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Topic 4: The Origin of Tetrapods

� Next two lectures will deal with:

� Origin of Tetrapods, transition from water to land.

� Origin of Amniotes, transition to dry habitats.

Topic 4: The Origin of Tetrapods

� What is the geological time scale and why is it important?

� Where do herps fit in the vertebrate phylogeny?

� What are the evolutionary origins of tetrapods?

� What changes were involved in the transition from water to land?

The geological time scale

� Organizes the history of the earth

� Based on geological and biological/fossil criteria

� Allows us to consider “________________”

� Time scale over which geological and ___________________ phenomena occur

� Organized into hierarchical ________, ________, ______________, and ______________

The geological time scale

Eon Era

Phanerozoic

Cenozoic

Mesozoic

Paleozoic

Precambrian

Proterozoic

Archaen

Hadean

MYA

0-65

65-245

245-570

570-2500

2500-3800

3800-4600

� Precambrian accounts for ~88% of Earth history

� We will concentrate on Phanerozoic

Age of oldest rocks

1st unicellular organisms

1st multicellular organisms

1st multicellular organisms

Age of oldest rocks

1st multicellular organisms

Age of oldest rocks

The Paleozoic Era

Period

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

MYA

245-286

286-320

360-408

408-438

438-505

505-570

Events

1st _________________

1st reptiles, amphibians specialize

1st amphibians

1st jawed fishes

1st jawless fishes & land plants

1st vertebrates

The Mesozoic Era

Period

Cretaceous

Jurassic

Triassic

MYA

65-144

144-208

208-245

Events

- Mass extinction at end of Cretaceous

- 1st modern Squamata

- 1st Urodela, Anura

- 1st Rynchocephalia

- High reptile diversity

- 1st birds

-1st Angiosperms, dinosaurs, mammals

- 1st Testudines, ___________

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The Cenozoic Era

Period Epoch

QuanternaryHolocene

Pleistocene

Tertiary

Pliocene

Miocene

Oligocene

Eocene

Paleocene

MYA

Recent

0.01-1.5

1.5-5

5-24

24-37

37-58

58-65

Comments

Modern humans

Evolution of humans

1st hominines

-1st Gymnophiona

- 1st pacental mammals

What are stem and crown groups?

� _________________ – smallest monophyletic group to contain the last common ancestor of all extant members of a taxon

� _________________ – contains crown group plus closely related extinct taxa. Last common ancestor is older than that for its extant members

What are stem and crown groups?

� Crown groups:

� Are monophyletic

� Contain all extant taxa

� Can contain extinct taxa

Benton Fig 9.11

� Stem groups:

� Are also monophyletic

� Contain all extant & extinct taxa

Where do herps fit into the

vertebrate phylogeny?

� Phylogeny of jawed vertebrates

� What are outgroups of Tetrapoda (3)?

� Actinopterygii (6)

� Actinistia (7)

� Dipnoi (8)

� ____________________ (6) are ray-finned fishes

� Extremely diverse (~25,000 spp.)

� Highly derived

� Fins have extrinsic muscle

Pough et al 2004, Fig 2-1

� ________________ (2)

� Lobe-finned fishes

� Contain Actinistia (7), Dipnoi (8), Tetrapoda (3)

� Fins/limbs are supported by bone and contain ___________ muscle

� Teeth have enamel

� What kind of characters are these?

Pough et al 2004, Fig 2-1

Where do herps fit into the

vertebrate phylogeny? Sarcopterygian lineages

Benton Fig 9.11

Vertebrate Life Fig 8-35

� _________________

� Coelacanth, Latimeria

� 2 extant species

� Symmetrical, 3-lobed ________

� “living fossil”

� _________________

� Lung fishes

� 3 extant genera

� Gondwanan distribution

� 1 in S. America, 1 in Africa, 1 in Australia

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Sarcopterygian lineages

� Tetrapoda

� Includes all modern amphibians and amniotes, and their last common ancestor.

� _________________ _________________

� Fully developed _____________ girdle

� Pectoral girdle free from the skull (not the case in non-tetrapods)

� Discrete shaft of ______________

� This phylogeny includes only extant taxa

� Many species of early tetrapods and stegocephalians are extinct

� Therefore, must look at both stem and crown Amphibia

Pough et al 2004, Fig 2-1

Where do herps fit into the

vertebrate phylogeny?

Stem and Crown Amphibia

Most groups of

Amphibians and all

early tetrapods are

____________

Benton 1997, Fig 4-21

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Early tetrapod relationships are contentious�

Pough et al. 2004 Fig 2-8

Traditional hypothesis

Laurin & Reisz 1997

Inclusion of only some taxa, assumption of Temnospondyl-Lissamphibian association

No a priori assumptions, many more taxa included

Stegocephali

Tetrapoda

Lepospondyls

Temnospondyls

How do these two hypotheses differ? How are they the same? Why might we prefer one over the other?

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What are the evolutionary origins of tetrapods?

Pough et al. 2004 Fig 2-8

Stegocephali

Tetrapoda

Lepospondyls

Temnospondyls

� Laurin & Reisz 1997

� Lepospondyls are:

� __________________

� Sister to Lissamphibia

� Amniotes (including reptiles) are:

� Sister group to Lissamphibia+Lepospondyls

Pough et al. 2004 Fig 2-3

Stegocephali

Tetrapoda

Lepospondyls

Temnospondyls

� Elpistostegidae (Panderichthys)

� Outgroup to Stegocephalia but closer than Dipnoi

� Dorsoventrally _____________, no dorsal fin, dorsal eyes

� _________________ water

� Tetrapod-like body but with fins

� Developed frontal bones a synapomorphy with stegocephalia

What are the evolutionary origins of tetrapods?

Pough et al. 2004 Fig 2-2

Stegocephali

Tetrapoda

Lepospondyls

Temnospondyls

� Acanthostega & Ichthyostega

� ___________ from Devonian (360mya) of Greenland and Russia

� Four limbs with digits, but also a branchial apparatus with gills �________________

What are the evolutionary origins of tetrapods?

Vertebrate Life Fig 10-3

Stegocephali

Tetrapoda

Lepospondyls

Temnospondyls

� Acanthostega & Ichthyostega

� _______________

� 6-8 digits per limb

� What does this mean for ancestral number of digits?

What are the evolutionary origins of tetrapods?

Pough et al. 2004 Fig 2-7

Stegocephali

Tetrapoda

Lepospondyls

Temnospondyls

� Temnospondyls

� Carboniferous

� Few centimeters to over 1m

� Limbed, posture more upright than Acanthostega/Ichthyostega

� Some aquatic, others (below) terrestrial

� Typically ________________

What are the evolutionary origins of tetrapods?

Stegocephali

Tetrapoda

Lepospondyls

Temnospondyls

� Amniota

� We will come back to these

� Mammals, birds, snakes & lizards, turtles, crocidilians

What are the evolutionary origins of tetrapods?

� Next slide

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Pough et al. 2004 Fig 2-4b,c; 4-14, 4-15

� Lepospondyls� Carboniferous & Permian

� Diverse� Microsauria – _________ & small

� Aisopoda – ___________� Nectridea – _________ & newt-like

What are the evolutionary origins of tetrapods?

Photos: KP Bergmann, Pough et al. Fig 3-8

Stegocephali

Tetrapoda

Lepospondyls

Temnospondyls

� Lissamphibia

� Urodela, Anura, Gymnophiona

� Extant (finally)

What are the evolutionary origins of tetrapods?

What changes were involved in the transition

from water to land?

What is involved in

this major evolutionary

transition?

Terrestrial

Obligate Aquatic� This transition involved aspects

of structure and function associated with:

� ____________________

� ____________________

� ____________________

� ____________________

� ____________________

� ____________________

� ____________________

� ____________________

What changes were involved in the

transition from water to land?

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� Body support� Moving onto land means coping

with _____________

� In the water, the body is supported by the water, on land by limbs

� Coping with ______________ __________________

� Small in water

� Large on land

What changes were involved in the

transition from water to land?

� Ground reaction forces

� Pectoral girdle became ____________ from the head

� Minimizes forces exerted on skull and brain, increases head ____________

Ichthyostega

Benton 1997 pg 183; Benton Fig 4-2a; Pough et al 2004 Fig 2-2

Panderichthys

Actinopterygii

What changes were involved in the

transition from water to land?

� Ground reaction forces

� Limb girdles became more ______________ to the vertebral column

� Allows limbs to become support structures and transfer forces to the axis

Benton 1997 Fig 3.22; Pough et al 2004 Fig 2-2a

Acanthostega

Protopterus(African lungfish)

What changes were involved in the

transition from water to land?

� Locomotion

� A switch from ____________ to walking and running

� Terrestrial locomotion is much harder on the ___________

� Fin-based swimming is replaced by pushing off a substrate with the limbs

� Lateral undulation plays a role in both

� ________________ less of an issue on land

What changes were involved in the

transition from water to land?

� Locomotion

� Limbs change dramatically:

� More ____________

� More ________________

� Switch from extrinsic to

intrinsic limb musculature

AcanthostegaPanderichthys & Acanthostega

Vertebrate Life Fig 10-3

What changes were involved in the

transition from water to land?

� Locomotion

� Vertebral morphology becomes more ______________

� Zygapophyses limit excessive intervertebral forces, especially _____________

Kardong & Zalisko 2002: Fig 5.8c, 5.11b

What changes were involved in the

transition from water to land?

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� Body shape

� Very _________________ in fishes

� Streamline but dorsoventrally flattened in Panderichthys

� Allows invasion of shallow

water

� Also dorsal eyes, no dorsal fin

� Decrease in hydrodynamics in terrestrial forms

What changes were involved in the

transition from water to land?

� Respiration� Gills only to gills & lungs to

lungs only

� Both gills and lungs are ancestral because lungs arose from ____________

� Acanthostega still had gills

� Ventilation increasingly powered by __________ of the ______________

What changes were involved in the

transition from water to land?

� Excretion

� Trend from excreting ammonia to __________ to ___________

� Ammonia is most toxic but also most water soluble

� Ammonia is voided quickly through ___________

� Urea and Uric Acid are concentrated and excreted by _______________

What changes were involved in the

transition from water to land?

� Water balance

� Fishes live in excess water

� Lissamphibia live in moist environments, but more water-_________________

� Mucous glands limit evaporation from an amphibian’s body

� Lissamphibia have various behaviors and postures that limit water loss

What changes were involved in the

transition from water to land?

� Hearing

� Sound travels differently in air than in water

� Other organisms are similar density to water

� In air there is a differential between densities

� Hyomandibula bone of the 1st gill arch gives rise to the _______________, an inner ear bone that connects to the ____________________

� This allows for better conduction of vibrations to the inner ear

Kardong & Zalisko Fig 5.29d

What changes were involved in the

transition from water to land?

� Feeding

� ______________ is prevalent in fishes, impossible on land

� On land, jaws and the tongue play a more important role

Pough et al 2004, Fig 11-19

What changes were involved in the

transition from water to land?