Vertebrate Structure and Development
Anatomy and morphology are often used synonymously, but
there is a difference. Anatomy describes body parts and how they
are organized. Morphology deals with anatomy plus explaining
these structures and how they work. Morphogenesis is the study
of how morphology develops.
Studying body parts and naming them is not very informative.
Therefore, most morphologists are interested in relating anatomy
to behavior, ecology physiology and other aspects of an animal's
When studying anatomy or embryology to determine relatedness
of species or functional adaptations one needs to study many
individuals. This eliminates faulty conclusions based on
individual variation, sexual dimorphism or age variation.
Statistical analyses as well as physical measurements are used
for comparisons. This may be difficult when studying extinct
animals. Morphologists may only have 1 specimen or pieces of
bone. Most animals decompose rather than fossilize.
How do animals change over time? There are 2 lines of
thought. 1) Phyletic gradualism - change is slow and steady.
2) Punctuated equilibrium - for long periods there is little
change then when geologic changes occur morphologic changes occur
We can talk about structures such as beaks or limbs as being
specialized or generalized. Specialized structures are highly
adapted for 1 function i.e. bat hand as opposed to monkey hand.
Generalized structures are better at performing a variety of
functions. If environmental changes occur a generalized
structure is more likely to have the capacity to change.
Two terms that refer to comparisons between the evolution of
species are parallelism and convergence. Parallelism refers to 2
groups that undergo similar changes over time and don't become
more or less similar. Convergence refers to a situation where 2
species become more similar over time. Marsupial and modern
mammalian herbivores have evolved similar traits for eating
grasses. The limbs of fishes and marine mammals have shown
convergence as each group became more adapted for living in the
ocean. Even if 2 lines start with very different structures, if
the 2 live in similar environments the structures may become more
similar to meet similar demands.
The vertebrates are only a small group of animals (~ 5%).
They can be categorized as follows:
What makes a chordate?
It is multicellular, has 3 germ layers, has bilateral
symmetry, has a mesoderm lined coelom, has a complete digestive
tract, is a deuterostome (anus comes from an early opening called
the blastopore - protostomes - mouth comes from blastopore).
Subphyla: Urochordata, Cephalochordata, and Vertebrata
Classes: Myxini, Cephalaspidomorphi, Chondrichthyes, Actinopterygii, Sarcopterygii
Amphibia, Reptilia including Aves and Mammalia
Two important characters shared with many invertebrates are:
1) cephalization and 2) metamerism. We also share 3 germ layers
(ectoderm, mesoderm and endoderm).
What do vertebrates and other chordates have in common?
Subphylum Vertebrata: true brain with a cranium, well developed vertebrae, solid liver with a heptaic portal system, may have a gallbladder,
pancreas, spleen, unsegmented gonads, mesodermal kidneys,
endoskeleton. Relative to most animals vertebrates usually are large. They have paired limbs.
1) Notochord - a dorsal, flexible rod used for support -
present during some life stage.
2) Dorsal nerve cord - derived from ectoderm.
3) Pharynx - with pharyngeal slits.
4) Ventral heart - and closed circulatory system.
5) Postanal tail
Classification or taxonomy is the science of placing animals
into like groups based on evolutionary and genetic relatedness.
A group is referred to as a taxon. Zoologists have 7 major
categories: Phylum - class - order - family - genus - species.
Every animal belongs to a unique assignment of these 7
categories. The science of taxonomy is very dynamic and animal
classification is frequently changed as new data and analyses
Review binomial nomenclature.
When comparing species we often talk about primitive or
derived characters. Primitive characteristics resemble the
ancestral state and derived characteristics depart from the
ancestral. A horse ancestor had fairly simple teeth and average
length digestive tract. Horses have very complex tooth surfaces
for grinding grass and long digestive tracts for processing
When we look at comparative anatomy the evolutionary history
of an animal is always part of the discussion. The term
phylogeny refers to the evolutionary history of a group.
Anatomical changes occur over a lifetime as well as on an
evolutionary time frame. Ontogeny means the developmental
history of a single individual. If you think about the changes
in head shape and proportions that occur from before birth to
death you can see how great these changes can be.
Embryology is also closely related to comparative anatomy.
Over 100 years ago this was discovered. Von Baer found that
certain traits which are common to large taxonomic groups like a
phylum develop earlier in an embryo than those traits common to
only a small group like a genus. The biogenetic law can be
summed as ontogeny recapitulates phylogeny. Put in English this
means that the evolutionarally oldest traits show up first in an
embryo. If you think back to the chordate traits they show up in
vertebrate embryos and most disappear in mammals.
Natural selection is the force that molds anatomy and
development. Animals can't decide to develop anything. Natural
selection works with the body parts that are available. An
example is ornamentation on male birds. Females like the
showiest males and mate with them. The showy males contribute
the most genes to the next generation. Over time only the showy
genes are left. There is no master plan in molding anatomy.
Rather it is which genes are passed on. The key is which
phenotype reproduces the most.
Another concept we need to discuss involves the timing of
sexual maturity. Paedogenesis is when a larval organism develops
adult gonads. Neoteny is when as adult retains larval traits -
like gills and no eyelids.
Before we wrap up discussing concepts that we need to know
for the semester there is one last term. Vestigial refers to a
structure that remains in a species, but in its ancestors had
greater function. We often say our appendix is vestigial. In
other mammals it serves an important role in digestion.
You all know what extinct means, but you may not be familiar with the term extant. It means that a species is still in living.
When we look at relationships amount groups we want to make sure that the groups we talk about are monophyletic. This
means that all of the animals in the group share a common ancestor.
Another term I want you to be familiar with is heterochrony. This refers to alterations in timing
and rate of development (usually embryonic) that change the body form of adults.
After years of trying to discover relationships between the
vertebrates and invertebrates it has become well established that
the vertebrates are most closely related to the protochordates.
These include the urochordates, cephalochordates, and another
group the hemichordates.
We lump the urochordates and cephalochordates into the
phylum chordata because of the shared characters they have with
vertebrates and because we believe all 3 subphyla share a common
The urochordates can be subdivided into 3 classes, 2 of
which have larvae with the chordate characteristics. The
remaining characteristic, the pharynx with pharyngeal gill slits,
allows the adults to filter feed. When you think of a typical
vertebrate, filter feeding seems very foreign, but the 1st
vertebrates, and many extant ones, are filter feeders.
Interestingly, the larvae don't feed. The notochord is in the
tail. (Fig 3.1, 3.3 - 3.5)
The cephalochordates resemble some of the larval fishes much
more than the urochodates do. They have small muscle segments,
myomeres, that show metamerism. So do many muscles in fish.
They are filter feeders. The gill slits are housed inside of a
chamber called the atrium. Notochord is in the head. (Fig. 3.3, 3.6 - 3.9)
The cephalochordates are unique in that their notochord
extends to the tip of the rostrum. This may be useful for
burrowing animals. Cephalochordates have a brain, but it is not
subdivided in the same way as a vertebrate brain. Vertebrate
brains are closely associated with many sense organs. This isn't
the case for cephalochordates. They have ocelli for sensing
light and some chemoreceptors and tactile receptors.
We have one group of small marine creatures that are hard to
place. These are the hemichordates. At times they have been in
the phylum chordata, but presently aren't. They have certain
characteristics that align them closely with the chordates, but
they also have many differences. Although they aren't in our
phylum we recognize that they are probably closely related to the
chordates. Their name means half chordate. (Fig. 3.2)
We feel confident that some protochordate was the ancestor
from which the earliest, filter feeding fish arose. We don't
know who the ancestor was, but it must have had the basic
Some folks split the hagfish from the rest of the vertebrates and use the name Craniata as the clade that includes both the hagfish and vertebrates.
When learning these names try to speak the language and not
Extant Vertebrate classes:
1) Myxini: jawless vertebrates. Hagfish.
2) Cephalaspidomorphi: jawless vertebrates. Lamprey.
3) Chondrichthyes: cartilaginous fish. Sharks, skates, rays
4) Actinopterygii and Sarcopterygii: bony fish. Typical fish.
5) Amphibia: salamanders, toads/frogs, and caecilians.
6) Reptilia: tuataras, amphisbaenians, lizards, snakes,
turtles, crocodilians, the extinct dinosaurs and Aves, the birds.
7) Mammalia: monotremes, marsupials and true mammals.
The first 4 extant and several extinct classes of
vertebrates belonged to the group known as fishes. All of the
members live in water and were or are adapted to moving, hunting,
breeding and living in water.
* Superclass Agnatha: The jawless fish. Most are extinct except
for the lamprey and hagfish. This group did not give rise to the
other vertebrates, but is the most primitive class.
This was the first living group of animals to posses bone.
The skeletons were largely cartilage, but many extinct agnathes
had bone in their armor-like scales. Unlike other vertebrates
they lack jaws and teeth. They also lack limbs and limb girdles.
They retain the notochord throughout life.
Class Myxini: These are the hagfish
Together these 2 extant forms are known as cyclostomes.
Order Petromyzontiformes - lampreys. The larval form
is called ammocoetes.
Superclass Gnathostomata - all jawed vertebrates. Jaws were derived from the
1st gill arch. This allowed biting and chewing and opened up new
niches for these animals. The gnathostomes have paired
* Class Placodermi: First class to possess jaws and paired
appendages. They had bony plates on their skin and retained a
notochord throughout life. These fish occurred world-wide and
became extinct before reptiles, birds and mammals evolved.
* Class Chondrichthyes: These are mainly marine. They have
little or no internal bone and rarely have it in the scales.
Subclass Elasmobranchii: sharks and rays. They have external gill slits and the 1st gill slit is a round opening -
the spiracle. Sharks are active and predaceous. Rays stay
on the bottom and eat sluggish prey. They are dorsoventrally
Subclass Holocephali: chimaeras. They are marine, lack a
spiracle, but have an operculum covering the gills.
* Bony fish:
They are found world-wide and have been on earth for 400 million years. They are
called the bony fish because most have bone in the skull,
vertebrae, limb girdles, fin supports and scales. The gills are
covered by a bony operculum.
Class Actinopterygii: the ray-finned fish. These are the
most numerous bony fishes. They have a gas bladder or lungs for
breathing and floatation.
Class Sarcopterygii: These have fleshy stalks for their
fins. This includes the dipnoans or lungfish (which at one time
seemed to be the ancestors to higher verts) and the
rhipidistians which may have given rise to higher vertebrates.
Superorder Dipnoi: the lungfish
Tetrapod means 4 legs. This includes amphibians, birds,
reptiles and mammals even though some (i.e. snakes) have lost
their legs. The 4 tetrapod classes are all land dwellers. Some
of them such as aquatic salamanders, sea turtles, dolphins etc
have become secondarily adapted for living in water.
The clade Stegocephali includes the tetrapods and closely related stem-taxa.
The details of this clade are still being resolved but recent fossil evidence shows that the presence of
limbs occurred before tetrapods became terrestrial.
The trek from an aquatic lifestyle -> terrestrial life style
involved a number of anatomical changes before tetrapods could be
In water fins and a streamlined body are needed for
efficient movement. Gills and a gill covering are needed for
respiration. The eye needs no lid or tear ducts. There's no
reason to have a nose with mucus glands for smelling and there's
no need to hear in air. Food is never dry so salivary glands
aren't needed. Joints don't need to support weight. Skin never
dries out and eggs need no protective covering and fertilization
can be external.
The Amphibians were the 1st verts on land. They still are
the most closely tied group to water. When they made the
transition the following major changes occurred:
1. The head became movable.
2. The head became flattened and animals developed a snout.
3. The front of the head became bigger than the rear of the
head - this allowed more room for sensory organs adapted for air.
4. The middle ear bones were modified for air hearing and a
tympanum (eardrum) developed.
5. Changes in the skeleton occurred (i.e. strengthening of
the vertebral column) to accommodate weight support.
6. The ribs were modified to aid in respiration.
7. Elbow, knee. wrist and ankle joints became movable for
8. Muscles were modified for movement in air.
9. A tongue was added to aid in feeding.
10. Mucus secreting salivary glands formed to help in swallowing.
11. Skin glands were added to prevent desiccation.
12. Eyes were modified for air vision and eyelids developed.
13. Internal fertilization developed.
14. With reptiles a shelled egg developed.
Many other changes occurred which allowed life on land.
The amphibians were the 1st tetrapods and probably came from
crossopterygian ancestors. Typically amphibians live in or near
fresh water, are secretive and lay eggs in water. They are found
in all habitats except marine and extremely cold areas.
There are 2 subclasses that we will deal with:
There are 3 orders: 1) Anura - frogs and toads - these
usually have external fertilization. The life cycle is: egg ->
tadpole -> adult. 2) Caudata (Urodela) - salamanders - usually
fertilization in internal. Egg -> larvae -> adult.
3) Gymnophiona (Apoda) - the caecilians - found in the southern
hemisphere and are poorly known and seem to have internal
Subclass Labyrinthodontia - extinct and contained the 1st
Amphibians. These were both aquatic and terrestrial.
Subclass Lissamphibia - all modern amphibians.
This class arose from labyrinthodont amphibians. All have
scales and reproduce without returning to the water. Probably
the most famous are the dinosaurs, but we still have 4 extant
Subclass Anapsida - The name refers to the lack of opening
in the temporal part of the skull. This group contained the stem
reptiles which were small and lizardish and gave rise to the rest
of the reptiles. Also, the order Chelonia which is the turtles
is one of our extant orders.
Subclass Diapsida - These have 2 openings in the temporal
area. All orders are extinct except for 3.
Order Squamata includes snakes, lizards and amphisbaenians.
Most lizards have 4 legs although some families show a reduction
or loss of 1 or both pairs. Amphisbaenians have a front pair or
no legs at all. Snakes, which arose from lizard, lack limbs.
Order Rynchocephalia, the tuatara, is a lizard-like reptile
restricted to several islands of New Zealand.
Order Crocodilia includes alligators, crocodiles, gavials
and garials. The dinosaurs belonged to this subclass as did the order
thecodontia which gave rise to the birds.
Subclass Synapsida - these reptiles had 1 temporal opening.
All are extinct, but the order therapsida - the mammal-like
reptiles - gave rise to the mammals.
It is recognized by many scientists that the class Reptilia is not monophyletic unless the
birds are included within it. Despite this, because of historical reasons the birds are usually
divided out as a separate class and birds are typically not included in herpetology classes.
This group is the most distinctive of the vertebrate classes
and it's hard to confuse a bird with anything else. In part this
is because of the demands for flight. All birds have wings and
feathers. All birds walk on 2 legs - bipedal. Staying in flight
requires a high use of energy and energy consumption by this
group is much higher than in lower vertebrates.
Subclass Archaeornithes - The ancient birds. These had many
reptilian traits, teeth and a long tail with feathers that stuck
out the side. The front limb had claws.
Subclass Neornithes - new birds. All the living birds.
These have a fan arrangement to the tail feathers, modifications
of bones for flight (keel on the breastbone for flight mm to
attach to, hollow bones to decrease weight), no teeth or front
claws. There are 2 superorders (and many orders). Superorder
Paleognathae - large flightless birds (ostrich). Superorder
Neognathae - all other birds.
The most distinguishing traits are hair and milk production.
Mammals have a high energy use like birds and need to eat
frequently. Only birds and mammals have the insulation to keep
body heat from escaping and allow them to maintain a high
constant body temperature.
Subclass Prototheria - Order Monotremata - platypus and
echidna of Australia. They are egg layers and have some
reptilian anatomical features, but they have hair and produce
Subclass Theria - These all give birth to live young. There
are 2 groups. The metatherians have 1 order Marsupialia which
includes all marsupials. The opossum is the only 1 in the U.S.
All marsupials retain the undeveloped young in a pouch after
birth. Eutherians are the typical mammals. All have a placenta
- but so do other groups. There are 16 extant orders (p. 70 -
Last updated on 10 Jan 2007
Provide comments to Lynnette Sievert at email@example.com
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