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I. Introduction

• life originated in sea
• most phyla, particularly algae & invertebrates are still predominately or exclusively marine
• life begins colonization of land - Silurian (435-395 m.y.a.)

A. Problems:

1. must obtain water
2. must obtain food
3. prevent body water evaporation
4. develop special structures for breathing oxygen
5. gamete desiccation

B. Advantages of Terrestrial Life:

1. for plants- abundant unoccupied space direct sunlight
2. for animals- abundant free oxygen after plants - almost unlimited food

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II. Transition Summary:

• most organisms made transition - via fresh waters of lakes and rivers

1st must adapt to - lack of salts
• all animals: same salt concentrations as seawater, in fresh water must develop special organs to prevent salt loss
• 9 Phyla make transition to fresh water.
• Only 4 Phyla make transition to land:
1. Mollusca (snails);
2. Arthropoda (arachnids and insects);
3. Tracheophyta (all) ;
4. Chordata - reptiles, birds, mammals (notochord, nerve cord, gill slits).
5. Note: Arachnids - class of Arthropods; separate from insects (spiders, scorpions, etc.)

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III. Transitional Stages of plant and invertebrate phyla to land:

A. PLANT TRANSITIONS: (Tracheophyta)

• Phylum Chlorophyta ancestor to Phylum TRACHEOPHYTA (Land plants: ferns, conifers, flowers).
• EVIDENCE:
1. same pigments
2. yield same carbohydrate
3. one of few algae - transition to fresh water, some moist soil

B &C. TERRESTRIAL INVERTEBRATES: (molluscs & arthropods)

• Many phyla make transition to fresh water
• only 2 phyla make transition to land:
1. molluscs - snails;
2. arthropods (Late Silurian) - crustaceans, arachnids *insects!!
• success to due to ancestors:
1. water-tight covering
2. mobile - to seek food
• respiration:
1. gastropods - lungs to oxygenate blood
2. insects & arachnids - tracheae : no complex circulatory - respiratory systems, thus a limit to size
• insects: develop wings: move rapidly to search for food & to flee predators
• adaptive advantage: rapid evolutionary radiation; 3/4 all living animal species
• fossils: few; do not live in areas of sediment accumulation
• Order of appearance
1. gastropods - 1st in Carboniferous
2. arachnids & insects - impressions or in amber.
3. 1st land animals - Silurian/scorpion-like arachnids,
4. Carboniferous & Permian - flying insects; some 3' wingspan

IV. ORIGIN OF VERTEBRATES

A. INTRODUCTION

• Certainly arose from invertebrate ancestor
• ancestral group uncertain;
• fossils give no clues
• 1st fossil (fish in Cambrian); fully differentiated.
• Therefore based upon indirect evidence:
1. first vertebrates appear related to organisms with a backbone-like structure or "notochord" (no separate segments)
2. "invertebrate chordates" - 3 notochord subphyla are 3/4 subphyla of Chordata.

B. INVERTEBRATE CHORDATES

• most revealing - lancelets
• Subphylum Cephalochordata - well developed notochord; gill slits; no jaws or teeth
• Descendants of E. Paleozoic ancestors of vertebrates
• also - Subphylum Hemichordata "acorn worms" - notched; gill slits
• most likely invertebrate ancestor to chordates - echinoderms!
1. 1)larval stages similar to "acorn worms"
2. biochemical & embryological parallels
• Notochord & internal skeletons - key adaptations: for evolutionary success of vertebrates
• internal skeleton
1. strengthening support system w/nervous system & muscles
2. development of large and mobile vertebrates.

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V. THE EVOLUTIONARY HISTORY OF FISHES

A. INTRODUCTION:

• earliest vertebrates were fish
• fish & amphibious weretransitionary to land vertebrates
• first fossil fragments: - Cambrian
• rare until - end Silurian,
• explosive radiation; throughout Devonian
• L. Silurian - M. Devonian.: evolutionary experimentation - classes: Agnatha & Placoderms
• Devonian - rapid evolutionary expansion: "Age of Fishes"

B. AGNATHS: "JAWLESS FISHES"

• most primitive- appeared in Late Cambrian
• almost all Ordovician & Silurian fishes were of this type
• lancelet - like ancestors
• deposit - or suspension feeders
• small; <1 foot
• covered by bony plates (Ostracoderms)- function uncertain, cartilaginous skeleton.
• decline rapidly in Early Devonian when jaw-bearing fish enter
• all armoured ones extinct by close of Devonian group survived as (unarmored) - lampreys & hagfish

C. PLACODERMS

• jaws - 2nd only to internal skeleton in importance - permitted carnivorous and herbivorous feeding
• NO transitional fossils - probably originated from agnaths - agnath embryological studies
• 1st fossils - Late Silurian
• rapid radiation - Early Devonian
• well diversified - Mid Devonian
• characteristics: most 1-3'; up to 40'; carnivorous; tough bony armour; partially calcified vertebrae but mostly cartilaginous skeleton.
• dominance brief - began decline in Late Devonian as replaced by dominant modern descendants - sharks & bony fishes
• few survive until end Permian
• By Early Devonian.... agnaths & placoderms adapted to fresh water; evolutionary radiation in rivers, lakes & oceans
• since the Devonian fresh & salt water populated by fish

D. AND E. SHARKS & BONY FISHES

• more efficient jaw mechanisms
• more efficient swimmers - loss of armor; fins & muscles
• both arise - Devonian - from placoderms NO intermediate fossils

Bony-Fish
(1) predatory, scavenging, herbivorous, & bottom dwelling
(2) arose from fresh water placoderms
(3) Cretaceous : re-invasion of sea
(4) two groups in Devonian - fins important distinction!!

• (A) RAY-FINNED
• (B) LOBE-FINNED - GAVE RISE TO AMPHIBIANS!!!!!!!

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VI. AMPHIBIANS - Transitional vertebrates

• Devonian- critical time in the evolutionary history of fishes; also time vertebrates made transition to land.
• Fossil evidence.
1. oldest in Upper Devonian of Greenland and Canada
• Faced same problems as plants & invertebrates
1. reproduction
2. water-retention
3. oxygen-respiration
4. Faced unique problem of locomotion
• Fishes adapted to swimming: profound modification required of land-dwelling descendants.
• The skeletal potential for developing a means of locomotion distinguished lobe-finned fishes from ray-finned relatives!

A. lobe-finned (see page 292, figure12.13)

1. fins move freely at point of attachment
2. muscles extended into fin;
• precise control of movements
• suited to develop into elongated, flexible limbs to support and move animal
• able to cope with lack of water because evolved auxiliary lungs (know they had lungs because some surviving air-breathing descendants - lungfish)

B. ray-finned (fin structure not weight supporting, illustration in class)

• Another Adaptation
• Lived in fresh water streams & ponds; sometimes became stagnant and dried up.
• some able to cope with lack of water because evolved auxiliary lungs

C. Lobe-finned fish to amphibian transition

• Why fishes found it advantageous to develop limbs:
1. searching for new streams & ponds
2. escaping aquatic predators
3. food
• LUNGS & FIN ARRANGEMENT - of living & fossils lobe-finned fishes; adaptations that made possible life on land.
• Lungfish but one group of lobed-fins.
• Another lobe-finned group, less specialized, gave rise to land-dwelling amphibians!
• Ichthyostega; the first amphibian (from the Late Devonian of Greenland) is remarkably similar to lobe-finned ancestors:
• Some similarities:
1. Skull & skeleton of Devonian (crossopterygians; earliest amphibians) - almost identical
2. similar and unusual complexity to teeth in cross section
3. similar skull structure
4. both had tails
5. bones structure in limbs similar, except modification of lower fins into stubby limbs
• The lobe-finned to amphibian transition one of best documented in fossil record!

D. Amphibian expanison

• Amphibians rapidly expanded & diversified in Carboniferous and remained abundant through the Early Permian, known as "Age of Amphibians".
• During the Carboniferous, large amphibians were more reptile-like in appearance - fat, stub-nosed, "alligators".
• Some of these amphibians were several hundred pounds and up to 10 feet long.
• Surviving modern amphibians - frogs, toads, newts, salamanders, etc.; originated in the Triassic and Jurassic as specialized descendants of late Paleozoic amphibians.
• These amphibians followed the first terrestrial plant life by 80 m.y...why?
• Amphibian versus Reptile Reproduction.
1. Amphibians must return to water to leave their eggs.
2. Their eggs are porous to allow oxygen in and wastes out but must be immersed to prevent desiccation.
3. Reptiles solved this problem with the amniotic egg ( with amniotic membrane and yolk).

E. Rise of the Reptiles.

• The first reptiles evolved from the anthracosaur amphibians during the Early Pennsylvanian.
• This event was the beginning of the end for amphibians!