CONICET | Buscador de Institutos y Recursos Humanos

The tetrapod assemblage coming from the upper levels of the Los Colorados Formation (late Norian-Rhaetian, La RiojaProvince) has provided a diverse and abundant record of an archosaur-dominated fauna. In particular, the currentlyavailable record shows that crurotarsans is the taxonomically most diverse group, with five described species, the aetosaurNeoaetosauroides Bonaparte, the rauisuchid Fasolasuchus Bonaparte, and the crocodylomorphs HemiprotosuchusBonaparte and Pseudhesperosuchus Bonaparte. In this contribution we provide the reappraisal of an ilium (PULR 076)found in direct association with the holotype of the basal neotheropod Zupaysaurus Arcucci and Coria, from Quebradade Los Jachaleros, upper levels of the formation. This bone was previously recognized as a probable basal saurischian,but clearly not pertaining to the holotype of Zupaysaurus. Nevertheless, we reinterpret this element as belonging to apoposaurid rauisuchian. The bone can not be referred to Dinosauria because it lacks a brevis shelf, brevis fossa, and ahigh and thin iliac blade. By contrast, PULR 076 presents the following synapomorphies of Poposauridae: first sacral ribarticulating extensively with the anteriorly pointing crest of the ilium, supra-acetabular crest with an anteriorly extendingcrest, and a postero-dorsally oriented post-acetabular process. In addition, this specimen exhibits a suite of autapomorphieswhich allow distinguishing it from other archosaurs (thick and laterally convex iliac blade and strongly dorsoventrallylow and slightly medially deflected post-acetabular process) Accordingly, this specimen seems to be a newpoposaurid species which increases the already high diversity of one of the youngest known crurotarsan-bearing assemblages.

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