Recovery of a large, articulate portion of Sphenophyllum costae Sterzel from lower Cantabrian strata of the Sydney
Coalfield, Nova Scotia, Canada, demonstrates that it is the largest sphenophyll yet known to inhabit clastic substrates of
Euramerica. The specimen shows four orders of branching, with each axis characterized by whorls of leaves having distinct morphologies. Foliage clearly displays a tendency to become less divided on axes of ascending order. Furthermore, each branch order can be distinguished based on its internodal lengths and widths, which indicates that the species has a marked hierarchy of ramification similar to that found in the calamitalean sphenopsids. Axes of all orders apparently show epidogenetic followed by apoxogenetic growth patterns, which contributes to the high degree of heterophylly in the taxon. Comparison of axes and foliage of S. costae with other members of the genus reveals that sphenophylls from clastic substrates followed a fundamental ontogenetic pattern of growth and architecture, although subtle variations existed between taxa. Climber hooks are documented for the first time in the species, and the presence of these modified leaves supports the interpretation that S. costae formed dense, multi-branched thickets up to 2 m high comprising mutually supportive axes that rested upon, clung to, or entwined with adjacent axes. Palaeoecological and taphonomic evidence suggests that the plant formed as ground cover beneath arborescent medullosalean pteridosperms, and probably occupied well-drained (but moist) clastic habitats such as elevated channel margins or topographically raised hummocks on floodplains.