Пенсильванский период: поздний карбон и эволюция жизни

Second subperiod of the Carboniferous

The Pennsylvanian (,/p//ɛ//n//s//əl/'/v//eɪ//n//i//.//ən/ pensəlVAYNiən, also known as Upper Carboniferous or Late Carboniferous) is, on the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period (or the upper of two subsystems of the Carboniferous System). It lasted from roughly As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain by a few hundred thousand years. The Pennsylvanian is named after the U. S. state of Pennsylvania, where the coal beds of this age are widespread. The division between Pennsylvanian and Mississippian comes from North American stratigraphy. In North America, where the early Carboniferous beds are primarily marine limestones, the Pennsylvanian was in the past treated as a full fledged geologic period between the Mississippian and the Permian. In parts of Europe, the Mississippian and Pennsylvanian are one more or less continuous sequence of lowland continental deposits and are grouped together as the Carboniferous Period. The current internationally used geologic timescale of the ICS gives the Mississippian and Pennsylvanian the rank of subperiods, subdivisions of the Carboniferous Period.

All modern classes of fungi have been found in rocks of Pennsylvanian age.

The major forms of life at this time were the arthropods. Arthropods were far larger than modern ones. Arthropleura, a giant millipede, was a common sight and the giant griffinfly Meganeura "flew the skies". It is commonly considered that is because of high oxygen level, however some of those large arthropod records are also known from period with relatively low oxygen, which suggest high oxygen pressure may not have been a primary reason for their gigantism.

Amphibians were diverse and common; some were several meters long as adults. The collapse of the rainforest ecology in the mid Pennsylvanian (between the Moscovian and the Kasimovian) removed many amphibian species that did not survive as well in the cooler, drier conditions. Amniotes, however, prospered due to specific key adaptations. One of the greatest evolutionary innovations of the Carboniferous was the amniote egg, which allowed for the further exploitation of the land by certain tetrapods. These included the earliest sauropsid reptiles (Hylonomus), and the earliest known "pelycosaur" synapsids (Archaeothyris). Small lizard like animals quickly gave rise to many descendants. Amniotes underwent a major evolutionary radiation, in response to the drier climate that followed the rainforest collapse. For some reason, pelycosaurs were able to reach larger sizes before reptiles could, and this trend continued until the end of the Permian, during which their cynodont descendants became smaller and nocturnal, as the reptilian archosaurs took over, although dicynodonts would remain megafaunal until their extinction at the end of the Triassic. Most pre rainforest collapse tetrapods remained smaller, probably due to the land being primarily occupied by the gigantic millipedes, scorpions, and flying insects. After the rainforest collapse, the giant arthropods disappeared, allowing amniote tetrapods to achieve larger sizes.