Tanaocrossus

Tanaocrossus (Ancient Greek for "outstretched fringe", referring to its distinctive dorsal fin) is an extinct genus of primitive freshwater ray-finned fish that inhabited southwestern and eastern North America during the Late Triassic period. It contains a single known species, T. kalliokoskii, known from the United States, with indeterminate species also known.^[1][2]

Tanaocrossus Temporal range: Carnian–Rhaetian PreꞒ Ꞓ O S D C P T J K Pg N
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	†Scanilepiformes
Genus:	†Tanaocrossus Schaeffer, 1967
Species:	†T. kalliokoskii
Binomial name
†Tanaocrossus kalliokoskii Schaeffer, 1967

It is primarily known from the Norian to Rhaetian of the Chinle Formation of Colorado and Utah.^[3][4][5] Indeterminate remains are known from the Dockum Group of Texas and New Mexico (Bull Canyon Formation).^[3][6] A single fragmentary but distinctive fossilized fin from the earlier Carnian-aged Doswell Formation of Virginia, which shares the genus's distinctive fin rays, was long the only record from the eastern United States, but a partial specimen was also later identified from the Manassas Sandstone.^[4][6][7]

Morphologically, it can be easily distinguished from other co-occurring fishes by its highly elongated dorsal fin that stretches across its body. Its taxonomic affiliations have long been enigmatic since its description due to its unusual morphology.^[2][7][4] More recent studies have found it to be a scanilepiform, an enigmatic order of fish which were later found to most likely be basal cladistians, which would make it a distant relative of modern bichirs.^[8][9] A 2008 study questioned this assignment and instead reclassified it to the Perleidiformes, but this has been disputed by later studies.^[10][8][11]

It was likely a slow-moving fish that inhabited quiet waters, with its dorsal fin allowing for undulation while swimming.^[2]