The Chordates

The Chordates

Possess all 5 Invertebrate Trends, Plus:

– Dorsal nerve (Spinal) cord

– Notochord or backbone/vertebrae

– Tail (at some stage of the life cycle)

– Muscle Blocks

– Gill Slits (at some stage of the life cycle

General Chordate Body Plan

The nerve or spinal cord connected the elongated body to the brain, and directed messages very quickly to distant parts of the body. This allowed organisms to grow larger and still be able to send nerve signals effectively.

The notochord/backbone/vertebrae added strength and rigidity to an elongated body, and provided protection to the spinal cord. This feature also allowed organisms to grow larger and provided attachment sites for muscles.

The tail is an adaptation for locomotion, both for balance and for propulsion.

Muscle blocks allowed for muscles to grow larger and more powerful, especially since the muscles could be attached to the internal skeleton.

Gill slits evolved as an adaptation for larger and/or faster aquatic creatures to be able to exchange larger amounts of respiratory gases. The gills are also thought to be the structures from which jaws evolved, a feature that is possessed by nearly all of the so called advanced vertebrates.

Compared to the Invertebrates, the Chordates are:

– Smarter

– Larger

– More coordinated

– Faster

Three Divisions of Chordates:

» Urochordates

» Cephalochordates

» Vertebrates

Urochordata- also known as Tunicates

or Sea Squirts.

- live in salt water (marine)

- are free-floating or sessile.

-chordate features are most evident in the larvae

(Illustration from Linzey, 2001)

Cephalochordata• - aka “lancelets”

ex. Amphioxus

• - are marine - live in sand or shallow water.

• - adults possess all chordate features

Vertebrata

-Some of the 5 chordate features are evident only during development

- Embryonic vertebrates have:

- a notochord

- dorsal nerve chord

- gill slits/pouches

(pharyngeal clefts)

Vertebrate Origins

What is the ancestral origin of the Vertebrates/Chordates?

- no fossil record, therefore only based on speculation

- one popular hypothesis puts the chordates as possibly descendant from the same common ancestor of echinoderms??

From Romer

Vertebrata

5 classes:• fish

• amphibians

• reptiles

• birds

• mammals

Fish

• First appear in the Paleozoic Era

• Devonian Period is “The Age of Fishes”

Fish are more advanced over their invertebrate ancestors, in that they

(the advanced fish) possess:

• Skull

• Bones (either cartilaginous or calcified)

• Jaws (which evolved from gill slits)

• Fins (paired appendages, different from the appendages of the

invertebrates)

Fish Subdivisions

•Agnathans

•Placoderms

•Chondrichthyes

•Osteichthyes

Agnathans

• No jaws – jawless

• Cartilage skeletons

• Lampreys and hagfish

Placoderms• First fish to have jaws, vertebrae and paired fins

• They are now extinct – below are pictures reconstructed from fossils:

Jaws evolved when gill openings in the head became enlarged and fitted with teeth - an important adaptation to consume huge chunks of food, allowing fish to grow to enormous size.

Placoderms

Vertebrae evolved to enclose the spinal cord – this added protection to the spinal cord and provided an attachment site for muscles - allowing for more powerful movements.

Paired fins improved swimming ability, allowing fish to swim faster and with great manoeuvrability.

Other Interesting Facts about Fish: • Have a 2 chambered heart & a closed circulatory

system

• Are cold blooded (ectotherms)

• Sexually reproduce, but use external fertilization (males don’t have a penis!)

• Produce jelly-like eggs that must be laid in water

Chondrichthyes• Skulls smaller – lighter weight

• Skeletons of cartilage – very flexible and light weight – for greater agility and speed

Sharks

and Rays

Osteichthyes

• The Bony Fishes

• Two types:

1) Lobe Finned Fish

2)Teleosts

Lobe Finned Fish• Fins in the shape of lobes, allowing them to walk on pond bottom or even on land• Also had primitive lungs for breathing – did not have to rely on gills, could

be out of water for short periods of time• Modern day lung fish and coelocanthes

evolved from the lobe finned fish• Amphibians likely evolved from lobe finned

fish too

Teleosts – Spiny Finned Fish• Modern fish - name a fish and it’s

probably a teleost !!

• Fins have spines in them for support

• Have a swim bladder to help them be buoyant at any depth

Amphibians

Frogs, toads, newts and salamanders In addition to the innovations accomplished by fish,

amphibians have:

• Legs that extend sideways – awkward for both swimming and walking – but better

than lobes• Lungs – (though the lobe finned lung fish also had lungs)• 3 chambered heart

Crocodiles & Snakes Reptiles Turtles & Lizards

And Don’t Forget…

Dinosaurs were reptiles too!!

In addition to the amphibian characteristics, reptiles also had the following characteristics that allowed them to be away from water for longer, move better and grow larger:

• Dry scaly skin (prevented drying out of skin, but also prevented breathing thru skin)

• Expandable rib cage (allowed them to inflate their lungs to a large volume)

• Leather shelled amniote eggs (allowed them to lay their eggs away from water)

• Copulatory organs (i.e. a penis - for internal fertilization – the only way for sperms to get to eggs when animals are living on land !!)

• Legs extending beneath the body (for better locomotion & supporting more weight)

• 3 and 1/2 chamber heart (for better separation of oxygenated and deoxygenated blood – richer blood – more energy)

Birds

Improvements over

reptiles include:• Feathers

(for insulation and flight)

• Hollow bones

(lighter weight for flight)

• Warm blooded

(allowing for greater energy production)• Air sacs

(for greater surface area and gas exchange)• 4 chamber heart

(for complete separation of oxygenated and deoxyg. blood = richer blood = more energy)

• Hard shelled amniote egg

(more protection from drying out and from predators)

Mammals• The Cenozoic Era is known as the “Age of

Mammals”

• In addition to warm blood and a 4 chambered heart, mammals also possess:

• Milk and Mammary glands (to nourish young)

• Body hair (for insulation)

• 3 groups: placentals, monotremes, and marsupials

Placentals

• Possess a placenta which supports the growth of the embryo until birth – better chance of survival

Monotremes

• Lay leathery eggs (like reptiles)

Echidna (spiny ant-eater)platypus

Marsupials

• Give birth to an immature live young - no larger than a kidney bean

• Wombats, opossums, kangaroos, koalas

The End!