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The cat gap is a period in the fossil record of approximately 25 to 18.5 million years ago in which there are few fossils of cats or cat-like species found in North America. The cause of the "cat gap" is disputed, but may have been caused by changes in the climate (global cooling), changes in the habitat and environmental ecosystem, the increasingly hypercarnivorous trend of the cats (especially the nimravids), volcanic activity, evolutionary changes in dental morphology of the Canidae species present in North America, or a periodicity of extinctions called van der Hammen cycles.

The Silurian hypothesis is a thought experiment which assesses modern science's ability to detect evidence of a prior advanced civilization, perhaps several million years ago. In a 2018 paper, Adam Frank, an astrophysicist at the University of Rochester, and Gavin Schmidt, director of the NASA Goddard Institute, imagined an advanced civilization before humans and pondered whether it would "be possible to detect an industrial civilization in the geological record". They wrote, "While we strongly doubt that any previous industrial civilization existed before our own, asking the question in a formal way that articulates explicitly what evidence for such a civilization might look like raises its own useful questions related both to astrobiology and to Anthropocene studies." The term "silurian hypothesis" was inspired by a 1970s Dr Who serial Doctor Who and the Silurians which featured a species called the Silurians.

According to Frank and Schmidt, since fossilization is relatively rare and little of Earth's exposed surface is from before the quaternary time period, the chances of finding direct evidence of such a civilization, such as technological artifacts, is small. After a great time span, the researchers concluded, we would be more likely to find indirect evidence such as anomalies in the chemical composition or isotope ratios of sediments. Objects that could indicate possible evidence of past civilizations include plastics and nuclear wastes residues buried deep underground or on the ocean floor.

Prior civilizations could have gone to space and left artifacts on other celestial bodies, such as the Moon and Mars. Evidence for artifacts on these two worlds would be easier to find than on Earth, where erosion and tectonic activity would erase much of it.

Frank first approached Schmidt to discuss how to detect alien civilizations via their potential impact upon climate through the study of ice cores and tree rings. They both realized that the hypothesis could be expanded and applied to Earth and humanity due to the fact that humans have been in their current form for the past 300,000 years and have had sophisticated technology for only the last few centuries.

The Azolla event is a scenario hypothesized to have occurred in the middle Eocene epoch, around 49 million years ago, when blooms of the freshwater fern Azolla are thought to have happened in the Arctic Ocean. As they sank to the stagnant sea floor, they were incorporated into the sediment; the resulting draw-down of carbon dioxide has been speculated to have helped transform the planet from a "greenhouse Earth" state, hot enough for turtles and palm trees to prosper at the poles, to the current icehouse Earth known as the Late Cenozoic Ice Age.

The Great Oxidation Event (GOE), sometimes also called the Great Oxygenation Event, Oxygen Catastrophe, Oxygen Crisis, Oxygen Holocaust, or Oxygen Revolution, was a time period when the Earth's atmosphere and the shallow ocean experienced a rise in oxygen, approximately 2.4 billion years ago (2.4 Ga) to 2.1–2.0 Ga during the Paleoproterozoic era. Geological, isotopic, and chemical evidence suggests that biologically induced molecular oxygen (dioxygen, O₂) started to accumulate in Earth's atmosphere and changed Earth's atmosphere from a weakly reducing atmosphere to an oxidizing atmosphere, causing almost all life on Earth to go extinct. The cyanobacteria producing the oxygen caused the event which enabled the subsequent development of multicellular forms.

The Schöningen spears are a set of eight wooden throwing spears from the Palaeolithic Age that were excavated between 1994 and 1998 in the open-cast lignite mine in Schöningen, Helmstedt district, Germany, together with an associated cache of approximately 16,000 animal bones. The excavations took place under the management of Hartmut Thieme of the Lower Saxony State Service for Cultural Heritage (NLD).

Originally assessed as being between 380,000 and 400,000 years old, they represent the oldest completely preserved hunting weapons of prehistoric Europe so far discovered. As such they predate the age of Neanderthal Man (by convention taken to have emerged 300,000 years ago), and are associated with Homo heidelbergensis. The spears support the practice of hunting by archaic humans in Europe in the late Lower Paleolithic.

The age of the spears was estimated from their stratigraphic position, "sandwiched between deposits of the Elsterian and Saalian glaciations, and situated within a well-studied sedimentary sequence." More recently, thermoluminescence dating of heated flints in a deposit beneath that which contained the spears suggested that the spears were between 337,000 and 300,000 years old.

St. Cuthbert's beads (or Cuddy's beads) are fossilised portions of the "stems" of crinoids from the Carboniferous period. Crinoids are a kind of marine echinoderm which are still extant, and which are sometimes known as "sea lilies". These bead-like fossils are washed out onto the beach and in medieval Northumberland were strung together as necklaces or rosaries, and became associated with St Cuthbert.

In other parts of England, circular crinoid columnals were known as "fairy money." Pentagonal crinoid columnals were known as "star stones", and moulds of the stems left impressions which were known as screwstones. In Germany, the columnals were known as Bonifatius pfennige (St Boniface's pennies) and in America they are known as Indian beads.

Spondylus is a genus of bivalve molluscs, the only genus in the family Spondylidae. They are known in English as spiny oysters or thorny oysters (though they are not, in fact, true oysters).

Dimetrodon ( (listen) or , meaning "two measures of teeth") is an extinct genus of non-mammalian synapsids that lived during the Cisuralian (Early Permian), around 295–272 million years ago (Ma). It is a member of the family Sphenacodontidae. The most prominent feature of Dimetrodon is the large neural spine sail on its back formed by elongated spines extending from the vertebrae. It walked on four legs and had a tall, curved skull with large teeth of different sizes set along the jaws. Most fossils have been found in the southwestern United States, the majority coming from a geological deposit called the Red Beds of Texas and Oklahoma. More recently, fossils have been found in Germany. Over a dozen species have been named since the genus was first described in 1878.

Dimetrodon is often mistaken for a dinosaur or as a contemporary of dinosaurs in popular culture, but it became extinct some 40 million years before the first appearance of dinosaurs. Reptile-like in appearance and physiology, Dimetrodon is nevertheless more closely related to mammals than to modern reptiles, though it is not a direct ancestor of mammals. Dimetrodon is assigned to the "non-mammalian synapsids", a group traditionally called "mammal-like reptiles". This groups Dimetrodon together with mammals in a clade (evolutionary group) called Synapsida, while placing dinosaurs, reptiles and birds in a separate clade, Sauropsida. Single openings in the skull behind each eye, known as temporal fenestrae, and other skull features distinguish Dimetrodon and mammals from most of the earliest sauropsids.

Dimetrodon was probably one of the apex predators of the Cisuralian ecosystems, feeding on fish and tetrapods, including reptiles and amphibians. Smaller Dimetrodon species may have had different ecological roles. The sail of Dimetrodon may have been used to stabilize its spine or to heat and cool its body as a form of thermoregulation. Some recent studies argue that the sail would have been ineffective at removing heat from the body due to large species being discovered with small sails and small species being discovered with large sails, essentially ruling out heat regulation as its main purpose. The sail was most likely used in courtship display with methods such as threatening rivals or showing off to potential mates.

The Boring Billion, otherwise known as the Mid Proterozoic and Earth's Middle Ages, is the time period between 1.8 and 0.8 billion years ago (Ga) spanning the middle Proterozoic eon, characterized by more or less tectonic stability, climatic stasis, and slow biological evolution. It is bordered by two different oxygenation and glacial events, but the Boring Billion itself had very low oxygen levels and no evidence of glaciation.

The oceans may have been oxygen- and nutrient-poor and sulfidic (euxinia), populated by mainly anoxygenic purple bacteria, a type of chlorophyll-based photosynthetic bacteria which uses hydrogen sulfide (H₂S) instead of water and produces sulfur instead of oxygen. This is known as a Canfield ocean. Such composition may have caused the oceans to be black- and milky-turquoise instead of blue. (By contrast, during the much earlier Purple Earth phase the photosynthesis was retinal-based.)

Despite such adverse conditions, eukaryotes may have evolved around the beginning of the Boring Billion, and adopted several novel adaptations, such as various organelles, multicellularity, and possibly sexual reproduction, and diversified into plants, animals, and fungi at the end of this time interval. Such advances may have been important precursors to the evolution of large, complex life later in the Ediacaran and Phanerozoic. Nonetheless, prokaryotic cyanobacteria were the dominant lifeforms during this time, and likely supported an energy-poor food-web with a small number of protists at the apex level. The land was likely inhabited by prokaryotic cyanobacteria and eukaryotic proto-lichens, the latter more successful here probably due to the greater availability of nutrients than in offshore ocean waters.

The Permian–Triassic extinction event, also known as the P–Tr extinction, the P–T extinction, the End-Permian Extinction, and colloquially as the Great Dying, formed the boundary between the Permian and Triassic geologic periods, as well as between the Paleozoic and Mesozoic eras, approximately 252 million years ago. It is the Earth's most severe known extinction event, with up to 96% of all marine species and 70% of terrestrial vertebrate species becoming extinct. It was the largest known mass extinction of insects. Some 57% of all biological families and 83% of all genera became extinct.

There is evidence for one to three distinct pulses, or phases, of extinction. Potential causes for those pulses include one or more large meteor impact events, massive volcanic eruptions (such as the Siberian Traps), and climate change brought on by large releases of underwater methane or methane-producing microbes.

The speed of the recovery from the extinction is disputed. Some scientists estimate that it took 10 million years (until the Middle Triassic), due both to the severity of the extinction and because grim conditions returned periodically for another 5 million years. However, studies in Bear Lake County, near Paris, Idaho, showed a relatively quick rebound in a localized Early Triassic marine ecosystem, taking around 2 million years to recover, suggesting that the impact of the extinction may have been felt less severely in some areas than others.