Abstract Detail

Horsetails Through Form, Space, and Time

Escapa, Ignacio Hernan [1], Tomescu, Alexandru [2], Rothwell, Gar W [3], Elgorriaga, Andres [4].

Equisetalean evolution in deep time.

Equisetalean evolution has been approached from multiple angles in numerous studies, and has generated a number of hypotheses over the last century. Many of these have focused on the origin and relationships of Equisetum in the context of the overall pattern of relationships within euphyllophytes. However, Equisetum is the single living representative of a lineage that is at least 350 million and morphologically diverse. Early hypotheses on the evolution of the group stemmed primarily from the extensive knowledge developed as a result of studies of the Paleozoic Calamitaceae, which include numerous taxa preserved in the classic coal ball floras of the Carboniferous. Arising from these, the evolutionary sequence that led to the morphology of modern Equisetum was thought to be a transition whose intermediate stages were illustrated by Archaeocalamitaceae, Calamitaceae, Neocalamites, and Equisetites. The evolution of stems, leaves, strobili, and sporangiophores as seen in Equisetum was subsequently explained in light of this hypothesized evolutionary progression. However, a vexing issue of all these hypotheses was a major discontinuity between the widely diverse morphologies of Paleozoic equisetaleans and the considerably less variable, seemingly canalized morphology of Mesozoic taxa. During the last two decades newly discovered Equisetales, particularly from the Upper Paleozoic of high latitude regions, has yielded new morphological information that brings back into focus some previously overlooked fossils (e.g., Cruciaetheca, Paracalamitina, Tschernovia), whose reproductive morphology is characterized by the presence of multiple sporangiophore whorls per internode. This morphology is different from that seen in the Equisetum strobilus, traditionally interpreted as one sporangiophore whorl per node. In this context, morphological phylogenetic studies concerning equisetalean evolution require detailed reevaluation and analysis of alternative hypotheses for the homology of reproductive organs in fossil and extant Equisetales, together with an updated evaluation of the rich fossil record of equisetalean morphologies. In this contribution we build and analyze morphological matrices based on vegetative and reproductive characters for extant as well as extinct taxa. An alternative scenario for the evolution and origin of Equisetum, involving herbaceous taxa from the Upper Paleozoic of Gondwana and Angara is discussed. To evaluate the impact of alternative homology hypotheses for Equisetum strobilus, two alternative sets of characters are used for scoring reproductive morphology. Understanding the evolution of Equisetum in deep time provides an excellent opportunity for identifying the plesiomorphic aspects of equisetalean morphology and for attempting more meaningful comparisons with other euphyllophyte groups, aimed at better resolved tracheophyte phylogenies.

1 - MEF-CONICET, Fontana 140, Trelew Chubut, N/A, 9100, Argentina
2 - Humboldt State University, Department Of Biological Sciences, 1 Harpst Street, Arcata, CA, 95521, USA
3 - Oregon State University, Botany and Plant Pathology, 2082 Cordley Hall , Corvallis, OR, 97331, USA
4 - Museo Palentológico Egidio Feruglio-CONICET, Avenida Fontana 140, Trelew, Chubut, 9100, Argentina

Keywords:
Equisetales
Equisetum
phylogeny
Fossils
Upper Paleozoic.

Presentation Type: Symposium Presentation
Session: SY03, Horsetails through form, space and time
Location: Oglethrope Auditorium/Savannah International Trade and Convention Center
Date: Monday, August 1st, 2016
Time: 3:00 PM
Number: SY03005
Abstract ID:734
Candidate for Awards:None