A WOODY LYCOPSID STEM FROM THE NEW ALBANY SHALE (LOWER MISSISSIPPIAN) OF KENTUCKY

A lycopsid axis from the New Albany Shale (Sanderson Formation) of Kentucky is described. The stem, which branches dichotomously, is 45 mm in diameter and is characterized by a relatively narrow parenchymatized protostele, a 3.0 mm-thick cylinder of secondary xylem, a tripartite cortex, and a periderm that is more than 5.0 mm thick. The secondary xylem is composed of uniseriate and biseriate vascular rays and narrow tracheids with scalariform wall thickenings on both radial and tangential walls. The periderm is characterized by elongate, thick-walled cells, some of which broaden tangentially in the outer part of the tissue forming zones that appear wedge-shaped in cross section. Surface features of the axis, including leaf bases, are not preserved. The stem is tentatively regarded as a member of the Lepidodendrales in accordance with the numerous anatomical characters that it shares with more recent representatives of the order. Because the external morphology is not known, however, the possibility exists that the axis corresponds to a protolepidodendralean taxon currently known only from compression and/or impression remains or some other nonlepidodendralean plant that produced secondary xylem. The extremely narrow profile of the secondary xylem tracheids (relative to other arborescent lycopsids) is interpreted as evidence that the plants grew in a habitat that was substantially drier than the Upper Carboniferous coal swamps. THE LEPIDODENDRALES is an extinct order of arborescent lycopsids known primarily from Carboniferous fossils. Stems included within this order exhibit unique, helically arranged leaf cushions and histologically distinct secondary xylem and periderm. Coincident with the formation of coastal peat swamps during the early part of the late Carboniferous, the lepidodendraleans underwent a period of extensive diversification, becoming one of the dominant vascular plant groups of the Upper Paleozoic. The plants were exceptional in size, number, and morphology and occupied a variety of ecological zones within the coal swamps (DiMichele and Phillips, 1985); the obvious evolutionary success of the group has been attributed to the presence of a suite of unusual vegetative (DiMichele and Phillips, 1985; Cichan, 1985, 1986) and reproductive (Phillips, 1979; Thomas, 1981) features. Despite the fact that the Lepidodendrales is one of the best-understood plant taxa of the Carboniferous, relatively little is known about the early evolutionary history of the group, in particular the phylogenetic events that oc' Received for publication 29 December 1986; revision accepted 11 March 1987. The research was supported in part by funds provided by the National Science Foundation (BSR-8614588) to M.A.C. and (DEB-8001803) to C.B.B. 4Michael Cichan died August 16, 1987. curred prior to the invasion ofthe coal swamps. The group reputedly arose during the late Devonian from plants with a Lepidodendropsistype morphology (Thomas, 1978), and it has been suggested that Valmeyerodendron, Cyclostigma, and Lepidosigillaria represent morphological intermediates between the Upper Devonian plants and the more massive Upper Carboniferous representatives (Stewart, 1983). Most of the uncertainty surrounding the early evolution of the Lepidodendrales can be attributed to the scarcity of anatomically preserved fossils from the late Devonian and earliest Carboniferous. Although compression and impression material from these times is known in some detail (Jennings, 1972; Chitaley, 1982) and has provided important information relative to stem morphology, anatomically preserved axes are generally required to unequivocally determine the presence of secondary tissues and thus establish lepidodendralean affinities (Thomas, 1978; DiMichele, 1983). In this contribution, a permineralized lycopsid stem from the New Albany Shale (Lower Mississippian) of Kentucky is described. The axis exhibits relatively thick zones of secondary xylem and periderm, both of which are histologically similar to tissues found in significantly younger members of the Lepidodendrales. In this respect, the axes represent the oldest, certifiably woody lycopsid fossils