Porostrobus nathorstii sp. nov.: A New Species of Lycopsid Cone from the Early Pennsylvanian of Illinois

Thirty-one specimens of a small megasporangiate lycopsid cone referable to the genus Porostrobus Nathorst and abundant associated dispersed megaspores have been collected from Early Pennsylvanian strata in the Allied Stone Company quarry, Milan, Illinois. Based on other elements in the flora, the deposit is considered to be part of the Morrowan Caseyville Formation and probably of Namurian age. This is the first reported occurrence of Porostrobus in North America and the cones are recognized as a new species, P. nathorstii. The environment of deposition indicates that the cones may have been transported from the parent plant prior to preservation. Cones are preserved as coalified compressions measuring 15-36 mm long by 2.57 mm wide, and are characterized by an apical tuft of leaves up to 20 mm long. Sporophylls are spirally arranged on a narrow cone axis, lack a heel or keel, and have a long distal lamina. Sporangia contain a single functional megaspore tetrad. Mature megaspores are 750-1,150 ,m in diameter, have prominent trilete sutures raised to form a gula, and have numerous branched hairs confined to an equatorial band. Megaspores correspond to the dispersed form Setosisporites praetextus (Zemdt) Potonie and Kremp. Porostrobus nathorstii s the only species of the genus described to date that is monosporangiate. IN RECENT YEARS, several sites in western Illinois have yielded significant information about Early Pennsylvanian nonswamp floras, both on their overall composition and on individual taxa within the floras. The Allied Stone Company quarry in Milan, Illinois (Leary, 198 1) has been a major collecting site for nearly ten years and continues to yield new species as well as new data on known taxa. Among the abundant and well preserved fossil plants from the Allied quarry deposits are thirty-one specimens of small lycopsid cone referable to the genus Porostrobus Nathorst and numerous dispersed associated megaspores of the Setosisporites Potonie and Kremp (1955) type. Porostrobus was originally erected by Nathorst (19 14) based on a single specimen which he had previously described as a new species, Lepidostrobus zeilleri, from the Lower Carboniferous of Spitzbergen (Nathorst, 1894). Nathorst reassigned this cone in the belief that it was associated with Porodendron stems from the same locality. This association has been l Received for publication 28 October 1988; revision accepted 10 May 1989. The authors thank Moline Consumers, operators of the Allied Stone Company quarry, for access to the deposits, and Debra Willard, University of Illinois, for SEM micrographs. This research was supported by NSF grants EAR-8108628 and EAR-8403856 to RLL. 2 Current address: Department of Botany, North Carolina State University, Raleigh, NC 27695. questioned because it is based solely on a single, unattached cone (Chaloner, 1967). Bharadwaj (1958) reexamined P. zeilleri and more fully described megaspores and microspores contained within the cone. Chaloner (1967) states that the megaspores are of the Setosisporites hirsutus var. brevispora (Zerndt) Potonie and Kremp (1955) type. Playford (1963) describes dispersed microspores from Spitzbergen that closely resemble those described for P. zeilleri (Bharadwaj, 1958), and designates them as Densosporites spitzbergensis. Chaloner (1962) named a second species of Porostrobus, P. canonbiensis, based on a single cone from the Westphalian of Scotland which he had previously named Selaginellites canonbiensis (Chaloner, 1958). Chaloner (1958) describes the megaspores as being of the S. hirsutus type and the microspores as being of the Densosporites loricatus type. The cones described in this report represent a new species, Porostrobus nathorstii, and constitute the largest known suite of specimens of the genus known to date. This is also the first reported occurrence of this genus in North America. The discovery of these cones provides an opportunity to expand the generic concept of Porostrobus to include monosporangiate cones. MATERIALS AND METHODS-Study material includes thirty-one cones and cone fragments