Deuterostomes and Echinoderms

Deuterostomes Overview

• Consists of Echinodermata, Hemichordata, and Chordata phyla.
• Echinodermata include sea stars and sea urchins.
• Hemichordata are acorn worms.
• Chordata includes vertebrates.

Echinoderm Body Plan

• Larvae exhibit bilateral symmetry.
• Adults have pentaradial (five-sided) symmetry.
• Possess an endoskeleton made of calcium carbonate plates secreted inside the skin.
• Have a branching, fluid-filled water vascular system.
• Tube feet are fluid-filled appendages that are part of the water vascular system.
• Podia are sections of tube feet that project outside the body for substrate contact.
• The water vascular system functions as a hydrostatic skeleton used for movement.
• Pentaradial symmetry, calcium carbonate endoskeleton, and water vascular system are synapomorphies.

Echinoderms as Consumers

• Sea stars are carnivores and major predators of sessile bivalves in rocky intertidal zones.
• Pisaster ochraceus is a keystone species that significantly impacts species distribution and abundance.
• Sea urchins are herbivores with a unique jaw-like structure called Aristotle's lantern.
• Sea urchins primarily feed on marine algae, especially kelp.

Major Echinoderm Lineages

• Crinoidea (feather stars, sea lilies) are mostly suspension feeders using tube feet and cilia.
• Sea lilies are stalked and sessile, while feather stars can swim short distances.
• Asteroidea (sea stars) are predators or scavengers that use tube feet to pull apart bivalves and evert their stomach.
• Ophiuroidea (brittle stars, basket stars) capture prey, scavenge, or suspension feed using flexible arms and tube feet.
• Echinoidea (sea urchins, sand dollars) are mostly herbivores (sea urchins) or suspension feeders (sand dollars).
• Holothuroidea (sea cucumbers) are suspension or deposit feeders using modified tube feet called tentacles.

Chordate Defining Features

• Pharyngeal slits or pouches.
• Dorsal hollow nerve cord.
• Notochord.
• Muscular tail that extends past the anus.
• These traits enable rapid swimming.

Chordate Groups

• Cephalochordates (lancelets) are small, mobile suspension feeders that burrow into the sand.
• Urochordates (tunicates) have a tunic that supports the body.
• Sea squirts suspension feed by pumping water through pharyngeal slits.
• Vertebrates include hagfishes, lampreys, sharks, bony fishes, amphibians, reptiles, birds, and mammals.
• The spinal cord is the dorsal hollow nerve cord.
• Pharyngeal slits become gill slits in fishes and appear during development in terrestrial vertebrate embryos.

Invertebrate Chordate Lineages

• Cephalochordata (lancelets) suspension feed using pharyngeal slits and swim using muscles along the notochord.
• Urochordata, Ascidiacea (sea squirts) suspension feed by pumping water through an incurrent siphon.
• Urochordata, Thaliacea (salps) suspension feed by pumping water and move via ciliary action or jet propulsion.

Vertebrate Characteristics

• Distinguished by vertebrae (protecting the spinal cord) and a cranium (protecting the brain).
• The vertebrate brain has a forebrain (smell, cerebrum), midbrain (vision), and hindbrain (balance, hearing).
• Gnathostomes (jawed vertebrates) have an enlarged cerebellum and medulla oblongata in the hindbrain.

Key Innovations in Vertebrate Evolution

• Urochordates are the closest living relatives and the sister group to vertebrates, according to molecular data.
• The oldest vertebrate fossils are from Chengjiang, China (530 mya).
• Metaspriggina walcotti from the Burgess Shale (505 mya) is a well-preserved early vertebrate.
• Early vertebrates had a cartilaginous cranium and lacked bone
• Early vertebrates had a cartilaginous cranium and lacked bone.
• Hagfishes have a three-part brain, paired eyes, and a cartilaginous cranium.
• Lampreys have cartilaginous vertebrae. Analysis of microRNAs supports the monophyly of hagfishes and lampreys.

Jawless Fish Lineages

• Hagfishes are scavengers and predators that swim by undulating and use the notochord for support.
• Lampreys are ectoparasites that use suckers to move upstream and have larvae resembling lancelets.

Origin of the Vertebrate Jaw

• Jawed fishes appeared in the Silurian period (430mya).
• Placoderms are early armored fishes with bony shields.
• Jaws and teeth improved prey capture.
• Paired fins and internal fertilization also evolved.
• Natural selection acted on developmental regulatory genes determining the morphology of gill arches.
• Gill arches and jaws consist of hinged bars of bone or cartilage and develop from similar cells.
• Hox and Dlx genes are similarly expressed in jaws and gill arches.

Jawed Fish Lineages

• Chondrichthyes (sharks, skates, rays) are mostly predators with internal fertilization.
• Actinopterygii (ray-finned fishes) have protrusible jaws and often a gas-filled swim bladder and are mostly oviparous with external fertilization.
• Coelacanths have a hinged skull and swim with lobed fins, using a tetrapod gait and internal fertilization.
• Dipnoi (lungfishes) are omnivorous with crushing tooth plates.

Origin of the Bony Endoskeleton

• Cartilaginous endoskeleton stiffened by bone in the Silurian period.
• Swim bladder evolved from paired lungs for neutral buoyancy.

Origin of the Limb

• First tetrapods appear in the fossil record 365 mya.
• Lungfish morphology and the fossil record show a transition from fins to limbs.
• Amphibians were the first tetrapods.
• Most amphibian adults feed on land but lay eggs in water.
• Amphibians undergo metamorphosis.
• Anura (frogs, toads) are carnivorous as adults and jump or swim using hind legs.
• Urodela (salamanders) walk on land and undulate to swim, mostly carnivorous as adults.
• Gymnophiona (caecilians) lack limbs and burrow or swim by undulating.

Origin of the Amniotic Egg

• Amniotes (reptiles, birds, mammals) are named for the amniotic egg.
• Amniotic eggs allow egg-laying on land.
• The egg has four membranes: amnion, yolk sac, allantois, and chorion.
• Amniotic eggs have leathery shells that are not waterproof in most reptiles, but in birds, the shell is hard and waterproof.
• In viviparous reptiles and mammals, the amniotic egg is modified and retained internally.

Origin of Lactation and Fur

• Mammals are named for mammary glands for lactation.
• Mammals are endotherms.
• Monotremata (platypus, echidnas) feed using a beak or bill and secrete milk from skin glands.
• Marsupiala (marsupials) complete development in the mother's pouch.
• Eutheria (placental mammals) have an extensive placenta.
• The placenta facilitates nutrient and gas exchange.
• Amnion and chorion support diffusion of gases, nutrients, and wastes, within the placenta
• The evolutionary advantage of viviparity and the placenta include constant temperature, protection, and portability.
• The earliest mammals had a single bone in the lower jaw and three bones in their middle ear.

Origin of Scales and Feathers

• Many non-avian reptiles are ectotherms.
• Lepidosauria (lizards, snakes) are mostly carnivorous with some parthenogenesis in reproduction.
• Testudinea (turtles) are mostly carnivorous marine turtles or herbivorous terrestrial turtles, and exclusively oviparous.
• Crocodilia (crocodiles, alligators) are carnivorous ambush predators.
• Aves (birds) are herbivores, carnivores, and detritivores.
• Feathers are made of keratin providing insulation, display, lift, and steering.
• Wings and flight evolved in pterosaurs, bats, and birds.
• Birds are a monophyletic taxon within dinosaurs.
• Further adaptations make powered flight efficient. Examples: keeled sternum, reduction in weight of skeleton, endothermy.

Parental Care

• Involves providing food, warmth, and protection.
• Most extensive in mammals and birds, lactation in mammals.
• Mammals and birds invest time and energy in parental care.