Chapter 34: Deuterostome Animals – Structure, Diversity, and Evolution

Objectives

• Identify major groups of deuterostomes and chordate subphyla, including their defining characteristics.

• Describe four defining characteristics of chordates.

• Explain major evolutionary steps and groupings leading to the majority of vertebrates known today.

• Recognize four major groups of Vertebrata and their defining characteristics.

• List major groups of Osteichthyes.

• Know four extra-embryonic tissues.

Deuterostome Characters

Defining Features

Deuterostomes are a major clade of animals characterized by unique embryonic development and high morphological diversity.

• Largest-bodied animals and some of the most morphologically complex.

• Variable morphology and behavior.

• Shared embryonic features:

  • Radial cleavage during early cell division.

  • Formation of anus from blastopore (mouth forms secondarily).

  • Enterocoelous coelom formation (from mesodermal pouches).

  • Indeterminate fate of early blastomeres (cells can develop into any tissue).

Phylum	Key Features
Echinodermata	Bilateral larvae, pentaradial adults, water vascular system, endoskeleton
Chordata	Notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail

Deuterostome Phyla

Major Groups

Deuterostomes include four phyla, with Echinodermata and Chordata being the most studied in detail.

• Echinodermata: Sea stars, sea urchins

• Hemichordata: Acorn worms (burrowing/deposit or suspension feeders)

• Xenoturbellida: Recently discovered, wormlike species

• Chordata: Vertebrates and related animals

Echinodermata

Synapomorphies

Echinoderms are exclusively marine animals with unique body plans and feeding mechanisms.

• Bilateral symmetry in larvae, but pentaradial symmetry in adults (five-sided).

• Calcium carbonate endoskeleton for support.

• Water vascular system:

  • Branching, fluid-filled tubes and chambers.

  • Forms a hydrostatic skeleton.

  • Tube feet with externally projecting podia for movement and feeding.

Life-Styles

• Podia are essential for feeding and movement.

• Feeding types:

  • Predatory species

  • Suspension feeders

  • Deposit feeders

  • Herbivores

Phylogeny

Class	Common Name	Feeding Type
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

Chordate Characters

Defining Traits

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

• Pharyngeal slits: Used for suspension feeding, gas exchange, or ear/neck structures.

• Dorsal hollow nerve cord: Central nervous system precursor.

• Supportive/flexible notochord: Provides structural support.

• Muscular post-anal tail: Locomotion and balance.

Chordate Phylogeny

Major Lineages

• Cephalochordates (invertebrates): Lancelets or amphioxus

• Urochordates (invertebrates): Tunicates, sea squirts, salps

• Craniates (vertebrates): Animals with skulls and backbones

Cephalochordates

Small, marine, mobile suspension feeders. Dioecious with external fertilization.

• Lancelets (amphioxus) are the main example.

Urochordates

Marine animals retaining only pharyngeal slits as adults. Possess an exoskeletal tunic, U-shaped gut, and two siphons. Reproduction can be sexual (internal or external fertilization) or asexual.

Craniates/Vertebrates

Distinguished by the presence of a skull (cranium), complex sensory organs, and higher metabolic rates.

• Complex musculature

• Heart with two or more chambers

• Erythrocytes with hemoglobin

• Kidneys

Vertebrate Synapomorphies

• Spinal column (cartilage or bone)

• Cranium (elaborate skull)

• Fin rays (in aquatic forms)

• Large brains divided into three regions:

  • Forebrain (olfaction & cerebrum)

  • Midbrain (vision)

  • Hindbrain (balance, autonomic functions, hearing)

Neural Crest Cells

Unique to craniates, neural crest cells originate near the dorsal margins of the closing neural tube in embryos.

• Produce craniofacial bones and cartilage

• Melanocytes (pigment cells)

• Smooth muscles

• Adrenal medulla

• Parts of the nervous system

Myxini (Hagfish)

Most basal group of craniates. Marine, bottom-dwelling scavengers.

• Cartilaginous skull

• Axial rod of cartilage from notochord

• Lack jaws and vertebrae

• Small brain, eyes, ears

• Tooth-like formations

Petromyzontids: Lampreys

Oldest living vertebrate lineage. Jawless, lack true vertebrae (arcualia), found in fresh and salt water, ectoparasites of live fish.

Vertebrate Key Innovations

Major evolutionary innovations documented by fossils:

1. Bony exoskeleton (~480 million years ago)

2. Jaws (~440 mya)

3. Bony endoskeleton (~410 mya)

4. Limbs capable of moving on land (~365 mya)

5. Amniotic egg (~345 mya)

Jaw Evolution

Jaws evolved from the anterior-most gill arch cartilage, which originates from neural crest cells.

• Gnathostome (jawed vertebrate) characters:

  • Enlarged forebrain (smell/vision)

  • Lateral line system in aquatic forms

  • Most diverse vertebrate group today

"Fishes" are Paraphyletic

"Fishes" do not form a monophyletic group; they are a grade, not a clade.

• Three jawed lineages today:

  • Chondrichthyes (cartilaginous fish)

  • Osteichthyes (bony fish)

    • Actinopterygii (ray-finned fish)

    • Sarcopterygii (lobe-finned fish, includes tetrapods)

Chondrichthyes

Cartilaginous skeleton, mostly marine, acute senses, paired fins, placoid scales, most predatory.

• Two major groups:

  • Elasmobranchii: Spiral valve, ampullae of Lorinzini, most diverse

  • Holocephali: Venomous spine, single gill opening, deep sea, no stomach

Osteichthyes

Includes bony fish (Actinopterygii and Sarcopterygii) and tetrapods. Characterized by bony endoskeleton, high species and habitat diversity.

Aquatic Bony Fish

• Operculum (gill cover)

• Lateral line system

• Paired fins

• Mostly oviparous (external fertilization)

• Primitive lungs or swim bladder (teleosts)

Actinopterygii: Ray-fins

• Fin rays in paired fins

• Most have swim bladder

• Greatest diversity among Osteichthyes (96%)

• Wide range of aquatic habitats and diets

• Bony scales (mostly reduced/ganoid or enamel-like)

• Protrusible jaws and mobile pharyngeal jaws

• Elaborate fin specialization

Sarcopterygii: Lobe-fins

• Scales of cosmine (dentine-like material)

• Muscular pectoral and pelvic fins supported by linear bone arrangement

• Diverse fossil record

• Only 8 living species: Lungfish (Dipnoi), Coelacanth (Actinistia)

Living Sarcopterygian Fish

• Coelacanths: Marine, deep sea, live-bearers, vestigial lung, intracranial joint

• Lungfish: Freshwater, lay eggs, can become dormant, found in Southern Hemisphere

Fin-Limb Transition

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

• Adaptations:

  • Four limbs/feet with digits

  • Neck for greater head motion

  • Pelvic girdle fused to backbone

  • Absence of gills

  • Ears for detecting airborne sounds

Tiktaalik: "Fishopod"

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

• Fins, gills, lungs, scales

• Ribs to breathe air and support body

• Neck and flattened skull with eyes on top

• Fins showing bone pattern of limb

Additional info: These notes cover the major topics of Chapter 34 (The Origin and Evolution of Vertebrates) and are suitable for General Biology students preparing for exams on animal diversity, phylogeny, and evolutionary innovations.