Phylum Chordata

Chordates are fun to take a look at because this group contains animals that you may be more familiar with than invertebrates.

Also, animals moved onto land during chordate evolution so some big evolutionary jumps were made within this group.

We are going to break the chordates down into their classes to take a closer look.

• • In an effort to speed things up a bit, we are going to bunch the first two chordates class together and call them the invertebrate chordates.

The

invertebrate chordates

(tunicates, lancelets)

These guys have no true backbone, or any bones for that matter. They represent the earliest chordates showing the four characteristics features, but not much else.

Tunicates (sea squirts) are sessile filter feeders that live in marine habitats. They have the characteristic notochord, nerve chord, and post anal tail as larvae, but lose all of these chordate features as adults. In fact, the adults could be easily mistaken for a cndaria – a mistake I am sure you will never make.

• lancelets are small marine eel like creatures. Unlike the tunicates, they maintain their chordate features into adulthood. In fact the classic diagram showing chordate features looks like a lancelet.

• • • •

Superclass

Agnatha

(hagfish, lamprey)

This groups is often referred to as the “jawless” fish. They have a cartilagenous skeleton but no jaw.

They are the first group to have a real head containing a skull.

Hagfish are deep sea marine scavengers, eating sick or dead fish. Hagfish produce slime as a defense mechanism. They can make several liters of slime in under a minute. This either repulses the predator or can actually suffocate them.

Hagfish slime hagfish

• Lamprey are fresh water dwellers that parasitize fish. Click to see gross video of hagfish feeding

Lamprey on fish

Lamprey mouth

Class

Chondrichthyes

(con-drick-thees) (Sharks, Skates, and Rays)

• This group is the first to have jaws • They have an internal skeleton ( endoskeleton ) made of cartilage.

• Sharks are sexual reproducers with internal fertilization .

• They vary in the amount of time the eggs stay in the body. Some sharks lay eggs, some hatch the eggs internally, some give birth to fully formed live young.

• All sharks are carnivores, however, some of the largest feed only on plankton. Skates and rays feed mostly on invertebrates.

Sharks Skates Rays

Skates and rays can be told apart by their tails. Rays have a skinny whip-like tail. Skates have a thicker tail with double lobed fins on either side.

Skates also have a sunnier disposition.

Class

Osteichthyes

(Bony Fish) (oz-tee-ick-thees)

• • • • • • • This is what you think of when you say “fish” The most numerous of all vertebrates in both actual number and number of species. Have a hard endoskeleton reinforced with calcium.

Bodies covered in flattened scales Most are sexual reproducers with external fertilization and laid eggs.

Breathe by drawing water over gills and extracting the dissolved oxygen.

Have an internal swim bladder that fills with air to aid in buoyancy.

Class

Amphibia

(Frogs, Toads, Salamanders, Newts)

• • • The first land dwelling animals and therefore the first to have legs.

Like the first land plants, amphibians moved onto land but still require water to lay their eggs. Most amphibians are carnivores eating insects, worms, and each other.

• Their jelly-like eggs (which are fertilized externally ) need to stay in water while they develop, and the young, which hatch with tails (tadpoles), live in water until they grow their legs.

Frogs

, which vary greatly in size, live mostly in water or very moist areas.

Toads

are terrestrial as adults and have drier, bumpier skin.

Salamanders

live in moist terrestrial areas under leaves and rocks The giant salamander, which lives in streams in Japan, is the largest amphibian.

Or can be found zipping around race tracks.

Newts

Are actually a subgroup of salamanders. They can be hard to distinguish, but tend to have rougher skin and are aquatic as adults.

They are also known to be politically active .

Class

Reptilia

and Class

Aves

(Snakes, lizards, turtles, crocodiles, and birds)

The classification of these two groups is currently in dispute. While we have known that birds had reptile ancestors for some time, it has more recently been determined that there are some fossil species that were previously thought of as reptile that are now understood to be more closely related to birds than other reptiles. • • • In the mean time .

Most reptiles are land dwelling, but some that live in water They are ectothermic (cold blooded) like their earlier chordate relatives.

Reptiles have amniotic eggs which do not need to be in water to develop. This enabled reptiles to colonize land further than their amphibian ancestors.

• Amniotic eggs have a hard or leathery covering to protect the egg from drying out with several internal membranes (including one called the amnion) that allow for gases to be exchanged through the shell.

W

e are going to assume that we do not need to show you pictures of snakes, lizards, turtles and crocodiles.

I

nstead we are going to discuss dinosaurs I f you investigate current ideas about reptile classification you generally will find something like this.

O n this diagram we see pictures of things that we have all grown up calling dinosaurs.

H owever, when we look at how they are actually related to the rest of the reptiles some • • interesting questions pop up.

How useful or accurate is the term dinosaur?

How accurate is the idea that dinosaurs are giant • lizards?

How accurate is the statement “birds are living dinosaurs”?

• Why did John Hammond ever think it was a good idea to bring his grandchildren to Jurassic Park and what parents in their right mind would let their children go while they were still working out the kinks?

N ow that we know reptiles and birds are related, and since birds are something we are all familiar with, it makes sense to discuss some of their shared characteristics.

• • B irds have many shared characteristics that help facilitate flight.

Forelimbs adapted to be wings with feathers Weight saving adaptations that include lighter skull, no urinary bladder, no teeth, and small gonads.

• • • O ther characteristics help birds generate the energy necessary for flight They are endothermic generating their own body heat which keeps muscles warm.

A four chambered heart that efficiently keeps oxygenated blood separate.

Efficient lungs While there are flightless birds, they most likely evolved from flying ancestors.

Ostrich Kiwi Juvenile penguin Flightless cormorant

Class

Mammalia

• You most likely learned the common • • • features of mammals in second grade.

Warm blooded ( Hair endotherms Mammary glands ) which they use to feed their young milk.

Give birth to live young.

However, that last one is vicious lie perpetrated by second grade teachers around the world. Not all mammals give birth to live young.

A few mammals ( monotremes ) lay eggs and are living links to mammal’s reptile ancestors..

Echidna a Platypus a

T

here is another group of mammals that grow their babies inside (internal gestation), but only for a very brief time until the nutrients in the egg are used up.

Marsupials

Like the monotremes, marsupials were much more common at the beginning of mammal evolution. They persist mostly in Australia which broke off from Pangea early and has few large predators. However, there are a few remaining marsupial examples almost everywhere, including North America. wombat koala

kangaroo

Marsupials give birth to tiny, fetus-like babies that finish their development in a special pouch on the mother.

opossum

T

he majority of mammals, however, are eutherians which grow their babies inside until they are much bigger (total internal gestation). They are able to do this because of the evolution of a new structure called the placenta , which allows the baby to be delivered oxygen and nutrients from the mother.

This nifty structure has led to the tremendous success of the eutherians.