Origin of the Vertebrates Early recognition that this

Origin of the Vertebrates

Early recognition that this forms a related grouping, based on certain characteristics in common: internal skeleton, circulation direction, some segmentation, etc.

Agnatha (no jaw) = most primitive as no jaw, no paired appendages, and only 2 semicircular canals, not three. Considered vertebrate as it has vertebrae = segmented backbone of either cartilage (here) or bone Also, dorsal nerve cord, other features.

Fossil Agnatha: no jaw, poor paired appendages, some with none, and 2 semicircular canals But: have bone as body armor and are filter feeders

2 semicircular canals in inner ear Gills for filter feeding and respiration No jaws Odd nerve area = electric sensitivity?

Seeking ancestors or relationships The great split among higher animals: deuterostome or protostome

Clevage pattern

Other major differences • Determinate vs intedeterminate clevage • Origin of coelom (schizocoel = protostome enterocoel = deuterostome Origin of mesoderm- outpocket, vs 4 d cell mesoderm skeleton(deutero) vs ectoderm skeleton (protostomes)

The other major Deuterostome group = Echinoderms spiny skin, pentaradiate, etc,

Fossils include sea lillies = filter feeders. Sessile. All living ones have a bilateral larvae, and are deuterostomes.

Other forms related to Vertebrates Amphioxus

Has segments, gills, dorsal nerve cord, also a notochord, Odd in no brain, segmented gonads, etc.

Why these features? Nerve cord – coordination of movement Notochord – organizer for the nerve cord Gills – here for feeding, only later for respiration.

So new group = Phylum chordata • Includes subphylum Vertebrata • Includes “odd” groups with notochord, nerve cord and gills. = amphioxus, tunicates,

Tunicate

Tunicate and larvae Note: origin of visceralsomatic dichotomy Visceral = adult Somatic = larval tail origin of need for dorsal nerve cord, notochord

Neoteny = keep immature features into adult Paedogenesis = become an adult Earlier in life Adult = can reproduce