygen) and leukocytes (immune response). VII. Excretory The functions of the excretory system are 1. the removal of metabolic waste products (mainly urea and its derivatives)

5 2. the maintenance of water balance 3. the regulation of various ions The main organ is the kidney in tetrapods but in aquatic organisms the gills and skin also play a large role. In freshwater, the kidney helps to rid the organism of excess water (hypotonic relative to the organism). VIII. Reproductive In vertebrates there are two types of gametes, sperm (small, motile and associated with males) and eggs (large, immobile and associated with females). Sex determination in fish and amphibians is by a complex set of genes, sex reversals are common and several types of hermaphroditism (functional male and female) are known. In amniotes, individuals are one sex or the other and often determined chromosomally. Organisms may lay eggs (oviparous) or bear live young (viviparous). IX. Sensory The function is that of integration of information and response throughout the body and the sensing of events in the external environment. The nervous system is divided into two parts: 1. central nervous system is the brain and spinal cord 2. peripheral nervous system is the individual nerves and sense organs. Peripheral nerves are either sensory (input to the CNS) or motor (output from the CNS). In addition, the autonomic part of the nervous system is divided into the 1. sympathetic (activates several processes for fight or flight) 2. parasympathetic (decreases metabolic rates and activities) The major sensory modalities are 1. mechanoreception (touch and body position) 2. chemoreception (taste and smell) 3. vision (detection of light including UV light and polarized light) 4. electroreception (detection of electrical/magnetic fields) 5. acousticolateralis (hearing and motion) X. Endocrine The regulation and control of various functions are performed through chemicals. The chemical messengers are hormones and they are released by endocrine glands. Hormomes are secreted by one organ and act upon a second organ. Hormones integrate many functions within the body and often work to modify the expression of whole suites of genes. Last updated on 14 January 2012 Provide comments to Dwight Moore at dmoore@emporia.edu