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: 1200 - 1600 Hz • Amphibian Papilla: 200 - 800 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.
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 • 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 Sol’n. 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 don’t 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 don’t 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.