Furcacauda
According to the Systema Naturae 2000 Taxonomic classification, Furcacauda is a genus within the family of Furcacaudidae, within the order Thelodonti, the Class Agnatha, and the Subphylum Vertebrata. Canadian fossils give rise to the Furcacaudiformes during the Devonian and Silurian time periods. Furcacaudiformes were deep water jawless vertebrates with symmetrical fork and lobed-finned tails and smaller than typical loganellid and nikoliviid thelodonti scales.
Furcacaudiformes are noted to having a laterally compressed body, large anterior eyes, slightly posterior, lateral, and vertical to a small mouth, and a condensed curved row of branchial openings (gills) directly posterior to the eyes. Many but not all had laterally paired fins. Wilson and Caldwell also note the presence of a caudal peduncle and a long caudal fin made of two large lobes, one dorsal and one ventral separated by 8 to 14 smaller intermediate lobes, giving the appearance of a striated half-moon shaped tail resembling the tail of a heterostracan. A large square cavity within the gut connecting a small intestine to a anal opening lead many to believe that it is this genus that exhibits the first vertebrate stomach.
According to Wilson and Caldwell their discovery, based on sediment infillings of fossils of the Furcacauda heintze, gives credence to the evolutionary development of stomach before jaws.
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