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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: | |
|---|---|
Scientific classification  | |
| Domain: | Eukaryota |
| 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]
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