(more below) Paleontologists are careful to distinguish the ammonoids from the ammonites.
Ammonoids lived from Early Devonian times until the end of the Cretaceous Period, or from about 400 million to 66 million years ago.
aeolian process - A geological modification, such as sand dunes or erosion, caused by wind activity. algal mat - A layered communal growth of algae observed in fossils and in present day tidal zones associated with carbonate sedimentation.
alkali metal - A strongly basic metal like potassium or sodium.
Like extant organisms, fossils vary in size from microscopic, even single bacterial cells one micrometer in diameter, to gigantic, such as dinosaurs and trees many meters long and weighing many tons.
A-horizon - The uppermost layer of a soil, containing organic material and leached minerals.
Ammonites were a suborder of ammonoid with heavy, ornamented shells that thrived starting in the Jurassic Period, between 200 and 150 million year ago.
Ammonoids have a coiled, chambered shell that lies flat, unlike gastropod shells.
But new research suggests that conditions were right for complex cells to evolve and die off at least once - or perhaps several times - before our lineage even got started. Billions of years ago, there could have been other complex life forms on the planet, totally unrelated to anything we see on Earth today. Earth has been around for an estimated 4.5 billion years.
Around 3.7 billion years ago, while the planet was still relatively fresh, two of the three kingdoms of life we see on Earth today - bacteria and archaea - arose.