REINVESTIGATION OF THE IRIDOPTERIDINAE OF ARNOLD FROM THE MIDDLE DEVONIAN OF NEW YORK AND VIRGINIA

The discovery of new specimens from the Middle Devonian of western New York and southwestern Virginia has prompted a reinvestigation of all available type and prominently figured material of three genera: Arachnoxylon, Iridopteris, and Reimannia, originally placed in the Iridopteridinae (Coenopteridales) by Arnold (1940). For Arachnoxylon, significant new information includes the presence of: (1) "typical" protoxylem strands as opposed to the "peripheral loops" originally ascribed to the genus, (2) a small type of vascular trace which is circular in transverse section and centrarch, borne from the tips of primary xylem ribs, (3) a large type of trace which is elliptical in transverse section proximally becoming four-ribbed distally and producing pairs of small, "subsidiary" traces, (4) a whorled order of trace departure with, in one case, both large and small traces comprising parts of a single whorl, (5) metaxylem elements with scalariform to circular bordered pit pairs, and (6) primary phloem containing cells with dark materials in their lumina.; For Iridopteris, a new information includes the presence of: (1) normal protoxylem strands instead of "peripheral loops", (2) small traces, similar to those in Arachnoxylon, which are produced radially but asymmetrically from the tips of primary xylem ribs in a regularly alternate manner at each node, (3) a larger type of trace which is elliptical in transverse section and contains two protoxylem strands, one near each end, (4) a whorled order of trace departure with both large and small traces comprising some of the whorls, (5) metaxylem tracheids with circular to elongate elliptical bordered pit pairs with elliptical apertures on all walls of the elements, and (6) features of primary phloem and inner cortex similar to that seen in Arachnoxylon.; In Reimannia, axes of three orders are observed to be in organic connection. The first order axis has a three-ribbed primary xylem column containing several protoxylem strands along the median plane of each xylem rib. Traces are apparently produced in a helical manner. The primary xylem of a second order axis is proximally elliptical or diamond shaped in transverse section and gives off a sub-opposite pair of ultimate appendage traces which may divide once through the course of their departure. Distally, the primary xylem of the second order axis assumes an increasingly three-ribbed configuration and probably produces a single abaxial trace.; The new information presented here suggests that Reimannia has little in common with the other members of Arnold's Iridopteridinae. Instead, I suggest that this genus should be considered a permineralized axis fragment form-genus within the Aneurophytales (Progymnospermopsida) perhaps related most closely to Triloboxylon or Cairoa.; Accumulating evidence presented in this work and elsewhere suggests that there existed in the Middle Devonian a taxonomically distinct group of plants currently placed within the genera Arachnoxylon, Iridopteris, Asteropteris, and Ibyka. For this group, I propose the elevation of Arnold's original suborder, Iridopteridinae, to ordinal rank. The Iridopteridales, as defined in this work, is designed to include the central concept of Arnold's original group, except for Reimannia, combined with the more readily identifiable aspects of the order Ibykales established by Skog and Banks (1973).