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

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 speciesLive deep – scavengers & predatorsStrictly 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 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

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

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

Entelognathus primordialis

Entelognathus primordialis,

Placoderms“Plate-skinned”

•  fresh & salt water

•  arose ~430 mybp

•  disappeared ~350 mybp

Synapomorphies:

Head and shoulder girdles with dermal bony plates

Five gill arches

Placoderms

- evolved towards reduced armament

Chondrichthyes - Cartilaginous fishes•  arose ~420 mybp

Chondrichthyes - Cartilaginous fishes

•  extant 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(sharks & rays)

Holocephali(ratfishes)

Chondrichthyes - Cartilaginous fishes

Holocephali

•  “whole-head”

•  oviparous

Holocephalans - Chimaeras - Ratfish

Synapomorphies:Gill cover over 4 gill openings

Upper jaw fused to cranium

Holocephalans - Chimaeras - Ratfish

- more speciose in the past

Some species possess head claspers