Fossil Leaves and Fruits of Tetramelaceae (Curcurbitales) from the Eocene of the Rocky Mountain Region, USA, and Their Biogeographic Significance

Abstract

Premise of research. Eocene sediments of Colorado, Utah, and Wyoming contain fossil leaves and infructescences that appear to represent Tetramelaceae (Cucurbitales). Today, this family consists of two genera (Tetrameles and Octomeles) of towering rain forest trees native to India, Southeast Asia, and Australia. Methodology. We examined the fruit, seed, and leaf morphology of extant Tetramelaceae as a basis for recognizing fossil representatives. Fossils were studied from localities in the Parachute Creek Member of the Green River Formation in Utah and Colorado and in the Bridger and Aycross Formations of western Wyoming. Extant and extinct material was studied by light microscopy and micro–computed tomography (micro-CT) scanning. Pivotal results. Infructescences of Parvaspicula lepidioidea (Cockerell) comb. nov. conform to Tetramelaceae in their spicate arrangement of helically arranged, capsular, ribbed fruits with inferior ovaries and parietal placentation. Each fruit typically has four styles and dehisces apically between the styles, reminiscent of extant Tetrameles, but they share an apical disk and the bulbous stigmas of extant Octomeles. Leaves of Punctaphyllum glandulosum (Brown) comb. nov. conform to Tetramelaceae in their long petioles, basal marginal and intramarginal veins, and regularly spaced circular laminar glands. These glands are small and cover the laminae or are larger and concentrated in a single file row along the lamina margin. Punctaphyllum glandulosum varies in its base shape, lobe number, and margin type, matching the variation seen in extant Octomeles leaves. Conclusions. The repeated co-occurrence of Parvaspicula and Punctaphyllum and the morphological similarities of both of these organs with extant Tetramelaceae lead us to infer that they represent parts of the same extinct plant. These records serve as the first documented leaf and infructescence fossils of Tetramelaceae and suggest that the family once lived in North America, indicating broader biogeographic distribution than previously recognized.