Metasequoia milleri is an extinct redwood species in the family Cupressaceae described from numerous permineralized remains in the Princeton Chert of British Columbia, Canada. The species was initially described from isolated pollen cones, with further papers released on the leaves, ovulate cones, branches, and mycorrhizal root communities. The pollen cones are known from specimens attached to twigs, while the other organs are included in the species based on close fossil association only. The descriptive history of the species spans from 1977 to 2001 with five major papers. It is one of three extinct species belonging to the redwood genus Metasequoia.
Distribution
Metasequoia milleri is known exclusively from the Princeton Chert, a fossil locality in British Columbia, Canada, which comprises an anatomically preserved flora of Eocene age, with rich species abundance and diversity. The chert is located in exposures of the Allenby Formation on the east bank of the Similkameen River, 8.5 km (5.3 mi) south of the town of Princeton, British Columbia. Notable in conjunction with the coal seams of the Allenby Formation are sections of chert which formed during silica-rich periods. The rapid cyclical changes from coal to chert and back are not noted in any other fossil locality in the world. An estimated 49 coal-chert cycles are known, though the exact conditions for this process are not well understood. Silica-rich volcanic episodes in the region during deposition would have been needed for formation of the cherts, while slowly moving waters and gently subsiding terrains would be needed for the peats and fens to accumulate. Rates of organic deposition in swamps have been estimated at 0.5–1 mm (0.02–0.04 in) in modern temperate climates, this suggests the time needed for each 10–20 cm (4–8 in) chert layer would be at least 100 years or more, with the full sequence of cycles taking place over no more than 15,000 years. The Allenby Formation is one of the southern-most in the Lagerstätten of the Eocene Okanagan Highlands, British Columbia, with the Canadian Penticton Group at the international border and the Klondike Mountain Formation of Republic, Washington, in northern Ferry County to the south. The highlands, including the Allenby Formation, have been described as one of the "Great Canadian Lagerstätten" based on the diversity, quality and unique nature of the biotas that are preserved. The Eocene Okanagan Highlands temperate-subtropical biome preserved across a large transect of lakes recorded many of the earliest appearances of modern genera, while also documenting the last stands of ancient lines.
History and classification In the early 1970s a series of papers were written noting the presence of permineralized plant fossils associated with the Allenby Formation south of Princeton, British Columbia. Paleobotanists Gar Rothwell and Jim Basinger reported the discovery of conifer pollen cones in the Princeton chert specimens via a 1977 paper in the Canadian Journal of Botany, identifying them as belonging to Metasequoia and Pinus. They noted the redwood pollen cones were often associated with woody axis, posited as from the same trees. Over a year later Rothwell and Basinger published a full description of the redwood pollen cones based on a series of 52 specimens which had been isolated and examined from the chert blocks. Using a cellulose acetate peel modified with a hydrofloric acid component microscope slid mounts of the pollen cones were created, while sections of chert were subjected to maceration and the free pollen grains obtained mounted for study. The chert blocks and microscope slides created were accessioned into the University of Alberta palaeobotanical collections with block number "P1195D top" designated as containing the holotype cone. The blocks top surface exposes the pollen cone in oblique longitudinal view, and the slide series "5271-5289" provides a serial section though the cone. Three paratype bearing blocks were also designated in conjunction with slide series from each. Paratypes UAPC P1013B2 bot a, with slides 5110-5139 and P1013B2 bot 6 with slides SL5093-5109 plus SL5229-5254 both provide pollen cones presented in longitudinal serial section at different cut angles. The third paratype P1269A with slides SL5329-5344 presents a cone in cross section profile. The formal description of the species from its pollen cones was published by Rothwell and Basinger (1979) in the Canadian Journal of Botany, with the species name milleri being chosen as a patronym honoring Charles N. Miller for his notable contributions to the understanding of confer evolution. In 1981 Basinger published a follow-up paper in which the scope and botanical definitions of M. milleri were expanded beyond the original pollen cone description. Anatomic descriptions were given for both mature and immature woods associated with the described pollen cones. Additionally shoots and leaves were described in detail from associated unconnected fossils present in the chert blocks. Three years later the species was again revisited by Basinger who documented associated seed cones from the chert. He noted that while the cones had not been found in full connection to other M. milleri organs, the consistent association of isolated parts and clear genus identity lead to placement in the same species. This outweighed what he ascribed as the usual caution which would be given to connecting plant organs not found in actual attachment under typical conditions. Approximately 17 years later, a short paper was published by paleobotanist Ruth Stockey and coauthors documenting fungal association with the roots of M. milleri. Based on slide series from several rootlets they detailed the repeated presence of symbiotic arbuscular mycorrhiza in the fossils. The group extrapolates the possibility of a high degree of evolutionary stasis based on comparison to living M. glyptostroboides which shows very high similarity with the fossils.
Description
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