Ch. 24 Flashcards
Animals: Cell Structure and Specialization
• Animals are multicellular eukaryotes
• They lack cell walls (unlike plants and fungi)
• Their bodies are held together by an extracellular matrix (ECM) - structural proteins such as collagen
• Nervous tissue and muscle tissue are unique, defining characteristics of animals →ability to move!
• Tissues are groups of cells that have a common structure, function, or both: true tissues separated by membranous layers
Characteristics of Animals
• Chemoheterotrophic: cannot make own carbon-based food source, must consume other organisms
• Sexual reproduction (mostly): motile haploid sperm fertilizes larger non-motile haploid egg to make diploid zygote
• Capable of movement: at least one stage of life cycle
Animal Diversity:
• Basal Phyla
• Protostomes
• Deuterostomes
Motility (movement) in Animals
• Some are only able to move around for part of their lives
• Some are sessile (stationary/not moving) for a portion of their life cycle
(e.g., during adulthood)
Key innovations in animal evolution
- Patterns of embryonic development
- Development of different tissues
- Type of body symmetry
- Presence or absence of a body cavity
Homeotic Genes in Animals
• Highly conserved nucleotide sequence
• Present in all eukaryotes with crucial role in
development of morphogenesis
• Regulatory genes (that produce proteins)
→turn other genes on and off
(subset of homeotic genes found in animals)
• Control anterior to posterior developmental sequence of embryo (i.e., pattern of formation during development)
✓order of Hox genes along chromosomes is similar among different groups of animals (homologous)
✓but number of repetitions can differ
• Number of Hox genes varies among animal phyla
─absent in sponges, few in jellyfish, more in arthropods and many in vertebrates (and chordates)
• Result of several gene duplication events over evolutionary time
Reproduction and Embryonic Development
• Most animals share similar pattern of early embryonic development:
• Have haploid gametes of different sizes - small motile sperm,
- large non-motile eggs
• Diploid zygote undergoes a number of mitotic cell divisions = cleavage
• Cleavage leads to formation of a multicellular, hollow blastula
• The blastula undergoes gastrulation, forming a gastrula with different layers of embryonic tissues
• During development, germ layers give rise to the tissues and organs of the animal embryo
Embryonic Development
Protostomia: first invagination of the gastrula (blastopore) becomes the mouth
• proto = first, stoma = mouth
Deuterostomia: second invagination becomes the mouth
- deutero = second
(first opening become anus, or closes up)
Protostomes vs. Deuterostomes
Protostomes:
(A) Cleavage: 4-cell embryo —> 8-cell embryo, spiral cleavage
(B) mesoderm and coelom formation: it just looks different, may have to draw it out somewhere
(C) origin of mouth and anus: blasotpore develops into mouth; anus forms later
Deuterostomes:
(A) Cleavage: 4-cell embryo —> 8-cell embryo, radial cleavage
(B) mesoderm and coelom formation:
(C) origin of mouth and anus: blastopore develops into anus; mouth forms later
Cell Cleavage
• Protostomes: the new row of cells is twisted slightly off centre:
— spiral cleavage
— determinate: new cell is destined to form some part of the later embryo (removal of some cells results in embryo missing organs)
• Deuterostomes: each cell division stacks the new cells directly above the previous ones
— radial cleavage
— indeterminate: early embryonic cells not differentiated (could split young embryo and get two complete later embryos→identical twins!)
Animal Development
• Direct development: embryo continues gradually on towards adult form
• e.g., humans
• Indirect development: intervening stages (larvae) whose morphology and behaviour differs greatly from sexually mature adult stage
• e.g., caterpillar→butterfly
Eumetazoa
• Animals with true tissues
• Tissue = integrated group of cells with common structure and function, isolated from other tissues by membranous layers Protostomia
• Parazoan design: aggregate of cells; specialized cells for different functions
• Eumetazoan design: specialized cells aggregated into distinct tissues; more advanced designs, tissues combine → organs
Origin of Embryonic Tissue Layers and Muscles
• While sponges have the genetic tool kit needed for cell-to- cell and cell-to-extracellular matrix (ECM) adhesion:
─ they do not form complex tissues
• Animals other than sponges are divided into two groups: ─ based on the number of embryonic tissue layers they have
─ And they do not form complex tissues
Germ layers
Germ Layers→Tissue
• Diploblastic → 2 germ layers
• Triploblastic → 3 layers
Diploblastic .vs. Triploblastic
• Germ layers:
— endo-→digestive tract (gut)
— ecto -→outer covering (skin & nerves)
— meso-→muscle and other organs
• “Radiata”: 2 embryonic cell layers
— diploblastic: endo- and ectoderm
• Bilateria: 3 embryonic cell layers
— triploblastic: endo-, ecto- and mesoderm (meso = middle)
Body plans: symmetry
• Asymmetric: lacking symmetry
– e.g., sponges
• Radial symmetry: no front and back, or left and right
– e.g., sea anemones, comb jellies
• Bilateral symmetry: two-sided symmetry
– e.g., lobster, humans
(a) Radial symmetry
— Radial animals are often sessile or planktonic (drifting or weakly swimming)
(b) Bilateral symmetry
— Bilateral animals often move actively and have a central nervous system
Bilaterally symmetrical animals have
– A right and left side
– A dorsal (top) side and a ventral (bottom) side
- Anterior (head) and posterior (tail) ends
- Note: Cephalization, the development of a head (central nervous system)
Body Symmetry
• asymmetric
# planes of symmetry: 0
locomotory abilities: sessile (no movement)
degree of cephalization: no head
Lifestyle: suspension feeders
• bilaterally symmetric
# planes of symmetry: 1
locomotory abilities: Highly mobile with lots of variation (some exceptions)
degree of cephalization: highly cephalized (some exceptions)
Lifestyle: Seek food (exceptions)
• radially symmetric
# planes of symmetry: >1
locomotory abilities: Usually poor
degree of cephalization: weak to none
Lifestyle: variable but often trap prey
Associations between Body Symmetry and Nervous System
Radial symmetry = Nerve network
Bilateral symmetry = Central Nervous system
Bilateral Symmetry and Cephalization
• Concentration of sensory organs in head
— e.g., sensory structures, receptors
— internal concentration of neural system → brain!
• Adapted for forward & direction movement
— quicker response to stimuli in environment
— better able to search for food
— better defenses capabilities
The Tube-within-a-Tube Body Plan
is common in triploblastic animals (especially in bilaterally symmetrical animals)
Animals May or May Not Have a Body Cavity
Body Cavities
• Most triploblastic animals possess a fluid-filled body cavity
• A true body cavity is called a coelom, derived from mesoderm
• Coelomates are animals that possess a true coelom
(A) Coelomate:
•Coelom
•Body covering (from ectoderm)
•Tissue layer lining coelom and suspending internal organs (from mesoderm)
•Digestive tract (from endoderm)
— Earthworms Arthropods Mollusks Echinoderms Vertebrates Etc…
• Pseudocoelomate: animals that lack complete mesodermal lining
• Triploblastic animals that possess a pseudocoelom are called:
(B) Pseudocoelomate
• Pseudocoelom
• Body covering (from ectoderm)
• Muscle layer (from mesoderm)
• Digestive tract (from endoderm)
• Phylum Nematoda
• Triploblastic animals that lack a body cavity are called acoelomates
(C) Acoelomate
• Body covering (from ectoderm)
• Tissue- filled region (from mesoderm)
• Wall of digestive cavity (from endoderm)
• Phylum Platyhelminthes
Function of body cavity?
• Fluid-filled cavity can be used as hydrostatic skeleton and help with movement
— by tensing muscles around incompressible fluid
• Cushions internal organs from blows to outside body (protection)
• Allows functioning of some internal organs
— e.g., digestive organs may only be able to function properly if they are
free to move, such as through muscle movements for moving food through the digestive tract
• Allows internal organs to shift without deforming outside of
body
— e.g., movement of guts, beating of heart
• Note: coeloms independently gained and lost (evolutionarily) in different animal lineages.
Evolutionary Relationships Among the Animals
Key shared derived characters
• Common ancestor resembled Choanoflagellates
• heterotrophic protist
• single flagellum surrounded by collar of microvilli (finger-like projections of cell membrane)
• cell morphology very similar to that of sponges (Porifera)
Three lines of evidence that choanoflagellates are closely related to animals
- Cell morphology similar in
choanoflagellates and sponges - Cell morphology unique to animal cells
- DNA sequence homology
Evolution of Multicellularity
What’s the advantage of multicellularity?
Hypothesis: ball-shaped colony of choanoflagellates may have evolved into a simple animal with endo- and ectodermal layers.
• If cooperative aggregations of cells are able to survive better and produce more offspring than their unicellular counterparts (higher relative fitness), then these various evolutionary pathways could all be possible.
— What is the role of Natural Selection in this progression?
What’s the advantage of multicellularity?
• cooperation and coordination among cells
• = cells can specialize for different functions (= different cells)
✓protective skin
✓enzyme secretion for trapping food ✓coordinated movement
✓specialized cells for reproduction
Diversity of Kingdom Animalia
Invertebrates are a Paraphyletic Group
Basal phyla Includes:
• Asymmetric body plans
• Radialsymmetrybody plans
Bilateria Includes:
• Mostly Bilateral
symmetry body plans
• Exceptechinoderms
with mostly radial symmetry
The Sponges: Phylum Porifera
Phylum Porifera
Phylum Porifera: characteristics
Lifestyle
• por = pore + fer = to bear
• Lack hox genes and no symmetry
• ~ 8000 extant species, 99% are marine
• Range from a few mm to a few m in
height
• Except for larval stage, they are mostly
sessile
─ attached to one spot, do not move
Phylum Porifera: characteristics
• No obvious tissues or organs
─e.g., no gut, no muscles, no nervous system
• Structural support comes from spicules ─tiny, hard needles or rods
─calcium carbonate or silica
• Some sponges have only tough collagen-protein network for support (= spongin)
• 2 major cell types
─ choanocytes – flagellated for creating stream of water for
feeding
─ amoebocytes – motile, moves nutrients between cells
Lifestyle
• They are suspension feeders → filter food out of the water
─ use cells called choanocytes that have flagella and draw water into the sponge through pores called porocytes
Eumetazoans
• specialized cells aggregated into distinct tissues; more advanced body plans, tissues combine → organs
Phylum Ctenophora
• Gelatinous body
• Have combs – fused cilia arranged
in plates, used in locomotion
• More complex than some other groups of animals (e.g., sponges)
─ have a nervous system
Phylum Cnidaria
• Includes jellyfish, hydras, sea anemones, corals
• ~10,000 extant species (99.9% marine)
• Eumetazoans: have true, differentiated tissues
• Diploblastic = two layers (ecto- and endoderm)
• Most have radial symmetry & simple body plan
• Cnidarians are named for unique cells = cnidocytes
─ A specialized cell used for feeding & defense
• Each cnidocyte contains very complex endocellular structure
- most common type is nematocyst
• Have a sac-like body, with a mouth surrounded by tentacles
• Have nerve cells
• Cnidocytes can send out a barbed nematocyst that can puncture and capture prey (feeding)
• Reproduction: many species alternate between the medusa and polyp forms
─ Often one form (medusa or polyp) is dominant
─ Produce planula (larvae) through sexual reproduction
Two basic body forms:
• Polyp (sessile form): oral end upwards, attached to a substrate
• Medusa (motile form): oral end downwards, moves freely through the water via hydrostatic skeleton
Life cycles in cnidarians
• Have asexual or sexual reproduction
• Haploid gametes (sperm and eggs) are
produced during sexual reproduction
• Some are polymorphic (like most “invertebrates”)
─ Have more than one form during their life cycle
→ Benefit: can exploit different types of environments during different life stages
Corals – specialized mutualism
Anthozoa – coral polyp
zooxanthellae
= dinoflagellates
KEY CONCEPTS
• Animals are a monophyletic group, with sponges as the basal animal
• The evolution of animals is more complicated than a smooth transition from simple to complex
─ Many key innovations did not arise all at once
─ Evolution did not stop within any of the lineages
• Body symmetry is a key morphological aspect of an animal’s body plan
…
Diploblastic:
Ancestral colonial flagellate —> Metazoa —> Eumetazoa —> Cnidaria + Ctenophora
Triploblastic:
Ancestral colonial flagellate —> Metazoa —> Eumetazoa —> Bilateria —> deuterostomia + protostomia
Deuterostomia —> ectoprocta + brachiopoda + (echinodermata + chordata)
protostomia —> platyhelminthes
protostomia —> rotifera + mollusca (Annelida + arthropoda) + nematoda
Kingdom Animalia: Protostomes
• monophyletic
•Lophotrochozoa
— platyhelminthes, rotifera, ectoprotca, brachiopoda, annlide
• ecdysozoa
— Nematoda, Arthropoda
• Protostome embryos typically have determinate, spiral cleavage as cells divide
• Protostome mesoderm differentiates near the blastopore and forms a coelom (schizocolom) within the mesoderm.
• In Protostomes the blastopore becomes the mouth and a second, later opening becomes the anus.
Clade Bilateria
• Bilateral symmetry
• Varying degrees of cephalization (differentiation of a head region)
• anterior concentration of neural ganglia (brain)
• reduced in sedentary animals (e.g., clams) • Triploblastic
• endo-,ecto-andmesoderm
• Nephrozoa refers to the presence of an excretory system
Protostomes are Monophyletic with two major lineages
• Fossils indicate both lineages originated in the ocean:
– Protostomes made the transition from water to land multiple times as they diversified
– Challenges:
1. Avoid drying out
2. Gas exchange
3. Hold up bodies
Lophotrochozoans and Ecdysozoans differ in their mechanism of growth
Lophotrochozoans: Incremental growth
Ecdysozoans: Moult between growth phases
Superphylum Lophotrochozoa
• includes ~18 phyla
• range from morphologically very simple (e.g., flatworms) to morphologically and behaviourally very complex (e.g., mollusks like octopuses)
• some have lophophore larval stage and others have trochophore larvae
• Relationships among lophotrochozoan phyla unclear
– Lophotrochozoa identified by molecular data
Clade Lophotrochozoa
Clade is polyphyletic!!
Very different animal types What do they share?
Protostome development
• first invagination = mouth • spiral cleavage
Unique molecular characters
Phylum Platyhelminthes
• Flatworms (~20, 000 species) • platy = flat, helminth = worm
• Acoelomate
• triploblastic
• but no fluid-filled body cavity
• basically solid tissue
• Most have eye spots (called ocelli)
• No special circulatory or gas-exchange system – How do they “breathe”?
• Nervous system has few ganglia (cluster of nerve cells)
• Do not have a complete digestive system
✓ have a mouth and a gut lumen, but no anus
✓ so, their excretory tube lacks an internal opening
✓ some parasitic ones lack mouths and guts
Consequences of body plan?
• bilateral symmetry → cephalization
• flattened body → larger surface area → nutrients and gases diffuse more efficiently (in water)
Phylum Annelida (~16,500 spp.)
• Segmented worms (annulus = ring)
• polychaetes, earthworm-like worms, leeches
characteristics
• Segmented
─ many repeated units with similar internal and external anatomy (divided by septa)
• Eucoelomate
─ body cavity completely lined with mesodermally derived tissues
─ hydrostatic skeleton
• Closed circulatory system
• blood contained in vessels
• Gas exchange via skin
• Complete digestive system with anus
• applies to all subsequent taxa
• Excretory system
• Nervous system
Phylum Mollusca
• The basic body plan is highly adaptive
• 2nd only to Arthropods
• ~93,000 named species
─ snails, clams, squids and many others
• 8 classes
• Most marine, but many freshwater and terrestrial
• Calcareous (i.e., calcium carbonate) shell enclosing soft body
• Shell lost or reduced in many taxa
─ moll = soft, originally named for octopuses and
cuttlefish
characteristics
• Mantle: thin layer of tissue that secretes shell • Muscular ventral foot used for movement
• Organs contained in a visceral mass above foot
• Mouthparts a straplike rasping organ called radula
• Gills for gas exchange and sometimes for feeding
─ e.g., bivalves like scallops
• Open circulatory system (most) ─ blood not in vessels
─ coelom reduced
• Not clearly segmented
• Many with trochophore larvae
• e.g., snails, nudibranchs, clams, mussels, squid, octopus
• very diverse phylum, with the following similarities…
— Visceral mass (internal organs)
— Mantle (tissue that may secrete shell)
— Foot (muscular, often aids in movement)
→these 3 structures are highly modified in each group of mollusks depending on their lifestyle!
Superphylum Ecdysozoa
(ecdysis = molting)
• Ecdysozoans are the most species-rich animal group
• 8 phyla
• Ecdysis: process of molting the cuticle in order to grow
– entire cuticle shed at once
– cuticle = non-living outer layer of skin
• Cuticlefrequentlysclerotized
– scler = hard
• Only particular regions are hardened
– called sclerites when shaped like plates
• Unsclerotizedcuticleinbetween
– flexible, allows for movement
About the cuticle
• Below cuticle is epidermis: living cells that secrete substances→cuticle
• Cuticle has 3 layers:
– endocuticle (endo = within) – exocuticle (exo = outside)
– epicuticle (epi = upon)
• very fine ducts run from epidermis to top of cuticle
Phylum Nematoda
• Roundworms, threadworms – nemato = thread
• 0.3 mm – 8+ meters long (most < 5mm)
• ~25,000 named species
– estimates of up to 100 x more
• Morphologically similar* – no segmentation
– no appendages
– no eyes
– slightly blunt at one end, slightly pointy at the other
• Must shed cuticle too - moult like other Ecdysozoa
*Note: Simplicity in this case is an example of “secondary” simplification derived from a more complex body plan.
• Pseudocoelomate
– body cavity, not completely lined by mesoderm
• Two sexes:
– internal fertilization
– some are hermaphrodites
• Provide an example of how the pseudocoelom can
help in movement:
1. longitudinal muscles occur along its length
2. to move, they contract the muscles
3. the pressure (called hydrostatic pressure) in the
pseudocoelom straightens the animal once the muscles relax
→ Move side-to-side by contraction of longitudinal body wall muscles
Phylum Nematoda vs. Phylum Annelida
• pseudocoelomate
— cavity between meso & endo
• coelomate
— cavity lined with mesoderm
Phylum Arthropoda
Origins of diversity?
• Morphological and physiological diversification has a genetic basis
•Until recently, biologists wrongly assumed that very different organisms required very different genetic instructions
• Earliest confirmed fossils:Cambrian explosion(~500Mya)
• Most species-rich phylum of any organism (~1 million living, named
species; estimated more than 3 millions!)
– estimates of 3-30 million in total
– 3 out of every 4 named species of animals is an arthropod
• Mostecologically important group of animals – especially decomposition and pollination
Characteristics
• Cephalization
– nervous tissue becomes concentrated toward head during
course of evolution
– head region composed of several segments specialized for feeding and sensing
• Open circulatory system
– hemolymph in hemocoel
• Jointed appendages
– arthros = joint, pod = foot
– specialized functions (walking, feeding, sensory, reproduction, defense)
• Segmented body
– Changes to regulation of Hox genes
• Sclerotized cuticle made of chitin – acts as an exoskeleton
– must moult to grow
– provides support (life on land!) and a rigid surface for muscle attachment
Morphological diversity
• Multicellular animals have a common tool kit of genes that establish the animal body plan during development (recall: Hox genes)
– Expressing the genes in the tool kit at different times and different places during development can lead to dramatic differences
• Variation in Gene expression + Ecological opportunity = Diversity
• Diversification of animal body plans can occur by the generation of new genes over time
• However, changing the expression pattern of existing genes likely had an even larger impact on animal body plan diversification
– a small set of elements can be reused and rearranged to produce a large variety of outcomes
What traits should Arthropods share?
Clade Metazoa
1. Collagen, multicellular, motile
2. Tissues, gastrulation, hox genes, symmetry
3. Bilateral, triploblastic, cephalization, coelom?
4. Protostome (spiral, determinate cleavage)
5. Ecdysis
6. Jointed appendages, segmentation, chitin exoskeleton
Tagmatization
— Specialization in the body regions of arthropods
— Segmented body & jointed appendages arranged into functional units (= tagmata) with specialized functions (division of labour)
→increase in arthropod diversity!
• head and thorax often merged as cephalothorax
Body plans of arthropods
Tagmatization = fusion of segments
Divide arthropods into two groups
Chelicerae:
- Fang like structure for mouth, don’t have antennae
Mandibles:
- Mendibles for mouth and have antennae on the head
5 Main Groups of Arthropods
Subphyla
• Trilobita
– trilobites, all extinct
• Chelicerata
– scorpions, spiders, mites, etc.
• Myriapoda
– millipedes, centipedes, etc.
• Crustacea
– crabs, lobsters, shrimp, etc.
• Hexapoda
– springtails, insects, etc.
Kingdom
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Subkingdom
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Superphylum
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Phylum
Kingdom: Metazoa
Subkingdom: Eumetazoa
Superphylum: Deuterostomia + Lophotrochozoa + Ecdysoza
Phylum: Portifera + Ctenophora + Cnidaria + Acoela + Exhinodermata + Chordata + Platyheiminithes + Mollusca + Annelida + Nematoda + Arthropoda
KEY CONCEPTS
• Modifications to the body plan through selection have lead to great diversity based on
─ feeding and locomotion
• Evolution resulted in innovation related to ─ respiratory mechanisms
─ excretory systems
─ food capture
─ morphology related to locomotion
Deuterostomia
blastopore → anus
• Based on molecular data (~ 4-5 phyla)
– 2 main phyla: Echinodermata and Chordata (they are sister clades. 1 does not evolve from the other)
-represents living organisms not the ones that are extinct
• Some shared developmental characters:
– triploblastic (3 germ layers) (embryo development)
– radial, indeterminate cleavage
• embryos have indeterminate, radial cleavage.
• mesoderm instead forms as outpocketings of the archenteron, forming the coelom (enterocolom) from the space in the outpocketings.
• The blastopore becomes the anus, and the mouth instead forms from a later opening.
Note: chordates did NOT evolve from echinoderms, they share a common ancestor!!
Echinoderms and Vertebrate origins
-Chordates have gill slits
-Echino do not = lost that trait —> based on phylogenetic tree
Phylum Echinodermata
• sea stars, sea urchins, sea lilies, sea cucumbers
• ~7000 extant species
• very good fossil record since the Cambrian – ~13,000fossilspecies
• Calcareous endoskeleton suited to fossilization
•large diversity, even larger in fossil record
— easier to fossilize b/c have hard exoskeleton
3 synapomorphic characteristics:
1. Radial symmetry in adults
- Calcareous endoskeleton (contains calcium carbonate) but always covered by a thin soft layer
- Water vascular system – with tube feet
Phylum Echinodermata
Basic characteristics
• Slow, sessile, marine
• Adults: pentaradial symmetry
– penta = 5
• Larvae - bilateral symmetry
- share with
• Mouth at centre of arms and faces down
• Thin epidermis covers endoskeleton (CaCO3)
• Diffuse nervous system with no central brain
-common to other groups of invertebrates
• Typically, both sexes engage in broadcast spawning
– fertilization occurs externally, in open water. By externally realizing gametes out into water
•say their ancestor had bilateral symmetry, hence they are categorized like they are in the phylogenetic tree
• Water vascular system
– network of hydraulic canals branching into extensions called tube feet
– madreporite: small calcified plate with a porous opening in the middle that allows water to flow in and out of system. (Is on top slighty on side, anus not far from it)
-reg flow of H2O across whole vascular system to eat, movements, filter H2O
–water pressure + adhesive→operate tube feet (ampulla + podium)
• Tube feet: locomotion, feeding, detection and gas exchange
Main Echinoderm Classes
Class Echinoidea
(urchins, sand dollars)
• Spines: locomotion and protection
• Eat seaweed
-have structures associated with a mouth
Main Echinoderm Classes
Class Holothuroidea (sea cucumbers (penta))
• Elongated Symmetry - secondarily bilateral
(Larva is bilateral?)
• Some tube feet around the mouth serve as feeding tentacles
• filter or suspension feeders
-f= filter at bottom of ocean
-S= take nutrients from h2o
Main Echinoderm Classes
Class Asteroidea (sea stars)
• Active, walk via tube feet
• Carnivorous predators
Phylum Chordata
• Bilateral symmetry
• Deuterostome:
✓triploblastic – 3 germ layers
✓radial, indeterminate cleavage
• Coelom
• Segmented bodies
•cephalochordatas considered the most primitive, basic body plan
4 chordate Synapomorphies
1) Notochord (noto = back, chord = cord)
• Cartilaginous skeletal structure
• Flexible rod located dorsally between the digestive tube and the nerve cord
• Stiff but not rigid support along body
• Present in all chordate embryos and some adults
• Most vertebrates - reduced to discs/pads between vertebrae
2) Dorsal hollow nerve cord • unique to chordates
- in other animals, nerve cord is solid and ventral
• develops from neural plate of ectoderm (dorsal to notochord)
• plate rolls into tube during embryonic development
• Nerve cord→spinal cord, nervous system & brain (among vertebrates)
3) Pharyngeal slits/clefts
In most chordates: clefts→pharyngeal slits
─ connects pharynx to external environment
─ allows water to exit w/o continuing through entire digestive tract
Suspension-feeding devices in many invertebrate chordates
Slits and supporting arches modified for gas exchange in vertebrates (“gills slits”)
4) Muscular, post-anal tail
• Most adult chordates have muscular tail
posterior to anus
• Contains muscles and skeletal elements
─ provides propulsive force in many swimming
species
─ acts as rudder, provides balance, grip, etc.
• Reduced in many species during embryonic development
Other Important Character States
• Bilateral symmetry
• Segmentation – what other phyla are also segmented?
• e.g.,muscle segments,vertebral column
— in Arthropoda and annelids —> externally seen
— In Chordata —> seen externally in muscles
Phylogenetic Relationship between Echinoderms, Hemichordates &Chordates
Lineages split a long time ago. Most likely in the Cambrian explosion, potentially pre-Cambrian, hence why we look so different.
SubPhylums of Chordata
• Includes two groups of invertebrates:
— Subphylum Urochordata
— Sybphylum Cephalochordata
• Includes animals with a backbone (= vertebral column):
— Subphylum Vertebrata
Subphylum Cephalochordata
(lancelets) 25 spp., all marine
• Adult has all 4 key synapomorphies of chordates
• Segmentation
─ serial arrangement of muscle ─ cephalization is minimal
• Sedentary suspension feeders
Subphylum Urochordata
(tunicates) ~1250 species, all marine
• Larvae stage has all 4 key synapomorphies of chordates
• Adults-donotlooklikechordates
─ metamorphosis→chordate characters
disappear
─ sessile, filter/suspension feeders
Subphylum Vertebrata (= Craniata)
• ~ 50,000 extant spp.
• Chordateswithahead
─ cranium = skull
─ more coordinated movement and complex feeding
behaviours
• FossilformsfromChinashowapparent morphological transition from non-craniate to craniate chordates
• Cambrian fossils from Haikou district
— Haikouella: eyes and brain but no braincase
— Haikouichthys about the same size and shape but had a braincase
Subphylum Vertbrata
synapomorphies
1) Two or more clusters of Hox genes
• non-craniate chordates have only 1 cluster
• greater genetic & morphological complexity
2) Neural crest
• group of embryonic cells that forms near dorsal margins
of the closing neural tube
• contributes to formation of bones and cartilages of the skull, and other structures
3) Endoskeleton and pronounced cephalization
• brain encased in protective plates (cranium ≈ skull)
• endoskeleton cartilage or bone
4) Elaborate skull – cranium
• Protects central nervous system & key sensory
structures
• Brain with 3 distinct regions
5) Circulatory system modified
• closed circulatory systems
• including a heart with two+ chambers
• Hemoglobin in red blood cells
• oxygenated by passing close to gills or lungs
6) Vertebral column = backbone
▪ a chain of skeletal elements surrounding
and protecting nerve cord
→ More complex nervous system
→ Replaces function of notochord
— providing rigidity
— provides attachment sites for muscles
and other skeletal elements (e.g., ribs)
7) Pharyngeal slits
→function as gill slits (aquatic vertebrates) →parts of braincase or ear (terrestrial vertebrates)
Agnatha
Jawless vertebrates = Agnathans
• Vertebrates that lack jaws
• Two extant lineages: Cyclostomes (for some not a monophyletic group)
• More agnathans in the fossil record
• Lack teeth
• long,flexible,tubularbodies
─ but no paired fins
─ skeleton made of cartilage (not bone)
Gnathostomata
Jawed Vertebrates = Gnathostomes
• Jawed fish evolved ~450 Mya
─ gnatho = jaw, stoma = mouth
• Have 2 pairs of fins
─ extinct agnathans (jawless) had 1 pair ─ extant agnathans have no paired fins
• Bigger, more developed brain than agnathans
• Further duplication of Hox genes: ─ 2 x 2 = 4 clusters of Hox genes ─ added genetic diversity
— Jaws → diverse feeding behaviour
— larger brain
— improved vision and sense of smell
Why is Hox gene duplication important?
— Greater segmentation of body and greater complexity = allow for greater genetic diversity
Where did jaws come from?
• Modifications of 2 pairs of gill arches (=skeletal rods) that used to support anterior pharyngeal slits
• With hinged jaws, more ways of getting food than suspension feeding using pharynx
─ grasp, kill, shred and crush large food items
— allowed modification of feeding = no longer limited too small food
• Posterior slits→specialized for gas exchange (gill slits) in aquatic vertebrates
The significance of jaws
-grooming, carrying young, building nests, allows for more diversity of what can eat
Fossil Gnathostomes
• The earliest gnathostomes in the fossil record is an extinct lineage of armored vertebrates called placoderms
• Appeared in the Ordovician- Silurian, at least 450 million years ago
• Had bony exoskeleton but still cartilaginous endoskeleton
OVERVIEW OF KEY EVENTS IN VERTEBRATE EVOLUTION
Key innovations
1. Pharyngeal gill slits.
2. Notochord dorsal, hollow nerve cord, post-anal tail
3. Cranium, neural crest, rudimentary, vertebrae, or vertebral column (fossil agnathans (🟠))
3. Jaws, fins. (🟣)
4. Internal skeleton, swim bladder or air breathing Organ (🟢)
5. Loped fins, lungs.
6. Paired limbs.(🔴)
7. Amniotic egg (🔵)
Carboniferous Amniotic egg (🔵)
Devonian Limbs capable of moving on land. (🔴)
Devonian bony endoskeleton (🟢)
Silurian. Jaws (🟣)
Ordovician. Bony exoskeleton (🟠)
KEY CONCEPTS
• Deuterostomes are a monophyletic group
• Echinodermatabodyplan
— Radial symmetry in adults, endoskeleton, and water vascular system
• Ancestral Chordate body plan has been retained but modified in diverse lineages
— Duplication of Hox genes led to development of complex body structures including limbs for dynamic locomotion
— Vertebrate evolution shows development of brain, and diversification of feeding methods
— Jaws likely evolved from the pharyngeal slits to aid feeding
• Know the key characteristics in chordate evolution and main groups covered in lecture slides
