Gilliodus is an extinct genus of eugeneodontid cartilaginous fish from the Carboniferous of North America. Two species, G. orvellei and G. peyeri, are known. The postcranial skeleton of the genus is identical in appearance to the distantly related caseodontid "Caseodus", and the genus is differentiated by the anatomy of its teeth. An isolated tooth from the Late Devonian of Poland has been suggested to belong to "Gilliodus", but later authors have concluded that this tooth more likely belonged to a different order of fishes.
Discovery and naming.
"Gilliodus" is known from fossils of Late Carboniferous (Moscovian to Kasimovian) age, found in shales in the Midwestern United States. "G. orvellei" is known from Indiana, while "G. peyeri" is known from both Indiana and Nebraska. The genus and both species were described in 1981 by paleontologist Rainer Zangerl. The genus name and the name of the type species honor Orville "Gillie" Gilpin, who was a fossil curator at the Field Museum of Natural History.
Additional teeth from the Late Carboniferous of Colorado may have belonged to "G. peyeri", and a tooth from the Late Devonian of the Świętokrzyskie Mountains, Poland have been suggested to belong to the genus as well. However, these Devonian remains have since been reclassified as those of an unrelated fish in a different taxonomic order by Michal Ginter and coauthors. 
Description.
The teeth of "Gilliodus" consisted of both rows of rectangular teeth along the lateral edges of the jaws, and a set of teeth along the midline of the lower jaw termed a tooth-whorl, which was similar to that of "Caseodus". The lateral teeth differed between the two species: in "G. peyeri" they were blade-like while in "G. orvellei" they were not. Both species are united by possessing well-developed ridges on their lateral teeth. The teeth of "Gilliodus" were covered in an outer layer of enameloid. The lower jaws (Meckel's cartilages) of both species were not fused at their anterior end. The postcranial skeleton of "Gilliodus" was identical in appearance to that of the caseodontid "Caseodus".
Classification.
"Gilliodus" is a member of the order Eugeneodontiformes (also spelled Eugeneodontida), and more specifically the family Eugeneodontidae. Rainer Zangerl considered the genus to be the sister taxon of "Eugeneodus", based on morphological analysis, in his 1981 description of the family and order.
Paleobiology.
In life, "Gilliodus" were nektonic carnivores. Based on the enameloid coatings on its teeth, Zangerl suggested that "Gilliodus" had a rapid rate of tooth-replacement.