THE ORIGIN OF VERTEBRATES

Who are the living vertebrates?

• Jawless fish: hagfish and lamprey

• Fish with jaws & cartilage skeletons: sharks and rays

• Fish with jaws & bony skeletons: all other fish (tuna, flounder, bass, etc.)

• Amphibians: frogs and salamanders: cold-blooded, lay eggs in water

• Reptiles: turtles, snakes and lizards: cold-blooded, lay eggs on land

• Birds: warm-blooded, feathers, lay eggs

• Mammals: warm-blooded, hair, eggs & live birth, nurse young.

What traits unite vertebrates?

• Spinal cord: A hollow nerve tube that runs on top or through backbone

• Heads: Brains and sense organs at the front end.

• Tails: Nerves, back support and muscles that extend past our butts.

What traits unite vertebrates?

• Hearts: A closed circulatory system with a muscular pump to move fluid.

• "Gill" Slits: Openings in pharnyx connecting throat to exterior.

• Segmented muscle on body wall: V-shaped muscles masses.

Where did all this stuff come from?

Non-Chordate Next of Kin

• Echinoderms: e.g., starfish, crinoids, sea cucumbers

• Range: Cambrian - Recent

• Habitat: Exclusively marine

• Unique characters: 5-fold symmetry in adults, water-vascular system, a uniquely constructed calcite skeleton

• Shared novelties: Embryonic traits (Radial pattern of embryonic cleavage, Deuterostome, Mesoderm formed by pouching); Skin-based nerve network; Bilateral, cilia-covered larvae.

• Hemichordates: Acorn Worms

• Range: Cambrian - Recent

• Habitat: Exclusively marine

• Unique characters: Acorn worms are large(up to 2 m), burrowing worm-like filter-feeders with a long muscular proboscis and a fleshy collar.

• Shared novelties: Adults are bilaterally symmetric; Closed circulatory system; Paired openings in the throat.

Chordate Next of Kin • Urochordates: Tunicates or sea squirts

• Range: No fossil record

• Habitat: Exclusively marine

• Unique characters: Tunicates are small, box-like filter-feeding animals that live either alone or in colonies cemented to the sea floor.

• Shared novelties: Notochord; Hollow nerve cord along back; Tail; Endostyle (a group of ciliated cells with alternating mucus cells; used to entangle food) an organ used for filter feeding.

• Cephalochordates: Branchiostoma

• Range: Cambrian (Pikaia from the Burgess Shale) - Recent

• Habitat: Exclusively marine

• Unique characters: Branchiostoma, also known as the lancelet, is a small, free-living fish-like animal that lives among sand grains and filter feeds.

• Shared novelties: Segmented muscles on upper body wall.

Craniates: the most primitive "vertebrates"

• Hagfish: the most primitive known "vertebrate“

• Range: Carboniferous-Recent

• Habitat: Exclusively marine

• Unique characters: Scavengers and carnivores that actively feed by rasping at prey with a bony tongue. They tie themselves into a knot to lever a chunk out of prey. They can coat themselves with mucous for defence. They contain no vertebrae and no bone.

• Shared novelties: A head (cartilage brain case; partial cranium); Sense organs on the head (weak eyes); A true heart; True gills for efficient oxygen retrieval from water; Cartilage gill supports to hold up these flimsy sheets.

• Heterostracans: the first truly abundant fishes

• Range: Cambrian - Devonian

• Habitat: Originate in marine waters, later invade fresh water

• Unique characters: Jawless, armored body with scales on the tail. The tail was the main source of propulsion. Bottom feeding hunters and detritus feeders. Still no vertebrae.

• Shared novelties: Improved sense organs (better balance & vision, lateral line system for motion detection and probably electroreception (used to hunt); Bone on the outer skull but not on the braincase.

Why Bones of Calcium Phosphate?

• Less soluble than calcite, not as subject to dissolution by metabolic acids.

• Store of an important nutrient, phosphate.

• May serve as insulation for electroreceptors.

• Protection.

The first true vertebrates

• Lamprey: another extant jawless "fish"

• Range: Carboniferous - Recent

• Habitat: Originate in marine waters, later invade fresh water.

• Unique characters: The adult is a parasitic bloodsucker. It is jawless, but its mouth has many hooks for latching onto prey, then they use the tongue to bore through the side of the prey. No bone on the body - an evolutionary reversal.

• Shared novelties: Vertebrae surrounding notochord (made of cartilage); Dorsal and anal fins; Endostyle turns into thyroid.

• Osteostracan: another group of extinct armored jawless fish

• Range: Silurian - Devonian

• Habitat: Some marine, mostly fresh water

• Unique characters: Similar to heterostracans, with a bony head shield, scales on the tail, propulsion from the tail, well developed sense organs, and elaborate plumbing for gill system. They were active swimmers. Many were bottom feeders.

• Shared novelties: Paired pectoral fins (source of forelimbs); Braincase covered in bone.

The first vertebrates with jaws • Placoderms: the most primitive jawed fish

• Range: Silurian - Carboniferous

• Habitat: Marine and fresh water

• Unique characters: Heavy armour on head and front of the trunk, scales on the tail, no teeth, just plates of bone for shearing food, heavy fish (probably slow swimmers).

• Shared novelties: Jaws; Paired pelvic fins (source of hind limbs); Paired nasal openings.

• Acanthodians: earliest known jawed fishes

• Range: Silurian - Permian

• Habitat: Initially marine, later invade fresh water.

• Unique characters: Their fins were supported by erectable spines. Some filter fed, others had teeth. Highly manueverable swimmers propelled by their tails.

• Shared novelties: Teeth; Advanced jaw joint.

• Chondrichthyes: Cartilaginous fish, e.g., sharks and rays

• Range: Silurian - Recent

• Habitat: Mostly marine, but some fresh water.

• Unique characters: No bone except in their scales (an evolutionary reversal). Fin and tail structures suggest an active, highly efficient swimming for a predatory life style. Sharks give birth to live young. This requires internal fertilization of eggs. They link up when breeding by using claspers.

• Shared novelties: Regular pattern of tooth replacement.

Vertebrates with skeletons made entirely of bone

• Osteichthyes: Bony fish - fish with bony skeletons.

• Range: Silurian - Recent

• Habitat: Marine and fresh water.

• Shared novelties: Skeleton completely composed of bone, including skull, vertebral column, fins, and ribs; Swim bladder for buoyancy control.

Two major groups within the bony fish

• Ray-finned fish

• Range: Silurian - Recent

• Habitat: Marine and fresh water

• Shared Novelty uniting bony fish: Fins made of bony spines connected by poorly muscled webs.

• Lobe-finned fish: lungfish, coelocanths, rhipidistians

• Range: Devonian - Recent

• Habitat: Mostly fresh water, some marine.

• Unique Characters: Torpedo shaped body with heavy scales, unusual bone with many pores, perhaps for electroreceptive cells.

• Shared novelties: Paired pectoral and pelvic fins that are fleshy and muscular; Peculiar convoluted dentin and enamel.

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