While it and the rest of its family were historically considered elasmobranchs related to sharks and rays, they are now regarded as holocephalans, a diverse subclass which is today only represented by chimaeras.

Romerodus is known from the Carboniferous and possibly Permian periods of North America, and the only named species, R. orodontus, was discovered in organic shale deposits in the U.S. state of Nebraska.

[1] R. orodontus lived between 307 and 304 million years ago,[2] during the latest Moscovian to Kasimovian stages (described by Zangerl as the equivalent Westphalian D substage)[1] of the Pennsylvanian subperiod.

[5][8]: 238–239 The incomplete but articulated specimen FMNH PF 8522 from the Stark Shale is designated as the holotype of the genus and species, and consists of the front half of the animal preserved in ventral (bottom) view.

Below the pectoral girdles were paired, sternal cartilages of unknown function,[1] which may have been homologous to the unpaired sternum-like structure observed in the related Ornithoprion and Fadenia.

Among eugeneodonts and the potentially related orodonts, the pavement teeth were typically arranged in tightly packed lateral rows along both the upper and lower jaws.

[1][16] Recent findings, however, strongly suggest that the order Eugeneodontida, to which Romerodus belongs, are a lineage of holocephalan (also defined as euchondrocephalan) fish distantly related to living chimaeras.

[1][6][17] Rainer Zangerl's morphological analysis of the group, published in 1981, indicates that R. orodontus is most closely related to Caseodus based on similarities in dentition, as well as the structure of their tails and upper jaws.

The postcranial anatomy of the caseodonts was apparently extremely conserved and varied little between genera, although features in their skulls and teeth indicate they were an ecologically diverse group.

[1]Fadenia Erikodus Ornithoprion Caseodus Romerodus Bobbodus Eugeneodus Gilliodus Campyloprion Parahelicoprion Helicoprion Agassizodus Toxoprion Sarcoprion Syntomodus Lestrodus Edestus Helicampodus Parahelicampodus The Stark Shale, where the type specimen of R. orodontus originated, is believed to have been a marine depositional environment.

[5][23] Upwellings would have caused nutrient-rich conditions in the upper water column, encouraging the growth of algae and other plankton which, as they died, would be deposited on the oxygen-poor seafloor to form shale.

[3][24] These include Cobelodus,[8]: 228–232 Heslerodus,[26] Listracanthus,[8]: 243 [25] several species of well-preserved iniopterygian,[24][27] and other eugeneodonts such as Gilliodus,[1] Agassizodus, and indeterminate genera known only from a pectoral fin or isolated tooth whorls.

It has been proposed that these crustaceans, which are found only in isolated regions, may have been part of a poorly known benthic fauna which was adapted to oxygen-poor deep waters, or alternatively originated from die-offs in a shallower ecosystem nearby.

The deposits which yield Romerodus-like fish are limestones preserving large numbers of unassociated teeth, bones, and fin spines from different species collected together, which may represent mass-death assemblages.

[29] Due to its short jaws and proportionally large tooth whorl, however, Helicoprion may have been more anatomically different from caseodonts than previously assumed,[18] and a subsequent publication has suggested lengths around 7 m (23 ft) are likely the most reasonable.

[29] The Idaho Museum of Natural History displayed murals and a life-sized replica of Helicoprion davidsii,[30] which featured body proportions and anatomy inspired by well preserved caseodonts such as Romerodus.