Sediment-cast calamitalean axes in growth position are one of the most common fossils in the Pennsylvanian coal measures. In this paper, we challenge the long accepted position that these fossils represent "pith casts". If correct, the hypothesis would require the sediment-cast pith to have been surrounded by a cylinder of secondary xylem during life, which later decayed away. However, sedimentary layers and structures developed around upright calamitaleans indicate that fluid flow was interacting directly with the preserved surface of the stem, not a hypothetical woody cylinder that lay external to it. Furthermore, stem diameter-density data for calamitalean stands already lies at the self-thinning threshold, and if actual stem diameters were significantly greater than preserved diameters, this threshold would be significantly exceeded. We also note that measured diameters for upright sediment-cast calamitaleans are more similar to stem diameter data for anatomically preserved calamitalean axes than for pith diameter data from the same axes. Our findings indicate upright calamitaleans are in fact stem-casts and their preservation involved a two-stage process. First, stems were buried in flood-deposited sediments, creating a mold of the external surface of the plant. Second, following near-total decay of the axis, which may have occurred in a matter of weeks under tropical conditions, further sedimentation infilled the mold, forming a cast. As such, the preservation of upright calamitaleans was identical to that for arborescent lycopsids, which are commonly found in the same beds. That said, we stress that some transported sediment-cast calamitalean axes preserved in fluvial channel facies are certainly "pith casts" in the traditional sense, however, their morphologies differ from those specimens traditionally called "pith casts". In this context, axes were buried in a single phase of sedimentation under energetic flow conditions, resulting in the pith becoming sediment-filled. However, intriguingly, a review of such genuine pith casts shows that only a tiny proportion preserves large woody cylinders surrounding the pith. This is not a taphonomic feature, but reflects our contention that the great majority of floodplain-based calamitaleans were reed-like plants with relatively small amounts of secondary xylem. Woody calamitaleans, including large tree forms, are documented almost exclusively from petrifactions, and thus from peat-forming environments (coal balls) and, more rarely, floodplain settings under exceptional conditions of preservation (volcanigenic deposits, for example). These may be difficult to recognize in adpression preservation due to the masking, by wood development, of node-internode features. The differing architectures may reflect adaptations to disturbed and undisturbed environments, respectively.
