The oldest known fossils are from rocks that were
deposited about 3.5 billion years ago?

The first abundant shelly fossils occur in rocks
that are about 570 million years old?

The last ice age ended about 10,000 ago?

hmmmm...

Earth's history is subdivided into eons, which are subdivided into eras, which are subdivided into periods, which are subdivided into epochs.

Fossils are the recognizable remains, such as bones, shells, or leaves, or other evidence, such as tracks, burrows, or impressions, of past life on Earth.

Rocks formed during the Proterozoic Eon may have fossils of relative simple organisms, such as bacteria, algae, and wormlike animals. Rocks formed during the Phanerozoic Eon may have fossils of complex animals and plants such as dinosaurs, mammals, and trees. Early explanations were built around the idea of successive natural disasters or catastrophes that periodically destroyed life. After each catastrophe, life began anew.

In the mid-nineteenth century, both Charles Darwin and Alfred Wallace proposed that older species of life give rise to younger ones. According to Darwin, this change or evolution is caused by four processes: variation, over-reproduction, competition, and survival of those best adapted to the environment in which they live. Darwin's theory accounts for all of the diversity of life, both living and fossil. His explanation gave scientific meaning to the observed succession of once-living species seen as fossils in the record of Earth's history preserved in the rocks.

Darwin's theory of evolution has been refined and modified continuously as new information has accumulated. All of the new information has supported Darwin's basic concept--that living beings have changed through time and older species are ancestors of younger ones.

Ammonites are perhaps the most widely known fossil, bearing the typically banded spiral formation shell. These creatures lived in the seas between 415 - 65 million years ago, when they became extinct along with the dinosaurs.

These sea creatures first appeared 415 million years ago in the form of a small straight shelled creature, known as Bacrites. They quickly evolved into a variety of shapes and sizes including some shaped like hairpins. During their evolution the ammonites faced no less than three catastrophic events that would eventually lead to their extinction. The first event occurred during the Permian (250 million years ago), where only 10% survived. These surviving species went on to flourish throughout the Triassic, however at the end of this period (206 million years ago) they faced near extinction, when all but one species survived. This event marked the end of the Triassic and the beginning of the Jurassic, during which time the number of Ammonite species grew once more. The final catastrophe occurred at the end of the Cretaceous period when all species were annihilated and the Ammonites became extinct. This event also contributed to the death of the dinosaurs.

Ammonites began life as tiny planktonic creatures less than 1mm in diameter. In their infancy they would have been vulnerable to attack from other predators including fish. However, they quickly assumed a strong protective outer shell that would shield their soft interior from damage. Evidence suggests that they gained in size rapidly, with females growing up to 400% larger than the males.

Ammonites moved by jet propulsion, expelling water through a funnel-like opening to propel themselves in the opposite direction. They typically lived for two years, although some species survived beyond this and grew very large as pictured above. Evidence of their short lives is estimated by looking at their living relatives - the Nautilus. These creatures exist within modern day seas and include characteristics similar to Ammonites (see picture below). Living Nautilus found in tropical seas around the world.

Nautilus shells comprise of individual chambers, each growing in size as the creature grows. These chambers are secreted by the creature at a rate of one every four weeks, equal to 13 each year. Using this as a guide an ammonite shell containing 26 chambers could be assumed to have housed the creature for two years. Like the Nautilus, Ammonites retained their original shell throughout their life. However it's worth noting that in comparison to modern day Nautilus which live in cold deep water, Ammonites preferred warm shallow waters and may have had a higher metabolism. Consequently, it's possible that Ammonites could reach larger sizes far quicker than modern day Nautilus.

Trilobite

The Paleozoic is often called the age of the trilobite.Trilobites particularly flourished in the oceans of the Cambrian and Ordovician periods, beginning around 540 million years ago, with a diminishing number of families persisting until the Permian. The number of families actually peaked in the Late Cambrian when an extinction event removed many. The morphological diversity actually peaked in the Ordovician. Many more families were removed at the end of the Ordovician 440 million years ago during a great ice age where ice sheets advanced to the equator. The diminished number of trilobite families that survived to the Silurian radiated into new and exotic forms, and still more exotic spiny and pustulose forms in the Devonian. The Devonian was punctuated by periods of rising seas that disrupted the reef systems where the trilobites flourished forcing selective adaptation. The end of the Devonian saw the Frasnian-Famennian event where only Proteus survived into the Carboniferous. Despite reduced ancestry, with decent with modification ruling, these trilobites filled the same ecological niches such that adaptation led to a repeating of many of the forms of their extinct cousins. While the genetic path was assuredly different, the newly evolved forms had recognizable morphological similitude with those long extinct. Regrettably, trilobites never truly recovered in the Carboniferous, with but a handful of genera extant by the Permian. Failing to adapt to deep-water habitats, their vulnerability to climatic change remained and led to their disappearance prior to yet another great mass extinction at the end of the Permian. The age of the trilobite yielded to the age of the insect.