Chapters 1 & 2
The Vertebrate Story: An Overview
Introduction
We really do not know how eactly, but life on earth began about 3.5 billion years
ago
First organic molecules, then viruses, bacterias, fungi, protozoans, plants and
multicellular animals, all inhabiting all parts of the planet, from the tropics to the
poles, from the deepest part of oceans to the tallest tip of mountains
The diversity of life is astounding
Vertebrates make up a tiny fraction of this biological diversity, but have still been
the focus of a large amount of study
There are ~ 1.5 million named species of extant animals (probably 3-30 million
total), of which only ~50,000 are vertebrates
Vertebrates are arguably among the best studied animal groups
why?
they are often large, visible, and “obvious” to observers
they have always been important human resources
because we are vertebrates, studying them, we are essentially studying
ourselves
People working with biological classification has employed seven basic
taxonomic categories
(listed in decreasing order of inclusiveness)
Kingdom
Phylum (= Division in plants)
Class
Order
Family
Genus
Species
We will talk more about classification, systematics, and taxonomy later
All forms of life are classified into 22 major groups known as kingdoms (11
eukariots and 11 prokariots)
Within each kigndom, each group of organisms with similar characteristics is
classified into a category known as a phylum
Chordates are characterize by the presence (transitory or permanent) of:
1- a dorsal, hollow nerve cord
2- a flexible supportive rod or notochord running longitudinally through the body
ventral to the nerve cord
3- pharyngeal slits or pouches
4- postanal tail
The phylum Chordata is divided into three subphyla: Urochordata (or “tailchordates”), Cephalochordata (or “head-chordates”), and Vertebrata
This class is about the study of the subphylum Vertebrata
Urochordata and Cephalochordata (both referred to as protochordates) are
small, “non-vertebrate” marine animals
To understand their evolutionary significance in relation to the focus of this class
(the vertebrates), we will study and briefly discuss their characteristics.
Much of what we will discuss throughout the semester is directly related to the
science of taxonomy - the process of organizing groups of organisms into a
hierarchical relationship
Classifying objects and events is a critically important way of dealing with the
overwhelming amount of information in our environments
Grouping related things together helps us identify important similarities and
differences among objects/events
Knowing the “rules” for grouping lets us communicate more efficiently with each
other
Biological classification can be a very controversial and complex field – we are
going to grossly oversimplify things here!
Points of consensus among taxonomists are:
-biological classifications should reflect, at least in part, the evolutionary
relationships among organisms
-evolutionary relationships can be inferred from patterns of similarity and
differences among organisms – some of which are more useful than others
Modern taxonomy is based on the founding work of Carolus Linnaeus (Carl von
Linne), an 18th Century Swedish naturalist, which developed the binomial
nomenclature to designate species
He arranged species into hierarchical categories (taxa) for classification
It forms the basis for much of the literature of biology
**We will be discussing the shortcomings of the traditional approach throughout
the course**
The binomial nomenclature consists of genus plus species
Latin or Greek words (or their transformation) are chosen to describe an animal
in some particular way
i.e., Petromyzon marinus - sea lamprey
(noun) + (adjective)
means "stonesucker" "of the sea"
species = groups of interbreeding natural populations that are reproductively
isolated from other such groups (Mayr, 1942)
genus (genera) = all similar species grouped together based on characters that
define the genus
Latin is usually used because it was considered a universal language - routinely
used by scientists instead of common names
The name of the author of a species is often included at the end of the scientific
name. This is not part of the name, but the authority citation
i.e., Sturnira lilium (Geoffroy, 1810) (he defined as Phyllostoma lilium)
Sturnira hondurensis Goodwin, 1940
Scientific names may include a name:
Rana catesbeiana - bullfrog (for Mark Catesby)
or a location where the species was discovered:
Sitta canadensis - Red-breasted Nuthatch (discovered in Canada)
Two primary reasons for taxonomy
• for simple convenience - organizational purposes
• it tries to show phylogeny or evolutionary history
Points of controversy among taxonomists include
• what methods are best for inferring evolutionary relationships
• whether or not other information besides evolutionary relationships should be
considered in constructing classifications
For our purposes, there are two “schools” of systematics we need to identify
(because these are the ones we will be using): “traditional” or evolutionary
systematics and “cladistics” or phylogenetic systematics
Key points about cladistics (phylogenetic) systematics
• cladists developed a very powerful method for inferring evolutionary relationship
based on similarities and differences among organisms:
– symplesiomorphies, (sym-) shared, (plesio-) primitive (morphies) characters =
traits unchanged from an ancestor that are inherited by a group of lineages
• tell us that members are related, but do not give us fine-scale resolution (i.e.,
which two are most closely related, etc.)
• i.e., backbones in sharks, lizards, mammals: because we all have them, we
are related, but does not tell us which two are most closely related
– apomorphies, (apo-) unique, derived (morphies) characters = traits that are
different from ancestors and unique for a particular lineage
• tells us that the lineage is different from others, but can not be used to “link”
lineages evolutionarily
• i.e., of sharks, lizards, and mammals, only mammals have fur – does not help
us figure out which is our closest relative
– synapomorphies, (syn-) shared, (apo-) derived (morphies) characters = traits
that are different from an ancestor and shared by two or more lineages
• tell us that those lineages are more closely related to each other than they
are to other lineages
• i.e., of sharks, lizards, and mammals, lizards and mammals have forelimbs
modified for weight-bearing on land; that feature unites mammals and
amphibians, indicating close evolutionary relationship
• as we study different groups of vertebrates, we will identify key shared primitive
(symplesiomorphies), shared derived (synapomorphies), and unique derived
(apomorphies) characteristics
• cladists hold that only evolutionary relationships should be reflected in
classification – nothing else
Results of using “strict” cladistic classification:
(a) formal names are given only to monophyletic groups = groups consisting of a
single common ancestor and all descendents (this is important because
monophyletic lineages are “real” evolutionary entities; need to identify them for all
kinds of evolutionary studies)
(b) in order to reflect complex patterns of relationship, strictly cladistic
classifications are extremely complex – too much so for us to worry about!
Key points about traditional (evolutionary) systematics:
• in addition to evolutionary relationships, classifications can legitimately reflect
other information:
– age of lineage (older lineages are placed “higher” in the classification)
– uniqueness of lineage (lineages that have changed most over time are
placed “higher” in the classification)
– biological similarities and differences that do not necessarily reflect close
evolutionary relationship (lineages that are not closely related may be grouped
together because of other kinds of similarities)
Result of evolutionary systematics
– formal names may be given to paraphyletic groups =not all descendants of a
common ancestor are included (this is important because paraphyletic groups
are not “real” evolutionary entities in the same way monophyletic groups are)
- classifications are simpler than those developed by cladists
We will use features of both schools of thought
• we will use cladograms and cladistic philosophy to discuss evolutionary
relationships among vertebrate groups
• we will use modified traditional classifications for our taxonomy
Illustration: i.e., reptiles and birds
• represent patterns of relationship using a diagram called a cladogram:
– nodes = presumed common ancestors
– lines = characteristics:
• unless the character is changed, every taxon above the line has that same
characteristic
• when lines are drawn at nodes, all lineages coming off the node have the
characteristic
• according to this diagram, Aves and Saurischia are more closely related to each
other than either is to Ornithischia; Aves and Saurischia are more closely related
to Ornithischia than they are to Crocodilia
Which characters are primitive, or derived?
These terms are relative, so need to specify that we are talking about all four
taxa:
• character #1 (diapsid skull, e.g.): all lineages have it unmodified, so it is shared,
primitive (symplesiomorphy) (and does not tell us which groups are most closely
related)
• character #2 (features of post-cranial skeleton) is new, and it is shared among
Ornithischia, Saurischia, and Aves – so it is a shared, derived character
(synapomorphy) for the lineage (O + S + A) and tells us that these three groups
shared a recent common ancestor (making them more closely related to each
other than they are to crocs)
• character #3 (more features of post-cranial skeleton) is new, and it is shared
between Saurischia and Aves – so it is a shared, derived character for the
lineage (S + A) and tells us that these two are the most closely related groups
• character #4 (feathers) is new, and it is unique to Aves – it is a unique derived
(apomorphy) trait and tells us that Aves is a novel lineage, but does not tell us
about its relationships to other lineages
How do cladists and traditional systematists classify these lineages?
Traditional systematists argue:
• feathers make birds biologically unique, so birds should be in their own class
(class Aves)
• Ornithischia and Saurischia had sufficient biological similarities to group them
together into the Dinosauria
• Crocodiles are old and relatively unchanged; they belong in their own group
(Crocodilia)
• Dinosaurs and crocs are similar enough to group together in the class Reptilia
• in this scheme, Reptilia is paraphyletic: it excludes Aves, one of the
descendents of the common ancestor of all four groups
Cladists argue that the only thing that matters is the evolutionary relationship
• Aves is one descendant of the common ancestor of all 4 groups; if crocs and
dinosaurs are included in Reptilia, Aves is part of Reptilia
• in fact, if Ornithischia and Saurischia are grouped as Dinosauria, Aves is also
part of Dinosauria
Who’s right?
both and neither; each way of doing things has its own advantages and
disadvantages, which is why we will use both!
Vertebrates in relationship to the Chordata and related phyla
Vertebrates are a monophyletic group and a subphylum of the phylum Chordata
Closest non-chordate relatives are echinoderms and hemichordates; shared
primitive characters (because they were present before earliest chordate
common ancestor) with those lineages (#1 on the diagram) include
1. bilateral symmetry
2. coelomate body plan
3. segmentation
4. deuterostome development
Vertebrates are placed in the Phylum Chordata (#2 on the diagram), which are
characterized as:
• multicellular, eukaryotic organisms
• having bilateral symmetry (two halves) and cephalization (a head end)
• coelomate (a tube within a tube)
• a complete digestive tract
• the blastopore develops into the anus (deuterostomes); remember: the
blastopore develops into the mouth in protostomes
• segmented - body plan of repeated structures
Four distinctive derived characteristics (apomorphies) of chordates distinguish
them from their ancestors:
1- Notochord, or a rod of vacuolated cells, encased by a firm sheath that lies
ventral to the neural tube in vertebrate embryos and some adults
2- Hollow nerve cord that lies dorsal to the notochord
3- Pharyngeal slits or pouches - thought to have evolved for filter-feeding, and
secondarily for respiration
4- Muscular postanal tail
Also share the derived Endostyle - elongated groove in the pharynx floor of
protochordates that may develop as the thyroid gland in chordates
• In the subphylum Vertebrata, all members possess the four chordate
characteristics at some time in development, but often these structures are
altered significantly in adult animals
– a cartilagenous or bony endoskeleton including a vertebral column and
braincase is a derived character (apomorphy) for all vertebrates (#3 on diagram –
it is derived because it is new for vertebrates)
• These characteristics may be found in some of the ancestors of chordates and
are commonly placed in an informal grouping called Protochordates
(urochordates & cephalochordates). These serve as living representations of the
missing fossils in vertebrate evolution
The Phylum Chordata may be subdivided into two groups as:
Acraniata - without a cranium
Includes the Subphyla Urochordata and Cephalochordata (part of the
protochordates)
-no brain or skull
-no paired appendages and no backbone
-generalized development
-egg => tadpole => sessile adult
Craniata - with a brain and braincase (skull)
Subphylum Vertebrata
-a vertebral column or backbone engulfs and/or obliterates the notocord
-paired appendages
PROTOCHORDATES OR ACRANIATES
Phylum Hemichordata - acorn worms and pterobranchs
Hemichordates are a group of organisms that show an affinity to the chordates,
but are lacking some key characteristics of chordates
They include two groups: Enteropneusta (acorn worms) and Pterobranchia
(pterobranchs)
Acorn worms are 2 cm to 1.5 m long; marine in shallow waters, solitary, live in
mud or vegetation; filter-feeders
They have well-developed gill slits, and a stomochord, at one time thought to be
homologous with the notochord
They also have a dorsal strand of nerve cells, believed to be the precursor to the
dorsal hollow nerve cord
Example genera: Saccoglossus, Dolichoglossus
Pterobrancs are tiny, deep-sea, colonial, moss-like animals. There is no trace of
dorsal nerve cord or notochord, and only one pair of gill slits in species of the
genus Cephalodiscus
Balanoglossus has some characteristics in common with chordates, such as gill
slits and a dorsal nerve cord; however, this genus also has a ventral nerve cord,
and the nerve cords in general are not hollow like most chordates, but instead
are solid
This particular genus has a worldwide distribution, lives in shallow sea water, and
can range between a few centimeters to up to two meters (6' 6" !!)
In addition, it also lacks a notochord. It does have a structure called the
stomochord, or a diverticulum (blind sac) that is made up of cells that resemble
those found in the notochord, but has a different developmental pathway. So, it
should be clear why these species are called Hemichordates, and are not
included with the true chordates
Subphylum Urochordata (Phylum Chordata)
Tunicates/sea squirts - Urochordates are all marine, and are enclosed in a tough
cellulose-like tunic (hence the common name tunicate)
Tunicates are members of the true chordates, and represent some of the most
primitive ancestors of the Subphylum Vertebrata
Most of the 2,000 species belong to the taxon Ascidiacea (or sea squirts)
This group undergoes complete metamorphosis from a mobile larva to a sessile
adult, resorbing the tail and notochord
Some are solitary, most are colonial. The few remaining species of tunicates
belong to the taxa Thaliacea and Appendicularia (larvaceans)
Thaliaceans lack a tail and notochord; they have no known larval stage. They are
small, free-swimming, pelagic barrel-shaped animals that use jet propulsion
Appendicularians do not metamorphose, and are able to reproduce as freeswimming larvae
Tunicates differ strongly in appearance between the adult and larva
The larval form possesses more of the chordate characteristics than the adult
form (see figure)
Adult is sessile (and sometimes colonial), and must obtain food by siphoning sea
water through its body and trapping food particles in the endostyle
Larvae are tadpole-like and free-living, and have an endostyle, gill slits, dorsal
nerve cord, and notochord. The larval stage lasts only a few days, and ends
when the larva attaches to a substrate and metamorphoses into an adult
***This example should illustrate to you that although chordates are said to have
four basic things in common, these characteristics need not be retained
throughout life for an animal to be considered a chordate. Rather, they must only
be present in an individual at some time during development***
Subphylum Cephalochordata (Phylum Chordata)
Amphioxus, the last group are the cephalochordates, which are usually
represented by one organism - Branchiostoma lanceolatum , commonly called
Amphioxus (which means "sharp at both ends")
Amphioxus are 2-3 inches in length, and live on seashores throughout the
temperate zone. Fish-like in appearance, it has a laterally compressed dorsal fin,
but it does not have complete organs, or any bony structures
Amphioxus shows some cephalization, in that the primary feeding structures are
concentrated at the anterior end, and it has a pigment spot on the anterior end
that may be used for orienting toward light
Origin of Free-Swimming Vertebrates
In contrast to protochordates (hemichordates, urochordates, and
cephalochordates), vertebrates are actively-feeding, predatory organisms that
move by lateral undulation of an elongate body
Cephalochordates are like vertebrates in having the derived feature of an
elongate body as adults, but are still (primitively) filter feeders; that is, they feed
while motionless, moving food-laden water by means of cilia on their gill bars
Hemichordates and most urochordates are also filter-feeders, moving water
through their gill slits, but are sessile as adults. When ascidian tunicates
metamorphose, the notochord is resorbed
Note, however, that ascidian and larvacean urochordates have a free-swimming
larval stage (with a notochord); ascidians metamorphose to sessile adults, but
larvaceans become sexually mature as mobile "larvae"
These observations have led workers to suggest that the freely-swimming mode
of locomotion of vertebrates (and cephalochordates) evolved by retaining the
form of the larvae of the "ancestors" (hemichordates and urochordates) as the
form of the adults of the descendants (cephalochordates and vertebrates)
This general phenomenon is called “paedomorphosis” (pedomorphosis): the
evolutionary retention of larval features of the ancestors as the adult features of
the descendants
CRANIATES
Subphylum Vertebrata
General characteristics of vertebrates
Vertebrates may be characterized by 12 general derived characteristics
You should become very familiar with these traits, and identify how they are
expressed in the representative vertebrates you are seeing in labs/field
1. Bilateral symmetry
2. Two pairs of jointed locomotor appendages, which can include fins (pectoral
and anal/dorsal fins, as well as the forelimbs and hindlimbs)
3. Outer covering of protective cellular skin, which can be modified into special
structures such as scales, hair, and feathers
4. Metamerism found in skeletal, muscular, and nervous system. This includes
ribs, vertebrae, muscles, and ganglia/peripheral nerves
5. Well-developed coelom or body cavity completely lined with epithelium
(cellular tissue of mesodermal origin) that may be divided into 2 to 4
compartments
6. Well-developed internal skeleton of cartilage and/or bone, separated into axial
skeleton (skull, vertebrae, ribs, sternum) and appendicular skeleton (girdles and
appendages)
7. Highly developed brain enclosed by skull and nerve cord enclosed by
vertebrae - these provide advanced neural structures that are highly protected
from damage
8. Well-developed sense organs (eyes, ears, nostrils) located on the head
(cephalization)
9. Respiratory system, including (primary) either gills or lungs, located closely to
the pharynx or throat
10. Closed circulatory system with ventral heart and median dorsal artery (aorta)
11. Genital and excretory systems closely related, utilizing common ducts and
pathways
12. Digestive tracts with two major digestive glands (liver and pancreas) that
secrete into it
Grouping the vertebrate classes
Within the Subphylum Vertebrata are seven recognized extant Classes
representing more than 57,000 species:
• Agnatha = the jawless fishes; ~ 90 species
– Myxini - hagfishes
– Cephalaspidomorpha - lampreys
• Chondrichthyes - cartilagenous fishes; ~ 950 species
– Elasmobranchii = sharks, skates and rays
– Holocephali = ratfish
• Osteichthyes - bony fishes; ~ 27,000 species
– Sarcopterygii = lobe-finned fishes (lungfish, coelocanth); ~ 4 species
– Actinopterygii = ray-finned fishes; all the rest
• Amphibia - frogs, toads, salamanders, and caecilians; ~ 5,500 species
– Caudata = Urodela = salamanders; ~ 500 species
– Anura = Salienta = frogs; ~ 4,800 species
– Gymnophiona = caecelians; ~ 165 species
• Reptilia - turtles, snakes, lizards, and crocodilians; ~ 8,700 species
– Testudinomorpha = turtles; ~ 300 species
– Crocodilia = crocodiles, alligators; ~ 23 species
– Lepidosauria = snakes, lizards, tuatara (~ 7,750 species)
• Aves - birds; ~ 9,700 species
• Mammalia - mammals; ~ 5,000 species
– Prototheria - Monotremata = egg-laying mammals (3 species)
– Metatheria = Marsupials = pouched mammals (~ 275 species)
– Eutheria = placental mammals; ~ 4,750 species
MORPHOLOGICAL CONCEPTS
To analyze design, concepts of form, function, and evolution I have developed a
list of concepts that will help you understand more the entire picture. These
concepts are grouped based on similarity, symmetry, and segmentation
Similarities - corresponding parts may be considered similar to each other by:
- Homology - two or more features that share a common ancestry: bird’s wing
and mole’s arm may be traced back to common ancestral reptile
- Serial homology - special case with similarities between successively repreated
elements in the same organism: vertebral collumn, muscle segments
- Analogy - features with a similar function: wings of bats and bees similar in
function but of different ancestral structural origin
- Homoplasy - features that simply look alike; may or may not be homologous or
analogous: turtle and dolphin flippers; insect wings which look like leaves but
cannot photosynthesize
Symmetry - how the body meets the surrounding environment:
-radial symmetry - the body is laid out equally from a central axis; any of several
planes passing through the center divids the animal into equal halves
-bilateral symmetry - only the midsagittal section divides the body into two equal
halves
Segmentation - a body built of repeated or duplicated segments (metameres)
separated by a series of septa
Body regions are described by basic terms of:
- anterior = head end (cranial/superior)
- posterior = tail (caudal/inferior)
- dorsal = back
- ventral = front
- the midline is medial; the sides lateral
- attached appendages have a distal (farther away) and proximal (closer) portion
- the pectoral region or chest supports the forelimbs
- the pelvis region refers to the hips which support the hindlimbs
- a frontal plane divides the body into dorsal and ventral sections, sagittal plane
into left and right, and transverse plane into anterior and posterior portions
Cephalization is the pronounced tendency for the anterior end of the body to
become more and more distinctly separated and differentiated from the rest of
the body as a head
During cephalization, the brain and sense organs become centralized at the
head, and there forms a greater elaboration of the feeding apparatus, which
includes jaws, musculature, teeth, beaks, tongues and glands
EVOLUTIONARY MORPHOLOGY
The concept of function covers both how a part works and how it serves
adaptively in the environment - cheek muscles of a mouse function both within an
organism (chewing) and by meeting environmental demands (resource
processing), which are defined by:
-function: the action or property of a part as it works in an organism
-biological role: how the part is used in the environment during the course of the
organism’s life history
Preadaptation
a structure or behavior posesses the necessary form and function before the
biological role arises that it eventually serves - feathers in birds probably served
as insulation to conserve body heat prior to development of flight
(thermoregulation now a secondary function)
Evolutionary change involves continuous renovations - old parts are altered but
new parts rarely added
Comparisons among characters require careful use of terminology defining
relationships - traits may be Primitive/Generalized/Derived/Specialized
An important distinction to make is among the terms. They are not necessarily
interchangeable, and should be used carefully when describing morphology
**Primitive and derived are antonyms**
-Primitive - structures that are similar to that of the ancestors or shared by all
living groups
-Derived - structures that are different from that of the ancestors
**as are generalized and specialized**
-Generalized - modified to perform a variety of functions
-Specialized - modified to perform restricted functions
As an example:
In mammals, the pentadactyl (five phalanges) condition is primitive, in that it is
found in all living groups. However, there is a derived condition in some
mammals, such as the bat wing, in which the first digit is elongated, or in the
horse foot, which is reduced completely to a single digit.
In contrast, our anterior phalanges (fingers) are generalized, in that they can
perform a number of different functions, from playing the piano to carving a
sculpture. However, our posterior phalanges (toes) are specialized, and can
usually only perform the function of balance and walking
PHYLOGENY
-The course of evolution (phylogeny) is often summarized in dendrograms
(schematic diagrams) that depict treelike branched connections between groups
-Phylogenies serve as a graphical representation of the evolutionary
relationships of organisms
-They may show which organisms branched off first from a common ancestor
-They may give information on the relative abundance of these taxa
Each branch in the dichotomous branching pattern signifies a point at which two
taxa diverge based on some morphological or other character trait
All extant species usually listed in a line at the top. Extinct species’ lines do not
meet up with those of extant species
PALEONTOLOGY
Vertebrate evolution was once referred to as the "Vertebrate Story" by
paleontologist Alfred Romer - unfolds across millions of years with roughly 99.9%
of all species which ever to have evolved now extinct
All that survives are their remnants, the fossils are sketchy vignettes they tell of
the structure and early history of vertebrates
Fossil remnants may include bones, teeth, eggs, small boney elements
(embryos, diet?), feces, DNA traces - fossil dating, restoration, and
reconstruction lead to an improved understanding of the past