Lecture 27 Flashcards
The Cambrian explosion
An event that marked the beginning of the Phanerozoic Eon - when most major animal phyla appeared in the fossil record. It lasted for 30 million years and resulted in the rapid appearance of most modern phyla.
When did the Cambrian explosion happen
542 million years ago
Three accepted explanations for the Cambrian explosion
environmental, developmental, and ecological
Environmental cause of Cambrian explosion
It is believed that only in the Cambrian did oxygen levels in the ocean reach a threshold allowing complex life forms to form. Additionally, calcium in seawater increased (due to increased continental erosion) allowing microscopic organisms to build shells.
The great unconformity
The weathering of the continent was so great between the Precambrian and Cambrian ages, that entire mountain and sediment layers were weathered away from the rock record.
Developmental cause of Cambrian explosion
- Finally, Hox genes occurred that allowed animals to evolve. Minor modifications to the animal’s development as they grow from embryo to adult may have been able to cause very large changes in the final adult form.
- Horizontal gene transfer aided in the acquisition of the biochemical capability of biomineralization.
Ecological cause of Cambrian explosion
- end-Ediacaran mass extinction
- evolution of eyes
- arms race between predators and prey
- development of large zooplankton
End-Ediacaran mass-extinction
Extinction explains why Ediacaran fauna disappeared. This was likely followed by adaptive radiations.
Evolution of eyes
The evolution of eyes meant that prey needed new defences against predation, such as the evolution of armour and spines.
The arms race between predators and prey
Each tried to grow bigger, and as prey became bigger they developed greater mobility or they biomineralized to prevent being eaten.
Development of large zooplankton
Large zooplankton meant that fecal pellets were deposited on the seafloor, providing a new energy source. This opened the door for more burrowing animals.
Lagerstatten
Cambrian fossil records include deposits that preserve soft tissue (fossil sites with a community berried together). They allow paleontologists to examine the internal anatomy of animals and a variety of different organisms.
Most significant lagerstatten deposits.
Cambrian Maotianshan shale beds (China), Sirius Passet (Greenland), the middle Cambrian Burgess Shale (Canada) - Alberta Shell, and the late Cambrian Orsten (Sweden) fossil beds.
Burgess Shale
Correlate to the Stephen Formation, a dark mudstone 508 million years old. They were deposited at the base of a submerged escapement composed of the limestone reefs of the Cathedral Formation.
The great proliferation of life in the Cambrian Period was entirely confined to the oceans. However, by ____ animal life also became established on land.
400 Ma
Order of evolution in terms of fish
FIsh (growing bigger) –> fish evolve fins and become inland (first amphibians) –> amphibians went back to water to reproduce –> soon reptiles and animals that live on land don’t need to go back to the water to reproduce –> these animals colonize land and are not stuck on land
Non-amniotic vertebrates
fish and amphibians must be in the water to reproduce. Their eggs are exposed and lack a covering.
Meiotic vertebrates
Vertebrates that have evolved internal fertilization and an amniotic egg (enclosed egg).
Before plants, ___ and ___ covered most of earth’s surfaces.
algae and bacteria
Plants likely began from the evolution of ____, which formed from _____, and later the next steps were: ____.
Stromatolites, cyanobacteria, Chlorophytes
Chlorophytes
green algae
The earliest fossil evidence consists of ____.
spores
Spores
plant reproductive structures
Three major advances occurred in plant history:
- seedless plants
- naked-seed producing, pollinating, but non-flowering plants
- Plants with both seeds and flowers (angiosperms)
seedless plants
Seedless (spore-bearing) plants evolved, such as ferns and mosses, and were so abundant that they formed world’s largest coal deposits.
gymnosperms
Naked seed-producing, pollinating, but non-flowering plants
Naked seed-producing, pollinating, but non-flowering plants
Evolved in non-moist environments allowing them to expand their habits, and had spread across the supercontinent Pangea.
Angiosperms
Plants with both flowers and seeds
Evolution of plants with flowers and seeds
Self-enclosed seed-producing plants evolved from gymnosperms that randomly spread their pollen. They evolved by having nectar that enticed insects to deliver pollen for them.
Early paleozoic plants profoundly altered the environment because they:
facilitated soil formation, provided food and shelter for emerging land animals, and paved for the evolution of large trees
The phanerozoic era was punctuated by ______, attributed due to:
major extinction events
meteorite impacts, massive volcanic eruptions
Two major extinctions
Permian-Triassic (252 Ma ago) and Cretaceous-Tertiary (66 Ma ago)
Permian Extinction
The worst extinction event occurred at the end of the Permian when more than 90% of marine species and 70% of animals on land perished. This is directly related to the large igneous province in Russia where large magma releases caused a rifting event and produced CO2 and SO2. Falling acid rain and chemical weathering produced plankton blooms making oceans anoxic and acidic. Dust falling blocked sunlight.
Cretaceous extinction
Wiped out 1/4 of all known species, and is likely caused by the impact of a large meteorite, having crashed somewhere off the coast of Mexico.
Why the cretaceous extinction was likely caused by
- abrupt manner of extinctions (usually takes tens of millions of years)
- A thin layer of clay found in many parts at the crustaceous-Tertiary boundary with high concentrations of iridium. Iridium on earth can only be found in the mantle and core. The source must have been extraterrestrial.
Flood Basalts in India
During the Mesozoic, India moved steadily northward and moved over a fixed hotspot in the mantle which led to a massive outpouring of flood basalts known as the Deccan Traps. Some suggest this led to an environmental catastrophe and the stressed ecosystem from volcanism was further harmed by the meteorite impact.
Two factors that caused the Cretaceous extinction
Meteorite and hot-spot volcanism in India
