A little digging into the literature
reveals the holotype had a different look (Fig. 2).
Colosteus is a sister to the pre-tetrapod, Osteolepis. in the large reptile tree (not yet updated). Which means if Colosteus is a tetrapod (it has a vestigial forelimbs with four fingers), then tetrapods had a second sterile origin and Colosteus is part of that short radiation, not the main line leading to amphibians, reptiles, birds and mammals that ran through Acanthostega and Ichthyostega.
Hook 1983 reports on the history of this specimen:
“The first report of fossil vertebrates from the Ohio Diamond Coal Company Mines at Linton was given by J. S. Newberry in 1856; it included a brief description of Pygopterus scutellatus, a supposedly new species of paleoniscoid fish. Although no figures were supplied or specific specimens adequately described, a general picture of a heavily scaled, elongate form with flattened head and pointed snout was established.”
“In 1869, E. D. Cope erected the batrachian genus Colosteus on the basis of Linton material lent him by Newberry. Cope cited three species, C. crassiscutatus, C. marshii and C. foveatus, and provided measurements by which the type specimens can be identified today. However, Cope later admitted (1871 a, p. 41) that in describing the type species, C. crassiscutatus, he had “overlooked” Newberry’s original account of P. scutellatus, thereby implying that both taxa were inadvertently based on the same specimen. Since Pygopterus was (and is) a valid fish genus, and the material in question was certainly not piscine (Cope, 1873), Cope recognized the proper combination of Colosteus scutellatus (Newberry, 1856) for the type species.”
Here’s a little information
the fish genus, Pygopterus (Permian – Triassic, Fig. 3).
There is much more to this story in Hook 1983.
PDF online here. If there are fossils that demonstrate that some specimens of Colosteus were indeed flat-headed, let me know. The flat-head reconstruction has to have some basis in the fossil record.
Cope ED 1869. Synopsis of the extinct Batrachia, Reptilia, and Aves ofNorth America. Trans. Amer. Phil. Soc., vol. 14, pp. 1-252.
Hook RW 1983. Colosteus scutellatus (Newberry), a primitive temnospondyl amphbian from the Middle Pennsylvanian of Linton, Ohio. American Museum Novitates 2770:1-41. PDF online
Newberry JS 1856. Description of several new genera and species of fossil fishes from the Carboniferous strata of Ohio. Proc. Acad. Nat. Sci. Philadelphia, vol. 8, pp. 96-100.