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Early Vertebrates (Chordata) (Characteristics of all Chordates (Pharyngeal…
Early Vertebrates (Chordata)
Characteristics of all Chordates
Muscular post-anal tail
always present in embryos
reduced in some adult chordates (like humans)
Dorsal Hollow Nerve Cord
All retain it
Hollow tube that turns into central nervous system, derived from ectoderm
Pharyngeal slits/clefts
invertebrate chordates are suspension feeders
aquatic: becomes gills; tetrapod: part of ears, head/neck
arches along pharynx, slits open into pharynx
Notochord
Present in all embryos and some adults
long, flexible tube between digestive tube and dorsal hollow nerve that provides skeletal support. Derived from Mesoderm
Basal (Invertebrate) Chordates
Tunicates
larvae of many species use tail and notochord to swim to suitable substrate
Radical Metamorphosis
remaining organs rotate 90º
nervous system degenerates
draws in water through siphon, water passes through pharyngeal slits into atrium and exits through excurrent siphon
Tail and notochord resorbed
all chordate characters present in larvae stage
Lancelets
Most basal chordate
Adult
swims down to seafloor and wriggles backward into sand, leaving only anterior exposed
Cilia draws seawater into mouth, net of mucus on pharyngeal slits captures food which goes into intestines
Retain key chordate traits
Larvae
feed on plankton in the water column
develop a notochord, dorsal hollow nerve cord, numerous pharyngeal slits, and a post-anal tail
Vertebrate Characteristics
Well-Developed nervous system
neural crest
give rise to a variety of structures: teeth, some parts of skull, types of neurons, sensory capsules for eyes etc
A collection of cells that appears along the edges of the closing neural tube of an embryo
spinal cord
No need for notochord anymore
dorsal hollow nerve cord inside spine
well-developed brain
63,000 species
Cartilaginous or bony vertebrae
Early Vertebrates
Lamprey
energy-efficient swimmers (create vortexes with fins)
notochord persists in adults
adults parasitize fish
Jawless, no backbone (cartilaginous vertebrae)
Hagfish
notochord persists in adults
reduced sensory structures
Jawless, no backbone (cartilaginous vertebrae)
produces slime as a defense
Gnathostomes (jaws)
Fossils
Acanthodians emerged around roughly the same time and radiated later
earliest include extinct lineages known collectively as placoderms (plate skinned)
Chondrichthyans - sharks, rays, and relatives
Adaptations that aided predation
can detect electric fields of animals
sharp vision, olfactory bulbs
streamlined bodies for hunting (bilateral symmetry helps)
Swim nearly constantly to keep from sinking to ocean floor
skeletons of cartilage and mineralized teeth
Derived Characters
lateral line system for organs
brain is enlarged compared to other vertebrates
4 pairs of Hox genes
Jaws
evolutionary innovation used to scare away predators, or allow predators to grip and tear at prey
Fossils from 440 MYA
Possibly derived from skeletal rods that supported gills
Osteichthyans
Ray-Finned
gills for gas exchange
have a swim bladder to maintain bouyancy
named for bony rays that support their fins
Lobe-Finned
have a swim bladder to maintain bouyancy
Presence of rod shaped bones surrounded by a thick layer of muscle in their pectoral and pelvic fins
bones that resemble wrists in their fins
Only 3 extant lineages: coelocanths, lungfish, tetrapods
Have an ossified (bony) endoskeleton with a hard matrix of calcium phosphate