Pronunciation: koh-shee-SOR-us
Meaning: Koshi lizard
Author/s: Shabata and Azuma (2015)
Synonyms: None known
First Discovery: Katsuyama, Japan
Discovery Chart Position: #892
Koshisaurus katsuyama
Japan's Kitadani Formation in the City of Katsuyama had already yielded Fukuiraptor, Fukuisaurus and Fukuititan by 2015. Then palaeontologists found a fourth dinosaur that not only bucked the trend of being named for Fukui Prefecture but also doubled the Japanese record of styracosternans and proved to be their first hadrosauroid.
Although hadrosauroids are primitive compared to hadrosaurids, they're more advanced than the hadrosauriformes, which are represented in Japan by Fukuisaurus: a close relative of Proa, Iguanodon and Ouranosaurus. Koshisaurus is from the same quarry as Fukuisaurus, and its bones belong to a juvenile, so it came as no surprise when the latter was initially identified as an adult version of the former. However, Koshisaurus is from a higher and presumably younger bonebed, and although there aren't many directly comparable bones, the pair differ in the characteristics of their jaws and teeth.
Koshisaurus is one of the most primitive members of Hadrosauroidea, judging by at least three small horizontal ridges between the primary vertical ridges on the out-facing surface of its upper jaw teeth, and an "antorbital fossa": a depression on each side of its skull, between the eye socket and nostril. Amongst its kind, the likes of Xuwulong, Jinzhousaurus and Altirhinus all have similar teeth, but their ridges are fewer in number. In addition, they lack the fossa: a feature that the most advanced family of hadrosauroids (the hadrosaurids) lost completely. Only one other hadrosauroid sports both grooved teeth and a fossa: China's Equijibus, and while it was initially trumpeted as the basal-most hadrosauroid, it may turn out to be an iguanodont, and they're more archaic still.
Although hadrosauroids are primitive compared to hadrosaurids, they're more advanced than the hadrosauriformes, which are represented in Japan by Fukuisaurus: a close relative of Proa, Iguanodon and Ouranosaurus. Koshisaurus is from the same quarry as Fukuisaurus, and its bones belong to a juvenile, so it came as no surprise when the latter was initially identified as an adult version of the former. However, Koshisaurus is from a higher and presumably younger bonebed, and although there aren't many directly comparable bones, the pair differ in the characteristics of their jaws and teeth.
Koshisaurus is one of the most primitive members of Hadrosauroidea, judging by at least three small horizontal ridges between the primary vertical ridges on the out-facing surface of its upper jaw teeth, and an "antorbital fossa": a depression on each side of its skull, between the eye socket and nostril. Amongst its kind, the likes of Xuwulong, Jinzhousaurus and Altirhinus all have similar teeth, but their ridges are fewer in number. In addition, they lack the fossa: a feature that the most advanced family of hadrosauroids (the hadrosaurids) lost completely. Only one other hadrosauroid sports both grooved teeth and a fossa: China's Equijibus, and while it was initially trumpeted as the basal-most hadrosauroid, it may turn out to be an iguanodont, and they're more archaic still.
(Koshi lizard from Katsuyama)Etymology
Koshisaurus is derived from the Japanese "koshi" (an ancient name for the region now known as Hokuriku, in which its fossils were found) and the Greek "sauros" (lizard).
The species epithet (katsuyama) refers to Katsuyama City.
Discovery
The remains of Koshisaurus were discovered in the Kitadani Formation (the uppermost unit of the Tetori Group), Katsuyama City, Fukui Prefecture, Hokuriku, Honshu Island, Japan, in 2007. The holotype (FPDM V9079) is a partial right upper jaw bone with teeth. Further bones — FPDM V9081 (a vertebra), FPDM V9080 (a piece left pubis) and FPDM V9077 (the underside of a left femur) — were found in the same area in 2010. All bones are modestly sized, and study of growth lines in a thighbone confirmed they belong to a juvenile.
