lecture 3 - early fishes · lecture 3 - early fishes 1. early chordates 2. ... placoderms 7....
TRANSCRIPT
Lecture 3 - Early Fishes
1. Early Chordates
2. Conodonts
3. Early Vertebrates
4. Jawless fishes
5. Agnatha/ Gnathostomes junction
6. Placoderms
7. Chondrichthyes
most primitivechordates?
Early Chordates
Urochordates(tunicates and salps)
Cephalochordates(lancelets)
notochord: stiffening rod that provides support• precursor to spinal chord
- lack vertebrae, brain, eyes, & heart
Craniata (= Vertebrates)Synapomorphies:Cranium present
Cartilage or bone or both are present
Heart chambered
Neural crests
Conodonts
• 600 – 200 mybp
• “elements” were abundant in fossil beds
• Not until 1980’s did we find fossilized soft body parts
• cartilaginous head skeleton
Agnatha - Jawless Fishes
Agnatha • appeared 530 mybp• previously given superclass
status• now recognized as
paraphyletic• Myxinomorphs now
considered separate in own superclass
• still used as informal adjective for jawless fishes
Agnatha – Ostracoderms
• Name means “shell-skinned” referring to bony shield that covered head and thorax
• heavily armored
• first ossified bones evolve
• jawless & no pelvic fin
Ostracoderms are paraphyletic
…making “ostracoderm” a false designation
• likely 4 superclasses of jawless fishes
Hagfishes - Class Myxini
77 speciesLive deep – scavengers & predatorsStrictly marine• isoosmotic - no osmoregulation
Hagfishes - Class Mixini
• 4 rudimentary hearts
• 70-200 pairs of slime glands
• eye spots
• lack vertebrae
Hagfishes - Class MyxiniReproduction….?• cash prize for information on the reproduction of
Myxine glutinosa remains unclaimed since 1854 from the Royal Danish Academy of Sciences
Development• No larval stage, direct development
from eggs
Lampreys - Class Petromyzontida
~ 42 speciesOsmoregulate - found in freshwater and oceanLarval stage called ammocoeteAmmocoete undergo metamorphosis Ammocoete filter feeds
Lampreys
Reproduction• semelparous• males build nest • eggs hatch after 12-14 days and ammocoete
emerges• ammocoete burrows into mud or silt in river or
stream, with head emerged filter feeding• can stay like this for up to seven years
Non-parasitic spp.• form spp. pairs with parasitic spp.
Agnatha - Gnathostomata Junction
Synapomorphies:
• Jaws modified from gill arches
• Paired limbs
• Vertebral centra usually present
Gnathostomes: jawed vertebrates
Previously thought that a shark like ancestor gave rise to all jawed vertebrates but this fossil debunks that hypothesis• characteristics of both Placoderms and Osteichthyans
Synapomorphies:
Head and shoulder girdles with dermal bony plates
Five gill arches
Placoderms
- evolved towards reduced armament
Synapomorphies:
• Cartilaginous skeleton, calcified but not ossified
• Skull with no sutures
Chondrichthyes - Cartilaginous fishes
Synapomorphies (cont’d):
• Internal fertilization with claspers
• High blood concentration of urea
Chondrichthyes - Cartilaginous fishes
Reproduction
oviparous: lay eggs• ~40% species
viviparous: live birth• ~10% species
ovoviviparous: egg hatches internally and give live birth• ~50% species
Chondrichthyes - Cartilaginous fishes
Split into two subclasses:
Elasmobranchii(sharks & rays)
Holocephali(ratfishes)
Chondrichthyes - Cartilaginous fishes
Holocephalans - Chimaeras - Ratfish
Synapomorphies:Gill cover over 4 gill openings
Upper jaw fused to cranium