Animals with a nerve cord along the back and gill slits during some developmental stage are called

Basic information about the vertebrate animals

Vertebrates make up a tiny fraction of total 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 ~50,000 are vertebrates

Vertebrates are arguably among the best studied animal groups � why?

they�re often large, visible, and �obvious� to observers

they have always been important human resources

because we�re vertebrates, we are essentially studying ourselves

• 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 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

• developed the binomial nomenclature to designate species
• arranged species into hierarchical categories (taxa) for classification
• forms the basis for much of the literature of biology

• usually Latin or Greek words that are chosen to describe an animal in some particular way

• 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

The name of the author of a species is often included at the end of the scientific name

Scientific names may include a name:

• Rana catesbeiana - bullfrog (for Mark Catesby)

• Sitta canadensis - Red-breasted Nuthatch (discovered in Canada)

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

Key points about cladistics (phylogenetic systematics)
� cladists developed a very powerful method for inferring evolutionary relationship based on similarities and differences among organisms:

� shared, primitive characters = traits  unchanged from an ancestor that are inherited by a group of lineages
    � tell us that members are related, but don�t give us fine-scale resolution (i.e., which two are most closely related, etc.)
    � e.g., backbones in sharks, lizards, mammals: because we all have them, we�re related, but doesn�t tell us which two are most closely related
� unique, derived characters = traits that are different from ancestors and unique for a particular lineage
    � tells us that the lineage is different from others, but can�t be used to �link� lineages evolutionarily
    � e.g., of sharks, lizards, and mammals, only mammals have fur � doesn�t help us figure out which is our closest relative
� shared, derived 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
    � e.g., 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

(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!

� in addition to evolutionary relationships, classifications can legitimately reflect other information, e.g.:
    � 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 don�t necessarily reflect close evolutionary relationship (lineages that aren�t 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�ll use cladograms and cladistic philosophy to discuss evolutionary relationships among vertebrate groups
� we�ll use modified traditional classifications for our taxonomy

� 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 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

� characteristic #1 (diapsid skull, e.g.): all lineages have it unmodified, so it�s shared, primitive (and doesn�t tell us which groups are most closely related)
� characteristic #2 (features of post-cranial skeleton) is new, and it�s shared among Ornithischia, Saurischia, and Aves � so it�s a shared, derived character 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)
� characteristic #3 (more features of post-cranial skeleton) is new, and it�s shared between Saurischia and Aves � so it�s a shared, derived character for the lineage (S + A) and tells us that these two are the most closely related groups.
� characteristic #4 (feathers) is new, and it�s unique to Aves � it�s a unique derived trait and tells us that Aves is a novel lineage, but doesn�t tell us about its relationships to other lineages

� 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

� 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�ll use both!

1. bilateral symmetry
2. coelomate body plan
3. segmentation
4. deuterostome development

� 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); the blastopore develops into the mouth in protostomes
� segmented - body plan of repeated structures

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
Hollow nerve cord that lies dorsal to the notochord
Pharyngeal slits or pouches - thought to have evolved for filter-feeding, secondarily for respiration
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

� a cartilagenous or bony endoskeleton including a vertebral column and braincase is a derived character for all vertebrates (#3 on diagram - it�s derived because it�s new for vertebrates)

The Phylum Chordata may be subdivided into two groups as:

Acraniata - without a cranium

• no brain or skull
• no paired appendages and no backbone
• generalized development

• Includes the Subphyla Urochordata and Cephalochordata

• Subphylum Vertebrata - a vertebral column or backbone engulfs and/or obliterates the notocord
• paired appendages

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): These 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

• Pterobranchia (pterobranchs): These 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 species also has a ventral nerve cord, and the nerve cords in general are not hollow like most chordates, but instead are solid. This particular species 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, this species 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

• 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 2000 species belong to the taxon Ascidiacea (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 free-swimming larvae.

• 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.

• 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.

• 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: the evolutionary retention of larval features of the ancestors as the adult features of the descendants.

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 lab.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 either gills or lungs, located closely to the pharynx or throat

10. Closed circulatory system with ventral heart and median dorsal artery

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

Within the Subphylum Vertebrata are eight recognized extant Classes representing more than 42,000 species:

� Agnatha = the jawless fishes; ~ 80 species
   � Myxini - hagfishes
   � Cephalaspidomorpha - lampreys
� Chondrichthyes - cartilagenous fishes; ~ 900 species
   � Elasmobranchii = sharks, skates and rays
   � Holocephali = ratfish
� Osteichthyes - bony fishes; ~ 24,000 species
   � Sarcopterygii = lobe-finned fishes (lungfish, coelocanth); ~ 7 species
   � Actinopterygii = ray-finned fishes; all the rest
� Amphibia - frogs, toads, salamanders, and caecilians; ~ 4300 species
   � Caudata = Urodela = salamanders
   � Anura = Salienta = frogs (4100 species)
   � Gymnophiona = caecelians
� Reptilia - turtles, snakes, lizards, and crocodilians; ~ 7000 species
   � Testudinomorpha = turtles
   � Crocodilia = crocodiles, alligators
   � Lepidosauria = snakes, lizards, tuatara (~6850 species)
� Aves - birds; ~ 9650 species
� Mammalia - mammals; ~ 4500 species
   � Prototheria - Monotremata = egg-laying mammals (3 species)
   � Metatheria = Marsupials = pouched mammals (~275 species)
   � Eutheria = placental mammals

• Pisces - a collective term for all fishes and includes the Myxini, Cephalaspidomorpha, Chondrichthyes, and Osteichthyes
• Tetrapoda - collective term for the terrestrial vertebrates which have four limbs unless some have been secondarily lost or converted to other uses; includes Amphibia, Reptilia, Aves, and Mammalia

• Agnatha - jawless vertebrates including the Myxini and Cephalaspidomorpha
• Gnathostomes - vertebrates with jaws derived from the mandibular arch, which may have (in primitive vertebrates) supported gills; includes Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, and Mammalia

• Anamniotes - vertebrates that lack an amnion, or extraembryonic membrane that surrounds the embryo and encases it in amniotic fluid; includes Myxini, Cephalaspidomorpha, Chondrichthyes, Osteichthyes, Amphibia
• Amniotes - vertebrates that possess an amnion; includes Reptilia, Aves, Mammalia

• no jaws
• no paired appendages
• a completely cartilagenous skeleton
• a single nostril
• 6 - 14 external or concealed gill slits
• a persistent notochord
• a two-chambered heart

• Class Myxini
• Order Myxiniformes - hagfishes
• restricted only to saltwater habitats
• Class Cephalaspidomorpha
• Order Petromyzontiformes - lampreys
• found in both fresh and saltwater habitats

• temperate, marine deep water
• feed on detritus and carrion, as well as polychaete worms
• use sensitive tentacles around their mouths in locating prey
• single nostril opens into pharynx

• temperate, anadromous (hatch/breed in fresh water, mature in marine and freshwater)
• parasitic as adults - attach to other fishes with suction-like mouths and rasp a hole in the skin
• buccal glands secrete an anticoagulant to ensure free-flowing food source
• larvae are called ammocoetes, resembling the amphioxus - primarily detritus feeders until they metamorphose into adults, sometimes after 6 or 7 years as a larva

• Evolution of jaws represents an advancement in morphology, expanding the function of the mouth to a wider range of potential prey types. Thus, the jaws are an example of a derived structure that is more generalized than its ancestral form.

• skeleton completely cartilaginous with no endoskeletal bone; jaw present
• paired appendages
• paired nostrils
• no swim bladder
• scales dermal placoid when present
• gill arches internal to gills
• approx. 800 freshwater and marine species - most filter-feeders

• 5-7 gill openings plus spiracle anterior to first gill
• upper jaw not attached directly to the braincase
• teeth derived from placoid scales, deciduous and continually replaced
• claspers present in males, internal fertilization, ovoviviparous (egg contained within the uterus, where the young develop and then hatch as miniature adults) or viviparous (embryos develop internally and then emerge as a miniature adult)
• modern species present by end of Mesozoic

• almost purely predaceous/marine
• heterocercal tail fin - caudal fin is longer on the dorsal side than on the ventral side

• greatly flattened bottom dwellers
• scales not over entire body
• pectoral fins winglike
• crushing teeth - mollusk eaters
• spiracles greatly enlarged
• oviparous - produce an egg pouch covered in a very tough shell

• upper jaw fused to braincase
• flat, bony plates instead of teeth
• operculum covering gillslits
• strictly marine feeding on mollusks

• endoskeleton made up of bone
• jaws and paired appendages
• gill arches internal to gills, single gill opening covered by bony operculum
• dermal scales not placoid
• many forms have swim bladder
• appeared in Devonian - dominant vertebrates since mid Devonian
• arose in freshwater, moved into saltwater
• approx. 20-25,000 species in fresh and salt water, some partially terrestrial

• fin rays attach directly to girdles
• internal nostrils - nares absent
• single gas bladder
• known from Devonian

• general primitive form
• typically small
• skeleton primarily cartilage
• heterocercal tail
• ganoid scales
• most died out by end of Mesozoic

• Division Ginglymodi - garpike
• moderate ossification of skeleton
• heterocercal tail
• elongated jaws
• ganoid scales
• dominant during Mesozoic
• Division Halecostomi
• Subdivision Halecomorphi - bowfin
• represented by single freshwater species Amia calva
• cycloid scales
• almost homocercal tail

• skeleton mostly bony
• tail typically homocercal
• no spiracle
• scales ctenoid or cycloid
• known from marine forms first, originating from Holosteans during Mesozoic
• major radiation associated with modifications in locomotor or feeding mechanisms and high fecundity, i.e.

• fleshy lobed fins so that fin rays do not articulate directly to girdles
• internal and external nares
• many retain the heterocercal tail
• the coelacanth is represented by a single species that lives off the Comoro Islands near Madagascar

The rise of the Tetrapod Classes and the movement from water to land represents one of the major evolutionary events in the history of vertebrates. New structural designs were required to make the transition to land in order to cope with increased oxygen levels, decreased water supply, more fluctuating ambient temperature, and slight changes in the way sensory information is obtained. Primary transition to land involved the development of four limbs.

Class Amphibia - Amphibians

• The name translates as "double life", referring to life on both land and in water
• arose from Crossopterygian, Rhipidistian ancestors
• three extant orders, two extinct subclasses with ~3,600 species
• lungs and skin used as adult respiratory organs
• gills present in larvae, retained into adulthood in some neotinic forms (salamanders)
• heart with two atria and one ventricle - "three chambered"
• skin is naked or with bony dermal elements
• ectothermic - must regulate body temperature by moving to different microclimates within its environment
• group includes smallest terrestrial vertebrates up to some 5� in length
• name implies a continued tie to water - eggs must be laid in water or at least in very moist environment; young develop as gill breathing, water-dwelling tadpoles
• embryos lack an amnion, but eggs are laid in a jelly-like protective coating

• tail maintained throughout life
• limbs 1 -2 "normal" pairs
• elongated trunk and long tail
• can retain larval characteristics (flattened, shovel-shaped head, fleshy tail, external gills) in adult forms (paedomorphic) - the result is a sexually mature individual with many other body parts in the larval or juvenile condition (neoteny)

• loose tail as adults
• caudal vertebrae fuse to form long inflexible urostyle - relates to saltatorial locomotion
• long hind limbs developed for saltatorial locomotion
• vocal cords well developed
• ear modified for reception of airborne sound waves

• elongated, snake-like, with no limbs or girdles
• no vocal cords or airborne sound detection
• some retain scales embedded in skin
• notochord persists
• minute eyes, lack lids
• chemosensory tentacle on head

• first fully terrestrial vertebrates with ~6,000 species
• development of cleidoic (closed, self-contained) egg; embryo with extra-embryonic membrane and relatively impermeable shell
• lungs for respiration
• heart with two atria and ventricle partially or totally (Crocodilians) divided
• one occipital condyle
• skin with epidermal scales or bony plates
• ectothermic, sometimes called heliotherms because they can regulate body temperature by using solar radiation
• first appeared in late Paleozoic, so numerous by Mesozoic known as "Age of Reptiles"

• ribs modified along with epidermal plates to form shell - carapace and plastron
• girdles inside ribs
• jaws covered with horny epidermal plates, no teeth
• little change since Triassic

• contains most modern reptiles
• lizards known from Cretaceous, snakes in Cenozoic
• skull has lost one or both temporal regions
• vertebrae usually procoelous
• abdominal ribs usually greatly reduced or absent
• body covered with horny epidermal scales
• quadrate bone moveable
• teeth set in sockets

• diapsid skull
• contains dinosaurs and ancestors to birds

• quadrate fixed
• bony plates embedded in epidermis
• teeth set in sockets
• abdominal ribs present in Gastralia
• ventricles completely separated
• developed secondary palate
• "crop" similar to birds

• second only to bony fish in diversity with ~9,000 species
• endothermic rather than ectothermic
• the reptile scale has evolved into a feather which is the only unique characteristic of this class
• modifications for flight include hollow bones, pectoral appendages modified as wings, air sacs, large eyes and large cerebellum
• four-chambered heart
• epidermal scales on bill, legs, feet
• bill instead of teeth; teeth absent in modern forms
• modifications for vocalization
• species distributed to both poles
• two primary groups of birds

• passerines (Passeriformes): ~ 5,500 perching birds
• non-passerines: the remainder of orders with ~4,500 species

• represented by ~4,100 species in almost all habitats
• possess hair/fur as an integumentary derivative
• mammary glands to nourish young
• endothermic
• viviparous (oviparous in one order)
• two occipital condyles
• zygomatic arch and secondary palate
• single dentary bone in lower jaw
• dentary-squamosal jaw articulation
• muscular diaphragm
• arose from synapsid reptiles which branched off at base of reptilian tree

• oviparous
• mammary glands without nipples
• cloaca still present
• pectoral girdle with separate precoracoid, coracoid, and interclavicle bones

• viviparous, young born extremely altricial
• abdominal skin pouch (marsupium) supported by epipubic bones
• lack typical chorioallantoic placenta, have yolk-type
• vagina doubled, no cloaca
• mammary glands located inside marsupium
• restricted to New World tropics and Australia

• viviparous
• chorioallantoic placenta
• vagina single
• mammary glands with external nipples
• precoracoid and interclavicle gone
• arose in Cretaceous, great radiation of insectivore-like ancestors during Cenozoic

To analyze design, concepts of form, function, and evolution have developed which address 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

• 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.

- 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

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.

Evolution and morphology have not always been happy companions - cooperation between disciplines has led to concepts of design and change in design.
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

Derived - structures that are different from that of the ancestors

Specialized - modified to perform restricted functions

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.

Phylogenies serve as a graphical representation of the evolutionary relationships of organisms. They may show:

• which organisms branched off first from a common ancestor
• may also give information on the relative abundance of these taxa

All extant species usually listed in a line at the top. Extinct species� lines do not meet up with those of extant species.

Vertebrate evolution was once referred to as the "Vertebrate Story" by paleontologist Alfred Romer - unfolds across 590 million years with roughly 99.9% of all species which ever to have evolved now extinct

All that survives are their remnants, the fossils and scetchy 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

What are structural similarities between species that are based on common function and not on common evolutionary descent called?

What is the evolutionary process that produces analogous structures called?

What animals are chordates but not vertebrates?

What characteristics are common among animals with backbone?