The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from 323.2 million years ago to 298.9 million years ago Ma. 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. The division between Pennsylvanian and Mississippian comes from North American stratigraphy. In North America, where the early Carboniferous beds are marine limestones, the Pennsylvanian was in the past treated as a full-fledged geologic period between the Mississippian and the Permian. In 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. Amphibians were common; the collapse of the rainforest ecology in the mid-Pennsylvanian removed many amphibian species that did not survive as well in the cooler, drier conditions. Reptiles, 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, the earliest known synapsid. Small lizard-like animals gave rise to many descendants. Reptiles underwent a major evolutionary radiation, in response to the drier climate that followed the rainforest collapse; the major forms of life at this time were the arthropods. Due to the high levels of oxygen, arthropods were far larger than modern ones. Arthropleura, a giant millipede relative, was a common sight and the giant dragonfly Meganeura "flew the skies"; the Pennsylvanian has been variously subdivided.
The international timescale of the ICS follows the Russian subdivision into four stages: Bashkirian Moscovian Kasimovian Gzhelian North American subdivision is into five stages, but not the same, with additional Appalachian series names following: Morrowan stage, corresponding with the middle and lower part of the Pottsville Group Atokan stage, corresponding with the upper part of the Pottsville group Desmoinesian stage, corresponding with the Allegheny Group Missourian stage, corresponding with the Conemaugh Group Virgilian stage, corresponding with the Monongahela Group The Virgilian or Conemaugh corresponds to the Gzhelian plus the uppermost Kasimovian. The Missourian or Monongahela corresponds to the rest of the Kasimovian; the Desmoinesian or Allegheny corresponds to the upper half of the Moscovian. The Atokan or upper Pottsville corresponds to the lower half of the Moscovian; the Morrowan corresponds to the Bashkirian. In the European subdivision, the Carboniferous is divided into two epochs: Silesian.
The Silesian starts earlier than the Pennsylvanian and is divided in three ages: Namurian Westphalian Stephanian. The Late Carboniferous a Time of Paleomap project. World map from this time period; the Carboniferous - 354 to 290 Million Years Ago, University of California Museum of Paleontology. Information on stratigraphies, localities and life; the Pennsylvanian Epoch of the Carboniferous Period: 318 to 299 Mya, Paleos.com US Geological Survey comparison of time scales
The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between 251.902 Ma and 247.2 Ma. Rocks from this epoch are collectively known as the Lower Triassic, a unit in chronostratigraphy; the Early Triassic is the oldest epoch of the Mesozoic Era and is divided into the Induan and Olenekian ages. The Lower Triassic series is coeval with the Scythian stage, today not included in the official timescales but can be found in older literature. In Europe, most of the Lower Triassic is composed of Buntsandstein, a lithostratigraphic unit of continental red beds; the Permian-Triassic extinction event spawned the Triassic period. The massive extinctions that ended the Permian period and Paleozoic era caused extreme hardships for the surviving species. Many types of corals, molluscs and other invertebrates had disappeared; the most common Early Triassic hard-shelled marine invertebrates were bivalves, ammonites, a few articulate brachiopods. The most common land vertebrate was the small herbivorous synapsid Lystrosaurus.
Early Triassic faunas lacked biodiversity and were homogenous throughout the epoch due to the effects of the extinction, ecological recovery on land took 30M years. The climate during the Early Triassic epoch was arid and dry and deserts were widespread; the hot climate of the Early Triassic may have been caused by widespread volcanic eruptions which accelerated the rate of global warming and caused the Permian-Triassic extinction event. Until the existence of an extinction event about 3 million years following the end-Permian extinctions was not recognised because there were few species left to go extinct. However, studies on conodonts have revealed that temperatures rose in the first 3 million years of the Triassic reaching sea surface temperatures of 40 °C in the tropics around 249 million years ago. Large and mobile species disappeared from the tropics, amongst the immobile species such as molluscs, only the ones that could cope with the heat survived. On land, the tropics were devoid of life.
Big, active animals only returned to the tropics, plants recolonised on land when temperatures returned to normal around 247 million years ago. Geologic time scale GeoWhen Database - Early Triassic Palaeos scotese
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series; the Cretaceous is named after the white limestone known as chalk which occurs in northern France and is seen in the white cliffs of south-eastern England, which dates from this time. During the Late Cretaceous, the climate was warmer than present, although throughout the period a cooling trend is evident; the tropics became restricted to equatorial regions and northern latitudes experienced markedly more seasonal climatic conditions. Due to plate tectonics, the Americas were moving westward, causing the Atlantic Ocean to expand; the Western Interior Seaway divided North America into western halves. India maintained a northward course towards Asia. In the Southern Hemisphere and Antarctica seem to have remained connected and began to drift away from Africa and South America. Europe was an island chain. Populating some of these islands were endemic dwarf dinosaur species.
In the Late Cretaceous, the hadrosaurs and ceratopsians experienced success in Asiamerica. Tyrannosaurs dominated the large predator niche in North America, they were present in Asia, although were smaller and more primitive than the North American varieties. Pachycephalosaurs were present in both North America and Asia. Dromaeosaurs shared the same geographical distribution, are well documented in both Mongolia and Western North America. Additionally therizinosaurs appear to have been in North Asia. Gondwana held a different dinosaurian fauna, with most predators being abelisaurs and carcharodontosaurs. Spinosaurids were present during this time. Birds became common and diverse, diversifying in a variety of enantiornithe and ornithurine forms. Early Neornithes such as Vegavis co-existed with forms as bizarre as Avisaurus. Though small, marine Hesperornithes became large and flightless, adapted to life in the open sea. Though represented by azhdarchids, other forms like pteranodontids, tapejarids and uncertain forms are present.
It has been assumed that pterosaurs were in decline due to competition with birds, but it appears that neither group overlapped ecologically, nor is it evident that a true systematic decline was in place with the discovery of smaller pterosaur species. Several old mammal groups began to disappear, with the last eutriconodonts occurring in the Campanian of North America. In the northern hemisphere, multituberculates and eutherians were the dominant mammals, with the former two groups being the most common mammals in North America. In the southern hemisphere there was instead a more complex fauna of dryolestoids and other multituberculates and basal eutherians. Mammals, though small, ranged into a variety of ecological niches, from carnivores, to mollusc-eater, to herbivores to atypical cursorial forms. True placentals only evolved at the end of the epoch. Instead, nearly all known eutherian and metatherian fossils belong to other groups. In the seas, mosasaurs appeared and underwent a spectacular evolutionary radiation.
Modern sharks appeared and giant-penguin-like polycotylid plesiosaurs and huge long-necked elasmosaurs diversified. These predators fed on the numerous teleost fishes, which in turn evolved into new advanced and modern forms. Ichthyosaurs and pliosaurs, on the other hand, became extinct during the Cenomanian-Turonian anoxic event. Near the end of the Cretaceous Period, flowering plants diversified. In temperate regions, familiar plants like magnolias, roses and willows could be found in abundance; the Cretaceous–Paleogene extinction event was a large-scale mass extinction of animal and plant species in a geologically short period of time 66 million years ago. It is known as the K–T extinction event and is associated with a geological signature a thin band dated to that time and found in various parts of the world, known as the Cretaceous–Paleogene boundary. K is the traditional abbreviation for the Cretaceous Period derived from the German name Kreidezeit, T is the abbreviation for the Tertiary Period.
The event marks the beginning of the Cenozoic Era. "Tertiary" being no longer recognized as a formal time or rock unit by the International Commission on Stratigraphy, the K-T event is now called the Cretaceous—Paleogene extinction event by many researchers. Non-avian dinosaur fossils are only found below the Cretaceous–Paleogene boundary and became extinct before or during the event. A small number of dinosaur fossils have been found above the Cretaceous–Paleogene boundary, but they have been explained as reworked fossils, that is, fossils that have been eroded from their original locations preserved in sedimentary layers. Mosasaurs, plesiosaurs and many species of plants and invertebrates became extinct. Mammalian and bi