MORPHOLOGICAL DESCRIPTION OF ORBICULOIDEA (LINGULIDA: DISCINIDAE) SPECIMENS FROM THE CARBONIFEROUS-PERMIAN BOUNDARY IN PARANÁ BASIN, SOUTHERN BRAZIL

Discinids are inarticulate brachiopods of exclusive marine habit. A well-known genus of this group is _Orbiculoidea_, whose records extends from Ordovician to Permian. This study aims to review the morphology of the Budó facies (Paraná Basin, Rio Grande do Sul state) discinids. This record has been attributed to the late Carboniferous-early Permian transition. Up until now, species from the Carboniferous-Permian boundary of Paraná Basin were attributed to _O. maricaensis_ and _O. guaraunensis_. Unfortunately, they were only briefly described, based on features that are not relevant to establish a trusty diagnosis. We have analyzed 100 specimens, 54 of them dorsal valves and 48 ventral ones. The shell is 1-7 mm wide and subcircular in outline. Shell width equals shell length; occasionally length exceeds the width. Shell height is about 0.7 mm. Dorsal valve is slightly convex, with a rounded apex. The apex is subcentral, closer to the posterior margin of the shell. The posterior slope is slightly concave, while the anterior slope is slightly convex. Ventral valve is flat, with central apex. The pedicle track is narrow, extending from the apex to near the posterior margin. No visceral areas were observed. The ornament consists of concentric rugellae, present on both valves, including on the apex. The rugellae are evenly spaced, low, 8-15 per mm. The interspaces bear thinner, more subtle concentric lines. Some specimens do not present the pedicle track well delimited, but a triangular opening instead, this feature is particularly common on flattened specimens. We suggest that these are taphonomic derived deformations (i.e., preservational artifacts), caused by compression on the valves, after their disarticulation and final burial. These features are enough to classify the specimens as _Orbiculoidea_. However, diagnostic characters that could define the species are missing, thus we are referring them as _Orbiculoidea_ sp. The present project is under development and the taphonomic and taxonomic views will be considered while reevaluating the holotypes and other specimens, in order to attribute new names or confirm the validity of the existing ones. [FAPESP 2018/20426-6][FAPESP 2017/10956-5].