Assaulting the land- Early Tetrapod Evolution

Question 1

When were sea levels at (or near) an all time high?

Answer 1

The Paleozoic

Question 2

What were the main geographical and environmental changes during the Cambrian- Silurian period? (570-409 Mya)

Answer 2

• Sea levels were at an all time high.
• High levels of CO2, hot and dry climates were probably unsuitable for terrestrial combustion
• During the Silurian glaciations, falling CO2 levels and rising O2 levels (25%) prepared the land for invasion
• Continents started coalescing during this period into Pangaea.

Question 3

Describe the Devonian-Carbiniferous period. (409-290Mya)

Answer 3

• Pangaea completely forms
• This lead to the spreading of land plants during the Devonian period and the subsequent modification of the soil
• Soil production profoundly affects the climate resulting in global cooling
• Associated sharp decrease in atmospheric CO2 levels, likely associated with photosynthesis; O2 levels get very high peaking at 30%

Question 4

What did the typical Devonian/Carboniferous scene look like?

Answer 4

• Terrestrial environment becomes substantially more complex.
• Horsetails, club mosses (lycopods) become common plants
• Dragonflies, Scorpions, mites, millipedes and euripterids among the first invertebrates exploiting this new habitat.

Question 5

What is described as the most dramatic event in mammal evolution?

Answer 5

Movement from water onto land

Question 6

Why is land a relatively dangerous habitat?

Answer 6

Because animals are composed mostly of water

Question 7

What organisms made the transition to land before early animals?

Answer 7

Vascular plants, pulmonate snails, and tracheate arthopods made the transition earlier.

Question 8

What creatures are the only living vertebrate that have a transition from land in both their ontogeny and phylogeny?

Answer 8

Amphibians

Question 9

Was the transition to land complete in the Quasterrestrial period?

Answer 9

No, there was hovering between aquatic and land environments.

Question 10

What physical differences had to be addressed when moving from water to land?

Answer 10

• Oxygen is 20 times more abundant in air
• Oxygen diffuses more rapidly in air
• Air is 1000 times less dense
• Air provides less buoyancy
• Limbs and skeleton must support weight
• Air fluctuates in temperature more rapidly than water; animals have to address these extremes
• Variety of terrestrial habitats allow greater opportunities for adaptation and diversification

Question 11

How did early terrestrial vertebrates evolve?

Answer 11

• 400 million years ago, bony fish diversified to include many freshwater forms.
• Combinations of characteristics that evolved in aquatic habitats made it possible to explore terrestrial habitats
• Two structures connected to pharynx: air filled cavity functioned as a swim bladder; paired internal nares functioned in chemoreception
• On land, combination would be used to draw in oxygen rich air through nares into the air-filled cavity.
• Bony elements of paired-fins were modified for support and movement underwater, on land would provide the same function.
• Multiple fish groups evolved some degree of terrestriality
• One transition in late Devonian period provided the ancestral linage of all tetrapod vertebrates
• Evolved adaptations for air breathing: increased vascularisation of the air-filled cavity
• Double circulation to direct deoxygenated blood into the lungs and oxygenated blood out of the lungs to other body tissues

Question 12

Why was support important for moving to land?

Answer 12

• A fish is buoyed up by the water and its body weight may effectively be zero.
• On land, body is usually held up by the limbs and all of the internal organs have to become structurally modified in order to cope with gravity.
• Vertebrae and muscles around the backbone have to become modified to prevent the body from sagging between the limbs; axial skeleton developed into a suspension bridge.

Question 13

Why was support important for moving to land?

Answer 13

• A fish is buoyed up by the water and its body weight may effectively be zero.
• On land, body is usually held up by the limbs and all of the internal organs have to become structurally modified in order to cope with gravity.
• Vertebrae and muscles around the backbone have to become modified to prevent the body from sagging between the limbs; axial skeleton developed into a suspension bridge.

Question 14

How was movement/locomotion altered when moving to land?

Answer 14

• Fish have a smooth gliding motion
• Terrestrial organisms have to operate in a jerky fashion to produce steps propelling the body forward
• Paired fins produced a form of ‘walking’ although different in detail from tetrapod walking
• Pelvic/pectoral girdle alter substantially to support weight and allow expansion of the lungs
• Movement is via flexon of axial skeleton predominantly (some reptiles retain this condition)
• Generating friction is energetically very expensive compared to flight and swimming
• Mode of locomotion often correlates with the size of the organism

Question 15

How did early tetrapods modify the way they feed?

Answer 15

• Fish have highly kinetic skulls; in water food is weightless and can be sucked via currents of water
• Terrestrial vertebrates use jaws and teeth to seize food/manipulate items in the mouth
• Early tetrapods lost much of the kinetic skull but developed musculature to feed on small fishes and the increasing numbers of terrestrial invertebrates

Question 16

How did early tetrapods develop their breathing?

Answer 16

• Air breathing needs lungs, or some equivalent supported vascular surface instead of gills. Lungs contain internal folds and pouches lined with heavily vascularised skin and bathed in fluid
• Living lungfishes have functional lungs and the same is assumed for most other early bony fishes.
• The first tetrapods may have been only marginally better than their fish ancestors at air-breathing

Question 17

What are the two main modes of breathing in tetrapods?

Answer 17

• Costal ventilation: where the ribs and costal muscles expand and contract the lungs
• Buccal pumping: where air is sucked into the mouth and throat, and then rammed into the lungs by raising the floor of the mouth

Amniotes rely on costal ventilation, but lliving amphibians use buccal pumping, and it is seen especially in frogs.

Early tetrapods likely breathed partly, or mainly by buccal pumping as suggested by their broad mouths and short straight ribs.

Question 18

How did the first tetrapods alter their sensory systems?

Answer 18

• As the lateral line system could be used only in the water, eyesight is even more important on land than in shallow ponds (tetrapods have larger eyes than their precursors)
• Early tetrapods had a poor sense of hearing in air, as did their ancestors; the bone associated with hearing in modern amphibians and reptiles (stapes) is present in early tetrapods but it is too massive to be effective in hearing high frequency sound.
• A modified version of the hyomandibular element, which forms part of the jaw-hinging apparatus in most fishes.
• Olfaction: becomes a critical tool for detecting mates and prey; development of the nostril and vomeronasal/Jacobson’s organ
• Proprioception: develops with associated muscle spindles for detecting stretch- allowing tetrapods to determine posture and balance.

Question 19

How did early tetrapods maintain water balance?

Answer 19

• In air, water can evaporate through the moist skin of the body, the lining of the mouth and nostrils. The early tetrapods risked desication.
• This was probably countered this by remaining close to fresh water which they could drink.
• Certain forms evolved semipermeable skin coverings that would have cut down water loss. Eventually leads to impermaeble skin coverings in the reptilian/mammal lineage of amniotes (might explain the abundance of early tetrapod fossils)

Question 20

How did early tetrapods evolve their forms of reproduction?

Answer 20

Living amphibians betray their ancestry in their mode of reproduction.

• Even highly terrestrial forms have to lay their eggs in water where young hatch out as aquatic larvae, tadpoles. After some time in water, breathing through gills, the tadpoles metamorphose into the adult form.
• Sufficient

Question 21

How did early tetrapods evolve their forms of reproduction?

Answer 21

Living amphibians betray their ancestry in their mode of reproduction.

• Even highly terrestrial forms have to lay their eggs in water where young hatch out as aquatic larvae, tadpoles. After some time in water, breathing through gills, the tadpoles metamorphose into the adult form.
• Sufficient specimens have been found in Carboniferous and Permian rocks to confirm that at least some early tetrapods passed through larval stages similar to those of modern amphibians.

Question 22

Outline the main differences between the Acanthostega and Icthyostega.

Answer 22

Acanthostega

• Postbrachial lamina on cleithrum indicates presence of operculum
• Non-bendable elbow
• Small ribs
• Weak zygapophyses
• Weak sacral connection
• Simple fin-like hind limb
• Large tail
• 4cm jaw

Icthyostega

• Bendable weight-supporting elbow
• Robust overlapping ribs
• Stronger zygapophyses
• Stronger sacral connection
• More robust paddle-like hind limb
• Small tail fin
• 10 cm jaw