Abstract
One of the youngest occurrences of anatomically preserved Sphenophyllum Brongn. 1828 is reported from the Permian Motuca Formation, Parnaíba Basin, central-north Brazil. At least 31 stems of this extinct sphenophyte, which are densely interwoven with each other and associated with tiny roots and leaves, occur in a silicified state within the pith of a stem of the arborescent calamitalean Arthropitys barthelii, which was additionally bored by putative xylophagous herbivores. Although restricted within natural limits, the Sphenophyllum plants show different developmental stages that encompass mostly juvenile stems but also a few with initial secondary growth. The stems are circular to elliptical in outline and consist of a central triarch actinostele made of primary vascular tissues and a variable amount of wood. Extraxylary tissues comprise a narrow innermost zone with putative phloem, internal periderm of rectangular cells arranged in radial files, inner parenchymatous cortex, outer sclerenchymatous cortex, and small-celled layers of bounding tissue. Furthermore, Sphenophyllum plants are frequently found as climbers on the trunk surfaces of Psaronius Tietea and Grammatopteris tree ferns, or they are trapped among the adventitious aerial roots of these plants. In addition, Sphenophyllum cauline members were also found dispersed with other plant organs in a particular type of fossil-rich, silicified fluvial sandstone or chert, reflecting rapid fossilisation of parautochthonous remains. The presented fossil assemblages provide evidence of a previously underestimated facet of the ecosystem in seasonally influenced, densely vegetated riparian forests bordering extended distal fluvial discharges in low-latitude Gondwana. Because of largely similar development of major anatomical traits in these late Paleozoic sphenophylls, taxonomic delimitation based on vegetative axes alone remains challenging and highlights the unexpectedly long-term stability of the primary cauline anatomy in particular and vascular architecture in general. As a result, the studied Brazilian specimens can only be identified as Sphenophyllum sp., although we emphasise close relationships to Sphenophyllum thonii, a nearly worldwide distributed species during the early Permian.