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 - lack vertebrae, brain, eyes, provides support & heart • precursor to spinal chord 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

77 species Live deep – scavengers & predators Strictly marine • isoosmotic - no osmoregulation Hagfishes - Class Mixini

• 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

~ 42 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

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, 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 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