Download Natural History of Vertebrates Lecture Notes Chapter 2

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
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
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.
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)
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 [email protected]