Invertebrates ChordataVertebrates

Chordate evolution is a history of innovations that is built upon major invertebrate traits.

They display many of the basic traits that first evolved in the invertebrates: bilateral symmetry, cephalization, segmentation, coelom, "gut" tube, etc.

Chordate evolution is marked by physical and behavioral specializations

• For example the forelimb of mammals has a wide range of structural variation, specialized by natural selection

Evolutionary innovations and specializations led to adaptive radiations - the development of a variety of forms from a single ancestral group.

Each kind of organism—including humans—is a mosaic of traits.

Many traits are conserved from remote ancestors, and others are unique to its branch on the family tree.

All groups are multicelled, bilateral, coelomate animals.

Have four unique features that appear in their embryos and persist into adulthood.

1. A strong but flexible rod called notochord.2. A nerve cord parallels the notochord and

gut which anterior ends develop into a brain

3. Embryos have gill slits on the wall of pharynx functions in both feeding and respiration

4. A tail that extends past the anus.

Notochord, dorsal hollow nerve cord, pharyngeal gill slits, blocks of muscle, postanal tail

Characteristics of the Chordates

The notochord• All chordate embryos have a notochord, a stiff but flexible rod that provides internal support • Remains throughout the life history of most invertebrate chordates; among, present only in the embryos of vertebrate chordates

Characteristics of the Chordates cont.

Dorsal Hollow Nerve Cord (=Spinal Cord)• A fluid-filled tube of nerve tissue that runs the length of the animal, dorsal to the notochord• Present in chordates throughout embryonic and adult life

Characteristics of the Chordates cont.

Pharyngeal gill slits• Pairs of opening through the pharynx• Invertebrate chordates use them to filter food• Juvenile fishes use them to them for breathing• In adult fishes the gill sits develop into true gills• In reptiles, birds, and mammals the gill slits are vestiges, occurring only in the embryo

Chordate Metamerism• Body segmentation (i.e. metamerism) appears to have evolved in two lineages of the chordates: the Cephalochordates and the Vertebrates -- probably occurred after divergence from the Urochordates• However, segmentation in the chordates does not involve the coelom.• The cephalochordates and the chordates movement is accomplished by contraction of muscle fibers that are arranged in segmented blocks - myotomes• Presumably, segmentation of muscles developed as an adaptation for undulatory swimming and rapid burrowing

Characteristics of the Chordates cont.

Blocks of Muscle - Myotomes • Surrounding the notochord and nerve cord are blocks of muscle

Postanal Tail• The notochord, nerve cord, and the myotomes extend to the tail• Found at some time during a chordate's development

Subphylum Cephalochordata

Urochordata

is composed of about 2 dozen blade-shaped animals known as lancelets.

retain all characteristic throughout life.

SubPhylum Cephalochordata

Exclusively marine animals ; they are capable of swimming but usually are buried in the sand with only their anterior end being exposed

SubPhylum Cephalochordata cont.•All chordate characteristics are present throughout their life history• They are filter feeders: inside of the oral hood is lined with cilia -wheel organ• These cilia, plus cilia in the pharynx help generate a water current • Water and suspended food particles pass through the oral hood, equipped with projections called cirri that strain larger particles• Feed by secreting a mucous net across the gill slits to filter out food particles that are present in the water.

is commonly known as tunicates ( sea squirts).

Larva posses all chordate characteristics while adults exhibit only Pharyngeal pouches.

Marine animals; some species are solitary, others are colonial.

Sessile as adults, but motile during the larval stages

Possess all 5 chordate characteristics as larvae

Settle head first on hard substrates and undergo a dramatic metamorphosis (e.g., tail, notochord, muscle segments, and nerve cord disappear)

•

Adult body is covered by an outer envelope or tunic; composed of fibers of tunicin embedded in a mucopolysaccharide matrix.

Tunic encloses a basket-like pharynx, that is perforated by gill slits.

Tunicates are filter feeders; plankton is trapped in a sheet of mucus and cilia later direct the food-laden mucus to the stomach.

Water leaves the animal via an excurrent siphon.

Tunicates

Notochord , tail and nerve cord occurs only in a larval stage.

During the adult stage, the pharynx with gill slits retain.

e.g. sea squirts attached to piers and boats

Lancelets

All the four characteristic chordate traits are visible in the adult.

Both the tunicates and lancelets filter food from water currents that pass through their gill slits.

All characteristics of chordates

Distinguishing characteristics◦ Vertebral column or spine◦ Cranium or skull that protects the

brain◦ Endoskeleton composed of bone or

cartilage

General Characteristics• Exhibit all 5 chordate characteristics at sometime in their life history .• Usually well cephalized, including a well developed brain and a number of anterior sensory structures.• Brain is usually encased in a skull, made of hard bone or a cartilage.• In most vertebrates, the embryonic notochord is replaced by a vertebral column. • Possess a distinctive endoskeleton consisting of vertebral column, limb girdles, two pairs of jointed appendages, and a head skeleton.• Muscles are attached to the skeleton to provide movement.• Often have a muscular perforated pharynx.• Closed circulatory system with a well developed muscular heart; blood is oxygenated as it flows through vascularized skin, gills or lungs.

Evolutionary Relationships of the Vertebrates

• Earliest vertebrate fossils (jawless ostracoderm fishes; 500 mya) share many of the novel structures observed in the living vertebrates.

When and from where did these vertebrate characteristics evolve?• May have evolved from an invertebrate chordate lineage.• This idea is supported by the discovery of a fossilized mid-Cambrain invertebrate chordate from the Burgess Shale formation – Pikaia.

•-Pikaia: A ribbon shaped, somewhat fish-like creature about 5 cm in length.• -It possessed a notochord and the V-shaped myomeres.•-Resembles Amphioxus, and may very well be an early cephalochordate.

Evolutionary Relationships of the Vertebrates

• Speculations regarding vertebrate ancestry have focused on living cephalochordates and tunicates.• One hypothesis on the evolution of the vertebrates is Garstang's Hypothesis suggested that sessile tunicates were an ancestral stock that evolved a motile larval stage.• Garstang speculated that at some point larvae failed to metamorphose into an adult, but developed gonads and reproduced in the larval stage. With continued larval evolution, a new group of free swimming animals evolved.• Garstang called this process paedomorphosis, a term that describes the presence (or evolutionary retention) of juvenile or larval traits in the adult body.

Garstang's Hypothesis

called this process paedomorphosis, a term that describes the presence (or evolutionary retention) of juvenile or larval traits in the adult body

Include fishes, amphibians, reptiles, birds and mammals.

Feature : a chamber of cartilage or bone that encloses the brain cranium.

Vertebrates are craniates with a vertebral column (cartilaginous or bony segments).

The first craniates arose 530 mya had fins, a notochord and segmented muscles – active swimmer larvae of lampreys –modern jawless fishes.

The earliest jawless fishes include the Ostraderms – have armor-like plates act as protection but less effective compared to ones with jaws –hinged bony feeding structure.

The jawed craniates began their radiation which include the Placoderms – have armor plates protected their heads and some evolved into huge predator due to their offensive and defensive body structures.

Vertebrates changed the course of the animal evolution.

When vertebrae replaced the notochord – with muscles promote maneuverability & more forceful contraction led to agile, fast moving fishes that began their dominance of the seas.

When jaws becoming part of the related trend, complex sensory organs and nervous systems also started among the ancient fishes and continued among vertebrates on land.

Evolution of paired fleshy fins began as a starting point for all legs, arms and wings that evolved among amphibians, reptiles, birds and mammals.

Gills as respiratory organ also evolved as they do not work out of water forming two small outpouches that evolved into lungs.

The ancestors of land vertebrates relied more on the paired lungs than on gills as the lungs are also accompanied by more efficient circulatory systems.

Jaws, paired fins and lungs were key innovations that led to the adaptive radiation of vertebrates.

Fishes are world’s most dominant vertebrates as their numbers exceed other combined vertebrate groups and also most diverse.

Two major groups:1.Skeleton of cartilage – cartilaginous fishes

(sharks and rays)2.Skeleton of bone – bony fishes (ray-finned

fishes, lung fishes and lobe-finned fishes)

All have skeleton of cartilage; 5-7 pairs of gill slits

Shading and replacement of teeth which are modified scales.

Sharks and rays.

Long, eel-like bodies Lack jaws Paired fins bones

Class Chondrichthyes◦Jaws◦Paired fins◦Skeleton of cartilage

skate

Have a skeleton of bone—heavier than cartilage.

Have a gas-filled swim bladder or lung to maintain buoyancy.

400 mya, 3 main lineages emerged:1.Ray-finned fishes 2.Lung fishes3.Lobe-finned fishes

Class Osteichthyes◦Jaws◦Most have a skeleton composed of bone

Flexible fins supported by rays derived from skin rather than bone or cartilage.

More than 21,000 sp. Diverse variety of colors and body shapes to

adapt to different aquatic habitats. Elongated eels , flattened flounders on

sandy bottoms and box-shaped fishes wedge among corals.

Edible fishes such as salmon, trout, sardines, snapper, carp and cod.

Have fleshy ventral fins reinforced with skeletal parts.

Large marine fish belonging to a group of primitive bony fishes.

The only living fishes is the coelacanths, Latimeria sp.

Similar fins with the lobe-finned fishes.

Have gills and one or two lunglike sacs.

Inflates the sacs by gulping air. Some fishes total dependent on this gas exchange or else drown if kept underwater!

South American Lungfish : a living species of Lepidosiren. look at the four slender "fins" of this fish, which are homologous to the four legs of terrestrial vertebrates.

Lung Fishes - Sarcopterygii

Lung Fishes - Sarcopterygii

Protopterus annectens –Slender African lungfish

•Carnivorous – fearless predator•has two lungs which enables them to breath when the water has dried up; survive drought by "hibernation“ by digging itself into a mud hole and curls up in a chamber that is lined with mucus and breaths out of the mouth.

Walking probably originated in water during the Devonian era, which occurs to the aquatic tetrapods, the Acanthostega and the Ichthyostega where their skull, caudal tail and fins were fishlike but they also both had short neck and four limbs with digits.

Proposed evolution indicate that the skeletal elements inside the fins may have evolved into limb bones of the early amphibians.

An extinct tetrapod genus, among the first vertebrates to have recognizable limbs which appeared in the Upper Devonian about 365 mya, and was anatomically intermediate between lobe-finned fishes and the first tetrapods fully capable of coming onto land.

An early tetrapod that lived in the Upper Devonian period, 367-362.5 mya. It was a labyrinthodont that represents an intermediate form between fish and amphibians. It possessed lungs and limbs that helped it navigate through shallow water in swamps.

A small genetic change could have transformed lobed fins into limbs.

Even a single mutation in one master gene can lead to a big change in morphology.

Vertebrates with four legs which were the first tetrapods on land.

Their body plan and reproductive mode are somewhere in between the fishes and the reptiles.

Most species are dependent on aquatic or moist habitats to complete their life cycle.

Include frogs, toads and salamanders.

◦Included frogs, toads and salamanders

◦Skin is moist and permeable to gases and water

◦Most lay eggs in water◦Most have an aquatic larval stage

Fire salamender - Salamandra salamandra

Leaf Green Tree Frog - Litoria phyllochroa

Toad - Bufo marinus

Amniotes were the first vertebrates to adapt to dry land habitats, through modifications in their organ systems and eggs.

The eggs have four membranes that conserve water and support the embryo’s development.

Amniote skin is rich in protein keratin acts as waterproof.

Have well developed kidneys to conserve water and the fertilization is internal.

Two major groups of amniotes:1.Synapsids – existing mammals and extinct

mammal-like2.Sauropsids – “reptiles” and birds “Reptiles” is not a formal taxonomic group

as it is not monophyletic as it also refers to several lineages which all share the basic animal features but not the derived traits that define birds or mammals.

Descendants of the surviving dinosaurs vanished 65 mya due to mass extinction.

Include lizards (most diverse), lay eggs develop outside the body.

Largest – monitor lizard (komodo dragon) ancestral lizard – snakes –modern snakes

are carnivores Most intelligent – crocodilians (e.g.

crocodiles, alligators) predators that have a four-chambered heart, lay eggs and show parental behavior.

◦Turtles, crocodiles, alligators, lizards and snakes

◦Skin is dry and scaly◦Eggs that can be laid on land

Vertebrates with feathers. Originated from Mesozoic reptiles began

their adaptive radiation. Diverged from a lineage of theropod

dinosaurs. Have a cloaca

◦Adapted for flight Feathers Hollow bones Unique respiratory system

Characteristics◦Endothermy◦Hair◦Completely divided four chamber heart◦Milk produced from mammary gland◦Single jawbone◦Specialized teeth

Egg-laying mammals Most primitive of mammals Only three species

◦ Two anteaters and one platypus

250 species The U.S. has only one species – the Virginia

opossum Young develop in a pouch

Diverse group composed of at least 18 orders

95% of mammals are placental They live on land, in water and the air