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Lecture 10

BISC 316 Lecture Notes - Lecture 10: Shoulder Girdle, Axial Skeleton, Tetrapod


Department
Biological Sciences
Course Code
BISC 316
Professor
Tammy Mc Mullan
Lecture
10

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Lecture 10
Evolution of fins (Review)
1. Fin Fold Theory
Lateral fold along length of body (anaspida)
Fold provided some stability
Some areas of fold lost, other enhanced to form fins
2. Fin Spine Theory
Early fish (acanthodii) has two or more ventral lateral spines
Spines (for support) and a fleshy web of tissue (increase surface area)
3. Gill Arch Theory
Interested renewed due to molecular evidence
o Same molecules and pathways involved in gill and fin growth
Regardless of theory, the evolution of paired fins was largely influenced by selective pressures related to
their role in locomotion
Paired fins
o Placoderms
Fin supported by dermal plates, some species had internal cartilaginous
skeleton
o Chondrichthyes
Axial or basal elements with parallel or radiating radials
o Actinopterygii (ray-finned fish)
Internal skeleton associated with fin
Radials close to body and fin rays in tissue of fin made the fin very flexible
o Sarcopterygii (lungfish)
Similar to fossil sharks (internal skeleton)
Central row of axials center of fin
Radials on both sides of axials
o Crossopterygii (coelacanth)
Fin bones similar to early tetrapods (Ichthyostegids) limb
Homologous to humerus, radius, ulna
o Acanthostega
Has 8 digits
o Ichthyostega
7 digits on its hind leg
Paired fins and limbs
o Homology between Crossopterygii and Ichthyostegids limb not due to direct evolution
of limb, but to a common pattern of development
Fin to Limb
1. Formation of stylopod and zeugopod (Sarcopterygian)
2. Formation of autopod region (sarcopterygian)
3. Determination of number of bones and digits (sarcopterygian)
4. Loss of fin rays (tetrapods)
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Sarcopyterygian fish share some characteristics with early tetrapods
Early tetrapods are more fish like than once thought
Why do the first limbs evolve?
Mutation must be selected for in the environment or neutral for it to stick around
Some environments with fluctuating water levels, or decreasing oxygen levels
Selective pressures favored the evolution of lungs and limbs
Limbs evolved from fins with axial skeleton
Fin rays allowed the fins to be flexible
Axial skeleton gave support
Selective pressures that favoured limbs
1. Exploit unoccupied habitats
2. Exploit abundant food in shallow water
3. Escape from unfavourable environments
4. Avoidance of predators in shallow water
5. Improve locomotion; anchor point as body undulates forward
6. Improve locomotion; strut to prevent body from tipping on side
Ordivician and Silurian Era
Extensive shallow seas
Ostacoderms, Placoderms, and Acanthodii
Osteichthyes (Devonian) radiation of fish
Tetrapods
o First skeletal evidence of tetrapods appears in the early Devonian
o Continents are moving closer together
o Fewer epicontinental seas
o Laurasia and Gondwanaland separated by the Tethys sea
The Tethys sea is a large sea that is very tropical with abundant plant life
Carboniferous
o Pangea present throughout the Permian and the Triassic
o 160 million years
o Coered 36% of earth’s surfae
o Few epicontinental seas
o Pangea colonized by vertebrates
Devonian
o Large horsetails and giant club mosses
o First fossil seeds appeared late Devonians
Carboniferous
o Increased diversity of terrestrial invertebrates and plants
Herbivores and predators
Land offered novel food and habitats
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High atmospheric oxygen levels
Class Amphibia
Early tetrapods Layrithodotia
1. ste tetrapods or Layrithodontia
a. Acanthostega and Ichthyostega
2. Batracomorpha
a. Lepospondyli and Temnospondyli
3. Reptiliomorpha
4. Lissamphibia
Origins of Tetrapods
Sarcopterygii (lobe-finned fish)
1. Osteolepiformes
2. Elpistostegidae (Pandricthyes)
a. Tiktaalik rosaea
1. Osteolepiformes
the gap etee fishes ad tetrapods has arroed
Cylindrical body with large heads
Thick scales
i. Would help protect them as they move through shallow water which prevents
abrasion or protection from obstacles that can cut them
Variable tail shapes
Paired crescent shaped vertebrae
i. Very similar to earliest tetrapods
Ribs project dorsally from vertebrae
Labyrinthodont teeth
Ancestral Osteoliepiformes
i. Long snout and teeth stalked or ambushed prey
Similar behaviour to extant frogfish
i. Walk and slow gallop
2. Elpistostegidae
Resemble the early tetrapods
No dorsal fins
i. Helps prevent rolling
Reduced tail
i. Suggests that they are not using the tail for locomotion
Large fish > 1 m long
Long snout
i. Longer the snout, usually feeds with a snapping motion
Dorsally flattened with the eyes on top of the head
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