Diversity of Vertebrates

This chapter will be a veritable "parade of taxa", as we start tracing the evolution of vertebrates and the derived characters that distinguish them from the chordates that we discussed in the last lecture.

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 vertebrates you will see 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 was described in a previous lecture - structures can include ribs, vertebrae, muscles and ganglia/peripheral nerves.

5. Well-developed coelom, or body cavity completely lined with epithelium (cellular tissue), that may be divided into 2 to 4 compartments.

6. Well-developed internal skeleton of cartilage and 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. This provides 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, and 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.

Grouping the vertebrate classes

Within the vertebrate classes, there are further ways of subdividing the groups based on derived characteristics. There are eight recognized extant classes of vertebrates(Figure 3.1, p. 81 in text):

Myxini - hagfishes

Cephalaspidomorpha - lampreys

Chondrichthyes - cartilagenous fishes

Osteichthyes - bony fishes

Amphibia - frogs, toads, salamanders, and caecilians

Reptilia - turtles, snakes, lizards, crocodilians

Aves - birds

Mammalia - mammals

These can be grouped based on their general habitat requirements: Pisces - collective term for all fishes; includes Myxini, Cephalaspidomorpha, Chondrichthyes, Osteichthyes

Tetrapoda - collective term for the terrestrial vertebrates; they have four feet unless some have been secondarily lost or converted to other uses. Includes Amphibia, Reptilia, Aves, Mammalia

Or based on their feeding habits: Agnatha - jawless vertebrates, including 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, Mammalia

Or based on their embryonic characteristics 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.

Don't let these different terms confuse you! They are all ways of distinguishing taxa based on primitive versus derived traits. Use them to help you memorize and classify the taxa we will be talking about.

Tracing vertebrate evolution through the fossil record

Keep in mind that the evolutionary relationships among the different taxa that we are discussing have been determined from the fossil record. Vertebrates, among all species of animals in the world, have the best fossil record.

Reasons include the presence of hard parts, such as bones and scales. The exception to this are the cartilaginous fish, because cartilage does not fossilize well, and the birds, due to the fact that they have hollow bones that can be crushed and lost. In contrast, the best fossil record among the vertebrates exists among the large mammals, whose bones are preserved well as fossils.

Superclass Agnatha

The Agnatha are in some texts referred to as a class, and in others as a superclass. In general this group shares the common characteristics of:

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

Because the fossil record is very poor for these species, it is unclear whether the two Agnathan groups should be described as classes or orders.

Ancestral forms of this class were the Ostracoderms, which are extinct, but were heavily-armored on their heads and trunk. The ostracoderms were believed to be detritus feeders, because of their jawless mouths.

Extant Agnathans include two groups, called cyclostomes, because of their circular mouths:

Class Myxini, Order Myxiniformes - hagfishes (Figure 3-8, p. 88 in text)

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

Class Cephalaspidomorpha, Order Petromyzontiformes - lampreys (Figure 3-10, p. 90 in text) temperate, anadromous (hatch/breed in fresh water, mature in marine) and freshwater

parasitic as adults - attach to other fishes with their 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.

The remaining vertebrate orders are Gnathostomes (possess true jaws). Evolutionary studies have shown that in most cases the jaw is modified from one of the gill arches that were used to support gills in more primitive species.

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.

Class Chondrichthyes - cartilagenous fishes characterized by:

paired nostrils

skeleton completely cartilagenous with no endoskeletal bone

no swim bladder

scales dermal placoid when present

gill arches internal to gills

freshwater and marine species

Contains three main groups: Subclass Elasmobranchii 5 - 7 gill openings plus spiracle anterior to first gill

upper jaw not attached to 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

Order Squaliformes - true sharks

almost purely predaceous/marine

heterocercal tailfin - caudal fin is longer on the dorsal side than on the ventral side

Order Rajiformes - Rays, skates, sawfishes 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

Subclass Holocephali - Chimaeras upper jaw fused to braincase

flat, bony plates instead of teeth

operculum covering gillslits

strictly marine feeding on mollusks

Class Osteichthyes - bony fishes endoskeleton made up of bone

jaws and paired appendages

gill arches internal to gills

gills 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

Subclass Actinopterygii: ray-finned fish fin rays attach directly to girdles

internal nostrils - nares absent

single gas bladder

known from Devonian

Superorder Chondrostei (sturgeons and paddlefish) general primitive form

typically small

skeleton primarily cartilage

heterocercal tail

ganoid scales

most died out by end of Mesozoic

Superorder Neopterygii (most bony fishes) 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

Subdivision Teleostei - true bony fish

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.

- maxilla and premaxilla independently mobile from rest of skull

- pelvic and pectoral fins adapted for speed and braking giving maneuverability

- fusiform bodies streamlined for speed

Subclass Sarcopterygii: species previously believed to be extinct, such as the coelacanths and lungfish. 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 Tetrapoda 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.

Class Amphibia - Amphibians

arose from Crossopterygian, Rhipidistian ancestors

three extant orders, two extinct subclasses

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 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

Order Urodela (Caudata) - salamanders 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)

Order Salientia (Anura) - frogs and toads 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

Order Gymnophiona (Apoda) - caecilians 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

Class Reptilia first fully terrestrial vertebrates

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"

Subclass Anapsida ("no opening"); Order Testudinata (turtles) 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

Subclass Diapsida ("two openings") Order Squamata - lizards and snakes 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

Subclass Archosauria - Ruling Reptiles diapsid skull

contains dinosaurs and ancestors to birds

Order Crocodilia - crocodiles 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

Class Aves

Subclass Neornithes - modern birds

endothermic rather than ectothermic

the reptile scale into a feather which is the only unique characteristic of this class

four-chambered heart

epidermal scales on bill, legs, feet

bill instead of teeth; teeth absent in modern forms

modifications for flight include hollow bones, pectoral appendages modified as wings, air sacs, large eyes and large cerebellum

modifications for vocalization

Class Mammalia possess hair/fur

mammary glands to nourish young


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

Subclass Prototheria - egg-laying mammals oviparous

mammary glands without nipples

cloaca still present

pectoral girdle with separate precoracoid, coracoid, and interclavicle bones

Subclass Theria

Infraclass Metatheria - marsupial mammals

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

Infraclass Eutheria - "true" or placental mammals 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

Final note: We will be using the terminology and phylogenies developed in this lecture as we go through the different systems. Try to become comfortably familiar with this phylogeny, as I will be referring to it later. Because this is comparative anatomy, the focus of this course will be on how different systems developed in different taxa, without emphasis on any single taxonomic group (such as humans).

General Definitions:

Adaptive radiation - evolutionary process in which descendants from an ancestral species multiply and diverge to occupy many different habitats and modes of life

Agnatha - jawless vertebrates, including Myxini, Cephalaspidomorpha

Amniotes - vertebrates that possess an amnion or extraembryonic membrane that surrounds the embryo and encases it in amniotic fluid, including Reptilia, Aves, and Mammalia

Anamniotes - vertebrates that lack an amnion . Includes Myxini, Cephalaspidomorpha, Chondrichthyes, Osteichthyes, Amphibia

Cleidoic egg - self-contained egg that allows animal to bypass larval stage

Cloaca - posterior chamber of most fishes, nonmammalian tetrapods, and monotreme mammals into which the digestive tract and urogenital passages discharge).

Coelom - body cavity completely lined with epithelium (cellular tissue

Gnathostomes - vertebrates with jaws derived from the mandibular arch, which may have (in primitive vertebrates) supported gills. Includes Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, Mammalia

Heliotherm - regulates body temperature by using solar radiation .

Marsupium - skin pouch in which immature young are carried

Neoteny - paedomorphosis that results from the slowing down of somatic development relative to reproductive development.

Oviparous - eggs are laid and the young develop outside of the mother

Ovoviviparous - egg contained within the uterus, where the young develop and then hatch as miniature adults

Paedomorphosis - retention of juvenile characters into the adult stage

Pisces - collective term for all fishes; includes Myxini, Cephalaspidomorpha, Chondrichthyes, Osteichthyes

Placenta - composed of parts of the uterine lining and fetal extraembryonic membranes through which exchanges between mother and embryo occur.

Swim bladder - sac of air that acts as a hydrostatic mechanism, allowing a fish to control its vertical position in the water

Tetrapoda - collective term for the terrestrial vertebrates; they have four feet unless some have been secondarily lost or converted to other uses. Includes Amphibia, Reptilia, Aves, Mammalia

Viviparous - embryos develop internally and then emerge as a miniature adult