Cambrian Period

 The Cambrian Period was the first geological period of the Paleozoic Era. It lasted 55.6 million years, from 541 million years ago to 485.4 million years ago. The Cambrian is known for sites of exceptional preservation where even 'soft' parts of organisms are preserved, so our understanding of the Cambrian biology surpasses that of some later periods.

A profound change in life on Earth happened during the Cambrian Period, in that mineralized multicellular organisms became common. The rapid diversification of life-forms in this period—known as the Cambrian Explosion—produced the first representatives of all modern animals, probably from a single common ancestor.

Although life prospered in the oceans, the land is thought to have been comparatively barren. Shallow seas flanked several continents and were relatively warm. Polar ice was absent for much of the period.

Large, high-velocity rotational movement of Gondwana appears to have occurred in the early Cambrian, and may have resulted in Laurentia (North America), Baltica and Siberia being 'tossed away' and forming isolated land masses. Most continental land was clustered in the Southern Hemisphere, but was drifting north.

With a lack of sea ice, the sea level was high, which led to large areas of the continents being flooded in warm shallow seas, which were ideal for sea life. The sea levels fluctuated, suggesting there were pulses of expansion and contraction of a south polar ice cap.

The article stated that the Earth was generally cold during the early Cambrian, and then said the average temperatures were 7 degrees Celsius higher than today. That doesn't seem very cold to me.

The Cambrian flora was little different from what had existed in the previous period. Primarily, there were marine macroalgae in the seas, and that was pretty much it. There were no land plants known from the Cambrian, although biofilms and microbial mats were well developed on tidal flats and beaches 500 million years ago. There were also microbes forming microbial Earth ecosystems, comparable with modern soil crust of desserts, which contributed to soil formation.

It was once thought that trilobites were the dominant life form of the time period. But it turns out that these had a heavy armor which fossilized far more easily than the bodies of other animals, so there were plenty of trilobite fossils, even though trilobites were only a minor part of the animal diversity.

Earth suffered a mass extinction at the start of the Cambrian Period. It is thought that animals that burrowed into the sea bed, destroyed the microbial mats covering the seabed, and many organisms dependent on the mats became extinct, while other species adapted to the changed environment.

Despite the 'Explosion' at the start of this period, the later half saw a sharp drop in biodiversity. 500 million years ago, oxygen levels in the oceans dropped dramatically, while the level of poisonous hydrogen sulfide increased, producing more extinction events, making the latter half of the period surprisingly barren.

However, some organisms did venture onto land, producing trace fossils of their movements. Some of these fossil trackways suggest a large, slug-like mollusc.

Just when you thought Earth pre-history was going to get interesting, it takes one step forward and two steps back. Dry land is still pretty barren, except for an occasional slug-mollusc looking for some tasty soil microbes.

Well, we're pretty sure humans arrive on the scene eventually, so we're just going to keep slogging forward until we find us.

 

https://en.wikipedia.org/wiki/Cambrian