Poster

         Paleobotany

First evidence of anisotomous dichotomy in Stigmaria ficoides

Presenting Author
Michael D'Antonio
Description
Stigmaria ficoides is the late Paleozoic form species of rooting organ for the members of the core of the arborescent lycopsid phylogeny (families Diaphorodendraceae and Lepidodendraceae). This organ has been traditionally described as four primary axes each dividing dichotomously twice to yield sixteen terminal axes, all bearing lateral appendages or “rootlets.” Until now, the dichotomies constituting the organ system architecture have only been known to be isotomous (i.e., the two daughter branches are roughly the same dimension), unlike the architecture of some sigillarian Stigmaria morphospecies such as S. asiatica, which bore minor geotropic axes on the underside of the stigmarian arms. Here we describe two S. ficoides specimens that branch by anisotomous dichotomies (i.e., the two daughter branches are of unequal dimension). The assignment is made to S. ficoides based on rootlet scar morphology and insertion pattern. Each specimen exhibits minor axes branching off a larger parent axis. This finding demonstrates a greater degree of developmental plasticity than had previously been known for the rooting system form species and suggests that there is still much to be learned about arborescent lycopsid developmental biology.