Cretaceous polar forests were temperate forests that grew at polar latitudes during the final period of the Mesozoic Era, known as the Cretaceous Period 145–66 Ma. During this period, global average temperature was about 10 °C (18 °F) higher and carbon dioxide (CO2) levels were approximately 1000 parts per million (ppm), 2.5 times the current concentration in Earth's atmosphere. The abundance of atmospheric carbon dioxide had a very significant impact on global climate and Earth's natural systems as its concentration is considered one of the main factors in the development of a pronounced greenhouse Earth during the Cretaceous, with a very low average global temperature gradient. As a consequence, high paleolatitudes in both hemispheres were much warmer than at present. This temperature gradient was partly responsible for the lack of continental ice sheets in polar regions.

As a response to elevated global temperatures, the Earth's hydrologic cycle was significantly enhanced due to greater volume of moisture evaporation from the surface of the ocean. In turn, the absolute sea level during this time period stood at elevations much higher than the present level. Continental encroachment of seawater formed widespread shallow seas, including expanses of epeiric seas.

An increase in surface area between shallow, warm epeiric seawater and the atmosphere permits higher evaporation rates and more precipitation at various latitudes, producing a more temperate global climate. A widespread temperate climate also had significant effects on high latitude ecosystems.

During the Cretaceous, temperate forests thrived at polar latitudes, as there was a notable difference from current conditions at high latitudes during the Cretaceous polar seasons. The duration of summer sunlight and winter darkness lasted for approximately 5 months each. This variation in light is thought to have played a critical role in the composition and evolution of polar forests. Fossilized flora evidence suggests the presence of paleoforests up to latitudes of 85° in both Northern and Southern hemispheres. The dominant forms of vegetation at these high latitudes during the previous 100 million years were rapidly evolving and ultimately being replaced during a time known as the Cretaceous Terrestrial Revolution. During the Cretaceous Terrestrial Revolution, conifers, cycads and ferns were selectively replaced by angiosperms and gymnosperms, becoming the main species dominating the high paleolatitudes. In this Cretaceous greenhouse world, Arctic conifer forests were considered predominantly deciduous, while those that grew on Antarctica contained a significantly greater proportion of evergreens.

The remains of the ancestor of modern birds, the Neornithes, are uncommon in the Mesozoic, with a large radiation occurring in the Neogene of Antarctica. However, the discovery of the Late Cretaceous Vegavis, a goose-like bird, on Vega Island indicates that the major modern bird groups were already common in the Cretaceous. A femur belonging to an unidentified seriema-like bird was also discovered on Vega Island. Bird footprints were preserved in Dinosaur Cove, and, being larger than most Cretaceous bird species, indicate an abundance of larger enantiornithe or ornithurine birds during the Early Cretaceous.

Two diving birds, possible primitive loons, were discovered in Late Cretaceous Chile and Antarctica: Neogaeornis and Polarornis. Polarornis may have been capable of both diving and flight. The earliest penguins, Crossvallia and Waimanu, are known from 61–62 Ma in the Paleocene, however molecular data suggests penguins first evolved in the Late Cretaceous. Given that these penguins were dated so close to the Cretaceous–Paleogene extinction event, the group either evolved before the event or very rapidly afterwards.

Dinosaur fossils are rare from the South Polar region, and major fossil-bearing locations are the James Ross Island group; Beardmore glacier in Antarctica; Roma, Queensland; Mangahouanga stream in New Zealand; and Dinosaur Cove in Victoria, Australia. The dinosaur remains of this region, such as those found in Victoria, consist only fragmentary pieces, making identification controversial. For example, disputed identifications of an allosaurid which may represent an abelisaurid, the ceratopsian Serendipaceratops which could be an ankylosaur, and the difficult-to-classify theropod Timimus have consequently been made.

dinosaur groups that achieved pan-Gondwanan distribution over the course of the Cretaceous would have had to have used the land bridge connecting Australia to South America via Antarctica in the South Polar region. The South Polar iguanodontian Muttaburrasaurus is most closely related to European rhabdodontids, which were the dominant group in Europe during the Late Cretaceous. The Cretaceous South Polar Kunbarrasaurus is identified as being part of a unique lineage of Gondwanan ankylosaurs, which suggests homogeneity in the Gondwanan fauna. The Dromaeosauridae are known from Antarctica, and represent a relict population from a previously world-wide distribution. Despite these apparent cross-continental migrations, it is unlikely that South Polar dinosaurs migrated out of the polar forests during the winter, as they were either too massive–such as ankylosaurs–or too small–such as troodontids–to travel long distances, and a large sea between East Gondwana and other continents impeded any such migrations in the Late Cretaceous. It is possible, to cope with the winter conditions, some dinosaurs hibernated, such as the theropod Timimus.

The most common and diverse group found so far are the hypsilophodont-like dinosaurs, making up half of the dinosaur taxa found in southeastern Australia, which is unseen in more tropical regions, perhaps indicating some kind of advantage over other dinosaurs in the poles. Being small with grinding dentition, they likely fed on low-lying vegetation such as lycopods and podocarp seed pods.

After an asteroid impact, the ensuing impact winter is thought to have killed off the dinosaurs along with much of Mesozoic life in the Cretaceous–Paleogene extinction event. However, the lack of an abrupt extinction horizon in Antarctic or Australian sediments for plant and bivalve fossils during this time period indicates a less powerful impact in the South Polar region. Given that the dinosaurs and other fauna of the polar regions of the Cretaceous were well adapted for living in long periods of dark and cold weather, it has been postulated that this community might have survived the event.

South Polar region of the Cretaceous

When Dinosaurs & Forests Ruled The Arctic and Antarctica

BBC Earth: Walking With Dinosaurs Episode 5 - Spirits of the Ice Forest

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