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 Hemichordata (formerly a subphylum within Chordata) and the next lowest branch point in the cladogram is the Echinodermata (figure 2-1). These three phyla 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 (successice cell divisions at right angles to each other) instead of spiral (not at right angles).
(shared derived characters of the chordates) Subphylum Vertebrata
Basically, a vertebrate is a chordate with a cartilaginous or bony endoskeleton.

(shared derived characters, table 2-1) Relatives of the vertebrates within the chordates

Subphylum Cephalochordata (amphioxus)
Basic chordate plan is similar to that of a larval lamprey (figures 2-2) The Cephalochordata is probably the sister taxon to the Vertebrata, with shared derived characters: Subphylum Urochordata (tunicates)
The larva of urochordates have chordate features, however, the adults are sessile and do not resemble any chordate (figure 2-5).
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 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

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.
  • 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

    1. Integument
      This is the covering of the body
      Divided into three parts
      1. epidermis
      2. dermis
      3. hypodermis

        The functions of the integument are:
      • protection from pathogens, predators, or chemicals
      • exchange of compounds
      • sensation (input to the nervous system
      • secretory (mucus, poison glands, sweat glands)
      • 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.

    2. Skeletal
        Functions for:
      • support
      • movement
      • protection
      • 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:
      • axial, which includes the cranium, vertebrae, and ribs
      • appendicular, which includes the front and hind limbs and the associated girdles.

      Bone is divided into two basic types: dermal which is formed in the skin and endochondral which is formed within cartilage

    3. Muscular
      The function of muscular tissue is movement, either of the body itself or organs within the body.
        Three types of muscle tissue:
      • smooth, which is involuntary muscle that lacks striations.
      • striated or skeletal, which is voluntary muscle that has striations caused by the arrangement of actin and myosin.
      • 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).

    4. Digestive
      Typically divided into mouth, esophagus, stomach, intestine, cecum, large intestine, anus (figure 3-13). 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.

    5. 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.
      • gills - aquatic
      • skin - aquatic
      • lungs - aquatic to terrestrial
      Ancestral vertebrates were small and used cutaneous respiration, while larger more active forms must rely on gills or lungs.

    6. 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.

    7. Excretory
        The functions of the excretory system are
      • the removal of metabolic waste products (mainly urea and its derivatives)
      • the maintenance of water balance
      • the regulation of various ions

      The main organ is the kidney (figure 2-15) in tetrapods but in aquatic organisms the gills and skin also play a large role.

    8. 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).

    9. 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:
      • central nervous system is the brain and spinal cord
      • 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
      • sympathetic and
      • parasympathetic.

        The major sensory modalities are
      • mechanoreception (touch and body position)
      • chemoreception (taste and smell)
      • vision (detection of light)
      • electroreception (detection of electrical/magnetic fields)
      • acousticolateralis (hearing and motion)

    10. Endocrine
      The regulation and control of various functions are performed through chemicals. The chemical messengers are hormones and they are released by endocrine glands.


    Last updated on 22 January 2008
    Provide comments to Dwight Moore at dmoore@emporia.edu
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