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Lecture 3 FishAgnatha, Placodermi, Acanthodii, Chondricthyes and Osteichthyes

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The First Vertebrates: Class Agnatha

the first vertebrates lived about 500 mybp in theCambrian era.

they are placed in the class Agnatha, or jawless

vertebrates

the early agnathans are often referred to as

ostracoderms shell skins in reference to their bony

armour

although ostracoderms have been extinct since about300 mybp, we still have some extant members of the

class Agnatha today

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Ostracoderm Characteristics

KK Figs. 3.9, 3.10, H&G Figs 3.3, 3.4

no jaws

single nostril onthe top of the head(rather than a pair)

tubular gillopenings

notochord in adults pharyngeal filter

feeding

endostyle organ

few or no paired

fins

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Modern Agnatha (cyclostomes)

Living Agnatha are

hagfishes (KK Fig.

3.5 a&b) and

lampreys (KK Fig.

3.5 c&d). H&G

Figs. 3.1, 3.2.

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Modern Agnatha - Hagfish

Hagfish are marinefish that feed ondead fish or marinemammals.

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Lamprey - mouth showing

teeth

Lampreys are freshwater or anadromous agnathans, many of which are

parasites on other fish as adults.

sea lamprey on lake trout

Modern Agnatha - Lampreys

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Lamprey larvae - ammocetes

The larval stages of lampreys live in streams, filter-feedwhile mostly buried in the sediment, and bear a

strong similarity to cephalochordates.

Recapitulation?

H&G Fig. 3.2, KK 3.5d

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Primitive features of cyclostomes

no jaws

single nostril on the top of the head

tubular gill openings no paired fins

notochord persists in adult

pharyngeal filter-feeding in ammocete larvaeThe mysterious conodonts?

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A Devonian Lamprey

From Gess et al. 2006, Nature 443: 981

Priscomyzon riniensis, from South

Africa 360 mybp

Lampetra fluviatilis, amodern lamprey

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Fishes with Jaws Fishes with jaws appear in the fossil record about 400 mybp,

or about 100 million years after Agnatha during the Silurianperiod.

By the Devonian age of fishes four classes of fishes with

jaws were diverse and abundant.

Jaws apparently evolved from bones supporting the anteriorgills, and allowed fish to become effective predators rather

than filter feeders (we will return to this later!).

monkfish

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Classes Placodermi and Acanthodii These two classes are both extinct, but were abundant during the Devonian

(approx. 400 mybp)

Both were heavily armoured, active predators.

Placoderms (KK 3.12, H&G 3.5) Acanthodians(KK 3.14, H&G 3.8)

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Class Chondricthyes - cartilaginous fishes

Jawed fishes with cartilage rather

than bony skeletons. Most with

unique scales (dermal denticles),

unique teeth, spiracles, and slit-

like gill openings. Known as

fossils since > 400 mybp.

Two subclasses, Elasmobranchii

(sharks and rays) and

Holocephali (ratfishes or

chimaeras)

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These are mostly active, free-swimming elasmobranchs with

streamlined bodies and large heterocercal tails.

Subclass Elasmobranchii 1 (Sharks)

Streamlined, fusiform,often pelagic.

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Subclass Elasmobranchii 2 - skates and rays

Dorso-ventrally flattened, many specialized

for bottom feeding.

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Subclass Holocephalichimaeras or ratfishes

KK 3.13b, H&G Fig. 3.7

Deep-sea chondrichthyans with a fleshy operculum covering the gills.

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Class Osteichthyes - bony fishes

more species than any other class ofvertebrates for the last 150 million years

bony skeletons, scales, operculum

two subclasses:

subclass Actinopterygii

(ray-finned fishes)

subclass Sarcopterygii

(fleshy-finned fishes)

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Living Actinopterygii with a longhistoryKK 3.17,

H&G 3.9, 3.10

These Actinopterygii go back as fossils for millions of years

and share some primitive traits, including:

- lungs- heterocercal (assymmetric) tails

- bony (ganoid) scales covered with enamel

- notochord in adults

Local fishes

Acipenser= sturgeon

Lepisosteus = gar

Amia = bowfin

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-soft, flexible (elasmoid) scales

- symmetrical (homocercal) tail

- gas bladder rather than lungs

Atlantic Salmon (Salmo)

KK 3.17

H&G 3.11

Modern Actinopterygii (teleosts)

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Functional Significance of Tail Shape

KK 1.1

H&G 27.18

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Sarcopterygii - lungfishKK 3.19, H&G 3.12

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Fish Evolutionary Tree

KK Figs 3.8 and 3.16

5 classes of fish:

Agnatha, Placodermi, Acanthodii,

Chondrichthyes,and Osteichthyes.

Which two are extinct? Which is largest?