11/11/14 1 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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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