Lecture 3 - Early Fishes
1. Early Chordates
2. Conodonts
3. Early Vertebrates
4. Jawless fishes
5. Agnatha/ Gnathostomes junction
6. Placoderms
7. Chondrichthyes Early Chordates Cephalochordates (lancelets) most primitive chordates?
Urochordates (tunicates and salps)
notochord: stiffening rod that provides support - lack vertebrae, brain, image • crucial to vertebrate forming eyes, & heart embryological development Cephalochordates are sister to vertebrates • embrace your inner lancelet! 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
81 species Live deep – scavengers & predators Strictly marine • isoosmotic - no osmoregulation Hagfishes - Class Myxini
• 4 rudimentary hearts
• 70-200 pairs of slime glands
• eye spots
• lack vertebrae Rasping tongue Hagfish feeding Hagfishes - Class Myxini
Reproduction….? • 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
~ 41 species Osmoregulate - found in freshwater and ocean Larval stage called ammocoete Ammocoete undergo metamorphosis Ammocoete filter feeds Lampreys 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
Gnathostomes: jawed vertebrates Agnatha - Gnathostomata Junction
Gnathostomes: jawed vertebrates Agnatha - Gnathostomata Junction
Gnathostomes: jawed vertebrates
Synapomorphies:
• Jaws modified from gill arches
• Paired limbs
• Vertebral centra usually present
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 Entelognathus primordialis Entelognathus primordialis, Placoderms “Plate-skinned”
• fresh & salt water
• arose ~430 mybp
• disappeared ~350 mybp Placoderms Synapomorphies:
Head and shoulder girdles with dermal bony plates
Five gill arches
- evolved towards reduced armament Chondrichthyes - Cartilaginous fishes
• arose ~420 mybp Chondrichthyes - Cartilaginous fishes
• extant cartilaginous fishes Chondrichthyes - Cartilaginous fishes
Synapomorphies:
• Cartilaginous skeleton, 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 • placental attachment ovoviviparous: egg hatches internally and give live birth • ~50% species • ancestral state Chondrichthyes - Cartilaginous fishes
Split into two subclasses:
Elasmobranchii Holocephali (sharks & rays) (ratfishes) Holocephali
• “whole-head”
• oviparous Holocephalans - Chimaeras - Ratfish
Synapomorphies: Gill cover over 4 gill openings
Upper jaw fused to cranium Holocephalans - Chimaeras - Ratfish Some species possess head claspers
- more speciose in the past