Paleobiogeography is study and description of the geographic distribution and abundance of organisms in the geologic past, especially with regard to ecologic and climatologic factors. A solid limestone structure standing above the surrounding seafloor and constructed by living organisms, usually corals . Coral reefs are restricted largely to shallow, clear, tropical marine environments of normal salinity. Algae are plants that require light. Reefs form near sea level and can be used as indicators of past sea levels .
Cyanobacteria are among the most primitive groups of organisms.
The fossil record of stromatolites extends back more than 3 billion years. Currently, stromatolites form in restricted settings, e.g. high intertidal and supratidal, hypersaline and current-swept settings, where other organisms, e.g., grazers, can t survive. May have been more widely distributed in the past before other organisms evolved. Pelagic sediments Fine-grained sediments of the open ocean, away from continental margins Microfossils Fossils that are too small to be studied without the aid of a microscope. The geographic distribution of certain species is limited by temperature and salinity conditions.