Equisetites from the Early Permian of North-central Texas

Equisetites is a genus of fossil plants allied with, if not identical to, modern Equisetum, which is the only extant genus of the ancient lineage Sphenopsida. This class-level group of lower vascular plants is characterized by whorled vegetative and reproductive appendages, distinctive nodal and internodal morphologies, and unique "sporangiophores," specialized reproductive organs of uncertain homology on which the sporangia are borne. Described, heretofore, from rocks ranging in age from Middle Permian through Pleistocene, Equisetites is primarily a genus of the post-Paleozoic. Fossils similar to isolated leaf sheaths of this genus have been found in a single deposit of Early Permian age in north-central Texas. Five specimens are known, each composed of up to 10 basally fused and apically free leaves that are straight-sided but taper sharply to an acute tip. Leaves have a single midvein. Attribution to Equisetites must remain tentative in the absence of any other organs of the plant, including stems. However, it is common in some species of extant Equisetum for leaf sheaths to fall away from the main stem as they die and dry, so such isolation is not entirely unexpected. INTRODUCTION Equisetites is a genus of fossil plants with similar gross morphology and anatomy to modern Equisetum. In addition to the general sphenopsid characteristics of whorled appendages and sporangiophores, these genera are characterized by small leaves fused into a leaf sheath, and strobili that lack bracts. The fossils are placed in a separate genus primarily because of their state of preservation, not because they share some distinguishable derived characteristics that unite them as a group while at the same time separating them from Equisetum. Equisetites is reported to have a range of mid-Permian through the Pleistocene (Stewart and Rothwell, 1993, p. 207). Most of the reported Paleozoic occurrences have proven to be cases of mistaken identity, however, a consequence of the vagaries of natural processes that alter the appearances of living organs and continue to modify them during the process of fossilization. For example, E. contractus Goppert (1864) is likely a calamitean stem (Schimper, 1869); E. lingulatus Germar (1845) (= E. priscus Geinitz) is possibly an Annularia, although at least one of the illustrations appears to show an Equisetites-\ike leaf sheath (Germar, 1845, plate X, fig. 5); E. rugosus Schimper (1869) is very large with connected leaves and appears to be a Phyllotheca; E. zeaeformis (Schlotheim) Andrae (Schlotheim, 1820) has very broad leaves with large teeth and may be a torn Cordaites. Possible candidates for Paleozoic occurrences include Equisetites vaujolyi Zeiller (1895) from the Permian of France, which has large leaves, up to 6 cm long and 6 mm wide, with a single vein per leaf (Doubinger, 1956). However, this species is quite different in appearance from better known Mesozoic forms. The bractless Equisetites hemingwayi is in appearance very much like strobili of modem Equisetum and, thus, could belong to this lineage. Based on specimens in the British Museum (Kidston, 1901), strobili were apparently sessile and borne at the nodes on stems that are very Equisetum-\ike in appearance. This species was described from what is now referred to as the Middle Pennsylvanian, or Westphalian, from the Yorkshire coal measures in Great Britain, which would make this most certainly the earliest, if uncorroborated, occurrence of this branch of the Sphenopsid family tree. Seward (1898) who previously had examined Kidston's specimen determined that the strobili, although Equisetum-like in construction, are lateral instead of terminally attached, and stems show no evidence of leaf sheaths; Seward thought the leaves to be Astemphyllites-\ike, putting the generic affinities of this species in question, as well. Specimens very similar to Equisetites leaf sheaths have been found in rocks of Early Permian age from north-central Texas. The specimens are not found in attachment to stems or in association with identifiably sphenopsid reproductive organs. Consequently, the identification remains tentative. Nonetheless, the Early Permian precocious occurrence of plants typical of the Late Permian and Mesozoic has been reported elsewhere in north-central Texas for cycad and conifer taxa (DiMichele et al., 2001a), and scraps of conifers are known to occur millions of years before the appearance of good specimens within a macrofloral context (Lyons and Darrah, 1989). Such occurrences suggest the existence of many vascular-plant evolutionary lineages well before their appearance in the fossil record.