The giants of the Carboniferous
On the scale of geological times, the Carboniferous is the fifth of the six periods into which the Paleozoic is divided, which in turn is the first of the three eras into which the Phanerozoic eon is divided.
The Carboniferous is between 359.2 ± 2.5 and 299.0 ± 0.8 million years ago, preceded by the Devonian and followed by the Permian.
The name of the Carboniferous derives from the fact that the deposits of fossil coals are widespread in the lands formed in this period, as a result of the great development of forests that took place in this period.
The denomination and the chronostratigraphic unit were introduced in 1822 by the English geologists William Daniel Conybeare and William Phillips, who however initially considered it at the level of epoch or series, with the name of Carboniferous Series.
The International Commission of Stratigraphy recognizes for the Carboniferous two sub-periods (Pennsylvanian and Mississippian, both of American origin), which give rise to six epochs and seven floors.
The Carboniferous flora, initially similar to that of the Upper Ordovician, rapidly developed new species and became extremely rich and vigorous.
It is divided into two groups:
- vascular cryptogams or pteridophytes (the large family of ferns), including Sphenophyllales (vine-like climbing plants), lepidodendrals, lycopodial, equisetal (horsetail or horsetail), filical (fern).
- the gymnosperm phanerogams, including pteridosperms (including Callistophyton), cycads, cordaits, conifers: these plants, some of which still exist but with reduced dimensions, were in some cases gigantic. Lepidodendrals were represented by large trees: the Lepidodendrons, the Sigillaria, the Stigmaria reached tens of meters height.
The lepidodendrals were represented by large trees: the Lepidodendrons, the Sigillarias, the Stigmarias reached tens of meters in height.
Among the vertebrates, the primitive bony fishes (including the paleonsciforms) and the cartilaginous fishes had a great expansion, which diversified giving life to strange forms that have now disappeared, such as the Eugeneodontida, the Chondrenchelyida and the Symmoriida, and the ancestors of today's sharks. Exceptional fossils of these forms are those found in the Bear Gulch deposit in Montana.
Placoderms, however, or armored fish similar to Ostracoderms, as well as most agnates, had already disappeared in the Devonian. The tetrapods thus experience a considerable expansion, even if in the fossil record they are absent for about 20 million years (the so-called "Romer's gap"). Alongside forms of uncertain collocation (Crassigyrinus, Loxommatidae) in the lower Carboniferous one can already recognize the first representatives of the temnospondyl amphibians, which became particularly prosperous towards the end of the period and in the Permian. The lepospondyl amphibians, for their part, immediately develop highly specialized forms (Aistopoda).
The reptiliomorphs evolve to produce semi-aquatic (Embolomeri) and terrestrial forms; the latter gave rise to the first true reptiles. In the Joggins deposit in Nova Scotia numerous remains of these primitive reptiles (e.g. the famous Hylonomus) have been found preserved inside hollow tree trunks. The first synapsids (Archaeothyris) also developed at the same time as these animals.
Among the invertebrates, in the marine fauna there are very numerous foraminiferans, above all fusulin, which constitute for the most part the limestones of Russia, Iran, China and Japan. Tetracorals and tabulates are widespread in the carbonate platform facies; among the echinoderms, the blastoids and crinoids are the most common forms; among the brachiopods, the productids and the spiriferids. Among the molluscs there is a continuous development of the cephalopods, while among the arthropods the trilobites are in decline. In the fresh waters lived phyllopod crustaceans and pulmonate gastropods. Large arthropods were present on land: scorpionids, arachnids, myriapods, and insects.
Arthropleura, in particular, was a genus of centipede that lived in the Carboniferous Period (340 to 280 million years ago) in Nova Scotia, Illinois, Ohio, Pennsylvania, and Scotland.
No intact fossils have been found but its length is estimated at about two meters and perhaps more. It was certainly one of the largest invertebrates ever to appear on land and its predators probably had to be few, with the exception, perhaps, of some large-sized amphibian genera that developed in the late Carboniferous - early Permian.
Arthropleura seems to have evolved from other terrestrial arthropods such as the common myriapods which, due to the high oxygen concentration perhaps due to the large amount of forests existing in that period, and the initial lack of large predators, increased in size. It can be assumed that these large terrestrial arthropods had a breathing system based on tracheas like their present-day relatives, but more easily the animal could have been equipped with primitive lungs (in fact the fossil remains do not show spiracles), apparatuses which in any case only in the presence of high concentrations of oxygen could allow him to breathe effectively given the size of the body.
The Arthropleura became extinct at the beginning of the Permian, that is when the climate became drier, destroying the humid forests where the animal lived and thus causing, in addition to a relatively rapid desertification, a certain decrease in the level of oxygen which, together with action of large predators (first amphibians and then reptiles), determined their extinction. The same fate probably befell all the other large coeval terrestrial arthropods.
One of these was the meganeura (Meganeura monyi), a prehistoric insect that also lived in the Carboniferous, about 300 million years ago.
The appearance of this animal closely resembled that of a dragonfly, but its dimensions were gigantic: with a wingspan of over 75 centimeters, the meganeura is one of the largest insects ever to appear on Earth (although the Permian species Meganeuropsis permiana is another good candidate). Meganeura was a predatory animal, probably feeding on small amphibians and other insects. Fossils of meganeura were discovered in the Commentry coal mines in France in 1880. Five years later, French paleontologist Charles Brogniart described and named the fossil. Another notable specimen was discovered at Bolsover, Derbyshire in 1979. The holotype is in the National Museum of Natural History of France in Paris.
Controversy has arisen over the question of how insects of the Carboniferous period were able to grow so large. The way in which oxygen is diffused through the tracheal respiratory system places an upper limit on body size, which Carboniferous insects seem to have exceeded, however. It was first proposed (Harlé & Harlé, 1911) that meganeura could fly because there was more oxygen in the atmosphere at the time than at present twenty percent, perhaps as much as thirty-five percent. This would have caused a greater probability of triggering fires much more frequent than today, and the fossils in fact reveal a high percentage of fire-resistant plants. This theory was abandoned by later scientists, but has gained credence more recently through further studies into the relationship between gigantism and oxygen availability (Chapelle & Peck, Nature, 1999). If this theory were correct, these giant insects would have been dangerously susceptible to suddenly decreasing oxygen levels, and certainly could not have survived in our atmosphere.
