''Wakinoichthys'' is a small freshwater fish from the
Early Cretaceous
The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...
(
Albian
The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...
) of
South Korea
South Korea, officially the Republic of Korea (ROK), is a country in East Asia. It constitutes the southern half of the Korea, Korean Peninsula and borders North Korea along the Korean Demilitarized Zone, with the Yellow Sea to the west and t ...
and
Japan
Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
. Two species are currently known, ''W. aokii'' and ''W. robustus''.
History and naming
The first specimens of ''Wakinoichthys'' have been discovered from the Gamou Formation (referred to as Third Formation) of the Wakino Subgroup, a part of the Kanmon Group from
Kitakyushu
is a Cities of Japan, city located in Fukuoka Prefecture, Japan. As of June 1, 2019, Kitakyushu has an estimated population of 940,978, making it the second-largest city in both Fukuoka Prefecture and the island of Kyushu after the city of Fuk ...
, Japan. Starting in 1977 Japanese and Korean researchers have cooperated in excavating fossils from the
Jinju Formation
The Jinju Formation (), also known as the Dongmyeong Formation () in some literature is an Early Cretaceous geologic Formation (geology), formation in South Korea.Wakino Subgroup and ichthys, the Greek word for fish. ''W. aokii'' was named for Mr. Tateyu Aoki, who collected and donated the specimens to the Kitakyushu Museum and Institute of Natural History, while ''W. robustus'' was named for its more robust build compared to its relative. The Jinju Formation is dated to the
Albian
The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...
age, and it is likely that the Gamou Formation is around the same age.
Description
''Wakinoichthys aokii'' was a proportionally long and slender fish, reaching lengths between which are 6.4 times longer than its body depth. The median fins are positioned relatively posterior and the dorsal fin's origin lies behind that of the anal fin. The base of the dorsal fin is about half the length of the anal fin's base. The pectoral fins are elongated with a stout first pectoral fin ray. The pelvic fin is located at the centre of the abdoman and the caudal fin is forked. ''Wakinoichthys'' had long lower jaws that housed small canine-like teeth, while those of the upper jaw were larger and more pronounced. The endopterygoid meanwhile housed villiform teeth.
''W. robustus'' differs from ''W. aokii'' due to its more robust build. Its total body length is only 3.8 times longer than its body depth and with a proportionally bigger head. The dorsal outline of the skull is slightly convex. The holotype specimen of ''W. robustus'' is long, while the other specimen are typically smaller.
Phylogeny
Yabumoto assigns ''Wakinoichthys'' to the order of the
Osteoglossiformes
Osteoglossiformes , meaning "bony tongues" in Ancient Greek, is a relatively primitive order of ray-finned fish that contains two sub-orders, the Osteoglossoidei and the Notopteroidei. All of at least 245 living species inhabit freshwater. They ...
, which includes the modern arowana, butterflyfish, elephantfish and knifefish based on a series of characters such as the position of the median fins, dentition of the premaxilla and dentary and villiform endopterygoid teeth among other morphological traits.