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Abstract Detail


Rothwell, Gar [1], Stockey, Ruth [2], Stevenson, Dennis [3].

Cyacas-like seeds in the Bajocian Stage of the Middle Jurassic.

More than 200 large permineralized cycad seeds have been recovered from shallow marine sediments of the Yukon Formation (Vancouver Group) on South Balch Island, near Queen Charlotte City, on the islands of Haida Gwaii, off the west cost of Canada. Two distinct morphologies of such seeds have been identified, including the radial Cycadeocarpus columbianus Dawson and a “bilateral” seed that closely resembles Cycas. The Cycas-like seeds are 3.3-7.1 cm long, 3.2-7.6 cm wide, and 2.4-4.2 cm thick. Some have a smooth outer surface while others have several longitudinally oriented ridges and furrows on the surface. Some are oval in cross section, while others have blunt wings in the major plane of section. All of the forms intergrade, with the differences having resulted from differential taphonomic modification. The integument is divided into two valves that are defined by a suture that extends from the endotesta to the periphery of the seed in the major plane of section, and gives the seeds 180 degree rotational symmetry. Opening at the suture is characteristic of the distinctive germination mechanism of the genus Cycas. The nucellus is adnate to the integument in the basal three quarters of the seed cavity and forms a distal pollen chamber that is sealed at the apex. The integument consists of an inner zone of small, thin-walled cells, and an outer zone of larger more isodiametric cells that form thick-walled sclereids toward the inside. Toward the outside, the thick-walled sclereids intergrade with thin-walled cells of similar dimensions. Two large vascular bundles of contorted tracheids enter the base of the seed in the major plane and extend distally between the two valves of the integument. Distal to the chalaza those bundles enter the inner zone integument and branch. One branch extends proximally to below the seed cavity where it converges with the bundle from the other side of the seed as transfusion tracheids. The other branch extends distally to near the micropyle, where it terminates within the inner zone of the integument. Incompletely preserved megagametophyte cells and an apparently dicotyledonous embryo are present in some specimens. These seeds compare favorably with those of Cycas spp. in most characters, including germination via splitting of the bivalved integument. However, novel seed vascularization and a distinctive zonation of the integument reveal that they most probably represent a stem-group member of the Cycadaceae.

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1 - Ohio University/Oregon State University, Department of Environmental and Plant Biology/Department of Botany and Plant Pathology, 2082 Cordley Hall, Oregon State University, Corvallis, OR, 97331, USA
2 - Oregon State University, Department of Botany and Plant Biology, 2082 Cordley Hall, Corvallis, OR, 97331, USA

Cycad seed
Haida Gwaii, Canada.

Presentation Type: Oral Paper
Session: 17, Paleozoic and Mesozoic paleobotany
Location: Sundance 3/Omni Hotel
Date: Tuesday, June 27th, 2017
Time: 9:00 AM
Number: 17005
Abstract ID:358
Candidate for Awards:None

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