# Duho Formation

The Duho Formation (Korean: 두호층; Hanja: 斗湖層; RR: Duho-cheung) is a Middle Miocene geological formation in Pohang, South Korea. This formation is deposited in a deep marine environment and is roughly dated to around 15 to 12 million years ago based on various analyses. It is the uppermost unit of the Yeonil Group of Pohang Basin and mainly composed of yellow-brown to dark gray mudstones.

## Geology

The deposition of the Pohang Basin began some time after 21.89 to 21.68 million years ago, corresponding to the Early Miocene, based on zircon U-Pb dating. This deep-marine basin was probably produced by rapid subsidence around 17 million years ago, following the abrupt ending of the strike-slip deformation and volcanic activity, and experienced tectonic inactivity for 5 to 7 million years until the crustal uplift in southeastern Korea around 10 million years ago which caused the sedimentation to cease. The basin was likely a pelagic zone around 17.6 to 11.5 million years ago based on biostratigraphy analysis.
The intrusive basalt of the Yeonil Group from the southern part of the basin is dated to the lower Middle Miocene around 15.16 to 14.82 million years ago based on K-Ar dating, which implies the evolution of back-arc basin in the seas of southeastern Korea, followed by the tectonic inversion which changed the geodynamic setting and basaltic magma composition. Paleomagnetic dating (analysis of the sediments' ancient magnetic fields) of the Duho Formation yielded the age estimate ranging from 14.7 to 11.6 million years ago. The age estimate of the Yeonil Group based on the same technique is around 17.3 to 11.3 million years ago for the lower and upper part respectively with a reliable pole position of 15 million years ago, while biostratigraphy analysis indicated that the age of the Duho Formation likely ranged from 14 to 12 million years ago. It is probably not younger than 11.95 million years ago based on the radiolarians recovered from the formation.

## Paleoecology

Notable fossils from the Duho Formation include a variety of aquatic invertebrates such as ophiuroids, bivalves and mantis shrimps, numerous extant genera of plants, indeterminate remains of toothed whales with some diagnostic to the genus level, and diverse ichthyofauna ranging from small bony fish to large cartilaginous fish like the giant lamniform shark Otodus megalodon which would have been the apex predator of the Miocene seas of Korea. While the fauna consists of species ranging from the shallow coastal waters (neritic and epipelagic zone) to the depths of over 1,000 metres (3,300 ft) (bathypelagic zone), the sediments were probably deposited in a pelagic deep-water setting as evidenced by the paleoecological and bathymetric features of the fossil assemblage.

## Fossil content
### Arthropods

The probable ant fossil named as Aphaenogaster "koreana" in a 2018 conference abstracts is a nomen nudum.
Arthropods of the Duho Formation :
| Genus | Species | Notes |
| Leesquilla | L. bajee L. sunii | Mantis shrimp belonging to the family Squillidae |
| Pohsquilla | P. neonica P. scissodentica | Mantis shrimp belonging to the family Squillidae |
| Squilla | S. sp. | Mantis shrimp belonging to the family Squillidae |
| Scarites | S. sp. | First officially described terrestrial animal (ground beetle) from the deep marine Duho Formation |

### Echinoderms

The probable sea urchin fossil named as Brissopsis "pohangensis" in a 2015 doctoral thesis is a nomen ex dissertationae.
Echinoderms of the Duho Formation :
| Genus | Species | Region | Member | Material | Notes | Image |
| Ophiura | O. pohangensis | | | | Brittle star fossils previously described from the same formation are probably conspecific with O. pohangensis | |

### Molluscs

Molluscs of the Duho Formation :
| Genus | Species | Notes |
| Acharax | A. tokunagai | |
| Conchocele | C. bisecta | |
| Delectopecten | D. peckhami | |
| Lucinoma | L. acutilineatum L. sp. | |
| Macoma | M. incongrua | |
| Mizuhopecten | M. kimurai ugoensis | Reported as Patinopecten kimurai ugoensis, but this species was already moved to a different genus Mizuhopecten by Mausda (1963) |
| Nuculana | N. pennula | |
| Panomya | P. simotomensis | |
| Patinopecten | P. sp. | |
| Phos | P. cf. minoensis | |
| Portlandia | P. cf. gratiosa | |
| Propeamussium | P. tateiwai | |
| Squiresica | S. yooni | Vesicomyid bivalve mollusc fossil originally assigned to as Calyptogena cf. elongata |
| Yoldia | Y. sagittaria | |

### Mammals

Mammals of the Duho Formation :
| Genus | Species | Material | Notes |
| Kentriodontidae | Indeterminate | Partial maxilla (DWFM 10001) | Extinct family of toothed whales |
| Pomatodelphininae | Indeterminate | Partial rostrum and mandible (KIGAM VP 201101) | River dolphin found in marine deposits and related to Pomatodelphis |
| Tursiops | T. sp. | Partial skeleton measuring 70 cm (2.3 ft) long, with a nearly complete skull | Resembles the modern bottlenose dolphin with the estimated complete length of the specimen around 1.5–2 m (4.9–6.6 ft) |
| Kogia | K. sp. | Partial skull measuring 40 cm (1.3 ft) long, associated with the tooth of an adult tiger shark | Resembles the modern pygmy sperm whale and dwarf sperm whale |

### Bony fish

Bony fish of the Duho Formation :
| Genus | Species | Material | Notes |
| Auxis | A. koreanus A. sp. | Two fragmentary, disarticulated specimens distinguished from the extant species by the osteological differences in the skull Partial caudal vertebrae (GNUE322001) associated with a leaf imprint | First and second fossil record of the genus Auxis in Korea |
| Cyclothone | C. duhoensis | | The oldest nominal species of Cyclothone |
| Pleuronectiformes | Indeterminate | | First record of flatfish from this formation |
| Pleuronichthys | P. sp. | | |
| Stenobrachius | S. sangsunii | | Lanternfish |
| Vinciguerria | V. orientalis | | Distinct from modern species of Vinciguerria in terms of caudal skeleton structure |
| Zaprora | Z. koreana | | Second fossil record of the family Zaproridae |

### Cartilaginous fish

The record of Otodus obliquus from this formation is most likely a misidentification of other otodontid or lamniform shark.
Cartilaginous fish of the Duho Formation :
| Genus | Species | Material | Notes |
| Carcharhinus | C. aff. C. plumbeus C. aff. C. amblyrhynchos C. aff. C. altimus | Teeth | |
| Cetorhinus | C. huddlestoni | Gill rakers and oral tooth mold | |
| Cosmopolitodus | C. hastalis C. planus | Teeth | Uncertain whether it is a distinct genus or a junior synonym of Carcharodon |
| Dalatias | D. orientalis | Teeth | Two teeth originally assigned to as D. licha (CNUNHM-F279) and D. cf. licha (CNUNHM-F392) respectively are now the paratypes of D. orientalis |
| Galeocerdo | G. aduncus | Teeth | |
| Hexanchus | H. griseus | Tooth | Miocene record of the modern bluntnose sixgill shark |
| Isurus | I. sp. 1 I. sp. 2 | Teeth | |
| Mitsukurina | M. owstoni | Teeth | Among the oldest known record of the modern goblin shark; one tooth (CNUNHM-F268) was originally assigned to as M. cf. lineata |
| Otodus | O. megalodon | Teeth | Largest known macropredatory shark that ever lived |
| Parotodus | P. benedenii | Tooth | |

### Plants

Plants of the Duho Formation :
| Genus | Species | Notes |
| Acer | A. ezoanum A. huziokae A. nordenskioeldi A. palaeoplatanoides A. pohangense A. protomiyabei A. prototrifidium A. pseudoginnala A. rotundatum (=A. subpictum & A. pictum) | |
| Alangium | A. aequalifolium | |
| Albizia | A. miokalkora | |
| Castanea | C. tanaii | |
| Castanopsis | C. pohangensis | |
| Carpinus | C. hokoensis C. kodairae-bracteata C. miofargesiana C. oblongibracteata C. stenophylla C. subcordata | |
| Cinnamomum | C. lanceolatum | |
| Cryptocarya | C. ennichiensis | |
| Cunninghamia | C. protokonishii | |
| Cyclobalanopsis | C. huziokai C. mandraliscae C. yabei | |
| Dipteronia | D. brownii | |
| Entada | E. mioformosana | |
| Fagus | F. cf. hayate | |
| Firmiana | F. sinomiocenica | |
| Fraxinus | F. oishii F. insularis F. sp. | |
| Keteleeria | K. ezoana | |
| Hemitrapa | H. yokoyamae | Extinct genus of aquatic plants related to water caltrop |
| Lindera | L. gaudini | |
| Liriodendron | L. meisenense | |
| Liquidambar | L. miosinica | |
| Paliurus | P. koreanus | |
| Pasania | P. miohypophaea P. protokonishii | |
| Parrotia | P. fagifolia | |
| Phoebe | P. mioformosana | |
| Picea | P. kaneharai | |
| Pinus | P. miocenica P. cf. prekesiya | |
| Platanus | P. guillelmae | |
| Pseudolarix | P. japonica P. sp. A P. sp. B | |
| Pterocarya | P. asymmetrosa | |
| Rhododendron | R. tatewakii | |
| Tilia | T. asiatica | |
| Zelcova | Z. ungeri | |

### Ichnofossils

Ichnofossils of the Duho Formation :
| Genus | Species | Material | Notes |
| Chondrites | C. isp. 1 C. isp. 2 | Three deep-water ichnofossils | C. isp. 1 and C. isp. 2 are deposited under oxygen-depleted and oxygenated conditions respectively |
| Palaeophycus | P. isp. | | |
| Planolites | P. isp. | | |
| Taenidium | T. isp. | | |