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.