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11/11/14
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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
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