South Polar region of the Cretaceous

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Paleogeography of the Late Cretaceous (90 Ma); the Antarctic Circle is at 60°S

The South Polar region of the Cretaceous comprised the continent of East Gondwana–modern day Australia,

Cretaceous Period. The southern region, during this time, was much warmer than it is today, ranging from perhaps 4–8 °C (39–46 °F) in the latest Cretaceous Maastrichtian in what is now southeastern Australia. This prevented permanent ice sheets from developing and fostered polar forests, which were largely dominated by conifers, cycads, and ferns, and relied on a temperate climate and heavy rainfall. Major fossil-bearing geological formations that record this area are: the Santa Marta and Sobral Formations of Seymour Island off the Antarctic Peninsula; the Snow Hill Island, Lopez de Bertodano, and the Hidden Lake Formations on James Ross Island also off the Antarctic Peninsula; and the Eumeralla and Wonthaggi Formations
in Australia.

The South Polar region housed many

labyrinthodont amphibian, Koolasuchus. The isolation of Antarctica produced a distinct ecosystem of marine life called the Weddellian Province.[1]

Landscape

Geology

Map showing current and previous positions of Australia, with Dinosaur Cove outlined in red and the equator and Antarctic Circle shown for reference
Dinosaur Cove has moved north over the last hundred million years[2]

The Latady Basin in the southern Antarctic Peninsula–

plate, causing volcanic activity.[5]

An important fossil-bearing

formation is the Santa Marta Formation at James Ross Island off the Antarctic Peninsula, one of the most significant formations of the Southern Hemisphere, representing a high diversity terrestrial flora and fauna of the Late Cretaceous polar region. Seymour Island is similar in this respect, the area of discovery for vertebrate and invertebrate creatures–such as plesiosaurs–of the Late Cretaceous, extending into the Eocene.[4] Other Cretaceous fossil-bearing formations in Antarctica are the Snow Hill Island Formation, the Lopez de Bertodano Formation, and the Hidden Lake Formation, also on James Ross Island.[6][7]

The

Cretaceous New Zealand.[7]

The Great Artesian Basin is a remnant of the inland Eromanga Sea

The Eromanga Sea was an inland sea across what would be Australia that formed in the Early Cretaceous. The sea reached the Eromanga Basin from the north via the Carpentarian Basin. The southern end of the sea comprised lagoons and rivers, and to the east in Surat Basin a bay. When India drifted away from Australia in the Early Cretaceous, the Perth Basin also filled with seawater. The area that would be the Coral Sea was a rift valley. As Australia and Antarctica drifted apart throughout the Cretaceous, a sea formed in between them.[8]

Vegetation

See also: Polar forests of the Cretaceous

The

geothermal heat, may have prevented glaciation and increased global temperatures. Temperatures may have been up to 15 °C (27 °F) warmer than they are in the modern age.[10][11][12]

Jurassic

The landscape of the

lycopods, bryophytes, fungi, and algae. It had a temperate climate with heavy rainfall.[13][14]

Early Cretaceous

In the

polar winters may have lasted from six weeks to four and a half months.[19]

East Gondwana may have been similar to present day South American temperate forests (above).[20]

However, rocks from the Early Cretaceous

glaciation and cold climates was discovered in sediments in the Eromanga Basin in modern-day central Australia, or 60 to 80°S in the Early Cretaceous. This basin was a large inland sea in the Cretaceous. Nonetheless, similar formations could have been created by simply debris flow, and so it is possible that glaciation did not ever occur there.[23][24] The total polar ice coverage during the Mesozoic may have been a third of the size as it is in modern times, though cold snaps of subfreezing temperatures possibly occurred throughout the Early Cretaceous.[25]

Late Cretaceous

The canopy of the polar forests around present-day Alexander Island, which was around 75°S in the Middle Cretaceous, were predominantly evergreen, and likely most South Polar forests were as well, and comprised mainly araucarian and podocarp conifers.[4][26][16] It is thought that these trees remained dormant throughout the polar winters until summers under the midnight sun.[27]

Evidence of flowering plants dating to around 80 Ma in the Late Cretaceous suggests the existence of temperate forests–similar to those in present-day Australia, New Zealand, and southern South America.[20] Some flower remains were discovered near 60°S, and it is possible at such a low latitude that this area was subject to polar winters and seasonal weather, though the flowers suggest an annual temperature range of 8–15 °C (46–59 °F) and a rainy climate.[16][18]

Pollen remains from southeastern Australia are identical to living plant species of Australia: conifers, flowering plants that inhabit areas with high rainfall and a

age 72 to 66 mya, from a mean annual temperature of 7 °C (45 °F) to a more seasonally extreme 4–8 °C (39–46 °F).[29] These plants likely survived the Cretaceous–Paleogene extinction event, which killed off most life 66 Ma, on the volcanic Antarctic Peninsula; plant fossils dating to 60 Ma in the Paleocene from Seymour Island off this peninsula are recognized as being the ancestors of temperate plants inhabiting modern-day Australia and South America.[27]

The Late Cretaceous (Campanian) Zamek and Half Three Point Formations of King George Island were located at 60°S and display a rich assemblage of fossil flora, such as Podocarpus; Araucaria; the leptosporangiate ferns Cladophlebis and Clavifera; and a variety of Magnoliopsida flowering plants, Dicotylophyllum, Myrciophyllum santacruzensis, Nothofagus, Sterculiaephyllum australis, Monimiophyllum, and so forth.[30][31][32] The Sobral Formation of Seymour Island spanning the Cretaceous–Paleogene boundary at a paleolatitude of 63°S provided a new genus of fossil flower in the family Cunoniaceae, Eucryphiaceoxylon eucryphioides.[33]

Ecology

See also: List of dinosaurs and other Mesozoic reptiles of New Zealand

Much as in Australia today, East Gondwana played host to many

endemic animals, which included many relict species of families that had gone extinct in the rest of the Cretaceous world. It is possible the polar regions of the Late Cretaceous had been inhabited by groups of plants and animals whose ancestry can be traced back to the Ordovician.[16][34] The gradual isolation of Antarctica in the Late Cretaceous created a distinct group of aquatic creatures called the Weddellian Province.[35]

Dinosaurs

Birds

Early penguins (Waimanu above) may have existed in the Late Cretaceous.[36]

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.[36]

Two diving birds, possible primitive divers, were discovered in Late Cretaceous Chile and Antarctica: Neogaeornis and Polarornis. Polarornis may have been capable of both diving and flight.[36] 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.[36][37]

Non-avian

See also: List of Australian and Antarctic dinosaurs and List of dinosaurs and other Mesozoic reptiles of New Zealand
theropod found in Australia.[38]

Dinosaur fossils are rare from the South Polar region, and major fossil-bearing locations are the

ankylosaur, and the difficult-to-classify theropod Timimus have consequently been made.[38]

The supercontinent Pangaea of the Jurassic allowed major dinosaur clades to achieve a global distribution before breaking up, and several closely related cognates existed between South Polar forms and forms found elsewhere despite separation by the

theropod Timimus.[45]

hypsilophodont-like Diluvicursor

The most common and diverse group found so far are the

ontogenesis, that is, relatively large eye-sockets may have been a feature only seen in juveniles or perhaps was a birth-defect, since there is only one specimen known.[43][19]

Though remains are scant and, consequently, taxonomic descriptions can be dubious,

Joan Wiffen's theropod", were discovered in the Late Jurassic/Early Cretaceous rocks of New Zealand.[51]

Wintonotitan likely inhabited the South Polar region.[50]

Four

forked ferns of the time.[46]

Paleocene

Main article:
Paleocene dinosaurs

After an

bivalve fossils during this time period indicates a less powerful impact in the South Polar region.[16] 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.[19]

Rivers and lakes

The last

temnospondyls–a group of giant amphibians which mainly died out after the Triassic–inhabited the South Polar region into the Early Cretaceous. Koolasuchus, perhaps the last of the temnospondyls, is thought to have survived in regions where it was too cold for their competitors, the neosuchians–a group of reptiles containing modern crocodilians–which are inactive in water below 10 °C (50 °F), to survive.[52] Though neosuchians are known from Cretaceous Australia, it is thought that they stayed away from the polar region, arriving to Australia over-seas rather than over-land.[34]

inland Eromanga Sea of the Early Cretaceous.[52]

It is likely the temnospondyls inhabited the freshwater systems of polar Australia until the

ray-finned fish which perhaps targeted their larvae. The migration of neosuchians into the region suggests that average winter temperatures were greater than 5.5 °C (41.9 °F), with an average annual temperature of more than 14.2 °C (57.6 °F). However, polar neosuchians are only known from an almost complete skeleton of Isisfordia, and other neosuchian remains are of undetermined species.[52]

rhomaleosaurids and Leptocleidus, which died out in the Early Cretaceous, indicating the polar freshwater systems may have been a refuge for the pliosaurs of the Cretaceous. In contrast to modern marine reptiles, these South Polar plesiosaurs probably had a better tolerance of colder waters.[53][24]

Oceans

The Australian Woolungasaurus being attacked by a Kronosaurus

Early to Middle Cretaceous

endothermic similar to modern day leatherback sea turtles (Dermochelys coriacea). The lower number of plesiosaurs and higher number of ichthyosaurs and sea turtles in more northerly areas of Australia indicates a preference for colder areas in plesiosaurs.[53]

Restoration of the New Zealand Kaiwhekea

Several Late Cretaceous oceanic plesiosaurs and mosasaurs have been discovered in New Zealand and Antarctica, with some, such as Mauisaurus, being endemic, while others, such as Prognathodon, having a cosmopolitan distribution.[54][55] Elasmosaurs and pliosaurs are known from one to three species from this area. The discovery of three cryptoclidids in the Southern Hemisphere–Morturneria from Antarctica, Aristonectes from South America, and Kaiwhekea from New Zealand–indicates a diversification of the family in the Late Cretaceous of this region and perhaps an increasing productivity of the early Southern Ocean.[56]

Pterosaurs

Pterosaurs similar to the Brazilian Anhanguera (above) inhabited the Eromanga Sea region

Two clades of pterosaurs are represented in Early Cretaceous Australia,

archaeopterodactyloids to survive into the Cretaceous. The only pterosaur tooth remains discovered in Australia deriving from the Early Cretaceous belong to Mythunga and a possible Late Cretaceous anhanguerid. Mythunga is estimated to have had a 4.5-meter (15 ft) wingspan, much larger than any other archaeopterodactyloid discovered, though it is possible the pterosaur is more related to the Anhangueridae or Ornithocheiridae.[57][58][59] However, pterosaur remains existing in what were the non-polar regions of Australia, given their ability to migrate by air, perhaps did not need to cross a land bridge through the polar regions to arrive there, meaning they did not ever inhabit the South Polar region.[16][34]

Of the Late Cretaceous pterosaurs, only the remains belonging to the family Azhdarchidae–found in the Carnarvon and Perth basins in Western Australia–were assigned to a taxon. A possible representative of Ornithocheiridae was found in Late Cretaceous Western Australia, though the family was previously thought to have gone extinct in the Early Cretaceous.[57][58][59]

Mammals

Restoration of Steropodon

Seven mammals have been discovered from Early Cretaceous Australia: an undescribed

Bishops, Steropodon, and Corriebaatar; all of which were endemic to Australia during this time. It is likely mammals crossed the Antarctic land bridge between Australia and South America in the Early Cretaceous, and likely the ancestors of the endemic mammals of Australia arrived during the Jurassic across the supercontinent Pangaea.[60][61]

Invertebrates

Several fossils of

Mangaotanean Monro Conglomerate was situated at 68°S and provided fossils of Helastia sp.,[62] and the crab Hemioon novozelandicum was found in the Swale Siltstone, located at 76°S during the late Albian.[63] Several specimens of insects were also found in the Tupuangi Formation of the Chatham Islands at a latitude of 79°S during the Cenomanian to Turonian.[64]

See also

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Further reading

External links

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