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15 Modern TraiTs in early Mesozoic sphenophyTes: The EquisEtum-like cones oF spaciinodum collinsonii wiTh in siTu spores and elaTers FroM The Middle Triassic oF anTarcTica andrew B. schwendemann, Thomas n. Taylor, edith l. Taylor, Michael krings, and Jeffrey M. osborn Structurally preserved cones of the early Middle Triassic sphenophyte Spaciinodum collinsonii have been discovered within permineralized peat from Fremouw Peak, Antarctica. Cones consist of whorls of peltate sporangiophores bearing approximately 10 sporangia each. Spores have a perispore and four elaters with spatulate ends, making Spaciinodum the earliest known Triassic sphenophyte with elater-bearing spores. These equisetalean cones occur alongside the vegetative stems, leaves, and dormant buds of Spaciinodum collinsonii. This close association and the absence of other sphenophytes at the locality indicate that the various plant organs comprise a single species. On the basis of information that has recently become available for Spaciinodum collinsonii, this taxon can today be regarded as one of the best-understood Mesozoic sphenophytes. The morphology and anatomy of S. collinsonii correspond to those of the modern Equisetum subgenus Equisetum, which suggests that the origin of the extant subgenus Equisetum dates as far back as 240 million years ago. Fossil remains previously described as reproductive axes of Spaciinodum are reinterpreted as vegetative axes with a fungal infection. The species diagnosis is therefore emended on the basis of the discovery of new reproductive axes that differ significantly from those previously defined. The Sphenophyta are a phylum of pteridophytes with a rich fossil history dating back to the Devonian, making it one of the most ancient lineages of plants. The group reached its peak diversity during the Late Carboniferous (Pennsylvanian), a time when most sphenophytes were arborescent. Only herbaceous species, however, survived into the Mesozoic (or perhaps not, see Schweitzer et al. 1997) and persist to the present. Much of our understanding concerning the evolutionary history of this introduction 2 Schwendemann, T. N. Taylor, E. L. Taylor, Krings, and Osborn 16 group is due to permineralized specimens found in Carboniferous coal balls being correlated with the compression and impression fossils. The ability to characterize the Carboniferous sphenophytes on the basis of their morphology and anatomy has led to increased understanding of their taxonomy and ecology (e.g., DiMichele and Phillips 1994; DiMichele et al. 2005). In contrast, although several Mesozoic sphenophytes have been described, relatively little is known about their phylogenetic relationships, how they evolved from Paleozoic forms, and how they gave rise to the extant representatives of the group (Taylor et al. 2009). This gap in our understanding is largely due to the lack of permineralized sphenophytes from the Mesozoic. To date, only a single anatomically preserved sphenophyte has been described from this era based exclusively on permineralized specimens, Spaciinodum collinsonii. Spaciinodum collinsonii is a permineralized sphenophyte from the lower Middle Triassic of the Fremouw Formation in Antarctica (Osborn and Taylor 1989; Osborn et al. 2000). The species was originally described from aerial stems and rhizomes, and the taxon is characterized by jointed and ribbed stems with diagnostic pith canals, carinal canals, and vallecular canals that are restricted to nodes (Osborn and Taylor 1989). Osborn et al. (2000) later described reproductive remains in organic association with the vegetative stems. The reproductive axes were described by Osborn et al. (2000) as having a vascular system consisting of 31 to 33 collateral vascular bundles that are continuous through successive nodes and internodes . This vascular condition is found in extant Equisetum cones (Browne 1912, 1915, 1920, 1933, 1941; Barratt 1920; Page 1972), but not in the vegetative axes (Golub and Wetmore 1948a, 1948b; Bierhorst 1959; Page 1972). Sporangia of S. collinsonii were described as occurring in a single whorl attached to the axis in association with cortical chambers, not as occurring on peltate sporangiophores, as in extant Equisetum and the majority of Mesozoic sphenophytes. Cell layers of the sporangial wall were unidentifiable as a result of preservation; however, the remains of a tapetal membranelike layer were suggested. Sporangia were reported as containing abundant spores averaging 10 µm in diameter. Spores were described as spheroidal, with rugulate surface ornamentation and a sporoderm averaging 1.0 µm in thickness. New evidence on the anatomy of dormant buds and vegetative axes of Spaciinodum (Ryberg et al. 2008), as well as additional recently discovered reproductive specimens have cast doubt on the interpretation of Osborn et al. (2000). The objective of our study is to describe recently discovered reproductive axes and spores from the Fremouw Peak locality that are comparable to those of extant...

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