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From Fins to Limbs
___ -> ___ -> ___
“Rhipidistia” -> “Labyrinthodonts” -> Tetrapods
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Acanthostega – “four-footed fish”
- Mostly aquatic – internal gills
- Radial fin rays supported tail
- Lateral line system
- Labyrinthodont teeth
- Intracranial joint
- Weight-bearing girdles
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Panderichthys (Elpistostegidae)
- Eyes on top
- No anal/dorsal fins, reduced tail fin
- Body and head – dorsoventrally flattened
- Long snout
- Derived humerus = powerful forelimbs for propping
- Ventrally-projecting ribs
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Late Devonian “Fishapod” -
- Tiktaalik
- Derived Epistostegid
- Claim to fame: Filled gap between most derived Sarcopterygian and first tetrapod.
- Intermediate characters between fish and tetrapod
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Tiktaalik fish like features
- Fin rays
- Well-developed gills
- Poorly ossified vertebrae
- Long body
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Tiktaalik tetropod-like features
- No operculum
- Large, overlapping ribs – support body out of water
- Long snout
- Pectoral fin with bend in middle – prop body up
- Fingerlike bones at end of fin
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The Earliest Tetrapods
Primarily aquatic:
Presence of groove on ventral surface of ceratobranchials: -> Supports gills in fishes and internal gills
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animal with fish-like internal gills;maybe had lungs
polydactyl
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Humerus branches to form
radius and ulna
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Preaxial =
segmentation of radius, radiale
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Postaxial =
segmentation & branching to form carpals, metacarpals, digits
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Hypotheses for terrestrial living
- Limblike fins of Devonian Sarcopterygians allowed walking from one dry pond to a larger one
- Searching for food
- Juvenile dispersal
- Laying eggs in moist environments
- Basking in sun to raise body Temp
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Adaptations to life in a shallow water environment
- Limbs with digits
- Development of ankles and wrists
- Attachment of pelvic girdle to vertebral column
- Development of a distinct neck (loss of opercular bones)
- Articulation between skull and vertebral column
- Longer, flatter snout
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Living fish that provides a model of tetrapod-like limb usefulness in water.
- Frogfish
- Modified pectoral fin to walk over substrate
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Extant non-amniotic tetrapods
(frogs, salamanders, caecilians)
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Primitive Paleozoic tetrapods:
- Larger than living amphibians
- Many had dermal scales(no cutaneous gas exchange)
- More closely related to amniotes than amphibians
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Acanthostega Primitive features
- Fin-like forelimb and hindlimb
- Operculum (internal gills)
- Weak zygapophyses
- Small ribs
- Non-bendable elbow
- Weak sacral connection
- Large tail fin
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Ichthyostega
- More specializations for both aquatic AND terrestrial life
- Distinct thoracic and lumbar regions
- Longer, overlapping ribs (only thoracic)
- Elongated ribs at base of tail
- Hindlimb like seal flipper
- Ear region specialized for underwater hearing
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Current classification of Paleozoic non-amniote tetrapod:
- Temnospondyls: more aquatic, immobile skull, flatskull, 4 fingers
- Anthracosaurs: more terrestrial, domed skull, 5 fingers
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Early Carboniferous : tetrapods had split into 2 lineages:
- Batrachomorphs
- Reptilomorphs
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Reptilomorphs
- non-amniotic relatives
- amniotes - Synapsid Testudine Archausoromorph Lepidosauromorpha
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Batrachomorphs
- Temnospondyls – nonamniotic tetrapods
- Largest, longest-lasting group Primitive, extinct
- amphibia
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Temnospondyls
only group of non-amniotic tetrapods(aside from amphibians) to survive the Paleozoic
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only group of non-amniotic tetrapods(aside from amphibians) to survive the Paleozoic
Temnospondyls
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Anthracosaur
(non-amniotic reptilomorph) tetrapods
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Lepospondyls
(Reptilomorphs) but may be moreclosely related to living amphibians or even amniotes.
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Great diversification of insects in the Late Carboniferous (in response to increasing diversity of vegetation?)
Supported diverse fauna of fully terrestrial vertebrate predators!
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amniotic egg advantages
- freedom from aquatic environment
- skip the stage between egg an adult
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Derived Features of Amniotes
- Amniotic egg
- Skin permeability: Keratin, Lipids, Skin elaborations (hair, scales, feathers)
- Costal (rib) ventilation of lungs
- More complex innervation of forelimb
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Structure and Development of the Amniotic Egg
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amniotic egg Shell
leathery or calcified, mechanical protection, butporous to allow movement of respiratory gases, water vapor
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amniotic egg albumin
(egg white) protection, reservoir of water, protein
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Large yolk
energy for the developing embryo
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Amniotes have 3 additional membranes:
- Chorion – surrounds entire contents
- Amnion – surrounds embryo
- Allantois – storage of nitrogenous waste, respir. organ
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