Mummified leaves of Metasequoia occidentalis (Newberry) Chaney from the middle Eocene Buchanan Lake Formation at Napartulik, Axel Heiberg Island, were examined by transmission electron microscopy to determine whether sun- and shade-type leaves could be identified. Where discernable, features related to thylakoid geometry, such as the degree of grana stacking, the number of thylakoids per granum and grana stack width, were measured. The chloroplast characteristics of M. glyptostroboides Hu & Cheng leaves have a highly significant response to light intensity such that the leaves can be characterized as sun or shade leaves based on these features, indicating that adequately preserved chloroplasts in M. occidentalis leaves can also be characterized in this manner. Using data from the response of chloroplast ultrastructure to sun and shade environments in M. glyptostroboides leaves, four classification analyses, each using different combinations of chloroplast ultrastructural features, were performed to assign adequately preserved leaves of M. occidentalis as having grown in either a sun or shade environment. The ability of the four classification methods to reliably predict whether the fossil leaves grew in sun or shade environments was tested using data from leaves of M. glyptostroboides growing in known light environments. On examining 108 sections of 36 fossil leaves, 9 leaves were found to have adequate chloroplast preservation for the purpose of classification. By the method of analysis deemed to be the most reliable, five of the nine were leaves classified as sun-type leaves and four were classified as shade types. The results further suggest that leaf senescence in M. occidentalis growing in the polar forests may have been initiated by temperature, rather than low-light conditions.
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