Vertebrate Structure and Development
Lecture Notes

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 life.

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 rather abruptly.

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.

Chapter 2

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).

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?
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
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.

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 arrive.

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 vegetation.

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 ancestor.

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 chordate characteristics.

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.

Extant Vertebrate classes:
1) Myxini: jawless vertebrates. Hagfish.
2) Cephalaspidomorphi: jawless vertebrates. Lamprey.
3) Chondrichthyes: cartilaginous fish. Sharks, skates, rays and chimeras.
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.
When learning these names try to speak the language and not just memorize.


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
Class Cephalaspidomorphi:
Order Petromyzontiformes - lampreys. The larval form is called ammocoetes.
Together these 2 extant forms are known as cyclostomes.

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 appendages.

* 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 flattened.
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
Superorder Crossopterygii:


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 fully terrestrial.

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 locomotion.
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.
Class Amphibia

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:
Subclass Labyrinthodontia - extinct and contained the 1st Amphibians. These were both aquatic and terrestrial.
Subclass Lissamphibia - all modern amphibians.
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 fertilization.

Class Reptilia

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 orders.

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.

(Class) Aves

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.

Class Mammalia

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 milk.
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 - 72).

Last updated on 10 Jan 2007
Provide comments to Lynnette Sievert at
Return to the Vertebrate Structure and Development Home Page at Emporia State University.