Research work · 1976
Paleozoic origin of the cycads
Sergius Harry Mamay
USGS professional paper
Abstract
Upper Paleozoic rocks in the southwestern United States contain several taxa of fossil plants that appear to represent a lineage from the Pennsylvanian pteridosperms. to the modern cycads.The oldest genus, Spermopteris Cridland and Morris, is a megasporophyll that has surficial seeds borne in two rows along the sides of presumably abaxial fac,es of taeniopterid foliage; it was first found in the Virgilian Lawrence Formation of Kansas, and a poorly preserved specimen is now known from the Leonardian Wellington Formation of Oklahoma.The putatively derivative cycadean genera Phasmatocycas Mam:a.y and A rchaeocycas Mamay occur in lower Leonardian rocks of Kansas and Texas.P hasmatocycas (type-species: P. kansana Mamay) is a fertile axis that has two lateral rows of naked ovules.In P. kansana, the ovules are represented by well-preserved double cuticles similar to those of the Jurassic cycad Beania; small resinoid spherules, interpreted as remains of glands, alternate with the ovules.Taeniopterid foliage associated with P. kansana bears abundant spherules identical with those on the fertile axis; on the basis of the spherules and the known seed habit of Taeniopteris, Phasmatocycas is reconstructed as a cycadlike.megasporophyll with an elaminar, fertile base and a t31eniopterid distal lamina.The glands may be primitive nectaries, poss.iblyinvolved in an early stage in the development of entomophily.Phasmatocycas is regarded as hav.ing been derived from a spermopterid ancestor through reduction of numbers of seeds and loss of the basal foliar lamina.Althoug'h the type specimen of Phasmatocycas is from' the Wellington Formation of Kansas, a poorly preserved specimen is known from the younger Vale Formation of Texas, where it is associated with structures strongly reminiscent of male cycadean cones.Archaeocyca.sMamay, typified by A. whitei Mamay, is approximately contemporary with Phasmatocycas kansana, occurring at two localities in the Belle Plains Formation of Texas.It resembles Phasmatocycas in having a basal s•eedbearing area and a sterile distal lamina.In Archaeocycas, however, the seeds are attached to the basal lamina, which is inrolled and at least partly encloses.the seed.A rchaeocycas probably represents a form intermediate between Spermopteris and Phasmatocycas.Alternativ•ely, Archaeocycas might be a fertile appendage evolving in the direction of a carpellike organ through proliferation and fusion of the laminar margins; this alternative, however, is not preferred.Associated with the cycadean megasporophylls are a single specimen of Cycadospadix (C.yochelsoni Mamay, n. sp.) and various other plants.C. yochelsoni cannot be determined as cycadalean or bennettitalean, although it is possible that it is either and that the two orders may have been distinct as long ago as the Early Permian.Among other plant associates, only Taeniopteris is common to all the cycad-bearing assemblages; this strengthens the hypothesis that Taeniopteris was the leaf of primitive cycads, as indicated by Spermopteris.Stratigraphic distribution of the plants supports the idea of derivation of the cycads from the Pennsylvanian spermopterids and of a lineage to the modern cycads through the Triassic genera Palaeocycas-Bjuvia and Dioonitocarpidium.Horizontal and vertical distribution of the Paleozoic taxa emphasizet the importance of the lower Leonardian of the southwestern United States as a temporal and geographic locale for rapid and dramatic plant evolution.Modern cycadean leaves are thought to be derived from taeniopterid stock in the Paleozoic and Mesozoic through progressive incision of the laminar margin.In one line of foliar evolution, shallow marginal dentation, in which each tooth receives only one• vein, becomes progressively deeper, resulting in the cycadaceous leaf type in which each leaflet has a :single vein; a parallel .series of known forms involved several veins to a segment and could have produced the zamiaceous leaf type•.The Stangeria leaf type could have arisen from a compound taeniopterid with little modification, whereas marginal incision of a compound taeniopterid might have produced the Bowenia type of leaf.Cumulative evidence indicates that the cycads extend back into the Late Pennsylvanian instead of only the Late.Triassic; thus they rival the conifers as the oldest extant seed plants.FIGURE !.-Reconstruction of part of fertile l~af of Spermopteris coriacea, showing oblique view of upper (ventral) surface, and at lower right, mode of seed attachment to lamina (from Cridland and Morris, 1960, fig.10).Approximately X 3.
DOI: 10.3133/pp934