Chibanian



The Chibanian, more widely known as Middle Pleistocene (its previous informal name), is an age in the international geologic timescale or a stage in chronostratigraphy, being a division of the Pleistocene Epoch within the ongoing Quaternary Period. The Chibanian name was officially ratified in January 2020. It is currently estimated to span the time between 0.770 Ma (770,000 years ago) and 0.129 Ma (129,000 years ago), also expressed as 770–126 ka. It includes the transition in palaeoanthropology from the Lower to the Middle Paleolithic over 300 ka.

The Chibanian is preceded by the Calabrian and succeeded by the proposed Tarantian. The beginning of the Chibanian is the Brunhes–Matuyama reversal, when the Earth's magnetic field last underwent reversal. Its end roughly coincides with the termination of the Penultimate Glacial Period and the onset of the Last Interglacial period (corresponding to the beginning of Marine Isotope Stage 5).

The term Middle Pleistocene was in use as a provisional or "quasi-formal" designation by the International Union of Geological Sciences (IUGS). While the three lowest ages of the Pleistocene, the Gelasian, Calabrian and Chibanian have been officially defined, the Late Pleistocene has yet to be formally defined.

Definition process
The International Union of Geological Sciences (IUGS) had previously proposed replacement of the Middle Pleistocene by an Ionian Age based on strata found in Italy. In November 2017, however, the Chibanian (based on strata at a site in Chiba Prefecture, Japan) replaced the Ionian as the Subcommission on Quaternary Stratigraphy's preferred GSSP proposal for the age that should replace the Middle Pleistocene sub-epoch. The "Chibanian" name was ratified by the IUGS in January 2020.

Climate
By early Middle Pleistocene, the Mid-Pleistocene Transition had changed the glacial cycles from an average 41,000 year periodicity present during most of the Early Pleistocene to a 100,000 year periodicity, with the glacial cycles becoming asymmetric, having long glacial periods punctuated by short warm interglacial periods. Millennial-scale climatic variability continued to be highly sensitive to precession and obliquity cycles.

The late Middle Pleistocene was a time of regional aridification in the Levant, with a shallow lake covering what is now the Shishan Marsh drying and developing into a marsh.

Along the northwestern Australian coast, the intensification of the Leeuwin Current resulted in an expansion of reefs coincident with the Great Barrier Reef's formation.

Events
The Early-Middle Pleistocene boundary saw the migration of true horses out of North America and into Eurasia. Also around this time, the European mammoth species Mammuthus meridionalis became extinct and was replaced by the Asian species Mammuthus trogontherii (the steppe mammoth). This was coincident with the migration of the elephant genus Palaeoloxodon out of Africa and into Eurasia, including the first appearance of species like the European straight-tusked elephant (Palaeoloxodon antiquus). With the extinction of Sinomastodon in East Asia at the Early-Middle Pleistocene boundary, gomphotheres became completely extinct in Afro-Eurasia, but continued to persist in the Americas into the Late Pleistocene. There was a major extinction of carnivorous mammals in Europe around the Early-Middle Pleistocene transition, including the giant hyena Pachycrocuta. The mid-late Middle Pleistocene saw the emergence of the woolly mammoth (Mammuthus primigenius), and its replacement of Mammuthus trogontherii, with the replacement of M. trogontherii in Europe by woolly mammoths being complete by around 200,000 years ago. The last member of the notoungulate family Mesotheriidae, Mesotherium, has its last records around 220,000 years ago, leaving Toxodontidae as the sole family of notoungulates to persist into the Late Pleistocene. During the late Middle Pleistocene, around 195,000-135,000 years ago, the steppe bison (the ancestor of the modern American bison) migrated across the Bering land bridge into North America, marking the beginning of the Rancholabrean faunal stage. Around 500,000 years ago, the last members of the largely European aquatic frog genus Palaeobatrachus and by extension the family Palaeobatrachidae became extinct.

Palaeoanthropology
The Chibanian includes the transition in palaeoanthropology from the Lower to the Middle Paleolithic: i.e., the emergence of Homo sapiens sapiens between 300 ka and 400 ka. The oldest known human DNA dates to the Middle Pleistocene, around 430,000 years ago. This is the oldest found,.

After analyzing 2,496 remains of Castor fiber (Eurasian beaver) and Trogontherium cuvieri found at Bilzingsleben in Germany, a team of scientists concluded that, around 400 ka, hominids in the area hunted and exploited beavers. They may have been targeted for their meat (based on cut marks on the bones) and skin.