DOLOMITE EXPOSURE near Summerville GA is on GA Hwy 48, about 0.85 miles south of US 27

MISSISSIPPI LIMESTONE near Rising Fawn GA is on I-59 about 0.5 miles south of EXIT 4 (Rising Fawn)

EVOLUTION OF NORTH AMERICA

ARCHEAN CRATONS > 2.5 BYABEAR COLLISION 2.5-2.0 BYAPASSIVE MARGINS 2.5-MAGMATIC ARC 2.0-1.8 BYACHURCHILL COLLISION 1.9-1.8 BYAYAVAPAI COLLISION 1.8-1.6 BYAGREENVILLE COLLISION 1.4-1.0 BYAPAIOZOIC COLLISIONS 0.5-0.2 BYA

The Michigan Basin is underlain by the same rocks as in Georgia’s NW Valley & Ridge Province, except in Georgia the rocks have been intricately folded and faulted whereas in Michigan they are still in their original horizontal depositional position.

CLIMATE CHANGE

Ten Hottest Years on Record 1979 1997 2016

Hottest Year 1944 1997 2016Second 1943 1995 2015Third 1941 1990 2014Fourth 1940 1988 2010Fifth 1937 1987 2013Sixth 1931 1981 2005Seventh 1926 1980 2009Eighth 1900 1944 1998Ninth 1882 1943 2012Tenth 1881 1941 2007

Time Interval of Top Ten 99 years 57 years 19 years

Frequency of Hot Years 1/9.9 1/5.7 1/1.9once per decade twice per decade every other year

SILURIAN & DEVONIAN EXTINCTION EVENTS

416 & 359 million years ago

PERMIAN TRIASSIC CRETACEOUS

DEVONIAN

ORDOVICIAN Mississippian Jurassic

Miocene

Silurian

Cambrian

PANGAEA Super Continent

TOTAL GENERA THROUGH THE PHANEROZOIC ERAThe Cambrian to the Present Holocene

The Present

From a human perspective, the Devonian period was a crucial time for the evolution of vertebrate life: this was when the first tetrapods climbed out of the primordial seas and began to colonize dry land. The Devonian occupied the middle part of the Paleozoic Era (542-250 million years ago), preceded by the Cambrian, Ordovician and Silurian periods and followed by the Carboniferous and Permian periods.Climate and geography. The global climate during the Devonian period was surprisingly mild, with average ocean temperatures of "only" 80 to 85 degrees (compared to as high as 120 degrees during the preceding Ordovician and Silurian periods). The North and South Poles were only marginally cooler than the areas closer to the equator, and there were no ice caps; the only glaciers were to be found atop high mountain ranges. The smallish continents of Laurentia and Baltica merged to form Euramerica, while the giant Gondwana (which was destined to break apart millions of years later into Africa, South America, Antarctica and Australia) continued its slow southward drift.

The Silurian only lasted 30 or so million years, but this period of geologic history witnessed at least three major innovations in prehistoric life: the appearance of the first land plants, the subsequent colonization of dry land by the first terrestrial invertebrates, and the evolution of jawed fish. The Silurian was the third period of the Paleozoic Era (542-250 million years ago), preceded the Cambrian and Ordovician periods and succeeded by the Devonian, Carboniferous and Permian periods.Climate and geography. Experts disagree about the climate of the Silurian period; global sea and air temperatures may have exceeded 110 or 120 degrees Fahrenheit, or they might have been more moderate ("only" 80 or 90 degrees). During the first half of the Silurian, much of the earth's continents were covered by glaciers (a holdover from the end of the preceding Ordovician period), with climatic conditions moderating by the start of the ensuing Devonian.

SILURIAN CLIMATE

DEVONIAN CLIMATE

Georgia

The Ordovician World 444 million years agoLAURENTIA

Proto North America

Georgia

The Silurian World 430 million years agoLAURENTIA

Proto North America

GeorgiaLAURENTIA

Proto North America

Devonian World400 million years ago

Invertebrates. The Silurian period followed the first major global extinction on earth, at the end of the Ordovician, during which 75 percent of sea-dwelling genera went extinct. Within a few million years, though, most forms of life had pretty much recovered, especially arthropods, cephalopods, and the tiny organisms known as graptolites. One major development was the spread of reef ecosystems, which thrived on the borders of the earth's evolving continents and hosted a wide diversity of corals, crinoids, and other tiny, community-dwelling animals. Giant sea scorpions--such as the three-foot-long Eurypterus--were also prominent during the Silurian, and were by far the biggest arthropods of their day.Vertebrates. The big news for vertebrate animals during the Silurian period was the evolution of jawed fish like Birkenia and Andreolepis, which represented a major improvement over their predecessors of the Ordovician period (such as Astraspis and Arandaspis). The evolution of jaws, and their accompanying teeth, allowed the prehistoric fish of the Silurian period to pursue a wider variety of prey, as well as to defend themselves against predators, and was a major engine of subsequent vertebrate evolution. The Silurian also marked the appearance of the first identified lobe-finned fish, Psarepolis, which was ancestral to the pioneering tetrapods of the ensuing Devonian period.Plant Life During the Silurian PeriodThe Silurian is the first period for which we have conclusive evidence of terrestrial plants--tiny, fossilized spores from genera like Cooksonia and Baragwanathia. These early plants were no more than a few inches high, and thus possessed only rudimentary internal water-transport mechanisms. Some botanists speculate that these Silurian plants actually evolved from freshwater algae (which would have collected on the surfaces of small puddles and lakes) rather than ocean-dwelling predecessors.

As a general rule, wherever you find terrestrial plants, you'll also find some kinds of animals. Paleontologists have found direct fossil evidence of the first land-dwelling millipedes and scorpions of the Silurian period, and other, comparably primitive terrestrial arthropods were almost certainly present as well. However, large land-dwelling animals were a development for the future, as vertebrates gradually learned how to colonize dry land.

SILURIAN LIFE 444-416 MYA

Terrestrial Life During the Devonian PeriodVertebrates. It was during the Devonian period that the archetypal evolutionary event took place: the adaptation of lobe-finned fish to life on dry land. The two best candidates for the earliest tetrapods (four-footed vertebrates) are Acanthostega and Ichthyostega, which themselves evolved from earlier, exclusively marine vertebrates like Tiktaalik and Panderichthys. Surprisingly, many of these early tetrapods possessed seven or eight digits on each of their feet, meaning they represented "dead ends" in evolution--since all terrestrial vertebrates on earth today employ the five-finger, five-toe body plan.Invertebrates. Although tetrapods were certainly the biggest news of the Devonian period, they weren't the only animals that lived on dry land. There was also a wide array of small arthropods, worms, flightless insects and other pesky invertebrates, which took advantage of the complex terrestrial plant ecosystems that started to develop at this time to gradually spread inland (though still not too far away from bodies of water).Marine Life During the Devonian PeriodThe Devonian period marked both the apex and the extinction of the placoderms, prehistoric fish characterized by their tough armor plating (some placoderms, such as the enormous Dunkleosteus, attained weights of three or four tons). As noted above, the Devonian also teemed with lobe-finned fish, from which the first tetrapods evolved, as well as relatively new ray-finned fish, the most populous family of fish on earth today. Relatively small sharks--such as the bizarrely ornamented Stethacanthus and the weirdly scaleless Cladoselache--were an increasingly common sight in the Devonian seas. Invertebrates like sponges and corals continued to flourish, but the ranks of the trilobites were thinned out, and only the giant eurypterids (invertebrate sea scorpions) successfully competed with vertebrate sharks for prey.Plant Life During the Devonian PeriodIt was during the Devonian period that the temperate regions of the earth's evolving continents became truly green. The Devonian witnessed the first significant jungles and forests, the spread of which was aided by the evolutionary competition among plants to gather as much sunlight as possible (in a dense forest canopy, a tall tree has a significant advantage over a tiny shrub). The trees of the late Devonian period were the first to evolve rudimentary bark (to support their weight and protect their trunks), as well as robust internal water-conduction mechanisms that helped to counteract the force of gravity.

DEVONIAN EVOLUTION

The Devonian seas were dominated by brachiopods, such as the spiriferids, and by tabulate and rugose corals, which built large bioherms, or reefs, in shallow waters. Encrusting red algae also contributed to reef building. In the Lower Devonian, ammonoidsappeared, leaving us large limestone deposits from their shells. Bivalves, crinoid and blastoid echinoderms, graptolites, and trilobites were all present, though most groups of trilobites disappeared by the close of the Devonian. The Devonian is also notable for the rapid diversification in fish. Benthic armored fish are common by the Early Devonian. Theseearly fish are collectively called "ostracoderms", and include a number of different groups. By the Mid-Devonian, placoderms, the first jawed fish, appear. Many of these grew to large sizes and were fearsome predators. Of the greatest interest to us is the rise of the first sarcopterygiians, or the lobe-finned fish, which eventually produced the first tetrapods just before the end of the Devonian.

By the Devonian Period, life was well underway in its colonization of the land. Before this time, there is no organic accumulation in the soils, causing these soil deposits to be a reddish color. This is indicative of the underdeveloped landscape, probablycolonized only by bacterial and algal mats. By the start of the Devonian, however, early terrestrial vegetation had begun to spread. These plants did not have roots or leaves like the plants most common today, and many had no vascular tissue at all. They probably spread largely by vegetative growth, and did not grow much more than a few centimeters tall. These plants included the now extinct zosterophylls and trimerophytes. The early fauna living among these plants were primarily arthropods: mites, trigonotarbids, wingless insects, and myriapods, though these early faunas are not well known. By the Late Devonian, lycophytes, sphenophytes, ferns, and progymnosperms had evolved. Most of these plants have true roots and leaves, and many are rather tall plants. The progymnosperm Archaeopteris, whose leaves are shown at right, was a large tree with true wood. In fact it is the oldest such tree known, and produced some of the world's first forests. By the end of the Devonian, the first seed plants had appeared. This rapid appearance of so many plant groups and growth forms has been called the "Devonian Explosion". Along with this diversification in terrestrial vegetation structure, came a diversification of the arthropods.

DEVONIAN EXTINCTION EVENT

The end of the Devonian period ushered in the second great extinction of prehistoric life on earth, the first being the mass extinction event at the end of the Ordovician period. Not all animal groups were affected equally by the End-Devonian Extinction: reef-dwelling placoderms and trilobites were especially vulnerable, but deep-sea organisms escaped relatively unscathed. The evidence is sketchy, but many paleontologists believe that the Devonian extinction was caused by multiple meteor impacts, debris from which may have poisoned the surfaces of lakes, oceans and rivers.

Silurian Fossils

Sea Scorpion

Ammonite

Crinoids

Osteolepis macrolepdotus Ischnacanthus gracilis

Stensiopelta zenaspid

LAND LIFE