Opisthiamimus

{{Short description|Extinct genus of reptiles}}

{{Automatic taxobox

| fossil_range = {{fossilrange|Tithonian}}

| image = Opisthiamimus.svg

| image_caption = Restored skull in lateral view

| taxon = Opisthiamimus

| authority = DeMar, Jones & Carrano, 2022

| type_species = †Opisthiamimus gregori

| type_species_authority = DeMar, Jones & Carrano, 2022{{Cite journal | doi = 10.1080/14772019.2022.2093139| title = A nearly complete skeleton of a new eusphenodontian from the Upper Jurassic Morrison Formation, Wyoming, USA, provides insight into the evolution and diversity of Rhynchocephalia (Reptilia: Lepidosauria)| journal = Journal of Systematic Palaeontology| volume = 20| issue = 1| pages = 779–786 | year = 2022| last1 = DeMar | first1 = D. G. | last2 = Jones | first2 = M. E. H. | last3 = Carrano | first3 = M. T. | doi-access = free | hdl = 2440/136608 | hdl-access = free }}

}}

Opisthiamimus ({{lit|Opisthias mimic}}) is an extinct genus of small-bodied eusphenodontian rhynchocephalian from the Upper Jurassic Morrison Formation of Northern Wyoming, United States. The type species, O. gregori, is known from four specimens, which together preserve a nearly complete skeleton. It is amongst the smallest known rhynchocephalians, with a skull length of {{Convert|2.05|cm|in}} and a snout–vent length (the length from the tip of the snout to the cloaca) of around {{Convert|8.5|cm|in}}. The genus was described in 2022 by David DeMar, Marc E. H. Jones, and Matthew Carrano. It was found that it was only distantly related to Eilenodon, Theretairus and Opisthias, the other known rhynchocephalians from the Morrison Formation.