Chapter 12Chordates

Chordates • In 1798, scientists from

the London Museum of Natural History were gathered around a strange looking beast

• They examined its flat duck like bill, its webbed feet and concluded this was not a real animal, but a joke someone was playing on them– In reality they were looking

at the duck-billed platypus

PlatypusThe platypus looks like no other animal, but has features

similar to other animalsSimilar features of the platypus:• Like mammals:

– Tail similar to a beaver and webbed feet similar to an otter– Like other mammals has hair and produces milk for its young

• Its milk though is secreted from its skin• Like birds and reptiles:

– has a single opening, called a cloaca, which functions for both reproduction and excretion

– Lays eggs which hatch in immature young– Like some reptiles it is venomous

Platypus and Young

Theory to the Madness• Even with its awkward look the platypus is very successful as

its structures are well adapted for its lifestyle and habitat– Structures allow for the platypus to be mobile on land and in

water• Tail acts as rudder and feet as paddles in the water• During day hides in underground burrows, dug with clawed hind feet

– It hunts mollusks in the water and its bill is highly sensitive making ideal for locating prey, but also for scooping it up• Bill has more than 80,000 sense organs in it• Some of these sense organs allow for detection of motion created by prey

– Dense oily fur and layer of fat under the skin protects from the cold of water, minimizing heat loss• This is key as platypus remains in water through the night and without this

system to maintain heat would chill, become sluggish and eventually succumb to hypothermia

– Poorly developed eyes as spends much of time underground, were large eyes would be useless and easily irritated by soil

Video of Platypus

Show video of platypus • Structures and info• Young and milk secretion

Section 12.1 Phylum Chordata

Phylum Chordata• When most think of animals

they think of a chordate as this is the phylum that contains most animals we are all familiar with

• Even though chordates usually considered the most familiar and interesting of all animals they only make about 5% of all known animal species and are one of the most recent groups of animal

Classification of Chordates

• Chordates make a distinct phylum containing organisms that at some time in their development have:– A notochord– Hollow dorsal nerve tube– Tail that extends beyond the anus– Muscular tube called the pharynx• The pharynx functions in feeding and/or respiration• At some time in embryonic development, chordates

have slits in the wall of the pharynx

4 Key Features of Chordates

1

2

3

4

Notochord• Is present in all chordate

embryos• Is a longitudinal, flexible

rod located between the digestive tube and the nerve cord.– It is composed of large,

fluid-filled cells encased in fairly stiff, fibrous tissue.

– It provides skeletal support throughout most of the length of the animal.

– While the notochord persists in the adult stage of some invertebrate chordates and primitive vertebrates, it remains as only a remnant in vertebrates with a more complex, jointed skeleton.• For example, it is the gelatinous

material of the disks between vertebrae in humans.

Hollow Dorsal Nerve Tube• Develops in the

vertebrate embryo from a plate of ectoderm that rolls into a tube dorsal to the notochord.– Other animal phyla have

solid nerve cord, usually located ventrally.

– The nerve cord of the chordate embryo develops into the central nervous system: the brain and spinal cord.

Tail that Extends Beyond the Anus

Most chordates have a muscular tail extending posterior to the anus.– In contrast, nonchordates have a digestive tract

that extends nearly the whole length of the body.– The chordate tail contains skeletal elements and

muscles.– It provides much of the propulsive force in many

aquatic species.

Pharynx

Pharyngeal gill slits connect the pharynx, just posterior to the mouth, to the outside of the animal.– These slits allow water that enters the mouth to exit

without continuing through the entire digestive tract.– In many invertebrate chordates, the pharyngeal gill

slits function as suspension-feeding devices.– The slits and the structures that support them have

become modified for gas exchange (in aquatic vertebrates), jaw support, hearing, and other functions during vertebrate evolution.

Phylum Chordata

• This phylum can be divided into 3 subphyla:– Cephalochordata• Such as lancelets

– Urochordata• Such as tunicates

– Vertebrata• Such as fish, amphibians, reptiles, birds, and mammals

Classification of Phylum Chordata

• The table shows the breakdown of the phylum and it classification• This is also seen on the next slide, but shown differently

Classification Example

Subphylum Cephalochordata Lancelets

Subphylum Urochordata Tunicates

Subphylum Vertebrata

•Class Agnatha Jawless fish (e.g. lamprey)

•Class Placodermi Jawed armoured fish (extinct)

•Class Chondrichthyes Cartilaginous fish (e.g. sharks)

•Class Osteichthyes Bony fish (e.g. perch)

•Class Amphibia Amphibians (e.g. frogs)

•Class Reptilia Reptiles (e.g. alligators)

•Class Aves Birds (e.g. blue jay)

•Class Mammalia Mammals (e.g. dogs)

Phylum Chordata Subphylum's

Subphylum Urochordata

• Most urochordates, commonly called tunicates, are sessile marine animals that adhere to rocks, docks, and boats.– Others are planktonic

• Some species are colonial, others solitary

Tunicates

• Tunicates are suspension-feeders.– Seawater passes inside the animal via an incurrent

siphon, through the pharyngeal gill slits, and into a ciliated chamber, the atrium.

– Food filtered from the water is trapped by an mucous net that is passed by cilia into the intestine.

– Filtered water and feces exit through an excurrent siphon.

– The entire animal is encased in a tunic of a cellulose like carbohydrate

Tunicates

Tunicates• Now from the diagram on

the past slide only 1 of the 4 chordata characteristics are present– The pharyngeal slits

• But as larva the tunicates have all 4 characteristics– Larva swim until they attach

themselves to a surface– At the surface undergo

metamorphosis turning into an adult and lose 3 of the 4 chordata characteristics

Tunicate Larva

Subphylum Cephalochordata

• Cephalochordates, also known as lancelets, closely resemble the idealized chordate.– The notochord, dorsal nerve cord, numerous gill

slits, and post-anal tail all persist in the adult stage.

– Lancets are just a few centimeters long.– They live with their posterior end buried in the

sand and the anterior end exposed for feeding.

Lancelets

Lancelets• Lancelets are suspension

feeders, feeding by trapping tiny particles on mucus nets secreted across the pharyngeal slits.– Ciliary pumping creates a flow

of water with suspended food particles into the mouth and out the gill slits.

– In lancets, the pharynx and gill slits are feeding structures and play only a minor role in respiration, which primarily occurs across the external body surface.

Lancelets• A lancet frequently leaves its burrow to swim to a

new location.• Though feeble swimmers, their swimming

mechanism resembles that of fishes through the coordinated contraction of serial muscle blocks.– Contraction of these chevron-shaped muscles flexes

the notochord and produces lateral undulations that thrust the body forward.

– The muscle segments develop from blocks of mesoderm, called somites, arranged serially along each side of the notochord of the embryo.

Link Between the Subphylums

• Molecular evidence suggests that the vertebrates’ closest relatives are the cephalochordates, and the urochordates are their next closest relatives.

• The key thing is the organisms of the first two subphylum of the phylum chordata (Cephalochordata and Urochordata) do have notochords, but do not have backbones or vertebral columns– As such these organisms in these subphylums represent

the transition from invertebrates to the chordates, which thus through study can give clues about the origin of the chordates

Subphylum Vertebrata

• Around 95% of the chordates belong to this subphylum

• Key feature of the subphylum:• All vertebrates have vertebrae (singular

vertebra) that are hollow, cartilaginous, or bony structures that surround and protect the dorsal nerve cord

Vertebrae in Humans

Other features of Vertebrates

• Other features shared by vertebrates:– Endoskeleton (internal skeleton)– A large brain protected by a skull– An advanced nervous system– Complex heart and circulatory system– Special outer covering– One or two pairs of appendages– A large coelom that houses the vital organs

Diagram with Timeline for Vertebrate Evolution

• This diagram shows the timeline for vertebrate evolution and the appearance of the classes (note mammals were last to show up)

• This diagram shows a brief outline of vertebrate evolution and key developments along the way

Cold Blooded vs Warm Blooded

Ectotherms• All vertebrates except birds

and mammals are cold-blooded

• Body temperature and metabolic rate changes with environment– Thus can regulate by

changing environment (e.g. getting to hot get some shade)

Endotherms• Birds and mammals are warm-

blooded organisms• Body temperature and

remains constant regardless of environment

• Able to vary metabolic rate (e.g. bears slow down when hibernate)

• Ability to regulate temperature makes more independent of environment

Ecological Role of Vertebrates

• Vertebrate are found all over– In the ocean, fresh water lakes and streams; on

the land; and in the air

• They are key ecologically as vertebrates include fish, amphibians, reptiles, birds and mammals

Ecological Role of Fish

• Are vital part to many living systems– As consumers help keep

some populations in check

• Also important food source for many animals– Both aquatic and land

animals

Ecological Role of Amphibians• Play pivotal role in

wetland ecology by preying on insects and eating algae and dead plants that can fill ponds and streams

• Good indicators of environment health

• Source of food for many:– Fish– Birds– Reptiles– Mammals

Ecological Role of Reptiles

• Important role in environment and agriculture– As predators keep many

pests in check

• Also source of food to other animals

Ecological Role of Birds

• Help maintain balance in nature– Eat large number of

insects, keeping populations in check

– Predatory birds help keep mice and rats in check

• Also help in pollination and seed dispersal for plants

Ecological Role of Mammals• Influence and alter

environments in many ways• Some examples:

– Moles aerate soil– Beavers dam up water

flooding areas– Elephants knock down trees

to help maintain grasslands• Some are well known for

transmitting disease• Humans have very

significant role in both positive and negative ways

Cladistics

Cladistics

• Cladistics is an alternative method used for analyzing and predicting the phylogeny (evolutionary relationship) of organisms– It is based on analysis of shared characteristics

• Since characteristics are always changing cladists (people who study cladistics) must distinguish between recent derived (apomorphic) characteristics and older, primitive (plesiomorphic) characteristics

Cladistics• Based on the characteristics they work with the cladists

then arrange organisms based on their hypothesized degree of sharing of apomorphic characteristics rather than plesiomorphic characteristics

• Example:– A recently derived feature is the single toe of a horse– The original primitive state of five-digit appendages seen in

monkeys, turtles and crocodiles indicates an ancient, but common ancestor (thus by the fact monkeys and turtles both have 5 digits does not mean a close kinship)

– Thus to a cladist more closely related organisms will share a greater number of unique derived features that were not present in their distant ancestors

Cladograms• Cladists show their predicted

evolutionary relationships via cladograms

• Cladograms are:– Branching diagrams used to display

the hypothesized relationships• Cladists though now have highly

benefited from genetic data as they can now compare genetic codes for shared genetic codes instead of just outward traits– This has given lots of information

and helped find new and compelling evidence for many phylogenetic relationships

• Example cladogram– Read page 451 as it will take

you through a sample cladogram

Homework

• Read pages 448-451

• Answer questions– 1-7 page 451-452