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Evolution of Amphibians
How do we define the Amphibia? Amphibians They have a 2-phase life
history.
There is usually a free-living aquatic developmental stage. There
is usually a terrestrial juvenile/adult stage. NOTE: this
represents a transition from water to land, just as in the
evolution of the group. Amphibians They have cutaneous respiration.
(Think of SA/V ratios, metabolic rates, and dependence on water).
Terrestrial adults have lungs. Aquatic larvae and neotenic forms
have gills. Lungs employ force pumps - they have no diaphragm.
Amphibians They have a pair of sensory papillae in the inner ear,
and double transmission channels. Caecilians, urodeles, and some
anurans lack a tympanum and middle ear cavity, but have a stapes
which is attached to the shoulder girdle or skin!!(This is good for
low frequency sound waves) Amphibians They have a 2nd ossicle, the
operculum, which with a small muscle joins the pectoral girdle to
the oval window. The stapes conducts high frequency sounds, while
the operculum conducts low frequency sounds. Amphibians Like other
tetrapods, amphibians have Basilar Papilla, but they also have a
larger, 2nd Amphibian Papilla. Basilar Papilla: Hz Amphibian
Papilla: Hz. Amphibian Vision They have specialized visual cells in
the retina.
Salamanders and frogs have green rods in the retina (absent in
caecilians and all other vertebrates). What is the function of
visual rods?What is the function of green rods? Amphibian Skin They
have 2 types of skin glands:
Mucous glands maintain moist skin for respiration. Granular glands
produce toxic secretions. Epidermis is minimal. Major Groups of
Vertebrates Monophyletic Clades A monophyletic clade is one in
which all the daughter groups are members of the clade. This is
true for amphibians, fish, and mammals. Note that the Aves are a
daughter clade of the Reptilia.Aves are paraphyletic.Aves are
covered in Herpetology II (Ornithology) Amphibian Teeth They have
pedicillate Teeth.
The tooth has a crown and a pedicel.As the crown wears, it breaks
off and is replaced by a new one. Terrestrialization Why make the
transition to an aquatic ecosystem?
Terrestrialization may be a secondary event. Water bodies may have
had low oxygen content, and thus may have favored air gulpers.
abundant arthropod and plant resources may have been close to
shore. Terrestrialization The old hypothesis was that they wanted
to move to new bodies of water.Does this make sense?
Terrestrialization What problems exist for a fish design on
land?
Integumentary water loss. Thick dermis, but thin epidermis.
Presence of 2 types of glands (mucous and poison) Temperature
control? Respiration? Bucal pump, skin, lungs. (Lungs were already
present in the osteichthys.Lungs actually preceed swim bladders.
Terrestrialization Skeletal support Locomotion Hearing
organization of myomeres in fishes water acts as an external
skeleton in fish. Locomotion Hearing Terrestrialization
Feeding
Once the pectoral girdle is free from the skull, greater mobility
is possible in the skull, permitting greater feeding.This is
enhanced by the architecture of the 2 occipital condyls and the
presence of the atlas.The snout and jaws become elongated (seen as
a relative shortening of otic-occipital region).There is improved
articulation of the jaw, and expansion of the primary palate.
Terrestrialization Smell Reproduction
Circulation with multiple respiratory structures. Skeletal
solutions to terrestrialization. Pectoral girdleis divorced from
back of skull (implications for sound conduction)
Terrestrialization Skeletal Soln. Cont. undulatory locomotion
suspension of vertebral column regionalization of vertebral column
organization of amphibian vertebrae. Suspension of internal organs.
Reorganization of the skull. Terrestrialization Reorganization of
the skull cont.
reduction of the otic-optic portion of brain. evolutin of the
dermocranium evolution of the splanchnocranium evolution of the
neurocranium. evolution of the mandibular suspensorium. One more
time: What are amphibians?
From last time: 2-phase life history. cutaneous respiration. pair
of sensory papillae in inner ear. Specialized visual cells in the
retina (green rods). Pedicillate teeth. 2 types of skin glands.
These features dont really help us in the fossil record.
We would like to understand the evolutionary history of amphibians
and reptiles.In terms of fossils, how can we define them? Usually:
Tetrapod (excluding ichthyostegalia), morph of centra,
zygapopheses, arches, ribs, girdles, and presence of 2 occipital
condyles and 1 atlas.Also, dual auditory pathways & skull
bones. What is our best guess as to the history of the
amphibia?
Middle to late Devonian: Plants: predominantly wetland and
watercourse edges. Plant ht less than 2m with no stratification.
Single species patches. Low diversity, but evidence of seasonality
of seasonality and Gondwana glaciation. Some evidence for a
Devonian extinction. What is our best guess as to the history of
the amphibia?
Middle to late Devonian cont. Trend to warm and drier, with
increased diversity of plants. By the end of the Devonian there is
increased structural diversity of plants and wetlands appear to be
refugia for plants. By the Carboniferous, all taxonomic groups of
plants have evolved.There is high level taxonomic radiation, and
architectural diversication.Structurally modern plants. Animals in
late Devonian to early Carboniferous.
Fossil amphibian trackways in Australia? Footprints in Brazil?
First fossils from Greenland and Russia, include Ichthyostegalids
and Anthracosaurs. Ichthyostegalids are from the Devonian of
Greenland.They are about 1m in length. Animals in late Devonian to
early Carboniferous.
Anthracosaurs are from the Devonian of Russia, Iowa, Scotland, and
W. Virginia, and range in size from .5m to 1.5m in length (why so
big?) They have dorsoventrally expanded tails, they lacked well
ossified carpals, and were probably insect predators. A word (or 2)
about monophyly, paraphyly, and tetrapods
Tetrapods are called tetrapods because they have 4 feet.So anything
with 4 feet is a tetrapod, except of course Ichthyostegalia.Also,
some tetrapods dont have any feet (snakes and caecilians). Major
vert. groups may not be mono-phyletic, including amphibians. A word
(or 2) about monophyly, paraphyly, and tetrapods
The problem is, many amphibian features are tied up with the
transition from water to land.These features may not represent
unique morphological inventions.Lots of groups have done exactly
the same thing. A word (or 2) about diversity.
Species diversity over time. Architectural diversity over time.
Mass extinctions. Anthracosaurs We now consider the Anthracosaurs
to include the lineage which leads to reptiles and mammals
(Seymouriamorpha, Cotylosauria, Batrachosauria, and
Anthracosauroideae). It no longer includes the loxommatoids and
lepospondyls.If it did, the Amphibia would be paraphyletic.
Lepospondyls This is an unnatural group (Aistopoda, Microsauria,
Nectridia) which shares morphological features associated with
small size and aquatic life history. Temnospondyls are amphibians,
and represent the first real amphibian radiation.The lissamphibia
are the 2nd radiation. Labyrinthodonts This is an unnatural group
including temnospondyls, anthracosaurs, and ichthyostegalians.The
group is defined on the basis of shared primitive characters, and
is not monophyletic.