The vegetative body of Metasequoia milleri from the Middle Eocene of southern British Columbia

Abstract

Anatomically preserved vegetative remains of Metasequoia milleri Rothwell and Basinger are common in the Princeton chert of the Allenby Formation in southern British Columbia. Deposition of the Allenby Formation and associated volcanics occurred during Middle Eocene time. The Princeton chert locality is in the upper strata of the Allenby Formation and is of late Middle Eocene age. The Princeton chert was formed by siliceous permineralization of marsh soil. Dissolved silicates were probably introduced by periodic influx of water from mineral springs or geysers.Anatomical features of stems, wood, and leaves are well preserved. Mature wood of the fossil resembles that of Metasequoia glyptostroboides in having traumatic resin cysts; opposite pitting on radial walls of tracheids; taxodioid cross-field pitting; tall, uniseriate rays; smooth-walled ray parenchyma; and diffuse, resinous, smooth-walled wood parenchyma. Leaves are linear, hypostomatic, and borne decussately, and have one or three resin ducts and slightly undulate to smooth epidermal cell walls. Leaves of living M. glyptostroboides differ in consistently having three resin ducts and much more pronounced undulations of epidermal cell walls. Metasequoia milleri has leaves of a generalized structure from which leaf types of many taxodiaceous genera could be derived. Roots associated with M. milleri are dimorphic. Primary tissues and secondary phloem are exceedingly well preserved. Cortex of both long and short roots contains mycorrhizal fungi.Compression remains of M. occidentalis are not distinguishable from M. glyptostroboides. Anatomical features of M. milleri, however, do reveal differences between Eocene and living Metasequoia. Wood rays of M. milleri are much higher than those of M. glyptostroboides. Other features of stem anatomy are similar in both species. Pollen cones differ in developmental and minor anatomical features. The close similarity of most organs of M. milleri to M. glyptostroboides and the dissimilarity of leaf structure indicate mosaic evolution within the genus.