Testing the telome concept: A modeling approach for understanding the growth of early vascular plants

The determination of morphological homologies is crucial to the phylogenetics of fossil taxa. Because of the antiquity of fossil taxa, a clear distinction of homologous organs is often difficult. Therefore, there is a need to understand the developmental processes that created these structures in order to interpret the homologous nature among plant organs. A study of developmental mechanisms in fossil plant taxa was first noted in the Telome Concept, which provides several viable mechanisms of morphological change in the Devonian fossil record. Unfortunately, many Zimmermann's ideas remain ambiguous and untested (Chapter 1). In order to test his ideas, a modeling technique has been employed to ascertain developmental information from the fossil record (Chapter 4). The algorithm's "if-then" statements and threshold variables act as developmental "rules" that constrain the ontogeny of a virtual plant. Studying the growth of living monilophytes, the parameters of the model have been adjusted to recreate the growth processes of early land plants. Specifically, the morphology and anatomy of Psilotum nudum was examined to understand the morphometric changes during aerial morphogenesis of this ancestral euphyllophyte (Chapter 2). In order to understand aerial shoot morphogenesis of the earliest land plants, morphometric data was collected from a anisotomous early euphyllophyte fossil taxon, Psilophyton dapsile (Chapter 3). We calibrated the variables and thresholds of the model to recreate a morphology of best approximation for this taxon. The calibrated algorithm represents a viable hypothesis for observing morphogenesis in the aerial shoots of Psilophyton dapsile. Alterations between this "Psilophyton" algorithm and an algorithm that creates an isotomous morphology represent possible macroevolutionary changes that occurred between ancestral-descendant pairs. Thus, the structures exhibited by both the ancestor and descendant taxa share a similar developmental mechanism and are homologous. The strength of the model lies in the ability to test the validity of the Telome Concept and propose a quantifiable process of change that occurred between ancestor-descendant groups.