Benton M. Stidd

The current status of medullosan seed ferns

The compression and permineralized fossil record of presumed medullosan plants is reviewed and evaluated. Recent studies of stelar architecture suggest that stems are monostelic rather than polystelic as commonly believed. Imparipinnate fronds are mostly of the Neuralthopteris/Alethopteris type with Myeloxylon anatomy; the affinity between medullosan stems and neuropterid fronds is uncertain. It is suggested that the reproductive organs of the family should be restricted to those pollen organs believed to have produced Monoletes pollen and to ovules, for the most part, of the Trigonocarpus/Pachytesta type. It is doubtful that reproductive organs replaced lateral pinnules; it is more likely that they terminated pinnae or were borne abaxially along the pinna rachis. In some cases they may have been borne directly on penultimate axes or were, as in Codonotheca and Goldenbergia, borne on fertile fronds or fertile portions of vegetative fronds. Recent research involving pollen organs is emphasized and a phylogeny of Monoletespollen organs is proposed. It is suggested that the fossil record of medullosans is best interpreted from a punctuational perspective.

THE FROND OF HETERANGIUM

Frond members belonging to the monostelic seed fern genus Heterangium have been found in Pennsylvanian age coal balls collected in the Illinois Basin. Petioles bear small pinnae below a dichotomy which produced a bipartite frond. Pinnules of the Sphenopteris obtusiloba type are borne on secondary pinnae. The anatomy of each pinna order and the pinnules is described.

On the frond of Sutcliffia insigns var. tuberculata

Abstract Sutcliffia foliar members have been found in Pennsylvanian coal balls from seven localities in the Illinois Basin. Large frond members appear to be very similar to those of Medullosa . With decrease in size, vascular bundles assume a bilateral arrangement and pinna rachises appear more similar to lyginopterid foliar members. No pinnules have been found attached but circumstantial evidence suggests Linopteris is the pinnule type.

A new species of Heterangium from the Illinois basin of North America

A Heterangium specimen from the Illinois Basin exhibits a prominent periderm and distinct exarch protoxylem groups from which leaf traces arise individually. As a trace passes to a leaf it remains single but is bilobed for as far as it can be followed into a winged petiole. These features place the new species in the subgenus Heterangium. The specimen lacks the sclerotic plates and sparganum outer cortex present in other species. Certain aspects of stelar structure suggests that Heterangium represents a pteridosperm lineage that retained the protostelic condition from ancestors that produced other lineages exhibiting various degrees and types of longitudinal stelar dissection.

Telomes, theory change, and the evolution of vascular plants

Abstract Evolutionary theory has been dominated by the so-called “modern synthesis” since its inception in the 1930s. Zimmermanns telome theory incorporates the same gradualistic assumptions as the modern synthesis and has nurtured and molded thought patterns of paleobotanists and students of plant evolution. Telome theory, like the modern synthesis, should not be faulted for what it includes so much as for what it excludes. Telome theory emphasizes evolution by terminal additions to successive ontogenies but is usually employed descriptively as if mature organs have undergone webbing, fusion, recurvation and the like. Thought patterns of this sort are necessarily gradualistic and do not recognize internal factors controlling development that might affect non-terminal stages. Interpretation of vascular plant phylogeny has been overly constrained by assumptions and mechanisms that occupy the core of the modern synthesis. Certain nongradualistic explanations for the evolution of selected organisms are reviewed and attention focused on the role of internal constraints controlling ontogenetic pathways and their macroevolutionary consequences.

Is species selection dependent upon emergent characters

The architects of punctuated equilibrium and species selection as well as more recent workers (Vrba) have narrowed the original formulation of species selection and made it dependent upon so-called emergent characters. One criticism of this narrow version is the dearth of emergent characters with a consequent diminution in the robustness of species selection as an important evolutionary process. We argue that monomorphic species characters may at times be the focus of selection and that under these circumstances selection at the organism level is by-passed due to the absence of critical variance. Selection therefore shifts to the species level where variability reemerges in a clade. The absence of critical variance among organisms prevents effect macroevolution from operating. If species-wide properties are important in macroevolutionary processes, as we contend, systematists should pay more attention to their elucidation.