The Evolution of Chordates

Phylum Chordata belongs to clade Deuterostomata. Deuterostomes have events of development in common with one another. 1. Coelom from archenteron surrounded by mesodermal tissue.2. Formation of the mouth at the opposite that of the anus which developed from blastopore.

3. Radial cleavage during embryonic development.*There are some deuterostomes that have been removed from the clade due to DNA analysis. This includes ectoprocts and brachiopods. These two phyla have been moved over to the Clade Lophotorchozoan.

Characteristics of Chordates (At sometime during their life)-Notochord-longitudinal rod of cartilage between digestive tube and nerve cord. Used for support. Many have only in the embryonic stage.

-Dorsal hollow nerve cord derived from ectoderm during neurulation. Other animals have a solid cord on the ventral side-Pharyngeal cleft (slits) grooves posterior to the mouth. Develops into gills, ears, head neck etc.-Post-anal tails that extend beyond digestive tract (sometime during their life). Can be reduced in embryonic stage. Used for movement.

Three subphylaI. Subphylum Cephalochordata-Lancets keeps all 4 char-acteristics of chordates through adulthood. Has series of muscles on trunk used for swimming. Does not have cranium.

II. Subphylum Urochordata-has 4 characteristics of chordates in larval stage but loses 3 (retains gill slits) as a sessile adult that undergoes extreme metamorphosis. Does not have a craniumIII. Subphylum Craniates-has a cranium or head with a brain with sense organ evolving. Has two sets of Hox genes.

-Evolution of neural crests forming between the neural tube and ectoderm. These cells give rise to some structures unique to vertebrate skull and jaw.-The pharyngeal gill slits become gill slits associated with breathing. Terrestrial craniates pharyngeal gill slits become other structures.

Phylum Chordata belongs to clade Deuterostomata. Deuterostomes have events of development in common with one another. 1. Coelom from archenteron surrounded by mesodermal tissue.2. Formation of the mouth at the opposite that of the anus which developed from blastopore.

Classes in III Subphylum Craniates1. Class Myxini (hagfishes) skull of cartilage, no vertebrae, no jaw, retains notochord into adulthood. Swims with segmented muscles attached to notochord. Produce slime as protection.

2. Class Petromyzontidae (lampreys)are primitive vertebrate with a skull of cartilage, it also retains notochord into adulthood. The notochord is surrounded by a cartilage like pipe with primitive like vertebrate extensions enclosing nerve cord, no jaw. Mouth is sucker like.

These are the classes that are found in the clade vertebrata.

The next important evolutionary development was jaws. This developed from the rods in the pharyngeal gill slits. Vertebrates that have jaws are termed gnathostomesJaws allowed vertebrates to eat larger and more varied types of food by shredding and tearing the food apart. Intermediates fossilized animals between jawless and jawed vertebrates have been found. Jawed vertebrates also have larger cranium.

3. Class Chondrichthyes- Include sharks and rays. The skele-ton is composed of cartilage not bone. (Evidence suggests once it was bone). Evolution of fins increased swimming efficiency. Dorsal fin stabilizes shark, pectoral and pelvic fins allows for lift and forward movement. Oil in liver provide buoyancy, no swim bladder or lungs. Intestine with spiral valve. Reproduction-Oviparous-Eggs laid outside the bodyOvoviviparous-Eggs retained in oviduct and develop thereViviparous-Eggs develop in the uterus with yolk sac placenta

4. Class Actinopterygii- Ray-finned fish have bony endoskeleton of calcium phosphate. Gills are covered with an operculum. Buoyancy controlled swim bladder (was once ancient lung) gas goes in and out by blood. Covered by scales covered by mucus. Fins are supported by bony rays. Evolved first in freshwater and moved to marine environment. Some live in both environments during their life history. Reproduction mostlyOviparous-Eggs laid outside the body with external fertilization

Next evolutionary advance was a bony skeleton. Animals with a bony skeleton are in the clade Osteichthyes

5. Class Actinistia- lobe-finned fish have bony endoskeleton of calcium phosphate.Fins are supported by a single rod-shaped bone surrounded by a thick layer of muscle in their pectoral and pelvic fins. Allowed them to swim and “walk” underwater. Only two species survive today and were thought to be extinct until a fisherman caught one in 1938 in the Indian Ocean.

6. Class Dipnoi- lung fish have bony endoskeleton of calcium phosphate and lobbed-fins. These fish are freshwater fish living in stagnant water in the southern hemisphere. They gulp air that goes to the lungs. This oxygen is to supplement the oxygen that gills are extracting from the water. Lobe-fin fish are the closest living relative to amphibians or tetrapods.

The next development is the evolution of four limbs (Clade Tetrapod). Tetrapods have fours limbs. The neck has two vertebrae instead of one giving rise to side to side motion as well as up and down motion. Pelvic girdle fused to the backbone to reduce to force put on the legs. Pharyngeal clefts (no longer slits) give rise to the parts of the ears, glands and other structures.

The next develop-ment was the evolution of four limbs (Clade Tetrapod). The closest living tetra-pod to the lungfishes are the amphibians. The fossil record indicates that many other clades evolved to give rise to the class Amphibia. The evolution of legs allowed the verte-brates to move on to land. Amphibia means dual life.

7. Class Amphibia- Most amphibians reproduce in water. 3 chambered heart, lungs sac-like. Uses skin to help breath. 3 orders of amphibians.

A. Oder Urodela-salamanders retain tail as adults-legs out to. Walks side to side.

B. Order Anura-tadpoles juvenile form “fish-like” with gills, coiled intestine, long tail. Later absorbs tails. Legs emerge. Digestive tract specializes.

C. Order Apoda- legless nearly blind. Look like earthworms. Lost legs in the course of evolution.

Most amphibians reproduce in water, then live on land but there are some amphibians entirely aquatic or entirely terrestrial. Most salamanders and caecilians do not undergo metamorphosis instead the juvenile form looks like the adult form.

Clade Amniotes- are tetrapods that have an amniotic egg which allowed tetrapods to be entirely terrestrial and not go back to the water to reproduce. Amniotic egg has extraembryonic membranes or membranes derived the embryo but no longer a part of the embryo.

Membranesa. Amnion-Contains fluid that surround the embryo.

Cushions against mechanical shock.b. Allantois-Garbage bag contains waste materials. Also

aids in gas exchange with the chorion c. Yolk sac-contains yolk or stockpile of nutrients for the

embryo. The egg white stores protein and is a source of amino acids for the embryo

d. Chorion combine with the allantois and aids in gas exchange. Surrounds the embryo and all the membranes.

e. Albumen found outside the chorion but inside the shell as a source of amino acids.

Clade Amniotes- gave rise the two clades- reptiles and synapsids. The Class Reptiles gave rise to diapsids and anapsida. These clades are based on the structure of the skull. The Class Mammalia in the clade synapsid.

The holes in the skull are thought to provide better attachment for jaw muscles which allowed the animals to capture and digest a greater variety of prey.

The reptiles have scales, and internal fertilization. Most are ectothermic except for birds which are endothermic and have an extreme high metabolic rate.

Turtles are thought to have been anapsids but other molecular evidence have them more closely related to crocodiles.

Snakes, lizards, and tuataras are diapsids belonging to the linage lepidosaurs. This group is greatly varied. Snakes evolved from tetrapods. Their scales are overlapping.

Crocodiles and birds are diapsids belonging to the linage archosaurs. This linage also includes dinosaurs. Birds are adapted for flight-Hollow bones-4 chambered heart (crocodiles also have 4 chambers)-Endotherms for high metabolic rate-Feathers which are modified scales and legs are covered with scales.-Beaks (adapted for food source) Once had teeth.

Class Mammals belong the clade synapsids which have two holes in the skull. -4 chambered heart-endotherms-mammary glands that produce milkThree clades

-Monotremes-Egg layers (only 5 species) no nipples but produce milk

The other two clades produce eggs and all amniotic membranes are present but offspring develop in the uterus forming a placenta

-Marsupials- Higher metabolic rate, nipples young complete development in pouch (marsupium).

-Eutherians- Higher metabolic rate, nipples, no pouch, development completed in the uterus. Embryo retained longer.

Marsupials were isolated from Eutherians in Australia. Adaptive radiation and convergent evolution occurred.

Many of the marsu-pials in Australia fill the same niche as a placental mammal in North America. Case of convergent evolution due to the same selection pressures.

This shows the relationship between the various orders mammals