Unexpected diversity of Orbiculoidea in the northern Paraná Basin: Insights into Devonian faunal dynamics in Gondwana

Recent fossil finds in the northern Paraná Basin (Brazil) allow us to reconsider the paleobiogeographic configuration of the Devonian (Pragian–Emsian) of southwestern Gondwana. Expanded distribution of Orbiculoidea xinguensis , associated with two new species O. matogrossensis sp. nov. and O. triangularis sp. nov., reveal a previously underestimated diversity in this basin. Morphometric analyses and comparative taxonomy were used to differentiate species within the Orbiculoidea genus, highlighting diagnostic features, including apex positioning, valve contour, and rugella patterns. O. triangularis sp. nov. presents distinct morphospaces from O. matogrossensis sp. nov. and O. xinguensis , possibly due to environmental stresses associated with transgression. The presence of O. xinguensis , originally restricted to the Lochkovian of the Amazon Basin, suggests initial marine connectivity between these regions during the Pragian-Emsian, facilitated by transgressive pulses that enabled faunal exchange. The co-occurrence of cosmopolitan ( O. baini and O. excentrica ) and endemic taxa denote a regime of moderate provincialism, with niche partitioning and speciation. The peak of diversity of the genus during the Pragian–Emsian is followed by a sharp decline in the Eifelian–Givetian in South America, and is directly correlated with transgressive events and habitat fragmentation during the Kačák Event, triggering the Lilliput effect in O. baini and O. excentrica . While marginal intracratonic basins faced local extinctions, they acted as refugia ( e.g. , the Bokkeveld Formation of South Africa). The Paraná Basin was an active center of speciation for Orbiculoidea , promoting faunal exchange and biogeographic connectivity of Gondwana during the Devonian.