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Respiration

• Problem was not how to acquire oxygen using air (solved by use of lungs)

– Rhipidistians and Dipnoi used air long before it was necessary for terrestrial life.

– Amphibians switched priority from gills to lungs.

– Later in vertebrate evolution, the lung greatly increased in complexity, especially with evolution of endothermy

• Increased metabolic rate with increased oxygen dependence

• Lungs of early amphibians were not very different from those of fish.

Respiration

• Problem was how to get the air into the lungs.

– Fish could gulp air at surface and force air into lungs, using buccal pump or by gulping air and diving head first, letting air rise and pass into the lungs.

– Land animals couldn’t use passive method (diving)

– Buccal pump would have to be very strong to pump air into lungs and lift the body off the ground. (Limbs were not very strong.)

– Buccal pump is force pump:

• Lower floor of mouth, close nostrils, raise floor, push air into lungs

Respiration

• Modern amphibians have further adaptations to increase respiration:

– use of cutaneous respiration

– Gas exchange supplemented through vascular system in thin skin.

Positive Pressure Breathing

Cutaneous Respiration Buccopharyngeal Membrane

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Gravity

• Fish body supported by water is weightless, especially if fish has a well-developed gas bladder.

• Very different stresses on aquatic vs terrestrial vertebrate:

• Very different anatomy

Desiccation

• Living on land poses problems of water balance, as moisture is continously lost from the lung’s moist membranes and from the skin.

• Early labyrinthodont amphibians had bony scales; some even had dermal plates.

• These structures likely reduced water loss as well as providing some protection.

• In modern amphibians, no scales or dermal plates are found, and the skin is permeable and moist.

• Skin is thin and covered by a thin layer of dead epidermis.

Desiccation

• Mucus glands in the dermis secrete mucus over the epidermis, keeping it moist to facilitate subcutaneous respiration

• Excessive water loss is prevented by a number of adaptations.

– High concentrations of salts can be tolerated in the body fluids. Amphibians have much greater tolerance for variation in salt concentration than do other vertebrates.

– Behavioral adaptations: shade seeking, selection of cool, moist habitats

– Permeable skin allows absorption of water from damp ground.

Desiccation

• Amphibians are still dependent on water for reproduction

• Amphibian eggs will readily dry out and die if not in water.

• Some forms have made partial adaptations and are able to lay eggs in moist locations on land or within gelatinous froth.

• However, in general, the amphibians are still highly dependent on aquatic environments.

• The next vertebrate group to arise - the reptiles - finally broke that dependence.

Reptiles

• Waterproof skin

• Amniotic Egg

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Devonian Period

• Two major animal groups appeared on land:

– First tetrapods, or land-dwelling vertebrates

– First terrestrial arthropods, including wingless insects and the earliest arachnids (spiders etc).

Devonian Fishes • Classification of Fishes

– Fishes with jaws (cont.)

• Bony fishes - Osteichthyes

– Lobe-finned fishes –Sarcopterygii

– Characterized by fleshy pelvic and pectoral fins with well developed bones and muscles.

– Muscular fins with articulating bones

» These fins join (or articulate) to the body via a single bone (humerus to the shoulder or pectoral girdle, and femur to the pelvis).

– Hinged braincase and a corresponding intracranial joint in the skull roof

» Gives extra bite force

Devonian Fishes

• Fishes with jaws (cont.) – Bony fishes - Osteichthyes

• Lobe-finned fishes –Sarcopterygii

• Gave rise to amphibians during Devonian

• Living representatives

– Coelocanth

» 2 species

» Water around S. Africa, Madagascar

– Lung fish – can breathe air

» Live in freshwater

» 6 species

» S. America, Africa, Australia

Devonian Fishes Lobe-finned: Sarcopterygii

Diplurus (upper left; Triassic), Eusthenopteron (upper right: Late Devonian)

and Holoptychius (bottom: Late Devonian) ©

Coelocanth

Latimeria

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Devonian - lungfish

Lungfish

Living Lungfish

Dipterus valenciennesi Devonian Lung Fish Fossil

Grand Dad

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Tiktaalik rosae

375 million-year-old transitional species between fish and the first legged animals. Neil Shubin, PhD

Tiktaalik roseae

• Tiktaalik roseae, better known as the "fishapod," is a 375 million year old fossil fish which was discovered in the Canadian Arctic in 2004.

– Discovery sheds light on a pivotal point in the history of life on Earth: when the very first fish ventured out onto land.

– Lived about 12 million years before the first tetrapods (which are approximately 363 million years old).

– Existence of tetrapod features in a fish like Tiktaalik is significant because it marks the earliest appearance of these novel features in the fossil record.

Tiktaalik roseae

• It's very unlikely that Tiktaalik lived entirely on land.

– Based on the structure of its front fin and its shoulder, we know that it was capable of swimming and propping itself up in a push-up position.

– From this we can infer that Tiktaalik lived most of its life in the shallow water close to shore, using its strong front fins to push along the bottom and stabilize it in moving water.

– Front fin's limited range of motion would have prevented Tiktaalik from swinging its fins forward to walk the way an amphibian does.

Tiktaalik roseae

• Tiktaalik has a flat head and a neck like tetrapods but fins and scales like fish

• Tiktaalik most likely inhabited shallow streams, using its fins to push off the bottom

• Living in the shallows would have been a good strategy for survival for several reasons.

– convenient way to escape the huge carnivorous fish that lived in the deeper waters.

– also probably helped Tiktaalik pursue some of the smaller fish hiding in the shallows and the bug-like critters inhabiting the dense foliage banking the rivers and streams.

Meet Tiktaalik roseae: An Extraordinary Fossil Fish

• Fish-like

– Scales

– Fins

– Gills and lungs

• Tetrapod-like

– Neck

– Ribs

– Flat head

– Ear notches

– Fin skeleton

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How did we know where to look for Tiktaalik?

1. We used the distribution of known fossils to determine where there was a gap in the fossil record

– To find a transitional fossil between land animals and fish, we start by looking at the very first tetrapods to show up in the fossil record.

– Then, we look for fish which had a similar pattern of bones in their fins as the tetrapods had in their limbs.

How to find transitional fossils

2. STEP 2: Determine the age of the rocks the transitional fossil should be in

– We plot the ages of the known fossils to isolate the transition from water to land

How to find transitional fossils

• STEP 3: Find where the right rocks are at the surface and exposed

– Geologic maps help us locate the areas where the right age rocks are at the surface

How to find transitional fossils

• STEP 4: Plan an expedition to the most promising site!

– Our search brought us to the Canadian Arctic

Your Inner Fish

• Neil Shubin, Ph.D.

– Book

– 3 episode video series PBS

– Available online