early vertebrate traits
jawless
distinct head with tripartite brain
pharyngeal muscles that were prev used for filter feeding, now used to draw water into mouth for respiration
more active than nonvertebrate chordates
when did the first vertebrates live
cambrian era - 500 million years ago
how were early vertebrates formed, what was their shape
soft bodied, small fish shaped specimens or eel-like in formation
how were early vertebrates known to be chordates
evidence of notochord and myomeres
how were early vertebrates considered vertebrates
sensory structures at head end -> vertebrates
2 super classes of subphylum vertebrata
- agnathans
- gnathostomes
2 major lineages of agnathans
- cyclostomes
- ostracoderms
cyclostomes
jawless and w/o mineralized tissues - considered “primitive”
lack specialized reproductive ducts and have a single nostril
not well represented in fossil record
unique features of cyclostomes
- single median nostril
- unarticulated gill arch skeleton
- velum
- tongue with keratinous teeth
- trunk muscles that extend into head and around eyes
- immune system that is diff from gnathostomes
when did the sister taxa of cyclostomes diverge
~460 million years ago
lampreys (petromyzontiformes)
freshwater/anadromous agnathans -> born in freshwater, go to ocean to grow, then return to freshwater to reproduce
many are parasitic of other fish as adults
single nasal opening on top of head
hagfish (myxiniformes)
around 75 species
worldwide distribution in marine environments
scavengers and predators of deep seas - can shoot out slime when threatened
pharynx is more posterior than other grps, most likely an adaptation to feeding mechanism
gill openings on side of body
how do stages of lampreys survive
larval: live in streams buried in sediment, filter feed for 3-7 years
adult: attach onto host via suctioned round mouth, contain a simple digestive system feeding on blood and tissue fluids from host
conodonts
agnathan
hypothesized to be vertebrates based on notochord, myomeres and large eyes
mineralized tissue may have evolved convergently with hard tissues of other vertebrates
ostracoderms
no jaws
extensive dermal bone - more derived than cyclostomes
tubular gill openings
notochord in adults
physical features of placoderms
type of gnathostome
jointed gill arches
jaws with teeth composed of dentine and enamel
tribasic pectoral and pelvic fins
epaxial and hypaxial muscles
antiarchs, arthrodires, ptyctodonts, qilinyu
4 lineages of gnathostomes
- placoderms: highly specialized extinct armoured fishes
- acanthodians: extinct “spiny sharks”
- chondrichthyans: cartilaginous fish
- osteichthyans: bony fishes (bony vertebrates)
jaws and teeth of gnathostomes
extant gnathostomes have teeth on jaws
bony fishes and tetrapods have teeth embedded in jaw
cartilaginous fish have teeth formed with skin
evolution of teeth must have happened after the jaws were formed - see placoderms
gnathostomes
derived characteristics include jaws ad 2 sets of paired limbs
jaws permitted new feeding behaviours and other manipulations (ex. carry pebbles to make a nest)
tooth whorl
rows of teeth, replacement teeth were in another row
chondrichthyan
acrodont
sharp, curved teeth, look like fangs, replacement teeth were simply curved at another angle but in the same row
bony fish, some lizards, amphibians
pleurodont
sharp, long, straight teeth, replacement teeth were embedded beside the tooth
most bony fish, some reptiles
thecodont
sharp, short teeth, replacement teeth were embedded beside the tooth inside
some reptiles, mammals
what was the first paired appendage to evolve in gnathostomes
pectoral fins
