before the sharks

Chordates

Chordates originated from metazoa which are all animals that are multicellular. From there came the coelomata which were bilaterally symmetric organisms that also had gastrulation. Gastrulation occurs when a hollow ball of cells invaginates (folds in) with a hole on the other side. From there the coelomata evolved into deuterostomata which also have an anus. The final step before the chordates was the hemichordates which are also known as acorn worms.

Chordates include any animals with a notochord at some point in the life cycle. The notochord is an elastic rod that prevents the body from telescoping and allows for movement. Chordates also all have a muscular post anal tail at some point in development which can be used for propulsion. Chordates all have a dorsal hollow nerve cord or spinal cord which synchronises muscle contraction, and was created through neurolation of the ectoderm which was created through gastrulation. Finally, all chordates have an endostyle which is the ciliated granular groove on the pharynx that later becomes the thyroid gland.

Cephalochordates are a type of chordate that looks almost exactly like the ancestral chordate. they are also commonly called branchiostoma. They exist buried int he sediment, and use pharyngeal slits for filter feeding. They are capable of gas exchange through the skin. They also have a circulatory system and excretory cells.

Olfactores are the other group of chordates that includes tunicates and vertebrates.

Tunicates are commonly known as sea squirts. The baby form of a tunicate looks like a chordate, while the adult form does not. They move from the free swimming baby lifestyle to a sedentary lifestyle through the loss of a tail, and other chordate identifiers. Tunicates also have the ability to make cellulose which shows horizontal gene transfer with plants

Vertebrates include all olfactores with a cranium or vertebrae. All vertebrates have a muscular pharynx which allows for occupation of new niches, due to better aeration, better ability to gather food and larger size making them able to be more active. Vertebrates include all cyclostomes and gnathostomes.

Cyclostomes

Cyclostomes include all vertebrates that lack a jaw.

Hagfish are a type of cyclostome that is marine. Hagfish show that a hox duplication occurred where two copies of a gene were made where one could maintain ancestral traits, and the other could occupy a new set of tasks. Hox duplication often works in collinearity where the top genes of a chromosome control upper parts of the organism and the bottom parts of the chromosome control bottom parts of organism. Hagfish only have a notochord, and a cranium made up 3 sections. Hagfish osmoconform to their environments to match external environment. Hagfish are located in deep sea, and cold water and are bottom dwelling scavengers. Hagfish feed by eating carcasses through tying their body in a knot around the food and ripping off chunks, and can also absorb nutrients through their skin. Hagfish use slime to escape from predators by clogging the gills, and escaping via sneezing. Very little is known about hagfish ecology, except for the high female:male ratio and that eggs are released straight into their body cavities

Lamprey are cyclostomes that do have vertebrae, which allows for attachment of muscles, and protects the spinal cord and reinforces the notochord. Lamprey are seen as living fossils or a jumble of ancestral and derived traits. Lamprey consist of cartilage, and lack fins and scales. Lamprey live a parasitic lifestyle by using their oral disk to attach to prey and create a wound to drink blood and interstitial fluid using anticoagulants. The oral disk also contains hooks to help attach better to prey. Lamprey use tidal ventilation to help push in and out water since they spend the majority of life attached to a host. Lamprey are anadromous where they move from salt to fresh water to reproduce, and are semelparous where they breed once and die. Baby lamprey are called ammocoete which undergo metamorphosis to become adults. Ammocoete filter feed. Sea lamprey are special because the sex of the lamprey is determined by how much food is eaten as a larvae, where more food equates to a female sea lamprey.

Gnathostome

Gnathostomes are vertebrates that do have jaws. Gnathostomes show a new hox duplication which allowed for the creation of jaws, teeth and paired fins which allow for better locomotion, predation and sensory skills. The jaw came from the gill arch skeleton which allowed for ventilation of water and more surface area for gas exchange. The jaw allowed for new food sources, and the stomach to becoem the main digestive organ. Paired fins allowed for more control of movement by preventing roll (tilt), yaw (sideways) and pitch (up and down) movement. Gnathostomes include two groups: chondrichthyes and osteichthyes