Origin of vertebrates And the evolution of “fish” The Deuterostomes Origin of vertebrates And the evolution of “fish”

Chordates (including vertebrates) and Echinoderms

Basic deuterostome characteristics (vs Protostome) Radial vs spiral clevage Mesoderm formation 2nd opening into gastrula = mouth not first

Echinoderm variation

Sea lilies; marine, sessile, filter feeders, external sieves.

The Chordates Have a dorsal nerve cord (sometime) Have a notochord = dorsal stiffening rod retained in adult vertebrates as organizer for dorsal nerve cord Have gills or pharyngeal clefts All three features are organizers for later development – so are retained in embryos.

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

Vertebrates – a subset of chordates with a vertebral column. Agnatha – most primitive vertebrates: vertebrates: have notochord, dorsal nerve cord, gills primitive: no jaws, no paired appendages, 2 semicircular canals in ear. Agnatha: living = lamprey, hagfish, Scavengers and parasites.

Fossil Agnatha: add bony armor; filter feeding.

Non-vertebrate Chordates. Amphioxus

No limbs, no jaws, gills a feeding device (become respiratory when armor added and skin respiration no longer possible. Also size increase means need something more than skin for respiration), but has a notochord, nerve cord and gills. Marine organisms.

Another non-vertebrate Chordate. Tunicate

Adult: filter feeder – basket of “gills” No nerve cord, no notochord, mostly eats and reproduces. Larvae – some have motile larvae = dispersal and ability to choose substrate.

Tunicate and larvae Larvae has tail = notochord, nerve cord, muscles. Note: origin of visceral- somatic dichotomy Visceral = adult Somatic = larval tail origin of need for dorsal nerve cord, notochord

Somatic organism = nerves, brain, muscle, tail,segmentation, notochord –voluntary nervous system = swimming portion of larva Visceral organism = gut, reproduction, respiration = involuntary nervous system = adult portion of larva

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

Agnatha (fossil ones) Armored – heavy Tail asymmetric no jaws Not much paired appendages Internal skeleton of cartilage Sea scorpion; arthropod, up to 6 feet long. predatory

How to become a better fish Add paired appendages Add jaws Loose the armor Strengthen internal skeleton

Jawless fish Armored fish Cartilaginous fish Spine fins Bony fish

Paired fins allow stabilization and maneuverability.

Fin types Fin fold Spiny fin Ray fin Fleshy fin Ray fin fleshy fin, fleshy fin symmetrical asymmetrical

Agnatha with fin folds

Shark with ray fin

Turn gills into jaws: what permits this? Gills become more efficient, not as many needed. Primitive fish – 7 gills, modern fish 3.5 tturnT

Class placodermi ; added jaws but still has armor on head and shoulders.

shark skin = denticles = small teeth, = same structure as dermal armor of primitive fish and of our teeth – enamel on outside, dentine underneath, then bone.

Class Osteichthyes Turn internal cartilage into bone during development Reduce armor to thin scales.

Note: dual origin of skelton Blue and red = internal skeleton Cartilage (blue) being replaced by bone (red) Pink – the skull is direct ossification, no cartilage precursor and is remnant of old dermal armor, protecting head and brain.

Crossopterygian; sarcopterygian with fleshy fin