User:The Morrison Man/sandbox:Opisthiamimus

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 2.05 cm and a snout–vent length (the length from the tip of the snout to the cloaca) of around 8.5 cm. The genus was described in 2022 by DeMar, Jones & Carrano. It was found that it was only distantly related to Eilenodon, Theretairus and Opisthias, the other known rhynchocephalians from the Morrison Formation.

History of Discovery
The holotype specimen of Opisthiamimus, USNM PAL 722041, was recovered from the Fox Mesa site in Bighorn County, Wyoming, United States along with the other three specimens attributed to the species. The holotype consists of an articulated skeleton which is nearly complete from the skull to the pelvic region. The site where these four specimens were discovered was found in 1997 by a group of geologists led by Erik Kvale. Work on the site by the Smithsonian Institution began in the year 1999, after which the teams returned for a number of subsequent field seasons, excavating a number of sediment blocks from a 2 x 3 m quarry. During this time the site was named the Fox Mesa locality after a nearby landform.

Due to a difficulty in sediment correlation with other sites and the inability to obtain a radiometric dating for the site, the age of the Fox Mesa beds cannot be judged beyond the temporal range of early Kimmeridgian-Tithonian (~156-145 mya). Accordingly, the age of the fossils that are found here cannot be narrowed down beyond this timeframe either.

Paleoecology
The Fox Mesa site is among very few microfossil assemblages that have yet been found in Morrison strata, and consists of two distinct layers.