Redescription and reassignment of the fossil wood Menendoxylon piptadiensis from the Pliocene Andalhuala Formation, South America

Abstract

A restudy of the holotype of the fossil wood Menendoxylon piptadiensis Lutz, 1987 has revealed that this taxon possesses several diagnostic anatomical features not associated with Leguminoseae-Mimosoideae to which it was originally referred. The combined presence of laticiferous tubes, axial parenchyma apotracheal diffuse and paratracheal vasicentric, strands of 2–7 cells, homocellular rays, simple perforation plates, vestured intervessel pits, and non-septate fibres with distinctly bordered pits are diagnostic characteristics that support its transfer to the family Apocynaceae and a particular resemblance with species of the tribe Willughbeieae within the subfamily Rauvolfioideae. The fossil is closest to Parahancornia fasciculata, Parahancornia peruviana and Couma guianensis, with anatomical characteristics especially similar those of Parahancornia fasciculata. The extant Parahancornia is most closely related to the fossil, supporting the hypothesis that this genus was more widespread in the past. Currently, it is distributed in the Neotropics, temperate Brazil and tropical South America (northern Bolivia, north-eastern Perú, Colombia and Guayana). The material studied here is the first fossil wood from South America with an affinity to Apocynaceae and a new combination is erected, Parahancornioxylon piptadiensis (Lutz) Moya & Brea gen. et comb. nov. Eco-anatomical characteristics, Carlquist's index and the nearest living relatives (NLRs) indicate a warmer and more humid climate in north-west Argentina during the Pliocene, probably associated with tropical forests.