Redescription of the type specimens for the Late Jurassic rhynchocephalian Opisthias rarus and a new specimen of Theretairus antiquus from Quarry 9, Morrison Formation, Wyoming, USA

摘要

We reexamine and redescribe the type specimens and other associated material of the Late Jurassic rhynchocephalian Opisthias rarus, from Quarry 9 of the Morrison Formation. We rediscover and describe a fragment of rock matrix belonging to the holotype that is presented for the first time, and we also comment on undescribed material from Quarry 9 that possibly belongs to an unnamed species of Opisthias. In addition, we describe a new specimen of Theretairus antiquus that helps to reject its proposed status as a junior synonym of O. rarus, and contributes to increase the knowledge of the microvertebrate fauna of the Late Jurassic of North America.