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Boreosomus; Temporal range: Early Triassic to Middle Triassic PreꞒ Ꞓ O S D C P T J K Pg N
	Boreosomus gillioti: fossil mold and latex cast
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	†Ptycholepiformes
Family:	†Ptycholepididae
Genus:	†Boreosomus; Stensiö, 1921
Type species
	†Acrolepis arctica; Woodward, 1912
Synonyms
	Diaphorognathus Brough, 1933;

Boreosomus (meaning: "boreal body") is an extinct genus of Triassic marine ray-finned fish. It was first described from the Arctic island of Spitsbergen (Svalbard, Norway), hence its genus name, but was later also discovered in other parts of the world. The type species is Boreosomus arcticus (= Acrolepis arctica Woodward, 1912).

Classification

*Boreosomus* slab and counterslab fossils at the Geological Museum in Copenhagen

Boreosomus belongs to the family Ptycholepidae (= Boreosomidae/Chungkingichthyidae). Other genera of this family are Acrorhabdus (Spitsbergen), Ardoreosomus (Nevada, United States), Chungkingichthys (China), Ptycholepis (global) and Yuchoulepis (China).^[1]

Some studies recover Boreosomus as a potential chondrostean.^[2]

Description

A characteristic feature of Boreosomus and other ptycholepids is the dorsal fin, which inserts at the level of the pelvic fins in the middle portion of the body. Most contemporary ray-fins have their dorsal fin in a more posterior position, often opposite to the anal fin. Also typical for ptycholepids are the somewhat rectangular, horizontally arranged suborbital bones.^[3]

Boreosomus gillioti could reach a body length of about 10–20 cm (3.9–7.9 in). Caudal fin was divided. Scales were strong and rectangular.^[4]

Fossil record

Boreosomus had a worldwide distribution during the Early Triassic and is also known from the Middle Triassic. Fossils of Boreosomus were found, apart from Spitsbergen (Svalbard), in Greenland, Madagascar, China (Shaanxi), Spain (Catalonia), United States (Arizona), and Canada (British Columbia).^[5]^[6]

Species

†Boreosomus arcticus (Woodward, 1912) [Acrolepis arctica Woodward 1912] (type species) - Early Triassic (Induan) of Svalbard
†Boreosomus gillioti (Priem, 1924) [Diaphorognathus gillioti (Priem 1924); Gyrolepis gillioti Priem 1924] - Early Triassic (Induan) of Madagascar
†Boreosomus piveteaui Nielsen, 1942^[3] - Earliest Triassic (Induan) of Greenland
†Boreosomus reuterskioeldi Stensiö, 1921 - Early Triassic (Induan) of Svalbard
†Boreosomus scaber Stensiö, 1921 - Early Triassic (Spathian) of Svalbard

Indeterminate species are known from the Early Triassic of Canada (British Columbia) and China (Guizhou), as well as the Middle Triassic of Svalbard, Spain, and possibly the United States (Arizona).^[5]

The species B. merlei is now placed in Australosomus.^[7]

References

^ Romano, Carlo; López-Arbarello, Adriana; Ware, David; Jenks, James F.; Brinkmann, Winand (2019). "Marine Early Triassic Actinopterygii from the Candelaria Hills (Esmeralda County, Nevada, USA)". Journal of Paleontology. 93 (5): 971–1000. Bibcode:2019JPal...93..971R. doi:10.1017/jpa.2019.18. S2CID 155564297.
^ Near, Thomas J; Thacker, Christine E (18 April 2024). "Phylogenetic classification of living and fossil ray-finned fishes (Actinopterygii)". Bulletin of the Peabody Museum of Natural History. 65. doi:10.3374/014.065.0101.
^ ^a ^b Nielsen, Eigil (1942). "Studies on Triassic fishes from East Greenland 1. Glaucolepis and Boreosomus". Palaeozoologica Groenlandica. 1: 1–403.
^ The Fossil Forum
^ ^a ^b "PBDB". paleobiodb.org. Retrieved 2024-04-01.
^ Romano, Carlo; Koot, Martha B.; Kogan, Ilja; Brayard, Arnaud; Minikh, Alla V.; Brinkmann, Winand; Bucher, Hugo; Kriwet, Jürgen (2016). "Permian-Triassic Osteichthyes (bony fishes): diversity dynamics and body size evolution". Biological Reviews. 91 (1): 106–147. doi:10.1111/brv.12161. PMID 25431138. S2CID 5332637.
^ Brinkmann, W.; Romano, C.; Bucher, H.; Ware, D.; Jenks, J. (2010). "Palaeobiogeography and stratigraphy of advanced Gnathostomian fishes (Chondrichthyes and Osteichthyes) in the Early Triassic and from selected Anisian localities (report 1863-2009): Literaturbericht". Zentralblatt für Geologie und Paläontologie, Teil II. 2009 (5/6): 765–812. doi:10.5167/uzh-34071. ISSN 0044-4189.