Devonian Period

 The Devonian Period spanned about 60 million years of the Paleozoic, from 419.2 million years ago to 358.9 million years ago. It is sandwiched between the Silurian Period, which came earlier, and the Carboniferous Period, which is more recent.

The first significant  adaptations of life on dry land occurred during this period, for life was well underway in colonizing the land. Moss forests and bacterial/algal mats of earlier were joined early in the period by primitive rooted plants that created the first stable soils and harbored such arthropods as mites and scorpions. By far the largest land organism at the beginning of this period was a poorly-understood plant which was possibly the fruiting body of an enormous fungus, a rolled liverwort mat, or another organism of uncertain affinities. This plant stood more than 8 meters (26 ft) tall, which means it towered over the low, carpet-like vegetation that covered the land. The first fossils of insects appeared around 416 million years ago, in the Early Devonian.

Many Early Devonian plants did not have true roots or leaves like modern plants. These were generally very short, growing hardly more than a few centimeters tall. By the middle of the period, forests of shrub-like plants existed, for plants like horsetails and ferns had evolved. These had true roots and leaves, and many were quite tall. Also, the earliest-known trees appeared in the Middle Devonian, although probably not any that we would recognize. Certainly I didn't recognize them from their descriptions, and they all seemed to have 'extinct' in their description. By the end of the Devonian, the first seed-forming plants had appeared. There was such a rapid appearance of so many plant groups and growth forms that it is called the "Devonian Explosion". Various terrestrial arthropods (which includes insects, spiders, and crustaceans) also became well-established.

Fish reached substantial diversity, so that Devonian is often called the Age of Fishes. Among the marine vertebrates, jawless armored fish declined in diversity, while jawed fish increased in both the sea and fresh water. Early cartilaginous and bony fishes also became diverse in the seas. The first abundant genus of shark appeared during this period. The ancestors of all four-limbed vertebrates began adapting to walking on land, as their pectoral and pelvic fins evolved into legs.

The first ammonites (an extinct specie of mollusc) appeared during the Devonian. Trilobites (which look something like armored roaches), brachiopods (similar to a mollusc) and great coral reefs were common in the seas. The Late Devonian extinction, which started about 375 million years ago, affected marine line severely, killing off—among other things—all trilobites save for a few species.

The Devonian was relatively warm, and probably lacked any glaciers. For this reason, the sea level was high. The temperature gradient from the equator to the poles was not as large as it is today. The weather was also very arid, particularly along the equator. Surface temperature of the tropical seas was probably 86°F in the Early Devonian, but CO2 levels dropped steeply throughout the period, because the newly evolved forests drew carbon out of the atmosphere. By Mid-Devonian, there was a cooling of about 9°F. However, there is evidence that the temperature rose again in the Late Devonian, which may have contributed to the extinction event.

The geography was dominated by the supercontinent Gondwana to the south, Siberia to the north, and the early formation of Euramerica in between. It was a time of great tectonic activity, as Euramerica and Gondwana drew closer together. In the early Devonian, Laurentia and Baltica collided, forming Euramerica, which rotated into the natural dry zone along the Tropic of Capricorn (appromately 23.3° South of the equator). Then the plate of Euramerica and Gondwana started to meet, beginning to form the supercontinent Pangaea. This raised the northern Appalachian Mountains and formed the Caledonian Mountains in Great Britain and Scandinavia. The west coast of Euramerica was low lying, with deep silty embayments, river deltas and estuaries (found today in Idaho and Nevada). However, a volcanic island arc approached the west coast in the Late Devonian, and began to uplift that coast in a prelude to mountain-building that happened later.

Hey, we're getting somewhere! If trees and tall bushes could grow during this time period, then the soil must be decent, right? So maybe we could grow crops, so long as we bring our own seeds. And maybe we could set up a farm with some barnyard animals, too, like chicken, ducks, cows and goats. There's insects for the fowl to eat. Do you think cows and goats would eat horsetails and liverworts? Because the article didn't say anything about grasses. And as long as we don't go swimming in shark-filled waters, I think we'd be relatively safe.

 

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

Silurian Period

After the Ordovician Period came the Silurian Period, lasting 24.6 million years, from 443.8 million years ago to 419.8 million years ago. It began approximately the same time as the major series of extinction events mentioned at the end of the Ordovician Period, when a large number of marine fauna were wiped out.

The Silurian saw wide-spread diversification of jawed fish as well as bony fish. Multi-cellular life also appeared on land as small plants similar to liverworts, hornworts, and mosses, which grew besides lakes, streams and coastlines. Invertebrate animals with an exoskeleton also found land during this time period. This includes insects, spiders and crustaceans. However, terrestrial life would not greatly diversify until later.

With the supercontinent Gondwana still covering much of the southern hemisphere, a large ocean occupied most of the northern half of the globe. High sea levels and relatively flat land (there were few significant mountain belts) produced a number of island chains, meaning there was a rich diversity of environmental settings.

What was left of Gondwana remained intact and continued to drift south, but there is evidence that the icecaps were less extensive than those of the late-Ordovician time period. The smaller continents of Avalonia, Baltica, and Laurentia drift together near the equator, starting the formation of a second supercontinent known as Euramerica.

When proto-Europe (Baltica) collided with proto-North America (Laurentia), the collision folded coastal areas from modern New York State through Europe and Greenland to Norway. At the end of the Silurian Period, sea levels dropped again, and the new mountain ranges were rapidly eroded.

The Silurian period enjoyed relatively stable and warm temperatures, in contrast with the extreme glaciations of the period before it, and the extreme heat that would follow it. Sea levels rose during the first half of this period, and fell during the second half.

Climate was warm because high CO₂ levels and warm shallow seas produced a greenhouse phase. Glaciers at the South Pole nearly disappeared entirely. There is strong evidence of a climate dominated by violent storms generated by warm sea surfaces.

The Silurian was the first period to have megafossils in the form of moss-like miniature forests along lakes and streams. The first fossil records of vascular plants (land plants with tissues that carry water and food) appeared in the second half of the Silurian period.

Fish reached considerable diversity. A diverse fauna of sea scorpions (some of them several meters in length) prowled the shallow Silurian seas of North America. Leeches made their appearance.

About the middle of the Silurian, the earliest-known animals fully adapted to terrestrial conditions appeared, including a millipede. There is also some evidence of predatory spiders and millipedes and centipedes. Predatory invertebrates indicate that simple food webs were in place that included prey animals. These may have included those who grazed on micro-organisms.

Yes, we are getting closer and closer modern Earth. That is what happens when you start at the beginning and work your way towards today. I'm still not seeing any fauna that would be worth hunting. Although a millipede several meters in length might have enough meat to make a stew. Is the ground fertile enough to grow carrots and potatoes to put in that stew? And just how bad did those storms get? Maybe I'll keep going before I try to colonize.

 

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