Nordenskioldia and Trochodendron (Trochodendraceae) from the Miocene of Northwestern North America

Abstract

The extinct trochodendraceous genus Nordenskioldia, well represented in the Paleocene of the Northern Hemisphere, is documented for the first time from the Neogene, based upon infructescences, fruits, associated twigs, and foliage from the Miocene of Idaho, Washington, and southern British Columbia. The infructescences and fruits, assigned to Nordenskioldia interglacialis (Hollick) comb. nov., are very similar to Paleocene N. borealis, but differ in ranging to a higher number of carpels per fruit and in being less regularly dehiscent. The leaves, Zizyphoides auriculata (Heer) comb. nov., formerly attributed to Populus and Cocculus, are clearly congeneric with the leaves associated with Nordenskioldia in Paleocene deposits. Zizyphoides auriculata leaves differ from Paleocene Z. flabellum in having generally more prominent dentations along the margin and a broader divergence of the lateral primary veins. Excellent preservation of the Miocene material reveals features not preserved in the Paleocene specimens, and in particular, lignified fruitlets clearly show aborted ovules in addition to the single mature seed. Infructescences of Trochodendron are also documented from the same Miocene localities at which N. interglacialis occurs. The close similarities between Paleocene and Miocene species of Nordenskioldia, and also between the Miocene and extant species of Trochodendron, suggest relative stasis in the morphological evolution of the Trochodendraceae over intervals of up to 45 million years.