Trochodendron

Abstract: Nordenskioldia borealis Heer is described and its systematic position is reassessed based on examination of the type material and specimens recently collected from three Paleocene localities in North America (Almont, North Dakota; Melville, Montana; Monarch, Wyoming). The morphology of Nordenskioldia infructescences and fruits is clarified, and in particular, silicified specimens from Almont provide new details of fruit and seed anatomy. Fruits are schizocarpic, and individual fruitlets also dehisce to release flat reticulate seeds. These seeds occur in many Paleocene floras but have not been linked previously to Nordenskioldia. Anatomical details of infructescence axes are identical to those of distinctive long and short shoot systems that cooccur with Nordenskioldia, and neither the infructescence axes nor shoots have vessels in the secondary xylem. Comparison of the floras at Almont, Melville, and Monarch with those at other Paleogene localities in Asia, Europe, and North America provides association evidence supporting earlier conclusions that the Nordenskioldia plant bore simple, entire- to crenatemargined leaves with actinodromous venation. Such leaves have been previously assigned to extant genera such as Cercidiphyllum, Cocculus, and Populus but are treated here as Zizyphoides flabella (Newberry) comb. nov. Based on the combined morphological and anatomical details now available, the Nordenskioldia plant is assigned to the Trochodendrales as an extinct genus most closely related to extant Trochodendron.

Abstract: Fossil remains of the family Trochodendraceae are found in the early middle Eocene (49–50 Ma) Republic flora of northeastern Washington, a flora that contains a highly diverse and extensive montane warm‐temperate assemblage. In this study, we document the earliest known fossil record of Trochodendron Sieb. & Zucc. (Trochodendraceae) based on the distinctive leaves of Trochodendron nastae Pigg, Wehr, & Ickert‐Bond sp. nov., and two infructescences, and an isolated fruit assigned to Trochodendron sp. The Republic Trochodendron fruits are smaller but otherwise closely resemble those of extant Trochodendron aralioides Sieb. & Zucc. and those of Neogene fossil fruits. Trochodendron nastae leaves have the features of extant and other fossil Trochodendron leaves, except for their palmate rather than pinnate primary venation. This feature is more typical of the trochodendralean sister genus Tetracentron Oliver. Trochodendron nastae leaves have venation that thus appears to be intermediate between these two genera...