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EVOLUTIONBiology 145

Lecture 7 –The Origin of Tetrapods

Time frame

Four Extant Groups of Vertebrates. Agnatha ‐jawless fish.Chondrichthyes ‐ cartilaginous fishes (sharks and rays).Osteichthyes ‐ bony fish.Tetrapods ‐ amphibians, reptiles, birds, mammals.

408 ma

362 ma

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Early fish ‐ Agnathans•First fish lacked jaws. a.Belong to the class, Agnatha. c.Living species (hagfish, lamprey) belong to the group called cyclostomes. d.Cyclostomes lack bony skeletons, in which regard, they are probably degenerate. b.Principal fossil representatives belonged to the group called ostracoderms -heavily armored with bone.

Origin of jaws

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Early Jawed fish ‐Placoderms

•Jaws first appear in an extinct group called placoderms.

•Modern fish derive from placoderms: Two modern lineages:

•a.Chondrichthyes (cartilaginous fishes) - sharks and rays.

b.Osteichthyes (bony fishes) –

If Chondrichthyes evolved from (bony) Placoderms, then cartilaginous skeletons must be secondary!

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Among bony fish the basic split is between actinopterygians(ray‐fin fishes) and sarcopterygians(lobe‐fin fishes).

a.Crossopterygians. i.Rhipidistians, the ancestors of all tetrapods. ii.Coelacanths, a marine offshoot of the basal stock, of which a single genus, Latimeria, survives as a much-modified deep sea form.

b.African and Australian lungfish.

•Sarcopterygians include

Lepidosiren – S. America

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Lung fish

Dipterus (Devonina); Epiceratodus (Australian, living)

Amphibians – the first tetrapods

1:  Lungs for breathing2: Limbs for locomotion3: Hearing (to replace lateral line system)

Pederpes – Lower Carboniferous – 345‐359 ma

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Origin of Lungs

Polypterus; African lungfish

History of the hyomandibular

sharks

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Legs

Crossopterygian Primitive extinct amphibian

Tiktaalik

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Tiktaalik

Mudskippers

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Ichthyostega –upper Devonian 359‐374 ma

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1:  Lungs for breathing2: Limbs for locomotion3: Hearing (to replace lateral line system)

Fully terrestrial tetrapods also need…

4: Scales (to constrain water loss)5: Eggs capable to developing in a terrestrial environment.

Completing the transition to land

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“Reptiles”•temporal fossae. 1.Fossae allow for larger jaw muscles — i.e., muscles need room to bulge when contracted. 2.Anapsids (no opening) mostly extinct — include modern turtles — the primitive condition. 3.Synapsids (single lower opening) include "mammal-like reptiles" (therapsids)

a.Dominant land animals in Permian (late Paleozoic). b.Some evolved into mammals.

4.Euryapsids (single upper opening) entirely extinct — include pleisiosaurs and other extinct reptiles. 5.Diapsids (two openings) include dinosaurs, living reptiles and birds (descended from small, carnivorous dinosaurs related to Velociraptor).

J= jugal, po = postorbital, sq=squamosal, p=parietal

Amniote egg

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