The term Carboniferous (meaning “coal bearing”) was proposed by the English geologist William Conybeare and William Phillips in 1822 to designate coal-bearing strata in north-central England. Subsequently in Continental Europe and Britain the system was divided into a Lower (a.k.a. “Early”) and an Upper (a.k.a. “Later”) Carboniferous. Meanwhile, American geologists proposed the name Mississippian for Lower Carboniferous strata and Pennsylvanian for the Upper Carboniferous strata. With some adjustments over time, the terms Mississippian, Lower Carboniferous and Early Carboniferous have come to mean the “first” epoch of the Carboniferous and similarly the terms Pennsylvanian, Upper Carboniferous, and Later Carboniferous have come to mean the “second” epoch of the Carboniferous.
During the Mississippian, animal life, both vertebrate and invertebrate, secured its position on land. Mineral layers of the Mississippian are mostly limestone which is composed of the remains of crinoids, lime-encrusted green algae, and/or calcium carbonate shaped by waves. During the Pennsylvanian, most of the world's coal deposits were laid down; the coal being formed from compressed layers of rotting vegetation from this time period.
Tectonics and Paleoclimate
During the Mississippian a glaciated Gondwana nears southern Euramerica and continues to collide with ancestral Europe, resulting in the Hercynian Orogeny and great mountains in southern Europe and what is now western North America. The climate, originally hot and dry became cooler and wet later in the Mississippian period. Later, during the Pennsylvanian, Laurussia and Siberia collide to form Laurasia and Gondwana moves up from the south on a collision course the result of which is the super continent of Pangea along with the Appalachian, Ouachita, Marathon, Ural, Variscan, and Hercynian orogenies which formed some of the largest mountains of all time. The climate during the Pennsylvanian underwent a pronounced cooling and glaciation triggered by Gondwanaland's southward migration. While the equatorial regions remain warm, wet and tropical, the poles are gripped in a massive ice age that lasts for many millions of years.
There were major changes in flora between Late Devonian and early Mississippian periods. Areas that were previously forest were now dominated by specialized, shrubby plants, mostly pteridosperms less than 2 meters in height which most would call “weeds”. Only much later during the period did lycopsid and calamite trees reappear. As the Pennsylvanian progressed, great forest swamps covered extensive equatorial areas. These forests consisted of diverse plants including tree ferns which grew 15 meters in height, Calmites (a giant version of the modern "horsetail" plant), lycopods (e.g. Lepidodendron which attained a height of 30 meters), the extinct group of plants called "seed ferns", and primitive Conifer-like plants (Cordaites) that reached 40 meters in height.
During the Mississippian period, arthropods, corals, bryozoa, crinoids, and mollusks flourished in warm shallow seas. Echinoderms (especially Crinoids) were extremely numerous, many types of Ammonoid cephalopods evolved, and the first ceratic ammonoids appear, with a more complex suture pattern. The first of the giant fusulinid foramnifers (i.e., marine amoebas) appear but are still tiny and relatively insignificant. The trilobites were much reduced in numbers and confined to a single super family (i.e., the Proetoidea), the last of the dendrite graptiloids died out as did the bulbous-shelled Oncocerida, the nautilida is the last of the nautiloid (palcephalopoda) cephalopods to flourish, and the giant straight-shelled Rayonnoceras is the sole survivor of the Actinocerida.
The vertebrates become numerous and diverse including sharks, actinopterygian (mostly of the "paleonisciforms”), and sarcopterygian. Meanwhile, terrestrial invertebrates are poorly known but it is likely that they consisted of mites, scorpions, and other arachnids, millipedes, arthropleurids, collembolans (i.e., springtails), and an increasing diversity of litter-reducing insects (e.g. blattoids). Some Eurypterids of this time may have been partially terrestrial. The major link between plant productivity and animal consumers seems to have been through detritivorous arthropods. Insect herbivory was only just beginning at the end of the Mississippian sub-period with most insects and arachnids scrounging for food in leaf litter and likewise serving as the primary food source for the early terrestrial tetrapodomorphs.
The Mississippian also experienced the greatest Tetrapodomorph evolutionary radiation. The early generations of aquatic Ichhyostegids were replaced by various parallel lineages of labyrinthodont and Lepospondyl amphibians. All the major ancient tetrapodomorph groups seem to have appeared at this time with the majority probably being semi-aquatic however early terrestrial forms and proto-reptiles appeared as well. The fresh-water Rhizodontiform fish, that like lungfish, were capable of breathing air on occasion and were probably the “super predators” of the swamps, streams and lakes.
The Pennsylvanian age was the high point of tetrapodomorph evolution. Tetrapodomorph were abundant, especially the "labyrinthodonts, and filled every available ecological niche from fully aquatic eel-like forms to large semi aquatic crocodile-like animals, from small forms like modern day newts and salamanders to terrestrial reptile-like creatures. Some types like the Aïstopoda evolved in such a way as to no longer require legs, superficially resembling snakes.
In the moist oxygen rich atmosphere flying insects were abundant and some became quit large (e.g., the Meganeura, with a wing span of 70 centimeters). The earliest reptiles also evolved at this time such as Hylonomus but remained relatively insignificant until the end of the period. Reptiles have a big advantage over tetrapodomorphs because they can lay their eggs on land without having to return to aquatic environments. With the appearance of reptiles, land animals were able to colonize the uplands for the first time, feeding on an abundance of insects. It was also during this time that the synapsids evolved and quickly diversified to such a degree that by the end of the Pennsylvanian, they had overthrown the tetrapodomorphs as the dominant life form on land.
Meteorite Impacts on Earth
I included a list of meteorite impacts relevant to this time period as a point of reference since many of the explanations for mass extinctions throughout Earth’s history include meteorite impact(s) as a possible cause. The meteorite impact information below was obtained from the ‘Earth Impact Database’ maintained by the Planetary and Space Science Centre, University of New Brunswick, Fredericton, New Brunswick, Canada ( www.passc.net/EarthImpactDatabase).