Deuterostome Animals

Overview of Deuterostomes

Deuterostomes are a major lineage of animals characterized by unique embryonic development and include some of the largest and most complex animal groups.

• Key embryonic features: Radial cleavage, formation of the anus from the blastopore, enterocoelous coelom formation, and indeterminate fate of early blastomeres.

• Major phyla: Echinodermata, Hemichordata, Xenoturbellida, and Chordata.

Deuterostome Phyla

• Echinodermata: Sea stars, sea urchins, and relatives.

• Hemichordata: Acorn worms, deposit/suspension feeders.

• Xenoturbellida: Recently discovered wormlike species.

• Chordata: Vertebrates and their relatives.

Echinodermata

Defining Characteristics

Echinoderms are exclusively marine animals with unique body plans and systems.

• Larval symmetry: Bilateral

• Adult symmetry: Pentaradial (five-sided)

• Endoskeleton: Made of calcium carbonate

• Water vascular system: Branching, fluid-filled tubes forming a hydrostatic skeleton

• Tube feet: Fluid-filled, externally projecting podia

Echinoderm Life Styles

Echinoderms display diverse feeding strategies, often using their podia.

• Predatory species: Use tube feet to open prey

• Suspension feeders: Trap particles with podia

• Deposit feeders: Consume organic material from substrate

• Herbivores: Feed on algae

Echinoderm Diversity

Five major living lineages:

• Crinoidea: Feather stars, sea lilies (suspension feeders)

• Asteroidea: Sea stars (predators, scavengers)

• Ophiuroidea: Brittle stars, basket stars (suspension/deposit feeders)

• Echinoidea: Sea urchins, sand dollars (herbivores/suspension feeders)

• Holothuroidea: Sea cucumbers (suspension/deposit feeders)

Chordata

Defining Characteristics

Chordates are defined by four key features present at some stage of development.

• Pharyngeal slits: Used for feeding, gas exchange, or as parts of the ear/head/neck

• Dorsal hollow nerve cord: Central nervous system precursor

• Notochord: Supportive, flexible rod

• Muscular post-anal tail: Locomotion

Chordate Phylogeny

Three major lineages:

• Cephalochordates: Lancelets (marine, mobile suspension feeders)

• Urochordates: Tunicates (marine, retain only pharyngeal slits as adults)

• Craniates (Vertebrates): Animals with skulls and often backbones

Cephalochordates

• Small, marine, mobile suspension feeders

• Dioecious with external fertilization

Urochordates

• Marine, retain only pharyngeal slits as adults

• Exoskeletal tunic, U-shaped gut, two siphons

• Sexual (internal/external fertilization), some asexual

Craniates/Vertebrates

• Head with skull (cranium), brain, eyes, sensory organs

• Higher metabolism, complex musculature, heart with 2+ chambers

• Erythrocytes with hemoglobin, kidneys

Vertebrate Synapomorphies

• Spinal column (cartilage or bone)

• Cranium (elaborate skull)

• Fin rays (in aquatic forms)

• Large brains divided into forebrain, midbrain, hindbrain

Neural Crest Cells

Neural crest is a unique feature of craniates, producing diverse cell types.

• Craniofacial bones and cartilage

• Melanocytes, smooth muscles, adrenal medulla

• Parts of nervous system

Myxini (Hagfish)

• Most basal craniates, marine, bottom-dwelling scavengers

• Cartilaginous skull, axial rod of cartilage from notochord

• Lack jaws and vertebrae, small brain/eyes/ears, tooth-like formations

Petromyzontida (Lampreys)

• Oldest living vertebrate lineage

• Lack true vertebrae, jawless, ectoparasites of fish

Vertebrate Key Innovations

Fossil evidence documents major evolutionary innovations:

• Bony exoskeleton (~480 mya)

• Jaws (~440 mya)

• Bony endoskeleton (~410 mya)

• Limbs capable of moving on land (~365 mya)

• Amniotic egg (~345 mya)

Jaw Evolution

Jaws evolved from anterior-most gill arch cartilage, a neural crest derivative.

• Gnathostome characters: Enlarged forebrain, lateral lines in aquatic forms

• Most diverse vertebrate group today

Major Vertebrate Groups

Jawed Lineages

• Chondrichthyes: Cartilaginous fishes

• Osteichthyes: Bony fishes (includes Actinopterygii and Sarcopterygii)

• Actinopterygia: Ray-finned fishes

• Sarcopterygia: Lobe-finned fishes (includes tetrapods)

Chondrichthyes

• Cartilaginous skeleton, mostly marine

• Cloaca, acute senses, paired fins, placoid scales

• Elasmobranchii: Most diverse group (sharks, rays)

• Holocephalians: Ancient, deep sea, single gill opening

Osteichthyes

• Bony endoskeleton, high species and habitat diversity

• Includes Actinopterygii (ray-fins), Sarcopterygii (lobe-fins), and tetrapods

Actinopterygii: Ray-finned Fishes

• Fin rays in paired fins, most have swim bladder

• Greatest diversity among Osteichthyes

• Bony scales, protrusible jaws, elaborate fin specialization

Sarcopterygii: Lobe-finned Fishes

• Muscular pectoral and pelvic fins supported by bones

• Scales of cosmine, diverse fossil record

• Living species: Lungfish (Dipnoi), Coelacanth (Actinistia)

Fin-Limb Transition

The transition from fins to limbs was a major event in vertebrate evolution, linking fish to tetrapods.

• Tetrapod adaptations: Four limbs with digits, neck, pelvic girdle fused to backbone, absence of gills, ears for airborne sounds

• Supported by fossil and molecular evidence

Tiktaalik: The "Fishapod"

Tiktaalik is a transitional fossil showing both fish and tetrapod features.

• Fins, gills, lungs, scales

• Ribs for breathing air, neck, flattened skull, eyes on top

• Fin bones showing limb pattern

Amphibians

Major Clades and Characteristics

Amphibians are the first tetrapods to colonize land, showing diverse adaptations.

• Three clades: Frogs & toads (Anura), Salamanders (Uredela), Caecilians (Apoda)

• Lungs and/or skin for respiration, moist skin, metamorphosis, cloaca

Amniota

Major Clades and Characteristics

Amniotes are distinguished by the amniotic egg and adaptations for terrestrial life.

• Major clades: Anapsida, Synapsida, Diapsida (distinguished by skull openings)

• Lay cleidoic egg, rib cage ventilated lungs, less permeable skin

Amniotic Egg and Extraembryonic Membranes

The amniotic egg allows reproduction independent of water and contains specialized membranes.

• Amnion: Protective membrane

• Yolk sac: Nutrients

• Allantois: Waste storage

• Chorion: Gas exchange

• Albumen: Water and protein supply

Reptilia

Major Lineages and Characteristics

Reptiles include four major lineages and show adaptations for terrestrial and aquatic life.

• Lizards & snakes (squamates), turtles (chelonians), crocodiles & alligators (crocodilians), birds (Aves)

• Ectothermy (except birds), scales, well-developed lungs, oviparous (most)

Birds: From Dinosaurs to Modern Forms

Evolution and Characteristics

Birds are descended from feathered theropod dinosaurs and are the only living endothermic reptiles.

• Early forms had teeth, clawed forelimbs, long tails

• All egg layers, feathers, scaly hind feet, hollow bones, clavicle, keeled sternum

• Lack urinary bladder, most have one ovary, efficient respiratory system, enlarged brains

Feathers and Flight

Feathers are made from β-keratin and evolved from reptilian scales, serving multiple functions.

• Insulation, camouflage, gliding, courtship display, flight

• Flight evolution hypotheses: Ground up or trees down

Mammalia

Major Clades and Characteristics

Mammals are the only living synapsids, with unique features and three major clades.

• Insulating hair or fur, three ear bones, mammary glands, enucleated red blood cells, seven cervical vertebrae, diaphragm

• Clades: Monotremata (egg-layers), Metatheria (marsupials), Eutheria (placentals)

Amniote to Mammal Jaw Evolution

• Reduction of jaw bones, increase of middle ear bones, modified jaw joint

• Mammals have single lower jaw bone (dentary), articular = malleus, quadrate = incus

Monotremes

• Most ancient group, lay shelled eggs, retain cloaca, low metabolic rates

• Platypus and echidnas (Australia/New Guinea)

Marsupials

• Young born early, nurtured in marsupium, diverse in Cretaceous

• Australia, New Guinea, Americas

Placentals

• Global distribution, most diverse modern mammals

• Young born later and more developed, diet correlates with tooth and digestive tract structure

Metatherian - Eutherian Convergences

• Australian marsupials converged in habit with many placentals