Unlocking the architecture of ancient charophyte fructifications: The utricle of Sycidium from the Upper Devonian of Armenia

Sycidium is one of the most emblematic charophyte genera of the Palaeozoic, characterised by its peculiar morphology, temporally persistent fossil record and cosmopolitan distribution. In this study, we present novel insights into the bilateral symmetry of the utricle, based on a previously overlooked character observed in a wellpreserved assemblage of Sycidium cf. spinuliferum from the Upper Devonian of Armenia. Notably, a distinct cell wall gap is present between adjacent polygonal cells within a row, facilitating intercellular connectivity and suggesting that these rows functioned as a cohesive structural unit. This configuration supports a revised symmetry model for the utricle, expressed as (2-1-3-1-2) × 2, where the numbers denote groups of cell rows interconnected via wall gaps. The Armenian material further shows that the presumed oospore was directly enclosed within the utricle, lacking a gyrogonite. Small canals in the poles of polygonal cells may have enabled limited exchange between the egg cell and the external environment through the thick utricle wall. The prominent central canal is interpreted as the last repository of the cytoplasm during the final stages of a centripetal calcification. Additionally, this study documents a previously unrecognised mode of calcification in charophytes characterised by a spongy microstructure. These findings collectively reinforce the hypothesis that Sycidium represents an independent evolutionary lineage within the Charophyta.