Chordates (phylum Chordata) are deuterostome coelomates-Nearest relatives are echinoderms (the only other deuterostomes)

Four features characterize chordates

1. Nerve cord

2. Notochord

3. Pharyngeal slits

4. Postanal tail

All chordates have all four of these characteristics at some time in their lives

Other characteristics also distinguish chordates1. Chordate muscles are arranged in segmented blocks called somites2. Most chordates have an internal skeleton against which the muscles work

The Chordates

Somites

Phylum chordata can be divided into three subphyla

1. Urochordata

2. Cephalochordata

3. Vertebrata

Nonvertebrates

Tunicates are marine animals

-Larvae are tadpole-like and have notochord and nerve cord

-Are free-swimming but do not feed

-Adults typically lose the tail and notochord

-Are immobile filter-feeders

-Many secrete a tunic (cellulose sac) that surrounds the animal

Lancelets are scaleless chordates-Notochord persists throughout animal’s life-Have no distinguishable head

-Feed on plankton using cilia-generated currents

-Closest relatives to vertebrates

Vertebrates: chordates with a spinal column-Distinguished from nonvertebrates by

1. Vertebral column – Encloses and protects the dorsal nerve cord

2. Head – Distinct and well-differentiated possessing sensory organs

Vertebrates also have-Neural crest – A unique group of embryonic cells that forms many vertebrate structures-Internal organs – Liver, kidneys, endocrine glands, heart and closed circulatory system-Endoskeleton – Made of cartilage or bone

-Makes possible great size and extraordinary movement

The first vertebrates appeared in the oceans about half a billion years ago-Jawed fishesJawed fishes soon became dominant-Their descendants, the amphibiansamphibians, invaded the land-ReptilesReptiles replaced them as the dominant land vertebrates. 300 MYA

-Gave rise to birds and mammals 220 MYA

Fishes are the most diverse vertebrate group-Over half of all vertebrates

• Vary greatly in size, shape, color and appearance

• 500 MYA• Paired fins• Jaws

Fishes have the following characteristics

1. Vertebral column2. Jaws and paired appendages3. Internal gills4. Single-loop blood circulation5. Nutritional deficiencies

The first fishes had mouths with no jaws -Agnatha (Superclass) extant as hagfish (class Myxini) and lampreys (class Cephalaspidomorphi)-Ostracoderms are now extinct

The development of jaws occurred in the late Silurian period (438-408 MYA)-Jaws evolved from the anterior gill arches that were made of cartilage

Skull

Gill slit

Anterior gill arch

The Devonian period (408-360 MYA) saw two types of jawed fishes-Spiny fishes (Acanthodii)

-Smaller fishes that dominated early on-Armored fishes (Placodermi)

-Larger fishes that dominated lateAt the end of the period, these early fishes were

replaced by sharks and bony fishes-These had more improved jaws

In the Carboniferous period (360-280 MYA), sharks (Class Chondrichthyes) became the dominant sea predators-A light, flexible, calcified, cartilaginous skeleton, and paired fins made them superior swimmers

Sharks were among the first vertebrates to develop teeth-Evolved from rough scales on mouth’s skin-Easily lost but continuously replaced

Sharks (and bony fishes) have a fully developed lateral line system-Series of sensory organs under the skin that detects changes in pressure waves

Most sharks have internal fertilization. Oviparous, ovoviviparous, and viviparous.

Bony fishes (Osteichthyes) evolved at the same time as sharks about 400 MYA-However, they adopted a heavy internal skeleton made completely of bone

Bony fishes are the most species rich group of all vertebrates (> 30,000 living species)

They developed significant adaptations that enabled them to dominate life in the water-Swim bladder and gill cover

Swim bladder -A gas-filled sac that allows bony fishes to regulate their buoyant density

-Gas addition = Gas gland-Gas release = Oval body

Gill cover-A hard plate, the operculum, covers gills-Its flexing permits water pumping over gills

Ray-finned fishes (class Actinopterygii) -Parallel bony rays support & stiffen each fin-There are no muscles within the fins

Lobe-finned fishes (class Sarcopterygii)-Have paired fins that consist of a long felshy muscular lobe

-Supported by central co-jointed bones-Almost certainly the amphibian ancestors

Amphibians (class Amphibia) are damp-skinned vertebrates-First vertebrates to walk on land

Successful invasion of lands by vertebrates required several adaptations1. LegsLegs to support body’s weight2. LungsLungs to extract oxygen from air3. Redesigned heartRedesigned heart to drive larger muscles4. Reproduction in HReproduction in H22OO to prevent egg drying

5. System to preventprevent whole body desiccation. desiccation. The The skin also aids in respiration. skin also aids in respiration. 6. 6. Plumonary veinsPlumonary veins. .

Amphibians evolved from lobe-finned fishIchthyostega was one of the first amphibians

-Efficient limbs for land crawling-Improved olfactory and auditory structures

-However, still fishlike-Spent much of life in water

In 2006, a transitional fossil was found between fish and Ichthyostega-Tiktaalik lived about 375 MYA

-Shoulder and limb bones like those of an amphibian

-Fins like those of a lobe-finned fish

-Spent most of its life in water

-Capable of moving onto land to capture food or escape predators

Modern amphibians are descended from three families of the Tertiary period (65-2 MYA)

-These accomplished a highly successful invasion of wet habitats all over the world

Today there are about 5,670 species in 37 different families, comprising three orders:

-Anura (“without tail”), Caudata (“visible tail”),Apoda (“without legs”)

Over 7000 species of reptiles (class Reptilia) now live on Earth

All living reptiles exhibit three key features1. Amniotic eggs, which are watertight 2. Dry skin, which covers body and prevents water loss3. Thoracic breathing, which increases lung capacity

Reptiles, birds and mammals are amniotes

The amniotic egg has four membranes

Chorion – Allows O2 entry

Amnion – Fluid-filled cavity

Yolk sac – Provides food

Allantois – Excretes wastes

Reptiles dominated earth for 250 million yearsReptiles are distinguished by the number of holes on

side of the skull behind eye orbit: -0 (anapsids), 1 (synapsids), 2 (diapsids)

The synapsids rose to dominance first

-Pelycosaurs were first land vertebrates to kill beasts their own size

-Therapsids replaced them about 250 MYA

-May have been endotherms

-Replaced by diapsids but gave rise to mammals.

Archosaurs (Diapsids) were the first land vertebrates to be bipedal

-Gave rise to dinosaurs about 220 MYA

Dinosaurs dominated for over 150 million years-Became extinct 65 MYA

-Asteroid’s impact

Modern reptiles developed two important characteristics:

1. Internal fertilization: Sperm fertilizes egg before protective membranes are formed

2. Improved circulation: Oxygen is provided to the body more efficiently

Comparing the circulatory systems of fish, amphibians, and reptiles.

All living reptiles are ectothermic-Obtain heat from external sources

There are four surviving orders of reptiles-Chelonia (turtles and tortoises) -Rhynchocephalia (tuataras)-Squamata (lizards and snakes)-Crocodylia (crocodiles and alligators)

In many ways, crocodiles resemble birds far more than they do other living reptiles -Build nests and care for their young-Have a four-chambered heart

Crocodile Gharial