Chordates 4 – Lepidosaurs
Tetrapoda – key adaptations of tetrapods are air breathing and limbs with fingers – evolve for life in
water. As tetrapods move onto land, they move into new niches. Tetrapod diversity – 30,000 species
(amphibia = 7250, mammalia = 5400 species…
Amniota
Amniotes (mammals, reptiles and birds) exhibit further specialisation for living and reproducing on
land.
Amniotic Egg = amniotes differ from amphibians and fish in having an amniotic egg. Egg encased in
membranes (amnion, chorion and allantois) that both protect the egg from drying out, and provide
gas exchange, allowing embryo to extract oxygen from air.
Internal Fertilisation = reproductive shift associated with internal fertilisation – eggs laid on land,
can’t be fertilised by releasing sperm into water. Instead, male and female genital openings contact
to exchange sperm. Often associated by an intromittent organ – penis of mammals, turtles,
crocodilians and some birds, hemipenes (double penis) of lizards and snakes.
Loss of the Larval Stage = juvenile amniotes have direct development – skip the larval stage seen in
lungfish and amphibians, hatch out as miniature adult with limbs and lungs – and lacking gills or a
tail fin.
Skin = tetrapods lack bony scales – instead they are covered by a skin that they can shed. In
amniotes, this skin is waterproof – not to keep water out, but to keep it in.
First amniotes = a little over 300 mya in the Carboniferous, the first amniotes appear. Small – a few
inches to a foot long. Reproductive mode lets them exploit habitats that amphibians (who must
return to water to breed) can’t. Amniotes stage adaptive radiations – the earliest amniotes have
already differentiated into
Synapsids (mammal lineage) take the lead = in the late Carboniferous and Permian, mammal-like
reptiles (forerunners of mammals) diversified and produce large herbivores and carnivores.
Synapsids dominate terrestrial ecosystems from the late Carboniferous to the Permian (299-252
mya) = mammal lineage produced specialised plant-eaters and meat eaters, which were the
dominant land animals for 50 million years. 252 mya, a massive extinction devastates the
environment – the Permo-Triassic Extinction. Massive volcanic eruptions devastated the planet,
wiping out
The Mesozoic (252-66 mya) = the Age of the Reptiles. Aftermath of the extinction sees reptiles take
over and diversify. Leads to the rise of the dinosaurs, as well as reptiles still with us today (lizards,
crocodilians and turtles). Mammals diversify as well, but are primarily small,
insectivorous/omnivorous and nocturnal forms.
Sauropsida and Synapsida
Sauropsid diversity = Lepidosauria 9000 species, Chelonia
Lepidosauria: tuatara, lizards and snakes
Tuatara = nocturnal and inhabit cool envirobments in New Zealand, introduction of rats by Maori
wiped them out on the mainland, so they exist only on tiny offshore islands. They have specialised
jaws with teeth that fuse to the jawbone, and shear their prey apart. Superficially lizard-like…
Triassic Origin = tuataras branch off from lizards in the Early Triassic – more or less modern looking
forms appear 240 mya. Tuataras were diverse in the Triassic before lizards appeared, and held their
own for a while, producing marine and plant-eating forms. They declined in the Cretaceous…
Squamata = lizards and snakes. The most diverse tetrapod clade. Roughly 9000 known species but
Tetrapoda – key adaptations of tetrapods are air breathing and limbs with fingers – evolve for life in
water. As tetrapods move onto land, they move into new niches. Tetrapod diversity – 30,000 species
(amphibia = 7250, mammalia = 5400 species…
Amniota
Amniotes (mammals, reptiles and birds) exhibit further specialisation for living and reproducing on
land.
Amniotic Egg = amniotes differ from amphibians and fish in having an amniotic egg. Egg encased in
membranes (amnion, chorion and allantois) that both protect the egg from drying out, and provide
gas exchange, allowing embryo to extract oxygen from air.
Internal Fertilisation = reproductive shift associated with internal fertilisation – eggs laid on land,
can’t be fertilised by releasing sperm into water. Instead, male and female genital openings contact
to exchange sperm. Often associated by an intromittent organ – penis of mammals, turtles,
crocodilians and some birds, hemipenes (double penis) of lizards and snakes.
Loss of the Larval Stage = juvenile amniotes have direct development – skip the larval stage seen in
lungfish and amphibians, hatch out as miniature adult with limbs and lungs – and lacking gills or a
tail fin.
Skin = tetrapods lack bony scales – instead they are covered by a skin that they can shed. In
amniotes, this skin is waterproof – not to keep water out, but to keep it in.
First amniotes = a little over 300 mya in the Carboniferous, the first amniotes appear. Small – a few
inches to a foot long. Reproductive mode lets them exploit habitats that amphibians (who must
return to water to breed) can’t. Amniotes stage adaptive radiations – the earliest amniotes have
already differentiated into
Synapsids (mammal lineage) take the lead = in the late Carboniferous and Permian, mammal-like
reptiles (forerunners of mammals) diversified and produce large herbivores and carnivores.
Synapsids dominate terrestrial ecosystems from the late Carboniferous to the Permian (299-252
mya) = mammal lineage produced specialised plant-eaters and meat eaters, which were the
dominant land animals for 50 million years. 252 mya, a massive extinction devastates the
environment – the Permo-Triassic Extinction. Massive volcanic eruptions devastated the planet,
wiping out
The Mesozoic (252-66 mya) = the Age of the Reptiles. Aftermath of the extinction sees reptiles take
over and diversify. Leads to the rise of the dinosaurs, as well as reptiles still with us today (lizards,
crocodilians and turtles). Mammals diversify as well, but are primarily small,
insectivorous/omnivorous and nocturnal forms.
Sauropsida and Synapsida
Sauropsid diversity = Lepidosauria 9000 species, Chelonia
Lepidosauria: tuatara, lizards and snakes
Tuatara = nocturnal and inhabit cool envirobments in New Zealand, introduction of rats by Maori
wiped them out on the mainland, so they exist only on tiny offshore islands. They have specialised
jaws with teeth that fuse to the jawbone, and shear their prey apart. Superficially lizard-like…
Triassic Origin = tuataras branch off from lizards in the Early Triassic – more or less modern looking
forms appear 240 mya. Tuataras were diverse in the Triassic before lizards appeared, and held their
own for a while, producing marine and plant-eating forms. They declined in the Cretaceous…
Squamata = lizards and snakes. The most diverse tetrapod clade. Roughly 9000 known species but
