Several Cenozoic endocarp remains from the northern hemisphere have been described with strong affinities to either Menispermum L. or Sinomenium Diels, a monophyletic group of menispermous vines. It has been proposed that all of these fossil species are synonymous and should be included within Sinomenium. In order to evaluate this suggestion, we have studied the morphological variation ranges in the menispermous endocarps with geometric morphometrics, and then the ranges of the selected fossil endocarps is compared to the ranges of modern endocarps. The shape of each endocarp is described using eight landmarks and 17 semilandmarks, accounting for the outline and the positions of lateral ridge and foramen on the endocarps. Endocarp ornamentation is studied by statistical comparisons of the number of transverse ridges. It is concluded that the ranges of variation within the fossil genera, all morphologically related to horseshoe-shaped endocarps in Menispermaceae, are found not to be greater than that in the only extant species of Sinomenium, S. acutum (Thunb.) Rehder & Wilson. Sinomenium macrocarpum Liu & Jacques, 2010 differs from the other fossil species of Sinomenium by its higher number of transverse ridges. All other fossil species of Sinomenium, except S. macrocarpum, and Wardensheppeya Eyde, 1970 are synonymous. Menispermum? taylori Chandler, 1964 is transferred to Sinomenium. The fossil genus Palaeosinomenium Chandler, 1961 is confirmed through the obliquity of its endocarp, but all species are found to be synonymous. Menispermicarpum rariforme Chandler, 1961 is also included in Palaeosinomenium.

The present study documents the first confirmed fossil record of Sinomenium in Menispermaceae, Sinomenium macrocarpum sp. nov., from the recently discovered Gray Fossil Site in Tennessee, southeastern USA. The fossil species is represented by more than 120 endocarps, all of which are characterized by their horseshoe-shaped form and occurrence of highly ornamented protuberances on both dorsal and lateral crests. A combination of their relatively large size and highly developed of protuberances on the surface of endocarp warrants the new species. The new species is justified through a detailed comparison with the related and published extant and fossil species. The discovery of the genus in North America appears to support that the Gray site in southern Appalachian region represented a forest refugium during the late Neogene when the global cooling was intensified and grasslands were globally expanded.