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Chordata

•  Finally, a phylum to call our own.

•  Deuterostomes •  Includes three

invertebrate lineages

Chordata •  Defined by characters that each

appears at some stage in a chordate’s life, often embryologically

•  notochord - longitudinal, flexible rod that serves as an internal skeleton, or axis of support. (replaced by bony segments in adult vertebrates)

•  dorsal hollow neural tube - located above notochord, develops as tube from ectoderm

•  pharyngeal gill slits –  posterior to mouth (pharynx) pharyngeal

slits function in filter feeding –  modified for respiration (gills) in

vertebrates •  post anal tail - muscular, functions in

locomotion (aquatic, marine)

Chordata

•  Probably evolved from larval form of deuterostome that evolved sexual maturity and could therefore reproduce

•  PAEDOMORPHOSIS

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Invertebrate Chordates

•  Paraphyletic •  Display some

plesiomorphic (ancestral) traits

•  Each display apomorphic (uniquely derived) traits

Cephalochordata: Lancelets •  Diverged from rest of

Chordata ~520 mya •  Simple, fusiform body

retaining all 4 basic chordate characteristics

–  What are these? •  Small (1-2 cm) shallow

marine filter feeders, usually buried tail-first in sand with oral cavity protruding.

•  Chevron-shaped muscle segments (myomeres) flex notochord for locomotion.

•  Is this an ancestral Chordata?

Urochordata: Tunicates

•  Also sea squirts & sea pork (?)

•  Larva is free-swimming filter feeder, possesses all four basic chordate characters

•  Life stage often as short as a few minutes

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Urochordata: Tunicates •  But adult undergoes

radical metamorphosis •  Becomes sessile, loses

notochord, neural tube, and tail

•  Pharynx is reduced •  Outer, epidermal wall or

“tunic” surrounds the adult

Myxini: Hagfishes

•  Last clade of invertebrates

•  First group of Chordata with a head

•  Monophyletic group Craniata

Craniata •  Craniata

–  Brain at anterior end of dorsal nerve cord

–  Eyes and other sensory organs concentrated

–  Skull as enclosure •  Neural crest

–  Cells that appear near dorsal margins of closing neural tube

–  Migrate to become a variety of structures:

•  teeth, much of skull, inner layer of skin of facial region, many neurons, other important cells

–  Has been called the fourth germ layer

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Myxini: Hagfishes •  Only extant animals which

have a skull and not a vertebral column

•  World’s most disgusting animal?

•  Enter both living and dead fish (through openings), feeding on the insides

•  Can exude copious amounts of slime as defensive mechanism

•  Will tie themselves in knots for defense or offense

Vertebrata: Animals with a backbone

•  Most successful group of chordates

•  Originated 513-542 mya

•  First fossils part of Cambrian Explosion

Vertebrata •  Evolutionary trend:

Notochord replaced by bony segments: vertebrae

•  Some lineages notochord still prominent, vertebrae just cartilaginous projections

•  Others (e.g. us), notochord only remnant as part of intervertebral discs

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Vertebrata •  BONE •  Specialized tissue

unique to vertebrates, forming an endoskeleton

•  Can be cartilage (e.g. lampreys), collagen-based cartilage (e.g. sharks & rays), or hard matrix of calcium phosphate (e.g. us)

Vertebrata: Major Events

•  Jaws •  Mineralized

skeleton •  Radiation of fish

and paired appendages

•  Tetrapod invasion of land

•  Amniotic egg

Agnathans: Jawless vertebrates

•  Include extinct Ostracoderms (oldest known vertebrates)

•  Include extant lampreys (Petromyzontida)

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Lampreys •  Only extant jawless

vertebrates •  Larvae filter feeders in

freshwater •  Adults parasitic in freshwater

or marine (catadromous) •  Skeleton is cartilage without

collagen •  Notochord is prominent axial

skeleton, vertebrae are cartilaginous pipe around notochord.

Relationships of the hagfishes

•  Are jawless fishes monophyletic?

•  What do these two alternatives say about the evolution of the backbone?

Relationships of the hagfishes

Heimberg, A.M. et al. 2010. microRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate. PNAS 107: 19379-19383.

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GNATHOSTOMES: vertebrates with jaws

•  The vast majority of Vertebrates

•  470 mya •  Paired fins and tail

allowed effective swimming

•  Jaws enhanced predation

GNATHOSTOMES: vertebrates with jaws •  Jaws evolved from

modifications of pharyngeal bars

•  Mechanism to increase efficiency of buccal pump

•  Move water through pharynx

•  Secondarily, jaws gave vertebrates the life of a predator

•  Teeth from modified dermal scales

GNATHOSTOMES: vertebrates with jaws

•  Placodermi earliest jawed fish

•  Dermal armor pronounced; true paired appendages (pectoral and pelvic) in most

•  Typically 1 m or less; some very large (10 m); all predaceous

•  Most diverse in Devonian, extinct by end of Paleozoic

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Modern Fish •  Chondrichthyes

–  Sharks, skates, rays

•  Osteichthyes –  Bony fish –  Actinopterygii

•  Ray-finned fishes

–  Sarcopterygii •  Lobe-finned fishes

and Tetrapods

Chondrichthyes

•  Sharks, skates, rays (elasmobranchs); chimaera (holocephalans)

•  Skeleton made of cartilage •  Internal fertilization: males

possess claspers (specialized structures of pelvic fins) –  Oviparous –  Ovoviviparous –  Viviparous

•  External gill slits open, not covered

Osteichthyes: Bony “fish”

•  Clade includes Tetrapods!

•  Ray-finned fishes make up most of “fish” diversity

•  Lobe-finned fish gave rise to tetrapods

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Osteichthyes: Bony “fish” •  Ancestrally: •  Operculum: bony flap

covering the gills externally

•  Swim bladder: modification of pharyngeal pouch, gas filled, regulates buoyancy

•  Homologous with lungs?

Actinopterygii: Ray-finned fishes

•  Ray-finned fishes make up most of “fish” diversity

•  Most diverse group of vertebrates

Actinopterygii: Ray-finned fishes

•  Pectoral and pelvic fins: webs of skin supported by bony or horny spines ("rays")

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Actinopterygii: Ray-finned fishes

•  Typical fish

Sarcopterygii: Lobe-finned “fishes”

•  Includes Tetrapods •  Two lineages of

truly aquatic forms

Sarcopterygii: Lobe-finned “fishes”

•  Fin bases bony, fleshy, robust, surrounded by thick layer of muscle

•  Not rayed.

Coelocanth limb

Lungfish

Tetrapod

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Sarcopterygii: Lobe-finned “fishes”

•  Aquatic forms never particularly diverse, two extant lineages: –  Actinistians

(Coelocanth) –  Dipnoians

(Lungfish) •  But gave rise to

tetrapods

Coelocanth

Lungfish

Tetrapod

Tetrapods and the Transition to Land

•  The fleshy, robust pectoral and pelvic fins “pre-adapted” the lobe-finned fishes to moving in a terrestrial environment.

•  Why is this a challenge? •  Used lungs to breathe air

in low oxygen water. •  Another pre-adaptation. •  The transition to land did

not come out of nowhere.

Tetrapods and the Transition to Land

•  Tetrapods: “Four feet” •  In place of pectoral fins,

have limbs that can support weight on land

•  Have digits that allow transmission of force to ground when walking

•  First appear in mid-Devonian (~380 mya)

•  Transition to tetrapod is gradual, no abrupt transition (including in limbs)

Acanthostega: Limbs with digits BUT… •  Limbs too weak to support weight on land •  Tail with fin •  Bones supporting gills

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Key Transformations in the Evolution of Tetrapods

•  Well-developed girdles (shoulder & pelvic) and limbs

•  Adaptations for respiration –  Loss of operculum –  Loss of internal gills –  Increased branching of lungs

•  Cranial-cervical joint –  Head moves independently of

axial skeleton

Key Transformations in the Evolution of Tetrapods

•  Story of evolution of tetrapods is, like ________________, the story of increased terrestriality

•  Least terrestrial extant Tetrapoda are the Class Amphibia

Amphibians

•  Amphibians traditionally defined as all Tetrapods without amniotic egg (later)

•  Extant members monophyletic

•  Extinct members paraphyletic

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Amphibians

•  Early amphibians diverse small to large (4m)

•  Generalized tetrapods with low, sprawling posture

•  Most extinct by end of Paleozoic

Modern Amphibians: Lissamphibia •  Little resemblance to

Paleozoic forms •  First appear in early

Mesozoic •  Generally terrestrial &

aquatic lifestyle –  Smooth, mucus-covered skin –  Various means of gas

exchange (gills, lungs, skin) –  But some have adaptations

that permit complete terrestriality

–  Unshelled eggs dehydrate quickly in dry air

–  Larval stage to brooding to viviparity to direct development

Anura: Frogs & Toads •  5420 species •  Specialized morphology

for hopping •  Adults are tailless •  From 10 mm to 300 mm •  Worldwide distribution •  External fertilization

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Urodela: Salamanders & Newts •  ~550 species, northern

hemisphere & northern South America

•  Generalized tetrapod morphology

•  2.7 cm to 1.8 m •  Paedomorphosis

common •  External fertilization

Apoda: Caecilians

•  Secondarily limbless •  Highly adapted to

burrowing –  Strong skull, pointed

snout –  Unique muscular

adaptations •  Pan-tropical •  Internal fertilization

Amniota and the Amniote egg •  Sister group to modern

amphibians •  Tetrapods with amniotic

egg •  Reptiles & mammals •  Increased adaptation to

dry land •  Monophyletic group

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Amniota and the Amniote egg •  Amniotic egg can be

deposited on dry land: resistant to desiccation

•  Extraembryonic membranes –  Amnion: surrounds embryo,

provides mechanical protection

–  Allantois: receives metabolic wastes

–  Chorion: gas exchange –  Calcareous or leathery shell

(plesiomorphic, what has lost this?)

Amniota: Terrestriality •  Amniotic egg •  Negative pressure inhalation

–  Rib cage ventilation –  More efficient than positive pressure inhalation (amphibians)

•  Keratinized skin: less permeable •  Internal fertilization

–  Oviparity: most reptiles, all birds, some mammals –  Ovoviviparity: some reptiles –  Viviparity: most mammal

Amniote Diversity

•  Two main extant lineages

1.  Mammals (derived Synapsids)

2.  Reptiles 1.  Chelonia (turtles) 2.  Archosaurs

(crocodilians + birds) 3.  Lepidosaurs (tuataras,

snakes, lizards)

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Reptiles: Testudines •  Turtles: 307 known

species •  First fossils ~210 mya •  Terrestrial, freshwater,

marine •  Carapace (dorsal) and

plastron (ventral) –  Derived from ribs

•  Head retraction evolved twice

Reptiles: Testudines •  Sister-group to

remaining Reptilia •  Lack openings in the

skull near the temple •  “Anapsida” (without

arch)

Reptiles: Diaspids

•  Distinguished by two ancestral skull openings (temporal fenestrae) posteriorly above and below the eye

•  Include Lepidosaurs and Archosaurs

•  Differ in numerous details of skull morphology.

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Lepidosauria

•  Reptiles with overlapping scales

•  Ectothermic –  Derive metabolic

heat from environment

•  Sphenodontia –  Tuataras only living

examples (2 species)

–  Part of a lineage that flourished ~200mya

Lepidosauria •  Reptiles with overlapping

scales •  Ectothermic

–  Derive metabolic heat from environment

•  Sphenodontia –  Tuataras only living

examples (2 species) –  Part of a lineage that

flourished ~200mya –  Now found only on islands

off of New Zealand –  Why might their

conservation be so important?

•  Squamata

Lepidosauria

•  Reptiles with overlapping scales

•  Sphenodontia •  Squamata

–  Lizards & Snakes –  ~7,800 species –  Ectothermic

•  Derive body heat from environment

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Lepidosauria

•  Reptiles with overlapping scales

•  Sphenodontia •  Squamata

–  Lizards & Snakes –  ~7,800 species –  Ectothermic

•  Derive body heat from environment

–  Snakes derived lizards –  One of four legless

lineages of lizards

Lepidosauria

•  Ancestry betrayed by vestigial limbs in early diverging snake groups

Archosauria: CROCODILIA

•  23 species survive today •  Most have long snouts with

numerous pointed teeth •  Nesting behavior and parental care

(synapomorphy of Archosauria?) •  In general, have legs splayed

somewhat to the sides, however they can pull the legs inward and gallop, can move quite fast if the need arises.

•  Ectothermic

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Archosauria: PTEROSAURIA •  Non-bird Dinosauria extinct

by end of Mesozoic •  What event? •  Pterosaurs:

–  First vertebrates with powered flight

–  25 cm to 10 m wingspan •  First evidence of

endothermy? –  Maintain body temperature

using metabolic energy

Archosauria: ORNITHISCHIA

•  “Bird-hip” dinosaurs –  (although birds derived

from “lizard-hip” dinosaurs)

•  Herbivores •  Extinct 65 mya •  Considerable

evidence of nesting behavior.

•  Endothermic?

Archosauria: SAURISCHIA

•  “Lizard-hip” dinosaurs •  Two lineages •  Sauropods: Long-necked

herbivores •  Theropods: Bipedal,

primarily carnivorous •  Only one lineage survived

K-T extinction •  Extant lineage has

nesting behavior and is endothermic

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Living Dinosaurs

•  Derived Saurischians descended from same lineage as T. rex (Coelurosauria)

•  BIRDS: Class Aves

AVES: Birds •  ~10,000 species:

most diverse tetrapod vertebrates

•  5 cm bee hummingbird to 2.7 m ostrich

•  Inhabit ecosystems from Antarctic to Arctic

•  Diverse feeding habits linked with diverse beak morphology

Modern Birds

•  Feathers •  Lightweight but

strong skeleton •  Beak with no teeth •  Hard-shelled eggs •  High metabolic rate •  Four-chambered

heart

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Modern Birds •  All of these intimately

associated with evolution of flight

•  Flight is plesiomorphic for modern birds

•  Large flight muscles attached to keeled sternum

•  Forelimb modified as aerofoil (wing)

Modern Birds •  Flight lost in some lineages •  Including Ratites

–  Ostriches, Rheas, Cassowaries, Emus, Kiwis, Moas†, Elephant Birds†

–  No flight muscle attachment (keel)

•  Including Penguins –  Flight muscles adapted for

swimming •  Flightlessness evolved

approximately 50 times in numerous island forms, 27 of which have gone extinct with colonization by Europeans

The Evolution of Flight

•  Birds are derived Coelurosauria (bipedal predatory archosaurs)

•  So, how did flight evolve? •  Recently discovered

(1990s) fossils in China show that feathers evolved well before flight

•  Why evolve a branched, 3-dimensional scale?

Sinosauropteryx with primitive hollow hair-like feathers Reconstructed Deinonychus

based on fossilized feathers

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The Evolution of Flight

•  First fossil with evidence of mechanical properties of flight is Archaeopteryx

–  Flight feathers indistinguishable from modern birds

–  Probably not powerful flier, probably downstroke glider

–  Braincase & inner ear synapomorphies with modern birds

•  But many plesiomorphic characters

–  Sharp teeth –  Forefingers with claws –  Long, bony tail

Summary: Reptilia •  Dominated terrestrial environments in Mesozoic •  Currently represented by lineages in three major

groups: –  Testudines: Turtles –  Lepidosaurs: Tuataras, lizards, snakes –  Archosaurs: Crocodiles, birds

•  Sister group to Synapsida, currently represented by Mammalia

Synapsids •  Single fenestration in

temporal region of skull •  Diverged from Reptilia

~300 mya •  Gradual transition in skull

morphology –  Increased control over jaws –  Specialized teeth –  Transition in hinge of jaw (to

squamosal hinge) and evolution of inner ear (from articular-quadrate hinge)

–  (see Fig. 25.6 & 34.31 in textbook)

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The Origin of Mammals

•  First true mammals appear during the Jurassic

•  True mammals: •  Hair •  Mammary glands & sweat

glands •  Deciduous, heterodont

dentition •  Three middle ear ossicles

(incus, malleus, stapes)

The Origin of Mammals •  Three extant groups present

by early Cretaceous –  Monotremes –  Marsupials –  Eutherians

•  ALL endothermic •  Adaptive radiation after K-T

extinction event •  From 30-40mm bumblebee

bat to 33m blue whale

Mammalian Diversity

•  Three extant clades distinguished by reproductive modes

1.  Monotremes = Protheria 2.  Marsupials = Metatheria 3.  Placentals = Eutheria

Reptilia

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Mammalian Diversity •  Three extant clades

distinguished by reproductive modes

1.   Monotremes/ Protherians –  Platypus & Echidna –  Australia, New Guinea (fossils

in Argentina) –  Five species

1. Monotremes/ Protherians •  Share numerous

plesiomorphic traits with Reptilia:

•  Lay eggs •  Urinary, defecatory, and

reproductive systems all open into a single duct, the cloaca

•  Lack nipples •  Legs to side rather than

underneath

1. Monotremes/ Protherians •  Why are they

mammals? •  What are the

synapomorphic traits that they must have?

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1. Monotremes/ Protherians •  Also have numerous

synapomorphies of their own •  Leg bears a spur in the ankle

region –  Non-functional in echidnas –  Powerful venom in male

platypus •  Capable of electroreception •  Adults lack teeth

Marsupials: Metatheria •  Kangaroos & wallabies1,

wombats2, koalas3, bandicoots & bilbies4, Tasmanian devils5, thylacines6, possums7, opossums8

•  234 species in Australasia •  100 species in Americas

1 2

3 4

5

6 8 7

Marsupials: Metatheria •  Distinctive pouch (marsupium), in

which females carry their young through early infancy

•  Give birth at a very early stage of development (about 4–5 weeks)

•  Why might this be adaptive? •  Newborn crawls up the body of the

mother and attaches itself to a nipple (in marsupium)

•  Have specialized sex orifices –  Cloaca is single urinary and

defecatory tract –  Females with two vaginas, male with

two-pronged penis; only function is sperm reception and discharge

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Marsupials: Metatheria

•  Fossils present in ALL continents (North American origin)

•  Declined as Eutherians diversified

•  Why dominant in Australasia?

•  Numerous convergent forms with Eutherians

•  Numerous forms extinct only 60,000-15,000 ybp

Placentals: Eutheria •  Embryo attaches itself to the

uterus via a large placenta via which the mother supplies food and oxygen and removes waste products.

•  Pregnancy is relatively long and the young are fairly well-developed at birth.

Placentals: Eutheria

•  No longer a cloaca •  Separate urinary

and defecatory tract •  But sexual orifice

shared with urinary tract in both males and females

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Eutherians: Four clades

with ~20 orders

Clade I: Afrotheria •  Golden moles5 & tenrecs8,

elephant shrews3, aardvarks1, hyraxes6, elephants7 and manatees2,4

•  Includes largest land animal and some not-so-large relatives

•  Believed to have originated in Africa when the continent was isolated from other continents –  Contradicts with some fossil

evidence

Clade I: Afrotheria

•  Originally grouped based on DNA sequences

•  Possible synapomorphies: –  Movable snout –  Testicondy (lack of a

scrotum in males) –  Descended testicle and

scrotum ancestral for Mammalia

–  Why would the scrotum have evolved in the first place?

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Clade II: Xenarthra

•  Sloths, Anteaters, Armadillos

•  Originated in South America

•  Colonized North America in Great American Interchange ~3mya

Clade III: Euarchontoglires •  First subclades: Glires •  Rodentia (rodents)

–  Mice, rats, squirrels, chipmunks, gophers, porcupines, beavers, hamsters, gerbils, guinea pigs, degus, chinchillas, prairie dogs, and groundhogs, capybaras

–  By far, most diverse order of mammals

–  Rodents and bats only Eutherian orders with species endemic to Australia

•  Lagomorpha –  Rabbits, hares, picas

Clade III: Euarchontoglires •  Second subclades:

Euarchonta •  Scandentia

–  Tree shrews •  Dermoptera

–  Flying lemurs •  Primates

–  Lemurs, the Aye-aye, lorids, galagos, tarsiers, monkeys, and apes

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Clade IV: Laurasiatheria

•  Hypothesis: evolved on the supercontinent of Laurasia, after it split from Gondwana when Pangaea broke up

•  Based on DNA sequence data

•  Fits well with zoogeography (distribution of fossils and extant lineages)

•  Six main orders

Clade IV: Laurasiatheria •  Eulipotyphia

–  Hedgehogs, shrews, moles

–  Insectivorous •  Chiroptera

–  Bats –  Forelimbs are

developed as wings –  Only mammals

naturally capable of flight

–  Only terrestrial mammals found on oceanic islands

Clade IV: Laurasiatheria •  Carnivora

–  Dogs & foxes, skunks, weasels, raccoons, bears, seals, cats, mongooses, hyenas, civets

–  Most diverse in size –  Predaceous

•  Pholidota –  Pangolins

•  Perissodactyla –  Horses, tapirs,

rhinos –  Odd-toed ungulates –  Hind gut fermenters

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Clade IV: Laurasiatheria •  Cetartiodactyla •  Consists of what had been

two orders: •  Artiodactyla

–  Even-toed ungulates •  Cetacea

–  Whales, dolphins, porpoises •  But whales sister-taxon to

Hippos •  Originally grouped based on

DNA sequence data •  Fossil evidence supporting

hypothesis, as is some morphology

Mammalian Phylogeny

•  Transition to internalization of egg •  Many orders of Eutheria were present at

K-T extinction event •  Underwent tremendous diversification in

species and in body form into the open niches formed by the extinction of most dinosaur lineages