The Furcacaudiformes, a new order of jawless vertebrates with thelodont scales, based on articulated Silurian and Devonian fossils from northern Canada

A new order of “fork-tailed” agnathans (jawless vertebrates), the Furcacaudiformes, is described from Wenlockian (Silurian) and Lochkovian (Devonian) fossils from the Northwest Territories of Canada. Six new species are classified in five new genera and two new families. One Silurian species is in the genus Pezopallichthys, family Pezopallichthyidae. One Devonian species is described in each of the genera Cometicercus, Drepanolepis, and Sphenonectris. The Devonian genus Furcacauda includes two species, one new and another that was previously described as Sigurdia heintzae Dineley and Loeffler. Preliminary phylogenetic analysis suggests the following relationships within Furcacaudiformes: (Pezopallichthys (Cometicercus (Drepanolepis (Sphenonectris (Furcacauda)))). Furcacaudiforms resemble some previously described thelodonts in structure of their scales, which are of loganellid and nikoliviid types, but differ in having generally smaller scales, as well as in having laterally compressed, hump-backed bodies, large eyes, branchial openings in an oblique, condensed row, a prominent anal notch, and large, nearly symmetrical, forked and lobed tails. The distinctive tail may indicate a close relationship with Heterostraci. A large, barrel-shaped stomachlike chamber dominates the gut and is connected to the anal opening by a short, tapered intestine. Several Devonian species possess paired, ventral ‘fin-flaps’ and/or a dorsal fin; these structures are not universally present and may have been secondarily lost in some lineages.