The Carboniferous is the period in which large plants evolved and diversified: tree ferns, giant horsetails, and giant clubmosses (popularly known as ground pines) dominated tropical swampy areas, such as theLepidodendron(‘scaly tree’). It was a member of the lycophytes, the group of vascular plants that reproduce by spores.It grew as a trunk without branches covered with leaves in the form of scales that carried out photosynthesis. It spent half its life as a branchless pole and when it reaches maturity, it develops a crown of long, thin branches growing directly from the trunk near the growth cup. The leaves were similar to those of grass but very large, almost a meter. As the tree grew, it lost the lower leaves and what remained were the traces they left in the form of scales.It could reach 40 meters in less than 20 years and was about two meters in diameter. Its ‘roots’ (really they were not, but an intermediate between root and trunk) were impressive: four or more arms that were arranged radially and could reach a length of 12 meters, but did not go deep into the ground. Another giant of the forests was theSigillaria , whose bark looked like a replica of a honeycomb, where each hexagonal ‘cell’ marks the place where a single herbaceous-like leaf sprouted. As the trunk grew, the old leaves fell to the ground and the new ones appeared at the top of the trunk, forming a crown.The Carboniferous was also the glorious age of fernsand of them none more graceful than thePsaronius, with its long fronds (so the fern leaf is called) and its resplendent trunk. Like modern tree ferns, they had a single branchless trunk topped by a crown of fronds that developed as the fern grew.
Among the most intriguing plants of the Carboniferous was the Medullosa , with its egg-sized seeds . Unlike other plants of its kind, its foliage was similar to ferns but developing seeds without flowers. Its reproduction is not very clear, since the seeds were too heavy for the wind to carry them, so it is thought that it was the insects that transported them between plants, anticipating what would happen millions of years later with the flowering plants. The pollen was produced in specialized organs that hung from the leaves – in various shapes, ranging from the very simple to those shaped like figs or shower heads – and they all had the same function: to disperse the pollen so that they could seeds form. Other huge trees were the Cordaitas , which have many features in common with conifers, with strap-like leaves that were up to a meter long. And it wasn’t too much for a tree that could easily reach 45 meters…
giant insects
The first terrestrial ecosystems were the Carboniferous swamps, which appeared near the coasts that belonged to what would later become Europe and North America (which were in the equatorial zone) due to a drop in sea level. There, among the huge trees and a ground covered with ferns and horsetails, lived all kinds of invertebrates that grew to sizes never seen before (and would never be seen again). Finding fossilized insects the size of a motorbike was somewhat surprising. That must have been the feeling of the miners of the small English town of Bolsover when in 1979 they found an impossible fossil: a dragonfly with a wingspan of half a meter.These giant dragonflies, called Meganeura , could have a wingspan of up to a meterand with a thoracic diameter of more than 3 cm, triple that of current dragonflies. Could he really fly? The mechanics of insect flight, to call it something, are very complicated. In this case we do not know in sufficient detail the interaction between the two sets of wings of theMeganeura to be able to deduce something of its aerodynamics. Large myriapods also crawled through Carboniferous soils, such asArthopleura(‘joined ribs’), which could exceed a meter in length, or aterrestrial scorpion of almost a meter in length, thePulmonoscorpius, in addition to some newcomers, the tetrapods, amphibians and reptiles, which appeared at the end of the Devonian and were essentially fish with legs. Unfortunately, we do not have many fossils from that transition becauseover 20 million years a significant and poorly explained scarcity of vertebrates is found in the fossil recordat the beginning of the Carboniferous: it is what is known as theRomer’s gap. But something must have happened because in later sediments fossils of amphibians have been found that, although they kept their tail fins, had lost their internal gills, which suggests that they could go inland, abandoning the shores of seas and lakes.
the end of an era
The plants were excellent food (an Arthopleura could consume a ton of ferns a year) and the plants had to find a way to defend themselves: lignin bark and cellulose. Lignin is very hard and resistant, insoluble in water and since at that time there was no fungus or bacteria capable of breaking it down, it was the perfect defense for Carboniferous trees. Something similar happened with the cellulose of the primitive leaves and stems. This evolutionary innovation had an unexpected consequence: dead trees did not rot and remained buried in the subsoil or at the bottom of swamps, so the carbon absorbed from the atmosphere was not returned to it. Consequence? A drop in atmospheric carbon dioxide concentration and an increase in oxygen levels to around 30% . This fact had a great impact on the animal world, as it allowed them to reach sizes never seen before: current insects have a natural growth limit because if they get bigger they cannot get enough oxygen for their bodies. Excess oxygen could diffuse naturally through the spiracles (its breathing holes) and thus increase its size. Of course, the increase in oxygen also led to the appearance of a new phenomenon, forest fires caused by lightning, which forced plants to develop strategies to regenerate quickly when the fire was extinguished.
But on earth nothing lasts forever, and about 395 million years ago the climate changed and the Carboniferous forests collapsed . Nobody knows why it happened, but the climate became much colder and drier and what were once great jungles were reduced to a minimum, with large deserts between them. Lycopods and horsetails, until then dominant, saw how almost all the members of their genera disappeared, as did the giant insects and amphibians. But there were also its winners: seed plants and reptiles , much better adapted to the new type of ecosystem that the Permian brought.
References:
Chaisson, E., The cosmic dawn, Salvat, 1989
Schwartz, J. H., Sudden origins, John Wiley and Sons, 1999
Shapiro , R. , Origins , Salvat , 1989
