Metasequoia Hu et Cheng (Cupressaceae) from the Eocene of Axel Heiberg Island, Canadian High Arctic
Liu, (Christopher) Yu-Sheng;Basinger, James F.
published: Dec 22, 2009
A large number of fossilized remains, consisting of leafy branchlets, seed cones, pollen cones, and seeds, of Metasequoia (Cupressaceae) recovered from Middle Eocene sediments of the Buchanan Lake Formation, Axel Heiberg Island, Canadian Arctic Archipelago are studied morphologically and anatomically. They are assigned to M. occidentalis (Newberry) Chaney with an emendation, which is proposed to accommodate all the published Metasequoia fossils but M. milleri Rothwell et Basinger. An emendation is made on the basis of detailed description of the associated leaf-bearing branchlets, leaves, pollen cones, pollen in situ, and seed cones, particularly the presence of helically arranged scales in some seed cones, which is a feature no longer represented in the extant Metasequoia. The most significant systematic emendation made is the morphology of pollen and arrangement pattern of seed cone scales. A comparison with extant and previously published fossil Metasequoia is made in detail and indicates a morphological stasis since Late Cretaceous/Paleocene. Slight differences from the extant species, however, have been found, such as narrower stomatal bands, smaller stomata, higher stomatal density, presence of hypodermis (like that of M. milleri), smaller pollen grains, gemmate exine but with more or less smooth gemmae, and seed cones with, in part, helically arranged scales in the fossil Metasequoia. The foliar cuticles of the Arctic fossil were compared with those from the populations representing all the natural distribution areas of Metasequoia in central China and the result demonstrates that the fossil shares morphological resemblance most closely to those of the extant Metasequoia from an isolated population in central China’s Hunan Province, about 100 km southeast of the type locality in Hubei Province, central China. The fossil pollen in situ were explained as relatively immature due to only being extracted from the smaller pollen cones and having distinct morphology and anatomy under scanning and transmission electron microscopy. The immature pollen grains have ontogenetic developmental significance, indicating that the spinulate processes of the mature Metasequoia pollen are probably developed from smooth gemmae. The helical arrangement of seed cone scales occurs among about 22% of the more than 3200 seed cones surveyed. The size range of seed cones is well correlated to three ecotypic variations in the extant Metasequoia and indicates that M. occidentalis of Axel Heiberg with larger seed cones (bigger than 2 cm, including large and medium cones) lived in a nearby upland in the Eocene, while those with smaller seed cones (smaller than 2 cm) were distributed in moist or inundated lowlands.